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We report a kind of coiled-up meta-liners with nonuniform cross sections (CMNC), which can efficiently attenuate low-frequency sound waves under grazing flow with a deep subwavelength thickness (e.g., ∼λ/17 at 500 Hz). At a grazing flow Mach number of 0.26, the average transmission loss of the meta-liner is 12.6 dB at 500–1000 Hz, which is twice as much as that of a double-degree-of-freedom acoustic liner of the same size. Physically, the nonuniform cross-sectional distribution and significant cross-sectional area ratio enhances vortex shedding, thus resulting in severe acoustic energy dissipation. The excellent low-frequency acoustic attenuation performance of CMNC is investigated thoroughly with experimental, theoretical, and numerical methods. This work provides an avenue for low-frequency noise reduction in grazing flow scenarios (e.g., in a high bypass ratio turbofan engine). [ABSTRACT FROM AUTHOR]

Flow-induced vibrations (FIV) were considered as unwanted vibrations analogous to noise. However, in a recent trend, the energy of these vibrations can be harvested and converted to electrical power. In this study, the potential of FIV as a source of renewable energy is highlighted through experimental and numerical analyses. The experimental study was conducted on an elastically mounted circular cylinder using helical and leaf springs in the wind tunnel. The Reynolds number (Re) varied between 2300–16000. The motion of the cylinder was restricted in all directions except the transverse direction. The micro-electromechanical system (MEMS) was mounted on the leaf spring to harvest the mechanical energy. Numerical simulations were also performed with SST k–ω turbulence model to supplement the experiments and were found to be in good agreement with the experimental results. The flow separation and vortex shedding induce aerodynamic forces in the cylinder causing it to vibrate. 2S vortex shedding pattern was observed in all of the cases in this study. The maximum dimensionless amplitude of vibration (A/D) obtained was 0.084 and 0.068 experimentally and numerically, respectively. The results showed that the region of interest is the lock-in region where maximum amplitude of vibration is observed and, therefore, the maximum power output. The piezoelectric voltage and power output were recorded for different reduced velocities (Ur = 1–10) at different resistance values in the circuit. It was observed that as the amplitude of oscillation of the cylinder increases, the voltage and power output of the MEMS increases due to high strain in piezoelectric transducer. The maximum output voltage of 0.6V was observed at Ur = 4.95 for an open circuit, i.e., for a circuit with the resistance value of infinity. As the resistance value reduced, a drop in voltage output was observed. Maximum power of 10.5μW was recorded at Ur = 4.95 for a circuit resistance of 100Ω. [ABSTRACT FROM AUTHOR]

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]

The present study explores the influence of protrusions on the combustion stability of a hybrid rocket motor. The primary aim is to analyze single, double, and triple-protrusion configurations taking into account variables such as their location, material, and spacing between them. Protrusions used were made of Nylon (regress during combustion) and graphite (non-regress during combustion). For the base-case (without protrusion), significant pressure oscillations were observed, arising from the interaction between vortex shedding and fundamental longitudinal acoustic modes of the chamber at the fuel grain exit. A protrusion at 0.8X/L (X-protrusion insertion point from head-end, L-fuel grain length) disrupted the interaction between acoustic modes and vortex shedding, substantially dampened its amplitude (around 96% reduction). In case of double (0.2–0.8X/L, 0.5–0.8X/L) and triple-protrusions (0.2–0.5-0.8X/L), the distance between protrusions is playing a key role in inducing the pressure oscillations. When protrusions are kept close (0.5–0.8X/L and 0.2–0.5-0.8X/L), the excitation of third mode acoustics were observed. This was attributed to the continuous increase in protrusion height. If protrusion height exceeds 6–7 mm, the flow over the front protrusion separated and impinges on the subsequent protrusion. This causes pressure oscillations in double and triple-protrusion cases. These pressure oscillations were mitigated using regressable protrusions made of nylon, as they keep the height within the range of 6–7 mm. The results of this research provide valuable perspectives on the significance of protrusions in controlling combustion instability in hybrid rockets. • Influence of Single and multi-protrusion on combustion stability of hybrid rocket was investigated. • Hybrid rocket combustion instabilities associated with acoustic modes-vortex shedding were detected. • A single protrusion near the fuel grain exit suppressed the excited acoustic mode instabilities. • For multi-protrusions, distance between the protrusions plays a key role in stability of the hybrid rocket. • Significance of protrusion material in controlling instability was demonstrated. [ABSTRACT FROM AUTHOR]

