Secondary Vortex Pair Research Articles

Topology optimization with vortex-intensity-based objective function enables novel film cooling hole with superior performance

Film cooling is widely adopted in the thermal protection of modern aerospace engines. The optimization of the film hole is an important issue in thermal designs. In this work, a density-based topology optimization is conducted to obtain a high-performance film cooling hole geometry. Unlike the traditional wall-temperature-based optimization objectives, a novel objective function based on the Q-criteria is proposed by taking the vorticity dynamics and flow mixing into account. Finally, a unique high-performance film hole is obtained, which shows superior performance compared to the standard cylindrical hole, particularly at high blowing ratios. The film cooling effectiveness can be elevated by 51 folds in the case of a high blowing ratio (M = 1.5). Analyses of the vortices and velocity fields suggest that the presence of the additional secondary vortex pair enhances the attachment of cooling air to the wall.

Flow and Heat Transfer Characteristics in the Entry and Stabilized Flow Regions of a Circular Channel Rotating About a Parallel Axis

Abstract Cooling of heavy-duty electrical machines such as generators and motors is crucial for the smooth operations without thermal runaway. A commonly employed technique for the cooling of rotors in these machines is to place channels at different radial locations for the continuous passage of coolant. These channels are therefore rotating about a parallel axis, and the rotation-induced forces alter the flow and thermal behavior of the coolant compared to stationary channels. The present study reports a detailed numerical investigation on a long circular channel rotating about a parallel axis. The objective is to analyze the flow, heat transfer, and rotation-induced forces (Coriolis and centrifugal forces) in the entry region as well as in the region where flow is stable (the term ‘stable’ is used rather than ‘developed’ due to the presence of secondary flows in this region). The rotating channel was subjected to constant wall heat flux and constant wall temperature conditions at different Rotation numbers of 0, 0.15, 0.4, and 0.6. The Coriolis force is observed to be strong enough in the entry region to influence the flow. In the “stable flow” region, the centrifugal force becomes more dominant and forms counter-rotating secondary vortex pair, which causes circumferential variation in the Nusselt number. The flow and heat transfer characteristics for constant wall heat flux and wall temperature boundaries are the same for rotation conditions with similar values of rotational Grashof number. A correlation is presented for the circumferential variation of the Nusselt number in the stable flow region.

The impact of ferrofluid actuated by an external magnetic field on heat exchange performances of curved channels

The development rules of ferrofluid flow and heat exchange in curved channels under the influence of external magnetic field were thoroughly investigated using the finite element method. The comprehensive strengthening performance of elbows under magnetic fields was evaluated based on the factors including the dimensionless pressure coefficient, heat transfer coefficient, and comprehensive strengthening factor. The results demonstrate that the heat transfer coefficient can be improved by increasing the volume fraction of ferrofluid in the presence of lower or no magnetic field, while it exhibits no significant variation with the increase of the volume fraction for higher magnetic fields. For a given volume fraction, a lower magnetic field may slightly inhibit heat transport of ferrofluid. A higher magnetic field can induce a complex swirling flow of the fluid along the curved axis. As the curvature decreases, the magnetic field effect becomes stronger, leading to the coupling of the magnetic field force with the centrifugal force. This coupling results in three pairs of secondary vortices on the cross-section and the heat exchange capacity is significantly enhanced. Within the study, the heat transfer coefficient of the curved tube with a dimensionless curvature of 0.067 for the magnetic field intensity of 1800G increased by 43.51%, and the comprehensive strengthening factor is 1.326, compared to without a magnetic field.

