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The dynamics of spin–orbit-coupled Bose–Einstein condensate with parity-time symmetry through a moving obstacle potential is simulated numerically. In the miscible two-component condensate, the formation of the Kármán vortex street is observed in one component, while 'the half-quantum vortex street' is observed in the other component. Other patterns of vortex shedding, such as oblique vortex dipoles, V-shaped vortex pairs, irregular turbulence, and combined modes of various wakes, can also be found. The ratio of inter-vortex spacing in one row to the distance between vortex rows is approximately 0.18, which is less than the stability condition 0.28 of classical fluid. The drag force acting on the obstacle potential is simulated. The parametric regions of Kármán vortex street and other vortex patterns are calculated. The range of Kármán vortex street is surrounded by the region of combined modes. In addition, spin–orbit coupling disrupts the symmetry of the system and the gain-loss affects the local particle distribution of the system, which leads to the local symmetry breaking of the system, and finally influences the stability of the Kármán vortex street. Finally, we propose an experimental protocol to realize the Kármán vortex street in a system. [ABSTRACT FROM AUTHOR]

In this paper, results of a flow visualization study on the flow around a square cylinder with corner chamfering are presented. The corners of the test cylinders were chamfered with each corner forming a triangle with chamfer dimension b. Experiments were conducted for b/Bo ratios of 0.05, 0.1, 0.2, and 0.3 where Bo is the side dimension of the uncut square cylinder. The flow structures, particularly the vortex shedding mode and mechanism around the uncut as well as chamfered square cylinders, were investigated in order to deduce the effect of corner modification. All the studies were conducted at a Reynolds number value of 2,100 (based on Bo) on both stationary and oscillating cylinders. For oscillating cases, a special mechanism was built to oscillate the cylinders (forced oscillation) at desired amplitudes and frequencies. Results indicates that corner chamfering brings notable changes in the near-wake flow structures of a square section cylinder both in stationary and oscillating conditions. In view of aerospace structures with similar geometries, the present results carry considerable practical significance. Many engineering structures carry square section geometry, such as buildings, bridge pylons, and so on, and are subjected to fluid flows. Flow around them can induce certain static and dynamic loads on these structures due to which eventually, they can fail if not designed properly. Flow-induced loads majorly depend on the body geometry. For square section cylinders (structures), corner shape is found to be a crucial factor that decides the nature of the flow field developed over them. This is because a properly designed corner shape can significantly bring down hydrodynamic/aerodynamic loads imposed on structures, ensuring structural safety. In this study, the corners of a square section cylinder are chamfered, and influence of this corner shape is studied through flow visualization. Flow visualization enables qualitative (mainly) as well as quantitative analysis of the flow field developed on the cylinder with different degrees of corner chamfering. This study brings out the effectiveness of corner chamfering in influencing the flow around a square cylinder, which represents an engineering structure in real world scenarios. [ABSTRACT FROM AUTHOR]

Design load and vibration for parked conditions are gaining in importance for large-scale modern wind turbines with increasing flexibility, especially edgewise vibration when the blade is at a high angle of attack. In this work, flow-induced vibration of the wind turbine airfoil at 90 degrees of attack angle is studied with the fluid-structure interaction (FSI) simulation. The unsteady aerodynamic force due to flow separation and vortex shedding at the high angle of attack causes the chordwise vibration of the airfoil. When the vortex shedding frequency f v gets close to the chordwise natural frequency f n of the airfoil, vortex-induced vibration (VIV) of high amplitude occurs accompanied with the frequency lock-in phenomenon. In the post lock-in regime, it is found that period-3 and torus bifurcation occur successively and the vibration response becomes aperiodic. Dynamic mode decomposition(DMD) technique is used to investigate the mechanism of bifurcation from the perspective of energy balance, through analyzing the vorticity field in the wake and pressure distribution on the airfoil surface. For the certain incoming velocity in the post lock-in regime, since the frequency of the DMD mode f = 2 f v / 3 is close to the natural frequency f n , both the vibration of frequency 2 f v / 3 and f v get excited, leading to the onset of bifurcation. The Lissajou curves are obtained through reconstructing the transient pressure of each DMD mode, which indicates that energy transfer mainly exists in modes f = f v . In addition, the reconstructed Lissajou curves based on the leading DMD modes agree well with the original time-domain Lissajou curves. [ABSTRACT FROM AUTHOR]

The energy harnessing from flow-induced vibrations (FIV) by an oscillating foil placed tandemly behind a circular cylinder (which serves as a vortex generator) is investigated. The foil is submerged in the wake produced by the fixed cylinder and could oscillate in the direction perpendicular to the incoming flow with single-degree freedom. The spacing ratio ranges from 1.0 to 5.0. The oncoming fluid velocity is U = 1–10 m/s, corresponding to the reduced velocity Ur = 3.81–38.08 and the Reynolds number Re = 9.58 × 103–9.58 × 104. Four harnessing damping ratios (ζharness = 0.0054–0.0216) are used to simulate the energy conversion conditions. The main conclusions are: (1) The optimal oscillation pattern related to the highest harnessed energy emerges as the spacing ratio close to 1.0. (2) The airflow energy converted by the foil is positively correlated with the harnessing damping ratio because the amplitude responses are similar at various harnessing damping ratios. A high velocity yields the highest harnessed power. (3) The harnessing efficiency of the foil could reach 48.89%, which is much more than that of an isolated flapping foil. [ABSTRACT FROM AUTHOR]

This paper describes the use of wireless smart sensors for examining the underlying mechanism for the wind-induced vibration of high-mast illumination pole (HMIP) structures. HMIPs are tall, slender structures with low inherent damping. Video recordings of multiple HMIPs showed considerable vibrations of these HMIPs under wind loading in the state of Kansas. The HMIPs experienced cyclic large-amplitude displacements at the top, which can produce high-stress demand and lead to fatigue cracking at the bottom of the pole. In this study, the natural frequencies of the HMIP were assessed using pluck tests and finite element modeling, and the recorded vibration frequencies were obtained through computer vision-based video analysis. Meanwhile, a 30.48 m tall HMIP with three LED luminaires made of galvanized steel located in Wakeeney, Kansas, was selected for long-term vibration monitoring using wireless smart sensors to investigate the underlying mechanism for the excessive wind-induced vibrations. Data analysis with the long-term monitoring data indicates that while vortex-induced vibration occurs frequently at relatively low amplitude, buffeting-induced vibration was the leading cause of the excessive vibrations of the monitored HMIP. The findings provide crucial information to guide the design of vibration mitigation strategies for these HMIP structures. [ABSTRACT FROM AUTHOR]

To study the aerodynamic performance and wind-induced response of carbon fiber-reinforced polymer (CFRP) cables, CFRP cable was designed by replacing a steel cable in a tied arch bridge based on stiffness, strength and area equivalent criteria, respectively. The aerodynamic performance and wind-induced response of CFRP cable and steel cable were studied and compared by computational fluid dynamics (CFD) model. Based on the computational results, optimal cable replacement criterion was proposed for CFRP cable to replace steel cable. In addition, surface modification was conducted by engrooving vertical symmetric (VS), vertical asymmetric (VA) and helical symmetric (HS) V-shaped grooves to improve the aerodynamic performance and wind-induced response of CFRP cable. Results showed that CFRP cables exhibited inferior aerodynamic performance and wind-induced response in most cases. However, CFRP cable based on stiffness equivalent criterion exhibited better aerodynamic performance and wind-induced vibration properties compared to the other two cable replacement criteria, thus is regarded as the optimal substitute for steel cable. In addition, HS grooves generated symmetric disturbances and caused approximately equivalent boundary layer separation delays uniformly and continuously along the cable length, thus exhibiting better effect in decreasing the reverse flow region, the maximum negative flow velocity, the vortex shedding frequency and the wind-induced vibration amplitude of CFRP cable. Hence, the stiffness equivalent criterion combined with surface modification with HS V-shaped grooves was proposed to replace steel cable with CFRP cable. This study can provide insights into the aerodynamic performance and wind-induced response of CFRP cable and instructions for cable replacement practice. [ABSTRACT FROM AUTHOR]

