Three-dimensional Vortical Structures Research Articles

Theoretical predictions and experimental measurements of novel aerostatic bearing with multi-inclined-orifice restrictors for the improvement of stability

This paper introduces a novel design featuring a multiple-inclined-orifice restrictor (MIOR) to address the challenge of turbulent vortices and the reduction of nano-vibrations in aerostatic bearings. This design optimizes the angle configuration to alter the dynamic airflow patterns within the chamber, suppress turbulent vortices, and mitigate nano-vibrations. Three-dimensional transient vortical structures, pressure fluctuation, and dynamic load-carrying capacity of aerostatic bearings with varying orifice angles were investigated through large eddy simulations. Additionally, load-carrying capacity and vibration tests were conducted on aerostatic bearings equipped with MIORs to validate the proposed model's accuracy and assess the effectiveness of MIOR. Remarkably, the experimental results were consistent with the simulation results, highlighting the exceptional stability and reduced vibration amplitude exhibited by MIORs with an angle of 110° while maintaining static performance.

Features and mechanisms of asymmetric wake evolution downstream of two parallel circular cylinders

Uniform flow past side-by-side circular cylinders is a classical fluid dynamic model that triggers rich phenomena, from which asymmetric wakes usually emerge. Such asymmetry arising from a completely symmetric geometric setting is of theoretical interest when exploring the system bifurcations. Using direct numerical simulation, a detailed parametric map of the wakes behind two side-by-side circular cylinders is presented with several asymmetric wake patterns. These include asymmetric anti-phase (AAP), typical and special deflected (DF), and in-phase (IP) flows, for which AAP and special DF flows are discovered for the first time. Additionally, the IP flow is simulated by both two- and three-dimensional grids to explore the effect brought by three-dimensional vortical structures. The evolution of these asymmetric wakes is analyzed in different phases, with the aid of the wavelet transform, Hilbert–Huang transform, and dynamic mode decomposition, to reveal their temporal variations of developing features. Interestingly, although revealing with distinct fully developed flow fields, there are several common dynamics identified among these wake patterns: AP and IP vortex shedding, wake transition, and gap flow oscillation. The vicissitudes of dynamic flow evolution allow us to further differentiate several wake patterns and ultimately contribute to a deeper understanding of asymmetric flows.

Tomographic PIV investigation on near-wake structures of a hemisphere immersed in a laminar boundary layer

The near wake of a hemisphere immersed in a laminar boundary layer is studied utilizing time-resolved tomographic particle image velocimetry (TPIV). Focus is placed on the three-dimensional vortical structures and the formation details of hairpin vortices before the onset of transition. The three-dimensional instantaneous pressure field of the hemisphere wake is reconstructed for better understanding the flow mechanism. Experiments are carried out with Reynolds number $Re_{r}=1370$ , based on the hemisphere radius $R$ . Features of periodicity of the near wake are analysed using proper orthogonal decomposition and Fourier transformation. The velocity fluctuation in the wall-normal direction is shown to be crucial to the formation of hairpin vortices in the near wake. By investigating the transport of mass and vorticity, and the correlation between pressure and hairpin vortex strength, the formation mechanism is revealed clearly. Specifically, the main hairpin vortices (MHVs) are formed within the reaction of outer high-speed flow and near-wall flow. The formation of the head portion is followed by the leg portion formation. The shedding of the MHVs is highly correlated with the pressure, as well as the pressure gradient in the wall-normal direction. For the side hairpin vortices (SHVs), the leg portion is formed first, followed by the generation of the head portion thanks to induction of the re-oriented standing vortices. The generation of the SHVs can be regarded as the downstream bridging of the standing vortices, similar to the generation of hairpin vortices due to the connection of streamwise vortices in turbulent boundary layers.

