Shock Turbulent Boundary Layer Research Articles

Extremely high wall pressure events in shock wave and turbulent boundary layer interactions using DNS data

This study investigates high-amplitude Extreme Wall Pressure fluctuation Events (EWPEs) in Shock wave/Turbulent Boundary Layer Interactions (STBLIs) through the conditional sampling of direct numerical simulation databases. The aim is to evaluate the effect of STBLIs and their strength on the statistical properties and associated turbulent structures of EWPEs using the conditional-averaging and clustering method. The temporal statistical results show that the occurrence probability and contribution ratio of EWPEs decrease downstream of strong STBLI, but their duration and interval time increase. Regarding two-dimensional wall pressure structures, the large population of small-scale structures becomes more elongated, but strong interactions induce a greater number of large-scale structures. The pairing of wall pressure events with a higher occurrence probability is verified by the joint probability density functions. Conditional analysis reveals that, as the interaction strength increases, the ejection motion associated with positive events occurs farther downstream and the spanwise vortex core locating above negative events is lifted up along the wall-normal direction. Moreover, analysis associates the paired wall pressure events with the sweep, ejection, and swirl motions in STBLIs, where hairpin eddies play an important role in the formation of positive–negative paired wall pressure structures.

Effects of herringbone riblets on shock-wave/turbulent boundary-layer interactions

In the experiment reported in this paper, a spanwise array of herringbone riblets strips is applied upstream of the interaction region between an impinging shockwave and a turbulent boundary layer developing along a flat plate boundary layer at a freestream Mach number of 1.85. High-speed schlieren photography, surface oil flow visualization and PSP are used to examine the effect of herringbone riblets on the characteristics of the shockwave-boundary layer interactions. It shows that despite that the height of the riblets is less than 2 % of the local thickness of the undisturbed boundary layer, a profound modification to the shockwave-boundary layer interaction zone is observed, and the overall size of the separation bubble is decreased. The observed control effects are attributed to the large-scale secondary flow structures produced by the directional riblets. To the best knowledge of the author, this is the first study evaluating the control effect of herringbone riblets on shock wave and turbulent boundary layer interactions in supersonic flow using the experimental method.

Wall pressure fluctuations in wall heated and cooled shock wave and turbulent boundary layer interactions

Wall pressure fluctuations in shock wave and turbulent boundary layer interactions on heated and cooled walls are investigated by performing direct numerical simulations. The objectives of this study are to identify flow dynamics causing low- and high-frequency wall pressure fluctuations and evaluate wall heating and cooling effects on the wall pressure fluctuations. The wall pressure spectra highlight two characteristic low-frequency spectral peaks at the separation point and below the expansion waves. Conditional analysis reveals that the two spectral peaks originate from the oscillations of the reflected shock and expansion waves, coupled with the expansion and contraction motions of the separated turbulent shear layer. Regarding wall temperature effects, wall cooling enhances the magnitude of the wall pressure fluctuations regardless of the low- and high-frequency components. Furthermore, influences of the expansion waves are also enhanced by wall cooling, which increases the wall pressure fluctuations locally after the separation.

Flow and surface loading induced by a supersonic jet over an aft deck

Aircraft designs may include an engine that exhausts over the upper fuselage of the aircraft. This design has many benefits including reduced noise below the aircraft. However, the scrubbing of the high-speed turbulent jet flow over the fuselage has the potential to cause structural fatigue. This paper describes a joint experimental/computational study of the surface pressure fluctuations induced by the supersonic jet flow. The model configuration consists of a rectangular convergent-divergent nozzle that exhausts over a flat plate. The plate is a continuation of the lower lip of the nozzle. Pressure measurements are made at several locations on the plate surface. In addition, schlieren visualization is used to identify the jet's shock cell structure. Numerical simulations are performed using Large Eddy Simulation (LES). Results are compared between the simulations and the experiments. The importance of shock turbulent boundary layer interaction to the surface loading is identified.

