Oblique shock

About: Oblique shock is a research topic. Over the lifetime, 6551 publications have been published within this topic receiving 119823 citations.

Numerical study on blast flowfields induced by supersonic projectiles discharged from shock tubes

Abstract: In this paper we report on a numerical study of the blast flowfield generated by a supersonic projectile released from the open-end of a shock tube into ambient air. The Euler equations, assuming axisymmetric flows, were solved using a dispersion-controlled scheme implemented with moving boundary conditions. Two initial test cases were calculated. One of them is for validation of the numerical method and the other for verification of the moving boundary conditions. After good agreement was achieved, four further cases were calculated for examining effects of various projectile speeds and different release times of the projectile after the precursor shock wave was discharged. The present numerical study confirms that complicated transient phenomena exist in the initial stages shortly after projectile release, and that the blast flowfield is much more complex than that which can be inferred from muzzle blast studies where combustion products obscure the flow.

Instability of shock-induced nozzle flow separation

Abstract: We investigate experimentally the causes of jet plume instability and enhanced mixing observed in the exhaust of shock-containing convergent-divergent nozzles. Key features of the internal flow are the separation shock, separation shear layers, and pattern of alternating expansion and compression waves downstream of the shock. We focus on two possible reasons for this instability—the motion of the separation shock and the wave pattern downstream of the shock. The nozzle flow was generated in a planar facility with variable area ratio and pressure ratio, and the motion of the shock was tracked using time-resolved wall pressure measurements. The isolated effect of the wave pattern was investigated in a separate facility wherein a sonic shear layer, simulating the nozzle separation shear layer, was disturbed with compression and expansion waves emanating from a wavy wall. In both instances, the instability of the shear layer was characterized by time-resolved measurements of the total pressure. In the nozzle flow, the amplitude of shock motion increases with shock strength. Correlation of shock motion with shear layer total pressure is virtually absent for weak shocks but becomes significant for strong shocks. However, impingement of stationary waves on the shear layer had no impact on its growth rate. We conclude that the enhanced shear layer instability is strongly coupled to shock motion, and that the wave pattern by itself is not a cause of enhanced mixing. The occurrence of asymmetric separation at large shock strengths is a further contributor to the enhancement of instability.

Shock Reflection in Gas Dynamics

Abstract: This paper is about multi-dimensional shocks and their interactions. The latter take place either between two shocks or between a shock and a boundary. Our ultimate goal is the analysis of the reflection of a shock wave along a ramp, and then at a wedge. Various models may be considered, from the full Euler equations of a compressible fluid, to the Unsteady Transonic Small Disturbance (UTSD) equation. The reflection at a wedge displays a self-similar pattern that may be viewed as a two-dimensional Riemann problem. Most of mathematical problems remain open. Regular Reflection is the simplest situation and is well-understood along an infinite ramp. More complicated reflections occur when the strength of the incident shock increases and/or the angle between the material boundary and the shock front becomes large. This is the realm of Mach Reflection. Mach Reflection involves a so-called triple shock pattern, where typically the reflection of the incident shock detaches from the boundary, and a secondary shock, the Mach stem, ties the interaction point to the wall. The triple shock pattern is pure if it is made only of the incident, reflected and secondary shocks, but of no other wave. As predicted by J. von Neumann, pure triple shock structures are impossible. A common belief was that this impossibility is of thermodynamical nature. We prove here that the obstruction is of kinematical nature, thus is independent either of an equation of state or of an admissibility condition. This holds true for all situations: Euler models, irrotational flows and UTSD, the latter case having been known for a decade. Because the Regular Reflection problem along a wedge gathers several major technical difficulties (a free boundary, a domain singularity, a solution singularity, a mixed-type system of PDEs, a type degeneracy across the sonic line), its solvability is still far from our knowledge, except in the simplest context of potential flows with small incidence, a problem solved recently by G.-Q. Chen and M. Feldman. Good though partial results have been obtained by S. Canic et al. for the UTSD model and by Y. Zheng for the Euler system. As far as the Euler equations are concerned, we improve and derive with higher mathematical rigour our pointwise estimates of 1994. Our improvements concern most of the estimates: • We give a now rigorous proof of the minimum principle for the pressure, whenever the flow is piecewise smooth, • Our new bound of the size of the subsonic domain applies now to data of arbitrary strength and incidence, • This together with the observation that the entropy increases, yields much better pointwise estimates of field variables, • We prove that there must exist a vortical singularity, at least in the barotropic case: the vorticity of the flow may not be square integrable, • Last but not least, we give a rigorous justification that the flow is uniform between the ramp, the pseudo-sonic line and the reflected shock, the latter being straight.

Effect of periodic blowing on attached and separated supersonic turbulent boundary layers

Abstract: Periodic blowing at frequencies up to 5 kHz was used to control the unsteadiness of two-dimensional shockwave/turbulent boundary-layer interactions. Two separate experiments were performed. In the first case, periodic blowing was introduced through a spanwise slot in the wall to produce an unsteady shock-wave/boundarylayer interaction boundary layer on the tunnel wall. In the second case, periodic blowing was introduced into the shock-induced separation bubble formed by a 24-deg compression corner interaction. The incoming flow conditions for both experiments were My. = 2.84, Rejl = 6.5 x 10 7/m, and 80 = 26 mm. Measurements of the fluctuating mass flux and wall pressure were made, and the unsteady flowfield was visualized through stroboscopic schlieren videography. The measurements were conditionally sampled based on the phase of the blowing cycle. The results suggest that (at least in this case) the naturally unsteady shock motion in the compression ramp interaction does not contribute significantly to the turbulence amplification, as had been previously thought. Instead, there is strong evidence to suggest that large-scale motions associated with the maxima in the angular momentum profiles in the downstream boundary layer are responsible for the large mixing observed.