Counter-rotating Vortex Research Articles

Suppression of flow separation around a finite wall-mounted square cylinder by suction at the side leading edge

We investigate the control of three-dimensional flow separation around a finite wall-mounted square cylinder by applying suction at the side leading edge. Direct numerical simulations are conducted at a Reynolds number of 250, with suction ratios Γ of 0–2 (where Γ is the absolute value of the suction velocity divided by the free stream velocity). The effect of Γ on the aerodynamic forces acting on the cylinder is studied. The results show that suction reduces the aerodynamic forces, with the best control effect for the fluctuating lift coefficient (corresponding to a reduction of over 70%) achieved at Γ = 0.375. As the suction ratio increases, the pressure drag experienced by the square cylinder decreases. Simultaneously, the mean frictional drag force exerted on the square cylinder increases. The optimal mean drag coefficient (corresponding to a reduction of nearly 20%) is achieved at Γ = 1. The effect of the suction ratio on the flow topology in the wake is also investigated. Suction significantly suppresses the flow separation. As the suction ratio increases, the spanwise counter-rotating vortices in the streamwise and transverse directions decreases in size, and the downwash vortex shrinks, and shifts toward the free end of the square cylinder. The far-wake streamwise base vortex disappears when active suction is applied to the side leading edge. However, a new pair of base vortices splits from the original base vortex and persists into the far wake flow field, forming a quadrupole vortex structure with the tip vortex.

Flow transitions of head-on vortex ring collisions with contaminated air–water interfaces

An experimental study was conducted on head-on collisions of Re = 2000 and 4000 vortex rings upon air–water interfaces to study the vortex dynamics and the effects of different vortex ring Reynolds numbers on the key vortex flow mechanisms. Unfiltered tap water was used where surface contaminants were present; hence, the interfacial stress levels are lower but not entirely zero like an idealized free surface. Results demonstrate that the vortex dynamics involve first, the resulting secondary and tertiary vortex rings transitioning into wavy states, before their upper loops disconnect/reconnect to the interface to form U-shaped vortex loops along the inner and outer peripheries of the primary vortex ring, respectively, in an alternating pattern. Second, tertiary vortex loops entangle around the primary vortex ring to produce counter-rotating vortex pairs that reorganize themselves along the primary vortex ring outer periphery, between the primary vortex ring and secondary vortex loops, as well as hairpin-like structures that aid ejection of primary vortex ring momentum. Third, secondary vortex loops rotate toward the collision axis before their lower segments are entrained by the primary vortex ring. A higher Reynolds number primary vortex ring would confer additional flow changes, such as a higher wave number for the secondary and tertiary vortex rings/loops, pairings of secondary vortex loops “side-arms” that reduce their instances by about half and formations of Tsai–Widnall–Moore–Saffman instabilities induced by flow perturbations. Finally, vortex flow models proposed to explain the flow mechanism at different flow stages are found to be in good agreements with the experimental visualizations.

Large Eddy Simulation of Pulsed Film Cooling with a Dielectric Barrier Discharge Plasma Actuator

The effects of the coolant pulsation and the plasma aerodynamic actuation (PAA) on the film cooling are herein explored via large eddy simulations. The electrohydrodynamic force derived from the PAA was solved through the phenomenological plasma model. The Strouhal number of the sinusoidal coolant pulsation and the averaged pulsation blowing ratio were 0.25 and 1.0, respectively. Comprehensive analyses were carried out on the time-averaged flow fields, and the results reveal that the pulsed cooling jet might cause a deeper penetration into the crossflow, and this phenomenon could be remarkably mitigated by the downward force of the PAA. Comparing steady film cooling to pulsed film cooling revealed a modest 15.1% reduction in efficiency, while the application of the dielectric barrier discharge plasma actuator (DBDPA) substantially enhanced the pulsed film cooling efficiency by 42.1%. Moreover, the counter-rotating vortex pair (CRVP) was enlarged and lifted off from the wall more poorly due to the coolant pulsation, and the PAA weakened the detrimental lift-off effect and entrainment of the CRVP. Then, the spatial–temporal development of the coherent structures was figured out by the alterations in the centerline temperature, reflecting the formation of the intermittent coherent structures rather than hairpin vortices due to the coolant pulsation, and their size and upcast behaviors were reduced by the PAA; thus, the turbulent integration of the coolant with the crossflow was suppressed fundamentally. Finally, the three-dimensional streamlines confirmed that the coherent structure dynamic behaviors were significantly regulated by the PAA for alleviating the adverse influences of the coolant pulsation. In summary, the PAA can effectively improve the pulsed film cooling efficiency by controlling the spatial–temporal development of the dominant coherent structures.

