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To achieve stable and continuous supercavitating flow over underwater vehicles, artificial ventilation is implemented, particularly effective at lower speeds. Previous research on supercavitation primarily focused on analyzing ventilated supercavitating flow with various cavitator types and/or ventilation rates. In this investigation, we examine the behavior of ventilated supercavitating flow over an axisymmetric model featuring both a disk cavitator and a Postdam propeller placed at the bow. Utilizing the Large Eddy Simulation turbulence model, Volume of Fluid method, and Kunz cavitation model, our simulation aims to capture the cavitating flow around the propeller and the ventilated supercavitating flow over the model. Validation of our numerical methods is achieved by comparing our results with experimental data of a ventilated model by Chung and Cho ["Ventilated supercavitation around a moving body in a still fluid: Observation and drag measurement," J. Fluid Mech. 854, 367–419 (2018)] and the cavitating Postdam propeller by SVA Postdam [S. Potsdam, "PPTC smp'11 Workshop," in Proceedings of the Workshop on Cavitation and Propeller Performance (2011)]. The results show that with a rotating propeller at the bow of the supercavitating model, the cavitating flow extends and stabilizes compared to configurations utilizing a traditional disk cavitator. The presence of the propeller accelerates the formation of supercavitating flow at a consistent incoming flow speed. Additionally, coupling the propeller with the disk cavitator results in significant increases in propeller thrust, torque, and efficiency. While there is an observed rise in model drag, the impact is not substantial. [ABSTRACT FROM AUTHOR]

Blade tip leakage is responsible for reducing the aerodynamic performance and increasing the thermal load. In order to obtain better cooling efficiency, the thermodynamic performance of different film hole arrangements and blowing ratios have been compared and analyzed in a cavity tip structure. Reynolds-averaged Navier–Stokes equation simulations at five blowing ratios from 0.25 to 2.0 have been carried out for three stream-wise and four pitch-wise film hole arrangements. Based on the reference flow field of the cavity without any coolant, the mechanism of the cooling air influence is analyzed by investigating the flow structure and the vortex development in the tip region. It is found that in the stream-wise arrangements and pitch-wise arrangements, the scheme SW1, in which the film holes are arranged along the suction surface, and the scheme PW2, in which the air film holes are arranged at 0.2 times the axial position, provide the maximum average film cooling efficiency of 0.235 and 0.282, respectively. However, the effective cooling area of SW1 is greater than that of PW2, which are 39.2% and 41.3%, respectively. At the lower blowing ratios, the cooling efficiency increases with the increase in the blowing ratio; however, at the higher blowing ratios, the cooling effect is affected by the clearance size and cavity depth when the coolant impinges on the casing wall. [ABSTRACT FROM AUTHOR]

Tomographic reconstruction of three-dimensional (3D) tracer particle distributions through light field particle image velocimetry (LF-PIV) faces challenges in low reconstruction resolution owing to the elongation effect and extensive computational cost incurred by the iterative process. To resolve these challenges, this study proposes a deep neural network-based volumetric reconstruction approach to alleviate the reconstruction elongation and enhance the reconstruction efficiency. A tailored deep learning model (namely, LF-DNN) incorporating residual neural network architecture and a novel hybrid loss function is established to reconstruct the particle distributions through LF images. The parallax information of the flow field decoded from the raw LF data is leveraged as the input features of the network model. Comparative studies between the proposed method and the traditional tomographic reconstruction algorithms (multiplicative algebraic reconstruction technique, MART and pre-recognition MART, PR-MART) are performed through synthetic datasets. Experiments on a cylinder wake flow are further conducted to validate the performance of the proposed LF-DNN. The results indicate that the LF-DNN outperforms MART and PR-MART in terms of the reconstruction quality, mitigation of elongation effect, and noise resilience. The LF-DNN also improves the reconstruction efficiency which is 9.6 and 7.1 times higher than the MART and PR-MART, respectively. The relative error of the cylinder wake flow achieved by the LF-DNN is 2% lower than the MART. It suggests that the LF-DNN can facilitate accurate volumetric particle reconstruction and hence the three-dimensional flow measurement by single camera-based LF-PIV. [ABSTRACT FROM AUTHOR]

The increase in power density of hydrodynamic torque converters (HTCs) leads to a sharp rise in temperature within flow channels, affecting the reliability. In order to accurately predict the thermal effect and temperature distribution characteristics of the HTC internal viscosity oil, a multi-physics computational fluid dynamics (CFD) model is proposed. A specialized test bench was established, and the macro and internal flow temperature data were obtained. HTCs with different working conditions and wheel sets were studied. The results indicate that CFD model considering energy equation can accurately predict the overall hydrodynamic performance and the flow field temperature characteristics under different rotating conditions. The prediction error of the overall temperature rise is within 4.92%, and the flow field temperature prediction error of the stator is under 14.3%. The hydraulic characteristics is improved by 6.02%. The analysis of internal flow and energy exchange characteristics indicates the thermal effects and temperature distribution mechanisms caused by energy loss in the flow field within the HTC. The study provides an effective computational model for the prediction and control of the heat generation of the HTC and enhances the depth of research on the flow mechanism of inhomogeneous flow fields caused by thermal effects. [ABSTRACT FROM AUTHOR]

Viscous oils, which are media commonly used for fluid power transmission, are characterized by high velocities, temperatures, and pressures when working in fluid components and mechanics. The transient nature of viscous oil makes it susceptible to complex operating conditions, which result in cavitation phenomena and can threaten the normal operation and safety of machinery and components. In this study, a three-dimensional computational fluid dynamics model that accounts for cavitation was developed to study the cavitation characteristics, formation conditions, and development of cavitating flow in viscous oil around a hydrofoil under various flow conditions. Moreover, a visual experimental system in which viscous oil flowed around the hydrofoil was proposed and developed to investigate the cavitation properties with regard to various flow conditions. Both numerical results and experimental data indicated that cavitation occurred on the suction surface of the hydrofoil head, and the cavitation characteristics in viscous oil are significantly influenced by the flow conditions. The maximum vapor volume change rate for the degree of effects on cavitation in viscous oil by flow conditions was calculated to be 1.78 cm3/(m/s), −130.66 cm3/MPa, 0.16 cm3/°C, and 4.52 cm3/°, respectively. Low velocities, high pressures, low temperatures, and small impact angles were proved to be able to suppress cavitation. This study provides a research method, an experimental mean, and data support for cavitation flow of viscous fluids, especially oil. It has significant engineering application significance for the development of fluid machinery. [ABSTRACT FROM AUTHOR]

