Numerical study of flow physics in supersonic base-flow with mass bleed

Numerical investigation of flow structures around a cylindrical afterbody under supersonic condition

Particle image v elocimetry tests were c onducted to assess the effect of core and fan nozzle chevrons on the flow field. A separate flow exhaust system was used to accurately simulate the discharge of a modern, medium bypass, turbo fan engine. Four nozzle configurations were tested; baseline (conical nozzles on both streams), core chevrons (chevrons on core stream and conical nozzle on fan stream), fan chevrons (chevrons on fan stream and conical nozzle on core stream), core and fan chevrons (chevron nozzles on both streams ). Far field acousti c measurements using the same configurations and cycle condition have been reported previously 1 . It was shown that configurations with chevrons on either core or fan nozzle reduce jet noise. It was also shown that a configuration with both core and fan n ozzle chevrons reduces jet noise to a greater degree. Additionally, that reduction was shown to be approximately additive when measured in decibel. That is, the reduction due to core chevrons plus the reduction due to fan chevrons was approximately equal to the reduction due to both chevrons. It was suggested that in general, chevrons modify the noise associated with the shear layer upon which they act; core nozzle chevrons affect the noise associated with the core/fan (inner) shear layer, fan nozzle che vrons affect the noise associated with the fan/free stream (outer) shear layer. The mixing region, which is created by the merging of the inner and outer shear layer, can be affected by either chevron nozzle. The total noise heard in the far field is thu s a superposition of the sources. In this way, jet noise is additive. Near -field acoustic measurements using the same configurations and cycle condition were also reported previously 2 . These results confirm this thesis. Core chevrons significantly redu ce the noise generation region that is typically associated with the inner shear layer. Fan chevrons reduce the noise generation region that is typically associated with the outer shear layer. Both chevrons reduce noise from the mixing region. These eff ects were observed when chevrons were used separately and when chevrons were used together. Results of the current tests demonstrate that in general, core chevrons modify the inner shear layer and the mixed region. Fan chevrons modify the outer shear lay er and mixing region. Variations in t urbulent kinetic energy have been correlated with variations in near field jet noise.

A large eddy simulation of a highly underexpanded supersonic jet impinging on a large plate is reported. The Favre filtered compressible Navier–Stokes equations in cylindrical coordinates have been solved by using a finite difference method. The large eddy simulation code employs a fifth-order weighted essentially nonoscillatory scheme known asWENO for the convective fluxes, a fourth-order centered difference approach for the viscous fluxes, and a four-stage third-order strong-stability-preserving Runge–Kutta technique in time. The numerical simulation successfully captures the shockwave structure in the jet plume and vortex structureswith different scales in the inner and outer shear layers. The evolution of annular shock waves and the vortex in the inner shear layer has been observed. Under the impact of the inner and outer shear layers, the interlacing vortex structures in the wall jet are formed. From this simulation, it can be concluded that the generation and disappearance of the annular shock arise from the actions of theMach disk, the oblique shock wave, vortices in the inner shear layer, and the impinging plate, all combined.

Particle image velocimetry and OH planar laser induced fluorescence are used to examine the flow and flame structure resulting from two adjacent fuel/air nozzles. The distance between nozzles is varied from 1.1 to 2.72 nozzle diameters to change the degree of interaction between the nozzles. Non-reacting PIV shows a flowfield which is nearly symmetric between nozzles for all four spacings. For all but the widest spacing, there exist differences in the flow structures between the inner and outer sides of the nozzles. Less distance between the nozzles results in more rapid merging of the shear layers and higher axial velocities between the nozzles. At a spacing of 1.89 nozzle diameters, the shear layers toward the adjacent nozzle are intermittently pulled into the center of the combustor, resulting in a wider and lower velocity average flow between nozzles compared to the other cases. When a flame is added, the flowfields become much more asymmetric, both between nozzles and between the shear layers toward and away from the interacting nozzle. The outer shear layers, away from the other nozzle, are pushed to the domeplate by the expanding recirculation region. With a higher airflow, this behavior is negated. At the two further spacings, the shear layers toward the adjacent nozzle also become different between swirlers. The flow from the nozzle located on the right stays near the domeplate and joins the shear layer from the left nozzle near the nozzle lip and becomes a strong and penetrating jet at an angle to the axial coordinate. OH PLIF imaging shows that the flame fronts are generally located in the shear layer between the incoming reactants and the recirculating combustion products. This includes cases where the flow is highly asymmetric and stays close to the domeplate or penetrates deep into the combustor. So there is no evidence that the addition of an adjacent nozzle has an effect on the local flow/flame interaction. However, it is also clear that the presence of interacting swirling flows with combustion can lead to very dramatic changes to the global flow behavior relative to a single nozzle experiment.

