Compression Ramp Research Articles

Relaminarization effects in hypersonic flow on a three-dimensional expansion–compression geometry

This experimental work explores the flow field around a three-dimensional expansion–compression geometry on a slender cone at Mach 8 using high-frequency pressure sensors, high-frame-rate schlieren, temperature-sensitive paint, shear-stress measurements and oil-flow visualizations. The $7^\circ$ cone geometry has a hyperbolic slice which acts as an expansion corner and suppresses the disturbances present in the upstream boundary layer. Downstream of the cone-slice corner, high-frequency boundary-layer disturbances attenuate in all cases. Under laminar conditions, second-mode instabilities from the cone diminish and lower-frequency second-mode waves develop on the slice at a frequency commensurate with the increased boundary layer thickness. For fully turbulent cases, the boundary layer over the slice shows evidence of a two-layered nature with a turbulent outer region and a near-wall region with strong attenuation of high-frequency disturbances and reappearance of lower-frequency instability waves. When a downstream compression ramp is added to the slice, the expanded boundary layer shows enhanced susceptibility to separation such that separation is observed at a $10^{\circ }$ deflection, which is smaller than expected for turbulent conditions. For a $30^{\circ }$ ramp, boundary-layer separation occurs further upstream where the heat flux contours show a decrease in heating that is characteristic of a transitional separation. These results demonstrate the effect of relaminarization caused by an upstream expansion on a subsequent shock-wave/boundary-layer interaction.

A synthetic particle group method for the unsteady interaction between shock wave and boundary layer past a compression ramp

To overcome the lack of resolved turbulent flow structures generated by the improved delayed detached eddy simulation (IDDES) model when simulating the shock wave and boundary layer interaction (SWBLI), a new, nonzonal, embedded synthetic turbulence generation method, named the synthetic particle group method (SPOM), is proposed and validated by simulating the supersonic flow past a flat plate at Mach 2.25. In the attached boundary layer, the SPOs are automatically introduced to trigger the turbulent perturbations. The turbulent boundary layer is successfully resolved and fully developed in a short streamwise range to produce the desired outcome. The effect of grid scale is also studied, and a suitable grid-scale value is recommended. Then, the SPOM is applied to the compression ramp flow at Mach 2.25 with unsteady SWBLI. The results show that the IDDES-SPOM is highly effective, accurate, and able to model scenarios where the original IDDES simulation fails. Furthermore, the effects of the inlet boundary layer thickness on local separation unsteadiness are studied. From the results, the lower frequency motion is attributed to a more fully developed boundary layer.

A Multifunctional Cementitious Composite for Pavement Subgrade.

Premature failure and degradation of layers are the main problems for transportation infrastructure. Addressing these issues necessitates implementing structural health monitoring (SHM) for pavement construction layers. To this end, this research investigated the stress/strain and damage detection capabilities of a self-sensing cementitious composite developed for potential utilization in the construction of an intelligent subgrade layer. The prepared self-sensing cementitious composite consisted of 10% cement and hybrid conductive fillers, including multiwalled carbon nanotubes (MWCNTs) and graphene nanoplatelets (GNPs) in sand. Initial findings reveal that the electrical resistivity of the composite is significantly affected by the concentration of MWCNTs/GNPs, with a minimum concentration of more than 0.5% needed to achieve a responsive cementitious composite. Moreover, the piezoresistive analysis indicates that an increase in the concentration of MWCNTs/GNPs and stress levels leads to an improvement in the stress/strain-sensing performance. When the self-sensing cementitious composite is subjected to equivalent stress levels, variations in the fractional changes in resistivity (FCR) exhibit an increasing trend with decreasing resilient modulus, stemming from a decrease in stiffness due to the increased concentration of MWCNTs/GNPs. Additionally, the electrochemical impedance spectroscopy (EIS) analysis demonstrates a contraction for the Nyquist plots under compressive ramp loading prior to failure, followed by the expansion of these curves post-failure. Scanning electron microscopy (SEM) images visually showcase the bridging effects of MWCNTs and the filling effects of GNPs within the composite structure.

