Flow Structures In Wake Research Articles

Flow pattern similarities in the near wake of three bird species suggest a common role for unsteady aerodynamic effects in lift generation.

Analysis of the aerodynamics of flapping wings has yielded a general understanding of how birds generate lift and thrust during flight. However, the role of unsteady aerodynamics in avian flight due to the flapping motion still holds open questions in respect to performance and efficiency. We studied the flight of three distinctive bird species: western sandpiper (Calidris mauri), European starling (Sturnus vulgaris) and American robin (Turdus migratorius) using long-duration, time-resolved particle image velocimetry, to better characterize and advance our understanding of how birds use unsteady flow features to enhance their aerodynamic performances during flapping flight. We show that during transitions between downstroke and upstroke phases of the wing cycle, the near wake-flow structures vary and generate unique sets of vortices. These structures appear as quadruple layers of concentrated vorticity aligned at an angle with respect to the horizon (named 'double branch'). They occur where the circulation gradient changes sign, which implies that the forces exerted by the flapping wings of birds are modified during the transition phases. The flow patterns are similar in (non-dimensional) size and magnitude for the different birds suggesting that there are common mechanisms operating during flapping flight across species. These flow patterns occur at the same phase where drag reduction of about 5% per cycle and lift enhancement were observed in our prior studies. We propose that these flow structures should be considered in wake flow models that seek to account for the contribution of unsteady flow to lift and drag.

High-Speed Particle Image Velocimetry of the Flow around a Moving Train Model with Boundary Layer Control Elements

Trackside induced airflow velocities, also known as slipstream velocities, are an important criterion for the design of high-speed trains. The maximum permitted values are given by the Technical Specifications for Interoperability (TSI) and have to be checked in the approval process. For train manufactures it is of great interest to know in advance, how new train geometries would perform in TSI tests. The Reynolds number in moving model experiments is lower compared to full-scale. Especially the limited model length leads to a thinner boundary layer at the rear end. The hypothesis is, that the boundary layer rolls up to characteristic flow structures in the train wake, in which the maximum flow velocities can be observed. The idea is to enlarge the boundary layer using roughness elements at the train model head so that the ratio between the boundary layer thickness and the car width at the rear end is comparable to a full-scale train. This may lead to similar flow structures in the wake and better prediction accuracy for TSI tests. In this case, the design of the roughness elements is limited by the moving model rig. Small rectangular roughness shapes are used to get a sufficient effect on the boundary layer, while the elements are robust enough to withstand the high accelerating and decelerating forces during the test runs. For this investigation, High-Speed Particle Image Velocimetry (HS-PIV) measurements on an ICE3 train model have been realized in the moving model rig of the DLR in Gottingen, the so called tunnel simulation facility Gottingen (TSG). The flow velocities within the boundary layer are analysed in a plain parallel to the ground. The height of the plane corresponds to a test position in the EN standard (TSI). Three different shapes of roughness elements are tested. The boundary layer thickness and displacement thickness as well as the momentum thickness and the form factor are calculated along the train model. Conditional sampling is used to analyse the size and dynamics of the flow structures at the time of maximum velocity in the train wake behind the train. As expected, larger roughness elements increase the boundary layer thickness and lead to larger flow velocities in the boundary layer and in the wake flow structures. The boundary layer thickness, displacement thickness and momentum thickness are increased by using larger roughness especially when applied in the height close to the measuring plane. The roughness elements also cause high fluctuations in the form factors of the boundary layer. Behind the roughness elements, the form factors rapidly are approaching toward constant values. This indicates that the boundary layer, while growing slowly along the second half of the train model, has reached a state of equilibrium.

Swirling dual-disk double-concentric jets at low annulus Reynolds numbers

The flow characteristics and mixing properties of swirling dual-disk double-concentric jets at low annulus Reynolds numbers were studied. The flow patterns, velocity characteristics, and mixing properties were characterized using the laser-assisted smoke flow visualization method, particle image velocimetry (PIV), and tracer-gas concentration detection technique. Three characteristic flow modes,annular jet-dominated wake, central jet-dominated wake, and central jet-dominated radial flow, were observed within different ranges of central jet Reynolds numbers for the annulus Reynolds and the swirl numbers were less than 270 and 0.245. The flow structure of the central jet-dominated wake (a dual-ring recirculation zone in the wake region and two pairs of counter-rotating vortices in the gap between the up- and downstream disks) allowed the annular jet fluids to be transported into the gap between the upstream and downstream disks to premix with the central jet fluids before further being transported to the disk wake. The flow fluctuation intensities in the wake of the central jet-dominated wake were significantly larger than those of the annular jet-dominated wake and central jet-dominated radial flow because the large-scale fluctuation eddies that possess large turbulence kinetic energy existed in the wake. These properties prominently promoted the mixing in the central jet-dominated wake.