Control of Turbulent Flow over a Circular Cylinder Using Tabs

In this study, we investigate tabs applied to turbulent flow over a circular cylinder for the reductions of the mean drag and lift fluctuations. Tabs are small and thin passive devices attached to the upper and lower surfaces of a circular cylinder near the flow separation. The Reynolds number considered is Re= 3900, based on the free-stream velocity and cylinder diameter. Large eddy simulations are performed using a dynamic global subgrid-scale eddy-viscosity model. A parametric study is carried out to find the optimal tab configuration for minimizing the mean drag and lift fluctuations. Parameters considered are the height (ly) and width (lz) of the tabs, and spanwise spacing (λz) between them. With the optimal parameters, the spanwise coherence of the vortex shedding behind the cylinder is effectively disrupted, resulting in three-dimensional vortical structures varying in the spanwise direction. As a result, the strength of the vortex shedding in the wake is successfully weakened, and the mean drag and lift fluctuations are significantly reduced by 14% and 95%, respectively, with the optimal tab configuration of ly/d=0.2, lz/d=0.3, and λz/d=4, where d is the cylinder diameter.

Large-Eddy Simulation of Boundary-Layer Transition over a Zigzag Trip

Wall-resolved large-eddy simulations (WR-LES) are conducted to study transitional and turbulent boundary-layer properties and flow structures downstream of a zigzag trip on the low-Reynolds-number SD7032 airfoil at and and angle of attack . Without a zigzag trip, the airfoil features a prominent laminar separation bubble on the suction side. First, a verification and validation study of the LES method is conducted for the zigzag trip at a roughness-height-based Reynolds number . The boundary-layer velocity profiles and the integral parameters are compared with experimental data acquired using a long-distance -particle image velocimetry setup in a wind tunnel. In the LES, if the zigzag tape is thick enough to trigger transition just behind the tape, the transition to turbulence is generally quicker than what was observed in the experiment. Although the general phenomenology and the development of the turbulent boundary layer after transition are consistent with the experimental data, there remain some differences in the exact length of the transitional region and also in the associated airfoil drag. A zigzag tape with or thicker induces transition quickly behind the trip through a shedding process of the shear layer created by the tape. The boundary layer behind the zigzag tape features prominent three-dimensional vortical structures emanating from the zigzags that persist over long chordwise distances and well into the turbulent boundary layer. Nevertheless, in a spanwise-averaged sense, the integral thicknesses of the newly created turbulent boundary layer are actually quite similar to a two-dimensional boundary-layer theory solution, given that the tape has a suitable thickness. In contrast, an overly thick tape will generate a thicker turbulent boundary layer, where the increased displacement and momentum loss are sustained and also contribute to total drag.

Reynolds number effects on the bistable flows over a wavy circular cylinder

The wake of wavy cylinder has been shown to exhibit bistability. Depending on the initial condition, the final state of the wake can either develop into steady flow (state I), or periodic shedding (state II). In this paper, we perform numerical simulations to reveal the Reynolds number effects (Re=30 – 300) on these two wake states. With increasing Reynolds number, the steady vortical structures in state I wake sways back and forth in the spanwise direction, resulting in low-frequency fluctuations in drag forces, but not in lift. For state II, the increase in Reynolds number is associated with the emergence of another spectral peak in the lift coefficient. The secondary frequency is associated with highly three-dimensional vortical structures in the wake. For both states, the wakes transition to turbulent flows at higher Reynolds numbers, with the development of small-scale vortices. We further study the streamwise gust flows over the wavy cylinder. The time-varying inflow velocity results in a wide range of instantaneous Reynolds number spanning from the absolutely unstable flow regime to the bistable regime. Depending on the period of the inflow velocity variation, the wake perturbations grown at the absolutely unstable flow regime can be damped out in state I wake, or grow large enough to trigger the transition to state II, resulting in loss of flow control efficacy. The above analyses reveal novel flow physics of the bistable states at unexplored Reynolds numbers, and showcase the complex transition behavior between the two states in unsteady flows. The insights gained from this study improve the understanding of the wake dynamics of the wavy cylinder.

