Shear Layer Vortices Research Articles

A Large Eddy Simulation Study on Hydrogen Microjets in Hot Vitiated Crossflow

Abstract In this paper, the results of a large eddy simulation (LES) study on hydrogen microjets (dj ~ 0.5 mm) injected into a hot (1600 K) vitiated crossflow at different angles?namely, normal (90°) and inclined jet (30°), are presented. The goal is to explore the effects of injection angle on coherent turbulent structure formations, flame-vortex interactions, and wall heat flux contributions. The LES identifies the presence of the horseshoe vortex, the shear layer vortices (SLV), and the counter-rotating vortex pair (CVP), along with the shedding of spanwise-symmetry hairpin vortices in both the normal and inclined jets. The structures in the latter, however, appear more convoluted. In the near field, SLV-induced flow is crucial for mixing and flame propagation on the windward side of the jet, stabilized by autoignition near the jet exits. A flame-shear layer offset is observed here. In the far field, CVP dominates hot vitiated crossflow entrainment, driving flame propagation and heat and species transfer to the leeward side near the injection wall. In the wake region, combustion remains closer to the normal jet exit due to a stronger recirculation zone, resulting in higher wall heat fluxes. The mean wall heat flux values of both the normal and inclined jets decrease and approach each other with moving away from the jet exit in the streamwise direction. The present LES study results are compared to experimental data from the literature by considering instantaneous hydroxide (OH) fields and mean wall heat fluxes.

Flow dynamics and mixing of jet in crossflow with cylindrical cavity

The flow dynamics and mixing characteristics of an air jet issued from a cylindrical cavity in an air crossflow are numerically studied using a large-eddy-simulation technique. The cavity, aligned concentrically with the jet, is located beneath the crossflow wall. The jet-to-crossflow velocity ratio is 4, and the Reynolds number for the jet flow is 1.39×104 based on its diameter and centerline velocity. The presence of the cavity significantly influences the early evolution of the jet and its interaction with the crossflow. Complex vortical structures are observed. Notably, windward vortices on the jet surface increase in size, accompanied by a reduction in the Strouhal number. For a deep cavity, these vortices break down and result in small vortical tubes in the jet streamwise direction due to secondary instability. Also examined are leeward shear-layer vortices, hanging vortices, wake vortices, and the recirculating flow within the cavity. Their roles in the mixing between the jet fluid and the crossflow are identified. The cavity enhances mixing. The effect is significant in the near field but diminishes in the far field. By adjusting the cavity radius and depth, it is determined that the cavity depth exercises a more profound impact on jet evolution and mixing than the cavity radius. The most substantial influence occurs when a narrow and deep cavity is implemented. These findings may serve as guidelines for optimizing cavity design for effective modulation of jet behaviors.

Experimental and numerical study on configuration, shape, distance and angle of attack in film cooling implementing vortex generator

The impact of vortex generator configuration, shape, angle of attack and distance between vortex generator and film cooling hole has been studied using both experimental and numerical methods for enhancing the gas turbine blade’s film cooling efficiency. Infrared (IR) thermography technique has been used for investigating the temperature field. In order to obtain the velocity field ANSYS FLUENT has been implemented. The diameter of the film cooling hole and the cross-flow velocity are used to calculate the Reynolds number, which is set at 2369. The blowing ratio of the jet to the cross-flow has been changed to 0.5, 1.0, and 1.5. Effect of CFU and common flow down configuration has been investigated. The angle of attack has been varied as 35° and 45°. It is observed that common flow down configuration of vortex generator performs better than Common Flow Up configuration. Common flow down configuration as well as lower distance between vortex generator and film cooling hole enhance the film cooling effectiveness due to generation of secondary longitudinal vortices which decrease the strength of the counter rotating vortex structures. Among all the shape (triangular, rectangular and trapezoidal) rectangular vortex generator performs better. Growths of shear layer vortices are modified due to the inclusion of vortex generator. Overall, Vortex Generator with common flow down configuration and 35° angle of attack performs better than Common Flow Up configuration due to generation of secondary longitudinal vortices which annihilates the counter rotating vortex structures due to having rotational tendency opposite to Counter rotating vortex pair.

