Characterization of hypersonic roughness-induced boundary-layer transition

IR thermography investigation on roughness induced transition in high-speed flows

“…From figure 6 it is clear that just behind the roughness element there is a localized region or streak of high heat transfer characterized by large values of K. It is visible that when going further downstream, the streaks expand toward the symmetry plane and the K intensity decreases gradually. These streaks are connected to the symmetry plane (SP) and off-symmetry plane (OSP) vortices also detected by Iyver and Mahesh [22] and Tirtey et al [11]. As a matter of fact, when moving in streamwise direction each SP vortex moves closer to the symmetry plane due to the induced velocity by the image vortex and the low-pressure region created behind the roughness.…”

“…For the high Reynolds test condition, a maximum wall temperature increase of 4 is observed in the laminar flow region, while the temperature rise reaches 7 in the turbulent wake. According to van Driest and Blumer [16] and Tirtey et al [11], this moderate wall temperature variation is not expected to have a significant effect on the following analysis.…”

“…To visualize the heat flux distribution around and downstream of the roughness element the K contour [11], defined in eq. 3, is used.…”

“…For the interpretation the convention introduced by Tirtey et al [11] is adopted and is reported here for sake of clarity: -< 30%: Laminar region; -30% ≤ ≤ 70%: Transitional region; -≥ 70%: Turbulent region; -≥ 110%: Local structure overheating.…”

“…A systematic work was carried out in support of the Hyper-X program [9] and in the frame of the ESA EXPERT program [10,11]. Most of the proposed correlations lack of some physic aspects that lead to transition.…”