Coexistence of different mechanisms underlying the dynamics of supersonic turbulent flow over a compression ramp

Supersonic turbulent flow over a compression ramp is studied using wall-resolved large eddy simulation with a freestream Mach number of 2.95 and a Reynolds number [based on δ0: the thickness of incoming turbulent boundary layer (TBL)] of 63 560. The unsteady dynamics of the present shock wave/turbulent boundary layer interaction (STBLI) flow are investigated by using dynamic mode decomposition techniques, linear and nonlinear disambiguation optimization, local stability analysis (LSA), and global stability analysis (GSA). By analyzing the dynamic system for the STBLI flow, three dynamically important modes with characteristic spanwise wavelengths of 2δ0, 3δ0, and 6δ0 are captured. The 2δ0 mode approximates the spanwise scale of the Görtler-like vortices and Görtler mode of LSA, suggesting the presence of Görtler instability, which is believed to be related to the unsteady motion of streaks downstream of reattachment in the flow. The features of the 3δ0 mode are also observed in large-scale motions of the incoming TBL, implying the existence of a convective mechanism that is excited and maintained by such motions. Additionally, the GSA results show the most unstable mode features a spanwise wavelength of around 6δ0, indicating the existence of global instability that is believed to be related to the oscillating motion of separation shock. The coexistence of these three mechanisms is confirmed. Discussions on the above findings provide an interpretation for low-frequency unsteadiness that the unsteadiness of surface streaks results from the combined effects of the Görtler instability near flow reattachment and the convection of large-scale motions in the incoming boundary layer, while the low-frequency shock motion may be related to a global mode driven by upstream disturbances.

Direct numerical simulation of turbulence amplification in a strong shock-wave/turbulent boundary layer interaction

In this study, the turbulence amplification mechanism within the strong shock-wave/turbulent boundary layer interaction is investigated using direct numerical simulation (DNS) over a 24° compression ramp with Mach 2.9 flow. A new in-house solver based on the compact finite difference scheme is introduced, and its accuracy is validated by comparing the flow statistics with existing DNS and experimental data. Within the DNS findings, two distinct turbulence kinetic energy (TKE) hotspots are identified. In contrast to previous studies, this study sheds light on shocklets, characterized by mid-frequency features, as a key factor contributing to the second TKE amplification, which occurs near the reattachment point. Streamline coordinate analysis reveals that shear effects dominate TKE production over the flow deceleration effect in the shock-wave/turbulent boundary layer interaction. The shear effect induced by the rolling up of the boundary layer initiates the first TKE amplification near the wall region in proximity to the separation point, followed by flow deceleration due to the main shock wave contributing to TKE generation. The initial detachment of the shear layer enhances the shear contribution. While TKE decreases above the separation bubble due to the positive mean velocity gradient, TKE amplifies again due to the flow deceleration caused by the secondary shock wave. In addition, the intermittently spawning shocklets above the bulge structures enhance the shear effect on the TKE production. Moreover, the generated TKE subsequently transfers to the local pressure minimum line, created by the bulges effect, thereby establishing a spatially converged maximum TKE line.

Complaint constant force mechanism using variable stiffness leaf-spring-like beam: Design and experiment

<p style='text-indent:20px;'>This research provides a design for a complaint constant-force mechanism employing a leaf-spring-like beam with variable stiffness to address the negative stiffness stroke underutilization issue of the bistable beam. By designing the compression of the leaf-spring-like beam at each given displacement and the compression ramp slope, this technique allows the constant force mechanism to potentially take full utilization of the bistable beam's negative stiffness component. Innovative design of a complaint constant force mechanism based on positive and negative stiffness is shown in this study. In addition, we conduct simulation analysis and experimental verification. The experimental results illustrate the feasibility and effectiveness of the proposed method.</p>

Direct numerical simulation of compression ramp shock wave/boundary layer interaction controlled by plasma actuator array

