Cross-flow-type breakdown induced by distributed roughness in the boundary layer of a hypersonic capsule configuration

Abstract

Direct numerical simulations are undertaken to investigate the nature of instability mechanisms induced by singular and distributed roughnesses on a blunt-capsule configuration. On the base of a capsule-like hemispherical forebody at wind-tunnel conditions ($M=5.9$), we analyse the development of unsteady disturbances behind a patch of two different roughness geometries. First, spanwise periodic roughness elements are considered and cross-validation with other methods of the stability analysis is achieved. Two main unstable modes are found in the roughness wake, corresponding to the symmetric and antisymmetric modes already known for single roughness elements. Second, the case of a patch of (pseudo-)randomly distributed roughness is presented. A new type of roughness-induced cross-flow-like instability is observed for the blunt-capsule configuration. The rapid growth of primary and secondary instabilities in the cross-flow-type vortex is analysed and quantified in both the linear and nonlinear stages up to the laminar–turbulent breakdown. Spatio-temporal Fourier analysis is performed to track the onset of secondary instabilities, whereas laminar–turbulent transition is identified by the steep increase of the wall heat flux.

Linear stability analysis of a boundary layer with rotating wall-normal cylindrical roughness elements

“…The two types of modes differ by their phase velocity , such that the TS-like mode travels at a lower speed () than the roughness mode (). The identified phase speed for the roughness mode is similar to the results of Di Giovanni & Stemmer (2018), who found that a roughness-induced mode travels at in a hypersonic boundary layer. For the TS-like mode, all streaks are capable of attenuating the TS-like mode except for the case C4 with high rotation rate (), where the roughness mode gets mostly amplified with a relatively high amplification rate.…”

Weak Non-Parallel Effects on Chemically Reacting Hypersonic Boundary Layer Stability

“…Transition studies in hypersonic flows by means of more advanced techniques are very limited. Examples of investigations by means of DNS are the works of Marxen et al [22], Ma et al [23], Mortensen et al [24] or Di Giovanni et al [25]. Concerning LPSE, the first application to chemically-reacting flows was done by Chang et al [26], where non-parallel effects on a wedge flow were studied for a single frequency, and found to give an N factor increment of 2.…”

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

“…The computational domain is located inside the shock layer established between the surface and the weak shock wave induced at the flat plate leading edge. This approach reduces the computational effort necessary to obtain the base-flow solution, and has already been employed in similar studies in the literature (see for example De Tullio et al 2013; Di Giovanni & Stemmer 2018). The top boundary has an angle to prevent roughness-induced compression waves and weak shock waves from impinging on it, thus avoiding potential reflections back into the 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).…”

“…An important difference with respect to the flat plate studies mentioned before is that, because of the strong bow-shock in front of the capsule, the boundary-layer modes upstream of the roughness elements are highly stabilized, and as a result their interaction with wake modes is not present. Very recently, Di Giovanni & Stemmer (2018) carried out linear stability and DNS computations for a patch of periodic distributed roughness and DNS computations for a patch of randomly distributed roughness on a blunt-capsule configuration at Mach 5.9. For the periodic case, the growth rates of the symmetric and antisymmetric wake modes were found to be in good agreement between 2D-LST, 3D-PSE and DNS.…”