Oyster reefs play a dual role in the ecological and economic sustainability of global estuarine resources. Due to human activity and climate change, the prevalence of cosmopolitan oyster reefs has noticeably declined in recent decades, triggering a global restoration movement. However, the hydrodynamic functions of oyster reefs during and after restoration, particularly the impacts of growth and morphology on the flow field, remain poorly understood. This study employs the lattice Boltzmann method coupled with large-eddy simulation to simulate unidirectional flow around restored oyster reef models using the open-source Palabos library. It examines the effects of unidirectional flow velocity and reef morphology on hydrodynamic characteristics. The research analyzes spatial and temporal variations in velocity, vorticity, and turbulence structure around the reef. The findings indicated significant flow field differences between the initially restored reefs and those post-restoration. The dimensionless wake region scale parameters of the initially restored reefs exhibit hysteresis effects, generating larger turbulence during the post-recruitment stage than in the initial stage. Areas of high turbulence in the wake are associated with above-canopy flow, bypass flow, and within-canopy flow. The presence of gaps and branches in the reef leads to complex turbulence structures and irregular vortex shedding in the reef's wake at the post-recruitment stage. These results are valuable for assessing oyster reef resilience and planning effective restoration interventions. [ABSTRACT FROM AUTHOR]

In this study, the low-frequency oscillations found in the compressible low-Reynolds-number regime, defined as low-Reynolds-number buffet, were investigated by numerical calculations and modal analysis. Dynamic mode decomposition (DMD) and compressive sensing methods were employed to extract periodic flow structures. Numerical simulation results showed low-Reynolds-number buffet and Kármán vortex shedding. Low- and high-frequency oscillations [ S t = O (0.01) , O (1.0) ] were extracted by DMD and named the buffet mode and vortex shedding mode, respectively. Low-Reynolds-number buffet does not necessarily exhibit supersonic regions or shock wave. Simulation results show that the thickness of the separated shear layer changes significantly under low-Reynolds-number buffet. The change in the thickness of the separated shear layer was confirmed by the buffet mode of DMD results. Two types of compression pressure waves, advecting upstream, were identified. DMD indicated that they resulted from vortex shedding. According to simulation and DMD results, the origin of the shock waves appears to be the condensation of compression pressure waves due to the vortex shedding mode and expansion in the supersonic region due to the buffet mode. The formation of the shock waves seems to be subordinated to the vortex shedding mode and the buffet mode. A feedback model for low-Reynolds-number buffet inherent in separated shear layers, which does not require supersonic regions or shock waves, was proposed. Supersonic regions highly condensed the compression pressure waves, inducing a larger separation region and amplifying the oscillation. The role of supersonic regions in determining oscillation amplitudes was evidenced, although supersonic regions are not essential to the mechanism of low-Reynolds-number buffet. [ABSTRACT FROM AUTHOR]

Investigating the subject offers a pioneering approach to enhancing thermal performance and aerodynamic efficiency, unlocking novel strategies for optimizing energy utilization and air dynamics in engineering applications. In this research, a numerical study of airflow control coupled to heat transfer around several heated square cylinders is carried out at a fixed Reynolds number (R e = 100). The effect of the positioning and length of the control partitions is examined. Numerical simulations are carried out using the lattice Boltzmann method with multiple relaxation times model. The obtained results reveal the existence of a critical position g = 0 where a significant improvement in the Nusselt number is observed. This improvement amounts to 31.1 % for the rear face of the top obstacle, 30.2 % for the rear face of the central obstacle, and 36.65 % for the rear face of the bottom obstacle compared to the uncontrolled case. Thus, a complete suppression of the vortex shedding is observed when the length of the partitions reaches a critical value (L p = 4 D). Furthermore, a maximum percentage of drag reduction is achieved by around 6.03 % for the central block and 16.67 % for the two end blocks when the length of the control partitions reaches this critical value. [ABSTRACT FROM AUTHOR]

The flow past three square cylinders of unequal size placed in an inline arrangement is studied using the lattice Boltzmann method at different Reynolds numbers [Re = (u∞ d)/ν] within the range of Re = 120, 150, 160, 175, and 200 for various gap spacings (g = s/d), ranging from 1 to 6. This study focused on the symmetric examination of flow behavior for various gap spacing within the three unequal-sized square cylinders. The main objective of this study was to investigate the effects of Reynolds numbers and gap spacing for flow structure mechanism and vortex shedding suppression in between the gap and down-stream position of all three cylinders. Results are obtained in terms of vorticity contours visualization, drag and lift coefficients, Strouhal number, and physical parameters. In vorticity contour visualization, different flow behaviors are observed, known as flow regimes, and are named according to their characteristics, and they are (i) steady flow regime, (ii) shear layer reattachment flow regime (SLR), (iii) fully developed vortex shedding flow regime, (iv) two-row fully developed vortex shedding flow regime, and (v) fully developed irregular vortex shedding flow regime. The present study also includes a discussion on aerodynamic forces, namely the mean drag coefficient (Cdmean), root mean square of the lift coefficient (Clrms), and Strouhal numbers (St) for three cylinders with sizes d = 20, d1 = 15, and d2 = 10, respectively. The maximum value of Cdmean for the first cylinder (C1) is obtained at (Re, g) = (200, 3) that is, 1.5156, where the existing flow regime is the SLR flow regime, while for C2 and C3, the maximum Cdmean values are examined at critical flow behaviors, where the existing flow regime is a fully developed irregular vortex shedding flow regime. Negative values of Cdmean are also examined for cylinders C2 and C3 at some combinations of (Re, g), attributed to the effect of thrust. Furthermore, it is noticed that the values of Strouhal number are increased with an increment in values of gap spacing. The highest value of the Strouhal number for all three cylinders is observed for C1 at (Re, g) = (120, 5), reaching 0.1556 along with a two-row fully developed flow regime. Furthermore, it is investigated from the present problem that the position of unequal sized square cylinders strongly influenced the flow structure mechanism. The information found and discussed in this study could be effective for structure designing arrangement in the case of three square cylinders of unequal size placed in a horizontal arrangement. [ABSTRACT FROM AUTHOR]

In this work, we studied the wave-induced vortex generation and shedding from a semi-submerged vertical square cylinder interacting with an upcoming undular bore wave train under a shallow water configuration. This unsteady and rapid process was investigated by means of numerical and experimental approaches. A numerical simulation, solving the full turbulent viscous Navier–Stokes equations, was carried out in order to study and characterize both the undular bore wave properties and the vortex dynamics triggered during this unsteady interaction. Starting with fluid at rest, the undular bore was generated by the impulsive translational motion of a piston wavemaker at laboratory scale in both a numerical and an experimental wave tank. When the undular bore impinges on the cylinder, filamentary vortex structures were formed at the four cylinder's edges synchronized with the propagating wave motion, leading to the vortex shedding phenomena at a frequency that matched the wave instantaneous frequency. These vortices extended along the entire cylinder span under the water column, from the free surface to the seabed. At the trailing edge of the cylinder, a pairing process of two shed vortices was observed, similar to a Lamb–Oseen vortex pair. These vortices were present during the whole undular bore wave train dynamic forcing. An overall agreement was found with the experimental version of the bore–cylinder interaction, carried out in a physical wave tank. Laser sheet bore profiling and particle image velocimetry measurements of the velocity field confirmed undular bore properties, the onset of vortex formation, subsequent shedding, and pairing in the experiments performed in similar conditions with the numerical approach. [ABSTRACT FROM AUTHOR]