Direct numerical simulation of compression ramp shock wave/boundary layer interaction controlled by plasma actuator array is performed to gain a comprehensive understanding of its control outcome and mechanism in this paper. The computation model adopts a 24-deg compression ramp at Mach number Ma∞ = 2.0, whose simulation setup all refers to the experimental reports. First, the plasma actuator array is introduced into the Navier–Stokes (N–S) equation by a heating source model, and the corresponding numerical validation is proved in detail. Then, the flow control effect of plasma actuator array is systematically revealed, including the wave drag reduction and flow separation suppression. The results show that the mean shock wave drag can be decreased by 15% and the characteristic length of separation bubble can be reduced by 28%. At last, a new shock wave/boundary layer interaction control mechanism, called as 3D structure reconstruction, is concluded to guide the compression ramp flow control. Namely, by manipulating the quasi-2D shock wave and separation bubble into a 3D structural morphology in spanwise direction, the overall negative effect induced by shock wave/boundary layer interaction can be mitigated.

Thermochemical non-equilibrium hypersonic flow over a rectangular cavity embedded on a compression ramp

This paper reports a systematic computational investigation that elucidates the fundamental thermochemical non-equilibrium physics that occurs when air at Mach number of 11 encounters a rectangular cavity of aspect ratio L/D = 2.0 embedded on a 25° compression ramp. The mechanistic details of this highly complex flow phenomenon are obtained by solving the compressible form of the Navier–Stokes equations in two dimensions using a finite-volume open-source library. Chemical and thermal non-equilibrium processes are treated using a five-species, 12-reaction chemical kinetics, and a two-temperature model, respectively. Following a detailed validation and grid sensitivity study, two simulations are conducted, one with isothermal boundary conditions and the other with conjugate heat transfer (CHT) to identify the effect of energy transmission to the material on surface heat flux. Fast Fourier transforms and near-wall velocity profiles inside and in the neighborhood of the cavity are used to identify primary oscillatory modes and shear layer dynamics. Two new descriptive states defined as “states I and II,” representative of the minimum and maximum deflection of the shear layer, are used to discuss the dynamical behaviors in the cavity, including the separation region before the cavity, trailing edge effects, frequency analysis of probe data collected at several key locations, and the effect of CHT on surface heat flux. It is found that the flow features at the cavity's center strongly influence the separation upstream of the cavity, and the transrotational temperature near the cavity's trailing edge is strongly correlated with the oscillations of the shear layer.

Starting Characteristics of Curved Compression Ramp Air Intakes with Overboard Spillage

An analytical framework is presented here for estimating the limiting area contraction for a family of curved compression ramp (CCR) supersonic air intakes with self-starting capability via overboard spillage. Flow compression in CCR intakes occurs through a combination of a curved ramp shock wave and a series of compression waves. In general, such a combination offers the best features of the canonical air intake families. That is, pressure recovery in CCR intakes is better than simple ramp (planar ramp) air intakes, and pressure ratios in CCR intakes are better than Prandtl–Mayer air intakes. A modified Kantrowitz criterion for startability is proposed to accommodate the nonuniform Mach number distribution in the cross-stream direction at the intake throat that is inevitably realized in CCR air intakes. The starting characteristics of different design configurations that fall within the family of CCR air intakes are analyzed, and a comparison is made with planar ramp and Prandtl–Meyer intakes. Inviscid flow numerical experiments of intake starting were carried out to successfully verify the starting characteristics predicted by the analytical model. The analytical framework presented here can be employed as a general tool for optimizing air intake design based on startability, efficiency, and other performance considerations.

Experimental investigation of the shock wave/boundary layer interaction at a compression corner and an incident shock wave generator

The separation of the boundary layer is one of the main failures in scramjets due to its influence on the amount of oxygen used in combustion, reducing thrust and specific impulse. The main objective of this paper was to perfom experimental investigations to study the shock wave/boundary layer interaction and boundary layer separation. The experimental investigation was done in the T2 shock tunnel using the throats/nozzles for theoretical Mach numbers of 6.5 and 8.3. Compression ramp (30° and 20°) models made of aluminum and printed on plastic filament in the 3D printer were designed and produced, as well as two shock generators models with 30°. The flow analysis was performed using pressure sensors and the schlieren image technique. In the cases of shock generators, the incident shock wave caused an increase in pressure and reached a plateau inside the separation bubble (from 81.84 kPa to 360.01 kPa). The highest pressure values were related to the reattachment line. Then, the expansion waves cause a reduction in pressure and adjacent phenomena cause increase in pressure. The ramps caused an increase in pressure and an effect close to a plateau. Then, the boundary layer is replaced and the pressure continues to increase until the end of the ramp, obtaining values from 15.9 kPa to 93.8 kPa. The highest pressure values were observed in the region of interaction of the incident shock wave with the reattachment shock. The pressure values between the aluminum and plastic filament models measured different values along the ramp (45.6 kPa and 28.8 kPa) in the last sensor. The images in schlieren were of excellent quality and the technique was highly efficient to visualize the phenomena. As results of these campaigns, it was possible to visualize the adverse pressure gradient, the flow separation structure, including the separation bubble and reattachment of the boundary layer.