Effect of mainstream Mach number on the flow characteristics and cooling effectiveness of supersonic film cooling

For supersonic film cooling, the interaction between the mainstream and coolant jet will influence the flow characteristics near the film cooling hole as well as the film cooling performance. To evaluate the influence mechanism of mainstream Mach numbers on the flow field structure and film cooling effectiveness, numerical simulations are performed under various mainstream Mach number conditions (Ma∞ = 0.8, Ma∞ = 2, and Ma∞ = 4). As the obstructing effect of the coolant jet, the complicated shock waves are induced upstream of the film cooling hole under the supersonic mainstream condition. Due to the influence of the oblique shock waves, the mainstream moves away from the central area along the spanwise direction, which results in diminishing of the interaction between the mainstream and the coolant. For supersonic mainstream condition, the secondary counter-rotating vortex pairs (SCVP) are induced in the outer area of the primal counter-rotating vortex pairs (PCVP), which weakens the action of the mainstream on PCVP and slows down the weakening of the vortex. The emergence of SCVP weakens the vertical diffusion of PCVP, while increasing the spanwise dimension of PCVP, resulting in better film flow adhesion and greater spanwise film coverage. Additionally, due to the shock waves induced uneven distribution of the static pressure occurs near the exit of the film cooling hole. The unevenness of the static pressure distribution increases with the mainstream Mach number, resulting in a relatively high mass fraction of the coolant at the leeward side of the film cooling hole. It is indicated that for supersonic mainstream condition, the mixing and dissipation of the coolant are diminished, leading to an enhancement in the film cooling performance at the downstream of film cooling hole. The film cooling effectiveness increases as the mainstream Mach number increases.

Numerical investigation into heat transfer augmentation in a square minichannel heat sink using butterfly inserts

Heat transfer augmentation in a square minichannel heat sink through introduction of various butterfly insert configurations is numerically investigated for a range of Reynolds numbers (200 – 900). The present study also examines the individual effects of insert, pitch of the insert, and perforations in the wings of the butterfly insert on the thermohydraulic performance of minichannel heat sinks. Incorporating any kind of insert in a typical square minichannel is found to have a positive influence on the overall thermohydraulic performance, yielding performance evaluation criteria consistently over unity for the whole range of tested Reynolds numbers. The effect of pitch is tested for unperforated inserts, wherein a reduction of pitch distance from 10 mm to 5 mm is found to increase heat transfer performance by 15.08 - 46.39% for the studied range of Reynolds numbers. Perforations in the wings of butterfly inserts are found to result in a reduction of friction factor, with a 4.01 – 20.78% decrease, when compared with an unperforated insert having an identical pitch. However, this reduction is achieved at the cost of a lower heat transfer performance (5.07 – 16.17%). Therefore, unperforated butterfly inserts are deemed to be superior in terms of the overall thermohydraulic enhancement. In all enhanced minichannel configurations, a redistribution of velocity and temperature fields due to the formation of a primary vortex pair and thinning of thermal boundary layers due to a second pair of secondary vortices are thought to be the major contributors behind the observed thermohydraulic enhancement. • Introduction of a butterfly insert with 10 mm pitch is found to enhance heat transfer performance in MCHS by 42.02% to 109.31% for the tested range of Reynolds numbers. • A reduction in pitch from 10 mm to 5 mm in an MCHS with unperforated butterfly inserts is observed to yield an augmentation of average Nusselt number by 15.08% to 46.39% for the studied range of Reynolds numbers. • Perforation on insert wings is found produce a reduced enhancement in average Nusselt number, i.e., 5.07%–16.17%, for the investigated range of Reynolds numbers. • Observed thinning and disruption of thermal boundary layers and enhanced fluid mixing at the central region of the minichannel due to a pair of primary vortices are thought to be the major reasons for enhanced thermal performance. • A maximum performance evaluation criterion of 1.66 is observed for the case of unperforated butterfly inserts with 5 mm pitch at a Reynolds number of 900.