Flow–acoustic resonance mechanism in tandem deep cavities coupled with acoustic eigenmodes in turbulent shear layers

This study presents the interplay of flow and acoustics within tandem deep cavities, focusing on the resonance mechanism occurring between turbulent shear layers and acoustic eigenmodes. The arrangement inside the tandem deep cavities includes both close and remote configurations. A combined fully coupled and decoupled aeroacoustic simulation strategy was devised. Employing an advanced high-order spectral/hp element method in conjunction with implicit large eddy simulation, the nonlinear compressible Navier–Stokes equations were solved to acquire internal flow–acoustic resonant field. In parallel, the linearized Navier–Stokes equations were tackled to determine coherent shear layer perturbations with external acoustic forcing. Based on acoustic measurements, the mainstream Reynolds number approaches approximately $R{e_{in}} = {O}({10^5})$ , where we identified the presence of frequency lock-in and a resonance range. Aeroacoustic noise sources were examined by implementing spectral proper orthogonal decomposition to decompose the pressure fields into hydrodynamic and acoustic components. As feedback intensified, the flow characteristics by the acoustic forcing effect and the flow-interactive effect were categorized according to the development of concurrent turbulent shear layers. Subsequently, the alternating and synchronous behaviours of concurrent shear layers resonated with the out-of-phase and in-phase acoustic eigenmodes were identified, and the corresponding large-scale counter-rotating vortex pairs and co-rotating vortex structures at the cavity entrances were extracted. The acoustic power generated by the Coriolis force was calculated using Howe's vortex-sound analogy, and the aeroacoustic energy transfer mechanism between large-scale shear layer vortices with acoustic eigenmodes was further explored. Finally, a linear response of coherent perturbations of the concurrent shear layers by external acoustic forcing was established. The amplification of flow in the streamwise direction toward the main duct led to the formation of coherent vortex structures, accompanied by separation bubbles into the main duct.

Tip effects on three-dimensional flow structures over low-aspect-ratio plates: mechanisms of spanwise fluid transport

Three-dimensional flows over low-aspect-ratio rectangular flat plates ( ${A{\kern-4pt}R} = 1.00$ – $1.50$ ) are investigated using tomographic and planar particle image velocimetry techniques. The chord-based Reynolds number is $5400$ , and the angle of attack is fixed at $6^\circ$ . This study reveals for the first time the interplay between spanwise fluid transport and downwash, both originating from the tip effects. Spanwise fluid transport promotes the formation and subsequent coherent development of leading-edge vortices, whereas downwash stabilizes the flow. Specifically, two mechanisms related to spanwise fluid transport are revealed. First, the spanwise fluid transport enhances the intensity of the reversed flow, promoting the shear layer roll-up and vortex shedding. Second, the near-wall spanwise flow interacts with the shed C-shape vortices, thereby strengthening the vortex heads. In particular, through these interactions, spanwise fluid transport can sustain the coherence of the C-shape vortices until the vortex heads split in a regular fashion. Consequently, the C-shape vortices are transformed into novel Þ-shape vortices for the plates of ${A{\kern-4pt}R} \leq 1.25$ , which supplements the previously discovered transformation from C-shape to M-shape vortices for larger ${A{\kern-4pt}R}$ plates. Downstream of this novel vortex-splitting transformation, two fundamental processes contribute to the formation of hairpin vortices. The above comprehensive understanding of complete vortex evolution routine provides valuable insights into the tip effects on the formation of three-dimensional flows over low- ${A{\kern-4pt}R}$ plates.

Effects of axial distance on flow behaviours of continuous jet impinging on a flat surface using a finite confined nozzle

