Nose-Tip Bluntness Effects on Transition at Hypersonic Speeds

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The existing database of transition measurements in hypersonic ground facilities has established that as the nose-tip bluntness is increased the onset of boundary-layer transition over a circular cone at zero angle of attack shifts downstream. However, this trend is reversed at sufficiently large values of the nose-tip Reynolds number, so the transition onset location eventually moves upstream with a further increase in nose-tip bluntness. This transition-reversal phenomenon was the focus of a collaborative investigation under the NATO Science and Technology Organization (STO) Applied Vehicle Technology (AVT)-240 group. The current Paper provides an overview of that effort, which included wind tunnel measurements and theoretical analysis related to modal and nonmodal amplification of boundary-layer disturbances. Because modal amplification is too weak to initiate transition at large bluntness values, transient growth has been investigated as the potential basis for a physics-based model for the transition-reversal phenomenon. Results indicate that stationary disturbances that are initiated within the nose-tip vicinity can undergo relatively significant nonmodal amplification that increases with the nose-tip bluntness. This finding does not provide a definitive link between transient growth and the onset of transition but is qualitatively consistent with the experimental observations that frustum transition during the reversal regime was highly sensitive to wall roughness and, furthermore, was dominated by disturbances originating near the nose tip. The present analysis shows significant nonmodal growth of traveling disturbances that peak within the entropy layer and could also play a role in the transition-reversal phenomenon if those disturbances are realizable in a natural disturbance environment.