A rectangular cylinder, with an aspect ratio of 5, is a widely used bluff body in engineering practice. It undergoes intricate dynamical behavior in response to minute alterations in the flow angle of attack (α). These modifications invariably precipitate the failure of wake control for classical flow control methods with various α values. In this study, global linear instability, adjoint method, and sensitivity analysis are employed to identify the optimal position for flow control. It is found that the sensitive region gradually transitions from the leeward side to the downwind side of the model as α and Reynolds number (Re) increase. So, we set up airflow orifices for flow control in both positions. Jet flow control on the leeward side effectively inhibits vortex shedding (α ≤ 2°). High-order dynamic mode decomposition is employed to reveal the inherent mechanism of control. Suction control on the downside effectively mitigates the shear layer separation phenomenon induced by the altered spatial structure associated with higher α. A novel zero-net-mass-flux wake control, bionics-based breathe-valve control (BVC), is proposed to optimize the control effect. BVC is applicable for various α and Re, with optimal effectiveness achievable through jet velocity adjustments. The prediction-control approach in this investigation provides a targeted method to mitigate flow-induced vibration. [ABSTRACT FROM AUTHOR]

Finite element computations were conducted to investigate the transverse vibrations of three identical tandem square cylinders and the associated wake patterns at a Reynolds number Re = 150. The reduced velocities ranged from U * = 3 to 20, and the angles of incidence were set at α = 0 ° , 22.5 ° , and 45 °. The streamwise gaps for these three α were L x = 5 H , 3.8 H , and 3.5 H , respectively, where H represents the projected dimension of the cylinder normal to the freestream. The mass ratio of the cylinders was fixed at m * = 2 , and damping was neglected to allow the cylinders to attain maximum amplitudes. In the presence of primary vortices being shed from all three cylinders, the upstream cylinder at α = 22.5 ° and 45 ° exhibits three distinct vibration regimes: initial excitation and upper and lower response regions. On the other hand, due to interaction with the upstream vortices, the two downstream cylinders display four response regions. In the case of α = 0 ° , the dynamic response of the upstream cylinder appears in only two regimes, but with a higher peak amplitude compared to α = 22.5 ° and 45 °. Vibration and shedding frequencies closely synchronize with the natural frequency of the spring-mass system in the second regime, leading to high amplitude oscillations for the most upstream cylinder with α = 22.5 ° and 45 °. The third response regime for the two downstream cylinders is associated with the lock-in phenomenon. In α = 0 ° and 22.5 ° configurations, the shedding mode is 2S in all response regimes, while at α = 45 ° , the shedding mode shifts to P + S during the second regime. Up to the second regime, lift and vortex forces are in-phase with the cylinder's oscillation for α = 0 ° and 22.5 ° , but they go out of phase beyond that. In the case of α = 45 ° , although lift remains in-phase with the displacement in the second regime, the vortex force is found to be out of phase. This study is expected to enhance the understanding of fluid–structure interaction phenomena involved in multiple structures and can aid in the design of stable structures in civil engineering, offshore engineering, and development of energy harvesting devices. [ABSTRACT FROM AUTHOR]

Inspired by a critical organelle called rough endoplasmic reticulum (RER), a wave‐energy attenuation nano‐carbon foam (WANF) with a unique hierarchical composite structure consisting of 3D‐lamelle, 2D‐perforations and 1D‐microspheres is constructed by incorporating graphene oxide (GO) and polydimethylsiloxane (PDMS) into a carbon nanotubes (CNTs)‐based aerogel. Benefiting by unusual mechanisms like vortex shedding, secondary reflection and resonance, WANF displays excellent acoustic wave absorption, achieving a sound absorption coefficient of over 0.9 with a bandwidth of 4.75 kHz at a thickness of 20 mm. Significantly, it features obvious acoustic absorption intensity in medium‐low frequency range. Moreover, the nano‐carbon aerogel skeleton (NCAS) of WANF endows it with good electromagnetic wave absorption through dielectric loss, exhibiting a minimum reflection loss of −48.61 dB and an efficient electromagnetic absorption with a bandwidth of 5.35 GHz. The excellent wave‐energy attenuation performance of WANF indicates an attracting double‐stealth effect in sonic and electromagnetic area, which shows great potential in integrate protection of advanced military equipment and sensitive electronic infrastructures. [ABSTRACT FROM AUTHOR]

We experimentally studied the wake flow behind a submerged vertical cylinder in an open channel rotating at a constant angular velocity around a vertical axis. We obtained the two-dimensional velocity field on the free surface of fluid by a Particle Image Velocimetry measurement. In some cases, we found the emergence of a steady vortex perpendicular to the surface and vortex-shedding behind it. The necessary condition for the emergence of such vortices is given by the excess of Rossby number below a critical value, which is determined by the relative height of the cylinder. Reynolds number must be large enough, and the thresholding value depends on the relative height. [ABSTRACT FROM AUTHOR]

In the present study, the effect of taper on the lateral sides on fluid flow and heat transfer characteristics on a square cylinder has been analyzed numerically. Two-dimensional simulations have been carried out in the laminar flow regime (Re 100–800) for divergent blockage ratios (ϵ = H/D) and taper angles on the forward and backward sides of the lateral cylinder. Compared to the square cylinder, the forward tapered modification cylinders led to significant total drag reductions, pressure drop, and higher heat dissipation for low Re. The backward taper cylinder illustrates a minor increase in heat dissipation; however, the high drag coefficient and pressure drop penalty cannot be neglected. Moreover, an optimized geometry has been identified as a function of all the parameters. The reduction in blockage ratio has been determined to suppress the vortex shedding for higher Re completely. Results also indicate that the decrease in blockage ratio reduces the critical Re of the domain. The slightly tapered surfaces on either side of the square geometry significantly influence heat transfer and apply to modern industrial electronics and their cooling. [ABSTRACT FROM AUTHOR]

Vibration causes compressor rotor blade failure that affects aeroengine safety. Therefore, a plane cascade model of rotor blades was developed to capture the vibration phenomenon. The blade vibration, vortex shedding, and aerodynamic noise were measured. The results indicate that flow and noise induce a natural vibration mode, resulting in torsional vibration fatigue damage. The frequency of trailing edge vortex shedding is consistent with the frequency of tip clearance leakage vortex pulsation and is close to the second-order modal frequency of the blade, which induces torsional vibrations. The triple frequency of the background noise peak frequency in the wind tunnel is consistent with the first standing wave acoustic resonance modal frequency of the cascade experimental section duct and is close to the third-order natural modal frequency of the blade, which induces bending and torsional vibrations. This research provides guidance for the study of rotor blade multifield coupling-induced vibration. [ABSTRACT FROM AUTHOR]

AbstractThis article presents the comparison between the unsteady flow characteristics of thick (t/c ≥ 6%) and thin (t/c ≤ 6%) airfoils performing pitching motion at Reynolds number Re=3.6×105. For the analysis, the considered airfoils are NACA 0012 as thick and symmetric airfoil and GOE 417A as thin and highly cambered airfoil. The analysis in this study is restricted to only two airfoils because from the preliminary investigations, similar aerodynamic characteristics with slight variation in their flow field development under same conditions were observed. The influence of reduced frequency, pitching amplitude and mean angle of attack on the airfoil’s aerodynamic performance in terms of instantaneous force coefficients is thoroughly investigated. Further, the flow surrounding the airfoils is also investigated. It is noticed that the aforementioned parameters substantially affect the instantaneous force coefficients (both qualitatively and quantitatively). Under certain conditions, the formation of reverse Karman vortex street was observed indicating airfoil’s thrust force generation. It must be noted that the conditions where the thrust force generation occurs for thick and symmetric airfoils like NACA 0012 are quite different from that of thin and cambered airfoils like GOE 417A. Additionally, a drastic change in the flow field around the airfoils is observed under similar conditions. For NACA 0012 airfoil, the formation and convection of the Leading Edge Vortex (LEV) under certain conditions has detrimental effects on its aerodynamic characteristics as it encourages early flow separation. However, the same cannot be true for thin and highly cambered airfoils like GOE 417A. Under the similar conditions with negligible variation, the LEV contributes toward providing extra suction over the GOE 417A airfoil’s suction side. This significantly improves airfoil’s aerodynamic characteristics. [ABSTRACT FROM AUTHOR]