The velocity fields and impinging pressure characteristics of confined jet impinges on a flat surface subject to the influence of various axial stages of a flat plate were experimentally examined. The instantaneous flow patterns, time-averaged flow patterns and velocity characteristics, velocity histories, and impingement pressure distributions were obtained using flow visualisation, particle image velocimeter (PIV), hot-wire anemometer measurement, and pressure detection techniques, respectively. From the analysis of the flow patterns, three characteristics of flow regimes are identified – laminar vortical wake, transition, and turbulent vortical wake – in the domain of Reynolds number and jet-to-wall distance. In laminar vortical wake mode, a ‘fat' pair of counter-rotating vortices appears between the confining surface of the nozzle and the flat plate. This results in a large region of turbulent intensities due to the entrainment of the jet flow. In transition mode, shear-layer vortices appear on the shear layer of the bifurcated jet. In turbulent vortical wake mode, the large momentum of jet fluids impinges the flat plate and deflects radially. The counter-rotating vortices and shear-layer vortices no longer exist in the wake of the bifurcated jet. The large eddies quickly break up into small eddies as the momentum of the jet-dominated flow. No obvious peak value of oscillating frequency was detected in this flow mode along the shear layers of the bifurcated impinging jet. The momentum of the fluid structures is converted into stagnation pressure on the flat plate when a strong jet impinges on it; therefore, the impingement pressure is large, consequently. The impingement pressure in the mode of the turbulent vortical wake was significantly greater than that in the modes of laminar vortical wake and transition.

Review[sbnd]Recent developments on applying acoustic waves for efficiency improvements of different thermofluids systems

This review classified recent works using proper acoustic waves to develop more efficient thermofluids systems for different industrial equipment such as swirl combustor, jet in crossflow, gas flare, different types of heat exchangers, cooling mechanism, aircraft wings, and turbine blades. For the swirl combustor, the range of proper excitation Strouhal number and jet pulsation intensity depends on the type of fuel, geometry of burner, swirl number, and central and annular Reynolds numbers. The combined effects of early jet break up, and centrifugal force induced by proper acoustic waves can lead to higher jet spread rate that can induce higher levels of mixing of the fuel and air in swirl combustor, improving combustion efficiency. For the transverse jet, the range of proper excitation Strouhal number and jet pulsation intensity depends on the type of fuel, geometry, and angle and direction of the burner, and jet and cross flow Reynolds numbers. Proper acoustic excitation can force the transverse jet flow to synchronized shear layer vortices. This phenomenon can cause the new vortices to appear in the upwind shear layer region of the deflected jet; then the spread, penetration, entrainment, and dispersion of the jet significantly increases, improving combustion efficiency. For the aircraft wing, vertical acoustic excitation significantly improves the lift force when the sound frequency is equal to the period of flow fluctuation in the separated boundary layer. For the heat exchanger, the ultrasonic transducer can be installed on the walls of a heat exchanger with direct contact to the fluid, improving the heat transfer coefficient. The physics of fluids behind the efficiency improvements of all mentioned cases were also clarified. Moreover, the acoustically excited impinging jets with higher cooling efficiency applied in different areas are discussed. Proper acoustic forcing can decrease the rate of unstable frequencies of disturbances related to impinging jets over a short distance. This phenomenon can significantly enhance heat transfer and improve cooling efficiency. This review paper provides insight into how acoustic waves could improve the efficiency of thermofluids systems in a variety of applications. Acoustic excitation could have the potential capability to improve the efficiency of new applications such as wind, gas or steam turbines, some types of industrial chemical mixers with parallel plane jets, aircraft wings or those of unmanned aerial vehicles, more efficient combustion chambers, and less polluting gas flares. However, the feasibility of this concept requires further investigation by the community in the future.

Aerodynamic performance increase over an A320 morphing wing in transonic regime by numerical simulation at high Reynolds number