Mean Flow, Turbulent Structures, and SPOD Analysis of Thermal Mixing in a T-Junction with Variation of the Inlet Flow Profile

This paper investigates the effects of different inlet flow profiles on thermal mixing within a T-junction using CFD simulations with the IDDES-SST turbulence model. The different combinations of inlet flow profiles are related to different stage in the flow entry region. The effects of the inlet flow profile on the mean and transient flow behaviour are assessed, while a spectral proper orthogonal decomposition and power spectral density analysis are performed to assess the underlying flow structures and the predominant frequency modes. It is found that the vortical structures associated with the horseshoe and hovering vortex systems consist of a single roll-up vortex for cases with uniformly distributed boundary conditions (BCs) at the branch inlet whereas a double roll-up vortex is observed for the other cases. The double roll-up vortex enhances the mixing locally due to the entrainment of fluid from the branch pipe in these vortical structures, which then results in a lower mean temperature distribution. The appearance of the secondary vortex pair and the nested vortices is delayed for cases with uniformly distributed BCs at the branch inlet, which again results in lower thermal mixing and consequently higher values of mean temperature when compared with the other cases. It is also found that the vorticity related to the counter-rotating vortex pair as well as to the second pair of vortices rotating in the opposite direction is higher for cases with uniformly distributed BCs at the branch inlet. Lastly, the combinations of inlet flow profiles lead to different coherent structures, and the dominant frequencies are of a Strouhal number of around 0.7 for uniformly distributed profiles at the branch inlet and in the range 0.4–0.5 for the other cases.

Research on Composite Drag Reduction Characteristics of ‎Biomimetic Microstructural Surface

The objective of this work is to investigate the effect of the bionic microstructure surface on flow field structure of the slab. The motivation behind this study is to investigate the effect of the bionic microstructure parameters including the height and intersection angle of microstructure in order to improve the drag reduction characters. The numerical simulation is performed on the bionic microstructure model of the V-shaped and serrated bionic microstructures using the RNG k-ε model. The drag reduction rates of two bionic microstructures under different dimensionless sizes are ob13.79 tained. The drag reduction efficiency is up to 8.76% when the dimensionless height of microstructure is at and the intersection angle for V-shaped microstructure. In addition, combined with wall temperature control of drag reduction technology, the influence of wall temperature on the drag reduction effect is also analyzed. Compared with the flow field structure of the surface boundary layer of the smooth plate, the wall microstructure divides the surface boundary layer into two parts: the bottom and the tip. The average velocity profile is moved up and the thickness of the linear bottom layer is increased. A large number of "quiet" fluids are gathered at the bottom of the surface boundary layer. In addition, the existence of wall microstructure can weaken the momentum exchange in the boundary layer and restrain the spreading vortex motion of the fluid in the near-wall region. The "secondary vortex pair" on both sides of the tip of the microstructures can effectively limit the lateral pulsation of fluid So as to achieve a good drag reduction effect‎.

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.

Advective mass transport in two side-by-side liquid microspheres

Gaseous entrainment from a contaminated outer air stream into a pair of side-by-side homogeneous and heterogeneous micro-sized water drops is numerically examined for varied gap ratio (ratio of interfacial gap to radius), Reynolds number , Weber number , and liquid-phase Peclet number . For and , the separation–attachment induced momentum exchange and imposed non-uniform interfacial shear stress lead to breakup of the primary Hill’s vortex ring and create a significant secondary vortex ring in each drop, which together construct a dominant advective transport mechanism therein. Beneath a three-dimensional (3-D) topological separation line, the study identifies an active advective mass entrainment process that is led by the ‘inflow’ natured local dynamics of this primary–secondary vortex ring pair. Mechanistically, the secondary and primary vortex rings regulate species transfer into a drop by maintaining the spontaneous inflow-type counter-rotating motion along the 3-D separation line, whereby the is entrained; and near the attachment points/nodes, two vortices distinctly repulse entry by virtue of their ‘outflow’ natured local dynamics. The blockage effect and nozzle effect on flow approaching and passing the narrow neck that formed in the presence of a second drop lead to the asymmetric growth of both primary and secondary vortex rings via the locally weakened and enhanced near-interfacial air flow and imposed variable shear stress, which induce the occurrence of an asymmetric mass transfer phenomenon plus biased saturation. The , once entrained, rotates mostly along a spiral orbit of a primary vortex ring, owing to its higher strength. For increased , the transport process is reinforced following increased strength of the inflow paired secondary–primary vortex dynamics that enhanced the net entrainment rate and also advanced its transport to the vortex core via augmented convective flow plus radial diffusion. A narrow gap facilitated faster near-gap saturation, while the quantitative transport rate is decreased by virtue of the produced tapered primary–secondary vortex pairs, associated inner flow bifurcation, and changed topology of the separated wake, which appear similar to what develops for a larger single drop. The gap induced inner vortical structures are characterized by a weaker secondary vortex and a tapered primary vortex near the neck. For heterogeneous drop pairs, the influence of varying 3-D surface flow topology on the two interfaces and the impact of solid fraction ( , with being the radius of the solid core) on the created advective mechanism by the primary–secondary vortex ring pair and resultant transport are exclusively elucidated.