AbstractThe present numerical investigation studies the momentum and heat transfer phenomena from a heated rotating patterned cylinder in the vortex shedding regime. In particular, the effect of surface patterns (ω,δ), fluid behavior (0.4≤n≤1.6), cylinder rotation speed (0.5≤α≤2), and Prandtl number (1≤Pr≤100) at Reynolds number 100 on the force coefficients and Nusselt number are studied. Vorticity and isotherm profiles demonstrate the flow dynamics and heat transfer characteristics. According to the findings of this study, the formation of the vortex (including its size, shape, and number of vortices) and the time period during which the vortex shedding pattern repeats itself demonstrates a dependency on the shape of the cylinder, fluid behavior, and rotating speed. It has been noted that a significant decrease in the frequency of vortex shedding occurs on adding topographical patterns. Additionally, it has been noticed that on increasing rotational speed, vortex shedding can be suppressed for all fluid behaviors. Numerical simulation results reveal that the average drag coefficient (C¯D) for a rotating patterned cylinder decreases with increasing rotational speed (α), and it further reduces on increase in pattern frequency (ω) and amplitude (δ) compared to a smooth circular cylinder. The shear-thinning fluid behavior helps to dissipate the heat from the cylinder to the surrounding fluid. Conversely, shear-thickening fluid behavior exhibits the opposite trend. [ABSTRACT FROM AUTHOR]

A comprehensive 2D numerical model was conscientiously developed to investigate the vortex-induced vibration phenomena in a cylindrical structure with rounded corners. The Navier-Stokes equation was adeptly solved under the specific condition of a Reynolds number (Re) of 150. The investigation reveals intricate details of the phenomena. The study aimed to systematically analyze the interaction between drag and lift force coefficients, cylinder vibration amplitude, and the patterns of vortex shedding modes under various conditions. This study systematically altered the radius of the cylinder's rounded corners to evaluate their effects on both structural and hydrodynamic responses. This variation was crucial in comprehending how slight alterations in the cylinder's geometry impact significant changes in the flow dynamics and correlated vibration behavior. The model's numerical results revealed the significant impact of the curved edge ratio on both the hydrodynamic forces acting on the cylinder and its vibration response. The variation in edge curvature resulted in changes in drag and lift coefficients, leading to a significant impact on the amplitude of vibration. This elucidates the crucial role of geometric design in controlling and optimizing the structural behavior of cylindrical structures under fluid flow conditions. [ABSTRACT FROM AUTHOR]

In this study, the whisker structure of a harbor seal is applied to the lip and the trailing edge of a turbine cooling blade with a pressure-side cutback. Four cases are studied in this article, including a prototype, a whisker trailing edge, a PP (The lip's peak and the trailing edge's peak are designed at the same blade height) case, and a PV (The lip's peak and the trailing edge's valley are designed at the same blade height) case. The DDES method is used to simulate the flow and thermal mixing process in the cascade. The coolant mass flow ratios ( c m ) of 0.5% and 1% at an outlet Mach number of 0.9 are simulated. The PP case can achieve the best aerodynamic performance, which reduces the thermodynamic energy loss coefficient of the prototype by 0.98% and 0.99% at c m = 0.5% and c m = 1%, respectively. The PP case and the PV case increase the lowest spanwise average adiabatic film cooling effectiveness on the cutback surface from 0.5 to 0.7 at c m = 0.5%. The film cooling effectiveness of the PP case and the PV case can remain close to 1 over the entire cutback surface at c m = 1%. The PP case transforms the trailing edge shedding vortices on the pressure side into hairpin vortices. The whisker lip suppresses the lip vortex and alters the closed separation bubble on the cutback surface, causing the coolant to converge toward Section 1. The convergence of coolant is more helpful for the PP case to control the pressure side shedding vortex, resulting in a lower thermodynamic energy loss. [ABSTRACT FROM AUTHOR]

Aquatic organisms have evolved exceptional propulsion and even transoceanic migrating capabilities, surpassing artificial vessels significantly in maneuverability and efficiency. Understanding the hydrodynamic mechanisms of aquatic organisms is crucial for developing advanced biomimetic underwater propulsion vehicles. Underwater tetrapods such as sea turtles use fins or flippers for propulsion, which exhibit three rotational degrees of freedom, including flapping, sweeping, and pitching motions. Unlike previous studies that often simplify motion kinematics, this study employs a specially designed experimental device to mimic sea turtle fins' motion and explore the impact of pitching amplitude, asymmetric pitching kinematics, and pausing time on lift and thrust generation. Force transducers and particle image velocimetry techniques are used to examine the hydrodynamic forces and flow field, respectively. It is found that boosting the fin's pitching amplitude enhances both its lift and thrust efficiency to a certain extent, with a more pronounced effect on thrust performance. Surprisingly, the asymmetrical nature of the pitching angle's pausing time within one flapping cycle significantly influences the lift and thrust characteristics during sea turtle swimming; extending the pausing time during the forward and upward flapping process improves lift efficiency; and prolonging the pausing time during the downward flapping process enhances thrust efficiency. Furthermore, the mechanism for high lift and thrust efficiency is revealed by examining the vortices shed from the fin during different motion kinematics. This research contributes to a more comprehensive understanding of the fin's hydrodynamic characteristic, providing insights that can guide the design of more efficient biomimetic underwater propulsion systems. [ABSTRACT FROM AUTHOR]

This study computationally investigates small-scale flickering buoyant diffusion flames in externally swirling flows and focuses on identifying and characterizing various distinct dynamical behaviors of the flames. To explore the impact of finite rate chemistry on flame flicker, especially in sufficiently strong swirling flows, a one-step reaction mechanism is utilized for investigation. By adjusting the external swirling flow conditions (the intensity R and the inlet angle α ), six flame modes in distinct dynamical behaviors were computationally identified in both physical and phase spaces. These modes, including the flickering flame, oscillating flame, steady flame, lifted flame, spiral flame, and flame with a vortex bubble, were analyzed from the perspective of vortex dynamics. The numerical investigation provides relatively comprehensive information on these flames. Under the weakly swirling condition, the flames retain flickering (the periodic pinch-off of the flame) and are axisymmetric, while the frequency nonlinearly increases with the swirling intensity. A relatively high swirling intensity can cause the disappearance of the flame pinch-off, as the toroidal vortex sheds around either the tip or the downstream of the flame. The flicker vanishes, but the flame retains axisymmetric in a small amplitude oscillation or a steady stay. A sufficiently high swirling intensity causes a small Damköhler number, leading to the lift-off of the flame (the local extinction occurs at the flame base). Under the same swirling intensity but large swirling angles, the asymmetric modes of the spiral and vortex bubble flames were likely to occur. With R and α increasing, these flames exhibit axisymmetric and asymmetric patterns, and their dynamical behaviors become more complex. To feature the vortical flows in flames, the phase portraits are established based on the velocity information of six positions along the axis of the flame, and the dynamical behaviors of various flames are presented and compared in the phase space. Observing the phase portraits and their differences in distinct modes could help identify the dynamical behaviors of flames and understand complex phenomena. [ABSTRACT FROM AUTHOR]

The effect of bionic fish swimming on the vortex-induced vibration (VIV) of a circular cylinder arranged in tandem at a low Reynolds number of 150 is numerically investigated in this work. The bionic fish placed upstream of the cylinder with gap ratios of 1, 3, and 5 and that located downstream of the cylinder with gap ratios of 3 and 5 are examined in the simulations that were carried out in the reduced velocity range of Ur = 2–15. It is found that both the gap ratio and the reduced velocity have a significant influence on the VIV response and wake flow structure. When the bionic fish is placed upstream, the maximum response amplitude of the downstream cylinder is much greater than that of an isolated one. Two flow regimes are identified in terms of the shear layer reattachment, i.e., the continuous reattachment and the alternate reattachment. Comparing the vortex shedding frequencies of the cylinder and the swimming fish, it is found that the frequency of the cylinder is always locked in the fish swimming frequency, and multiple frequencies occur at Ur = 5. When the bionic fish is arranged downstream, four flow regimes are observed, including the extended-body, continuous reattachment, alternate attachment, and co-shedding regimes. Furthermore, the time-mean energy transfer coefficient of the cylinder is considerably higher at Ur = 5 than that when the fish is placed upstream of the cylinder. [ABSTRACT FROM AUTHOR]

