Experimental Study On Crossflow Instabilities In The Boundary-Layer Of Swept Wing

The experimental investigations are performed on a three-dimensional boundary layer of a 45 degree swept back flat plate, which is subject to cross-flow instability. Experiments are mainly focused on initial growth of unsteady primary instability, high frequency secondary instabilities both in natural condition and artificial high frequency excitation.Initial growths of disturbances are measured by means of hot-wire anemometry. Due to the detected signals with low signal to noise ratio, the coherence function is used to eliminate background noise during data reduction. The initial instability development has been investigated for different modes of artificial disturbances and oncoming wind speeds.Experimental studies in natural conditions are performed, where only artificial primary unsteady and steady disturbances are introduced. A high frequency secondary instability mode is detected, which rides on the primary instability wave in time domain and appears at the inflectional point of mean streamwise velocity profile. It is found to locate at the top of stationary vortex. Considerable growth rate of the mode is found and is responsible for the transition of boundary layer.Experiments with controlled excitation of primary and secondary instabilities have been performed to study the process leading to the final breakdown of laminar swept wing flow. Two types of high frequency secondary instabilities are identified. The most amplified is centered around the inflection point of streamwise velocity profile, which is interpreted as inflectional instability and has highly amplification rate. The other occurs in lower location near to wall , which is interpreted as viscous instability and has growth rate similar to primary disturbance. It only appears when high frequency disturbance is imposed. The growth rate of inflectional instability and viscous instability also saturate similarly to the primary and nonlinearly generated disturbances. The main effect of inflectional instability mode in the final breakdown seems to interact with the disturbances having developed and thus widens the frequency spectrum to turbulent state.