Large-scale modern wind turbines at standstill are prone to vortex-induced vibration (VIV). In this study, coupled fluid–solid dynamics of the wind turbine airfoil at a 90° attack angle are performed using the detached eddy simulation. The fully developed vibration responses with different structural dampings are explored in detail. The frequency lock-in regime is determined, and the corresponding phase differences between the lift and displacement are presented with the Lissajous curve. The dominant surface pressure mode and wake flow exhibit significant three-dimensional flow characteristics in unlock-in conditions, while a strong spanwise correlation in lock-in conditions is detected. The pressure fluctuation on the suction side in the lock-in state is observed to be more significant than in the unlock-in state. The effect of the distributed airfoil surface pressure on VIV is evaluated by considering the contribution value and the cyclic aerodynamic work density. With the decrease in structural damping, the aerodynamic work near the leading edge gets enhanced and the negative work region is reduced, leading to a higher amplitude of VIV. The beat vibration and hysteresis behavior at the critical reduced velocity are also analyzed in both the time domain and frequency domain. [ABSTRACT FROM AUTHOR]

As the modern industry rapidly advances toward digitalization, networking, and intelligence, intelligent fault diagnosis technology has become a necessary measure to ensure the safe and stable operation of mechanical equipment and effectively avoid major disaster accidents and huge economic losses caused by mechanical equipment failure. As the "power heart" of hydraulic transmission systems, hydraulic piston pumps (HPPs) occupy an important position in aerospace, navigation, national defense, industry, and many other high-tech fields due to their high-rated pressure, compact structure, high efficiency, convenient flow regulation, and other advantages. Faults in HPPs can create serious hazards. In this paper, the research on fault recognition technology for HPPs is reviewed. Firstly, the existing fault diagnosis methods are described, and the typical fault types and mechanisms of HPPs are introduced. Then, the current research achievements regarding fault diagnosis in HPPs are summarized based on three aspects: the traditional intelligent fault diagnosis method, the modern intelligent fault diagnosis method, and the combined intelligent fault diagnosis method. Finally, the future development trend of fault identification methods for HPPs is discussed and summarized. This work provides a reference for developing intelligent, efficient, and accurate fault recognition methods for HPPs. Moreover, this review will help to increase the safety, stability, and reliability of HPPs and promote the implementation of hydraulic transmission technology in the era of intelligent operation and maintenance. [ABSTRACT FROM AUTHOR]

The analytical model for tornado vortices is crucial in both the wind field characterization and the tornado-resistant design of civil structures. The objective of this study is to derive a novel three-dimensional analytical tornado model from the vortex governing equations simplified based on the force balance analysis in tornado-like vortices (TLVs). First, TLVs with different swirl ratios are generated in a numerical simulator utilizing the large-eddy simulation. Then, the forces in the axisymmetric vortex governing equations are calculated for time-averaged TLVs. The governing equations in the single-cell TLV are simplified by ignoring some significantly small terms. Finally, a novel three-dimensional analytical tornado model, which contains the radial, tangential, and vertical velocity as well as the pressure, has been proposed and validated. The result shows that the force balance in the single-cell TLV is simpler than that in TLVs with larger swirl ratios. In the single-cell TLV, the viscous forces in the radial and vertical directions can be neglected, while the tangential viscous force remains to play an important role in the force balance. The proposed model mitigates the limitations of existing models in describing single-cell tornado vortices, such as only two-dimensional velocity being given, the neglection of the vertical shear effects near the ground, and the infinite velocity at high altitudes. It shows good agreement with the numerical and experimental TLVs as well as the real tornado. [ABSTRACT FROM AUTHOR]

Large-scale modern wind turbines at standstill are prone to vortex-induced vibrations. In this study, we propose the use of the aero-damping map to investigate the complex vibration responses of the wind turbine airfoil at 90° of attack angle with different levels of structural dampings. The vibration amplitude and response frequency in the lock-in condition and soft lock-in conditions agree well with the contour line on which the sum of aerodynamic damping and structural damping is equal to zero. The mechanism of frequency soft lock-in is explored from the aspect of energy transfer that when the equilibrium state cannot be maintained at the natural frequency due to high structural damping, the system locks to a frequency between the natural frequency and vortex shedding frequency of the stationary airfoil to achieve lower aerodynamic damping and more energy absorption from the air. The transient response of the beat vibration is also investigated with the aero-damping map combined with the dynamic mode decomposition method. It is found that the lock-in mode and von Kármán mode coexist in the unsteady flow field during beat vibration. The competition between the two modes causes the system to be in an intermittent state of alternating frequency lock-in stage with lower aerodynamic damping and unlock-in stage with higher aerodynamic damping, hence resulting in the amplitude amplification and attenuation alternately. [ABSTRACT FROM AUTHOR]

Light field particle image velocimetry (LF-PIV) can measure the three-dimensional (3D) flow field via a single perspective and hence is very attractive for applications with limited optical access. However, the flow velocity measurement via single-camera LF-PIV shows poor accuracy in the depth direction due to the particle reconstruction elongation effect. This study proposes a solution based on a dual-camera LF-PIV system along with an ordered-subset simultaneous algebraic reconstruction technique (OS-SART). The proposed system improves the spatial resolution in the depth direction and reduces the reconstruction elongation. The OS-SART also reduces the computational time brought by the dual-camera LF-PIV. Numerical reconstructions of the particle fields and Gaussian ring vortex field are first performed to evaluate the reconstruction accuracy and efficiency of the proposed system. Experiments on a circular jet flow are conducted to further validate the velocity measurement accuracy. Results indicate that the particle reconstruction elongation is reduced more than 10 times compared to the single-camera LF-PIV and the reconstruction efficiency is improved at least twice compared to the conventional SART. The accuracy is improved significantly for the ring vortex and 3D jet flow fields compared to the single-camera system. It is therefore demonstrated that the proposed system is capable of measuring the 3D flow field fast and accurately. [ABSTRACT FROM AUTHOR]

Road noise varies with the vehicles' load and speed, ranging from low to medium frequencies. The existing absorbers, including acoustic metamaterials, cannot fully mitigate the variable noise due to limited bandwidth. To target the variable noise but not occupy much space, this paper presents a tunable acoustic absorber that can transform from Helmholtz resonance to membrane resonance. This tunable absorber consist of double micro‐slit layers, namely an electro‐adhesive membrane and a flexible plate spaced closely in front of a shallow cavity. The boundary‐layer thick gap between the two forms a network of lateral orifices to enhance the acoustic damping at medium frequencies. When the gap closes under electrostatic attraction force, the membrane and plate zipped together and resonated like a drum at a much lower frequency. This transformation can shift the resonant frequency down by 1.6 octaves from 898 hetz (Hz) to 281 Hz, thanks to the structural resonance of the zipped parts. Interestingly, the peak absorption coefficient at the lowest tuned 281 Hz was as high as 0.93. Such distributed tunability based on a fault‐tolerant electro‐adhesive membrane will also be applicable to other acoustic meta‐materials that embody the components of micro‐perforated or micro‐slit panels. [ABSTRACT FROM AUTHOR]