We compare the space-time correlations calculated from direct numerical simulation (DNS) and large-eddy simulation (LES) of turbulent channel flows. It is found from the comparisons that the LES with an eddy-viscosity sub-grid scale (SGS) model over-predicts the space-time correlations than the DNS. The overpredictions are further quantified by the integral scales of directional correlations and convection velocities. A physical argument for the overprediction is provided that the eddy-viscosity SGS model alone does not includes the backscatter effects although it correctly represents the energy dissipations of SGS motions. This argument is confirmed by the recently developed elliptic model for space-time correlations in turbulent shear flows. It suggests that enstrophy is crucial to the LES prediction of space-time correlations. The random forcing models and stochastic SGS models are proposed to overcome the overpredictions on space-time correlations.

The present article reports experimental and numerical analyses of the macrostructures featured by a stratified swirling flame for varying stratification ratio (SR). The studies are performed with the Beihang Axial Swirler Independently-Stratified (BASIS) burner, a novel double-swirled full-scale burner developed at Beihang University. Experimentally, it is found that depending on the ratio between the equivalence ratios of the methane-air mixtures from the two swirlers, the flame stabilizes with three different shapes: attached V–flame, attached stratified flame and lifted flame. In order to better understand the mechanisms leading to the three macrostructures, large eddy simulations (LES) simulations are performed via the open source Computational Fluid Dynamics software OpenFOAM using the incompressible solver Reacting Foam. Changing the SR, simulation results show good agreement with experimentally observed time-averaged flame shapes, demonstrating that the incompressible LES are able to fully characterize the different flame behaviours observed in stratified burners. When the LES account for heat loss from walls, they better capture the experimentally observed flame quenching in the outer shear layer. Finally, insights into the flame dynamics are provided by analysing probes located near the two separate streams.

The statistical and structural characteristics of two plane jets, one developing along a real wall and the other along a frictionless wall (equivalent to a zero-shear, nondeformable free surface) are compared by way of highly resolved LES (large eddy simulation) solutions at computational conditions close to those of DNS (direct numerical simulation). The aim is to distinguish between two types of influence of the wall on the outer shear layer: one inviscid, arising from wall blocking, and the other, associated with the near-wall shear in the boundary layer. Results are presented for mean-flow properties, second moments and budgets thereof, structural characteristics, and the integral length scale. The comparisons demonstrate that the wall affects a significant proportion of the outer shear layer to a depth of approximately 3 times the thickness of the boundary layer, with or without wall shear. Outside the immediate near-wall layer, in the interaction region, the influence of the wall is affected by an i...