In this study, the wake of a cylinder was actively controlled by the cylinder's oscillatory morphing surface. Experiments were conducted in a closed-loop water channel. A cylinder of diameter 36 mm was placed in 0.09 m/s water flow, resulting in the Reynolds number 3240 and the vortex shedding frequency around 0.5 Hz. The cylinder's morphing surface oscillated at four different frequencies, i.e., 0.5, 1, 2, and 4 Hz. It was found that, compared to the rigid circular cylinder, the cylinder with oscillatory morphing surface can generally produce a smaller vortex formation length, especially at intermediate oscillation frequencies. The shear layers developed from the cylinder transit and roll up earlier due to enhanced flow instabilities. With the highest-frequency oscillations, the shear layer develops into a train of many small vortices that follow the trace of undisturbed shear layer. This study reveals some physical insights into this novel flow control method, which could be useful in future engineering applications. [ABSTRACT FROM AUTHOR]

Reduced order modeling (ROM) techniques, such as proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD), have been widely used to analyze stationary flows. Neural networks such as autoencoders are effective to reduce the dimension of non-stationary flow, but their larger storage requirements compared to POD and DMD compromise the expectations on ROM. The present work aims at compressing the autoencoder model via two distinctively different approaches, i.e., pruning and singular value decomposition (SVD). The developed algorithm is then applied to reconstruct the flow fields of typical stationary (i.e., a laminar cylinder flow and two turbulent channel flows) and non-stationary (i.e., a laminar co-rotating vortex pair) examples. It is shown that pruning and SVD reduce the size of the autoencoder network to 6% and 3% for the two simple laminar cases (or 18% and 13%, 20%, and 10% for the two complex turbulent channel flow cases), respectively, with approximately the same order of accuracy. Therefore, the proposed autoencoders optimized by the network pruning and SVD lead to effective ROM of both stationary and non-stationary flows although they require more iterations to converge than conventional methods. [ABSTRACT FROM AUTHOR]

The optimization method provides an effective approach to enhance the low-speed lift of the vortex lift waveriders by deforming the aerodynamic shape refinedly. However, the vortex lift enhancement mechanism of the optimized configuration is unclear. In this study, the flow evolutions of the original and the optimized configurations are studied by employing the delayed detached-eddy simulation. Results indicate that the convex deformation of the leeward surface plays a dominant role in enhancing the vortex lift by enhancing the low-pressure suction at the upstream breakdown location and delaying the vortex breakdown. For the enhancement of the low-pressure suction, the convex deformation intensifies the streamwise vorticity below the axis of the primary vortex of the leading-edge vortex, in turn enhancing the downwash effect and causing the primary vortex to move downward. This reduces the pressure coefficient induced by the primary vortex on the leeward surface, thus enhancing the vortex lift. In terms of the delay of the vortex breakdown, the convex deformation compresses and accelerates the flow between the spanwise convex and the leading edge. These intensities enhance the washing effect along the spanwise direction on the outward wing and cause the primary vortex to deflect toward the outboard wing. Subsequently, the primary vortex and the shedding vortices generated by the shear layer instability merge, which increase the primary vortex intensity, and enhance the streamwise velocity in the vortex axis. Correspondingly, the primary vortex breakdown is delayed. Ultimately, the increased low-pressure region caused by the delay of the vortex breakdown enhances the vortex lift. [ABSTRACT FROM AUTHOR]

