Streamwise Vortical Structures Research Articles

Experimental investigation on effects of herringbone riblets on shock wave/boundary layer interactions of a compression ramp at Mach 3

In this paper, the effects of herringbone riblets (HRs) on a turbulent boundary layer and the shock wave/boundary layer interactions (SWBLIs) are experimentally investigated at the Mach number of 3. An array of three varied lengths of HRs strips are applied upstream of the separation zone of SWBLIs on a compression ramp model. High-speed schlieren, oil-flow visualization, and the planar laser scattering technique are used to examine the shock pattern and boundary layer developing over the surface of the model. The snapshot proper orthogonal decomposition technique and the Fast Fourier Transform method are applied to study the impact of HRs on the interaction between the shock wave and the boundary layer. The experiments provide convincing evidence that HRs make the separation line wavy and shrink the separation zone by about −39.54% for the longest HRs in the present experiments. Furthermore, it is also revealed that these microscale HRs induce large-scale streamwise vortical structures within the boundary layer as found in incompressible flows. It is believed that these vortices promote momentum transfer within the boundary layer hence providing the dominant mechanism for suppressing flow separation.

Characteristic spanwise length scales of streamwise vortical structures in the wake of a circular cylinder at Re = 1500 measured via global and local approaches

Wake characteristics of the flow past a circular cylinder are analysed in detail at Reynolds number Re=1500 via direct numerical simulation. A periodic spanwise domain of length 1.5πD has been found to yield correct first- and second-order wake statistics in remarkable agreement with published results at the same and closeby Re. A Kelvin–Helmholtz instability with a frequency fKH≃0.666 is observed to occur intermittently in the shear layers issued from the top and bottom of the cylinder. The three-dimensional patterns in the wake have an estimated spanwise length scale lz1/D≃0.70 (D is the cylinder diameter) in the near-wake at (x,y)/D=(3,0.5), downstream from the cylinder, when quantified by autocorrelation (global approach). When using the Hilbert-transform (local approach) instead, the predicted length scale of streamwise vortical structures is distributed around λz/D≃0.33 at the same sampling location. Our results show that the two approaches measure different aspects of three-dimensionality: while the former informs of the typical spanwise spacing of streamwise vortices, the latter quantifies the local spanwise size of these same flow structures.

On the Wake Dynamics of an Oscillating Cylinder via Proper Orthogonal Decomposition

The coherent structures and wake dynamics of a two-degree-of-freedom vibrating cylinder with a low mass ratio at Re=5300 are investigated by means of proper orthogonal decomposition (POD) of a numerical database generated using large-eddy simulations. Two different reduced velocities of U*=3.0 and U*=5.5, which correspond with the initial and super-upper branches, are considered. This is the first time that this kind of analysis is performed in this kind of system in order to understand the role of large coherent motions on the amplification of the forces. In both branches of response, almost 1000 non-correlated in-time velocity fields have been decomposed using the snapshot method. It is seen that a large number of modes is required to represent 95% of the turbulent kinetic energy of the flow, but the first two modes contain a large percentage of the energy as they represent the wake large-scale vortex tubes. The energy dispersion of the high-order modes is attributed to the cylinder movement in the inline and cross-stream directions. Substantially different POD modes have been found in the two branches. While the first six modes resemble those observed in the static cylinder or in the initial branch of a one-degree of freedom cylinder in the initial branch, the modes not only contain information about the wake vortexes in the super-upper branch but also about the formation of the 2T vortex pattern and the Taylor–Görtler structures. It is shown that the 2T vortex pattern is formed by the interplay between the Taylor–Görtler stream-wise vortical structures and the cylinder movement and is responsible for the increase in the lift force and larger elongation in the super-upper branch.