Dynamics of three-dimensional vortical structures behind a barchan dune based on tomographic particle image velocimetry

In this study, tomographic particle image velocimetry (PIV) was used to measure three-dimensional (3D) flow structures behind a fixed-bed barchan dune model at a Reynolds number of 2528 in a circulation water tunnel. The topological evolution of the 3D instantaneous vortex structures and their dynamic characteristics in the dune wake were analyzed. 3D instantaneous arch-shaped vortex “chains” shedding from the barchan and typical quasi-streamwise vortex structures induced by the development of an internal boundary layer were found to be located both before and after the reattachment region, which differs a little from the results of previous studies. Nevertheless, both conditionally averaged and typical instantaneous 3D flow fields revealed that the arch-shaped vortex system dominates the barchan dune wake, and the inclination angle of these spanwise-oriented structures was found to change as it propagates downstream. Additionally, the dynamic characteristics of the dune wake were found to be similar to a flapping phenomenon such as oscillation of the recirculation in a backward-facing step flow. To provide complementary insight into this phenomenon, the separated and reattached shear flow were also examined using two-dimensional time-resolved planar PIV. It was found that the basic unsteady flapping motion, vortex roll-up, and complex vortex interactions can all be analyzed with ease in a Lagrangian framework.

Fluid structure interaction of a subaqueous pendulum: Analyzing the effect of wake correction via large eddy simulations

The dynamic behavior of a subaqueous cylindrical pendulum and corresponding flow dynamics are investigated. The objectives were twofold: (i) to examine whether the two-dimensional model equations sufficiently capture the three-dimensional dynamics and (ii) to investigate the emerging three-dimensional vortical flow structures. Large eddy simulations with two-way coupling fluid structure interaction were carried out using the immersed boundary method to simulate the motion of the pendulum and its interactions with the initially stagnant water. The resulting pendulum motion is compared against measured data obtained in a series of experimental tests to validate the simulation results and the model equations with and without wake corrections. An analysis of the flow vorticity revealed the development of a vortex ring during the first swing and the formation of tip vortices. The evolution of the vortex rings emerging from the motion of the subaqueous cylindrical pendulum was visualized using Q-criteria showing a reasonable agreement with vortical structures observed in the experiment using particle imaging velocimetry. The hydrodynamic moments acting on the simulated pendulum and the moments calculated from the model equations are analyzed. Using the insights from these numerical simulations, a modification of the wake correction is proposed to enhance the accuracy of the rate of decay and period. The transient effect of coherent flow on pendulum dynamics, especially the added mass effect, is discussed.

Numerical Investigation of Plasma Actuator Effects on Flow Control Over a Three-Dimensional Airfoil With a Sinusoidal Leading Edge

Abstract The impetus of the present bio-inspired work is to investigate the impact of simultaneously using wavy leading-edge (WLE) airfoils in combination with curved multidielectric barrier discharge (DBD) plasma actuators as hybrid passive and active flow control mechanisms, respectively. A precise distinction of the produced frequency and noise signals, altogether with the acoustic effect of using WLE and DBD plasma actuators, is herein analyzed with precision. Two specific DBD plasma actuators are designed to actuate at x/C = 3% and x/C = 30% on a NACA 634-021 airfoil with sinusoidal WLE that bears a wavelength of 25% and an amplitude of 5% of the mean chord length and straight-leading-edge (SLE). A large eddy simulation (LES) turbulence model was used. This includes the dynamic control of unsteady flow separation, the three-dimensional vortical structure and induced trains of vortices, the aerodynamic forces, the velocity variation, and also the spanwise flow. The momentum transfer between the main flow and boundary layer was improved by the DBDs-induced vortices train and formed streamwise counter-rotating pair-of-vortices over the tubercle. Also, both the continuous wavelet transform (CWT) and fast Fourier transform (FFT) methods were used to investigate the induced plasma flow spectral content for the WLE and SLE geometries. We witnessed an optimized flow control, by using DBD plasma actuators with the WLE airfoil, that resulted in less massive flow separation, faster turbulent transition, and a robust earlier flow reattachment. This modification was beneficial in increasing the efficiency and decreasing the noise for low Reynolds number operational conditions.

Numerical model and hydrodynamic performance of tuna finlets

Finlets, a series of small individual triangular fins located along the dorsal and ventral midlines of the body, are remarkable specializations of tuna and other scombrid fishes capable of high-speed swimming. In this study, a symmetric model containing nine finlets of tuna is proposed to overcome the limitation of measurement without losing authenticity. Hydrodynamic performance along with three-dimensional flow structures obtained by direct numerical simulation are demonstrated to disclose the underlying hydrodynamics mechanism of finlets. Complex interactions of leading-edge vortices (LEVs), trialing-edge vortices (TEVs), tip vortices (TVs) and root vortices (RVs) are observed from the three-dimensional vortical structures around the finlets. Two more cases consisting of the 3rd to 9th (without the first two) and the 3rd to 7th (without the first two and the last two) finlets are also simulated to examine the effects of the first two and the last two finlets.