PurposeThis study aims to investigate the morphing concepts able to manipulate the dynamics of the downstream unsteadiness in the separated shear layers and, in the wake, be able to modify the upstream shock–boundary layer interaction (SBLI) around an A320 morphing prototype to control these instabilities, with emphasis to the attenuation or even suppression of the transonic buffet. The modification of the aerodynamic performances according to a large parametric study carried out at Reynolds number of 4.5 × 106, Mach number of 0.78 and various angles of attack in the range of (0, 2.4)° according to two morphing concepts (travelling waves and trailing edge vibration) are discussed, and the final benefits in aerodynamic performance increase are evaluated.Design/methodology/approachThis article examines through high fidelity (Hi-Fi) numerical simulation the effects of the trailing edge (TE) actuation and of travelling waves along a specific area of the suction side starting from practically the most downstream position of the shock wave motion according to the buffet and extending up to nearly the TE. The present paper studies through spectral analysis the coherent structures development in the near wake and the comparison of the aerodynamic forces to the non-actuated case. Thus, the physical mechanisms of the morphing leading to the increase of the lift-to-drag ratio and the drag and noise sources reduction are identified.FindingsThis study investigates the influence of shear-layer and near-wake vortices on the SBLI around an A320 aerofoil and attenuation of the related instabilities thanks to novel morphing: travelling waves generated along the suction side and trailing-edge vibration. A drag reduction of 14% and a lift-to-drag increase in the order of 8% are obtained. The morphing has shown a lift increase in the range of (1.8, 2.5)% for angle of attack of 1.8° and 2.4°, where a significant lift increase of 7.7% is obtained for the angle of incidence of 0° with a drag reduction of 3.66% yielding an aerodynamic efficiency of 11.8%.Originality/valueThis paper presents results of morphing A320 aerofoil, with a chord of 70cm and subjected to two actuation kinds, original in the state of the art at M = 0.78 and Re = 4.5 million. These Hi-Fi simulations are rather rare; a majority of existing ones concern smaller dimensions. This study showed for the first time a modified buffet mode, displaying periodic high-lift “plateaus” interspersed by shorter lift-decrease intervals. Through trailing-edge vibration, this pattern is modified towards a sinusoidal-like buffet, with a considerable amplitude decrease. Lock-in of buffet frequency to the actuation is obtained, leading to this amplitude reduction and a drastic aerodynamic performance increase.

Analysis of vortex structure, temperature distribution, and near-wall flow development for an inclined jet interacting with crossflow of different boundary layer conditions

In modern gas turbines, film cooling confronts complex near-wall flow conditions. Because of the low velocity ratio and the inclined injection in film cooling, the jet is more attached to the wall, making the influence of the local boundary layer critical. This paper investigates the interaction between the inclined jet and the mainstream boundary layer using large eddy simulation (LES). Four inflow boundary layer conditions were investigated, including a thin laminar case (δ/D = 0.5) and three turbulent cases with different thicknesses (δ/D = 0.5, 1.0, and 2.0). The jet velocity ratios are 0.23, 0.46, and 0.91 for each inflow condition. To consistently extract vortices of varying intensities, a local threshold was proposed using λci criterion. Based on the extracted vortices, a comprehensive analysis of the vortical strength, size, and position for horseshoe vortex (HSV), counter-rotating vortex pair (CRVP), and shear layer vortices (SLV) is performed under different inflow conditions. The results provide a clear picture of how HSV and CRVP form and evolve. Quantitative patterns are disclosed for the vortex lifting and vortical decay. Moreover, the thermal transport effects of HSV, CRVP, and SLV are examined. It was proven that these vortices dominate the coolant coverage, coolant core lifting, and thermal diffusion, respectively. Meanwhile, the jet has a significant impact on the near-wall flow development. The length of transition and the magnitude of thickening were discovered to be correlated with the jet velocity ratio and inflow thickness. Overall, these findings present a fresh perspective in understanding the flow and heat transport processes for inclined jet-in-crossflow.

A numerical study on dynamic flows past three tandem inclined elliptic cylinders near moving wall

This numerical study focuses on the dynamic flows past three tandem inclined elliptic cylinders of equal spacing parallel to a moving wall using a lattice Boltzmann method. The gap ratio (G/D=0.6–2.5, where G and D are the gap between the wall surface and cylinder center and major axis, respectively), spacing ratio (L/D=1.5–10, where L is the distance of two adjacent cylinder centers), and inclination angle (α=±15°,±30°,±45°—the angle between normal vector and cylinder's major-axis) are explored at Reynolds number Re = 150 (based on D). The intended analysis links hydrodynamic coefficients, wake structures, and spectral analysis in parameter space of α−G/D−L/D to fluid mechanics. The flow is highly adjustable in this space, dividing into seven regimes: overshoot, continuous reattachment, alternative reattachment, wavy, meandering, quasi-coshedding, and coshedding, which are spatially classified into four modes due to flow interference: shear layer, primary, two-layered, and secondary vortex shedding modes. Transitions between adjacent modes determine three boundaries; and hydrodynamic coefficients differ substantially in parameter space. Due to shadowing, the upstream cylinder has a larger drag coefficient than the middle and downstream cylinders, reducing the drag coefficient of upstream cylinder and the lift coefficient of middle and downstream cylinders. α=±45° has the highest lift oscillation among the three cylinders and a small drag coefficient of the upstream cylinder. The moving wall's proximity effect increases the upstream cylinder's lift coefficient for α<0°, being negligible for high G/D across the full L/D range and stabilizing the lift oscillation of three cylinders.