The structural information about the tip leakage vortex at the design point remains largely unknown. Here, the dynamic mode decomposition method is utilized to visualize the main coherent structures corresponding to unsteady disturbance frequencies induced by the tip leakage vortex of an isolated rotor at the design point. The results show that the tip clearance size has a significant impact on unsteady disturbance characteristics at the blade tip region. The flow field within the blade tip region can be categorized into four distinct regions: the formation region of the main tip leakage vortex (MTLV), the formation region of the secondary tip leakage vortex (STLV), the merging zone where the MLTV and the STLV interact, and the vortex shedding zone induced by the leakage vortex breakdown. The disturbance peak in the frequency domain decreases from 121.3 RF to 70.96 RF as the tip clearance size increases from 1.5% blade height to 2%, resulting in a reduction of 41.36%. The increase in the tip clearance size amplifies unsteady disturbances caused by the MTLV and STLV. The STLV exhibits more pronounced oscillatory characteristics than the MTLV. The unsteady disturbance induced by the MLTV mainly occurs at around 0.5 blade passing frequency (BPF). In contrast, high-frequency unsteady disturbances (>1 BPF) in the flow field are caused by vortex shedding resulting from the interaction and collision between the STLV and the MTLV. A better understanding of the unsteady disturbance characteristics induced by leakage vortex benefits the study of stall warning technology. [ABSTRACT FROM AUTHOR]

To address the challenge of reducing aerodynamic drag while further enhancing the speed of high-speed trains, this research employs the concept of flow control for the bottom parts and draws inspiration from the front wings of Formula 1 (F1) race cars. Three kinds of two-multistage wing deflectors are designed and systematically analyzed by unsteady Reynolds-averaged Navier–Stokes (URANS) turbulent model. The most suitable design is determined by the single bogie model with a simplified train body. Using the improved delayed detached eddy simulation method, the aerodynamic drag of 1:8 three-car train models with or without two-multistage wing deflector is studied at different operational speeds. The results present the total drag reduction is higher at higher speeds. The reductions of 4.26%, 3.92%, 3.63%, and 3.49% are obtained at the operating speeds of 400, 350, 300, and 200 km/h, respectively. The two-multistage wing deflector desirably improves the flow structure at the bottom of the train, which leads to the reduction of aerodynamic drag and a corresponding reduction in the positive pressure zones within the bogie area. Furthermore, the deflector restricts shedding vortices, effectively narrowing the interference range of airflow under the train, which will provide a potential drag reduction method for the next generation high-speed train. [ABSTRACT FROM AUTHOR]

When two cylinders submerged in a uniform flow are arranged in tandem, the downstream cylinder can oscillate in response to the wake from the upstream cylinder. In this investigation, the downstream cylinder is allowed to oscillate freely around the center of a fixed upstream cylinder, mimicking a pendulum-like motion. Our findings suggest that the wake stiffness concept, initially verified for relatively large cylinder spacing (ℓ ≥ 4) and linear transverse cylinder motion, is also relevant for characterizing the wake-induced vibration (WIV) response observed for two tandem tethered cylinders situated in close proximity (2 ≤ ℓ ≤ 4) for Reynolds numbers in the range 1.0 × 10 4 ≲ Re ≲ 1.2 × 10 5 , with tests conducted up to R e ≈ 1.4 × 10 5 . For small cylinder spacing (ℓ ≤ 2.5), the downstream cylinder attains a maximum oscillation angle amplitude and exhibits consistent vibration, providing reliable potential for energy harvesting. We also explore hysteresis in the WIV response, which is observed to depend on the history of Reynolds number variation. Our findings reveal hysteresis at both the onset and termination of oscillation. [ABSTRACT FROM AUTHOR]

Large eddy simulation is adopted to analyze the interaction between the tip vortices shed by two contra-rotating propellers, by using a computational grid consisting of 4.6 × 109 points. Despite the complexity of the wake topology, the results of the computations show an excellent agreement with the measurements from an earlier experimental study on the same system. The interaction between the tip vortices shed by the two propellers produces vortex rings. Each of them consists of six helical sides, which are connected by U-shaped vortex lobes. The three upstream lobes of each vortex ring move to outer radial coordinates, as a result of their shear with the downstream lobes of the upstream vortex ring. In contrast, the downstream U-shaped lobes move to inner radial coordinates, as a result of their shear with the upstream lobes of the downstream vortex ring. This interaction results in an overall expansion of the wake of the contra-rotating propellers. The regions of shear between the U-shaped lobes of consecutive vortex rings are the areas of the largest turbulent stresses, which achieve higher levels than those produced in the wake of the two front and rear propellers working alone. This complex flow physics also triggers a faster instability of the wake system, breaking its coherence at more upstream coordinates, in comparison with the isolated propellers. [ABSTRACT FROM AUTHOR]