Aerodynamic performance of several passive vortex generator configurations on an Ahmed body subjected to yaw angles

We evaluate the aerodynamic performance of several passive vortex generators (VGs) placed on a standard Ahmed body, with a slant angle (α = 35º), subjected to different yawing angles (β) using RANS-based models. Rigorous validation of the numerical results is performed with previously published experimental data for (β ≤ 8º) for the Ahmed body. Our model results depict a good overall agreement with several experimental data sets. An array of different vortex generators such as the delta-winglet (DVGs), the cylindrical (CVGs) and trapezoidal (TVGs) types are introduced on to the validated model. The introduction of CVGs and DVGs tends to have a beneficial aerodynamic performance for (β = 0º). In contrast, the TVGs tend to impair the performance by producing massive flow separation over the slant for (β = 0º). Conversely, for (β > 0º), a swift transition happens with TVGs wherein the high-energy streamwise vortices that are produced tend to improve the pressure footprint, thereby reducing the overall drag. A deterioration in the performance of DVGs is predicted during (β > 4º), wherein the ‘c’-pillar vortex on the leeward side interferes with the streamwise vortical structure, which adversely influences the flow over the roof-slant edge. Overall, a maximum of ~ 8.5% and ~ 7.7% drag reduction appears to be possible with the designed CVGs and TVGs at smaller vehicle yawing conditions.

Computational study on an Ahmed Body equipped with simplified underbody diffuser

The Ahmed body is one of the most studied 3D automotive bluff bodies and the variation of its slant angle of the rear upper surface generates different flow behaviours, similar to a standard road vehicles. In this study we extend the geometrical variation to evaluate the influence of a rear underbody diffuser which are commonly applied in high performance and race cars to improve downforce. Parametric studies are performed on the rear diffuser angle of two baseline configurations of the Ahmed body: the first with a 0° upper slant angle and the second with a 25° slant angle. We employ a high-fidelity CFD simulation based on the spectral/hp element discretisation that combines classical mesh refinement with polynomial expansions in order to achieve both geometrical refinement and better accuracy. The diffuser length was fixed to the same length of 222 ​mm similar to the top slant angle that have previously been studies. The diffuser angle was changed from 0° to 50° in increments of 10° with an additional case considering the angle of 5°. The proposed methodology was validated on the classical Ahmed body considering 25° slant angle, found a difference for drag and lift coefficients of 13% and 1%, respectively. For the case of an 0° slant angle on the upper surface the peak values for drag and negative lift (downforce) coefficient were achieved with a 30° diffuser angle, where the flow is fully attached with two streamwise vortical structures, analogous to results obtained from [1] but with the body flipped upside down. For diffuser angles above 30°, flow is fully separated from the diffuser. The Ahmed body with 25° slant angle and a diffuser achieves a peak value for downforce at a 20° diffuser angle, where the flow on the diffuser has two streamwise vortices combined with some flow separation. The peak drag value for this case is at 30° diffuser angle, where the flow becomes fully separated.

Interaction of turbulences with non-breaking divergent waves in an open channel

This paper presents a direct numerical simulation of turbulent flows over a bump in an open channel to examine the turbulence characteristics near divergent waves emanating from the bump and to investigate the interaction of the turbulences with the divergent waves. To verify the reliability of the simulations, the mean velocity profile and root-mean-square of velocity fluctuations are compared with previous data. The anisotropic invariant maps show that the ratio of the streamwise to spanwise velocity fluctuations plays an important role in characterizing the anisotropic nature of the separated shear layer behind the bump in the vicinity of the free surface. The vortex identification discloses a large-scale streamwise vortical structure from the mean velocity field and a cluster of small coherent structures from the instantaneous velocity field, which are responsible for the anisotropic characteristics of the turbulence beneath the free surface.

Density ratio effects on the topology of coherent turbulent structures in two-way coupled particle-laden channel flows

This investigation considers the effect of density ratio on the modification of coherent turbulent structures in particle-laden channel flows at a shear Reynolds number of Reτ = 180. Direct numerical simulation and Lagrangian particle tracking are used to accurately predict the motion of particles dispersed within the flow at three Stokes numbers, St+ = 0.1, 50, and 92. Particle–fluid coupling is achieved through a local element-based force feedback field in the Navier–Stokes equations, which are solved using a seventh-order accurate spectral element method. After an initial transitory period wherein the effects of particle–fluid interaction are emphasized, the low density ratio particles are found to enhance the turbulence field, increasing the frequency of Q-criterion satisfying regions, while the inertial particles suppress the turbulence, reducing the number of quasistreamwise vortices. Results indicate that the topology of the quasistreamwise vortices is altered by the presence of the particles in the viscous sublayer, the buffer layer, and the log-law region such that the distribution of the third invariant of the deviatoric tensor, R, is widened by the presence of tracer-like particles and made thinner by the inertial particles. This effect reduces the amount of unstable focus/compressing regions and stable focus/stretching regions, which account for the streamwise vortical structures observed in these types of flow. Investigating the instantaneous coupling force field surrounding turbulent structures and low speed streaks shows that particles exert their greatest influence on the fluid in the regions noted, and mechanisms by which the particulate phase interacts with turbulent vortices are analyzed.

