Roughness-induced attachment-line transition on a swept cylinder in supersonic flow

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Roughness-induced attachment-line transition on a swept cylinder in supersonic flow

Coleman, C.P. / Poll, D.I.A.

The incomplete understanding of the attachment-line and crossflow transition processes is one of the fundamental obstacles to the development of an economically viable, supersonic, laminar flow aircraft. In the presence of small surface roughness, the flow along an attachment line may become turbulent at the lowest Reynolds number of all of the transition mechanisms that can be active on a swept wing. Moreover, because the resulting turbulence propagates in both the spanwise and chordwise directions, when the attachment line becomes turbulent, the flow over the whole wing, both upper and lower surfaces, will also become turbulent. Therefore, attachment-line transition is an important consideration in the design process. Experiments were conducted on a 76-deg swept cylinder to establish the conditions for transition onset at the attachment line with and without surface roughness in a Low-disturbance (quiet), Mach 1.6 airflow. Local flow parameters were estimated from surface pressure measurements, and hot wires and schlieren photography were used to determine the state of the boundary layer. It was found that, for a near-adiabatic wall condition and a smooth surface, the attachment-line boundary layer remained laminar up to the highest Reynolds number (Re of 790) attainable in the tunnel. Transition under the influence of trip wires was found to depend on wind-tunnel disturbance levels, and the onset conditions have been established.