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Nosetip Bluntness Effects on Transition at Hypersonic Speeds: Experimental and Numerical Analysis
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Abstract
The existing database of transition measurements in hypersonic ground facilities has established
that the onset of boundary layer transition over a circular cone at zero angle of attack
shifts downstream as the nosetip bluntness is increased with respect to a sharp cone. However,
this trend is reversed at sufficiently large values of the nosetip Reynolds number, so that the
transition onset location eventually moves upstream with a further increase in nosetip bluntness.
This transition reversal phenomenon, which cannot be explained on the basis of linear
stability theory, was the focus of a collaborative investigation under the NATO STO group
AVT-240 on Hypersonic Boundary-Layer Transition Prediction. The current paper provides
an overview of that effort, which included wind tunnel measurements in three different facilities
and theoretical analysis related to modal and nonmodal amplification of boundary layer
disturbances. Because neither first and second-mode waves nor entropy-layer instabilities are
found to be substantially amplified to initiate transition at large bluntness values, transient
(i.e., nonmodal) disturbance growth has been investigated as the potential basis for a physics based
model for the transition reversal phenomenon. Results of the transient growth analysis
indicate that stationary disturbances that are initiated within the nosetip or in the vicinity of
the juncture between the nosetip and the frustum can undergo relatively significant nonmodal amplification and that the maximum energy gain increases nonlinearly with the nose radius of
the cone. This finding does not provide a definitive link between transient growth and the onset
of transition, but it 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 that originated near the nosetip. Furthermore, 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