Three-dimensional Wake Structure Research Articles

Numerical study of a fish swimming in hydrokinetic turbine wake

The environmental effects of hydrokinetic turbines are still under investigation, reflecting the emerging status of this technology. This study investigates the interaction between hydrokinetic rotor wakes and fish swimming, revealing insights into fish biomechanics in complex flows and assessing the environmental implications of marine energy solutions. We conducted numerical simulations with the URANS approach and k−ω−SST turbulence closure model to predict three-dimensional turbulent flow in the OpenFOAM software. The hydrokinetic rotor wake was simulated employing the actuator line method, providing a computationally efficient alternative to full geometry simulations. For accurate replication of the motion of a fish-like tuna (Thunnus atlanticus), dynamic adaptive mesh discretization was employed. The results offer a comparative analysis of fish swimming performance within the wake rotor, particularly when immersed in the tip blade vortex, contrasted with scenarios where fish swim in undisturbed flow conditions. The analysis encompasses three-dimensional wake structures, force generation, efficiency, and equilibrium states (balancing drag and thrust) across varying Swimming numbers (Sw). Key findings include the enhanced attachment of the leading-edge vortex due to the caudal fin’s interaction with the tip blade vortex, resulting in improved auto-propulsive force production; a reduced tail stride frequency observed in fish swimming downstream of the rotor to achieve longitudinal force balance compared to unperturbed flow; and transverse hydrodynamic forces pushing fish radially away from the wake’s influence zone, potentially mitigating the risk of collision with turbine blades.

Three-dimensional wake structures over a short cylinder having a fore angled hole

The complex three-dimensional (3D) flows around a short cylinder on the wall, acted by a flow exiting from a fore angled hole (FAH) that penetrates the cylinder from the fore to the top end, were experimentally investigated. The 3D instantaneous velocity distributions were measured based on tomographic particle image velocimetry (Tomo-PIV). The 3D flow structures behind the FAH and no-hole cylinders are compared in terms of the measured 3D velocity and vorticity distributions, Q-criterion, and tip vortices. A 3D W-type arch vortex, which shows a larger peak or convexity on the horizontal part center than for the no-hole cylinder, is found around the FAH short cylinder. The exiting flow from the FAH increases the separation zone, and FAH height effectively controls the eddy structures. The instantaneous 3D velocity data obtained from Tomo-PIV are decomposed using 3D wavelet multiresolution analysis and proper orthogonal decomposition. With the FAH cylinder, the large-scale streamwise wake vortices are smaller than with the no-hole cylinder, and this exhibits more evident as increasing the FAH height. The time-mean 3D large-scale arch vortical structures, however, become stronger. Proper orthogonal decomposition indicates that the energetic flow structures of the FAH cylinders are stronger than those of the no-hole cylinder.

Wake of Elongated Low-Rise Building at Oblique Incidences

We investigated the turbulent wake of an elongated low-rise building at oblique wind incidence via wind tunnel experiments and numerical simulations. The deflection phenomenon of mean building wake is clearly supported by the downwind trajectory of the point of maximum velocity deficit. A two-step mechanism is proposed for the understanding of the wake deflection process and its evolution in the building wake. The oblique wind incidence leads to a location shift of shear layer flow competition in the near-wake region (“WD1”) and then the deflected prevailing wind extends its effect in the far-wake region (“WD2”). The streamwise development of lateral wake deflection predicted from this mechanism, as well as the variations with height due to the three-dimensional wake structure, compares well with the measurement and simulation results. For aviation safety assessment of wake effect of the present building on aircraft landing, the data are compared to the “1:35 rule” and “7-knot criterion”. In addition, the importance of velocity fluctuation is demonstrated with an exceedance probability analysis.

