Turbulent Pipe Flow Subjected To A Mild Straining

Turbulent pipe flow passing through a contraction is measured using planar PIV. The considered Reynolds numbers based on the pre-contraction pipe diameter are between Re_D = 12,300 and 47,900. The dimensionless strain rate imposed by the present contraction is restricted to 2.0, which means that the staining is rapid with respect to the integral time scale. When normalized with the Kolmogorov time scale, the strain is approximately unity at the center of the pipe. Therefore, the imposed strain is considered to be mild. It is found that the response of the streamwise velocity fluctuations (large scales) follow the predictions by Rapid Distortion Theory (RDT) within the experimental uncertainty. However, the radial velocity fluctuations, which are associated with smaller scales as compared to streamwise fluctuations, show a delayed response to the imposed strain. Such a delay is inconsistent with RDT. The present measurements, therefore, suggest a scale-depend response in agreement with some observations in other flows. The anomalous behavior of radial velocity fluctuations is discussed in relation to the coherent structures present in a turbulent pipe flow.