Fluctuating velocity plays an essential role in tornadic winds and the induced transient loads, while its characteristics are rarely considered in existing tornado models. Based on the coherent structure extraction technology, this study investigates the characteristics of the fluctuating velocity in a single-cell tornado-like vortex (TLV) and proposes a unified wind spectrum formula accordingly. First, the performance of proper orthogonal decomposition (POD) and dynamic mode decomposition is compared and validated using synthetic vortices. A single-cell TLV is then generated by large eddy simulation. The relationship between the fluctuating velocity and the coherent structures is analyzed. Finally, a wind spectrum formula is obtained from the fluctuating velocity reconstructed by the first two POD modes that are almost unchanged with height. Thus, it is a unified formula suitable for different heights. The results show that at a lower height in the single-cell TLV, more than 90% of the velocity fluctuation is induced by vortex wandering and size variation. The first two POD modes can accurately reconstruct the fluctuating velocity with an error of less than 8%. The power spectral density of the reconstructed fluctuating velocity agrees well with the Kaimal wind spectrum in the low-frequency subrange and the proposed formula in the high-frequency subrange. [ABSTRACT FROM AUTHOR]

The flow of cells in curved vessels is often accompanied by a secondary flow, which plays an important and practical role in various biomedical and bioengineering applications. However, there have been few attempts to investigate how the cells affect the development of the secondary flow in those curved microvessels. In this work, we use a particle-based model, smoothed dissipative particle dynamics, to numerically simulate the flow of red blood cells (RBCs) in a U-bend, with a diameter comparable to the RBC diameter. We first carry out three validation studies on the flow field, the cell deformation, and the cell aggregation, respectively, to establish the model predictive capability. Then, we study the formation and development of the secondary flow in a U-bend for the suspending (Newtonian) fluid, followed by exploring the disturbance of a single RBC and multiple RBCs to the secondary flow. The simulation results show that a secondary flow is developed in the U-bend for the suspending fluid, with a pair of Dean vortices. When a single RBC is suspended in the fluid, the secondary flow is disturbed, which is implemented by a transition from two to four and then back to two vortices again. This is the first time to show that cells can initiate such transition in a curved bend. When multiple RBCs are suspended in the fluid, the secondary flow becomes less likely to occur as the RBC number increases. On the contrary, the flow becomes more developed with increasing intercellular interactions. [ABSTRACT FROM AUTHOR]

The ever-increasing electromagnetic (EM) noise and acoustic noise are threatening public health in modern cities. Though materials are being developed, there is a lack of simple solution to address these two types of noise at the same time. Herein, flexible and ultralight (∼15 mg cm−3) silk fibre derived carbon fibre aerogels (SAs) are developed. The silk fibre mats, carbonized at different temperatures, are stacked together to form a compressible multi-layer structure. With optimized gradient impedance, the SA could achieve low-reflection coefficient (R < 0.02) electromagnetic interference (EMI) shielding in X and Ku bands (8.2 to 18 GHz). Moreover, the SA demonstrates an outstanding sound absorption performance (average absorption coefficient > 90%) from 1000 to 6000 Hz. Besides, the aerogel also shows a low thermal conductivity of ∼0.026 Wm−1 K−1, implying a potential thermal insulator. With such excellent performance and facile fabrication, the SA is expected to serve as a promising building material to be applied on the surface of architectures for the demand of both noise mitigation as well as energy conservation. The strategy to achieve multiple functions by using a multi-layered fibrous aerogel could also be applied to other natural fibres. [ABSTRACT FROM AUTHOR]

Multiphase flows are encountered in various industries, and the Coriolis flowmeter (CFM) is considered a high potential flowmeter for the metering of these flows. However, the decoupling effect and asymmetrical gas distribution in a CFM might decrease the accuracy of its multiphase flow metering The asymmetry of gas distribution in a CFM and its influence on the metering accuracy have only been qualitatively investigated in a few studies. The present paper quantitatively describes the gas distribution asymmetry in several CFMs under different flow conditions by numerical simulation. The simulation methodology is developed and validated by a results comparison with a conducted experiment and published data for bubbly, stratified and transitional flow regimes. U-shaped and triangle-shaped CFMs of different diameters are investigated at different gas volume fractions and flow rates. It is shown that the increase in the gas volume fraction and the reduction in the mixture flow rate lead to the increase in the gas distribution asymmetry. The strong correlation between the gas distribution asymmetry and the experimentally observed CFM error is demonstrated. The correction of the CFM error is proposed based on this correlation allowing the metering error to be decreased from 34% to 10% for the investigated conditions. [ABSTRACT FROM AUTHOR]

The light field particle image velocimetry (LF-PIV) has shown great potential for three-dimensional (3D) flow measurement in space-constrained applications. Usually, the parameters of the cross correlation calculation in the LF-PIV are chosen based on empirical analysis or introduced from conventional planar PIV, which lowers the accuracy of 3D velocity field measurement. This study presents an approach to selecting optimal parameters of the cross correlation calculation and thereby offers systematic guidelines for experiments. The selection criterion of the interrogation volume size is studied based on the analysis of the valid detection probability of the correlation peak. The optimal seeding concentration and the size of tracer particles are then explored through synthetic Gaussian vortex field reconstruction. The optimized parameters are employed in a cylinder wake flow measurement in a confined channel. A comparative study is conducted between the LF-PIV and a planar PIV system. Results indicate that the LF-PIV along with the optimized parameters can measure the 3D flow velocity of the cylinder wakes accurately. It has been observed that the mean and max errors of velocity decrease by 32.6% and 18.8%, respectively, compared to the related LF-PIV techniques without consideration of optimal parameters. Therefore, it is suggested that the optimized cross correlation parameters in the LF-PIV can improve the accuracy of 3D flow measurement. [ABSTRACT FROM AUTHOR]

Hydraulic torque converters are prone to cavitation due to their high impeller rotational speeds and their complex three-dimensional flow characteristics. Since the blades are the core components of torque converters, the shapes of the blades are important to the hydraulic performance and cavitation characteristics. Different cavitation computational fluid dynamics (CFD) models for a torque converter were developed to simulate the internal cavitation flow for different pump and turbine blade exit angles, and the influence of the blade angles on the cavitation characteristics and cavitation flow field in the torque converter was investigated. Experimental prototypes were produced and tested for verification. The results indicate that the pump and turbine blade exit angles had significant effects on the cavitation number of the torque converter. Increasing the pump and turbine blade exit angles promotes the generation and intensification of cavitation, resulting in severe changes in the shapes and locations of the cavitation bubbles due to changes in the fluid impact angles. Additionally, cavitation is quickly suppressed and the performance is improved when the blade exit angles are reduced within an appropriate range, in particular, that of the turbine blade. These research results can provide guidance for the design of a high-performance hydraulic torque converter cascade system and the suppression of cavitation for practical engineering applications. [ABSTRACT FROM AUTHOR]