Numerical study on the kerosene deflection characteristics and regulation methods of supersonic combustor equipped with a 75° swept strut

To promote the application of the swept strut in the supersonic combustor to solve the problem of thermal protection and resistance reduction of the strut injector at high flight Mach number, the kerosene mixing characteristics in the combustor equipped with a swept strut were studied by numerical simulation under the condition of Ma = 2.8, Pt = 1.68 MPa and Tt = 1680K. The results show that the kerosene deflection will occur in the combustor, and the kerosene is mainly enriched in the boundary layer of the combustor wall, which makes the kerosene's local mixing performance poor and the combustion efficiency low. Further analysis indicates that the kerosene deflection characteristics are mainly caused by the transport effect of the counter-rotating vortex pair (CVP) and the pressure distribution characteristics of the strut surface. To improve the phenomenon of kerosene enrichment and the uniformity of kerosene distribution, the regulation of kerosene deflection characteristics was studied from strut configuration design and kerosene distribution of strut injection orifices. The results show that the uniformity of the kerosene distribution can be improved by selecting the appropriate strut height, the trailing-edge swept angle of the strut and the step angle of the strut. The kerosene mixing efficiency and the uniformity of kerosene distribution can be improved by grouping the kerosene injection orifices of the strut and assigning the appropriate equivalence ratio to each group of injection orifices.

Analysis of the local equivalence ratio under fuel-feeding excitations in a lean gas turbine model combustor: Large eddy simulation approach

The synchronization process between the fuel-feeding excitation (ff) and the local equivalence ratio (ϕlocal) is a key issue that has not been well studied numerically under lean conditions. Therefore, in this study, a large eddy simulation (LES) with a wall-adapting local eddy-viscosity (WALE) subgrid model was utilized to analyze the interaction between various fuel-feeding excitations and the local equivalence ratio in a lean partially premixed gas turbine model combustor. The model combustor was simulated using five values of fuel-feeding excitation: 0 Hz, 15 Hz, 50 Hz, 100 Hz and 200 Hz. The results demonstrated that the complex interaction between the swirling flow and the fuel-feeding excitation affected the flow structure. These changes enhanced the excitation of toroidal counter-rotating vortices in the region between the fuel jet exit and shear layers, which improved the homogeneity of ϕlocal inside the inner recirculation zone (IRZ). Furthermore, the fuel-feeding excitation modulated the shear layer thickness by increasing the rolling-up of the instantaneous counter-rotating vortices, leading to spatial regularity for ϕlocal inside the IRZ. The coupling process between the fuel-feeding excitation and the precessing vortex core (PVC) significantly increased the fluctuations of ϕlocal around the fuel jet exit and the shear layer region. An analysis of the probability distribution showed a nonlinear relationship between ff and ϕlocal, where ϕlocal was high when ff = 50 Hz, enhancing the uniformity of ϕlocal inside the IRZ. However, ϕlocal was more spread radially when ff = 0 Hz in the downstream direction. The merging of vortex shedding and the PVC increasingly excited the self-oscillation of ϕlocal, while the fuel-feeding excitation dominated the oscillation of the counter-rotating vortices by increasing the subharmonic vortices. Moreover, the fuel-feeding excitation controlled and organized the small vortical structures, leading to stabilization of ϕlocal when ff = 50 Hz, 100 Hz, and 200 Hz.

