On the Utilization of a Flexible Bar as a Continuous Spatial Filter

Abstract

In order to determine empirically the wavenumber-frequency spectra of the wall pressure fluctuations beneath a turbulent boundary layer at nonconvective wavenumbers, a spatial filter with high wavenumber resolution is necessary. Such a filter can be realized by using a continuous system such as a flexible bar or by using a discrete array of finite-sized transducers. The bar offers the advantage that high wavenumber resolution can be achieved without the use of the elaborate equipment necessary for the transducer arrays. In order to use the bar as a spatial filter all that is required is a measurement of the modal displacement response at some spatial position. In this paper, the theoretical formulation of the bar as a continuous spatial filter is developed. Calibration procedures and measurement problems such as those associated with vibrational noise are discussed. Finally, measurements of the low-wavenumber (nonconvective) spectra of the wall pressure fluctuations obtained by using the bar as a spatial filter are reported. One important result of this study is that it demonstrates that the primary wavenumbers associated with the pressure fluctuations contributing to the bar response are not the convective wavenumbers. This implies that Corcos's model, which has been empirically verified only for wavenumbers near the convective wavenumbers [D. M. Chase, J. Acoust. Soc. Amer. 46, 1350–1365 (1969)], may not be adequate for predicting panel response. The measured low-wavenumber (nonconvective) spectra indicate the limitations of Corcos' model in this region. [Work supported by grant from the National Aeronautics and Space Administration.]