Journal of Hydrodynamics最新文献

Cavitation noise around propellers has many adverse effects. It is still very limited nowadays to inhibit propeller cavitation noise in engineering. In this study, the cavitation noise around a PPTC propeller is simulated using the large eddy simulation (LES) coupled with the porous Ffowcs Williams-Hawkings (PFW-H) equation. The investigation aims to find a strategy to suppress cavitation noise and analyze the noise suppression mechanism. The predicted hydrodynamic results agree well with the experimental data and are utilized in the hydroacoustic analysis. The hydroacoustic results indicate that the pseudo-thickness noise dominates the dominant frequency component of the total cavitation noise due to the effect of cavity evolution, which is one of the reasons why the pseudo-thickness noise dominates the total cavitation noise. A method is found to weaken the cavitation noise through ventilation at the generation location of the sheet cavity (SC). It is worth noting that ventilation inhibits the generation and development of SC by changing the pressure distribution on the suction surface of the blade and pushing away the cavities around the ventilation holes. Moreover, cavity evolution noise dominates the fluid volume evolution noise under the ventilated cavitating condition. Ventilation significantly attenuates the vapor volume pulsation and thus the cavity evolution noise, which leads to a reduction in pseudo-thickness noise and total cavitation noise. The ventilation mainly reduces noises at the dominant frequency of the pseudo-thickness noise and the total cavitation noise.

Vortices, which appear as swirling behaviour of a flow field, are an important phenomenon in fluid dynamics, and the extraction of vortex cores is necessary for the research of vortex evolution in many areas of fluid mechanics. The Liutex method is a milestone in vortex identification and provides a reasonable mathematical definition for a vortex core. Based on this definition, a novel integration-based method is presented, which can reduce the numerical error in the integration process through location optimization. Two typical test cases, the wake vortices of an A320 in the near-ground stage and a helicopter rotor, are examined to show that the proposed method can extract continuous vortex core lines with accuracy and efficiency for vortex parameter study.

This paper presents a numerical study on focused wave and current interactions with a cylinder. The cylinder is moving in the opposite direction to the wave propagation. An effective computational decomposition method is adopted to reduce the calculation resources. A potential solver high-order spectral (HOS) method is applied to generate focused wave field, while our in-house computational fluid dynamics (CFD) solver naoe-FOAM-SJTU with overset grid takes the charge of achieving the viscous effect around the moving cylinder. The viscous domain moving with the cylinder thus the size and mesh grids in computational domain is greatly reduced. The pressure on cylinder surface and wave fields around cylinder are compared with experimental data, shows a well agreement. Meanwhile, the scattering wave field and vortex shedding are discussed. With the existence of moving cylinder, the classical scattering wave types are still observed.

Many species of fish and birds travel in intermittent style, yet the combined influence of intermittency and other body kinematics on the hydrodynamics of a self-propelled swimmer is not fully understood. By formulating a reduced-order dynamical model for intermittent swimming, we uncover scaling laws that link the propulsive performance (cursing Reynolds number Re_c, thrust T̄, input power P̄ and cost of transport COT to body kinematics (duty cycle DC, flapping Reynolds number Re_f). By comparing the derived scaling laws with the data from several previous studies and our numerical simulation, we demonstrate the validity of the theory. In addition, we found that Re_c, T̄, P̄ and COT all increase with the increase of DC, Re_f. The model also reveals that the intermittent swimming may not be inherently more energy efficient than continuous swimming, depending on the ratio of drag coefficients between active bursting and coasting.

In this paper, the dynamic behaviors of the cavitation bubble near a fixed spherical particle during the second oscillation period are analyzed based on the high-speed photographic system. The deformation and motion of the bubble during the second period are investigated by changing the distance between the particle and the bubble and the maximum radius of the bubble. Meanwhile, the variation of the equivalent radius and the centroid motions are analyzed, and the dynamic behaviors of the bubble are categorized according to the bubble morphological characteristics during the second period. Through this research, it is found that (1) The dynamic behaviors of the bubble during the second oscillation period could be divided into three typical cases: For case 1, a bulge would exist on the bubble interface away from the particle, and for case 2, a bulge would appear on the bubble interface and evolve towards the particle, while for case 3, the bubble would be divided into two parts. (2) The larger the dimensionless distance between the particle and the bubble, the smaller the maximum bubble equivalent radius in the second period, and the shorter the second oscillation period. (3) When the bubble is close to the particle, a counter-jet appears at the bubble interface away from the particle during the rebound stage.

The objective of this paper is to investigate the dynamic behavior of cloud cavitating flow around a flexible hydrofoil with experimental and numerical methods. The tightly coupled fluid-structure interaction (FSI) modeling is applied and validated with the experimental data. The Q - criterion and ω - criterion are applied to illustrate the interaction between the vortex structures and cavities. The flexibility is seen to result in nose-up twist deformation, causing a reduction of the shedding frequency from an increase in the attached cavity length. Due to the flexibility, the fluctuation of load coefficients of the flexible hydrofoil is larger than that of the rigid hydrofoil. Moreover, the re-entrant jet propagation speed of the flexible hydrofoil is greater than that of the rigid hydrofoil. The shed cloud cavity is observed to be uniform along the flexible hydrofoil span under the combined influence of the strong vibration and the gap flow.

