On the Non Linear Spanwise Interaction of Disturbances Emanating from Two Point Sources in a Blasius Boundary Layer.
TEL-AVIV UNIV (ISRAEL) DEPT OF FLUID MECHANICS AND HEAT TRANSFER
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Localized disturbances in a laminar boundary layer simulate transition more realistically than the extensively studied, two dimensional perturbations regardless of the fact if they evolve in a linear manner or not. Localized disturbances can originate by surface imperfections, insects, or dust. The disturbances can be harmonic i.e. containing a single frequency and a band of spanwise wave numbers or pulsed i.e. containing a band of both streamwise and spanwise wave numbers. At sufficiently low amplitudes, localized disturbances evolve in accordance with the linear stability theory and the assumption of parallel flow presents no difficulty. The non linearity could arise from the finite amplitude of the perturbation or may be caused by a resonant wave triad. Non linear processes in a wave packet lead to breakdown and to the formation of turbulent spots. when the amplitude of the localized harmonic disturbance saturates, the non linear processes widen the band of the amplified lower frequencies adjacent to the excitation frequency. Experimental results describing the spanwise interaction of two harmonic or two pulsed localized disturbances leading to breakdown are presented and discussed. A comparison to the evolution and breakdown of single localized disturbance is provided. It was observed that spanwise interaction of localized disturbances promotes transition. The interaction between disturbances emanating from two harmonic point sources results in a rapid broadening of the spectral peak surrounding the excitation frequency while the interaction of two wave packets generates two new bands of frequencies at half and twice the dominant frequency of the dominant frequency of the wave packet. These new spectral peaks become broad and fill the rest of the spectrum.
- Fluid Mechanics