POD analysis on vortical structures in MVG wake by Liutex core line identification

The Liutex core line method, first combined with the snapshot proper orthogonal decomposition (POD), is utilized in a supersonic micro-vortex generator (MVG) wake flow at Ma = 2.5 and Reθ = 5 760 to reveal the physical significance of each POD mode of the flow field. Compared with other scalar-based vortex identification methods, the Liutex core line identification is verified to be the most appropriate approach that is threshold-free and provides full information of a fluid rotation motion. Meanwhile, the Liutex integration is employed to quantitatively track the evolution of the vortices in MVG wake and is applied to the determination of the effective control section of the MVG wake for the optimization study of MVG design. The physical mechanism of each POD mode for multi-scale and multi-frequency vortical structures is investigated by using Liutex core line identification to give some revelations. For the mean mode (mode 0) indicating the time-averaged velocity flowfield of the MVG wake flow, a pair of primary counter-rotating streamwise vortices and another pair of secondary vortices is uniquely identified by two pairs of Liutex core lines with Liutex magnitude. In contrast, mode 1 is featured by a fluctuated roll-up motion of streamwise vortex, and the streamwise component of the MVG wake is demonstrated to be dominant in terms of the total kinetic energy contribution. Meanwhile, a dominant shedding frequency of St = 0.072 is detected from the temporal behavior of mode 2, which has the organized arc-shaped vortex structures shedding from MVG induced by the K-H instability. Additionally, mode 4 subjects to low-frequency oscillations of the wall vortices and thus takes a relatively lower frequency of St = 0.044.

Numerical study of Taylor–Couette flow with longitudinal corrugated surface

This study investigates the Taylor–Couette flow (TCF) with a longitudinal corrugated surface on a stationary outer cylinder and a rotating smooth inner cylinder using large eddy simulation for three values of amplitude to wavelength ratios (A*) (0.1875, 0.2149, and 0.25) to explore the influence of the corrugated surface on the flow structures and the variation of torque for a wider range of Reynolds numbers (Re) (60–650). From the results, four flow regimes are observed. At Re = 60, initially, a pair of secondary vortices appears at the inner wall of the minimum gap region and it evolves to a pair of axisymmetric stationary wall induced vortices (ASSWIVs) in the maximum gap region. As Re increases to 80, 85, and 103 for the three values of A* (0.1875, 0.2149, and 0.25), respectively, another pair of axisymmetric stationary secondary vortices is seen at the minimum gap region of the inner wall. A further increase in Re (Re > 125, 130, and 138 for the three values of A*, respectively) results in the appearance of axisymmetric periodic secondary axial flow. Increasing Re further (Re > 225, 240, and 260 for A* = 0.25, 0.2149, and 0.1875, respectively) leads to the emergence of non-axisymmetric and non-periodic secondary axial flow (NANPSAF) with an azimuthal wave. Generally, the torque in TCF with the corrugated surface is found to be lower than TCF with a smooth surface except for the occurrence of the ASSWIV flow regime and weak axial secondary flow in the NANPSAF regime.