Numerical investigate on flow process and mixing characteristics of hydrogen/aluminum powder two-phase fuel under different inlet and injection conditions

The flow and mixing process of a hydrogen/aluminum powder two-phase fuel transverse jet in a supersonic cross flow are simulated in this work using the Euler-Lagrangian scheme. The effects of the inlet Mach numbers (Ma) and jet-to-crossflow momentum flux ratios (J) on the mixing process and flow characteristics of the gas-solid two-phase fuel are investigated. The results show that there are substantial differences in the flow and mixing properties of the two-phase fuel consisting of hydrogen and aluminum powder. While the powder fuel has a minimal impact on the flow field, the interaction of the hydrogen jet with the supersonic cross flow creates complex wave structures in the flow field. Hydrogen is influenced by the large-scale structures of the counter-rotating vortex pair (CVPs) and surface-trailing counter-rotating vortex pairs (TCVPs) in the flow field. In contrast, the powder fuel is influenced by various initial conditions, including the momentum of the hydrogen jet, the momentum of the supersonic mainstream, and the size of the combustor. The mixing efficiency of hydrogen is 7.381%–33.933 % higher than that of the powdered fuel under the same inlet and injection conditions. The comparison showed that the mixing efficiency of hydrogen decreases with the increase of injection momentum and inlet Mach number, while the mixing efficiency of powder fuel increases with increase of injection momentum and inlet Mach number. Among them, the mixing efficiency of hydrogen and powder fuel under the conditions of Ma = 1.6 and J = 0.86 are 0.63 and 0.29, respectively. The mixing efficiency of hydrogen and powder fuel under the conditions of Ma = 3.0 and J = 1.72 are 0.48 and 0.41, respectively.

Flow fields around asymmetrical micro vortex generators in supersonic flow

Motivated by the favorable effects of asymmetrical vortical structures on shock-wave boundary layer interactions, we performed implicit large-eddy simulations to analyze the ability of micro mechanical vortex generators (VGs) to generate such structures. For this purpose, we studied modified VG geometries, by skewing and varying the orientation of a microramp and introducing pairs of unevenly sized ramped-vanes. The modified VGs allow to study the effectiveness of these devices when operating in off-design conditions. The flow physics of all devices were investigated in a supersonic crossflow at Mach 2.5 and a momentum-thickness Reynolds number of 7000. The flow topology around each VG is very similar: the VG acts as an obstacle to the incoming flow and induces a complex system of shocks, recirculation regions, and vortical structures. We present a detailed study of the evolution of the dominant major counter-rotating vortex pair (CVP) to estimate the VGs' flow-control effectiveness. All modified configurations introduce asymmetry into the flow, yet to varying degree for different device types. The modifications made to the microramp have minimal impact on the boundary layer, while the uneven ramped-vane configurations have significant effect. Lateral spread of mechanical VG induced major CVP is not larger than the spanwise dimension of the VG. Consequently, the favorable effect of interactions between VGs arranged in arrays on flow-control effectiveness observed for air-jet VG arrays cannot be reproduced with mechanical VGs. Considering the large design offsets we introduced to the microramp and ramped-vane configurations, both device types are robust to off-design flight conditions.

Boundary layer transition due to distributed roughness: effect of roughness spacing

The influence of roughness spacing on boundary layer transition over distributed roughness elements is studied using direct numerical simulation and global stability analysis, and compared with isolated roughness elements at the same Reynolds number $Re_h=U_eh/\nu$ ( $U_e$ is the boundary layer edge velocity, h is roughness height and $\nu$ is the kinetic viscosity of the fluid). Small spanwise spacing ( $\lambda _z=2.5h$ ) inhibits the formation of counter-rotating vortex pairs (CVP) and, as a result, hairpin vortices are not generated and the downstream shear layer is steady. For $\lambda _z=5h$ , the CVP and hairpin vortices are induced by the first row of roughness, perturbing the downstream shear layer and causing transition. The temporal periodicity of the primary hairpin vortices appears to be independent of the streamwise spacing. Distributed roughness leads to a lower critical roughness Reynolds number for instability to occur and a more significant breakdown of the boundary layer compared with isolated roughness. When the streamwise spacing is $\lambda _x=5h$ , the high-momentum fluid barely moves downward into the cavities and the wake flow has little impact on the following roughness elements. The leading unstable varicose mode is associated with the central low-speed streaks along the aligned roughness elements, and its frequency is close to the shedding frequency of hairpin vortices in the isolated case. For larger streamwise spacing ( $\lambda _x=10h$ ), two distinct modes are obtained from global stability analysis. The first mode shows varicose symmetry, corresponding to the primary hairpin vortex shedding induced by the first-row roughness. The high-speed streaks formed in the longitudinal grooves are also found to be unstable and interact with the varicose mode. The second mode is a sinuous mode with lower frequency, induced as the wake flow of the first-row roughness runs into the second row. It extracts most energy from the spanwise shear between the high- and low-speed streaks.