Large‐Eddy Simulation of Turbulent Oscillatory Flow Over Three‐Dimensional Transient Vortex Ripple Geometries in Quasi‐Equilibrium

AbstractVortex ripples induced by oscillatory flow display straight crests at equilibrium, but they develop transient three‐dimensional (3‐D) bedform geometries when transitioning from one equilibrium state to another due to changes in the oscillatory flow. Large‐eddy simulations were performed of oscillatory flow over 3‐D bedform defects associated with an increase of the orbital motion amplitude, similar to what might happen during the waxing phase of a storm. The objective is to determine the intercorrelation between flow processes and evolution of 3‐D bedform defects toward an equilibrium two‐dimensional (2‐D) state of vortex ripples. Results are presented for oscillatory flows over fixed ripples with sinusoidal crests in the spanwise direction, which introduce slope asymmetries on opposite sides of the crests, during different transient states. It was found that steep‐sloped regions of the bedforms are associated with thicker recirculation areas, whereas in more gently sloped regions thinner recirculation vortices exist. Furthermore, streamwise vortical structures and significant secondary flow develop in the vicinity of sinusoidal crests, relative to straight crests. These flow structures might be responsible for the gradual reformation of the rippled bed, as they induce higher bed shear stresses, that is, increasing bed sediment transport, and they diffuse turbulent kinetic energy to higher levels above crests, that is, increasing suspended sediment transport, relative to equilibrium 2‐D ripples. Finally, the period‐averaged currents around the sinusoidal crests exhibit higher velocities on the steep‐sloped side than on the mild‐sloped one, also likely contributing to sediment transport and bed reformation toward a 2‐D configuration.

Characterization of three-dimensional vortical structures in the wake past a circular cylinder in the transitional regime

The flow past a circular cylinder in the transitional regime at Re = 2000 has been thoroughly investigated via well resolved direct numerical simulation with a spectral element code. Spanwise periodic boundary conditions of at least Lz ≥ 2.5D are required to properly reproduce first and second order turbulent statistics in the cylinder wake. A Kelvin–Helmholtz instability can already be detected at this relatively low Reynolds number at the flapping shear layers issued from either side of the cylinder. The instability, with a frequency fKH ≃ 0.84 that is in excellent agreement with published experimental results, arises only occasionally and the associated spanwise vortices are subject to spanwise localization. We show that while Kármán vortices remain predominantly two-dimensional, streamwise vortical structures appearing along the braids connecting consecutive vortices are mainly responsible for rendering the flow three-dimensional. These structures may appear in isolation or in vortex pairs and have a typical spanwise wavelength of around λz ≃ 0.20–0.28 at a location at (x, y) = (3, 0.5), as measured via Hilbert transform along probe arrays with spanwise orientation. In line with experimental and numerical results at higher Re = 3900, the size of the structures drops in the very near-wake to a minimum at x ≃ 2.5 and then steadily grows to asymptotically attain a finite maximum for x ≳ 20. A time-evolution-based stability analysis of the underlying two-dimensional vortex shedding flow, which happens to be chaotic, shows that the fastest growing perturbations in the linear regime have a spanwise periodicity λz ≃ 0.3 and are located in the very near-wake, right within the braid that connects the last forming Kármán vortex with the previous one, thus hinting at a close relation with the fully developed vortical structures observed in full-fledged three-dimensional computations.