Analysis of flow characteristics downstream delta-winglet vortex generator using stereoscopic particle image velocimetry for laminar, transitional, and turbulent channel flow regimes

The evolution of flow structures downstream a single pair of delta-winglet vortex generators (VGs) is investigated experimentally using stereoscopic particle image velocimetry. In addition, the laser Doppler anemometer technique is performed to characterize the upstream flow. Experiments are conducted in a bounded channel flow (height H) for the Reynolds numbers (ReDH, based on the hydraulic diameter height) ranging from 400 to 12 000. The purpose of this study is to provide detail insight into the generation and the dissipation of longitudinal vortices over a wide flow regime range including the laminar–turbulent transition. With a focus on transverse sections, the flow field is detailed. For all flow regimes, the main flow topology shows that the two main counter-rotating vortices are generated at a certain streamwise distance downstream the VG and then are advected gradually toward the channel lateral-walls. A secondary vortex pair is induced closer to the wall. Our results show that close to the VGs, local regions (1 > z/H > −1) are strongly defined as the inception of the turbulence production. The intensity of this latter is shown to vanish beyond a certain distance far from the origin of the perturbation (when x/H is greater than 3). The instantaneous flow structure describes the mechanism of vortex generation, relying on the intermittence of the flow organization and the sweep and ejection event balance. Detailed analysis on the turbulence properties and wall shear stress has been assessed and revealed that the flow transition induced by the perturbation of the VG is achieved at a Reynolds number no greater than 1500.

Effects of the cell and triangular microwell size on the cell-trapping efficacy and specificity

The single-, double- or multiple-cell entrapment efficacy is crucial in various aspects of biological studies. In this study, both computational and experimental approaches were conducted to explore the effect of the cell and microwell sizes on the ability of cell-trapping in a triangular microwell. From computational studies, it was found that the interaction between a spanwise vortex on the upper part and counter-rotating streamwise vortices at the leading edge helped to form a pair of secondary streamwise vortices deeper inside the microwell. The strength and size of these secondary streamwise vortices, which depended on the size of the microwell, played an important role in the arrangement of entrapped cells. The experimental results, with both microbeads and white blood cells (WBCs), were in good agreement with the simulated ones, and suggested that the ratio between the cell and microwell sizes was an important factor in the efficacy of single-, double- and multiple-cell cell-trapping. Entrapment of canine WBCs (size distribution between 7–15 µm) attained a highest single-cell trapping efficiency of 20.4 % in the array of 40-µm triangular microwells of 30 µm depth at a flow rate of 0.1 mL/h, but this was reduced to 16.5 and 10.6 % in the 60- and 80-µm microwells, respectively, under the same conditions.

Analysis of the wake dynamics of a propeller operating before a rudder

This paper analyzes the characteristics of the wake dynamics of a propeller operating before a rudder under open water conditions. The analysis is conducted using a numerical method based on the detached eddy simulation (DES). In particular, the evolution mechanism of the propeller wake is explored under different advance coefficient and rudder angle conditions. Under heavy load conditions, the topology of the vortex system becomes more complex and is accompanied by a stronger interaction between the wake and the rudder. In addition, a secondary vortical structure bridging the tip vortices is formed in the vicinity of the tip vortex, followed by the growth of a secondary vortex and the formation of a secondary vortex pair. At nonzero rudder angles, the interaction between the tip vortex and the rudder becomes stronger, yielding a more complex vortex topology. Furthermore, an intertwined vortical structure is formed on the suction side and the Ω-shaped vortical structures form much earlier. Analysis of the kinetic energy (KE) and pressure spectra on the rudder surface provides additional insights into the evolution pattern of the vortical structures. The objective of this study is to further identify the dynamic characteristics of the propeller vortices before the rudder under multiple operating conditions.

Numerical evaluation on single-row trenched-hole film cooling performances on turbine guide vane under engine-representative conditions

A numerical investigation is performed to study the effects of transverse trenches on the film cooling performances of single row film cooling holes on the turbine guide vane, under the engine-representative conditions. Two trenches are investigated in the current study, including a conventional straight trench and a novel serrated trench, both having the same slot width of 2.5d and depth of 0.75d. With the presence of trenches, the kidney vortices are effectively destroyed at the suction side, replaced by a pair of secondary vortices originating from the edge of trench slot, with a sense of rotation opposite to the kidney-vortex pair. However, at the pressure side, the kidney vortices remain clearly in the trenched-hole cases. The roles of trenched-hole on the film cooling enhancement behave more pronounced on the pressure surface when compared to the respective one on the suction surface. On the pressure surface, the serrated trench displays a more obvious role in the film cooling enhancement when compared to the straight trench. Combined with the effects of trenched-hole on the enthalpy loss coefficient, it is suggested that the trenches are more reasonably used on the pressure surface.

