Numerical Analysis of the Effects of Periodic Gust Flow on the Wake Structure of Ventilated Supercavities

A computational model is established to investigate the effects of a periodic gust flow on the wake structure of ventilated supercavities.The effectiveness of the computational model is validated by comparing with available experimental data.Benefited from this numerical model,the vertical velocity characteristics in the entire flow field can be easily monitored and analyzed under the action of a gust generator;further,the unsteady evolution of the flow parameters of the closed region of the supercavity can be captured in any location.To avoid the adverse effects of mounting struts in the experiments and to obtain more realistic results,the wake structure of a ventilated supercavity without mounting struts is investigated.Unsteady changes in the wake morphology and vorticity distribution pattern of the ventilated supercavity are determined.The results demonstrate that the periodic swing of the gust generator can generate a gust flow and,therefore,generate a periodic variation of the ventilated cavitation numberσ.At the peakσ,a re-entrant jet closure appears in the wake of the ventilated supercavity.At the valleyσ,a twin-vortex closure appears in the wake of the ventilated supercavity.For the forward facing model,the twin vortex appears as a pair of centrally rolled-up vortices,due to the closure of vortex is affected by the structure.For the backward facing model,however,the twin vortex appears alternately as a pair of centrally rolled-up vortices and a pair of centrally rolled-down vortices,against the periodic gust flow.

Effect of the asymmetric geometry on the wake structures of a pitching foil

The two-dimensional wake produced by a time-periodic pitching foil with the asymmetric geometry is investigated in the present work. Through numerically solving nonlinear Navier–Stokes equations, we discuss the relationship among the kinematics of the prescribed motion, the asymmetric parameter K ranged as-1 ≤ K ≤ 1, and the types of the wakes including the mP＋nS wake, the B′enard–von K′arm′an wake, the reverse B′enard–von K′arm′an wake, and the deviated wake.Compared with previous studies, we reveal that the asymmetric geometry of a pitching foil directly affects the foil＇s wake structures. The numerical results show that the reverse B′enard–von K′arm′an wake is easily deviated at K 〈 0, while the symmetry-breaking of the reverse B′enard–von K′arm′an wake is delayed at K 〉 0. Through the vortex dynamic method,we understand that the initial velocity of the vortex affected by the foil＇s asymmetry plays a key role in the deviation of the reverse B′enard–von K′arm′an wake. Moreover, we provide a theoretical model to predict the wake deviation of the asymmetric foil.

Measurement of the flow structures in the wakes of different types of parachute canopies

We measured flow structures with stereoscopic particle image velocimetry(stereo-PIV) in the turbulent wakes of three parachute canopies, which had the same surface area, but different geometries. The tested parachute canopies included ribbon canopy, 8-branches canopy, and cross canopy. The obtained results showed that the geometry of the parachute canopies had significant influences on the flow structures in the wakes of these three canopies. In addition, the variation of Reynolds number did not lead to a dramatic change in the distributions of velocity, vorticity,Reynolds stress, and turbulent kinetic energy.

Three-dimensional dynamics of a single bubble rising near a vertical wall:Paths and wakes

In order to clarify the migration mechanism and wake behavior of a single bubble rising near a vertical wall,three-dimensional direct numerical simulations are implemented based on the open-source soft-ware Basilisk and various types of migration paths like linear,zigzag and spiral are investigated.The volume of fluid(VOF)method is used to capture the bubble interface at a small scale,while the gas-liquid interface and high-velocity-gradient regions in the flow field are encrypted with the adaptive mesh refinement technology.The results show that the vertical wall has an obstructive effect on the diffusion of the vortex boundary layer on the surface of the bubble migrating in a straight line,and the resulting reaction force tends to push the bubbles away from the wall surface.For the zigzag or spiral movement of a bubble in the x-y plane,the perpendicular wall is an unstable factor,but on the contrary,the motion in the z-y plane is stabilized.