Three-dimensional wake structures controlled by the flow issuing from a horizontal hole in a wall-mounted short cylinder

This paper examines the three-dimensional (3D) wake flow structures induced by a wall-mounted short circular cylinder. The structures are controlled by the flow issuing from a horizontal hole (HH) drilled from the front to the rear of the cylinder. The 3D velocity fields are instantaneously measured at a Reynolds number of 10,720 using high-resolution tomographic particle image velocimetry (Tomo-PIV) in a water tunnel. The 3D vorticity fields, Q criterion, and characteristics of arch-type and tip vortices are compared between the HH cylinders and a standard cylinder based on the measured instantaneous 3D velocity fields. A 3D W-type arch vortex appears behind the short HH cylinders. Compared with the standard cylinder, the 3D W-type arch vortex and large-scale vortices break down more rapidly in the wake of the HH cylinders. The flow from the HH may reduce the rear recirculation region to 15%, effectively controlling the wake structures of short cylinders. A 3D wavelet multiresolution analysis is used to decompose the 3D velocity fields given by the Tomo-PIV data. The large-scale streamwise vortices in the HH cylinder wakes are smaller than those in the standard cylinder wake, and this becomes more evident as the hole height increases. The intermediate-scale vortices are also controlled by the flow issuing from the hole.

Effects of approach flow conditions on the unsteady three-dimensional wake structure of a square-back Ahmed body

We investigate the effects of approach flow conditions (uniform flow (UF): Case A and thick turbulent boundary layer (TBL): Case B) on the unsteady three-dimensional wake characteristics and bimodality of a square back Ahmed body. Using improved delayed detached eddy simulations we show that TBL induces a much higher ground clearance momentum deficit than UF, which significantly alters wake asymmetry orientation in the wall-normal plane and completely suppresses bimodal wake behavior in the spanwise plane. Both time-averaged and time-resolved turbulence statistics are used to explore the differences between the wake structure of the Ahmed body subject to the two approach flow conditions.

Experimental study on the three-dimensional wake structure of the DrivAer standard model

An automatic particle imaging velocimetry (Auto-PIV) measuring technique was developed based on the external triggering and automatic control technology. Measurements of the DrivAer model wake were performed using Auto-PIV in the spanwise and vertical direction. Three-dimensional time-averaged velocity distribution was reconstructed and the spatial distribution of the large coherent structures was described. The results indicate that regions with high velocity fluctuations mainly locate at the shear layer of the upper and lower edge of the recirculation bubble. Further proper orthogonal decomposition results show that the velocity fluctuations account for a very little proportion of kinetic energy compared with the time-averaged velocity, supporting that the DrivAer model has steady aerodynamic characteristics. The smooth outlines of the DrivAer notchback model can delay the flow separation on the top and the C-pillar of the model, which inhibits the formation of the back recirculation bubble at the rear window and reduces the size of the recirculation bubble behind the baggage compartment. These promote the pressure recovery at the rear of the DrivAer notchback model, and thus reduces the pressure drag. At the rear of the model, the airflow on both sides converges towards the center due to the pressure difference, producing the streamwise vorticity by shearing the baggage compartment, then separates at the rear of the baggage compartment and generates a pair of secondary vortices. Overall, the aerodynamic behavior of the DrivAer model is robust.

The Effect of Single Dielectric Barrier Discharge Actuators in Reducing Drag on an Ahmed Body

The feasibility of a single dielectric barrier discharge (SDBD) actuator in controlling flow over an Ahmed body, representing a simplified car model, has been numerically and experimentally investigated at Reynolds numbers of 7.68×105 and 2.25×105. The Ahmed body had slant angles of 25∘ and 35∘. The results showed that SDBD actuators could significantly enhance the aerodynamic performance of the Ahmed body. Several arrangements of the actuators on the slant surface and the rear face of the model were examined to identify the most effective arrangement for drag reduction. This arrangement resulted in an approximately 6.1% drag reduction. This improvement in aerodynamic performance is attributed to the alteration of three-dimensional wake structures due to the presence of SDBD, which coincides with surface pressure variations on the slant and rear faces of the Ahmed body.