Expanding pipes with orifice plates are often utilized as silencers for fluid machinery. However, intense tonal sounds can be generated from a flow through such expanding pipes. To clarify the mechanism of tonal sound from a flow through a circular expanding pipe with two orifice plates and the conditions for intense acoustic radiation, the flow and acoustic fields are directly solved based on the compressible Navier–Stokes equations. Phase-averaged flow fields indicate the occurrence of periodic vortex shedding in the free shear layers of the expanding pipe, resulting in acoustic radiation. The effects of the orifice radius and freestream Mach number on the acoustic radiation are focused on. The computational results demonstrate that vortex rings or spiral vortices are generated in the cavities formed by the orifice plates, where the primary vortical shape changes, depending on the freestream Mach number and orifice radius. The collision of the vortex ring and spiral vortex with the orifice plate or downstream edge of the expanding pipe leads to the occurrence of circumferentially in-phase and one-wavelength-mode pressure fluctuations, respectively. The orifice radius also affects the convective velocity of vortices and the position of the acoustic source, varying the frequency of the acoustic radiation. The findings of this research provide the first clarifications of fluid–acoustic interactions in an expanding pipe with orifice plates. [ABSTRACT FROM AUTHOR]

Combining tandem cascades and curved vanes can mitigate the separation in highly loaded compressor cascades. Flow loss mechanisms in curved tandem cascades are crucial for fundamental design and optimization. This study investigates the flow losses and vortex structures in curved tandem cascades with different curved angles and an original straight tandem cascade. Numerical simulations are conducted at variable incidence angles. The results indicate no consistent effect of curved angles on the aerodynamic performance of curved tandem cascades. The tandem cascade with a curved angle of 5° effectively reduces losses at all incidence angles, while the tandem cascade with a curved angle of 15° extends the critical incidence angle to 3°. The comparative analyses reveal that curved vanes increase spanwise pressure gradients, causing low-energy fluid near endwalls to move toward the midspan. The weighting coefficient of losses quantitatively describes that curved vanes weaken the trailing edge shedding vortices of both vanes and passage vortex while enhancing the end wall spanwise vortex of the front vane, thus affecting the loss distribution. Finally, this study innovatively applies topological analysis methods to curved tandem cascades, establishing the relationship between topological structures and vortex structures in corner regions of tandem cascades, which provides a valid research approach to reveal the vortex dynamics mechanisms of the loss distribution in tandem cascades influenced by curved vanes. [ABSTRACT FROM AUTHOR]

Tower-like structures with circular section are prone to unexpected across-wind vortex-induced vibration (VIV), and the along-wind response is usually neglected due to its amplitude being much smaller than that of VIV. Recent research pertaining to tall buildings has revealed, nevertheless, that the structural responses in the along-wind and across-wind directions are coupled in a way that results in a decrease in the across-wind response at the wind speed of VIV along with an increased along-wind response. However, the impact of coupling on tower-like structures has not yet been examined. Moreover, tower-like structures with a circular cross section undergo tip-induced vibration at the wind speed higher than vortex-resonance wind speed. However, the impact of this coupling effect on the tip-induced vibration remains uncertain. In this paper, a two-degree-of-freedom rocking model wind tunnel test is carried out under low turbulence and turbulent flow to investigate the coupling effect of tower-like structures. Experiment results show that the coupling effect exists at both the vortex-resonance vibration wind speed and the tip-induced vibration wind speed. Besides, when the along-wind response is coupled with the across-wind responses, it illustrates a steady harmonic vibration with a hardened non-Gaussian characteristic, and its kurtosis decreases to 1.5. Finally, the measured tip-associated vortex shedding frequency suggests that there is a "lock-in" phenomenon of the tip-induced vibration, this new finding improves the understanding of the wind-induced response at high wind speeds. [ABSTRACT FROM AUTHOR]

This study investigates the particle dispersion characteristics in the turbulent wake of a simplified vehicle model (Ahmed body) for two values of the rear slant angle ϕ (25 ° and 40 °) to study the effect of flow separation. In the experiments (Reynolds number Rel=1.90 × 10 5 ), smoke particles were released from a source and visualized with a laser sheet. Concentration fields were analyzed to calculate the vertical (Sy) and lateral (Sz) smoke spread. The findings indicate that the flow topology and concentration fields in the wake are highly dependent on ϕ. In the ϕ = 40 ° case, separation on the rear slant disrupts the trailing vortices originating on the rear slant edges and significantly alters the wake topology. The growth of vertical smoke spread saturates after the recirculation region. The uniform mixing and absence of trailing vortices concentrate the smoke particles in the model midplane. In the ϕ = 25 ° case, the signatures of trailing vortices were observed in the concentration fields behind the model. The vertical smoke spread is less, and the lateral smoke spread is more compared to the ϕ = 40 ° case. The growth of the smoke spread ( S y , S z ) driven by the trailing vortices persists for a long distance, even after the recirculation region. The disruption of trailing vortices brought about by the flow separation appears to be an important effect driving the vertical smoke spread in the wake. The connection between the turbulent velocity structures and concentration structures will need to be explored with combined velocity and concentration measurements in the wake. [ABSTRACT FROM AUTHOR]

This study investigates the steady and unsteady behaviors of near-wake flow for axisymmetric models acquired with conical boattails with different slant angles under low-speed conditions. Delayed detached eddy simulation (DDES) was applied for the numerical processes. The Proper Orthogonal Decomposition (POD) was then used to reconstruct the dominant modes of the pressure at the base and velocity fields on the symmetry plane. Both near-wake and far-wake flows were considered in this study to obtain a full pattern of the wake. It was shown that the DDES allows for obtaining a highly accurate wake flow field. The large-scale structure of the base flow with two dominant modes of vortex shedding at StD = 0.19–0.24 and bubble pumping at StD = 0.06–0.08 occurs for the axisymmetric blunt-based model. At boattail angles up to 16°, those frequencies are clearly distinguished from the time series of POD modes of the velocity fluctuations. However, for higher angles, the motions of the "bubble pumping" and vortex shedding combine to generate frequencies in the range of StD = 0.07–0.15. The fully separated flow disturbs a ratio of one-third for the bubble pumping and vortex shedding frequencies, which was shown widely in previous studies. The reduction of frequency band and large-scale features for boattail angles of 16° explains the low drag generated for that configuration. [ABSTRACT FROM AUTHOR]

Cavitation-induced vortex and energy loss are critical topics in the field of hydraulic machinery. Through a combination of experimental and numerical analysis, this paper investigates how blade loading affects vortical structures and energy loss during cavitation in a waterjet pump. The flow rate and cavitation conditions changed blade tip pressure loading, which significantly affected the trajectories of the primary tip leakage vortex cavitation and secondary tip leakage vortex cavitation. A considerable pressure gradient at the attached cavity closure region leads to a wall reentrant jet and a side-entrant jet, resulting in an attached vortex under the cavitation developing stage and severe stage. The development of the attached vortex leads to the shedding of attached cavities. Furthermore, the blade tip pressure difference loading significantly alters the distribution of tip leakage vortex and gives rise to a perpendicular leakage vortex (PLV). The PLV entrains the shedding cavities and forms a perpendicular cavitation vortex. These complex vortical structures induced by cavitation inevitably enhance enstrophy and lead to entropy production in the pump. The entropy production terms of viscous dissipation, turbulent dissipation, and the wall effect react differently to the development of cavitation. As the cavitation stages developed and became more severe, wall entropy production (S3) decreased. While turbulent dissipative entropy production S2 due to mixing losses occurring in the tip region increased sharply, which dominates the total entropy production S and results in an increase in S. [ABSTRACT FROM AUTHOR]