Simulations of Compression Ramp Shock Wave/Turbulent Boundary Layer Interaction Controlled via Steady Jets at High Reynolds Number

Shock wave/turbulent boundary layer interaction (SBLI) is one of the most common physical phenomena in transonic wing and supersonic aircraft. In this study, the compression ramp SBLI (CR-SBLI) was simulated at a 24° corner at Mach 2.84 using the open-source OpenFOAM improved delayed detached eddy simulation (IDDES) turbulence model and the “Rescaling and Recycling” method at high Reynolds number 1.57×106. The results of the control effect of the jet vortex generator on CR-SBLI showed that the jet array can effectively reduce the length of the separation zone. The simulation results of different jet parameters are obtained. With the increasing jet angle, the reduction in the length of the separation zone first increased and then decreased. In this work, when the jet angle was 60°, the location of the separation point was x/δ=−1.48, which was smaller than other jet angles. The different distances of the jet array also had a great influence. When the distance between the jet and the corner djet=70 mm, the location of the separation point x/δ=−1.48 was smaller than that when djet=65/60 mm. A closer distance between the jet hole and the corner caused the vortex structures to squeeze each other, preventing the formation of a complete vortex structure. On the other hand, when the jet was farther away, the vortex structures could separate effectively before reaching the shock wave, resulting in a better inhibition of SBLI. The simulation primarily focused on exploring the effects of the jet angle and distance, and we obtained the jet parameters that provided the best control effect, effectively reducing the length of the CR-SBLI separation zone.

Experimental and numerical investigation of shock wave/boundary layer interactions induced by curved back-swept compression ramp

When integrated with supersonic/hypersonic flight vehicles the ramp shock/boundary layer interaction occurs in streamline-traced inward-turning inlets exhibit complex three-dimensional characteristic. Two simplified flow models named the single-sided model and the double-sided model were abstracted to investigate the flow phenomenon in this paper. The single-sided model was designed to focus on curved ramp shock/boundary layer interaction near the corner of the inlet while the double-sided model aimed to expose the coupling mechanism and the evolution process near the symmetry plane of the inlet. The separation region of the single-sided model was observed to develop along the leading edge of the three-dimensional compression ramp using oil flow visualisation. It started at the junction of the ramp and the sidewall, spanned the whole test model, and appeared as a ‘scimitar’ shape with a maximum streamwise length of 0.58δ0. On the other hand, a symmetry separation pattern was observed in the double-sided model. The separation region was ‘crescent-shaped’, originated from the junctions of the ramp and two sidewalls, and coalesced at the symmetry plane, inducing a maximum streamwise separation scale of 0.78δ0 on the symmetry plane. The measured surface pressure distributions showed that the shock wave/boundary layer interactions in the two models behaved similarly around the ramp-sidewall junction.

On the coupling between wall-modeled LES and immersed boundary method towards applicative compressible flow simulations