Characterization of vortex regeneration mechanism in the self-sustaining process of wall-bounded flows using resolvent analysis

The regeneration mechanism of streamwise vortical structures in the self-sustaining process of wall-bounded turbulence is investigated. Resolvent analysis [1] is used to identify the principal forcing mode which produces the maximum amplification of the response modes in the minimal channel for the buffer [2] and logarithmic layer [3]. The identified mode is then projected out from the nonlinear term of the Navier-Stokes equations at each time step from the direct numerical simulations (DNS) of the corresponding minimal channel. The results show that the removal of the principal forcing mode is able to significantly inhibit turbulence for the buffer and logarithmic layer while removing the subsequent modes instead of the principal one only marginally affects the flow. Analysis of the dyadic interactions in the nonlinear term shows that the contributions toward the principal forcing mode come from a limited number of wavenumber interactions. Using conditional averaging, the flow structures that are responsible for generating the principal forcing mode, and thus the nonlinear interaction to self-sustain turbulence, are identified to be spanwise rolls interacting with meandering streaks.

Cyclist Reynolds number effects and drag crisis distribution

The Reynolds number effects on body limbs of a cyclist model, namely leg and arm, are investigated via robotic volumetric Particle Image Velocimetry measurements in the velocity range from 5 ​m/s to 25 ​m/s. The near wakes of such body limbs feature recirculation regions whose width and length are governed not only by the taper of the body parts, but also by the presence of coherent streamwise vortical structures. Moreover, the interaction with the wakes of the upstream body parts plays a role in the local wake properties. While reductions of the wake width are observed on both lower leg and arm with increasing free-stream velocity, the wake of the upper leg follows an opposite trend increasing in size at higher velocity. Such variations of wake width with the Reynolds number are related to the behaviour of the local drag coefficient, indicating a drag crisis behaviour on both leg and arm. The distribution of the so-called critical velocity upon these body segments is discussed, as it determines the freestream speed where a minimum value for the drag can occur.

Dynamics of the Tip Vortices in the Wake Behind a Circular Cylinder of Finite Length

The dynamics of the tip vortices in the wake behind a wall-mounted finite-length circular cylinder of the aspect ratio 2 was studied experimentally using time resolved stereo PIV technique. The cylinder was mounted normal to a ground plane and it was subjected to a cross-flow with thin boundary layer developed on the wall, the Reynolds number based on inflow velocity and cylinder diameter was 9.7 thousands. The dynamics of tip vortices were analysed using the POD method applied to the plane perpendicular to the flow close to the cylinder. Besides the decaying power spectrum, slower that the Kolmogorov-type one, the two distinct frequencies were detected on Strouhal numbers 0.09 and 0.15. These frequencies could be linked to the vortical structures dynamics in the wake. The frequency Sh = 0.15 corresponds to predominantly spanwise vortices dynamics with anti-symmetrical patterns with respect to the cylinder axis, while the frequency Sh = 0.09 corresponds to mainly streamwise vortical structures dynamics with symmetrical patterns respectively. Thus, the von Kármán vortex street was detected on Strouhal frequency 0.15.

Large-Scale Forcing of a Turbulent Channel Flow Through Spanwise Synthetic Jets

The investigation focuses on the forcing of a fully developed turbulent channel flow through a linear array of synthetic jets injected tangentially to the wall and orthogonal to the mean flow direction. Forcing configurations are varied by differently combining the number of actuated jets working in an opposing blowing–suction configuration. Instantaneous wall shear stress and streamwise velocity fluctuations evidence drag reductions as well as turbulence attenuation up to 20%. The forcing effects are persistent up to at least a 150 half-channel height downstream of the injection section. Particle image velocimetry investigations in planes perpendicular to the channel axis highlight the presence of a large-scale streamwise vortical structure covering the whole height of the channel. This structure is thought to be responsible for the significant drag reduction, which is similar to the typical behavior evidenced in the case of colliding jets. The nondimensional forcing frequency of the synthetic jets producing the maximum drag reduction and turbulence attenuation is 0.0074 for the investigated Reynolds number (). A statistical analysis of the near-wall structures demonstrates that the control mechanism acts in a way to reduce them in the forced configuration. It is conclude that the effect of the forcing is such that the near-wall structures merge and become less prone to inducing new structures, thus effectively reducing their number, and consequently the near-wall turbulence activity.