Three-dimensional flow past two stationary side-by-side circular cylinders

We conducted three-dimensional direct numerical simulations of flow past two stationary side-by-side circular cylinders. Based on the cylinder diameter, the Reynolds number was set as Re = 500 and the center-to-center spacing ratio was s/D = 1.0–5.0. In the studied parametric space, we identified five distinct flow regimes, namely, single bluff body flow at s/D = 1.1, deflected flow at s/D = 1.2–1.8, flip-flopping flow at s/D = 2.0–2.4, hybrid flow at s/D = 2.5, and antiphase flow at s/D = 2.7–5.0. The intrinsic features of each flow were elucidated using instantaneous vorticity contours, time-averaged flow fields, three-dimensional vortical structures, time histories and spectral frequencies of the drag and lift coefficients. Furthermore, the statistics of hydrodynamic forces, vortex shedding frequency, and wake three-dimensionality within each flow regime were examined thoroughly. Finally, we discussed two critical issues, namely (a) the effects of the three-dimensional vortical structures on the hydrodynamic forces and flow behaviors where both two- and three-dimensional results are compared, and (b) cylinder length effects and underlying mechanism for the switching behavior of the gap flow. We found that (1) the existence of the three-dimensional vortical structures significantly undermines the vortex interactions by absorbing energy from the spanwise vortices, and therefore, both the hydrodynamic and wake behaviors are considerably altered. (2) An increase of the cylinder length leads to the longer switching period of the gap flow in FF regime, while the transform of the gap flow direction is strongly dependent on the phase of the gap side vortices of the two cylinders.

Numerical simulation of the wake instabilities of a propeller

Propeller wake instability mechanisms are investigated using the improved detached-eddy and large-eddy simulation approaches under heavy loading condition. Simulations involving a rotating propeller are performed using a dynamic overset technique in order to compare experimental propeller loads and phase-averaged flow fields and validate the modeling approach and grid system. Analysis of the three-dimensional vortical structures, out-of-plane vorticity, velocity components, Reynolds stresses, and kinetic energy power spectral density reveal the flow physics that drive the tip vortex instability process. The present study extends knowledge of propeller wake instability inception mechanisms under heavy loading conditions. It also helps to promote further understanding of propeller wake dynamics and provides a reference for the assessment of turbulence modeling approaches used to study propeller wake instability.

Usefulness of Inviscid Linear Unsteady Lifting-Line Theory for Viscous Large-Amplitude Problems

Unsteady lifting-line theory (ULLT) is a low-order method capable of modeling interacting unsteady and finite wing effects at low computational cost. Most formulations of the method assume inviscid flow and small amplitudes. Although these assumptions might be suitable for small-amplitude aeroelastic problems at high Reynolds numbers, modern engineering applications increasingly involve lower Reynolds numbers, large-amplitude kinematics, and vortex structures that lead to aerodynamic nonlinearities. This paper establishes that ULLT still provides a useful solution for low-Reynolds-number, large-amplitude kinematics problems, by comparing ULLT results against those of experimentally validated computational fluid dynamics simulations at . Three-dimensional effects stabilize leading-edge vortex (LEV) structures, resulting in a good prediction of whole wing force coefficients by ULLT. Although the inviscid spanwise force distributions are accurate for small-amplitude kinematics, the ULLT cannot model three-dimensional vortical structures, and thus it cannot correctly predict the force distribution due to the LEV. It can, however, predict the shedding of LEVs to a limited extent via the leading-edge suction parameter criterion. This can then be used as an indicator of the usefulness of the force distribution results.

