Hypersonic boundary layer transition on a 7 degree half angle cone at Mach 10
Hypersonic boundary layer transition experiments are performed in the low enthalpy Longshot wind tunnel with a free stream Mach number ranging between 12 M 9.5 and Reynolds number between 12x 10 /m Reunit, 3.3x 10 /m. The model is an 800 mm long 7 half angle cone with nosetip radii between 0.2 and 10 mm. Instrumentation includes flushmounted fast response thermocouples and pressure sensors. Boundary layer transition onset location is determined from the wall heat flux distribution. Nose bluntness has a strong stabilizing effect. No transition reversal could be observed at RB 10RN for a Reynolds number based on the nosetip radius of ReRN, 123, 000. Increasing freestream unit Reynolds number results in larger RexB,e. Wavelet analysis of the boundary layer fluctuations shows that numerous wave packets are present during the transition process. Comparison with Linear Stability Theory results for second mode waves shows an excellent agreement for the most amplified frequencies. The N factor of the wind tunnel is 5 based on these computations and on the transition location measured experimentally. The convection velocity of the disturbances is closely approximated by the local boundary layer edge velocity for all conditions investigated. Schlieren flow visualization of the instabilities exhibits the typical rope shape of second mode disturbances for the sharpest nosetips. For nose bluntness larger than 4.75 mm, disturbances are mainly present at the edge of the boundary layer and within the inviscid shock layer. Their shape no longer presents the second mode typical structure although a frequency analysis of the disturbances is still compatible with second mode instabilities. Present results confirm the dominance of second mode waves in the transition process along a conical geometry for Mach numbers larger than 10.