Experimental investigation of the flow structures in the wake of a cross parachute canopy

In the present study, an experimental study was conducted to characterize the flow structures in the turbulent wake of a cross parachute canopy by utilizing Stereoscopic Particle Image Velocimetry (Stereo-PIV) technique. Both flow through gap and flow over arm of the cross parachutes were carefully measured and analyzed. The obtained results showed that the turbulent wake of the flow through gap was larger than that of flow over arm. Besides, the distributions of velocity, vorticity, and Reynolds stress of flow through gap differed significantly with those of flow over arm. In addition, the effect of the arm ratio of the cross canopy on the transverse profiles of velocity, vorticity, and Reynolds stress was also discussed.

Numerical simulation of stratified flow around a tall building of a complex shape

Results of large-eddy simulations of stably stratified atmospheric flow around an isolated, complex-shaped tall building are presented. The study focuses on the identification of flow structures in the building wake in high and low Froude number regimes. A qualitative comparison of results with available literature data and existing theories is presented. In addition to flow structures identified in earlier studies such as the horseshoe and recirculation eddy vortices, we analyze a stationary disturbance akin to mountain gravity wave, and a complex vortex structure associated with this wave, consisting of multiple symmetric pairs of vortices. The Froude number appears to be the principal parameter controlling the structure of the wake, waves and vortex pattern.

Dynamic leg-motion and its effect on the aerodynamic performance of cyclists

In this wind-tunnel based experimental study, the flow topology of the near wake of a generic anatomically accurate model cyclist is mapped for a range of reduced pedalling frequencies. Wake flow fields for both static leg and pedalling cyclists are compared over the full 360° rotation of the crank using both time- and phase-averaging. The primary wake flow structures and aerodynamic forces are quantified and analysed under dynamic pedalling conditions representative of an elite-level time-trial cyclist. Over the range of reduced pedalling frequencies studied, only minor variation was detected between the instantaneous drag and primary vortical structures of a pedalling cyclist compared to a stationary cyclist with the pedals in the same position. A simplified model of the aerodynamic forces acting on the legs under motion is presented to provide insight into how the motion of the legs influences aerodynamic drag. A comparison of predicted forces from this model with those from experiments provides a new perspective on how the aerodynamics of cyclists may be optimised.

Control of boundary layer flow and lock-on of wake behind a circular cylinder with a normal slit

A two-dimensional flow simulation around a circular cylinder with a normal slit at a Reynolds number of 200 is conducted using Ansys/Fluent software. The slit ratio (S/D) ranges from 0.03 to 0.3. The scenarios of the characteristics of the slit flow, the evolution of the boundary-layer flow, and the wake flow are investigated and disclosed by modified phase-averaged method. Some critical outcomes are outlined. Induced by the shedding vortex street with shorter formation length, the pressure difference between the two slit openings drives the fluid at each slit opening, in form of periodic blowing/suction with zero net mass flux. Periodic blowing/suction at the slit opening serves as a perturbation that significantly delays the boundary-layer flow separation along the rear surface of the slit cylinder and reduces the vortex formation length. With proper slit ratio, this perturbation synchronizes with the evolution of the boundary-layer flow and the formation of vortex-street, leading to the primary lock-on of the wake. In this study, the optimal slit ratio is S/D=0.18 at Re=200. For all slit ratios studied, the frequency and amplitude responses, as well as the slight phase shift of the wake flow structures clearly demonstrate the primary lock-on of the wake. In this study, the one-sided lock-on frequency range lies only within fa/fso>1 because the primary lock-on is caused by the synchronization between the periodic blowing/suction at the slit opening and the shedding vortex street with higher frequency (or reduced vortex formation length) behind the slit cylinder than that of a base-line (or non-slit) cylinder.

Experimental study comparing racecar aerodynamic downforce-generating devices using scale model NASCAR COT

The performance and safety of the rear wing and spoiler employed on the National Association of Stock Car Auto Racing (NASCAR) COT (car of tomorrow) racecar are experimentally studied using 10 % scale models in a water channel. Particle image velocimetry is used to qualitatively examine the differences in flow structures between the two downforce-generating devices under 0 and 180-degree yaw cases. The latter is important due to an issue with the COT flipping into the air when at extreme yaw (i.e. during a crash). At zero yaw, it is observed that smaller length scales of the flow structures in the wake of the wing compared to those in the wake of the spoiler, provide more predictable handling for racecars in close proximity and may allow more safe and competitive racing. At 180-degree yaw, it is observed that wake-structure interactions may not allow proper operation of anti-flipping devices (roof flaps) on the winged car. In the extreme yaw case, local flow scales are examined and show much stronger Reynolds number (Re) dependence for the wing than the spoiler.