The performance of torque converters, which use hydraulic oil as the working medium to transmit power, are severely affected by the operating temperature because various oil properties, such as viscosity, density and saturated pressure, vary drastically with temperature change. To study the thermal effect on the cavitation characteristics of viscous oil, we developed a full flow passage geometry and computational fluid dynamics (CFD) model with cavitation based on the finite volume method (FVM) to predict and analyze cavitation behavior and thermal effect in a torque converter. The results show that the critical cavitation number and critical cavitation temperature of the tested torque converter are 9.2 and 30 °C, respectively, and increasing the temperature reduces the cavitation number and induces the generation and intensification of cavitation. Besides, although higher temperature improves the transmission efficiency and capacity constant of the torque converter, it greatly magnifies the influence of cavitation on the hydrodynamic performance. In severe cases, it reduces the hydraulic performance by 9.3% and the flow velocity by 11.8% at 100 °C. Meanwhile, the cavitation flow field and the configuration of cavitation are also greatly affected by temperature. About 11.9% of the surface of the stator blade was covered by cavitation bubbles with the vapor volume fraction of 88.1% at 100 °C. Moreover, 30–60 °C is the optimum operating temperature range in terms of cavitations effect with no pronounced cavitation occurrence or that the cavitation degree is too weak to affect the performance in the mentioned temperature range. [ABSTRACT FROM AUTHOR]

A two-phase (air and water) smoothed particle hydrodynamics (SPH) method is employed to study the hydrodynamic-aerodynamic and wave interaction with fixed and floating structures in a wave basin. The method is first verified for a classical two-phase dam-breaking. A mirror-open boundary is implemented at the top and left sides of a two-phase wave basin with a piston to generate a second-order regular wave. It is observed that, compared to the single-phase simulation, the two-phase one obtains a smoother water surface and prevents the non-physical water splash when interacting with the sloped dissipative beach. This wave basin is also used to investigate wave-structure problems such as wave interaction with a rigid cantilever beam fixed to the basin bottom and downstream of the wave-maker mechanism and the dynamics of a single floating box and two floating boxes in the waves. A typical wave-structure interaction period is captured and described using pressure contours and velocity vectors at three selected instants for the wave-rigid cantilever beam case. With the increase of the structure's height, the wave height after the structure decreases, but no evident variation is found when changing its thickness. Besides the hydrodynamics interaction, a periodical collision is observed between the two floating boxes on the wave surface. [ABSTRACT FROM AUTHOR]

Cavitation is a transient phase transition between liquid and vapor, and it often occurs in fluid machinery, especially in a hydraulic torque converter that uses oil as the working medium to transmit speed and torque. The complex and strongly coupled fluid flow in the torque converter is prone to cavitation due to high rotating speed and high-temperature working conditions. Cavitation seriously affects the working performance, transmission smoothness, and service life of the torque converter. The flow pressure in the stator of a torque converter under various charging conditions and high rotating speeds was measured. The pressure data on the stator blade were analyzed in the time domain and frequency domain to identify and evaluate the cavitation characteristic. The transient cavitation flow inside the torque converter was also simulated with the computational fluid dynamics model. The results show that the shedding of cavitation seriously reduced the hydraulic performance, hindered the fluid flow, and destroyed the stability of the flow field. Moreover, cavitation aggravates the complexity and nonlinearity of the pressure frequency and hydraulic performance oscillation of the torque converter, and seriously affected the shaft/blade interaction frequency between the pump and stator. Meanwhile, the occurrence and degree of cavitation in the torque converter can be evaluated by APS.shaft/APS.blade (the amplitude ratio of the shaft interaction frequency and blade interaction frequency between pump and stator) with spectrum analysis of the dynamic pressure, and the critical value was 1.6 for the test torque converter. The research revealed the influence of cavitation on the internal flow field of the torque converter and provided a novel practical cavitation evaluation technique. [ABSTRACT FROM AUTHOR]

The perturbation technique based on the retardation-motion expansion is a simple method to obtain flow solutions at low Weissenberg number. In this context, this perturbation analysis is used to develop simple expressions for the motion of fibers suspended in viscoelastic fluids. In particular, the suspending fluid is characterized by a second-order fluid, Giesekus and PPT (Phan–Thien–Tanner) models, and their derivatives, such as the upper and lower convected Maxwell models. The first-order perturbation results in a similar effective velocity gradient that is exploited to express the translation and rotational motion of a single fiber and the associated extra stress tensor. In terms of a parameter related to the various viscoelastic fluid models, it is found that a fiber aligns along the vorticity direction when subjected to a shear flow. However, when a lower convected Maxwell model is considered, the elongated particle orients in the flow direction, as basically predicted by the Jeffery solution for a Newtonian suspending fluid. Furthermore, the conservation equation for particle concentration leads to particle migration in a pressure-driven flow channel and good agreement is observed with experimental data. [ABSTRACT FROM AUTHOR]

Recently, our direct numerical simulations [Yuan et al., "Hydrodynamic interaction and coalescence of two inline bubbles rising in a viscoelastic liquid," Phys. Fluids 33, 083102 (2021)] indicated that a stable chain can be formed for a pair of bubbles rising in a viscoelastic liquid, consistent with experimental observations. Motivated by the fact that the flow in bubble chains is still poorly understood, this Letter extends the investigations to multiple small bubbles ascending in a vertical file in a viscoelastic medium with different configurations. With an increasing bubble number, it is found that the rising velocity of the bubble group increases and the vertical chain of bubbles becomes unstable due to the distinct oscillation of the uppermost bubble. The terminal separation distance between two adjacent bubbles decreases in the upward direction, diminished by the neighborhood rising bubbles due to increasing loading. By probing the polymeric stresses and deformation, our results demonstrated that the accumulation of viscoelastic normal stresses promotes the aggregation of rising bubbles, while the successive chain of bubbles is stable because of the near-field repulsion induced by the non-monotonic polymer stretching among the bubble chain. In addition, the large bubble deformation appears to enhance the accumulative polymeric normal stress effect, and the bubbles can form more stable vertical chains at increasing initial spacing. Our findings provide insights into the mechanism of bubbles clustering in viscoelastic fluids, as chaining of bubbles is believed to be more prevailing in highly elastic flows. [ABSTRACT FROM AUTHOR]

In this paper, direct numerical simulations (DNS) are performed to investigate the inline rise of a pair of three-dimensional (3D) air bubbles in a viscoelastic liquid using the volume-of-fluid approach with an adaptive mesh refinement technique. The exponential Phan-Thien–Tanner model is used as the non-linear viscoelastic constitutive equation for the liquid. The numerical model has been validated by comparison with previously published results, including the terminal velocity jump discontinuity of an isolated bubble rising in a viscoelastic fluid, when its volume exceeds a certain critical value. Focusing on the inline rising bubble pair in such a viscoelastic medium with different configurations, we found that the wake of the small leading bubble attracts a larger trailing bubble, whereas for a supercritical bubble in front of a subcritical bubble, they tend to further separate. Before reaching a critical volume, the two subcritical bubbles remain close to each other after approaching each other, forming a stable chain. For pairs containing a supercritical trailing bubble, however, a drafting–kissing scenario occurs before the bubble–bubble coalescence. The long-range repulsion and the short-range attraction due to fluid elasticity are critical to the aforementioned bubble pair interactions. Interestingly, the terminal rise velocities of the stable bubble chain and the coalesced bubble both increase with the initial spacing. The squeezing flow near the growing bubble neck seems to delay the coalescence process. The capillary wave propagating down to the coalesced bubble tip together with the extensional flow behind the stretched bubble determines the generation of satellite microdroplets along the tail of the coalesced bubble. To the best of our knowledge, this is the first 3D DNS on a bubble pair ascending in viscoelastic fluids. [ABSTRACT FROM AUTHOR]