Three-dimensional viscous wake flow in fish swimming - A CFD study

In the present paper, the assessment of the hydrodynamics of fish swimming wake-flow is carried out to identify the main features related to the propulsion performance. Two different fish models with caragiform kinematics are studied: tuna fish (Thunnus atlanticus) and lambari (Astyanax bimaculatus). CFD methods were used to simulate laminar and turbulent flow regimes and a dynamically adaptive mesh discretization (ALE) was employed to reproduce fish motion. Simulations were run in a typical range of Reynolds (Re) and Strouhal (St) numbers of the fish swimming flow investigating hydrodynamics forces and the flow patterns of the induced fish-swimming wake. Firstly, equilibrium condition (drag balancing thrust) for Strouhal was achieved for each Reynolds number and three-dimensional wake structures were evaluated concluding that fish swimming experiencing a forward locomotion movement with a reversal von-Kármán vortex street and a high-velocity jet through the downstream direction. Finally, vorticity dynamics analysis showing that the leading-edge vortex produced by the caudal fin is associated to the peak of thrust in the motion cycle.

Multi-scale wake structures around the dune

The three-dimensional wake structures behind a barchan dune model are investigated by the high-speed particle image velocimetry (PIV) at the Reynolds number of 5530. The measured velocity fields are first evaluated by time-averaged flow patterns and power spectra analysis. Then the one-dimensional orthogonal wavelet analysis is applied to reveal multi-scale turbulent structures and their corresponding second order statistics in the dune wake. The instantaneous multi-scale structures suggest that the large-scale structure is responsible for the formation of separation region. The intermediate-scale structure tends to be more active at the boundary of separation region, which may be due to the secondary vortex movement caused by the interactions between the wake and main flow. The appearance of small-scale vortices at the downstream of separation region indicates the breakdown process of large-scale vortices. The Reynolds stress and kinetic energy distribution suggest that the large-scale structures dominate the generation of turbulence in the dune wake. The lower kinetic energy contained in intermediate- and small- scale structures is considered to be related to smaller-scale interactions that reflect energy cascading phenomenon. The Reynolds shear stresses distributions suggest that the intermediate-scale structures also make significance in the separated boundary layer. As for small-scale structure, it is worthy to note that a region of high Reynolds shear stresses with an opposite sign is observed around the dune crest. The cross-correlation function of multi-scale structures suggest the quasi-periodicity of large-scale structure. The intermediate-scale structures also show a certain periodicity, however, such structures tend to be more unstable and the strength of them reduces as they travel downstream. When decreasing to small-scale, the vortical structures decays quickly at the downstream with larger convective velocity. This may indicate the breakdown process of large-scale vortices and the quick dissipation of small-scale vortices.

Experimental observations of the three-dimensional wake structures and dynamics generated by a rigid, bioinspired pitching panel

The wake created by a trapezoidal pitching panel displays a variety of dynamics and behaviors. Results show that a chain of highly three-dimensional, alternating vortex rings is shed from the trailing edge. These rings interact with each other in a manner that is dependent on Strouhal number.

Discussion about the differences in mass transfer, bubble motion and surrounding liquid motion between a contaminated system and a clean system based on consideration of three-dimensional wake structure obtained from LIF visualization

Mass transfer from a bubble to the surrounding liquid plays an important role in industry processes. To improve the efficiency of the industrial processes, a deep understanding of the mass transfer mechanism is essential. In the present study, the relationship between the instantaneous mass transfer and the motions of the bubble and three-dimensional bubble wakes is discussed, on the basis of precise measurement. Moreover, since some industrial applications are contaminated system, the authors consider influences of bubble-surface contamination on the above. In the present experiments, purified water, and water contaminated with a very small amount of surfactant (1-pentanol) were employed. LIF (Laser Induced Fluorescence) technique and HPTS (a pH-sensitive dye) that was calibrated through a photoelectric optical fiber probe in advance were employed to visualize the mass transfer of the bubble and the wake structure of the bubble. The authors investigated highly reproducible single bubbles (considered to be almost the same bubble) launched from a special bubble-launch device; successfully they acquired pseudo-time-series sliced bubble wakes' images from every y-z plane at an equidistant interval. By processing the pseudo-time-series images through their own new image-processing, they challengingly reconstructed three-dimensional bubble wakes. From careful observations on the three-dimensional bubble wakes’ structure, they indirectly found out that the Marangoni convection, which was induced by the non-uniform distribution of surfactants desorption on the bubble surface, raised changes in the boundary layers in the contaminated water. The authors discussed the influences of gas-liquid interface contamination on the bubble motions, bubble wakes, and mass transfer process based on the reconstruction of three-dimensional bubble wakes in the purified water and contaminated water.