Influence of streamwise vortical structures on heat transfer in the far cylinder wake in a slot channel flow

We report on Large-eddy simulations (LES) of a flow around a cylinder in a narrow duct channel (slot) at the Reynolds number 3750 based on the bulk velocity and the diameter of the cylinder. Secondary streamwise vortical structures in the far wake are investigated appearing near the mixing layer at the midplane and near the duct channel surface. We investigate the correlation between the heat flux and friction velocity fluctuations and evaluate the propagation speed of these disturbances, which appears to be different.

Streamwise and spanwise vortical structure merging inside the wake of an inclined flat plate

The aim of this paper is to study the physics related to lift generation on an airfoil. A new hypothesis [1] of physical mechanism of flight relies on existence of streamwise vortical structures above the wing and inside the wake. The vortices origin as a consequence of flow instability inside the boundary layer developed under adverse pressure gradient. These structures are highly dynamical in nature, they change position and size very rapidly. A simple airfoil in the form of a flat plate with moderate angle of attack is considered in the presented research that generates suitable flow at rather low Reynolds number. Stereo PIV time resolved measurement technique is used to capture high-dynamic data in several planes which are located in the wake and are perpendicular to freestream or parallel to the airfoil. The overall image of the flow field dynamics will be created using POD decomposition. Distinct flow patterns with associated kinetic energy are to be described as well as their role in the studied case. Existence of streamwise vorticity is proved, topology and other parameters are estimated in the paper; however related pressures and forces are not evaluated.

Attenuation of Flow Separation Using Herringbone Riblets at

In this Paper, the effect of herringbone riblets as a means of suppressing shockwave-induced flow separation is investigated. A spanwise array of ribleted strips is applied behind the leading edge of a double ramp model placed in a Mach 5 freestream. The high-speed schlieren technique is used to examine the shockwave pattern and the state of the boundary layer developing along the surface of the model. Both pressure-sensitive paints and temperature-sensitive paints are employed to assess the impact of riblets on the interaction between the shockwave and the boundary layer. The results from this experiment show that herringbone riblets are capable of suppressing shock-induced flow separation and hence improving pressure recovery. Furthermore, it is also revealed that these microscale riblets induce large-scale streamwise vortical structures within the boundary layer similar to those observed in incompressible flows. It is believed that these vortices promote momentum transfer within the boundary layer, hence providing the dominant mechanism for suppressing flow separation.

Turbulent dynamics of sinusoidal oscillatory flow over a wavy bottom

A direct numerical simulation study is conducted to investigate sinusoidal oscillatory flow over a two-dimensional wavy wall. The height and wavelength of the bottom profile, and the period and amplitude of the free-stream oscillation, are selected to mimic a wave-driven boundary layer over vortex ripples on a sandy seabed. Two cases with different Reynolds numbers$(Re)$are considered, and the higher-$Re$case achieves a fully developed turbulent state with a wide separation between the energy-containing and dissipative scales. The oscillatory flow is characterized by coherent columnar vortices, which are the main transport agents of turbulent kinetic energy and enstrophy. Two classes of coherent vortices are observed: (i) a primary vortex formed at the lee side of the ripple by flow separation at the crest; (ii) a secondary vortex formed beneath the primary vortex by vortex-induced separation. When the free-stream velocity weakens, these vortices form a counter-rotating vortex dipole and eject themselves over the crest with their mutual induction. Turbulence production peaks twice in a half-cycle; during the formation of the primary vortex and during the ejection of the vortex dipole. The intensity of the former peak remains low in the lower-$Re$case, as the vortex dipole follows a higher altitude trajectory limiting its interactions with the bottom, and leaving minimal residual turbulence around the ripples for the subsequent half-cycle. Flow snapshots and spectral analysis reveal two dominant three-dimensional features: (i) an energetic vortex mode with a preferred spanwise wavelength close to the ripple wavelength; (ii) streamwise vortical structures in near-wall regions with a relatively shorter spanwise spacing influenced by viscous effects. The vortex mode becomes strong when the cores of the vortices are strained to an elliptical form while moving towards the crest. Following the detachment of the vortices from the ripple, the vortex mode in the higher-$Re$case breaks down the spanwise coherence of the columnar vortices and decomposes them into intermittent patches of turbulent vortex clusters. The distribution of wall shear stress over the ripple is also analysed in detail. The peak values are observed near the ripple crest around the ejection of the vortex dipole and the maximum free-stream velocity. In the former, both the vortex mode and streamwise vortices have strong footprints on the wall, yielding a bimodal wall-shear-stress spectrum with two distinctive peaks. In the second high-stress regime, decaying coherent vortices impose strong inhomogeneity on the wall shear stress as their wall-attached parts sweep the ripples. These spanwise variations in the wall shear provide insights into the instability of two-dimensional sand ripples.