Numerical Investigation on the Evolution Process of Different Vortex Structures and Distributed Blowing Control for Dynamic Stall Suppression of Rotor Airfoils

The influencing characteristic for the evolution mechanism of a dynamic stall vortex structure and distributed blowing control on rotor airfoils was investigated. Based on the moving-embedded grid method, the finite volume scheme, and Roe’s FDS scheme, a simulation method for the unsteady flow field of a pitch-oscillating airfoil was established. The flow field of the NACA63-218 airfoil was calculated using Reynolds-averaged Navier–Stokes equations. The evolution processes of different vortex structures during dynamic stall and the principal controlled vortex mechanism affecting aerodynamic nonlinearity were analyzed based on the pressure contours Cp and Q of the flow field structure and the spatiotemporal evolution characteristics of the wall pressure distribution. The research indicated that dynamic stall vortices (DSVs) and shear layer vortices (SLVs) were the major sources of the increase in aerodynamic coefficients and the onset of nonlinear hysteresis. Building upon these findings, the concept of distributed blowing control for DSVs and shear layer vortices (SLVs) was introduced. A comparative analysis was conducted to assess the control effectiveness of dynamic stall with different blowing locations and blowing coefficients. The results indicated that distributed blowing control effectively inhibited the formation of DSVs and reduced the intensity of SLVs. This led to a significant decrease in the peak values of the drag and pitch moment coefficients and the disappearance of secondary peaks in the aerodynamic coefficients. Furthermore, an optimal blowing coefficient existed. When the suction coefficient Cμ exceeded 0.03, the effectiveness of the blowing control no longer showed a significant improvement. Finally, with a specific focus on the crucial motion parameters in dynamic stall, the characteristics of dynamic stall controlled by air blowing were investigated. The results showed that distributed air blowing control significantly reduced the peak pitching moment coefficient and drag coefficient. The peak pitching moment coefficient was reduced by 72%, the peak drag coefficient was reduced by 70%, and the lift coefficient hysteresis loop area decreased by 46%. Distributed blowing jet control effectively suppressed the dynamic stall characteristics of the airfoil, making the unsteady load changes gentler.

Aerodynamic Characterization of a Generic High-Speed Projectile Configuration

A combined experimental and numerical study was conducted on a generic projectile configuration with low-aspect-ratio fins. The main objectives of this study were to characterize the aerodynamic behavior and validate numerical simulations for a range of Mach numbers (0.4–4) to facilitate an understanding of major flow features such as forebody and fin-generated shock waves, crossflow shear layer vortex, and fin vortex interactions. Measurements included forces and moments, surface oil flow visualization, and high-speed shadowgraph imaging. Numerical simulations were performed using the CFD++ solver. The results showed an excellent match between the experimental and numerical force and moment data. Pressure contours obtained using numerical simulations were integrated to obtain the contributions of individual components toward the total normal force on the body. Flow visualization results show a few complex and interesting flow features, such as shear layer roll-up, crossflow and fin tip vortex interactions, and shock-wave–boundary-layer interactions. The effects of vortex strength and location were analyzed to determine their contributions to the overall forces on the model. The database generated will be very useful for further validation of the numerical tool and a better understanding of vortex-dominated supersonic flows.