This paper presents an experimental study on the mechanism of interaction between a cavitation bubble and an air bubble. The cavitation bubble was generated by means of the low-voltage discharge method, and the combination of high-speed photography and a pressure measurement system allowed for simultaneous observation and measurement of the evolution of the shock wave and the change in shock wave strength with the presence of the air bubble in the vicinity. The high-speed imaging revealed the predominant roles of the relative distance φ and relative size ε between the cavitation and air bubbles in the determination of the stratification effect that the air bubble exerted on the shock wave produced from the first collapse of the cavitation bubble. The pressure measurement indicated that, when the air bubble did not merge with the cavitation bubble, the aforementioned factors, together with the angle $\alpha $ formed by the air bubble, cavitation bubble and the measuring point, would jointly affect the attenuation of the pressure peak and energy of the shock wave. Quantitatively, the attenuation magnitude was proportional to $a{(\alpha \varphi /\varepsilon)^b}$ , where the values of the coefficients a and b depended on whether the shock wave was stratified or not. When the cavitation bubble and the air bubble merged, the energy and the pressure peak of the shock wave decreased to less than 40 % of the values in the absence of the air bubble. With the new insight into bubble–bubble interaction mechanisms, the findings will facilitate a better understanding and development of cavitation utilization and prevention technology in water--air two phase systems. [ABSTRACT FROM AUTHOR]

In this paper, we numerically investigate the propulsive performance of three-dimensional pitching flexible plates with varying flexibility and trailing edge shapes. We employ our recently developed body-conforming fluid-structure interaction solver for our high-fidelity numerical study. To eliminate the effect of other geometric parameters, only the trailing edge angle is varied from 45 ° (concave plate), 90 ° (rectangular plate) to 135 ° (convex plate) while maintaining the constant area of the flexible plate. For a wide range of flexibility, three distinctive flapping motion regimes are classified based on the variation of the flapping dynamics: (i) low bending stiffness K B low , (ii) moderate bending stiffness K B moderate near resonance, and (iii) high bending stiffness K B high . We examine the impact of the frequency ratio f * defined as the ratio of the natural frequency of the flexible plate to the actuated pitching frequency. Through our numerical simulations, we find that the global maximum mean thrust occurs near f * ≈ 1 corresponding to the resonance condition. However, the optimal propulsive efficiency is achieved around f * = 1.54 instead of the resonance condition. While the convex plate with low and high bending stiffness values shows the best performance, the rectangular plate with moderate K B moderate is the most efficient propulsion configuration. To examine the flow features and the correlated structural motions, we employ the sparsity-promoting dynamic mode decomposition. We find that the passive deformation induced by the flexibility effect can help in redistributing the pressure gradient, thus, improving the efficiency and the thrust production. A momentum-based thrust evaluation approach is adopted to link the temporal and spatial evolution of the vortical structures with the time-dependent thrust. When the vortices detach from the trailing edge, the instantaneous thrust shows the largest values due to the strong momentum change and convection process. Moderate flexibility and convex shape help to transfer momentum to the fluid, thereby improving the thrust generation and promoting the transition from drag to thrust. The increase in the trailing edge angle can broaden the range of flexibility that produces positive mean thrust. The role of added mass effect on the thrust generation is quantified for different pitching plates and the bending stiffness. These findings are of great significance to the optimal design of propulsion systems with flexible wings. [ABSTRACT FROM AUTHOR]

Maintaining stability remains a crucial issue for the safety of underwater vehicles, especially during high-speed navigation where flow cavitation may occur. Cavitators are small protrusions on the hydrofoil surface, which can be used to control the patterns of flow cavitation. In this study, we investigate the effect of cavitators on supercavitating flow and hydrodynamic forces for high-speed hydrofoil. The volume of fluid method and the large eddy simulation turbulence model with the Kunz cavitation model are used in the simulations in order to accurately capture the interface between two phases (water and vapor) and the region of flow supercavitation. To validate the numerical method, the time-averaged simulated results of the supercavitating flow over a wedge-shaped hydrofoil are compared and validated against the experimental data by Kermeen ("Experimental investigations of three-dimensional effects on cavitating hydrofoils," Technical Report No. 47-14, Engineering Division of the California Institute of Technology, Pasadena, CA, 1960). Then, the numerical method is used to simulate the supercavitating flow of two-dimensional and three-dimensional cases with and without the cavitator placed on the lower side of the hydrofoil. The simulations were conducted for the cavitator located at the 1/32, 1/16, 1/8, and 1/4 chord of the hydrofoil at various angles of attack 1°–12° and cavitation numbers at 0.1, 0.2, and 0.4. A detailed analysis is made to examine the relations between cavitation pattern, hydrodynamic forces, and cavitator placement location. Compared to the cases without the cavitator, the supercavitating flow over the hydrofoils controlled by the cavitator demonstrates a significant difference in lift, whereas the drag does not change much. The changes in lift are closely related to the cavitator location. These findings can serve as a good guidance on how to improve the hydrodynamic performance and stability condition of a high-speed hydrofoil/wing using the cavitator. [ABSTRACT FROM AUTHOR]

The deformation and breakup of viscoelastic drops in simple shear flows of Newtonian liquids are studied numerically. Our three-dimensional numerical scheme, extended from our previous two-dimensional algorithm, employs a diffusive-interface lattice Boltzmann method together with a lattice advection–diffusion scheme, the former to model the macroscopic hydrodynamic equations for multiphase fluids and the latter to describe the polymer dynamics modeled by the Oldroyd-B constitutive model. A block-structured adaptive mesh refinement technique is implemented to reduce the computational cost. The multiphase model is validated by a simulation of Newtonian drop deformation and breakup under an unconfined steady shear, while the coupled algorithm is validated by simulating viscoelastic drop deformation in the shear flow of a Newtonian matrix. The results agree with the available numerical and experimental results from the literature. We quantify the drop response by changing the polymer relaxation time λ and the concentration of the polymer c. The viscoelasticity in the drop phase suppresses the drop deformation, and the steady-state drop deformation parameter D exhibits a non-monotonic behavior with the increase in Deborah number De (increase in λ) at a fixed capillary number Ca. This is explained by the two distribution modes of the polymeric elastic stresses that depend on the polymer relaxation time. As the concentration of the polymer c increases, the degree of suppression of deformation becomes stronger and the transient result of D displays an overshoot. The critical capillary number for unconfined drop breakup increases due to the inhibitive effects of viscoelasticity. Different distribution modes of elastic stresses are reported for different De. [ABSTRACT FROM AUTHOR]

