Effect of isolated roughness element height on high-speed laminar–turbulent transition

Abstract

Understanding of the roughness-induced laminar-turbulent transition of supersonic and hypersonic flows is partly challenged by the intricate sensitivities presented by different correlation criteria. We investigate experimentally the effect of height for an isolated roughness element of quadrilateral planform. Heat transfer measurements document the enhancement of roughness-induced disturbances -here the associated heat flux perturbation -along a downstream axisymmetric laminar separation. With increasing element height k, a gradual intensification in wake disturbance levels is found for subcritical elements (k/δ k <0.15, where δ k is the undisturbed boundary layer thickness) while elements taller than the effective condition (k/δ k 0.32) bypass the more moderate transition mechanisms to produce a fully turbulent element wake. Results exhibit high sensitivity to flow properties at roughness height between critical and effective conditions. A reduction in wake disturbance levels with increasing height is documented within 0.23 k/δ k 0.32. This effect coincides with a decrease in kinematic viscosity at roughness height ν k (as Mach number at height M k increases from 1.52 to 1.96) and is restricted to elements with strong local separation, whereby the influence of local shear effects is enhanced.

Analysis of the instabilities induced by an isolated roughness element in a laminar high-speed boundary layer

“…In recent years, a significant number of researchers have focused their efforts on studying the stability characteristics of the wake induced by three-dimensional isolated roughness elements in high-speed flow, using both experimental and numerical techniques (Choudhari et al 2010;Groskopf et al 2010a;Kegerise et al 2012;Choudhari et al 2013;De Tullio et al 2013;De Tullio & Sandham 2015;Groskopf & Kloker 2016;Theiss et al 2016;Estruch-Samper et al 2017;Di Giovanni & Stemmer 2018). Given the strong inhomogeneity of the wake flow field, numerical analyses based on stability theory generally employ two-dimensional amplitude functions, leading to two-dimensional local linear stability theory (2D-LST), also known as BiGlobal stability theory (Theofilis 2003), or three-dimensional parabolized stability equations (3D-PSE) (Paredes et al 2015b).…”

Direct numerical simulation of roughness-induced transition controlled by two-dimensional wall blowing

“…As roughness-induced transition is one of the main forms of transition for high-speed vehicles, its control is of great importance for the reduction of surface heat flux and friction drag (Lin 2008; Estruch-Samper et al. 2017). To obtain an effective approach for the control of roughness-induced transition, its mechanism should first be understood.…”