A Comparison of the Wake Structures of Scale and Full-scale Pedalling Cycling Models

This paper presents a novel approach to better understand the unsteady aerodynamics associated with a dynamically pedalling cyclist. Using high resolution Particle Image Velocimetry (PIV) in a water channel, the large-scale wake structure is analysed for various phases of the crank cycle of a 1:4.5 scale-model cyclist/bicycle under both static and pedalling conditions. Both quasi-steady and dynamic pedalling leg results are compared with detailed velocity field surveys made in the wake of a full-scale pedalling cyclist mannequin of similar geometry and position in a wind tunnel. A time-averaged and phase-averaged analysis of the various flow regimes that occur throughout the pedal stroke shows good agreement between scale-model and full-scale mannequin investigations. This highlights the robustness of the formation of the primary wake flow structures when subjected to varying Reynolds number, bicycle/rider geometry and quasi-steady/dynamic pedalling conditions.

Effect of aspect ratio on the near-wake flow structure of an Ahmed body

Through numerical simulations, the interaction between the longitudinal c-pillar vortices and the flow over the rear slant surface is established for an Ahmed-body geometry as its aspect ratio is varied. In turn this affects the flow structure and topology of the near wake, and has a significant effect on the drag.In particular, aspect ratio was found to influence the critical angle at which flow fully separates on the rear slant surface due to the interaction of the c-pillar vortices with the rear slant flow. This appears to be a consequence of the influence that downwash generated by the c-pillar vortices has on promoting rear slant flow reattachment. By isolating the pressure drag associated with individual surfaces, it is shown that the drag discontinuity is almost entirely due to a sudden change in the pressure field on the rear slant surface. On increasing the aspect ratio, once the flow becomes fully separated, c-pillar vortex strength and position change considerably, inferring a mutual relationship between flow reattachment and c-pillar vortex generation mechanisms for the Ahmed-body geometry.

U-shaped fairings suppress vortex-induced vibrations for cylinders in cross-flow

We employ three-dimensional direct and large-eddy numerical simulations of the vibrations and flow past cylinders fitted with free-to-rotate U-shaped fairings placed in a cross-flow at Reynolds number $100\leqslant \mathit{Re}\leqslant 10\,000$. Such fairings are nearly neutrally buoyant devices fitted along the axis of long circular risers to suppress vortex-induced vibrations (VIVs). We consider three different geometric configurations: a homogeneous fairing, and two configurations (denoted A and AB) involving a gap between adjacent segments. For the latter two cases, we investigate the effect of the gap on the hydrodynamic force coefficients and the translational and rotational motions of the system. For all configurations, as the Reynolds number increases beyond 500, both the lift and drag coefficients decrease. Compared to a plain cylinder, a homogeneous fairing system (no gaps) can help reduce the drag force coefficient by 15 % for reduced velocity $U^{\ast }=4.65$, while a type A gap system can reduce the drag force coefficient by almost 50 % for reduced velocity $U^{\ast }=3.5,4.65,6$, and, correspondingly, the vibration response of the combined system, as well as the fairing rotation amplitude, are substantially reduced. For a homogeneous fairing, the cross-flow amplitude is reduced by about 80 %, whereas for fairings with a gap longer than half a cylinder diameter, VIVs are completely eliminated, resulting in additional reduction in the drag coefficient. We have related such VIV suppression or elimination to the features of the wake flow structure. We find that a gap causes the generation of strong streamwise vorticity in the gap region that interferes destructively with the vorticity generated by the fairings, hence disorganizing the formation of coherent spanwise cortical patterns. We provide visualization of the incoherent wake flow that leads to total elimination of the vibration and rotation of the fairing–cylinder system. Finally, we investigate the effect of the friction coefficient between cylinder and fairing. The effect overall is small, even when the friction coefficients of adjacent segments are different. In some cases the equilibrium positions of the fairings are rotated by a small angle on either side of the centreline, in a symmetry-breaking bifurcation, which depends strongly on Reynolds number.