The hydrodynamic loads and wake dynamics of the ducted propeller in oblique flow are investigated by detached eddy simulation (DES), with particular emphasis on the characteristics of wake fields and kinetic energy (KE) spectra. The influence of the duct, together with the strengthened induction of the propeller, cause the non-uniform velocity distributions in the inflow plane, resulting in unstable and periodic hydrodynamic loads on the blades. Shedding vortices from the leading edge of the duct dominate the leeward wake, and strengthen the self- and mutual induction of tip vortices. Large deformation of the tip vortices is observed due to the complex interference of the shear-layer vortices, shedding vortices and secondary vortices. The trajectories of the tip vortices are found distorted but still recognisable on the windward wake. The spiral-to-spiral distances of tip vortices increase and the re-alignment effect of the main flow enhances in the downstream wake. [ABSTRACT FROM AUTHOR]

The aerobatic maneuvers of swifts could be very useful for micro aerial vehicle missions. Rapid arrests and turns would allow flight in cluttered and unstructured spaces. However, these decelerating aerobatic maneuvers have been difficult to demonstrate in flapping wing craft to date because of limited thrust and control authority. Here, we report a 26-gram X-wing ornithopter of 200-millimeter fuselage length capable of multimodal flight. Using tail elevation and high thrust, the ornithopter was piloted to hover, fly fast forward (dart), turn aerobatically, and dive with smooth transitions. The aerobatic turn was achieved within a 32-millimeter radius by stopping a dart with a maximum deceleration of 31.4 meters per second squared. In this soaring maneuver, braking was possible by rapid body pitch and dynamic stall of wings at relatively high air speed. This ornithopter can recover to glide stability without tumbling after a 90-degree body flip. We showed that the tail presented a strong stabilizing moment under high thrust, whereas the wing membrane flexibility alleviated the destabilizing effect of the forewings. To achieve these demands for high thrust, we developed a low-loss anti-whirl transmission that maximized thrust output by the flapping wings to 40 grams in excess of body weight. By reducing the reactive load and whirl, this indirect drive consumed 40% less maximum electrical power for the same thrust generation than direct drive of a propeller. The triple roles of flapping wings for propulsion, lift, and drag enable the performance of aggressive flight by simple tail control. [ABSTRACT FROM AUTHOR]

In this work, we adopt the integration of the L-system fractal tree generation, 3D printed wind tunnel modeling, and computational fluid dynamics (CFD) simulation approach to model the wind effect on a single tree. We compare the agreement between CFD simulations and wind tunnel measurements of rigid branched structures resembling trees. First, fractal tree mesh models based on species growth and branching patterns are developed to represent tree species for wind–tree modeling. Subsequently, a scaled-down fractal tree is generated with 3D-printing and subjected to tunnel testing with load cell and particle image velocimetry measurement data under the wind speed of 10 m/s and 15 m/s. Finally, CFD based on Reynolds-Average Navier–Stokes (RANS) simulation with a full closure model and Large Eddy Simulation (LES) using appropriate momentum sink and turbulence source terms for the volumetric tree is carried out. We use both the volume-average porous media and the volume-splitting discretized zones (split number 10 × 10 × 10) to reproduce the momentum sink effect in the numerical simulation. Three tree species, namely, Peltophorum pterocarpum (yellow flame), Khaya senegalensis (African mahogany), and Hopea odorata (ironwood), are tested, and a reasonable agreement of drag force prediction and velocity profiles is obtained when comparing the CFD simulation results with wind tunnel data. The RANS modeled drag force results exhibit 20% of over-prediction, while the normalized velocity profiles display a good match of velocity decay at the tree leeward sides. On the other hand, LES produces much better results with only 3% discrepancy with the experimental results. A comparison of experimental results among the tree species is also carried out. Due to the actual random wind direction, tree slenderness representation, and structural flexibility issues, the current methodology still has the limitation for validation with urban on-site measurement. Nonetheless, this integrated approach is the first step in establishing modeling tool applicability to examine the effect of the forest structure and composition on wind loads. [ABSTRACT FROM AUTHOR]

Green urbanism has stimulated more research on the aerodynamics of tree in recent years. The insight gained in studying wind load on trees would mitigate risk of tree falling and enable sustainable landscape planning. However, deciphering the effect of wind on trees is a daunting task because trees come in various species, shapes and sizes. In this study, we aim at conducting wind tunnel tests on various species of trees, including measuring the respective drag coefficient and turbulent flow field using a force balance and particle image velocimetry system. The wind tunnel experiment is conducted using scaled down fractal tree model at 10 and 15 m/s. The 3D-printed tree model is grown based on the data collected on the species-specific tree parameters, such as the height, trunk diameters, crown box dimensions, etc. In this paper, the wind tunnel result of Yellow Flame (Peltophorum pterocarpum) is presented. Results show that the drag coefficient for this inflexible tree model is not sensitive to wind speed. The Reynolds shear stress and turbulence kinetic energy are observed to be the largest at the top and bottom of the crown where the velocity gradients are the highest. [ABSTRACT FROM AUTHOR]

A smoothed particle hydrodynamics method is employed to study the mechanical and thermal behaviors of a fiber-filled composite with an anisotropic thermal conductivity (which is coupled to the orientation of the fibers) in a three-dimensional printing process for one- and two-layer deposition. Using a microstructure-based fiber suspension model with a fiber orientation-dependent thermal conductivity model, a temperature-shear-thinning viscosity model, and a microstructure constitutive model, the effect of the nozzle temperature on the fiber alignment when printing one layer and the mechanical and thermal interactions between two printed layers are investigated. It is found that the anisotropic thermal conductivity (fiber-orientation-dependent) enhances the fiber alignment in the printing direction in the upper half layer and reduces it in the lower half at a relatively high fiber concentration (Φ = 0.2). For the one-layer deposition, the fiber alignment in the printing direction is enhanced in the lower half of the layer with an increase in the nozzle temperature. This tendency is more pronounced with the increase in both the fiber concentration and the aspect ratio. On the two-layer deposition, the fiber alignment of the first layer experiences a "reciprocating" evolution due to the squeezing from the second layer, thus creating an enhancement in the upper half and a reduction in the lower half in the fiber alignment in the first layer (with respect to the printing direction). Increasing the fiber concentration or the aspect ratio amplifies this variation for the first layer. Increasing the substrate velocity also leads to some variations in the fiber alignment. [ABSTRACT FROM AUTHOR]

Professor John Blake spent a considerable part of his scientific career on studying bubble dynamics and acoustic cavitation. As Blake was a mathematician, we will be focusing on the theoretical and numerical studies (and much less on experimental results). Rather than repeating what is essentially already known, we will try to present the results from a different perspective as much as possible. This review will also be of interest for readers who wish to know more about the boundary element method in general, which is a method often used by Blake and his colleagues to simulate bubbles. We will, however, not limit the discussion to bubble dynamics but try to give a broad discussion on recent advances and improvements to this method, especially for potential problems (Laplace) and wave equations (Helmholtz). Based on examples from Blake's work, we will guide the reader and show some of the mysteries of bubble dynamics, such as why jets form in collapsing bubbles near rigid surfaces. Where appropriate, we will illustrate the concepts with examples drawn from numerical simulations and experiments. [ABSTRACT FROM AUTHOR]