Evaluation of novel wall function approach for supersonic flow problems involving separations induced by geometry and shocks

Supersonic combustion ramjet engine involves complex flow physics including shock-turbulent boundary layer interaction (STBLI) and shock-induced separations. The general approach while using an averaged Navier-Stokes equations to capture these flow separation, due to the flow physics and/or geometry accurately, is to use a low-Reynolds turbulence model. This requires the placement of first grid point in the viscous sublayer ( y + ≃ 1 ), which implies a need of finer mesh near the wall leading to an overall increase in number of cells in the computational domain and thus an increase in overall computational time. The wall-function method is a substitute to this approach which allows for the use of a coarser mesh in the region near to the wall. However, the standard wall-function methods are generally not very successful in capturing flow separations. In this work, we have employed a generalized wall function approach which incorporates the effect of both pressure gradient and compressibility in its formulation and have evaluated its performance for complex supersonic flow cases involving shock-turbulence boundary layer interactions (STBLI) and shock induced separations, which have not been done previously. We have shown that this method captures the flow physics accurately even with much coarser meshes near the wall. Further, insensitivity of the method to the placement of first grid point has also been demonstrated.

Experimental investigation on the flame stabilization modes of liquid kerosene in a cavity-based scramjet combustor

The flame stabilization modes of liquid kerosene is investigated in a model scramjet combustor with flight condition of Ma 6. The effects of equivalence ratio and cavity configuration schemes on the flame stabilization modes are studied. Flame propagations and the evolution of the flow field from the ignition to the establishment of stable flame in the supersonic flow are acquired through high-speed photography and schlieren photography. Only cavity-recirculation-region stabilized combustion can be obtained in the single-cavity scramjet combustor. The intensity of combustion is weak and is termed as local ignition with low combustion efficiency. In the tandem-cavity scramjet combustor, the flame stabilization mode is largely determined by the total equivalence ratio. The flame could spread into the mainstream and even propagate against the supersonic flow as the equivalence ratio increases. The positive feedback mechanism between combustion and precombustion shock train is responsible for the evolutions of the flame stabilization modes, where the interactions between shock waves and turbulent boundary layer play a key role in the flame propagation process. The large-scale eddies in the shear layer of the upstream cavity shed at the cavity aft wall and could enhance the turbulent levels of the incoming flow of the downstream cavity. Consequently, an intense kerosene flame can be ignited in the tandem-cavity combustor. The flame could not spread to the mainstream in the single-cavity combustor due to the lower turbulence level weakening the interaction of combustion products in the recirculation zone with unburned reactants in the supersonic flow.

Hypersonic shock wave and turbulent boundary layer interaction in a sharp cone/flare model

A direct numerical simulation of hypersonic Shock wave and Turbulent Boundary Layer Interaction(STBLI) at Mach 6.0 on a sharp 7° half-angle circular cone/flare configuration at zero angle of attack is performed. The flare angle is 34° and the momentum thickness Reynolds number based on the incoming turbulent boundary layer on the sharp circular cone is Reθ = 2506. It is found that the mean flow is separated and the separation bubble occurring near the corner exhibits unsteadiness. The Reynolds analogy factor changes dramatically across the interaction, and varies between 1.06 and 1.27 in the downstream region, while the QP85 scaling factor has a nearly constant value of 0.5 across the interaction. The evolution of the reattached boundary layer is characterized in terms of the mean profiles, the Reynolds stress components, the anisotropy tensor and the turbulence kinetic energy. It is argued that the recovery is incomplete and the near-wall asymptotic behavior does not occur for the hypersonic interaction. In addition, mean skin friction decomposition in an axisymmetric turbulent boundary layer is carried out for the first time. Downstream of the interaction, the contributions of transverse curvature and body divergence are negligible, whereas the positive contribution associated with the turbulence kinetic energy production and the negative spatial-growth contribution are dominant. Based on scale decomposition, the positive contribution is further divided into terms with different spanwise length scales. The negative contribution is analyzed by comparing the convective term, the streamwise-heterogeneity term and the pressure gradient term.

Compressibility effect on interaction of shock wave and turbulent boundary layer

The compressibility effect when an oblique shock wave impinges on a turbulent boundary layer was analyzed with a direct numerical simulation using Helmholtz decomposition. The turbulent intensity near the impinging region is significantly enhanced by the interaction of the shock wave and boundary layer. In particular, the interaction behavior can enhance the dilatational components of the Reynolds stress and velocity fluctuations. This also enhances the negative correlation between the dilatational components of streamwise and normal velocity fluctuations. The dilatation fluctuation in the impinging region increases significantly. Moreover, in the impinging region, the dilatational components of the production term and transport term contribute to the production and transport terms of the kinetic energy equation mainly in the vicinity of the interface. This simulation shows that the interaction behavior of an oblique shock wave and turbulent boundary layer can enhance flow compressibility significantly. In the interaction region, the turbulent intensity of the flow field is stronger than those upstream and downstream. This study provides a theoretical basis for the improvement of other simulation methods and turbulence modeling for the interaction of an oblique shock wave and turbulent boundary layer.