Characteristics of Flow Structures in the Wake of a Bed-Mounted Bluff Body in Shallow Open Channels

The characteristics of the flow structures observed in the wake of a bluff body mounted vertically on the bed and normal to the flow in a shallow open channel are investigated using detached eddy simulation (DES). The flow structures in the shallow wake are identified using the λ2-criterion. A distinctive feature in the time-averaged flow field, referred to as the owl face of the first kind, is observed. The position of this spiraling structure is stable at locations close to the bed, while its rotation sense switches from stable inward to unstable outward spiraling as it moves toward the free surface, where the bed friction becomes insignificant and the flow develops into a traditional two-dimensional (2D) wake. A three-dimensional (3D) structure resulting from a horizontally oriented secondary roll-up process is observed immediately downstream of the base of the bluff body in the center of the near-wake region. In addition to the horseshoe vortex, a new structure that wraps around the bluff body in the toe region is identified, referred to as a collar vortex. The presence of the coherent structures in the near-bed region is highlighted and their influence on the wake region is discussed.

Characteristics of flow past a circular cylinder with a rectangular groove

In the present study, features of the flow past a circular cylinder with single longitudinal groove pattern placed on its surface were investigated. Six different rectangular groove sizes were tested for angular position of the groove from the forward stagnation point of the circular cylinder within 0°≤θ≤150°. The particle image velocimetry (PIV) technique were employed to measure flow field downstream of the cylinder immersed in a uniform flow field with the Reynolds number, Re=5000. Time-averaged flow data such as vorticity, 〈ω〉 streamline, 〈Ψ〉, streamwise, 〈u′u′〉 and transverse, 〈v′v′〉 Reynolds normal stresses, turbulent kinetic energy, TKE and RMS of streamwise, urms and transverse, vrms velocity components were obtained from PIV data to demonstrate flow features. Moreover, frequency of Karman vortex shedding was explored using single point spectral analysis. It is concluded that presence of the groove on a cylinder surface significantly affects the near wake flow structure and turbulence statistics. Karman vortex shedding frequency, fk strongly depends on the groove size. Moreover, the shear layer instability is induced on the grooved side with additional frequencies.

Suppression of vortex-induced vibrations by fairings: A numerical study

Fairings are nearly neutrally buoyant devices, fitted along the axis of long circular risers to suppress vortex-induced vibrations (VIV) and possibly reduce the drag force. Here we study numerically how VIV can be practically eliminated by using free-to-rotate fairings. Since the rotational inertia is low for the fairings, direct numerical simulations based on standard fluid–structure interaction algorithms may fail because of the so-called added mass effect. To resolve this problem we introduce fictitious methods and successfully stabilize the simulations. We then investigate the effect of rotational friction Cf on the stabilization effect of the fairings. In particular through two-dimensional (2D) simulations we find that when the Reynolds number is low (Re=100), Cf=0 is the most effective choice in suppressing VIV. Moreover, at this low Reynolds number there exists a critical value of Cf around which large oscillations and non-symmetric trajectories are observed. On the other hand, at higher Reynolds number (Re=500) a different behavior emerges, i.e. VIV are suppressed continuously as Cf increases. At Re=1000, we perform 3D simulations to investigate the effects of three-dimensionality of the flow on the vibration and rotation responses. In this work we quantify numerically for the first time various salient features of free-to-rotate devices for VIV suppression and relate them to modified flow structures in the near wake.

Distributed forcing flow control in the wake of a blunt trailing edge profiled body using plasma actuators

A modern flow control technique for reducing the drag associated with the periodic shedding of von Kármán vortices in the wake of a blunt trailing edge profiled body is presented. The technique involves distributed forcing of the wake flow using an array of dielectric barrier discharge plasma actuators, with a spanwise spacing matched to the spanwise wavelength of the dominant secondary wake instability. The experiments include measurement of the velocity field in multiple vertical and horizontal planes in the wake using particle image velocimetry, as well as base pressure, at Reynolds numbers of 2000, 3000, and 5000 based on trailing edge thickness. The flow control technique causes elongation of the vortex formation region across the span, and significant reduction of the fluctuating and total drag forces, up to a maximum of 94% and 18%, respectively. The effectiveness of the flow control technique is shown to be dependent on the induced momentum coefficient. Proper orthogonal decomposition analysis is used to investigate the mechanism of interaction of the flow control technique with the wake flow. Two distinct flow regimes are observed depending on the induced momentum coefficient. The effect of the control on the wake flow structure in the first regime is similar to those observed in previous studies involving mild spanwise-periodic geometric perturbations at the trailing edge, where control leads to streamwise displacement of the vortices and a shift in shedding frequency. However, an incremental increase in the momentum coefficient leads to a second flow regime similar to those previously observed in the case of large-amplitude geometric perturbations, with an almost complete attenuation of vortex shedding in the near-wake region.