The flow characteristics for gas jet in liquid crossflow with special emphasis on the vortex-cavity interaction

The objective of this paper is to investigate the complex three-dimensional vortex structures created by the interaction of gas jet with liquid crossflow. A high-speed camera technique is used to record the evolution patterns of the jet cavity. High-precision numerical methods with the Chorin projection method and volume of fluid method (VOF) are employed to understand the complex flow features associated with jet–freestream interaction. The results present that three distinct regions of the jet cavity could be observed, referred to as the transparent cavity region (TCR), the transition region (TR), and the foam cavity region (FCR). The interaction between the flow of gas jet and liquid crossflow creates multiscale vortex structures, including the counter-rotating vortex pair (CVP), the upper-deck counter-rotating vortex pair (up-CVP), the horseshoe vortices, the shear layer vortices, and the fine-scale vortices, respectively. The relationship between multiscale vortex structures and the pulsation of the gas-liquid interface is analyzed in detail by analyzing the spatial distribution of different vortex structures and the fluctuations of the gas-liquid interface. In addition, the effect of the gas entrainment coefficient on cavity flow patterns and vortex structures is compared and analyzed.

Flow-induced vibrations of ten tandem cylinders at low Reynolds number

Flow-induced vibrations of ten cylinders in tandem arrangement are numerically investigated by using the immersed boundary method with a low Reynolds number (Re = 100). Seven spacing ratios L/D (where L = center–center spacing between tandem cylinders and D = diameter of the cylinder) are selected from 1.1 to 4.0, and the reduced velocity Ur ranges from 2.0 to 13.5 with an increment of 0.5. Small (L/D < 2.0) and large (L/D > 2.0) spacing ranges are identified, both including two types of responses: wake-induced vibration (WIV; Ur = 2.0–9.0∼10.5 for a small L/D and Ur = 2.0–6.0∼6.5 for a large L/D) and wake-induced galloping (WIG; Ur > 9.0∼10.5 for a small L/D and Ur > 6.0∼6.5 for a large L/D). The largest vibration amplitude of each cylinder is obtained in the WIG region for the small L/D condition. The presence of downstream cylinders suppresses the vortex shedding of upstream cylinders and thus postpones the vibration of upstream cylinders at a small Ur, whereas the downstream cylinder enhances the vibration at a larger Ur due to the wake interference. For a small L/D, three flow regimes with the extended-body, reattachment, and co-shedding patterns are successively presented as Ur increases. For a large L/D, four types of flow regimes, namely, EB-2S (the extended-body with “2S” pattern), RT-2S (the reattachment with “2S” pattern), TR-2S (two-row vortex street with “2S” pattern), and CS-VS (co-shedding with variation shedding), are classified. Two new vortex shedding patterns, “2G (two counter-rotating vortices shed from each side per vibration cycle)” and “2C (two co-rotating vortices shed from each side per vibration cycle),” have been identified. In the WIV region, there is only one dominant vibration frequency for upstream cylinders (C1–C7), while a sub-harmonic frequency emerges and dominates C8–C10 when L/D is large. The fluctuating lift force spectra show a broad-band frequency distribution due to the irregular positions of the vortex generation and merging, and the dominant frequency in the WIG region decreases consecutively from C1 to C10.