The current work presents an innovative numerical technique for high-Reynolds/high-Mach number compressible flow simulations in complex configurations. In particular, the research combines a novel wall-modeled large-eddy simulation technique with a sharp-interface immersed boundary method in the framework of high-order numerical schemes. The proposed approach enables the best from wall modeling and the immersed boundary. The first is concerned with accurately dealing with wall flows while avoiding direct control of near-wall resolution; the second is related to efficiently treating arbitrarily complex geometries in Cartesian grids. In particular, the paper extends a well-established wall model already presented by the research group to arbitrarily-shaped bounds. The method yields a minimally invasive technique that switches between wall-resolved and wall-modeled large-eddy simulations according to the local near-wall resolution. Thus, easily fitting aerodynamic simulation needs. After discussing the numerical procedure, the paper provides several benchmarks to demonstrate the validity of the proposed approach. In particular, literature-available direct numerical datasets are mined to compare acquired outcomes. The results’ accuracy is found to be remarkable from low-Mach-channel and -pipe flow configurations up highly-compressible flows concerning the spatial deployment of the boundary layer over a flat plate and the shock-wave/boundary-layer interaction over a compression ramp. Thus, the proposed method appears to be a promising framework for dealing with engineering-relevant flow simulations along with the efficient path of Cartesian meshes combined with massively parallel GPU-accelerated architectures.

Laminar hypersonic boundary layer flow over planar compression ramp with sharp leading edge and upstream influence

The present work aims to study the effects of shock wave — boundary layer interaction in 2D hypersonic flow at the sharp leading edge of a flat plate and a flat plate followed by a compression ramp. A key parameter in this work is the hypersonic interaction parameter χ, defined as χ≡CM∞3/Re∞1/2. This flow is modeled as a perfect gas and its behavior is studied over various compression ramp configurations, ranging from 0°–6°, using a semi-analytic reduced-order model. In this shock-layer analysis we focus on characterizing the viscous boundary layer for an adiabatic flow over an insulated wall, improving on existing semi-analytic methods by obtaining a non-singular solution, and using the Tangent Wedge approximation to match the pressure on the boundary layer edge. Upstream influence resulting from the compression ramp is also investigated with newly developed correlations.

Simulation of High-Enthalpy Turbulent Shock Wave/Boundary Layer Interaction Using a RANS Approach

In the era of space exploration, the scientific community is strongly focusing on the analysis of hypersonic flows in the presence of shock wave/boundary layer interaction. In these conditions, the flow field presents a complex shock structure due to the interaction of different shock waves with the boundary layer. The strong adverse pressure gradient makes the boundary layer separate, giving rise to a separation bubble. In the reattachment zone, the temperature can reach very high values, inducing thermochemical non-equilibrium effects. This research field is recently achieving more and more relevance in aerospace research, as the analysis of turbulent shock wave/boundary layer interaction so far has been mainly focused on perfect gas flows. In this manuscript, a Reynolds averaged Navier–Stokes (RANS) approach is considered, the shear stress transport (SST) model being coupled with the multitemperature approach proposed by Park to investigate thermochemical non-equilibrium effects in hypersonic turbulent shock wave/boundary layer interaction. The first part of the manuscript is devoted to the validation of the solver, and results for low enthalpy flat plate and compression ramp flows are presented. The numerical results are shown to be in good agreement with numerical solutions and experimental measurements. Afterward, the free stream conditions are modulated to make non-equilibrium relevant and analyze a reacting flow.

Coherent structures and turbulent model refinement in oblique shock/hypersonic turbulent boundary layer interactions

In the present study, we investigate the evolution of turbulent statistics and coherent structures in hypersonic turbulent boundary layers at the Mach number of 5 impinged by oblique shock waves generated by the wedge with the angles of 14°, 10°, and 6°, inducing strong, mild, and incipient flow separation, by exploiting direct numerical simulation databases, for the purpose of revealing the underlying flow physics that are of significance to turbulent modeling. We found that the large-scale structures are amplified within the interaction zone, manifested in the form of large-scale low- and high-speed streaks with the spanwise length scale of boundary layer thickness, and gradually decay downstream, the process of which is extremely long. The abrupt variation in the characteristic length, time, and velocity scales as well as the incompatible viscous dissipation of the mean and turbulent kinetic energy results in the incorrect predictions by the Reynolds-Averaged Navier–Stokes (RANS) equation simulations, provided the models are established based on solving the transport equations of the turbulent kinetic equation and its viscous dissipation (k−ε or k−ω models, for instance). To amend this issue, we propose to refine the parameters in the model as the functions of wall pressure, the flow quantities related to multiple flow features. The RANS simulations with the k−ω SST model utilizing the proposed refinement improve greatly the accuracy of the skin friction, wall heat flux, and Reynolds shear stress downstream of the interaction zone, and the wall pressure distributions in hypersonic turbulence over compression ramp, suggesting its promising prospect in engineering applications.

