A PHASE-LOCKED HIGH-SPEED REAL-TIME INTERFEROMETRY SYSTEM FOR LARGE-AMPLITUDE UNSTEADY FLOWS

A high-speed phase-locked interferometry system has been designed and developed for real-time measurements of dynamic stall flow over a pitching airfoil. Point diffraction interferograms of incipient flow separation over a sinusoidally oscillating airfoil have been obtained at framing rates of up to 20 kHz and for free stream Mach numbers of 0.3 and 0.45. The images were recorded on 35 mm ASA 125 and ASA 400 films using a drum camera. Special electronic timing and synchronizing circuits were developed to trigger the laser light source from the camera and to initiate acquisition of the interferogram sequence from any desired phase angle of oscillation. The airfoil instantaneous angle of attack data provided by an optical encoder was recorded via a FIFO data buffer into a microcomputer. The interferograms have been analyzed using software developed in-house to get quantitative flow density and pressure distributions.