Characteristics of reattached boundary layer in shock wave and turbulent boundary layer interaction

The reattached boundary layer in the interaction of an oblique shock wave with a flat-plate turbulent boundary layer at Mach number 2.25 is studied by means of Direct Numerical Simulation (DNS). The numerical results are carefully compared with available experimental and DNS data in terms of turbulence statistics, wall pressure and skin friction. The coherent vortex structures are significantly enhanced due to the shock interaction, and the reattached boundary layer is characterized by large-scale structures in the outer region. The space-time correlation of fluctuating wall shear stress and streamwise velocity fluctuation reveals that the structural inclination angle exhibits a gradual decrease during the recovery process. The scale interactions are analyzed by using a two-point amplitude modulation correlation. A possible mechanism is proposed to account for the strong amplitude modulation in the downstream region. Moreover, the mean skin-friction is decomposed to understand the physically informed contributions. Unlike the upstream Turbulent Boundary Layer (TBL), the contribution associated with the Turbulence Kinetic Energy (TKE) production is greatly amplified, while the spatial growth contribution induced by the pressure gradient largely inhibits skin-friction generation. Based on bidimensional empirical mode decomposition, the turbulence kinetic energy production contribution is further split into different terms with specific spanwise length scales.

Characteristics of wall-shear stress fluctuations in shock wave and turbulent boundary layer interaction

The wall-shear stress (WSS) fluctuations in the interaction of an oblique shock wave with a flat-plate turbulent boundary layer are investigated by means of direct numerical simulation (DNS) at Mach 2.25. The numerical results agree very well with previous experiments and DNS data in terms of turbulence statistics, wall pressure, and skin friction. The fluctuating WSS characteristics, including probability density function (PDF), frequency spectrum, space–time correlation, and convection velocity, are analysed systematically. It is found that the positively skewed PDF shape of the streamwise WSS fluctuations is significantly changed due to the presence of a separation bubble, while the PDF shape of the spanwise component is slightly affected, exhibiting a symmetric behaviour across the interaction. The weighted power-spectrum density map indicates that the low-frequency unsteadiness associated with the separated shock - exhibits little influence on the spectrum for either component, and no enhancement of the low-frequency energy is observed. A significant reduction in the spatial extent of the two-point correlation is observed, causing spanwise elongated coherence for the streamwise WSS fluctuations in the separation region. Moreover, the elliptic behaviour of the space–time correlations is essentially preserved throughout the interaction, and this is accompanied by a sudden reduction of the convection velocity in the separation bubble.

STATISTICAL CHARACTERISTICS OF PRESSURE FLUCTUATION IN SHOCK WAVE AND TURBULENT BOUNDARY LAYER INTERACTION

Shock wave and turbulent boundary layer interaction widely exists in the internal and external flow of high-speed aircraft. The aerodynamic performance and flight safety of aircraft are seriously affected by the strong pressure fluctuation in the interaction region. To investigate statistical characteristics of fluctuating pressure, the interaction between an incident shock of 33.2° and a spatially developed Mach 2.25 turbulent boundary layer is analyzed by means of direct numerical simulation (DNS). The numerical results have been carefully validated against with previous experiment and DNS at similar flow conditions in terms of mean velocity profile, turbulence intensity and wall pressure distribution. Statistics at the wall and in the outer layer, including fluctuation intensity, power spectral density, two-point correlation and space-time correlation, are quantitatively compared. The differences between them are analyzed in detail. It is found that the effect of the shock interaction on the wall-pressure fluctuation and the fluctuating pressure in the outer layer are utterly different. Based on the analysis of the power spectra density, the fluctuations in the separated region are both characterized by the low-frequency content, but in the reattachment region, the peak frequency of outer pressure fluctuations quickly shifts to higher frequency, with the low-frequency energy of wall-pressure fluctuation still being predominant. It is identified that the two-point correlations of pressure fluctuation at the wall and in the outer layer are both more elongated in the spanwise direction than that in the streamwise direction. The integral scale at the wall is generally increased, while the one in the outer layer increases sharply after passing the shock and then gradually decreases. The analysis of space-time correlation indicates that the iso-correlation contours are similar to the elliptical distribution and the convection velocity deduced by the correlation is dramatically decreased. Downstream of the interaction, the convection velocity in the outer layer is higher than that of wall-pressure fluctuation.