High-frequency instabilities in supersonic compression-ramp flow

We consider high Reynolds number supersonic flow over a compression ramp in the triple-deck formulation. Previous studies of compression-ramp stability have shown rapid growth of high-frequency disturbances in initial-value computations; however, no physical or numerical origin has yet been identified robustly. By considering linear perturbations to steady compression-ramp solutions, we show that instabilities observed in previous studies do not have a growth rate that is described by the integral eigenrelation of Tutty & Cowley (J. Fluid Mech., vol. 168, 1986, pp. 431–456) for a (long-wave) Rayleigh instability. We solve both the temporal and spatial instability problems in the limit of asymptotically large wavenumber $K$ (or equivalently frequency) and show that the growth rate of the instability remains $o(K)$ , being dominated by higher-order terms in the expansion at moderate ramp angles.

The determination of physical dimensions of a hypersonic three-stage compression system considering panel vibration effects

The panel vibration of air intake system that excitated by fluid-structural interaction, engine thrust variation and noise has decisive influences on the performance of an air-breathing hypersonic vehicle. The design of such type of intake system should contain the determination of configurational parameters and physical dimensions by considering both flow and structural vibrations, and a clear design path is still absent. In this paper, an intake system with three-stage external compression ramps is designed under Mach number of 6 and flight altitude of 24 km, and the configurational parameters are determined based on the shock-on-lip criterion, Oswatitsch criterion and viscous effects modification. The mode vibration characteristics of external compression ramps with panel thicknesses of 0.002 to 0.03 m are analyzed, and appropriate natural frequencies are obtained by constraint conditions of fixed four side boundaries with an additional and more realistic constraint of strengthened internal boundary. The flow field of the intake system with forced panel vibrations of compression ramps is simulated, and the effects of vibration amplitude of 0 to 0.11 m and frequency of 10 to 250 Hz on the intake performance, i.e., total pressure recovery coefficient and mass flow coefficient are clarified. The vibration effects of hypersonic flow on the compression ramp structure and intake performance are studied by fluid-structural coupling simulations. Finally, the available physical dimensions of external compression ramps are determined and a design path of intake system is developed by considering both the structural and intake performance.

Experimental investigation on effects of herringbone riblets on shock wave/boundary layer interactions of a compression ramp at Mach 3

In this paper, the effects of herringbone riblets (HRs) on a turbulent boundary layer and the shock wave/boundary layer interactions (SWBLIs) are experimentally investigated at the Mach number of 3. An array of three varied lengths of HRs strips are applied upstream of the separation zone of SWBLIs on a compression ramp model. High-speed schlieren, oil-flow visualization, and the planar laser scattering technique are used to examine the shock pattern and boundary layer developing over the surface of the model. The snapshot proper orthogonal decomposition technique and the Fast Fourier Transform method are applied to study the impact of HRs on the interaction between the shock wave and the boundary layer. The experiments provide convincing evidence that HRs make the separation line wavy and shrink the separation zone by about −39.54% for the longest HRs in the present experiments. Furthermore, it is also revealed that these microscale HRs induce large-scale streamwise vortical structures within the boundary layer as found in incompressible flows. It is believed that these vortices promote momentum transfer within the boundary layer hence providing the dominant mechanism for suppressing flow separation.

Wavelet analysis of supersonic shock-boundary-layer interaction

We present a wavelet analysis of supersonic shock-boundary-layer interaction. We have used direct numerical simulation data for supersonic flow over a compression ramp and performed orthogonal anisotropic wavelets. The wavelet-based method of extracting coherent structures is applied to the flow vorticity field, decomposed into coherent and incoherent contributions using thresholding of the wavelet coefficients. The statistics of the coherent part of vorticity are close to the statistics of the total field. The study aims to improve our understanding of the shock-boundary-layer interaction, the role of vorticity, and the relationship between the flow's coherent and incoherent vorticity components with the near-wall sound. Our analysis shows a substantial correlation between the incoherent part of vorticity components and wall-pressure fluctuations.