The test speed of high-speed maglev trains (HSMT) exceeds 600 km/h, requiring higher braking performance and technology. Plate braking technology, which is a suitable choice, has been applied for engineering the high-speed test vehicles. However, the unsteady aerodynamic response during the opening process of HSMT under crosswind needs to be studied. This study explores the unsteady aerodynamic characteristics of a HSMT with a train speed of 600 km/h during plate braking at different crosswind speeds. The plate motion is achieved based on the dynamic grid technology, and the unsteady flow field around the train is simulated using the unsteady Reynolds time averaged equation and the shear stress transport k-omega (SST k–ω) turbulence model. This calculation method was verified using wind tunnel test data. The peak aerodynamic drag (AD) of the braking plates overshot during opening. Under a crosswind of 20 m/s, the AD peak of the first braking plate was 11% larger than that without crosswind. The middle braking plates were significantly affected by upstream vortex shedding, and the AD fluctuation was the most severe. The AD of the head and tail coaches is significantly affected by crosswind. With an increase in the crosswind speed, the AD of the head and tail coaches decreased and increased, respectively. Compared with no crosswind, under a crosswind of 20 m/s, the AD of the head coach decreased by 43%, and the AD of the tail coach increased by a factor of approximately 1.1 times. Furthermore, the AD fluctuation of the tail coach was the most severe. [ABSTRACT FROM AUTHOR]

This study analyzes the influence of nozzle geometry on the vortex and cavitation cloud structures. The differences between the coherent structures of the Helmholtz nozzle, organ pipe nozzle, and venturi nozzle jets are investigated through large eddy simulation. The vorticity transport equation is used to investigate the relationship between the cavitation cloud and diagonal pressure torque terms. The cavitation and vortex structure shedding frequencies of the jets are investigated using the dynamic mode decomposition method. Three distinct stages of the cavitation bubbles are illustrated: priming, expansion, and collapse. The nozzle structure determines the shape of the primary cavitation bubbles. Moreover, turbulent kinetic energy convergence facilitates the maintenance of the coherent structure. Organ pipe nozzle jets have a high peak velocity at the center axis. Their vortex structure only exhibits a stretched state in the downstream and collapses later than the vortex structures of other nozzles. Advantageously, organ pipe nozzles maintain the stability of the coherent structure. The jets generated by the three nozzles have similar static modes. Helmholtz nozzles produce jets with higher energy and periodically shedding small-scale vortex structural modes. These modes are coupled to the static flow field, resulting in quasi-periodic oscillations of the Helmholtz nozzle jets. The periodic oscillation effect of the Helmholtz nozzle jets is superior to that of the other nozzle jets. The high-energy modes of the venturi nozzle jets have anisotropic and small-scale vortex structures. Furthermore, the venturi nozzle jets exhibit good dispersion and cavitation properties. This study provides guidance for the use of jets with different properties in the respective engineering fields. [ABSTRACT FROM AUTHOR]

The cavitation phenomenon can induce non-uniformity in the fluid, impacting fluid dynamic performance. This paper focuses on the cavitation shedding of the Delft Twist 11 hydrofoil. First, the reliability of numerical simulations is verified by computational fluid dynamics results. Utilizing the variational mode decomposition method, the cavitation signals on two cavitation paths are decomposed. Finally, the cavitation pulsation tracking network method is proposed to extensively investigate the spatial propagation patterns of cavitation signals at various sections above the twisted hydrofoil. The results reveal that typical frequencies at different monitoring planes are 30, 58, and 88 Hz. The corresponding amplitude analysis at these frequencies provides insight into the spatial propagation and attenuation process of cavitation vortices shedding. This study offers a novel perspective for a deeper understanding of cavitation mechanisms. Simultaneously, this provides references for enhancing the performance of mechanical engineering systems, reducing energy consumption, and improving structural reliability. [ABSTRACT FROM AUTHOR]

We numerically investigate the forced synchronization of the self-excited flow behind a plunging airfoil in ground effect at a Reynolds number of Re = 1000. On varying the plunging amplitude and frequency, we find a rich array of nonlinear dynamics, such as a period-1 limit cycle due to natural vortex shedding as well as two-frequency quasiperiodicity on a torus attractor ( T 2 ). For certain non-resonant plunging frequencies without a ground surface, we find that low-dimensional chaos emerges via the Ruelle–Takens–Newhouse route. However, we find that the chaos can be suppressed by introducing a ground surface, inducing a direct transition from T 2 quasiperiodicity to 1:1 phase locking as the plunging amplitude rises over the boundaries of the Arnold tongue. Apart from suppressing chaos, the ground surface also causes the lift and drag coefficients to become less sensitive to the plunging motion itself. Knowledge of the critical plunging conditions required for forced synchronization and chaos could be useful in various engineering applications, such as the design of pico air vehicles. [ABSTRACT FROM AUTHOR]

The location of the aerodynamic center, with respect to the center of gravity, determines the longitudinal static stability of an aerial vehicle. While most studies in the past focused on high Reynolds number (Re) flows, we consider flows at low Re (100 ≤ R e ≤ 6000) past an end-to-end wing and a finite wing of various semi-aspect ratio (0.25 ≤ sAR ≤ 5). It is shown that two-dimensional computations, even for an end-to-end wing, are inadequate for accurate prediction of aerodynamic coefficients. The chordwise pressure distribution as well as the variation of aerodynamic coefficients with angle of attack (α) at low Re are significantly different from that at high Re. Beyond a certain α, the flow is associated with a large recirculation zone, resulting in a non-linear variation of the aerodynamic coefficients with α. The conventional definition of the aerodynamic center is too restrictive at low Re. Owing to the non-linear variation of the pitching moment with change in α, it is not possible to determine one single location where it is invariant over a large range of α. The aerodynamic center is, therefore, determined over various sub-regimes of α. It varies significantly over the sub-regimes. The wing-tip vortex significantly affects the vortex shedding, flow structures, and pressure distribution on a finite wing. The locations of the aerodynamic center and center of pressure move fore, toward the leading edge, with a decrease in aspect ratio compared to that for an end-to-end wing. [ABSTRACT FROM AUTHOR]

A channel with one backward-facing step and one forward-facing step is a typical configuration in engineering applications. In the channel, good heat transfer performance is often required, and the enhancement is usually achieved by employing different passive control methods, such as modification of geometric configuration or application of nanofluid. However, the other control method, i.e., active flow control (AFC), which is likely more effective, has been rarely applied in such a scenario. This study aims to bridge this gap by exploring how a rigid plate affects the heat transfer of the channel. The plate either is stationary or actively rotates, corresponding to passive flow control or AFC. The influences of the horizontal position of the plate (S) and its orientation angle (θ) on the heat transfer performance are studied when the plate is stationary to provide a baseline. Compared to the baseline, the effects of S, θ, and the rotation frequency (fr) are revealed when the plate undergoes a sinusoidal rotation. Such a thermo-fluid dynamic problem is numerically simulated by the immersed-boundary lattice Boltzmann method. The results show that the plate can improve the heat transfer performance no matter whether it rotates or not, compared to the case without a plate. The rotating plate outperforms the stationary one when θ and fr are properly chosen at each S. Substantial improvement can be achieved when vortex synchronization or resonance occurs in the channel, i.e., when the natural vortex shedding frequency is close or equal to fr. [ABSTRACT FROM AUTHOR]

