Low Wavenumber Pressure Content of Turbulent Boundary Layer Flows

The measurement technique of Martin has been repeated and extended to determine the low wavenumber content of the pressure fluctuations in turbulent boundary layer (TBL) flows. Thin metal and plastic plates have been used in a wind tunnel to determine their vibration response to the TBL pressures. A scanning laser vibrometer is used to measure the mode shapes of the plate vibration responses which, when transformed to the wavenumber domain, provide the transfer function from the pressure spectrum. Flush mounted microphones are also used to verify that the acoustic pressure content does not contaminate the measurement. By using 2D plate modes, the 2D content of the low wavenumber region of the TBL spectrum has been determined for smooth flows as well as flows over steps and with pressure gradients. The TBL pressure spectrum model developed by Chase has been extended to cover these cases.