Experimental investigation of a grid plasma jet array in a turbulent boundary layer for skin-friction drag reduction

Toward turbulent skin-friction drag reduction, a novel layout of plasma actuator with a grid anode is devised (grid edge length: L), capable of producing an array of wall-normal plasma jets. The characteristics of this plasma jet array and its interaction with a turbulent boundary layer are investigated experimentally with a particle imaging velocimetry. Results show that the quiescent flow field of the plasma actuator is dominated by a standing vortex ring attached at the inner sides of the square grid, a wall-normal jet flow issued from the grid center, and a downwash flow between adjacent grids. When a tandem array of 11 plasma jets are issued into the cross flow, an equilibrium stage is reached after the third jet. In this stage, the main body of the wall-normal jet bends noticeably to the cross flow, and its leeward side hosts a reverse flow zone, extending downstream to form a slender low-speed wedge (LSW). Two vortical structures are prominent: the streamwise counter-rotating vortex pair residing in the two sides of the jet body, and the arch-shape negative spanwise vortex situated on the LSW root. In the grid-middle plane, the production of turbulence is enhanced across the entire boundary layer, and a second production peak is identified at y+=26. Reduction of the spanwise-averaged wall shear stress is achieved downstream of plasma actuation at x/L≥19.4, and part of the drag reduction fruit earned by ejection is offset by the spanwise transportation of high-velocity fluids toward the middle plane.

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.

Effect mechanisms of leading-edge tubercle on blade cavitation control in a waterjet pump

The bio-inspired leading-edge tubercles (LET) are specifically designed and exploited to control the blade cavitation dynamics under the inlet guide vans (IGVs) wake impacts in a waterjet pump. The experiments, numerical simulation, and modal analysis are jointly carried out to elaborate on the effects mechanisms and cavitation oscillations of the LET pump. It is observed that the tubercle blade cavitation is stably confined in the troughs with a tubular-like pattern and the spanwise coherent development is broken. The tubercles drive the flow turbulent and then form low pressure in the troughs to give rise to cavities. Weak counter-rotating vortex pairs can be identified in the tubercle trough. The analysis of vortex transportation demonstrates that vortex stretching and dilatation would be induced by tubercle cavitation. The loading instabilities obtained by Dynamic Mode Decomposition (DMD) indicate that the dominant oscillations emerge around the tubercles in the blade middle part regarding heavy loading. Due to the large-scale cavities depression by the tubercles, the thrust extremes in some revolutions could be efficiently avoided. In comparison with smooth and tubercle pumps, the thrust energy regarding the tonal and broadband frequency for a single blade and impeller are closing, which need to be further studied to degrade excited force instabilities. This work fundamentally investigates the cavitation instabilities and mechanisms in a tubercle waterjet pump, which could provide a guide on pump design to stabilize cavitation oscillations under uncertain inflow perturbations.

Far field behaviour of wingtip vortices under synthetic jet-based control

Wingtip vortices have been proven to be detrimental to both aircraft efficiency and safety due to their adverse effects such as wake hazard, blade vortex interaction noise and induced drag. Despite the extensive literature on the subject, the number of experimental works featuring far field velocity measurements under active control is very limited. The present work deals with the experimental investigation of the effectiveness of synthetic jet actuation on the control of wingtip vortices and their wake hazard. In order to preserve the mutual induction of the counter-rotating vortices during their evolution, an unswept, low aspect ratio, squared-tipped, finite-span wing is employed. The synthetic jet actuation is based on triggering the inherent instabilities of Crow and Widnall at different values of the momentum coefficient Cμ with the goal of reducing the vortex strength and obtaining an anticipated vortex break up. Different exit geometries of the synthetic jets have been tested to analyze the effects of the jet velocity and position on the wingtip vortices. Phase-locked measurements of the velocity field in the far wake at a distance from the wing trailing edge from 26 to 80 chord lengths have been performed via stereoscopic particle image velocimetry. The effects of blowing at high momentum coefficient Cμ=1% are demonstrated to be remarkable on the mitigation of the wingtip vortices. On the other hand, both the time and phase-averaged results suggest that, at relatively low values of Cμ (0.2%), using a larger synthetic jet exit section area allows to greatly affect the wingtip vortices' features causing a striking alleviation of the vorticity distribution up to 90% with respect to the baseline reference case. In fact, due to the actuation at low frequency, the vortex instability is prematurely risen up and amplified, leading to early vortices linking and their consequent dissipation.