We report on a numerical study of the critical velocity for creation of quantized vortices by a moving Gaussian obstacle in a trapped Bose–Einstein condensate, modeled by the Gross–Pitaevskii equation. We pay attention to impact of density inhomogeneity associated with the global inverted parabolic profile by a trapping potential as well as the local density suppression around the Gaussian obstacle. When the width of the Gaussian potential is large, the wake dynamics is significantly influenced by the nonuniformity around the obstacle potential. The critical velocity, estimated through the time interval between the first and second vortex emission, can be explained by the local sound velocity by taking into account the above two contributions. We find that the ratio of the critical velocity for vortex creation to the sound velocity at the center of the system is minimally influenced by the nonlinear coefficient in the Gross–Pitaevskii equation. This result implies that the analysis in the homogeneous system is directly applicable to the inhomogeneous trapped condensates under the local density approximation. [ABSTRACT FROM AUTHOR]

This study investigates wave-stratified flow in a horizontal pipe at high pressure, and flow characteristics are obtained, such as flow pattern map, liquid film thickness, and pressure drop. Compared with a flow pattern map of a gas-liquid two-phase flow carried out at atmosphere, stratified flow zone is depressed with increasing system pressure and the critical gas superficial velocity decreases for smooth-wave-stratified flow transition, while the critical liquid superficial velocity increases for stratified-intermittent transition. On one hand, the compressed air results in an increase in momentum transfer between gas and liquid phases, which accounts for the smaller gas superficial velocity that is encountered in both smooth-wave and stratified-annular flow transition at higher pressure. One the other hand, it slows down the liquid below the crest, and it makes the interface wave crest unstable and split for the vortex shedding behind the wave crest, which accounts for flow regime transition in gas-liquid two-phase flows in pipelines. As a result, stratified-intermittent flow transition is depressed and delayed. The pressure influence on the liquid film profile is analyzed, and relationships between film thickness and dimensionless numbers are studied, such as liquid Weber number and gas Weber number. Friction factors on different interfaces at high pressure are studied, and new empirical formulas are deduced. [ABSTRACT FROM AUTHOR]

Moving vehicles pick up the particulates/dust on the road surface, and the wake structures behind the vehicle determine the dispersion of these particles. This study aims at understanding the characteristics of particle dispersion in the turbulent wake of a moving vehicle. The flow over a simplified vehicle model known as the Ahmed body is analysed. Experimental measurements and computational simulations of particle dispersion in the near wake of Ahmed body are presented. Unsteady simulations are performed for the prediction of the flow topology and particle dispersion around the Ahmed body, at R e l = 0.7 × 10 5 (Reynolds no. based on vehicle model length 'l'), the rear slant angle of Ahmed body considered is ϕ = 25 o . Governing equations solved are unsteady Reynolds Averaged Navier–Stokes (URANS) equations, and the turbulence model employed was SST k - ω . Experiments involved the release of fog particles (Ethylene-Glycol, d p ≈ 1 μ m ) from a source behind the model. Dispersion of the particles was measured in the wind tunnel using a laser sheet for fog particle visualisation at three downstream locations, X=1, 2, and 3, at R e l = 1.4 × 10 5 . It was observed that the flow structure in the near wake of the model consisted of two counter-rotating spanwise vortices and a pair of trailing vortices. URANS simulations capture both the turbulent flow features in the near wake. Experimental results were compared with the URANS simulation results, and it was found that the dominant turbulent structures (recirculation vortices and trailing vortices) in the near wake contribute to particle dispersion. The dispersion patterns obtained are dependent on the size and strength of turbulent structures. The URANS simulations perform reasonably well in capturing the vertical dispersion of particles. However, they do not accurately estimate the lateral dispersion of smoke particles. The lateral dispersion is underestimated by the URANS simulations. [ABSTRACT FROM AUTHOR]

Deep reinforcement learning (DRL) is gaining attention as a machine learning tool for effective active control strategy development. This study focuses on employing DRL to develop an efficient active control strategy for flow around three circular cylinders arranged in an equilateral-triangular configuration in a two-dimensional channel. The analysis of control outcomes reveals that DRL induces vortices of varying sizes between the cylinders, resulting in large elliptical vortices at the rear. This enhancement in flow stability leads to a significant 40.40% reduction in cylinder drag force and an approximate 8.23% decrease in overall drag oscillations. Our research represents a pioneering application of DRL for stabilizing complex flow around multiple cylinders, yielding remarkable control effectiveness. The noteworthy outcomes in controlling the stability of complex flows highlight the capability of DRL to grasp intricate nonlinear flow dynamics, showcasing its potential for investigating active control strategies within complex nonlinear systems. [Display omitted] • Demonstrating deep reinforcement learning in non-linear, complex flow problems. • Remarkable 40.40% drag reduction, 8.23% less overall drag fluctuations. • Efficient implementation of real-time, multi-point active control for complex flow. [ABSTRACT FROM AUTHOR]

A new bio-inspired flapping wing rotor (FWR) configuration combining the high lift of a flapping wing (FW) with the vertical take-off and landing characteristics of a rotor has proved its concept feasibility in the last decade and can be used for reconnaissance and aerial photography missions in complex and confined environments. During high-frequency flapping motion, the leading and trailing edge vortices of the flapping wing rotor alternately shed and form vortex sheets in the wake, resulting in periodic lift and a highly unsteady flow field. In the past, unsteady potential flow aerodynamic methods were derived based on a two-dimensional (2D) airfoil and extended to three dimensions by the blade element theory, neglecting the spanwise evolution effect of the vortices. In this paper, an unsteady vortex ring method (UVRM) model was developed based on two-dimensional (3D) potential flow theory for the aerodynamic calculation of a flapping wing rotor, which takes into account the spanwise evolution of shedding vortices at the leading and trailing edges of the flapping wing. The calculation results of the UVRM for several typical cases were compared with those of the computational fluid dynamics (CFD) method to verify the reliability of the UVRM. Then, fluid–structure interaction analysis for the flapping wing rotor was conducted based on the UVRM model, and the phase difference between twist deformation and flapping angle was found to be approximately 90°. In addition, the effects of wing chordwise and spanwise deformation on the aerodynamic performance of the flapping rotor were investigated based on the UVRM. The results show that the wing chordwise deformation has a positive effect on the mean lift coefficient of the flapping rotor. The wing spanwise deformation has a negative effect on the mean lift coefficient of the flapping rotor. The unsteady vortex ring method model proposed in this paper provides a solution to obtain the aerodynamic force of the flapping wing rotor in an efficient and accurate way. [ABSTRACT FROM AUTHOR]

The current study experimentally investigates the influence of nonflat plate trailing edge serrations on airfoil noise characteristics. Experiments are carried out at different flow velocities in the range of 25–40 m/s , corresponding to the Reynolds number range of 2.4×105 to 3.9×105. The wavelength (λ) of the serrations considered is in the range of 5–30 mm, and the serration height (2h) is in the range of 10–30 mm. The acoustic spectra show that the conventional nonflat plate trailing edge serrations generate a narrowband vortex shedding noise. The narrowband noise is observed to decrease with an increase in the wavelength; however, it is observed to increase with an increase in the serration height or root thickness by around 16 dB. Inclinations (θ) provided at the root of the serrations reduce the narrowband noise up to 20 dB and the broadband noise up to 5 dB. When a perforated plate inserts the sawtooth gap of the serration, the hybrid configuration effectively eliminates the vortex shedding phenomenon at the roots with a significant reduction in the low frequency range as well as the high frequency broadband noise. Airfoil noise poses a significant challenge in aviation, wind energy, and other sectors where aerodynamic noise reduction are crucial. When an airfoil is subjected to turbulent flow, the primary source of noise arises from the interaction of the flow with its trailing edge. The geometrical modifications made at the trailing edge helps to minimize the intensity of the noise generation. This paper presents the results of an experimental investigation on the impact of nonflat plate trailing edge serrations. The major findings of the study reveal that employing a nonflat plate trailing edge serrations can substantially mitigate airfoil trailing edge noise levels. [ABSTRACT FROM AUTHOR]