On the 3D structure of the flow-field in the vicinity of inclined plate

The motivation of the presented study is supporting new ideas about principle of flight by Hoffman and Johnson, see [1]. The new hypothesis of physical mechanism of flight relies on existence of streamwise vortical structures on the suction side of the airfoil and within its wake. The vortices origin is supposed to be the instability of the boundary layer subjected to adverse pressure gradient. The vortices are of highly dynamical nature, changing theirs position, size and other parameters in time very rapidly. For this experiment the simplest airfoil possible was chosen represented by a flat plate in uniform flow and moderate angle of attack. In the suggested paper detailed measurement of the zone of interest will be carried out using stereo PIV method. System of measuring planes perpendicular to the flow will be explored. POD analysis is to be utilized.

Dynamics and stability of gap-flow interference in a vibrating side-by-side arrangement of two circular cylinders

In this work, the coupled dynamics of the gap flow and the vortex-induced vibration (VIV) of a side-by-side (SBS) arrangement of two circular cylinders is numerically investigated at Reynolds numbers $100\leqslant Re\leqslant 500$ . The influence of VIV is incorporated by allowing one of the cylinders to vibrate freely in the transverse direction, which is termed as a vibrating side-by-side (VSBS) arrangement. A comparative three-dimensional study is performed between the stationary side-by-side (SSBS) and the VSBS arrangements to examine the characteristics of the complex coupling between the VIV and the gap flow. The results are also contrasted against the isolated configurations without any proximity and gap-flow interference. Of particular interest is to establish a relationship between the VIV, the gap flow and the near-wake instability behind bluff bodies. We find that the kinematics of the VIV regulates the streamwise vorticity concentration, which accompanies a recovery of the two-dimensional hydrodynamic response at the peak lock-in. Moreover, the near-wake instability may develop around an indeterminant two-dimensional streamline saddle point along the interfaces of a pair of imbalanced counter-signed vorticity clusters. The interaction between the imbalanced vorticity clusters and the gap-flow momentum are closely interlinked with the prominence of streamwise vortical structures. In both SSBS and VSBS arrangements, the flip-flopping frequency is significantly low for the three-dimensional flow, except at the VIV lock-in for the VSBS arrangement. While an early onset of VIV lock-in is observed for the vibrating configuration, a quasi-stable deflected gap-flow regime with stably deflected gap flow is found at the peak lock-in. The increase of the gap-flow proximity interference promotes the energy transfer and stabilizes the VIV lock-in. Finally, we employ the dynamic mode decomposition procedure to characterize the space–time evolution of the vortex wake system behind the cylinders.

Development of vortex structures in the wake of a sharp-edged bluff body

The development of streamwise vortical structures in the wake of an infinitely long inclined flat plate is investigated using detached eddy simulation. The numerical model is validated by comparing with available experimental results. The spanwise vortices shed from the leading and trailing edges are captured near the body in the midspan plane and are shown to quickly dissipate in the downstream wake. The generation of streamwise vortical structures and their interaction with the large-scale spanwise vortices are elucidated using the λ2-criterion to identify the vortices. The spanwise wavelength of the streamwise vortical structures is found to lie in the range corresponding to the mode B instability reported in the previous studies of wake transition. The temporal evolution of the streamwise vortices is studied in the near-wake region. The spanwise evolution of the flow clearly indicates the role of the streamwise vortices in dissipating the spanwise structures. The streamwise vortical structures also exert an influence on the shear layer at the trailing edge resulting in a corrugated pattern during their development. Cross-sectional planes normal to the plate are also used to examine the interaction between the streamwise and spanwise vortices. This combination of two-dimensional velocity fields and contours (in planes normal and parallel to the freestream and to the plate) and three-dimensional iso-surfaces provides a more complete picture of the formation of the complex fluid structures in the flow around a bluff body.