Vorticity forces of coherent structures on the NACA0012 aerofoil

This study examined the impact of coherent structures on the aerodynamic forces exerted on a NACA0012 aerofoil with angles of attack $7.5^{\circ }$ and $10^{\circ }$ , and a chord-based Reynolds number $50\,000$ . The study utilized the spectral proper orthogonal decomposition (SPOD) algorithm to identify the coherent structures, and vorticity force analysis to quantify their impact on lift and drag forces. Results showed that at $10^{\circ }$ , the zeroth frequency of the first SPOD mode had a significant impact on drag and lift forces due to a large vortex structure that caused a strong flow along the suction side of the aerofoil. The first and second frequencies of the first SPOD mode represented asymmetric vortex pairs and a series of vortex pairs that determined the leading-edge separation, respectively. At $7.5^{\circ }$ , the zeroth frequency of the first mode corresponded to an oscillating near-wall stream that followed the reattachment flow pattern, while the first frequency corresponded to a counter-rotating vortex pair that originated where the flow reattaches. Finally, the second frequency of the first mode corresponded to smaller counter-rotating vortex pairs at the shear layer originated near the reattachment point. These findings suggest that coherent structures have a significant impact on aerodynamic forces exerted on aerofoils, and can be identified and quantified using the SPOD algorithm and vorticity force analysis.

Effects of inflow velocity on transverse jet injection in a supersonic cavity combustor

The flow, mixing, and combustion mechanisms in the wide range scramjet engine are complex and far from clear. In the present work, the mixing flow of a sonic transverse jet injection in a supersonic cavity combustor is numerically investigated at two typical inflow velocities. The basic flow structures, unsteady flow dynamics, average flow structures, and several significant mixing performance parameters are well captured and compared based on high-resolution large eddy simulation. The simulation results show that separation shock induced by the jet is gradually merged with the bow shock at low Mach inflow so that the curved shock flow patter is produced. In addition, smaller large-scale coherent structures at the windward side and slower large-scale vortex transport are observed at low Mach inflow. At low Mach inflow, moreover, much narrower range of jet species mass fraction distributions and more upstream large-scale vortices breakdown and dissipation can be observed. The low Mach inflow generates weaker pair of counter-rotating vortices and some trailing counter-rotating vortices, which primarily leads to the weaker jet/cavity interaction. The baroclinic term effects are considerably weaker at low Mach inflow in the near field. In view of mixing efficiency and flammability efficiency, the effect of the cavity in enhancing mixing is more evident at low Mach inflow.

Large-Eddy simulations on interaction flow structures of pulsed jets in a crossflow

Numerical investigations of pulsed jets in a crossflow are carried out using large-eddy simulation. The flow mechanism and coherent structure evolution of pulsed jets with different frequencies (f = 20 Hz, 50 Hz, and 100 Hz) are analyzed and compared with the steady-state jet. The proper orthogonal decomposition (POD) method is used to analyze the effect of different vortex structures in the flow field, which further elaborates the influence mechanism of frequency on pulse jets in the crossflow. The results show that under the same velocity ratio, the normal penetration depth of pulsed jets to the crossflow are greater than that of the steady-state jet. Due to the pulsating effect, the pulsed jets will form a large-scale shear layer vortex, which strengthens the turbulence pulsation and promotes the mixing between jets and the crossflow. As the pulse frequency increases, the scale of the vortex rings in the downstream shear layer will become smaller but the number will increase, and fine vortex clusters will be formed around them. At f = 50 Hz, the counter-rotating vortex pair (CVP) and the jet front shear layer vortex have a greater influence in the flow field, followed by the downstream shear layer vortex, and the wake vortex has the least influence in the flow field. At f = 20 Hz and 100 Hz, the wake vortex has a greater influence in the flow field than that at f = 50 Hz.