Near-moving-wall flows past three tandem elliptical cylinders at low Reynolds number of 150

Dynamic flows past three tandem elliptic cylinders of equal spacing in parallel arrangement with a moving wall are numerically conducted by the lattice Boltzmann method. The gap ratio (G/D, where G and D are the gap between the wall surface and cylinder center and major axis, respectively) from 0.6 to 2.5 and spacing ratio (L/D, where L is the distance of two adjacent cylinder centers) from 1.5 to 10 are examined at Reynolds number of Re = 150 (based on D). The desired analysis correlates variations of hydrodynamic coefficients, wake structures, and spectral analysis in wide space of G/D and L/D with underlying fluid mechanics. The flow is highly adjustable in G/D−L/D space, dividing into six distinct regimes: overshoot, continuous reattachment, alternative reattachment, wavy, meandering, and coshedding, which are spatially classified into four modes because of flow interference, namely, shear layer, primary, two-layered, and secondary vortex shedding modes. The transition between adjacent modes defines three boundaries. The first boundary always occurs behind the middle or downstream cylinder, whereas the second and third boundaries occur between the upstream and middle cylinders and the downstream cylinder wake. Due to the near-wall flow interference, the hydrodynamic coefficients are highly changeable in L/D−G/D space, signifying the crucially lower drag coefficient of the middle and downstream cylinders than that of a single cylinder. A small G/D determines the increase in lift coefficient of the upstream cylinder as a result of the ground effect, while the shadowing effect of the upstream cylinder induces identical Strouhal numbers of the middle and downstream cylinders.

Seal vibrissa-based three-fourths open-jet wind tunnel nozzle optimization

This study investigated the effects of using bionics in reducing the low-frequency fluctuation phenomenon in three-fourths open-jet automobile wind tunnels. Low-frequency fluctuation in these wind tunnels can cause pulsations of velocity and pressure in the test section, leading to measurement accuracy reduction. The unique shape of the seal vibrissa had been demonstrated to effectively suppress vortex-induced vibrations. Here the wind tunnel nozzle was optimized with seal-vibrissa-shaped protrusions. The effects and corresponding mechanism of seal-vibrissa-shaped nozzle toward jet shear layer vortex structures and low-frequency fluctuation phenomenon were analyzed. A 1:15 scaled three-fourths open-jet wind tunnel was used in the study. The results indicated that after the implementation of the seal-vibrissa-shaped nozzle, the turbulence intensity was decreased at the bottom of the test section and the pressure and velocity fluctuations were effectively suppressed. The three-dimensional vortex structures separating from the surface of vibrissa destroyed the original two-dimensional vortex structures produced by the original nozzle. Many small vortex structures with dispersed frequencies and energies were formed, which decreased the low-frequency fluctuations. The study showed that the seal-vibrissa-shaped nozzle had the potential of inhibiting the large-scale vortex separation in the jet shear layer, which could effectively reduce the low-frequency fluctuations.

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.

Numerical simulations of flow past a backward-facing step under a strong transverse magnetic field

The flow of liquid metal over a backward-facing step (BFS) exhibits unique flow characteristics due to the influence of strong magnetic field. In this study, direct numerical simulation of the BFS flow under a strong magnetic field is conducted based on a quasi-two-dimensional model (with a large interaction number, N≫1, and a large Hartmann number, Ha≫1). The Reynolds number (Re), Hartmann number (Ha), and expansion ratio (ER) are investigated within the ranges of [100−80 000], [100−40 000], and [1.67−5], respectively. Three typical flow regimes are defined based on the evolution of the free shear layer vortices and the separation state of the boundary layer. Furthermore, a comprehensive flow regime map is presented for different ER, revealing a positive correlation between the critical Reynolds number (Rec1) and Ha at the onset of instability. Specifically, Rec1 is proportional to Ha0.5, Ha0.54, and Ha0.56 for ER = 5, 2.5, and 1.67, respectively. Moreover, the maximum relative thickness of the free shear layer at Hac1 appears in the range of approximately 0.7–0.78Lr for ER = 5, while for ER = 2.5, it appears in the range of approximately 0.80–0.85Lr, indicating that the instability position of the free shear layer occurs earlier for large ER. Our numerical investigation also demonstrates that an increase in the transverse magnetic field compresses the free shear layer and delays the process of vortex pairing, thereby suppressing the oscillatory behavior of the shear layer.