Instability, extinction and re-initiation of rotating detonation in a two-dimensional Rotating Detonation Engine (RDE) configuration are numerically investigated, and the emphasis is laid on the effects of variable total pressures on the above combustion dynamics. Two conditions, i.e. different constant pressures and time-varying pressures, are considered to investigate the combustion instability in RDE. It is seen that under constant pressure condition the rotating detonation is more prone to instability at low pressure, due to the instability from the deflagrative surface. The intrinsic frequency for the unstable cases with different pressures is close, and maybe related to the RDE configuration and/or fuel properties. For the time-varying pressures with various specified frequencies, the RDE shows the different levels of instability characterized by multiple frequencies. The dominant frequencies vary, depending on the competition between the forced external frequency and the intrinsic one induced by the RDE system. About the extinction of continuously rotating detonation, it can be found that when the total pressure is reduced to a relatively lower value the detonation is quenched and left with deflagrative combustion. Furthermore, when the total pressure is increased based on the extinguished flow fields, detonation fronts are re-initiated due to the local high pressure. The stochasticity in RDE re-initiation has been found, in terms of the location and number of initial detonation front, the propagation direction, their interaction and final stabilization. [ABSTRACT FROM AUTHOR]

This study aims to provide valuable insight into the irrigation in curved root canals with CFD method. Numerical simulation of the flow originated from a side-vented or an open-ended flat needle located in the prepared root canal with two different insertion depths was performed. The distribution of velocity, wall shear stress, apical pressure, and the detailed flow structures were all evaluated. The results show that both the side port of the side-vented needle and the curved portion of the target root canal tends to make circumferentially non-symmetric wall shear stress distribution and asymmetrical streamlines. However, the impact of the root-canal curvature variation on the flow pattern is apparent only when the open-ended flat needle was selected for irrigation or the needle was inserted into the curved part of the root canal. The insertion depth and the size of the gap between the outer surface of the needle and the root canal internal wall are critical to the apical pressure, except the former is simultaneously crucial for the micro-velocity zone decreasing. Overall, the optimal method for effective irrigation in a curved root canal is the side-vented needle with the same curvature to the root canal, deeper insertion, and reasonable gap size. [ABSTRACT FROM AUTHOR]

This work presents systematical investigations on the skin-friction drag reduction (DR) of turbulent channel flow subjected to spanwise wall oscillation using direct numerical simulation. Altogether 12 different oscillatory cases have been studied with a reference at Reτ = 200, varying the controlling parameters characterized by maximum wall velocity Wm+ and oscillation period T+. Some of the previously established facts have been reproduced by our analysis with a new focus on the phase-space dynamics of the near-wall streaks, on the basis of statistical data over entire oscillation periods and over phasewise variations. It is revealed that streamwise vortices are generated in the vicinity of oscillation walls, disrupting the formation of near-wall low-speed streaks. Although the overall turbulence is weakened, the Stokes layer is thicker within wall acceleration phases for larger Wm+, which causes the turbulence intensity to increase in the upper viscous sublayer. In addition, regarding the effect of T+, a long oscillation period promotes the formation of energetic near-wall structures, while for short T+, the streak-generation time scale preferentially restricts the growth of spanwise streaks. From a new vorticity-transport perspective of the Reynolds shear stress, our results further indicate that high drag-reducing phenomena are connected to the near-wall sweep events, and the shear stress variation is principally driven by the distortion of the spanwise transport of wall-normal vorticity, i.e., vortex tilting/stretching. The DR process is seen to be linked to the increase in enstrophy and turbulence-energy dissipation in the near-wall region. [ABSTRACT FROM AUTHOR]

The mechanical and thermal behavior of nonisothermal fiber-filled composites in a three-dimensional printing process is studied numerically with a smoothed particle hydrodynamics method. A classical microstructure-based fiber suspension model with a temperature-dependent power-law viscosity model and a microstructure constitutive model is implemented to model a fiber-filled system. The fiber microstructure is described by a second-order tensor A2 which describes the spatially averaged orientation of the fibers. Two benchmark cases are presented to validate the reliability of the present implementation. Three typical printing modes are tested to assess the characteristics of printed layers. The results show that the printed layer becomes thicker, and the fiber alignment in the printing direction is enhanced in the bottom half of the layer and reduced in the top half due to the existence of nonisothermal effects in the process. The variation in fiber orientation becomes larger with increasing fiber concentration. By increasing the Peclet number, the deposited layer thickness reduces and the fiber alignment in the printing direction is enhanced in the top half and reduced in the bottom half. The evolution of the orientation and the velocity gradient tensors projected along several streamlines are discussed to illustrate the effects of the temperature and different printing modes on the deposited layer. [ABSTRACT FROM AUTHOR]

A Smoothed Particle Hydrodynamics (SPH) formulation and implementation of the classical two-phase mixture model are reported, with a particular focus on the turbulent sediment transport and the sediment disturbances generated by moving equipment operating near or on the seabed. In the mixture model, the fluid-particle system is considered to be an equivalent medium whose evolution is described by a set of equations for the mixture continuity and momentum conservation, with the particle volume fraction being tracked by a transport equation. The governing equations are adapted to a Lagrangian, weakly-compressible SPH framework, the turbulence is modeled by a Reynolds-averaged Navier-Stokes approach, and adaptive boundary conditions for shear stress and turbulent quantities are implemented to account for laminar or turbulent local flow conditions. The complex rheological behavior of clay sediment/water mixtures is modeled using a volume fraction, shear rate-dependent viscosity which accounts for the existence of a yield stress. Hence, the proposed work encompasses several challenging modeling aspects: turbulence, non-Newtonian fluid behavior, sediment transport, and fluid-structure interactions. It is then illustrated on diverse cases of interest: a fluid-particle mixture column release, its subsequent turbulent transport and return to a hydrostatic equilibrium, the settling of particle clouds and two cases of particle-driven gravity currents, and their comparisons with available results. Finally, SPH simulation results for the disturbance of a bed of clay sediment/water mixture induced by a moving plate are reported and compared with experiments performed in our laboratory. The proposed SPH two-phase mixture model agrees well with the existing results considered in this study. [ABSTRACT FROM AUTHOR]

In this paper, a data driven approach is presented for the prediction of incompressible laminar steady flow field over airfoils based on the combination of deep Convolutional Neural Network (CNN) and deep Multilayer Perceptron (MLP). The flow field over an airfoil depends on the airfoil geometry, Reynolds number, and angle of attack. In conventional approaches, Navier-Stokes (NS) equations are solved on a computational mesh with corresponding boundary conditions to obtain the flow solutions, which is a time consuming task. In the present approach, the flow field over an airfoil is approximated as a function of airfoil geometry, Reynolds number, and angle of attack using deep neural networks without solving the NS equations. The present approach consists of two steps. First, CNN is employed to extract the geometrical parameters from airfoil shapes. Then, the extracted geometrical parameters along with Reynolds number and angle of attack are fed as input to the MLP network to obtain an approximate model to predict the flow field. The required database for the network training is generated using the OpenFOAM solver by solving NS equations. Once the training is done, the flow field around an airfoil can be obtained in seconds. From the prediction results, it is evident that the approach is efficient and accurate. [ABSTRACT FROM AUTHOR]