Improvement of turbulent flow in a bend using rotational guide vane: A shape optimization

The present study aims to minimize velocity disruption using rotational guide vane (RGV) before the 90° pipe bend where the curvature ratio is r/D = 1.0 at Re = 3 × 104. The combination of computational fluid dynamics and genetic algorithm is used as a tool for the geometric optimization of RGV. The SST k–ω turbulence model was preferred in flow analysis. The number of blade, the blade angle, and the location of RGV are the geometric parameters to be optimized. A new evaluation method of the velocity distribution after the bend is proposed. The objective function is defined as velocity distribution mean squared error that is based on the comparison of the velocity distribution on the cross section after the bend with the fully developed flow in a straight pipe under the same conditions. As a result of a series of optimization processes, velocity distribution at the 90° bend exit is compared between optimized RGV and without guide vane. The results show that the optimized RGV improves velocity distribution at the bend exit. Compared to the without guide vane case, the optimized RGV has delayed flow separation from α = 37° to 56°, and the flow reattachment point moves from x/D = 0.5 to 0 at the bend exit. Thanks to optimized RGV, the counter-rotating Dean vortices merged to form a single vortex at the center of the pipe. In addition, significant reduction in turbulent kinetic energy was observed, approximately 50% when using the optimized RGV compared to the without guide vane case.

Forced Injection of a Spanwise-Inclined Jet in Supersonic Crossflow

Beneficial effects of forced air-jet vortex generators in controlling shock-induced flow separation motivated us to study forced jet injection in detail to understand the advantageous control effects. We performed implicit large-eddy simulations to examine the influence of sinusoidally forced injection of a spanwise-inclined jet in supersonic crossflow. Two forcing frequencies were considered (with and 0.0062) in a crossflow at Mach 2.5 and momentum-thickness Reynolds number of 7000. The underexpansion of the jet has a near-linear dependence on the injection pressure. Consequently, in the near field, the major vortices pulsate with the injection pressure. Statistical analysis using triple decomposition revealed that periodic oscillations of the major counter-rotating vortex pair (CVP) result in momentum transfer by major CVPs, also on their lateral sides, leading to enhanced mixing and lateral spreading, which is beneficial for separation control. Three-dimensional dynamic mode decomposition revealed that forced injection induces a large-scale clockwise rotation of the major vortex. These effects are weak at low forcing frequency. High-frequency jet forcing thus leads to more efficient control of flow separation. On the other hand, periodic oscillations cause these vortical structures to dissipate and breakdown faster in the midfield region than for steady jets. The placement of forced jets is therefore an important parameter when used for separation-control purposes.

Effect of fuel-feeding excitation on the isothermal turbulent swirling flow structure in a lean gas turbine model combustor

Fuel-feeding excitation (ff) is an important factor in lean combustors because it controls the combustion process and maintains a stable mixing process. In this work, the structure of turbulent swirling flow is analyzed via large eddy simulation (LES) for a lean partially premixed gas turbine model combustor. The fuel injection is artificially excited at various frequencies of 0 Hz, 15 Hz, 50 Hz, 100 Hz, and 200 Hz. The LES findings show that all fuel-feeding excitation cases have a bubble vortex breakdown as the predominant flow topology. The size of the corner recirculation zone (CRZ) is constant at ff values of 50 Hz, 100 Hz, and 200 Hz, stabilizing the swirling flow in the upstream direction. The fuel-feeding excitation increases the swirl intensity by a rate of 35% and the instantaneous counterrotating vortices spiral further, creating a modulation for the shear layers that increases the rolling-up of small vortices. A mushroom-shaped vortex is formed near the fuel jet, where its position is fixed when ff = 0 Hz and oscillates periodically toward the downstream direction when fuel-feeding excitation is applied. The mushroom-shaped vortex shrinks to a balloon-shaped vortex and merges with the fuel jet when ff = 200 Hz. For all fuel-feeding excitation cases, the precessing vortex core (PVC) considerably increases the velocity fluctuations in the axial directions by 20%. The fuel feeding excitation improves the production of turbulence near the shear layers and the fuel jet exit by rates of 30% and 20%, respectively. In addition, the fuel-feeding excitation enhances the vorticity generation by small structures with a characteristic size that highly depends on the PVC. According to the proper orthogonal decomposition (POD), the first mode has the most energy compared to the other modes due to the prevalence of the PVC and Kelvin-Helmholtz instability in the shear layer region.