Experimental Investigation of Hypersonic Laminar Flow Over a Compression Ramp

An experimental investigation on the flow separation of a hypersonic laminar boundary layer flow over a compression ramp with angles of 32°, 36° and 40° is carried out in a Mach 5 hypersonic wind tunnel. The detailed structures are measured by particle image velocimetry (PIV), and some typical flow structures, such as a shear layer, separation shock, recirculation zone and reattachment shock, are clearly captured. In the 32° compression ramp flow, the hypersonic laminar flow does not experience flow separation, and the boundary layer always attaches to the ramp surface. When the ramp angle increases to 36°, a typical flow separation appears in the hypersonic laminar flow, and a shear layer and reattachment shock arise within the flow field. As the ramp angle increases to 40°, the separation shock moves upstream, the reattachment shock moves downstream, and the recirculation zone expands. Proper orthogonal decomposition (POD) analysis is performed on the velocity contours for three cases, revealing the spatial structure of the flow field. As the ramp angle increases, the coherent flow structures are captured more effectively by less POD modes, and there are more coherent structures in the flow field of a large-angle compression ramp. Finally, numerical investigations of the flow separation on three different compression ramps are carried out, and the simulation results are consistent with the measurement results.

Investigation on Supersonic Flow Control Using Nanosecond Dielectric Barrier Discharge Plasma Actuators

In this paper, the effects of streamwise Nanosecond Dielectric Barrier Discharge (NS-DBD) actuators on Shock Wave/Boundary Layer Interaction (SWBLI) are investigated in a Mach 2.5 supersonic flow. In this regard, the numerical investigation of NS-DBD plasma actuator effects on unsteady supersonic flow passing a 14° shock wave generator is performed using simulation of Navier-Stokes equations for 3D-flow, unsteady, compressible, and k ‐ ω SST turbulent model. In order to evaluate plasma discharge capabilities, the effects of plasma discharge length on the flow behavior are studied by investigating the flow friction factor, the region of separation bubble formation, velocity, and temperature distribution fields in the SWBLI region. The numerical results showed that plasma discharge increased the temperature of the discharge region and boundary layer temperature in the vicinity of flow separation and consequently reduced the Mach number in the plasma discharge region. Plasma excitation to the separation bubbles shifted the separation region to the upstream around 6 mm, increased SWBLI height, and increased the angle of the separation shock wave. Besides, the investigations on the variations of pressure recovery coefficient illustrated that plasma discharge to the separation bubbles had no impressive effect and decreased pressure recovery coefficient. The numerical results showed that although the NS-DBD plasma actuator was not effective in reducing the separation area in SWBLI, they were capable of shifting the separation shock position upstream. This feature can be used to modify the structure of the shock wave in supersonic intakes in off-design conditions.

Numerical Analysis of Side-loads Reduction in a Sub-scale Dual-bell Rocket Nozzle

A calibrated delayed detached eddy simulation of a sub-scale cold-gas dual-bell nozzle flow at high Reynolds number and in sea-level mode is carried out at nozzle pressure ratio NPR = 45.7. In this regime the over-expanded flow exhibits a symmetric and controlled flow separation at the inflection point, that is the junction between the two bells, leading to the generation of a low content of aerodynamic side loads with respect to conventional bell nozzles. The nozzle wall-pressure signature is analyzed in the frequency domain and compared with the experimental data available in the literature for the same geometry and flow conditions. The Fourier spectra in time and space (azimuthal wavenumber) show the presence of a persistent tone associated to the symmetric shock movement. Asymmetric modes are only slightly excited by the shock and the turbulent structures. The low mean value of the side-loads magnitude is in good agreement with the experiments and confirms that the inflection point dampens the aero-acoustic interaction between the separation-shock and the detached shear layer.

Injection of hydrogen sonic multi-jet on inclined surface at supersonic flow

The efficient fuel injection system is a vital part of scramjet for a high-speed flight. In this scientific study, the effects of the multi fuel jets on the inclined surface are investigated at the supersonic air stream with Mach[Formula: see text]. This study employed the CFD technique for the simulation of the interaction of multi hydrogen-jets with free stream airflow. The impressions of inclined surface angle and hydrogen jet total pressure on the fuel mixing zone are disclosed. The Mach contour on the jet plane is presented to describe the chief terms on the distribution and penetration of hydrogen jet inside the combustion chamber. To perform a 3D computational study, the RANS equation with the SST turbulence model is used. Our study shows that the mixing performance of multi jets enhances in high jet pressure when the angle of the tending exterior is raised. Our findings also demonstrate that power and slant of the separation shock is highly effective on the size of the mixing zone.