On the topography-driven vorticity production in shallow lakes

We analyse the vorticity production of lake-scale circulation in wind-induced shallow flows using a linear elliptic partial differential equation. The linear equation is derived from the vorticity form of the shallow-water equation using a linear bed friction formula. The features of the wind-induced steady-state flow are analysed in a circular basin with topography as a concave paraboloid, having a quadratic pile in the middle of the basin. In our study, the size of the pile varies by a size parameter. The vorticity production due to the gradient in the topography (and the distance of the boundary) makes the streamlines parallel to topographical contours, and beyond a critical size parameter, it results in a secondary vortex pair. We compare qualitatively and quantitatively the steady-state circulation patterns and vortex evolution of the flow fields calculated by our linear vorticity model and the full, nonlinear shallow-water equations. From these results, we hypothesize that the steady-state topographical vorticity production in lake-scale wind-induced circulations can be described by the equilibrium of the wind friction field and the bed friction field. Moreover, the latter can also be considered as a linear function of the velocity vector field, and hence the problem can be described by a linear equation. doi:10.1017/S1446181119000051

ON THE TOPOGRAPHY-DRIVEN VORTICITY PRODUCTION IN SHALLOW LAKES

We analyse the vorticity production of lake-scale circulation in wind-induced shallow flows using a linear elliptic partial differential equation. The linear equation is derived from the vorticity form of the shallow-water equation using a linear bed friction formula. The features of the wind-induced steady-state flow are analysed in a circular basin with topography as a concave paraboloid, having a quadratic pile in the middle of the basin. In our study, the size of the pile varies by a size parameter. The vorticity production due to the gradient in the topography (and the distance of the boundary) makes the streamlines parallel to topographical contours, and beyond a critical size parameter, it results in a secondary vortex pair. We compare qualitatively and quantitatively the steady-state circulation patterns and vortex evolution of the flow fields calculated by our linear vorticity model and the full, nonlinear shallow-water equations. From these results, we hypothesize that the steady-state topographical vorticity production in lake-scale wind-induced circulations can be described by the equilibrium of the wind friction field and the bed friction field. Moreover, the latter can also be considered as a linear function of the velocity vector field, and hence the problem can be described by a linear equation.

Flow Over the Tip Region of a Flexible Cantilever Wing With the Effect of Angle of Attack

The investigation focused on the conversions of flow structures with a change in angle of attack (AOA) for a flexible cantilever wing, which experienced a self-excited vibration. Stereoscopic particle imaging velocimetry (Stereo-PIV) was utilized to measure the velocity field in the wing-tip region as AOA varied from 0 deg to 12 deg. At the Reynolds number (Re) of 3 × 104, instability waves shedding from the wing were amplified as they propagated and developed into Karman Vortex Street in the far downstream region at low AOAs (AOA = 4 deg and 6 deg). As AOA increased to 8 deg with the wing model was still steady, the Karman Vortex Street no longer existed. The wing started to vibrate at AOA = 10 deg owing to the self-excited vibration, and the Karman Vortex Street appeared again. The inception location of the Karman Vortex Street moved further upstream than in the cases at AOA = 4 deg and 6 deg. A new vortex structure, secondary vortex-pairs, appears outside the main wing-tip vortex (WTV).