Stable state of Mode A for flow past a circular cylinder

A stable three-dimensional (3D) wake structure for flow past a circular cylinder has been discovered through 3D direct numerical simulations (DNS). The stable 3D wake structure occurs over a small range of Reynolds number (Re) below the critical Re for Mode A∗ (i.e., Mode A with large-scale vortex dislocations, where Mode A is the first 3D instability mode which will evolve into Mode A∗) instability. It is believed that the stable 3D wake structure discovered in this study is the stable state of Mode A wake structure inferred by Williamson [“Three-dimensional wake transition,” J. Fluid Mech. 328, 345–407 (1996)]. This confirms the wake transition sequence of 2D → A → A∗ → B suggested by Williamson. Compared with conventional Mode A structure, the stable state of Mode A structure has much weaker amplitude and does not evolve into large-scale vortex dislocations. The stable state of Mode A structure is triggered by small-scale spanwise disturbance introduced upstream of the cylinder, due to energy amplification through convective instability of the flow. The stable state of Mode A is transient and is damped out eventually under a transient initial disturbance condition, but is sustained throughout under a persistent disturbance condition. The emergence of the stable state of Mode A structure is correlated with both Re and the disturbance level. With the decrease of Re, the stable state of Mode A structure gradually becomes less well-defined and eventually disappears. With the decrease of the disturbance level, the stable state of Mode A structure emerges at a higher Re.

Wake structure visualization of a flapping-wing Micro-Air-Vehicle in forward flight

The present study provides an experimental visualization of the three-dimensional wake structure of a small flapping-wing Micro Air Vehicle (MAV) using a stereoscopic Particle Image Velocimetry (stereo-PIV) technique, in combination with simultaneous force measurements. Variations in several kinematic parameters and flow conditions (flapping frequency, flow speed and incidence angle) are examined to identify their effects on the wake topology. The three-dimensional wake structures were reconstructed using the temporal information of one transverse measurement plane by a ‘frozen flow’ approach, which was qualitatively validated by a spatial reconstruction method using information from different spatial planes. For the first time, the complete wake topology and evolution of this specific four-wing configuration MAV in forward flight condition is characterized experimentally. The general wake topology contains two predominant vortex structures: a U-shaped vortex ring shed from the trailing edges after instroke and an arc-shaped vortex shed from the wing tips during outstroke. These two vortex structures are shed periodically from the wing and appear quite isolated from each other. As expected, a higher reduced frequency will result in an increased unsteadiness of the wake behavior, reflected by a shorter wavelength and vortical connection. Application of an incident angle of the incoming free-stream introduces geometrical changes of the wake structure.

Three-dimensional vortex wake structure of a flapping-wing micro aerial vehicle in forward flight configuration

This paper investigates the formation and evolution of the unsteady three-dimensional wake structures generated by the flapping wings of the DelFly II micro aerial vehicle in forward flight configuration. Time-resolved stereoscopic particle image velocimetry (Stereo-PIV) measurements were carried out at several spanwise-aligned planes in the wake, so as to allow a reconstruction of the temporal development of the wake of the flapping wings throughout the complete flapping cycle. Simultaneous thrust-force measurements were performed to explore the relation between the wake formation and the aerodynamic force generation mechanisms. The three-dimensional wake configuration was subsequently reconstructed from the planar PIV measurements by two different approaches: (1) a spatiotemporal wake reconstruction obtained by convecting the time-resolved, three-component velocity field data of a single measurement plane with the free-stream velocity; (2) for selected phases in the flapping cycle a direct three-dimensional spatial wake reconstruction is interpolated from the data of the different measurement planes, using a Kriging regression technique. Comparing the results derived from both methods in terms of the behavior of the wake formations, their phase and orientation indicate that the spatiotemporal reconstruction method allows to characterize the general three-dimensional structure of the wake, but that the spatial reconstruction method can reveal more details due to higher streamwise resolution. Comparison of the wake reconstructions for different values of the reduced frequency allows assessing the impact of the flapping frequency on the formation and interaction characteristics of the vortical structures. For low values of the reduced frequency, it is observed that the vortex structure formation of instroke and outstroke is relatively independent of each other, but that increasing interaction occurs at higher reduced frequencies. It is further shown that there is a phase lag in the appearance of the structures for increasing flapping frequency, which is in correlation with the generation of the forces. Comparison of thrust generated during the instroke and the outstroke phases of the flapping motion in conjunction with the development of the wake structures indicates that wing–wing interaction at the start of outstroke (peel motion) becomes a dominant feature for reduced frequencies greater than 0.62.

