Experimental study of the high-frequency instability in the hypersonic boundary layer over a cone at 6° angle of attack

The high-frequency instability on a cone at 6 degrees angle of attack is measured in a hypersonic quiet wind tunnel with Reynolds number of 6.90 x 10(6) m(-1), and Mach number is 6. Fast-response pressure sensors are used to measure the disturbance waves on the surface of the cone. The nano-tracer-based planar laser scattering (NPLS) technique is used to visualize the coherent structures of the three-dimensional boundary layer. At the plane of azimuthal angle of theta = 30 degrees from the leeward ray, low- and high-frequency disturbance waves with the characteristic frequency of f = 10-20 kHz and f = 120-140 kHz are detected. From the NPLS image, the regular large-scale traveling crossflow waves structures are observed, which are related to the low-frequency instability. On the top of the traveling crossflow waves, there are a series of small-scale structures, which suggests there is strong shear on the top of the traveling crossflow waves. These small vortices likely are the secondary instability of the traveling crossflow waves, which are associated with the high-frequency instability. The disturbance waves characteristics in different planes are measured using PCB transducers. The result shows that the high-frequency instability occurs in the planes of theta = 15 degrees-60 degrees, and the characteristic frequency is between f = 106.97-181.08 kHz. With the increase of azimuthal angle, the characteristic frequency increases obviously, which is related to the thinner boundary layer near the windward side. With the increase of the x-coordinate, the circumferential range of the high-frequency instability gradually widens.