Assessment of Turbulence Models for Predicting the Interaction Region in a Wall Jet by Reference to LES Solution and Budgets

In this paper, the effect of subgrid-scale (SGS) modelling, grid resolution and anisotropy of the subgrid-scales on large eddy simulation (LES) is investigated. LES of turbulent channel flow is performed at Re τ=934, based on friction velocity and channel half width, for a wide range of resolutions. The dynamic Smagorinsky model (DS), the high-pass filtered dynamic Smagorinsky model (HPF) based on the variational multiscale method and the recent explicit algebraic model (EA), which accounts for the anisotropy of the SGS stresses are considered. The first part of the paper is focused on the resolution effects on LES, where the performances of the three SGS models at different resolutions are compared to direct numerical simulation (DNS) results. The results show that LES using eddy viscosity SGS models is very sensitive to resolution. At coarse resolutions, LES with the DS and the HPF models deviate considerably from DNS, whereas LES with the EA model still gives reasonable results. Further analysis shows that the two former models do not accurately predict the SGS dissipation near the wall, while the latter does, even at coarse resolutions. In the second part, the effect of SGS modelling on LES predictions of near-wall and outer-layer turbulent structures is discussed. It is found that different models predict near-wall turbulent structures of different sizes. Analysis of the spectra shows that although near-wall scales are not resolved at coarse resolutions, large-scale motions can be reasonably captured in LES using all the tested models.

The central bleed jet in an axisymmetric, compressible base flow with a Mach 2.46 free stream was visualized using an acetone planar laser-induced fluorescence (PLIF) technique. Three bleed flow rates, a low-bleed case (I=0.0038) and two others that bracket the optimal bleed case (I=0.0113 and I=0.0226), were examined. The injection parameter, I, is defined here as the bleed mass flow rate normalized by the product of the base area and freestream mass flux. This study shows that the bleed jet in the low-bleed case is almost instantly deflected outward toward the outer shear layer after it exits from the bleed orifice. When I=0.0113, the bleed fluid carries enough momentum that the bleed jet remains coherent and generally aligned along the symmetry axis for approximately one base radius before it is deflected outward by the primary recirculation region. For the highest bleed rate examined (I=0.0226), the bleed jet remains coherent and aligned along the symmetry axis for approximately 2.5 base radii downstream, where the impingement of freestream fluid causes the bleed jet to symmetrically eject mass downstream. Because the behavior of the bleed jet is much different for pre- and post-optimal bleed rates, some general conclusions can be made about the effect of bleed rate on base drag. For the two lower-bleed cases, there is a base pressure increase because the bleed fluid partially isolates the primary recirculation region from the outer flow, and provides part of the mass necessary for entrainment in the outer shear layer. The bleed jet in the high-bleed case is not as effective at increasing the base pressure, because the jet is shielded from the outer shear layer by the secondary recirculation region, which exists in the annulus between the bleed orifice and the base corner.

The present work investigates the stability of compressible binary planar jets. Different from a homogeneous jet, where a single chemical species is present, the binary jet may have strong density gradients due to the choice of the chemical species considered in each stream. The goal is to identify the possible instability modes for simple and co-flowing jets and investigate the effect of density gradients on the flow structure, growth rates, unstable frequency range and disturbance phase speed for each mode. The effect of species concentration on free shear layer stability has been reported previously in the literature, but detailed comparisons between stability modes and characteristics for a range of density ratios typical of oxygen and hydrogen mixtures as well as the identification of inner and outer sinuous and varicose modes are new. Linear stability theory is used to determine the stability characteristics of the different configurations. For the co-flowing jet four different modes are found, the inner and outer shear layers both have sinuous and varicose modes. Both for the sinuous and varicose modes the simple jet is more unstable when the fluid with the highest density is at the inner jet, with amplification rates twice as high as the lowest density ratio considered, but the range of unstable frequencies can be four times lower. The sinuous mode is less dispersive than the varicose and the disturbance speeds may vary by one order of magnitude with density ratio. For co-flowing jets the external mode is up to seven times more unstable, but this is due to the choice of the velocity ratio considered. For the inner mode the density gradient has a stabilizing effect regardless of which species is at the center. The co-flowing jet is more dispersive, except for the varicose inner mode. The variation of phase speed with density gradient is not as strong as in the simple jet. The ratio of larges to lower phase speeds are of the order of 2 for the co-flowing jet and 4 for the simple jet.