Three-dimensional vortex wake structure of flapping wings in hovering flight

Flapping wings continuously create and send vortices into their wake, while imparting downward momentum into the surrounding fluid. However, experimental studies concerning the details of the three-dimensional vorticity distribution and evolution in the far wake are limited. In this study, the three-dimensional vortex wake structure in both the near and far field of a dynamically scaled flapping wing was investigated experimentally, using volumetric three-component velocimetry. A single wing, with shape and kinematics similar to those of a fruitfly, was examined. The overall result of the wing action is to create an integrated vortex structure consisting of a tip vortex (TV), trailing-edge shear layer (TESL) and leading-edge vortex. The TESL rolls up into a root vortex (RV) as it is shed from the wing, and together with the TV, contracts radially and stretches tangentially in the downstream wake. The downwash is distributed in an arc-shaped region enclosed by the stretched tangential vorticity of the TVs and the RVs. A closed vortex ring structure is not observed in the current study owing to the lack of well-established starting and stopping vortex structures that smoothly connect the TV and RV. An evaluation of the vorticity transport equation shows that both the TV and the RV undergo vortex stretching while convecting downwards: a three-dimensional phenomenon in rotating flows. It also confirms that convection and secondary tilting and stretching effects dominate the evolution of vorticity.

Volumetric visualization of the near- and far-field wake in flapping wings

The flapping wings of flying animals create complex vortex wake structure; understanding its spatial and temporal distribution is fundamental to animal flight theory. In this study, we applied the volumetric 3-component velocimetry to capture both the near- and far-field flow generated by a pair of mechanical flapping wings. For the first time, the complete three-dimensional wake structure and its evolution throughout a wing stroke were quantified and presented experimentally. The general vortex wake structure maintains a quite consistent form: vortex rings in the near field and two shear layers in the far field. Vortex rings shed periodically from the wings and are linked to each other in successive strokes. In the far field, the shed vortex rings evolve into two parallel shear layers with dominant vorticity convected from tip and root vortices. The shear layers are nearly stationary in space compared to the periodic vortex rings shed in the near field. In addition, downwash passes through the centers of the vortex rings and extends downward between the two shear layers.

Shape optimization of the caudal fin of the three-dimensional self-propelled swimming fish

Shape optimization of the caudal fin of the three-dimensional self-propelled swimming fish, to increase the swimming efficiency and the swimming speed and control the motion direction more easily, is investigated by combining optimization algorithms, unsteady computational fluid dynamics and dynamic control in this study. The 3D computational fluid dynamics package contains the immersed boundary method, volume of fluid method, the adaptive multi-grid finite volume method and the control strategy of fish swimming. Through shape optimizations of various swimming speeds, the results show that the optimal caudal fins of different swimming modes are not exactly the same shape. However, the optimal fish of high swimming speed, whose caudal fin shape is similar to the crescent, also have higher efficiency and better maneuverability than the other optimal bionic fish at low and moderate swimming speeds. Finally, the mechanisms of vorticity creation of different optimal bionic fish are studied by using boundary vorticity-flux theory, and three-dimensional wake structures of self-propelled swimming of these fish are comparatively analyzed. The study of vortex dynamics reveals the nature of efficient swimming of the 3D bionic fish with the lunate caudal fin.

