Minimization of farfield acoustic effects in turbulent boundary layer wall pressure fluctuation experiments

The complications caused by facility-generated acoustic pressure fluctuations superimposed on turbulent fluctuations of interest in experimental studies are discussed. Two methods are put forward for addressing this problem; together, they permit wall pressure fluctuation experiments of high quality to be carried out in pipe and annular flows of water over a bandwidth of 1 kHz. One method is an improved technique for using three flush-mounted pressure transducers within a coaxial plane; this provides more than 20 dB reduction without using electronic filters below 300 Hz. For frequencies above 300 Hz, where axisymmetric nonplanar modes prevail, it is shown that the other method, involving composite polymeric materials within the facility, provides exceptional noise attenuation, reaching 60 dB at 1000 Hz. Experimental results reveal that the two techniques permit measurements of both the nearfield and farfield pressure components.