Reflected shock/turbulent boundary layer interaction structure analysis based on large eddy simulation

In order to understand the physical phenomenon of the reflected shock/turbulent boundary layer interaction, the Large Eddy Simulation (LES) is conducted to investigate shock wave and turbulent boundary layer interaction in a 12° compression ramp with inlet high Mach number of 2.9. Rescaling/recycling method is used as inflow turbulence generation technique and validated on a supersonic flat plate turbulent boundary layer. The flow field of recycling plane in the plate computation domain is obtained to give the inlet boundary condition for the LES computation. This paper focuses on the reflected shock/turbulent boundary layer interaction region, where the fine flow structure and instantaneous flow field are analyzed in detail. It is found that the unsteady motion of the shock wave leads to the increase of wall pressure fluctuation.

Experimental study on shock and turbulent boundary layer interactions under different incident shock waves

In this paper, the interactions between shock wave and the turbulent boundary layer of an incoming wall were experimentally studied. The experiments were performed in a Mach 3.4 supersonic low-noise wind tunnel at the unit Reynolds number of 6.30×106 m−1. Under different incident shock wave conditions, fine flow field structure, wall pressure, and wall temperature distribution were determined. Relationship was also analyzed. Instantaneous fine structures of typical flow fields with shock wave and turbulent boundary layer interactions were investigated using the nano-tracer planar laser scattering technique, and the transient flow images were analyzed statistically. The oscillation position of the induced shock wave satisfies a normal distribution. Simultaneously, the wall pressure distribution under different incident shockwave conditions of the interaction region was determined using the flush air data sensing system, and the relationship between flow the structure of the separation region and the wall pressure distribution with different incident shock wave intensities is analyzed. Using a temperature sensitive paint technology, the wall temperature distribution under different incident shockwave conditions of the interaction region model was determined. Also, the relationship between the strip structure caused by Gortler vortices and the incident shock wave intensity in the shockwave/turbulent boundary layer interaction was analyzed.

Experimental Study on Unsteady Characteristics of Shock and Turbulent Boundary Layer Interactions

In this paper, the experimental study on the shock wave and turbulent boundary layer interactions was performed in the Mach 3.4 supersonic low-noise wind tunnel. The angle of the shock generator was θ = 15°, and the unit Reynolds number of 6.30 × 106 m–1. The wall temperature and pressure distribution during the disturbance process of the shock wave and the turbulent boundary layer were obtained based on the temperature-sensitive paints technique and the flush air data sensing system, and the basic flow field of the interactions region was partitioned. Meanwhile, based on the nano-tracer planar laser scattering technique, and the instantaneous fine structures in the interaction region were obtained and the spatiotemporal evolution characteristics of the flow structure were analyzed. The flow visualization images showed that the oscillation position of the induced shock wave satisfied the normal distribution. Compared with the flow visualization images and the temperature results, the correlation between the flow structure of the interactions region and the wall temperature change was obtained. At the same time, the wall fluctuation pressure of the center surface of the shock wave and turbulent boundary layer interactions region was measured by the high-frequency pulsating pressure sensor. The power spectrum density results showed that under the action of the shock wave incident by the shock generator, there were two characteristics frequency signal of 12 and 30 kHz in the induced shock oscillation interval. For the signal of 12 kHz, the frequency value and the amplitude were increased from the turbulent boundary layer to the separation bubble, and the oscillation energy of the induced shock wave was enhanced. The amplitude of the peak signal of each measurement point gradually decreased from the separation bubble to the reattachment zone, and the energy was gradually attenuated. For the high frequency signal of 30 kHz, the frequency variation of each channel was relatively small, relatively stable, and the energy was concentrated.

Computational Analysis of Transverse Sonic Injection in Supersonic Crossflow Using RANS Models

AbstractTransverse injection at sonic speed from a rectangular slot into a supersonic crossflow is numerically explored with an indigenously developed parallel three-dimensional (3D) Reynold-averaged Navier–Stokes (RANS) solver for unstructured grids. The RANS models used for turbulence closure are the one-equation Spalart–Allmaras (SA) model and the two-equation shear stress transport (SST) model. For each model, the influence of compressibility corrections is assessed. Due to the presence of shock-turbulent boundary layer interaction (STBLI) in the flow, various STBLI corrections are assessed for both the models. Most of the simulations are two-dimensional (2D), but three-dimensional simulations are also performed to investigate the mismatch between the experimental dataset and the numerical results. The SA model is less sensitive to STBLI corrections, but some improvement in its prediction of the separation distance is found with the compressibility corrections. The SST model results are insensitive to the compressibility corrections, but the STBLI correction improves its results. Improved agreement with the experimental dataset is found when simulations are done in 3D, suggesting that the experiments were not so close to 2D as previously believed.