Vortex formation behind a cylinder in a fluctuating flow

A visual investigation of the flow structure in the near wake behind a transverse cylinder in a fluctuating flow is carried out. The experiments were performed on a special setup, on wide ranges of the frequencies and amplitudes of superimposed fluctuations. Characteristic flow patterns are revealed and the corresponding wake flow structures are described. On the basis of the generalization of the information obtained the flow pattern map is constructed.

Experimental investigation of the effect of Reynolds number on flow structures in the wake of a circular parachute canopy

In the present study, an experimental study was conducted to characterize the effect of Reynolds number on flow structures in the turbulent wake of a circular parachute canopy by utilizing stereoscopic particle image velocimetry (Stereo-PIV) technique. The parachute model tested in the present study was attached by 28 nylon suspension lines and placed horizontally at the test section center of the wind tunnel. The obtained results showed that with the increase of Reynolds number, the intensities of the vortices near the downstream region of the canopy skirt were found to increase accordingly. However, the increase of Reynolds number did not result in a significant change in ensembleaveraged normalized x-component of the velocity, ensembleaveraged normalized vorticity, normalized Reynolds stress, and normalized turbulent kinetic energy distributions in the turbulent wake of the circular parachute canopy. The obtained results are very useful to further our understanding about the unsteady aerodynamics in the wake of flexible circular parachute canopies and to constitute a reference for CFD computation.

An experimental research on the wake flow structure of serrated cascade

This study concerns the mechanisms of the turbulent broadband noise reduction for cascade trailing edge serrations while the inlet velocity changed from 30 m/s to 50 m/s. The turbulence spatio-temporal information are measured with 3D hot-wire. The experiment is carried out in the Northwestern Polytechnical University low speed open jet wind tunnel. It showed the spreading rate of the wake and the decay rate of the wake centerline velocity deficit increased with serrated edge compared to the straight edge, and the three components of velocity changed differently with serrated trailing edge. It is also found that the turbulence peak occurs further from the airfoil surface in the presence of the serrations, and the serrations widened the mix area, which allowed the flow mixed together ahead of the schedule.

The importance of rear pillar geometry on fastback wake structures

The wake of a fastback type passenger vehicle is characterised by trailing vortices from the rear pillars of the vehicle. These vortices strongly influence all the aerodynamic coefficients.Working at model scale, using two configurations of the Davis model with different rear pillar radii, (sharp edged and 10mm radius) the flow fields over the rear half of the models were investigated using balance measurements, flow visualisations, surface pressure and PIV (Particle Image Velocimetry) measurements.For a small geometry change between the two models, the changes to the aerodynamic loads and wake flow structures were unexpectedly large with significant differences to the strength and location of the trailing vortices in both the time averaged and unsteady results. The square edged model produced a flow field similar to that found on an Ahmed model with a sub-critical backlight angle. The round edged model produced a flow structure dominated by trailing vortices that mix with the wake behind the base of the model and is weaker. This flow structure was more unsteady than that of the square edged model. Consequently, although both models can be described as having a wake dominated by trailing vortices, there are significant differences to both the steady state and unsteady flow fields that have not been described previously. This also shows that the fastback wake structure described by Ahmed is not definitive.

Subgrid-scale eddy viscosity model for helical turbulence

A novel eddy-viscosity closure is proposed for the subgrid-scale (SGS) stress tensor in large-eddy simulation of helical turbulence. The eddy-viscosity is shown to be proportional to the product of the large-scale velocity strain rate tensor and the symmetric part of the large-scale vorticity gradient (or vorticity strain rate tensor) based on a SGS helicity dissipation balance and a spectral relative helicity relation. The new SGS model is first tested and validated in simulation of homogeneous and isotropic helical turbulence. The statistical results demonstrate that the present model can predict both the energy and helicity spectra more precisely than the dynamic Smagorinsky model and a mixed helical model as compared with the results calculated in direct numerical simulation. Then, a compressible version of the new SGS model is parameterized and utilized to simulate the compressible flow past a circular cylinder. It is found that the present eddy-viscosity model can reproduce the skin friction force much more accurately than the standard Smagorinsky model, and can simulate more realistic flow structures in the near wake of the cylinder than the detached-eddy simulation approach. The surprising findings in simulation of flow past a circular cylinder suggest that the inclusion of contribution from helicity in SGS modeling is important for large-eddy simulation of separated turbulent flows.