Technical activities on seafloor for harvesting polymetallic nodules result in a displacement of a large amount of sediment, which is convected away from the site by the underlying currents and turbulent diffusion, with a possible impact on the benthic communities living in the neighborhood. To better understand the dispersion mechanism of the resuspended sediment, a smoothed particle hydrodynamics technique augmented by a message passing interface parallel algorithm to address the intensive demand on the three-dimensional simulations is developed. Our numerical results show that the resuspended sediment would occupy a downstream area extending to about 5 km, for a nominal current speed of 5 cm/s. The evolution of the sediment plume occurs mainly along the current direction, while the turbulent diffusion disperses the sediment laterally. Coarse sediment particles are found to return to the seafloor fairly quickly after being resuspended, while fine particles are more persistent in the suspended state and travel much further downstream. In 900 tons of sediment resuspended for 18 h, 318 tons have returned to the bottom at the end of the simulation period. The majority of the deposited sediment is composed of coarse sediment particles (d > 60 μm), and almost half of the deposited sediment is distributed within the harvesting region. The sediment deposition rate reaches up to 48% of the resuspension rate and is still rising after 18 h. The horizontal turbulent diffusivity, which is supposed to be weak at the ocean bottom, does not have any obvious influence on the dispersion of the resuspended sediment; it only slightly reduces the deposition rate. [ABSTRACT FROM AUTHOR]

We present a numerical study of the fused deposition modeling 3D printing process of fiber-reinforced polymers by means of Smoothed Particle Hydrodynamics (SPH). For this purpose, a classical microstructure-based fiber suspension model coupled with a constitutive model for the suspending polymer is implemented within an SPH framework. The chosen model is reviewed, together with the details and specificities of its implementation in SPH. The results for several representative cases are then presented, mainly in terms of contours of fiber orientation tensor components and orientation distributions across the deposited layer thickness. The impact of the fiber concentration and its aspect ratio in a semi-concentrated regime and the effect of the ratio between extrusion and substrate velocities are investigated. Some insights into the link between the flow field and fiber orientation evolution within the printing head and as the material exits the nozzle are given. The main findings lie in the prediction of a skin/core structure in the deposited layer in which the skin regions exhibit a higher fiber alignment with respect to the core region. This effect is found to be enhanced by an increase in fiber concentration and to be sensitive to the substrate-to-extrusion velocity ratio. It is indeed enhanced in cases where the substrate velocity is low compared to the extrusion velocity and accompanied by a larger swelling of the deposit at the nozzle exit. [ABSTRACT FROM AUTHOR]

During the operation, the pulse detonation engine (PDE) may have to work with different fuel/air equivalence ratios (from a lean to rich fuel mixture) as in the cold start-up operation, which would strongly affect the engine performance characteristics and the outputs of interest. For a better operational control, it is necessary to gain understanding of the effect of the equivalence ratio on the dynamics of the processes of PDE. Thus, in this study, numerical simulations are performed for different ethylene fuel/air equivalence ratios to study its effect on the dynamics of the deflagration-to-detonation transition (DDT) and detonation processes. In particular, the density-based solver with a shock-capturing scheme is employed to solve for the viscous, compressible, and reacting flows governed by reacting Navier-Stokes equations. The computed flame propagation speed, run-up distance, and Chapman-Jouguet detonation velocity are comparable to the current experimental results. In addition, the numerical results show that the minimum values of the run-up distance and run-up time, as well as the maximum value of the detonation velocity occur at the equivalence ratio of about 1.1. Analysis of the computed results associate these findings to the firm correlation of the flame speed with equivalence ratio, which is in turn function of the temperature, pressure, and mixture composition. The shifting of the outputs of interest to the richer fuel side from the stoichiometric point can be attributed to the combustion product dissociation, mixture heat capacity, and oxidizer enrichment. [ABSTRACT FROM AUTHOR]

The flow of the laminar boundary layer on a flat plate is studied with the simulation of Navier-Stokes equations. The mechanisms of flow instability at external edge of the boundary layer and near the wall are analyzed using the energy gradient theory. The simulation results show that there is an overshoot on the velocity profile at the external edge of the boundary layer. At this overshoot, the energy gradient function is very large which results in instability according to the energy gradient theory. It is found that the transverse gradient of the total mechanical energy is responsible for the instability at the external edge of the boundary layer, which induces the entrainment of external flow into the boundary layer. Within the boundary layer, there is a maximum of the energy gradient function near the wall, which leads to intensive flow instability near the wall and contributes to the generation of turbulence. [ABSTRACT FROM AUTHOR]

Detached eddy simulation (DES) is investigated as a means to study an array of shallow dimples with depth to diameter ratios of 1.5% and 5% in a turbulent channel. The DES captures large-scale flow features relatively well, but is unable to predict skin friction accurately due to flow modelling near the wall. The current work instead relies on the accuracy of DES to predict large-scale flow features, as well as its well-documented reliability in predicting flow separation regions to support the proposed mechanism that dimples reduce drag by introducing spanwise flow components near the wall through the addition of streamwise vorticity. Profiles of the turbulent energy budget show the stabilising effect of the dimples on the flow. The presence of flow separation however modulates the net drag reduction. Increasing the Reynolds number can reduce the size of the separated region and experiments show that this increases the overall drag reduction. [ABSTRACT FROM AUTHOR]

The knowledge of weather conditions at the stratosphere is important for the planning and execution of high-altitude balloon flights, which require an accurate modeling of weather data over a period of time. Various methods based on statistical analysis, artificial neural networks, and cluster analysis have been employed to model the temporal variation of weather parameters. In the present study, a proper orthogonal decomposition (POD) method has been used to study the spatial as well as temporal variations of wind data in Singapore. The use of POD facilitates a compact representation of the weather dataset and aids in faster computation of wind profiles for use in balloon trajectory simulation. Further, the results reveal the existence of the quasi-biennial oscillation phenomenon, which is characteristic of equatorial easterly-westerly winds. This phenomenon enables the development of a Fourier prediction model, which can be used in real-time balloon trajectory simulations. The Fourier model is observed to be sensitive to wind velocity fluctuations, especially in the vicinity of alternating wind directions. However, it provides a reasonable projection of balloon trajectory, which can be used in preliminary planning and testing of high-altitude flights. Thus, a prior knowledge of wind profiles based on POD or a Fourier model aids in balloon station keeping. A simple case of altitude-controlled balloon flight is presented, and the results highlight the advantages of the present method in balloon station keeping. [ABSTRACT FROM AUTHOR]

Acoustic absorbers, acoustic metamaterials, electro-adhesion, electrostatics, Helmholtz resonators, Helmholtz resonators, membrane resonators, self clearing, sound absorbers Transformation from Helmholtz to Membrane Resonance by Electro-Adhesive Zip of a Double-Layer Micro-Slit Acoustic Absorber (Adv. A transformable absorber that switches from the Helmholtz resonance at a medium frequency to the membrane resonance at a low frequency is developed. [Extracted from the article]