A numerical study on the effect of the flow control using bleeding systems in a bump-type inlet

In many supersonic inlets, several oblique shock waves are followed by a terminal normal shock wave. The normal shock-wave/turbulent boundary-layer interaction is critical with respect to its influence on the development of boundary layer throughout the subsonic diffuser and the total pressure recovery at the engine face. In the current study, the bump-type inlet was designed for two oblique shock waves and a normal shock wave. In addition, a porous surface was installed underneath the root of the normal shock wave. The effect of flow control on the interaction between the normal shock wave and turbulent boundary layer in supersonic inlets by using the bleeding system was investigated numerically and was evaluated with respect to the inlet performance parameters.

Characteristics of the cylinder-induced shock wave and turbulent boundary layer interactions

Characteristics of the cylinder-induced shock wave and turbulent boundary layer interactions

Wall modeled large eddy simulation of supersonic flow physics over compression–expansion ramp

In the present work, wall modeled large-eddy simulation (WMLES) in the Fluent software is used to investigate the flow physics of a three-dimensional shock–turbulent boundary layer interaction, as an important phenomenon in aerospace science, on a compression–expansion ramp with the angle of 25°. Fine flow structures are obtained via Laplacian of density that called shadowgraph, in which shock wave structures are visible distinctly. The results are compared with the experimental data of Zheltovodov et al., 1990 [33], in the same condition regarding geometry, boundary conditions, etc. as those used by them. Results show that not only there are a good agreement with experimental trends concerning wall pressure, friction coefficient distribution and mean velocity profiles, but also in comparison with those presented by Grilli et al., 2013 [24]. LES simulation, used in this study, presents more accurate results with fewer computational costs. Afterwards, we investigated the influence of discontinuity in wall temperature, varying stagnation pressure and Reynolds number on physics of flow in order to control the shock behavior. Our simulations shows that, discontinuity in wall temperature, varying free stream stagnation pressure and Reynolds number (the free stream Mach number remained essentially constant) influences the starting point of shock, shock strength, separation length and the collision angle of separated and reattachment shock waves.

A Modified Gas-Kinetic Scheme for Turbulent Flow

AbstractThe implementation of a turbulent gas-kinetic scheme into a finite-volume RANS solver is put forward, with two turbulent quantities, kinetic energy and dissipation, supplied by an allied turbulence model. This paper shows a number of numerical simulations of flow cases including an interaction between a shock wave and a turbulent boundary layer, where the shock-turbulent boundary layer is captured in a much more convincing way than it normally is by conventional schemes based on the Navier-Stokes equations. In the gas-kinetic scheme, the modeling of turbulence is part of the numerical scheme, which adjusts as a function of the ratio of resolved to unresolved scales of motion. In so doing, the turbulent stress tensor is not constrained into a linear relation with the strain rate. Instead it is modeled on the basis of the analogy between particles and eddies, without any assumptions on the type of turbulence or flow class. Conventional schemes lack multiscale mechanisms: the ratio of unresolved to resolved scales – very much like a degree of rarefaction – is not taken into account even if it may grow to non-negligible values in flow regions such as shocklayers. It is precisely in these flow regions, that the turbulent gas-kinetic scheme seems to provide more accurate predictions than conventional schemes.

Visualization of Mach 3.0/3.8 Flow around Blunt Cone with Supersonic Film Cooling

Fine instantaneous flow structures of different scales around a blunt cone with or without supersonic film cooling were visualized via nanotracer planar laser scattering (NPLS), which has a high spatiotemporal resolution. The Mach number of the freestream is 3.0 and 3.8 respectively and the air injection is at Mach 2.5. Lots of typical flow structures were visible clearly, such as shock wave, expansion fan, shear layer, mixing layer, K-H vortices and turbulent boundary layer. With injection, the model wall surface can be covered by a thin film layer. While no injection, the flow is similar to the supersonic flow over a backward-facing step and the structures are simpler relatively and there is a longer laminar region. Flow structures with or without film cooling at Mach 3.0 and 3.8 were compared.