Experimental investigation of the hypersonic boundary layer transition on a 7° straight cone

In this paper, the experiments about the boundary layer transition on a 7° half-angle straight cone are carried out in a Mach 6 low-noise wind tunnel. The wall fluctuation pressure is measured by the transducer with megahertz response frequency, and the development process of the disturbance in the hypersonic boundary layer is investigated. The peaks in power spectrum density of the fluctuation pressure are related to the second mode wave, which is indicated through verifying the existence of the longitudinal acoustic second mode waves reflected between the relative sonic line and the solid wall by the flow visualization result. The wavelength and the characteristic frequency of the second mode wave in the hypersonic boundary layer are found to be greatly influenced by Reynolds number. The characteristic frequency of the second mode wave changes from 55 kHz to about 226 kHz when the Reynolds number increases from 2×106 m-1 to 8×106 m-1. The second mode wave appears at the position closer to the upstream with a higher disturbance growth speed under higher unit Reynolds number. As the second mode wave propagates downstream, its characteristic frequency gradually decreases. The freestream noise level also has a great influence on the development of the disturbance wave. The characteristic frequency of the second mode wave decreases significantly in a relatively quiet environment. The cross-correlation analysis results show that the propagation velocity of the second mode wave in the boundary layer is about 0.8-0.9 times the local mainstream velocity. The wavelength of the second mode wave is about 5.01 mm at the location from X=380 mm to X=440 mm when the unit Reynolds number is 5×106 m-1. At 1° angle of attack, the development of the boundary layer on the windward side and the leeward side of the cone are significantly different. The characteristic frequency of the second mode wave in the leeward surface is almost the same as the result at zero angle of attack under the same unit Reynolds number. However, the position of the second mode wave is greatly advanced. Results show that the disturbance development in the boundary layer of the leeward surface is accelerated, and the second mode wave appears at the position closer to the upstream. The velocity of the second mode wave in the leeward surface rapidly increases when it propagates downstream. While on the windward side, the disturbance development is inhibited and the second mode wave has a higher characteristic frequency. The wavelength of second mode wave also decreases obviously.