Effects of injection on the instability of boundary layers over hypersonic configurations

Abstract

A high-order numerical method to study hypersonic boundary-layer instability including high-temperature gas effects Phys. Fluids 23, 084108 (2011); 10.1063/1.3614526Direct numerical simulation of breakdown to turbulence in a Mach 6 boundary layer over a porous surface Phys. Fluids 22, 094105 (2010);Computations are performed to study the boundary layer instability mechanisms pertaining to hypersonic vehicles with significant ablative effects. The process of laminar-turbulent transition over vehicles with ablative heat shields can be influenced by both the out-gassing associated with surface pyrolysis and the resulting modification of surface geometry including the formation of micro-roughness. To isolate the effects of out-gassing, this paper examines the stability of canonical boundary layer flows over smooth surfaces in the presence of gas injection into the boundary layer, with an emphasis on the case of massive injection that is relevant to previous laboratory experiments. For a slender cone, the effects of strong out-gassing on the predominantly second mode instability are found to be weakly stabilizing. This new, somewhat surprising result is confirmed by computations carried out on a flat plate boundary layer at high Mach numbers. In contrast, for a blunt capsule flow dominated by first mode instability, the effect of out-gassing is shown to be strongly destabilizing, consistent with the well-known behavior of subsonic boundary layers. C 2013 AIP Publishing LLC. [http://dx.

Blunt-Body Paradox and Improved Application of Transient-Growth Framework

“…Transient growth analysis is performed for a blunt, spherical-section forebody at 28 to iteratively adapt the computational grid to the shock. 1 The axial and wall-normal grid distribution was based upon a prior study of a similar blunt-body configuration involving axisymmetric flow over a hemispherical body at zero angle of attack, 1,17 wherein the computed solution was shown to be insensitive to changes in grid resolution. In the present case with a fully three-dimensional flow field, similar grid resolution was achieved along both surface coordinates and the singularity along the geometric axis of the spherical-section forebody was avoided by using a surface mesh with nonsingular topology.…”

“…Ablation leads to out-gassing and surface roughness, both of which are known to influence the transition process. The effects of out-gassing on disturbance amplification in boundary layer flows over hypersonic configurations were studied by Li et al 1 The present paper seeks to isolate the effects of distributed surface roughness on blunt forebodies in the absence of any outgassing.…”

Numerical Investigation of Roughness Effects on Transition on Spherical Capsules

“…The boundary-layer flow over a hemisphere at Ma = 7.32 is considered to cross-verify the two different transientgrowth implementations used in this work. Details on the basic flow computations were reported by Li et al 32 and the non-modal disturbance growth characteristics of the boundary layer in downstream direction are given in Ref. 22.…”

Measurements of Instability in Supersonic Flow with Injection by Time-Resolved Flow Visualization

“…1 Historically, researchers have been interested in injection for cooling applications in hypersonic flows, both in film and transpiration cooling [2][3][4] and as a surrogate for ablation. [5][6][7][8] More recently boundary-layer injection has been studied as a potential means of delaying laminar-turbulent transition in hypervelocity boundary layers due to acoustic absorption 9, 10 and in scramjet inlets for a variety of benefits. [11][12][13] Injection as a means of cooling on a hypersonic vehicle remains a promising application.…”