Numerical study on wake characteristics of high-speed trains

Intensive turbulence exists in the wakes of high speed trains, and the aerodynamic performance of the trailing car could deteriorate rapidly due to complicated features of the vortices in the wake zone. As a result, the safety and amenity of high speed trains would face a great challenge. This paper considers mainly the mechanism of vortex formation and evolution in the train flow field. A real CRH2 model is studied, with a leading car, a middle car and a trailing car included. Different running speeds and cross wind conditions are considered, and the approaches of un- steady Reynold-averaged Navier-Stokes （URANS） and de- tached eddy simulation （DES） are utilized, respectively. Re- suits reveal that DES has better capability of capturing small eddies compared to URANS. However, for large eddies, the effects of two approaches are almost the same. In conditions without cross winds, two large vortex streets stretch from the train nose and interact strongly with each other in the wake zone. With the reinforcement of the ground, a complicated wake vortex system generates and becomes strengthened as the running speed increases. However, the locations of flow separations on the train surface and the separation mechanism keep unchanged. In conditions with cross winds, three large vortices develop along the leeward side of the train, among which the weakest one has no obvious influence on the wake flow while the other two stretch to the tail of the train and combine with the helical vortices in the train wake. Thus, optimization of the aerodynamic performance of the trailing car should be aiming at reducing the intensity of the wake vortex system.

A Forced System of Two Cylinders with Various Spacings

The spectrum characteristics and wake structures for a circular cylinder oscillating in a wake are investigated by use of the currently modified virtual boundary method. A forced system of two cylinders with a small spacing (the downstream one is made to oscillate in the transverse direction) is studied and interesting flow characteristics are observed. A vortex switch and the change of vortex modes (between 2S mode and 2P mode) are observed in the “lock in' region. Vortex bands are formed and lost with the increasing excitation frequency. Information concerning saddle points in the flow field is obtained for different excitation frequencies. For a forced system of two cylinders with a large spacing, the upstream cylinder sheds vortexes because there is no downstream cylinder oscillating in the wake. No distinct “lock in' response is found for the downstream cylinder.

EXPERIMENTAL STUDY OF FLOW STRUCTURE BEHIND A CIRCULAR CYLINDER WITH WAVY SURFACE BY PIV

The wake behind a circular cylinder with wavy surface was investigated using single-frame PIV technique at Re=5200. The mean flow field and spatial distribution of turbulence statistics in the nodal, saddle and middle planes were presentcd and compared with those of a smooth cylinder. The near wake behind the wavy cylinder shows pronounced spanwise periodicity of flow structures. Compated with a smooth cylinder, the vortex formation region considerably expands in the streamwise direction, particularly reaching maxi mum in the saddle plane of the wavy cylinder. However, the longitudinal vortices in the nodal plane were noticeably suppressed in the transverse direction. In addition, the wake in the nodal plane contains the peak value of turbulent kinetic en ergy and the Reynolds shearing stress due to the intensive entrainment of free flow and the momentum exchange in the spanwise direction.

The numerical and experimental investigations of the near wake behind a modified square stay-cable

The near wake structure, the wake-flow characteristics and the drag coefficients behind a modified square stay-cable （MSC） with sinusoidal variations of the cross-section area along the spanwise direction are investigated experimentally and numeri- cally. The Reynolds numbers are chosen as 100 and 500 for the laminar flow and Re = 6 000 and 22 000 for the turbulent flow. The detailed near wake structures, the velocity fields and the force coefficients for the MSC are captured, the effect of the Reynolds number on the flow structure for the MSC is studied. The numerical and experimental investigations show that the free shear layers from the leading edge are widened and prolonged and then roll up into vortices further downstream the MSC, unlike a straight square stay-cable （SSC） under the same flow conditions. As a result, the distinct mean drag reduction and the fluctuating lift suppression are observed for all Reynolds numbers, a drag reduction of at least 15.8% and the rms lift coefficient reduction of up to 95% are observed, as compared with the case of a straight square stay-cable at Re = 500.

Numerical Simulation of a Three-Dimensional Fish-like Body Swimming with Finlets

The swimming of a 3Dfish-like bodywith finlets is numerically investigated at Re=1000(the Reynolds number is based on the uniform upstream flow and the length of the fish-like body).The finlets are simply modeled as thin rigid rectangular plates that undulate with the body.The wake structures and the flow around the caudal peduncle are studied.The finlets redirect the local flow across the caudal peduncle but the vortical structures in the wake are almost not affected by the finlets.Improvement of hydrodynamic performance has not been found in the simulation based on this simple model.The present numerical result is in agreement with that of the work of Nauen and Lauder[J.Exp.Biol.,204(2001),pp.2251-2263]and partially supports the hypothesis ofWebb[Bull.Fish.Res.Bd.Can.,190(1975),pp.1-159].