Three-dimensional wake structures and aerodynamic coefficients for flow past an inclined square cylinder

Numerical investigation has been performed for flows past an inclined square cylinder in the range of Re≤300 to elucidate the characteristics of the laminar three-dimensional wake behind the cylinder depending upon Reynolds number and angle of incidence (θ). An immersed boundary method was used for implementation of the cylinder on a Cartesian grid system. Both Floquet stability analysis and full three-dimensional simulation were employed to detect the onset of the secondary instability leading to three-dimensional flow, and to provide quantitative flow data. It was revealed that mode A becomes more unstable for “symmetric” flow configuration, while mode C is dominant for asymmetric flows in the range of 10°≤θ≤25°. The most unstable three-dimensional modes predicted by the Floquet stability analysis were well confirmed by the full 3D simulations which were conducted at Re=150, 200, 250, and 300, with varying angle of incidence. The full 3D simulations also provided the key flow characteristics such as mean flow-induced force/moment coefficients and Strouhal number of vortex shedding. It was seen that they are sensitive to slight inclination of the square cylinder, and the Re effects are insignificant except for mean lift coefficient. Vortical structures in the wake, taken from the results of the Floquet stability analysis and the full 3D simulations, respectively, were visualized by Q-contours, revealing good agreement between the two.

Numerical simulation of 3D flow past a real-life marine hydrokinetic turbine

We simulate three-dimensional, turbulent flow past an axial-flow marine hydrokinetic (MHK) turbine mounted on the bed of a rectangular open channel by adapting a computational framework developed for carrying out high-resolution large-eddy simulation (LES) in arbitrarily complex domains involving moving or stationary boundaries. The complex turbine geometry, including the rotor and all stationary components, is handled by employing the curvilinear immersed boundary (CURVIB) method [1,2]. Velocity boundary conditions near all solid surfaces are reconstructed using a wall model based on solving the simplified boundary layer equations [2]. To demonstrate the capabilities of the model we apply it to simulate the flow past a Gen4 axial flow MHK turbine developed by Verdant Power for the Roosevelt Island Tidal Energy (RITE) project in the East River in New York City, USA. We carry out systematic grid refinement studies, using grids with up to 185 million nodes, for only the turbine rotor placed in an infinite free stream to show that the computed torque converges to a grid insensitive value, which is in good agreement with field measurements. We also carry out LES for the complete turbine configuration, including the pylon, nacelle and rotor, mounted on the bed of a straight rectangular open channel. The computed results illustrate the complexity of the flow and show that the power output of the complete turbine is primarily dependent on the rotor geometry and tip speed ratio, and is not affected by the stationary components of the turbine and the presence of the channel bed. The complete turbine simulation also reveals that the downstream wake of the turbine consists of three main regions: (1) the outer layer with the spiral blade tip vortices rotating in the same direction as the blades; (2) the counter-rotating inner layer surrounded by the spiral tip vortices; and (3) the core layer co-rotating with respect to the tip vortices. This study is the first to report the three-dimensional wake structure of MHK turbines.

The effect of small-amplitude time-dependent changes to the surface morphology of a sphere

Typical approaches to manipulation of flow separation employ passive means or active techniques such as blowing and suction or plasma acceleration. Here it is demonstrated that the flow can be significantly altered by making small changes to the shape of the surface. A proof of concept experiment is performed using a very simple time-dependent perturbation to the surface of a sphere: a roughness element of 1% of the sphere diameter is moved azimuthally around a sphere surface upstream of the uncontrolled laminar separation point, with a rotational frequency as large as the vortex shedding frequency. A key finding is that the non-dimensional time to observe a large effect on the lateral force due to the perturbation produced in the sphere boundary layers as the roughness moves along the surface is=tU∞/D≈ 4. This slow development allows the moving element to produce a tripped boundary layer over an extended region. It is shown that a lateral force can be produced that is as large as the drag. In addition, simultaneous particle image velocimetry and force measurements reveal that a pair of counter-rotating helical vortices are produced in the wake, which have a significant effect on the forces and greatly increase the Reynolds stresses in the wake. The relatively large perturbation to the flow-field produced by the small surface disturbance permits the construction of a phase-averaged, three-dimensional (two-velocity component) wake structure from measurements in the streamwise/radial plane. The vortical structure arising due to the roughness element has implications for flow over a sphere with a nominally smooth surface or distributed roughness. In addition, it is shown that oscillating the roughness element, or shaping its trajectory, can produce a mean lateral force.