An Estimated Search and Matching Model of the Croatian Labor Market: Post-crisis Analysis

The paper specifies a simple search and matching model of the labor market and studies how well the model can describe aggregate Croatian labor market dynamics. The model developed is a discrete-time search and matching model with convex vacancy posting costs and two types of shocks: productivity and separation shocks. The model is estimated on unemployment and vacancy data during the period from 2012 to 2020 by using Bayesian methods. The model fits the data well and the estimation shows that productivity shocks are the main driving force of the fluctuations in the labor market, especially for the case of vacancies and output, while the separation shock process accounts for a large percentage of unemployment fluctuations.

Shock–Boundary Layer-Interaction Control Through Recirculation in a Hypersonic Inlet

This technical brief presents a flow separation mitigation device, called cavity-recirculator that can be used to control flow separation during shock wave–boundary layer interaction (SBLI) in high-speed intake flows. The cavity-recirculator isolates the flow separation bubble generated at the SBLI spot, thereby thinning the boundary layer and reducing the blockage of the inviscid stream in the duct. The device has a potential application in scramjet engine intakes/isolators. The cavity-recirculator was tested on a single-ramp-compression intake model in a hypersonic shock tunnel, in a freestream of Mach 8 (±0.1). The device operation and effectiveness were assessed by flow visualization and pressure measurements in the test model. The measurements and visualization displayed a mitigation of flow separation through an improved flow field with a single shock train and the absence of flow separation shock, in the inlet.

Two-dimensional unsteadiness map of oblique shock wave/boundary layer interaction with sidewalls

The low-frequency unsteadiness of oblique shock wave/boundary layer interactions (SBLIs) has been investigated using large-eddy simulation (LES) and high-frequency pressure measurements from experiments. Particular attention has been paid to off-centreline behaviour: the LES dataset was generated including sidewalls, and experimental pressure measurements were acquired across the entire span of the reflected shock foot. The datasets constitute the first maps of low-frequency unsteadiness in both streamwise and spanwise directions. The results reveal that significant low-frequency shock motion (with $St\approx 0.03$) occurs away from the centreline, along most of the central separation shock and in the corner regions. The most powerful low-frequency unsteadiness occurs off-centre, likely due to the separation shock being strengthened by shocks arising from the swept interactions on the sidewalls. Both simulation and experimental results exhibit asymmetry about the spanwise centre. In simulations, this may be attributed to a lack of statistical convergence; however, the fact that this is also seen in experiments is indicative that some SBLIs may exhibit some inherent asymmetry across the two spanwise halves of the separation bubble. There is also significant low-frequency power in the corner separations. The relation of the unsteadiness in the corner regions to that in the centre is investigated by means of two-point correlations: a key observation is that significant correlation does not extend across the attached flow channel between the central and corner separations.

On the mean structure of sharp-fin-induced shock wave/turbulent boundary layer interactions over a cylindrical surface

Interactions between an oblique shock wave generated by a sharp fin placed on a cylindrical surface and the incoming boundary layer are investigated to unravel the mean features of the resulting shock/boundary layer interaction (SBLI) unit. This fin-on-cylinder SBLI unit has several unique features caused by the three-dimensional (3-D) relief offered by the cylindrical surface that noticeably alter the shock structure. Complementary experimental and computational studies are made to delineate both the surface and off-body flow features of the fin-on-cylinder SBLI unit and to obtain a detailed understanding of the mechanisms that dictate the mean flow and wall pressure features of the SBLI unit. Results show that the fin-on-cylinder SBLI exhibits substantial deviation from quasi-conical symmetry that is observed in planar fin SBLI. Furthermore, the separated flow growth rate appears to decrease with downstream distance and the separation size is consistently smaller than the planar fin SBLI with the same inflow and fin configurations. The causes for the observed diminution of the separated flow and its downstream growth rate were investigated in the light of changes caused by the cylinder curvature on the inviscid as well as separation shock. It was found that the inviscid shock gets progressively weakened in the region close to the triple point with downstream distance due to the 3-D relief effect from cylinder curvature. This weakening of the inviscid shock feeds into the separation shock, which is also independently impacted by the 3-D relief, to result in the observed modifications in the fin-on-cylinder SBLI unit.