Detecting vortical structures in time-resolved volumetric flow fields

The detection of three-dimensional coherent vortical structures that get advected as well as deformed with time is a challenge. However, it is critical for the statistical analysis of these vortices, for example, the quasi-streamwise vortices (QSVs) in the near field of a turbulent shear layer, where cavitation inception typically occurs. These structures exhibit underlying correlations among different properties that can be derived from the velocity gradients. Exploiting these correlations, a pseudo-Lagrangian vortex detection method is proposed that uses k-means clustering based on vorticity magnitude and direction, values of λ2, strain rate structure, axial stretching, and location. The method facilitates the finding that QSVs have pressure minima that are lower than those in the surrounding flow, including the primary spanwise vortices. These minima typically appear after a period of axial stretching and before contraction events.

Numerical study of hole formation in a thin flapping liquid sheet sheared by a fast gas stream

This study reports aerodynamics effects of a fully developed double-shear layer gas flow in the perforation and disintegration of a thin liquid sheet using three-dimensional numerical simulations. A double-shear layer gas flow, where a thin quiescent gas layer is sandwiched between the two fast moving gas layers, is first simulated up to a statistically steady state. The downstream dynamics of the gas shear layers shows vortex shedding and three-dimensional rib-like vortical structures, similar to the wake of a bluff body. This gas flow is then used as the initial condition for a liquid-gas simulation, where the liquid phase is sandwiched by the top and bottom fast gas streams. A thin liquid sheet with a thickness of 25 μm is considered using air/water conditions. The liquid sheet oscillates and small holes, preceded by craters surrounded by ripples, are formed. The thinning of the liquid sheet, spanwise corrugations, and vortex separations within the liquid-gas boundary layer are shown to govern the dynamics of the liquid sheet. A localized pressure jump is also observed inside the liquid sheet and precedes the rupture of the liquid sheet by pushing the liquid away in the span direction. The holes formed subsequently grow in size, collide and merge with each other, and break the liquid sheet into multiple droplets.

Vortex dynamics of veno-arterial extracorporeal circulation: A computational fluid dynamics study

Veno-arterial extra corporeal membrane oxygenation (VA-ECMO) is a modified heart lung machine used for patients with both heart and lung failure. This results in retrograde supply of oxygenated blood through the femoral artery in which the unsteady pulsating antegrade flow from the aorta interacts with a steady, uniform, and retrograde flow from the femoral artery, creating a mixing zone. This work aims to provide a mechanistic interpretation of VA-ECMO by developing an in-silico framework using computational fluid dynamics. We performed several numerical simulations to investigate the effects of aortic geometry on VA-ECMO by implementing two idealized full aorta models and studied the formation of secondary flow features and vortices. We used vortex identification methods to capture the three-dimensional vortical structures formed under various ECMO support levels. Our results show that under pulsatile aortic flow and 80% of ECMO support, the streamwise vorticity and aortic arch geometry strongly influence the mixing zone. Furthermore, we found that pulsatility at the aortic inlet causes oscillation in secondary flow structures at the abdominal aorta leading to unsteadiness in ECMO flow and differential wall shear stress. We also examined the effects of VA-ECMO flow rates on secondary flow and vortical structures. We show that the location and complexity of secondary flows and vortical structures are affected by ECMO support levels and geometry of aortic segments. Together, we believe that this computational framework is a crucial step in understanding flow features and vortical structures formed during VA-ECMO administration, which can improve patient care and ECMO management.

Study of the Plasma Actuator Effect on the Flow Characteristics of an Airfoil: An LES Investigation

<div class="section abstract"><div class="htmlview paragraph">The impetus of this study is to investigate the effect of using multi dielectric-barrier-discharge plasma actuator (DBDPA) over the 3D-airfoil surface and improve the performance of it. Two designed DBDPAs are placed at the leading edge and a distance of <i>x</i>/<i>c</i> = 0.3 chord of the NACA 63<sub>4</sub>-021 three dimensional (3D) airfoil. We solved the flow at different chord-based Reynolds numbers and in a wide range of angles of attack (AoA) (12<sup>∘</sup> ≤ <i>α</i> ≤ 24<sup>∘</sup>), using a large eddy simulation (LES) turbulence model, which is implemented under the OpenFOAM package framework. Here, detailed flow mechanism analyses, i.e., three-dimensional vortical structure, separation phenomenon, lift and drag coefficients, variation fluctuations and spanwise flow, are investigated around the airfoil 3D. Massive flow separation and transient aerodynamic loads acting on the airfoil have been significantly suppressed by optimized flow control using the DBDPAs in the airfoil. Wavelet analysis with and without the presence of the plasma flow over an airfoil is reported. The spectral content of the induced plasma flow is analyzed using Fourier and continuous wavelet transformations. Also, the priority of using airfoil with multi-DBDPAs at pre-stall and post-stall AoA, especially about the force coefficient are analyzed precisely. Multi DBDPA modification for low Reynolds number on wings and blade turbines can increase efficiency and decrease the noise.</div></div>

