Noise transmission along shock-waves (original) (raw)
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Some physical aspects of shock wave/boundary layer interactions
2009
When the flow past a vehicle flying at high velocity becomes supersonic, shock waves form, caused either by a change in the slope of a surface, a downstream obstacle or a back pressure constraining the flow to become subsonic. In modern aerodynamics, one can cite a large number of circumstances where shock waves are present. The encounter of a shock wave with a boundary layer results in complex phenomena because of the rapid retardation of the boundary layer flow and the propagation of the shock in a multilayered structure. The consequence of shock wave/ boundary layer interaction (SWBLI) are multiple and often critical for the vehicle or machine performance. The shock submits the boundary layer to an adverse pressure gradient which may strongly distort its velocity profile. At the same time, in turbulent flows, turbulence production is enhanced which amplifies the viscous dissipation leading to aggravated performance losses. In addition, shock-induced separation most often results in large unsteadiness which can damage the vehicle structure or, at least, severely limit its performance. The article first presents basic and well-established results on the physics of SWBLI corresponding to a description in terms of an average two-dimensional steady flow. Such a description allows apprehending the essential properties of SWBLIs and drawing the main features of the overall flow structure associated with SWBLI. Then, some emphasis is placed on unsteadiness in SWBLI which constitutes a salient feature of Communicated by A. Hadjadj.
Propagation of weak shock waves through turbulence
Journal of Fluid Mechanics, 1972
The effect of turbulence on the structure of weak shock waves is investigated. The equilibrium structure is shown to be governed by a balance between nonlinear steepening and the turbulent scattering of acoustic energy out of the main wave direction. The scattered energy appears as perturbations behind the shock front. For conditions typical of sonic booms in atmospheric turbulence the wave structure is governed by a Burgers equation similar to that describing viscous shocks, except that parameters related to the turbulence appear instead of the viscosity coefficient. The magnitude of the perturbations following a shock is estimated from first-order scattering applied to a thickened shock. Predictions of shock thicknesses and perturbations compare favourably with available experimental data. The approach used in the analysis of shock structure is to account for energy scattered from a single wave propagating a long distance through turbulence. This avoids difficulties of physical in...
Near-wall effects on global flow behaviour behind a diffracted shock wave
Accurate prediction of the effects of an adjacent wall on the global flow behaviour behind a diffracting shock wave plays a crucial role in describing high speed flow around complex structures. This research studied the near-wall effects on flow patterns behind a diffracting shock wave on curved walls using both numerical and experimental analysis. The development of the three-shock configuration/s and the unsteady development of secondary and recompression shocks were examined. The flow did not separate behind the diffracting shock for incidence shock Mach numbers less than 1.5. A three-shock configuration was observed from incident shock Mach number 1.5 while two three-shock configurations were observed at higher Mach numbers, though the upper three-shock interaction point faded away with time when the Mach number approached 3.0. For the range of incident shock Mach numbers between 1.5 and 2.0 both secondary and recompression shocks existed, however, the secondary shock did not ex...
A transonic interaction between a shock wave and a turbulent boundary layer is experimentally and theoretically investigated. The configuration is a transonic channel flow over a bump, where a shock wave causes the separation of the boundary layer in the form of a recirculating bubble downstream of the shock foot. Different experimental techniques allow for the identification of the main unsteadiness features. As recognised in similar shock-wave/boundary-layer interactions, the flow field exhibits two distinct characteristic frequencies, whose origins are still controversial: a low-frequency motion which primarily affects the shock wave; and medium-frequency perturbations localised in the shear layer. A Fourier analysis of a series of Schlieren snapshots is performed to precisely characterise the structure of the perturbations at low-and medium-frequencies. Then, the Reynolds-averaged Navier-Stokes (RANS) equations closed with a Spalart-Allmaras turbulence model are solved to obtain a mean flow, which favourably compares with the experimental results. A global stability analysis based on the linearization of the full RANS equations is then performed. The eigenvalues of the Jacobian operator are all damped, indicating that the interaction dynamic cannot be explained by the existence of unstable global modes. The input/output behaviour of the flow is then analysed by performing a singular-value decomposition of the Resolvent operator; pseudo-resonances of the flow may be identified and optimal forcings/responses determined as a function of frequency. It is found that the flow strongly amplifies both medium-frequency perturbations, generating fluctuations in the mixing layer, and low-frequency perturbations, affecting the shock wave. The structure of the optimal perturbations and the preferred frequencies agree with the experimental observations.
A Study of the Complex Flow Features Behind a Diffracted Shock Wave on a Convex Curved Wall
Journal of Applied Fluid Mechanics, 2015
The complex flow features behind a diffracted shock wave on a convex curved wall is investigated using large scale experimentation complemented by numerical computation. The study aimed at explaining the global flow behavior within the perturbed region behind the diffracted shock wave. Experiments were conducted in a purpose built shock tube that is capable of generating a range of incident shock Mach numbers Mn ≤ 1.6. Analysis of higher Mach number shocks on different wall geometries were carried out using numerical code that has been validated by earlier authors. Many flow features that were only distinct at high Mach numbers are clearly identified at low Mach numbers in the present investigation. The separation point moves upstream at incident shock Mach number Mn = 1.5 but moves downstream at higher Mach numbers and is nearly stationary at Mn = 1.6. At incident shock Mach number 3.0 the movement of the separation point tends to be independent of the wall curvature as the wall radius approaches infinity. The present investigation is important in the design of high speed flow devices and in the estimation of flow resistance on supersonic devices and space vehicles.