Flow characterization and boundary layer transition studies in VKI hypersonic facilities (Invited)

Abstract

A summary of the activities performed over the last years at the von Karman Institute for Fluid Dynamics in the frame of hypersonic boundary layer transition studies is presented. Free-stream noise levels have been determined in the H3 Mach 6 conventional wind tunnel using double hot-wires and modal analysis. In the Longshot wind tunnel at Mach 10, an improved free-stream characterization method, based on the use of free-stream static pressure probes, has been applied, alleviating the needs for the limiting adiabatic and isentropic nozzle flow assumptions. Based on these improved flow characterization, natural transition experiments have been performed in both wind tunnels on 7 • half-angle conical geometries at 0 • angle of attack and with different nosetip radii. Measurements techniques include either infrared thermography or flush-mounted fast response thermocouples in order to determine the transition onset location. Boundary layer instabilities are visualized using a LIF-based Schlieren technique at Mach 10, revealing rope-shape structures typical of the second mode disturbances. Wall measurements using fast-response pressure sensors complete the investigations. Dominant boundary layer disturbances at various locations along the cone are determined and compared with theoretical predictions. The corresponding N-factor is inferred for each wind tunnel. A comparison of the different measurement techniques is finally reported. Nomenclature Symbols c Specific heat, J/(kg.K)

Comparative Assessment of Modified γ - Re θ t

“…The results depicted in Figures 19(a) and 19(b) reveal that the N factor reaches the value of 2.1 and 3.8, respectively, at x ≈ 0:6 m, where the first ramp ends for the SCP-01 configuration. For the e N method, N ≈ 5:5 for Mack's 2 nd mode shows a good correlation with the transition onset locations of hypersonic boundary layers, measured from the wind tunnel experiments using the ground facilities [66][67][68]. In addition, it is well known that the transition onset in the ground test conditions using a quiet tunnel or in the flight test conditions corresponds to a higher N value than those obtained in conventional noisy tunnels, owing to the good flow quality of freestream vorticity, entropy, and low acoustic pressure levels [69].…”

Stabilization of hypersonic boundary layers by linear and nonlinear optimal perturbations

“…The length of the nearly sharp cone is L * = 0.305 m and the nose radius is r * n = 0.126 mm. The freestream conditions are selected to replicate those of a previous experiment in the VKI H3 hypersonic tunnel, 22 i.e., Mach 6 flow at a unit Reynolds number of 18 × 10 6 /m, and freestream temperature of T * ∞ = 60.98 K. Freestream values are used as reference values for nondimensionalization. For this problem, the computational coordinates, (ξ, η, ζ), are defined as an orthogonal body-fitted coordinate system.…”

“…The freestream parameters (M = 6, Re = 18 × 10 6 /m, T * ∞ = 60.98 K) are selected to match the flow conditions of a previous experiment in the VKI H3 hypersonic tunnel. 22 Experimental measurements and theoretical predictions based on quasiparallel, linear stability theory (LST) and the nonparallel, parabolized stability equations (PSE) have confirmed that laminar-turbulent transition in this flow is driven by the modal growth of planar Mack mode instabilities. 22 The array of actuators shown in Fig.…”

“…22 Experimental measurements and theoretical predictions based on quasiparallel, linear stability theory (LST) and the nonparallel, parabolized stability equations (PSE) have confirmed that laminar-turbulent transition in this flow is driven by the modal growth of planar Mack mode instabilities. 22 The array of actuators shown in Fig. 1 is purely notional, as the analysis presented herein is based on boundary layer streaks resulting from the transient growth of an optimal initial perturbation.…”

“…Experimental measurements and theoretical predictions based on quasiparallel, linear stability theory (LST) and the nonparallel PSE have confirmed that laminar-turbulent transition in this flow is driven by the modal growth of planar Mack mode instabilities. 22 The instability of the unperturbed flow is examined by PSE to establish the transition behavior in the absence of stationary streak perturbations. The onset of laminarturbulent transition in the unperturbed boundary layer flow is estimated using N -factor evolution of the planar Mack modes computed with the PSE.…”