Deep learning of the spanwise-averaged Navier–Stokes equations

Simulations of turbulent fluid flow around long cylindrical structures are computationally expensive because of the vast range of length scales, requiring simplifications such as dimensional reduction. Current dimensionality reduction techniques such as strip-theory and depth-averaged methods do not take into account the natural flow dissipation mechanism inherent in the small-scale three-dimensional (3-D) vortical structures. We propose a novel flow decomposition based on a local spanwise average of the flow, yielding the spanwise-averaged Navier–Stokes (SANS) equations. The SANS equations include closure terms accounting for the 3-D effects otherwise not considered in 2-D formulations. A supervised machine-learning (ML) model based on a deep convolutional neural network provides closure to the SANS system. A-priori results show up to 92% correlation between target and predicted closure terms; more than an order of magnitude better than the eddy viscosity model correlation. The trained ML model is also assessed for different Reynolds regimes and body shapes to the training case where, despite some discrepancies in the shear-layer region, high correlation values are still observed. The new SANS equations and ML closure model are also used for a-posteriori prediction. While we find evidence of known stability issues with long time ML predictions for dynamical systems, the closed SANS simulations are still capable of predicting wake metrics and induced forces with errors from 1-10%. This results in approximately an order of magnitude improvement over standard 2-D simulations while reducing the computational cost of 3-D simulations by 99.5%.

Continuous and discontinuous gravity currents in open-channel embayments.

Embayments, as major storage zones in riverine environments, could be surrounded by gravity currents associated with industrial pollution, heavy impurities, and sewage. The accumulated contaminated matter in embayments significantly impairs aquatic habitats and influences the embayment performance. In this study, the three-dimensional vortical structures of continuous and discontinuous gravity currents in channels connected to an embayment are investigated by solving unsteady Reynolds-averaged Navier-Stokes with algebraic Reynolds stress model (ASM) which has been improved by the buoyancy effects. The lateral embayment with different configurations is simulated to investigate the exchange processes of the dense fluid between the lateral embayment and main stream. The accuracy and consistency of the developed model are checked using the experimental data of continuous and discontinuous gravity currents in the straight channel and mass exchange in the lateral embayment. The findings have revealed that the agreement between measured and simulated flow and concentration fields is reasonable. The current head propagation of the continuous gravity current inside the embayment is similar to the discontinuous gravity current head propagation. At early time, the continuous gravity current leads to rapid diffusion of the concentration in the embayment and main channel downstream. In the discontinuous gravity current with the embayment aspect ratio equal to 1.0, the dense fluid firstly fills the lateral embayment as the most volume of the dense fluid is trapped and then it flushes out into the main channel. The geometric aspect ratio of the lateral cavity slightly affects the exchange coefficient between the main channel and lateral cavity. The time-averaged exchange coefficient is 0.33 in the continuous gravity current.

Volumetric flow characterisation of a rectangular orifice impinging synthetic jet with single-camera light-field PIV

The formation and evolution of volumetric flow structures from a rectangular orifice synthetic jet (AR=10, Re=550, Sr=0.0117, and Lo=0.0855m) impinging on a flat plate is investigated using phase-locked single-camera light-field particle image velocimetry (LF-PIV). An impinging plate is located 24 orifice widths away from the rectangle orifice and this volumetric flow data is used to capture the mechanisms involved in the interaction. Flow statistics and structure information obtained by time-averaging and phase-averaging the volumetric data reveal the formation and evolution of three-dimensional vortical structures and sweeping vortex that characterise the flow-surface interaction.