New data was obtained for a previously studied T-junction experimental setup [1] for a range of flow ratios between hot and cold flows in order to validate new Large Eddy Simulations (LES). The instantaneous velocity field downstream of the T-junction was measured with two-component Particle Image Velocimetry (PIV) in several horizontal and vertical planes at the centre line downstream of the T-junction. The generated PIV database enables a thorough validation of CFD turbulence statistics. The turbulence statistics are shown to be well predicted despite the fact that the mesh in the LES is rather coarse. By usage of time resolved PIV the temporal evolution of the predominant low frequent large-scale structures, responsible for much of the mixing and the high amplitude temperature fluctuations on the walls, were captured. Those structures are, however, weaker in LES than in PIV, being in line with the fact that the wake region behind the penetrating vertical hot jet is underpredicted in LES. Tests regarding the influence of the LES-results to the shape of the inlet boundary conditions (developed or flat symmetric mean-velocity profiles) were carried out and the sensitivity in the results was shown to be small. Furthermore, the results show good agreement with the experimental data independent of the flow ratio between the hot and the cold flows.

The structure of turbulent flows along a transition between tall-forested canopies and forest clearings continues to be an active research topic in canopy turbulence. The difficulties in describing the turbulent flow along these transitions stem from the fact that the vertical structure of the canopy and its leaf area distribution cannot be ignored or represented by an effective roughness length. Large-eddy simulation (LES) runs were performed to explore the effect of a homogeneous variation in the forest leaf area index (LAI) on the turbulent flow across forest edges. A nested grid numerical method was used to ensure the development of a deep boundary layer above the forest while maintaining a sufficiently high resolution in the region close to the ground. It was demonstrated that the LES here predicted first-order and second-order mean velocity statistics within the canopy that agree with reported Reynolds-Averaged Navier–Stokes (RANS) model results, field and laboratory experiments. In the simulations reported here, the LAI was varied between 2 and 8 spanning a broad range of observed LAI in terrestrial ecosystems. By increasing the forest LAI, the mean flow properties both within the forest and in the clearing near the forest edge were altered in two fundamental ways: near the forest edge and into the clearing, the flow statistical properties resembled the so-called back-facing step (BFS) flow with a mean recirculation zone near the edge. Another recirculation zone sets up downstream of the clearing as the flow enters the tall forest canopy. The genesis of this within-forest recirculation zone can be primarily described using the interplay between the mean pressure gradients (forcing the flow) and the drag force (opposing the flow). Using the LES results, a simplified analytical model was also proposed to explain the location of the recirculation zone inside the canopy and its dependence on the forest LAI. Furthermore, a simplified scaling argument that decomposes the mean velocity at the outflow edge into a superposition of ‘exit flow’ and BFS-like flow with their relative importance determined by LAI was explored.

Understanding of the structure of complex supersonic flows requires high-resolution, nonintrusive measurements across the whole field. The measurement requirements are even more challenging when dealing with small-scale systems. In this study, we apply the miniature rainbow schlieren deflectometry system to measure the density field in underexpanded microjets from an orifice injector of 500 μm diameter. The injector is used to replicate the practical scenario of accidental leakage from a compressed gas storage facility. Experiments were conducted for a range of supply pressures P s , although the majority of the results are presented for P s = 860 kPa. Experimental schlieren images were analyzed to determine the density contours in an axisymmetric domain with a field of view of 1.5 mm radius and 7.5 mm length, at a spatial resolution of 25 μm. Pressure, temperature, and Mach number profiles (normalized by the values at the orifice) were also obtained along the jet centerline. Results show features similar to those observed in underexpanded macrojet from a sonic nozzle; multiple shock-cell structures with expansion and compression fans, freejet boundary, incident, normal, and reflected shock waves, subsonic slip strip, and outer shear layer. The miniature rainbow schlieren deflectometry technique is shown to provide high-quality quantitative data to explain the structural details of underexpanded microjets.

Burner geometry effect on coherent structures and acoustics of a confined swirling partially premixed methane flame