Radiated noise and wall pressure measurements in turbulent boundary layers in dilute polymer solutions

Abstract

Measurements of radiated noise and wall pressure fluctuations in a turbulent boundary layer in water are described. A comparison is made between measurements in pure water and in dilute solutions of high molecular weight polymers. To obtain these measurements, a new experimental geometry was developed. The principle of the experiment is as follows: A flat steel plate 205 cm long by 80 cm wide is rolled into a single-turn spiral, with a radial gap of 4.5 cm between the two overlapping ends. The spiral is submerged in water and rotated about its axis, creating a boundary layer on the inner surface which leaves the interior of the spiral through the radial gap. The fluid leaving the interior through the gap is replaced through the two open ends of the spiral by means of stationary honeycomb filters which remove residual turbulence and vorticity. Measurements of the mean velocity profile show that the turbulent boundary layer on the inside surface of the spiral resembles that on a flat plate in a uniform free stream. A Reynolds number based upon plate length of 5 x 10^6 is obtained. Wall pressure fluctuations under the boundary layer are measured with piezoelectric transducers mounted flush in the wall of the spiral. Radiated noise is measured with a stationary transducer located outside of the boundary layer, near the center of the spiral. It is shown that the polymer additives cause significant reductions in both the radiated noise and wall pressure spectra. The reductions are greatest at high frequencies, or at Strouhal numbers greater than one.