Unsteady three-dimensional simulations are performed to elucidate the hidden transitional flow dynamics and Hopf bifurcations along the topological corelines of the created von Kármán streets behind a pair of square cylinders positioned in tandem. Our simulations provide significant new insight into the three-dimensional wake evolution and governing flow physics. We explain how pressure, velocity, and vorticity fluctuation along the Kármán vortex corelines in the increasingly unstable wake amplify, facilitating the growth of various modal instability patterns. The existing knowledge of wake transition through the intertwining of modes A, B, and C instabilities and associated linear stability analysis helped to gain some insight into the overall wake feature. The current study explains how exactly the transitional disturbances physically spread behind a pair of inline and tandem cylinders through the self-excited spanwise-periodic oscillation of the wake and created a sequence of variable length scaled Hopf bifurcations and their swapping for varied gaps between the cylinders and Reynolds number. The growth of a slow mode of the spectral frequency at the bifurcation points seemed crucial in initiating the near-transitional flow irregularity. [ABSTRACT FROM AUTHOR]

In this work, a National Advisory Committee for Aeronautics 66 hydrofoil with leading-edge protuberances is designed. The large eddy simulation combined with the Schnerr–Sauer cavitation model is used to obtain a satisfactory result as compared with the experimental measurement, integrating the permeable Ffowcs Williams–Hawkings equation for cavitation-induced noise analysis. It is found that the special leading-edge geometric structure deflects the incoming flow, creating two counter-rotating streamwise vortices at the peak shoulders. These lead to upwash and downwash effects and alter the pressure distribution on the suction side. The low pressure localized in the trough facilitates the advancement of the leading-edge cavitation while severely limiting the spanwise development of the cloud cavity, shortening the cavitation evolution by about 20% and reducing the maximum cavitation volume by about 35%. Analysis using the vorticity transport equation indicates that different vorticity transport equation splitting terms play dominant roles at different stages of cavitation evolution. Although the cavitation induces disturbances in the primary vortex, the effect is limited. Acoustic simulation shows that the bionic structure can reduce the total sound pressure level by 7.8–8.3 dB. The spherical noise reduction is not as effective as expected due to the similar cavitation volume acceleration processes of the two hydrofoils. However, the pressure fluctuation caused by the collapse of the cloud cavity is reduced by cavitation suppression, which reduces the linear noise. In addition, the protuberances suppress the generation of large-scale vortex systems and transform them into smaller ones, resulting in reduced spanwise correlation and coherence of the shedding vortices. This is a critical factor in noise reduction. Finally, we hypothesize that the unstable noise reduction is related to the streamwise vortices in the trough regions. These vortices increase the momentum exchange within the boundary layer, affecting its stability and weakening the acoustic feedback loop. [ABSTRACT FROM AUTHOR]

We address the linear stability of the two-dimensional incompressible flow past a square cylinder immersed in the wake of an upstream splitter plate, which separates two streams of different velocities, UT (top) and UB (bottom), to three-dimensional perturbations. The analysis is done in the so-called wake transition regime, across which two-dimensional vortex shedding incorporates spanwise modulation. In addition to the top and bottom stream Reynolds numbers (R e T , B = D U T , B / ν , with D the square cylinder side and ν the kinematic viscosity of the fluid), a parametric analysis is conducted to gauge the effects of splitter plate length (S) and the gap between the trailing edge of the splitter plate and the front face of the cylinder (G) on the leading three-dimensionalizing instabilities. Modes akin to the A- and B-type instabilities that characterize the wake transition regime past circular and square cylinders in homogeneous incoming flow are observed only at very small values of the top-to-bottom Reynolds number ratio R = R e T / R e B , while a third mode, mode C, is ubiquitous beyond more than very mild values of R. Increasing S at constant small G has a stabilizing effect on mode C, whose onset is pushed to higher values of R. Only for long S is mode A observed. Fixing a short S and increasing G results instead in a destabilization of mode C, and mode B is favored over mode A. [ABSTRACT FROM AUTHOR]