Analysis of High-Frequency Dynamics of a Reacting Jet in Crossflow Based on Large Eddy Simulation

Abstract Distributed combustion systems have shown the potential to reduce emissions as well as increase load and fuel flexibility. A characteristic feature of such systems is a reacting jet in crossflow, which exhibits complex vortical structures. In this paper, a generic combustion chamber with elliptic reacting jets in crossflow is examined, operating under lean-premixed conditions at elevated pressure and exhibiting high-frequency transverse mode shapes. It can be seen that depending on the orientation of the elliptical shape of the jet to the crossflow, thermoacoustic modes can be suppressed. A multidimensional fast Fourier transform shows that low aspect ratios (major axis of the jet aligned with the crossflow) result in the mixed 1L1T mode of first longitudinal and first transverse structure, while this mode disappears at high aspect ratios. To get a more detailed insight into the different vortex systems of the various aspect ratios, dynamic mode decomposition is applied. This modal decomposition technique reveals for low aspect ratios a shear layer mode that oscillates at a frequency close to the acoustic mixed mode. For this configuration, a mode representing a flapping motion is also identified. For high aspect ratios, the shear layer vortex increases its frequency and a higher-frequent mode appears in the acoustic spectrum.

Inflow-velocity and rotational effects on revolving and translating wings

An aspect ratio 9.5 rectangular wing is articulated in revolving and translating motions at a 45° angle of incidence and Reynolds number Re=O(300). The effects of rotational (Coriolis and centripetal) accelerations and relative inflow velocity profile on vorticity transport within the leading-edge vortex (LEV) system are independently investigated. For the range of displacements studied (180° rotation and corresponding translational displacement), a stably attached leading-edge vortex (LEV) is observed when rotational accelerations and/or a linearly varying inflow velocity profile is present; however, the inflow velocity profile has a stronger effect on stability of the LEV. LEV vorticity magnitude and lift are significantly augmented when both factors are included (i.e., the full revolving wing case). Vorticity transport analyses are conducted in a planar control region two chords from the axis of rotation, where LEV stability is typically observed on revolving wings at high incidence and at an equivalent spanwise position in the translating case. The fully revolving wing case exhibits a substantially larger leading-edge shear-layer vorticity flux than the other cases, whereas Coriolis tilting makes little contribution to regulation of LEV strength. A correlation is found between the spanwise convective flux and tilting flux contributions in all cases. Decomposition of the spanwise convective flux term demonstrates that the two phenomena are kinematically linked and, together, define a new out-of-plane convective flux term that captures the essence of the spanwise convective flux. The role of this term and the effect of rotational accelerations on it are examined.

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.

Mixing enhancement of a compressible jet over a convex wall

A highly compressive effect would suppress the mixing of the shear layer in a convex wall jet. The spanwise distributed protrusions at the nozzle lip are employed to achieve mixing enhancement in this study. The mixing characteristics and enhancement mechanisms are numerically investigated by the delayed detached-eddy simulation method based on the two-equation shear-stress transport model. A widely applicable flow spatiotemporal analysis method, called proper orthogonal decomposition (POD), is used to gain further insight into the dynamical behaviours of the flow instability mode. The results reveal that the centrifugal effect maintains and amplifies the initial perturbations induced by the spanwise distributed heterogeneities, resulting in forced streamwise vortices. The instabilities induced by the streamwise vortices significantly increase the growth rate of the jet half-width and the shear layer vorticity thickness. The spanwise wavelength of the streamwise vortices is consistent with the spanwise distributed forced excitation. In addition, the spanwise meandering motion of the streamwise vortices is observed, which is usually associated with the streamwise travelling wave. This is further confirmed by the POD analysis of the spanwise velocity fluctuation in both stream-radial and stream-span sections. Also, the spatial distributions of the POD modes with the highest energy provide information on the secondary instability modes. Both sinuous and varicose types of disturbances are observed in the unforced jet, whereas the forced jet seems to be dominated by the sinuous type instability, which is more easily excited than the varicose type instability. Moreover, the turbulence intensity in the forced jet is also significantly enhanced as expected due to the earlier and stronger streamwise vortices and associated instabilities. The enhanced turbulent characteristics of the highly compressible condition tend to be isotropic, whereas in the unforced jet, it is anisotropic due to the strong compressibility suppressing the spanwise turbulent fluctuations.