Effect of velocity ratio on the interaction between plasma synthetic jets and turbulent cross-flow

Plasma synthetic jet actuators (PSJAs) are particularly suited for high-Reynolds-number, high-speed flow control due to their unique capability of generating supersonic pulsed jets at high frequency (${>}5$ kHz). Different from conventional synthetic jets driven by oscillating piezoelectric diaphragms, the exit-velocity variation of plasma synthetic jets (PSJs) within one period is significantly asymmetric, with ingestion being relatively weaker (less than $20~\text{m}~\text{s}^{-1}$) and longer than ejection. In this study, high-speed phase-locked particle image velocimetry is employed to investigate the interaction between PSJAs (round exit orifice, diameter 2 mm) and a turbulent boundary layer at constant Strouhal number (0.02) and increasing mean velocity ratio ($r$, defined as the ratio of the time-mean velocity over the ejection phase to the free-stream velocity). Two distinct operational regimes are identified for all the tested cases, separated by a transition velocity ratio, lying between $r=0.7$ and $r=1.0$. At large velocity and stroke ratios (first regime, representative case $r=1.6$), vortex rings are followed by a trailing jet column and tilt downstream initially. This downstream tilting is transformed into upstream tilting after the pinch-off of the trailing jet column. The moment of this transformation relative to the discharge advances with decreasing velocity ratio. Shear-layer vortices (SVs) and a hanging vortex pair (HVP) are identified in the windward and leeward sides of the jet body, respectively. The HVP is initially erect and evolves into an inclined primary counter-rotating vortex pair ($p$-CVP) which branches from the middle of the front vortex ring and extends to the near-wall region. The two legs of the $p$-CVP are bridged by SVs, and a secondary counter-rotating vortex pair ($s$-CVP) is induced underneath these two legs. At low velocity and stroke ratios (second regime, representative case $r=0.7$), the trailing jet column and $p$-CVP are absent. Vortex rings always tilt upstream, and the pitching angle increases monotonically with time. An $s$-CVP in the near-wall region is induced directly by the two longitudinal edges of the ring. Inspection of spanwise planes ($yz$-plane) reveals that boundary-layer energization is realized by the downwash effect of either vortex rings or $p$-CVP. In addition, in the streamwise symmetry plane, the increasing wall shear stress is attributed to the removal of low-energy flow by ingestion. The downwash effect of the $s$-CVP does not benefit boundary-layer energization, as the flow swept to the wall is of low energy.

Effect of vane sweep angle on vortex generator wake

The effect of varying the vane sweep angle of different vortex generators (VGs) on the statistics of the wake flow and the coherent structures was investigated. Pairs of rectangular, trapezoid, and delta vanes with sweep angle varying in ascending order were arranged at an equal but opposite angle with respect to the flow. A single rectangular vane was also investigated to identify the effect of vane pairing. The VGs were installed in the thin laminar boundary layer of a flat plate at Reynolds number of 930, based on VG height and free-stream velocity. Time-resolved tomographic particle image velocimetry (tomo-PIV) was carried out in a volume covering the wake, and stereo-PIV was applied to two cross-flow planes of the wake. The measurements showed two counter-rotating streamwise vortices, which induce a strong upward motion along the centerline of the wake. A pair of secondary counter-rotating streamwise vortices were also observed. The single rectangular vane showed a primary streamwise vortex and a weaker secondary streamwise vortex with opposite rotation. The evaluation of wall-normal momentum transport along the wake centerline showed that the paired configuration enhanced flow mixing. The investigation of the instantaneous coherent structures and proper-orthogonal-decomposition of the three-dimensional velocity fluctuations indicated that the coherence and strength of the vortices were inversely proportional to the sweep angle of the VG. The delta vane VG, with the largest sweep angle, produced more small-scale turbulence while the rectangular VG, with an upswept vane, produced the most coherent streamwise vortices. The investigation of wall-normal momentum transport showed that the rectangular VG had the best performance in improving flow mixing, followed by the trapezoidal, single rectangular, and delta VG. When evaluating the performance of VG using the ratio of mixing enhancement over drag, the trapezoidal VG was the most efficient one. The investigation confirmed the possibility of lowering the device drag while maintaining the effectiveness of VG using an optimum sweep angle. The performance of delta VG was about 60% of the trapezoidal VG, which suggests a large sweep angle can adversely affect the VG performance.