The present study applies a Deep Reinforcement Learning (DRL) algorithm to Active Flow Control (AFC) of a two-dimensional flow around a confined square cylinder. Specifically, the Soft Actor-Critic (SAC) algorithm is employed to modulate the flow of a pair of synthetic jets placed on the upper and lower surfaces of the confined squared cylinder in flow configurations characterized by Re of 100, 200, 300, and 400. The investigation starts with an analysis of the baseline flow in the absence of active control. It is observed that at Re = 100 and Re = 200, the vortex shedding exhibits mono-frequency characteristics. Conversely, at Re = 300 and Re = 400, the vortex shedding is dominated by multiple frequencies, which is indicative of more complex flow features. With the application of the SAC algorithm, we demonstrate the capability of DRL-based control in effectively suppressing vortex shedding, while significantly diminishing drag and fluctuations in lift. Quantitatively, the data-driven active control strategy results in a drag reduction of approximately 14.4%, 26.4%, 38.9%, and 47.0% for Re = 100, 200, 300, and 400, respectively. To understand the underlying control mechanism, we also present detailed flow field comparisons, which showcase the adaptability of DRL in devising distinct control strategies tailored to the dynamic conditions at varying Re. These findings substantiate the ability of DRL to control chaotic, multi-frequency dominated vortex shedding phenomena, underscoring the robustness of DRL in complex AFC problems. [ABSTRACT FROM AUTHOR]

In this paper, we present a physics-informed approach to tailor the lift profile of an unsteady airfoil through the execution of an appropriate maneuver. In previous research, a low-order aerodynamic model based on the unsteady thin airfoil theory was developed for predicting the flowfield and loads on airfoils undergoing arbitrary motions. The theory was phenomenologically augmented using the concept of leading edge suction parameter (LESP) to incorporate the capability to predict intermittent leading edge vortex (LEV) shedding. The criticality of LESP was used to predict the onset and termination of LEV shedding and thus model the effect of LEVs on the flowfield and loads for a prescribed motion. In the current work, an inverse aerodynamic formulation is developed based on this framework for tackling the inverse problem: to obtain the motion kinematics required for generating a prescribed lift profile for an airfoil operating in the dynamic-stall regime. The LEV-modeling capability of the aerodynamic model enables the motion-design algorithm to take into account the effect of complex phenomena, such as dynamic stall and LEV shedding, which are not taken into account in previous research approaches. Several case studies are presented to demonstrate various scenarios such as lift tracking using pitching and heaving motions, lift cancellation during unsteady motion, and the generation of a given lift profile using two equivalent motions. The kinematic profiles generated by the inverse formulation are also simulated using a high-fidelity unsteady computational fluid dynamics solver to validate the predictions. [ABSTRACT FROM AUTHOR]

This paper presents the analysis of particle-laden liquid metal flow around a cylindrical obstacle at different obstacle Reynolds numbers. Particles in liquid metal are imaged using dynamic neutron radiography. We present the results of particle tracking velocimetry of the obstacle wake flow and demonstrate the capabilities to assess both temporal and spatial characteristics of turbulent liquid metal flow, validating our methods against theoretical expectations, numerical simulations and experiments reported in the literature. We obtain the expected linear vortex shedding frequency scaling with the obstacle Reynolds number and correctly identify the universal algebraic growth laws predicted and observed for trajectory curvature in isotropic homogeneous two-dimensional turbulence. To our knowledge, this is the first such result for liquid metals. Particle residence times within the obstacle wake and velocity statistics are also derived and found to be physically sound. Finally, we outline potential improvements to our methodology and plan for further research using neutron imaging of particle-laden flow. [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]

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]

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]

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]

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]