The fluid kinematics of Liutex decomposes a velocity gradient tensor (VGT) of ∇v into four components, including rotation (R), stretching/compressing (SC), anti-symmetric shear (S_anti-sym) and symmetric shear (S_sym), as oppose to the traditional Cauchy-Stokes decomposition where a VGT was decomposed into the strain rate and vorticity tensors. The current study limpidly clarified the physical meanings of these deformations in the newly-proposed decomposition from the perspectives of both fluid kinematics and dynamics. With in-depth understanding the physical connotations of these deformations, the present study further suggests that the S_sym be the only deformation appropriately correlated to the stress tensor, leading to the establishment of a new constitutive relation for Newtonian fluids with the modified model assumptions originated from Stokes in 1845. Moreover, the present research finds that the “principal decomposition” proposed by Liu is not mathematically unique when a VGT has three real eigenvalues (TR). Within the context, a new decomposition method is introduced to avoid the non-uniqueness issue arising from using the principal decomposition to establish fluid dynamics equations. Based on the modified Stokes assumptions and the novel VGT decomposition method, a set of new fluid dynamics momentum equations are obtained for Newtonian fluid. The added stress tensor of F^add is identified as the key difference between the newly-derived governing equations and the conventional Navier-Stokes (N-S) equations, which is caused by excluding the SC correlation to the stress tensor in the new constitutive equation. Finally, a preliminary analysis of F^add is conducted using the existing channel turbulence direct numerical simulations (DNS) data based on the traditional N-S equations. The F^add is found widely existing in turbulence and is of the same order of magnitude with the other force terms. Therefore, the F^add is expected to have some nonnegligible effects on altering the current DNS data based on the traditional N-S equations, which will be further verified by performing the “DNS” simulation using the newly-derived fluid dynamics equations in near future.

Atmospheric boundary layer (ABL) flow over multiple-hill terrain is studied numerically. The spectral vanishing viscosity (SVV) method is employed for implicit large eddy simulation (ILES). ABL flow over one hill, double hills, and three hills are presented in detail. The instantaneous three-dimensional vortex structures, mean velocity, and turbulence intensity in mainstream and vertical directions around the hills are investigated to reveal the main properties of this turbulent flow. During the flow evolution downstream, the Kelvin-Helmholtz vortex, braid vortex, and hairpin vortex are observed sequentially. The turbulence intensity is enhanced around crests and reduced in the recirculation zones. The present results are helpful for understanding the impact of topography on the turbulent flow. The findings can be useful in various fields, such as wind energy, air pollution, and weather forecasting.

This article investigates the effect of a synthetic jet (SJ) on the flow over a backward-facing step (BFS) in the weakly turbulent flow regime using the lattice Boltzmann method. The SJ operates with various momentum coefficients C_μ and forcing frequencies f ^*_jet . As C_μ increases, the reattachment length decreases, whereas increasing f ^*_jet causes the reattachment length at first decrease and then increase. A minimum reattachment length appears at C_μ = 0.3125, f ^*_jet = 1.6, corresponding to a 40% reduction compared with the uncontrolled case. Two mechanisms for the mediated flow are found: (1) A suitable control frequency leads to a lock-on state that prompts vertical momentum transfer and laminarizes the flow near the separation point, (2) Regular vortices emerge after wall reattachment in controlled cases. Fast Fourier and wavelet transform of the velocity near the separation point reveal that the monitored frequency becomes locked-on when f ^*_jet > 1.6, making the flow quasi-periodic and dramatically reducing the reattachment length. Turbulent kinetic energy spectra indicate that the monitored frequencies are dominated by the forcing frequency and that active control laminarizes the local flow. Proper orthogonal decomposition is used to extract coherent structures at multiple scales. In the dominant mode, reattaching wake vortices are regulated by active control. In the second mode, irregular wake vortices emerge after f ^*_jet = 2, which attenuates the SJ forcing and increases the reattachment length. This study provides insights on typical flows past a BFS and will shed more light on the design of closed-loop control strategies for separation flows.

The purpose of this paper is to investigate the performance improvement mechanism of a high power vertical centrifugal pump by using numerical calculations. Therefore, a comparative study of energy losses and internal flow characteristics in the original and optimized models was carried out with special attention to the hydraulic component matching. The optimized model (model B) was obtained by optimizing the vaned diffuser and volute based on the original model (model A), mainly the diffuser inlet diameter, diffuser inlet vane angle, volute channel inlet width and volute throat area were changed. Firstly, the comparative results on performance and energy losses of two models showed that the efficiency and head of model B was significantly increased under design and part-load conditions. It is mainly due to the dramatic reduction of energy loss P_L in the diffuser and volute. Then, the comparisons of P_L and flow patterns in the vaned diffuser showed that the matching optimization between the model B impeller outlet flow angle and diffuser inlet vane angle resulted in a better flow pattern in both the circumferential and axial directions of the diffuser, which leads to the P_L3 reduction. The meridian velocity V_m of model B was significantly increased at diffuser inlet regions and resulted in improvements of flow patterns at diffuser middle and outlet regions as well as pressure expansion capacity. Finally, the comparisons of P_L and flow characteristics in the volute showed that the turbulence loss reduction in the model B volute was due to the flow pattern improvement at diffuser outlet regions which provided better flow conditions at volute inlet regions. The matching optimization between the diffuser and volute significantly reduced the turbulence loss in volute sections 1–4 and enhanced the pressure expansion capacity in sections 8–10.