This study conducted wind tunnel tests to verify a passive jet flow control method for suppressing the aerodynamic force on a chamfered square cylinder at a moderate Reynolds number, focusing on the influence of the opening parameters and the spanwise spacing ratio of hollow pipes on the jet flow control effect. The pressure coefficient distribution on the surface of the cylinder and the overall aerodynamic coefficient were considered in order to systematically analyze the influence of opening shape, opening number, opening height, pipe height, and pipe radial thickness, and to determine the optimal combination of opening parameters. Subsequently, we analyzed the influence of the spanwise spacing ratio on passive jet flow control. Furthermore, the optimal arrangement of the hollow pipe was derived by comparing the aerodynamic coefficients of uncontrolled and controlled cylinders. The results revealed that the passive jet hollow pipe significantly reduces the root mean-square value of the pressure coefficient and the fluctuation of the lift coefficient. Moreover, the platform area of the mean pressure coefficient at the leeward side of the cylinder increases and the mean drag coefficient decreases. The spectrum analysis results showed that the passive jet flow control pipe alters the frequency and intensity of vortex shedding in the wake region. All results indicated that the aerodynamic forces and wakes of a chamfered square cylinder are effectively controlled through jet flow control with perforated hollow pipes. [ABSTRACT FROM AUTHOR]

The three-dimensional turbulent flow around a Flettner rotor, i.e. an engine-driven rotating cylinder in an atmospheric boundary layer, is studied via direct numerical simulations (DNS) for three different rotation speeds (α ). This technology offers a sustainable alternative mainly for marine propulsion, underscoring the critical importance of comprehending the characteristics of such flow. In this study, we evaluate the aerodynamic loads produced by the rotor of height h, with a specific focus on the changes in lift and drag force along the vertical axis of the cylinder. Correspondingly, we observe that vortex shedding is inhibited at the highest α values investigated. However, in the case of intermediate α , vortices continue to be shed in the upper section of the cylinder ( y / h > 0.3 ). As the cylinder begins to rotate, a large-scale motion becomes apparent on the high-pressure side, close to the bottom wall. We offer both a qualitative and quantitative description of this motion, outlining its impact on the wake deflection. This finding is significant as it influences the rotor wake to an extent of approximately one hundred diameters downstream. In practical applications, this phenomenon could influence the performance of subsequent boats and have an impact on the cylinder drag, affecting its fuel consumption. This fundamental study, which investigates a limited yet significant (for DNS) Reynolds number and explores various spinning ratios, provides valuable insights into the complex flow around a Flettner rotor. The simulations were performed using a modern GPU-based spectral element method, leveraging the power of modern supercomputers towards fundamental engineering problems. [ABSTRACT FROM AUTHOR]

This study presents a numerical investigation of the swirling flow inside a vertical conical container. The container is filled with liquid metal (Prandtl number, Pr=0.015), with the upper disc rotating at a fixed angular velocity while the bottom disc remains stationary. The sidewall of the container is rigid and inclined. In such a configuration, a phenomenon called vortex breakdown can occur, acting like a stagnation point against the axial flow pattern; the purpose of this study is to illustrate how the inclination of the cylinder's sidewall can affect the vortex breakdown and their emergence and suppression, The study encompasses a range of Reynolds numbers (1000≤Re≤2500) and various aspect ratios (1≤H/R≤2.4) where three different angles of inclination (α =5°, 10°, and 15°) are considered. The results are obtained by resolving numerically the governing equations using the finite volume technique. Our findings indicate that, for a given Reynolds number (Re=2000) and aspect ratios in the range of 1.2≤H/R≤1.6, the inclination of the sidewall leads to an earlier occurrence of vortex breakdown. On the other hand, for aspect ratios in the range of 1.8≤H/R≤2.2, the inclination of the sidewall results in the suppression of vortex breakdown. Furthermore, we have observed that the critical Reynolds number (Re_c) at which vortex at which vortex breakdown occurs for aspect ratios of A=H/R=1.4, 1.8, and 2.2 increases with the increasing angle of inclination α. [ABSTRACT FROM AUTHOR]

The influence of incoming freestream turbulence on an elastically mounted circular cylinder undergoing vortex-induced vibrations is investigated through simultaneous displacement and flow field measurements at a constant Reynolds number of approximately 4400. The results show that the effects of freestream turbulence are more prominent within the lower branch and desynchronization, where earlier desynchronization is observed with elevated freestream turbulence. Conversely, freestream turbulence minimally affects the structure and wake dynamics within the initial and upper branches. A comparison between a stationary cylinder wake and the wake of a cylinder free to vibrate at very low amplitudes reveals that even minor cylinder vibrations (≈ 1% of cylinder diameter) modify the separated shear layer and near wake dynamics, the result of which is a decreased sensitivity of the near wake to freestream turbulence. Proper orthogonal decomposition is used to identify changes in the vortex shedding characteristics that result in earlier desynchronization. [ABSTRACT FROM AUTHOR]

With an increasing demand for small energy generation in urban areas, small-scale Savonius wind turbines are growing their share rapidly. In such an environment, Savonius turbines are exposed to low mean velocity with highly turbulent flows made by complex geographies. Here, we report the flow-induced rotation of a Savonius turbine in a highly turbulent flow (18% turbulence intensity). The high turbulence is realized by using the far-field of an open-jet. Compared to low turbulence inflow (1% turbulence intensity), the turbine rotates 4% faster in high turbulence since the torque/power increases with turbulence intensity. The wake measurement by hot-wire anemometry and particle image velocimetry reveals the suppression of vortex shedding in high turbulence. In addition, a newly developed semi-empirical low-order model, which can include the effect of turbulence intensity and integral length scale, also confirms high turbulence intensity contributes to the rotation of the turbine. These results will boost more installation of small Savonius turbines in urban areas in the future. [ABSTRACT FROM AUTHOR]

The distortion of turbulence approaching a bluff body plays a dominant role in determining the unsteadiness of the fluctuating surface pressures. A three-dimensional (3D) spectral approach is proposed to decouple the effects of the distorted turbulence into the blocking effect, distortion effect, and 3D effect. Wind tunnel tests are conducted to investigate the unsteady behavior of the windward fluctuating surface pressures on a rectangular bluff body, considering the effect of the ratio of the turbulence scale to the structural feature size λ = Lu/D. The results show that the fluctuating pressure on the surface of a rectangular cylinder is mainly affected by the low-frequency blocking effect, the high-frequency turbulent distortion effect, and the full-frequency turbulence 3D effect. The low-frequency blocking effect is related to the surface spatial position, but is less affected by λ; the high-frequency distortion effect does not depend on the spatial position of the pressure but is affected by λ; the 3D effect is affected by both the spatial position of the pressure and λ. When λ approaches infinity, the distortion and 3D effects can be ignored, and the quasi-steady theory is valid for the fluctuating pressure. Finally, one-wavenumber and 2D fluctuating pressure generalized spectrum models considering the unsteady effects and the spatial position of the surface pressure are proposed, and their accuracy is effectively verified by the test results. [ABSTRACT FROM AUTHOR]

This paper describes an active control technique that uses symmetric downward slot blowing from the free-end side edges of a finite-height square cylinder to suppress the aerodynamic forces. The width (d) of the tested cylinder is 40 mm and the aspect ratio H/d is 5. The Reynolds number based on the oncoming flow velocity U∞ and d is 2.67 × 104. The tested blowing ratio Cb (=Ub/U∞, where Ub is the blowing velocity at the slot exit) ranges from 0 to 4, and two typical included angles of θ = 5° and 45° are considered. The experimental results indicate that free-end slot blowing effectively suppresses the aerodynamic forces on the cylinder. The maximum reduction in the aerodynamic forces occurs with θ = 5° and Cb = 3.0, whereupon the mean drag, fluctuating drag, and fluctuating lateral force are reduced by 6.80%, 48.52%, and 69.38%, respectively. Furthermore, slot blowing control introduces a strong downward entrainment into the near wake of the cylinder, weakening its spanwise vortex shedding. This successfully converts the alternating spanwise vortex shedding into symmetric shedding, especially near the free end from which the downward blowing issues. [ABSTRACT FROM AUTHOR]