We numerically investigate the fluidic pinball under symmetric forcing and find seven flow regimes under different rotation speeds. The fluidic pinball consists of three rotatable cylinders placed at the vertices of an equilateral triangle pointing upstream in a uniform oncoming flow. The starting point is the unforced asymmetric periodic vortex shedding at Reynolds number Re = 100 based on the cylinder diameter. The flow is symmetrically actuated by rotating the two rear cylinders at constant speed |b| up to three times the oncoming velocity in both directions. Counterclockwise (b > 0) and clockwise (b < 0) rotation of the bottom cylinder correspond to boat tailing and base bleeding, respectively. A total of seven distinct flow regimes are observed, including a steady flow, three symmetric/asymmetric periodic types of shedding, two symmetric/asymmetric quasi-periodic behaviors, and a chaotic dynamics. The vortex shedding features multiple coupled oscillator modes, including in-phase, anti-phase, and out-of-phase synchronization and non-synchronization. These shedding regimes are analyzed employing the temporal evolution of the aerodynamic forces and a dynamical mode decomposition of the wake flow. The kaleidoscope of unforced and forced dynamics promotes the fluidic pinball as a challenging modeling and control benchmark. [ABSTRACT FROM AUTHOR]

The stability of the flow past a circular cylinder in the presence of a wavy ground is investigated numerically in this paper. The wavy ground consists of two complete waves with a wavelength of 4D and an amplitude of 0.5D, where D is the cylinder diameter. The vertical distance between the cylinder and the ground is varied, and four different cases are considered. The stability analysis shows that the critical Reynolds number increases for cases close to the ground when compared to the flow past a cylinder away from the ground. The maximum critical Reynolds number is obtained when the cylinder is located in front of the waves. The wavy ground adds layers of clockwise (negative) vorticity due to flow separation from the wave peak, to the oscillating Kármán vortex. This negative vorticity from the wave peak also cancels part of the positive (counterclockwise) vorticity shed from the bottom half of the cylinder. In addition, the negative vorticity from the wave peak strengthens the clockwise (negative) vorticity shed from the top half of the cylinder. These interactions combined with the ground effect skewed the flow away from the ground. The base flow is skewed upward for all the near-ground cases. However, this skew is larger when the cylinder is located over the wavy ground. The vortex shedding frequency is also altered due to the presence of the waves. The main eigenmode found for plain flow past a cylinder appears to become suppressed for cases closer to the ground. Limited particle image velocimetry experiments are reported which corroborate the finding from the stability analysis. [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]

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]

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]

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]

A series of laboratory experiments was conducted to study the dynamics of sand–water coaxial jets in stagnant water. The effects of the velocity ratio of the annular to the core jets on the sand concentration, velocity, mixing properties, and energy distribution of sand–water coaxial jets were studied. In comparison to sand jets, the existence of carrier fluid in sand–water coaxial jets decreased the concentration fluctuations of sand particles. Proper orthogonal decomposition (POD) and spectral proper orthogonal decomposition (SPOD) techniques were utilized to study the low-rank dynamic behavior on flow entrainment and particle oscillations. It was indicated that the first two POD modes have the most coherent and energetic quantitative flow features during the entrainment process. The energy contribution of the first five POD modes was reduced by 50% in sand–water coaxial jets with relatively low velocity ratios of Ru<0.35. The SPOD analysis revealed that particle oscillations can be characterized by the Kelvin–Helmholtz type wave instabilities, and sand–water coaxial jets with small velocity ratios (i.e., Ru<1.24) had weaker vortex shedding and particle velocity fluctuations. The SPOD analysis indicated that most of the turbulence kinetic energy was concentrated in the low-frequency modes and coaxial jets with high-velocity ratios. The power spectral density (PSD) of concentration signals demonstrated that the dominant frequency of vortex shedding increased by increasing the radial distance from the jet axis. The PSD analysis of sand concentration for Ru=0.62 and 0.74 was consistent with the Kolmogorov scaling law at dominant frequencies ranging between 30 and 40 Hz. [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]

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]

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 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]

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]

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]

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]