The effect of mildly rapid strain on turbulent pipe flow

Abstract

The way in which mean strain affects the turbulent structures is imperative to understand various natural flows such as flow over a hill, the flow of a river in the delta, jet streams in the upper atmosphere etc. Further, it also has industrial implications viz; flow over bodies such as airfoil, turbomachinery, gas pipelines.
The strained pipe flows, in particular, have huge engineering interest due to its prevalence in industrial fittings wherein a larger pipe diameter is connected to a smaller one and vice versa. This subject also has a fundamental interest as strain highlights the interaction of various scales of turbulence. This scale interaction
essentially dictates transfer of energy in turbulence and hence is fundamental in understanding turbulence dynamics itself. The present work deals with the experimental study of the response of pipe turbulence to axisymmetric, irrotational strain using high-resolution planar Particle Image Velocimetry (PIV). A mildly rapid strain (s^*s ~ 3.2) is imposed on turbulence via a spatial contraction. It is seen that turbulence is suppressed upon straining. As a response to mean strain, transverse Reynold stress increases at the expense of streamwise Reynolds stress and anisotropy is induced in the turbulence. Despite strain being only mildly rapid, Rapid Distortion Theory (RDT) is found to predict the correct trend of normal Reynolds stress although traverse Reynolds stress is over-predicted. The effect of strain on different scales of turbulence is discerned. The large scales of turbulence are seen to get compressed in the radial direction although they do not get affected significantly in the streamwise direction. Near-wall coherent structures which were initially inclined w.r.t. the wall are seen to get aligned with the flow as they also get severely compressed in the radial direction. On the other hand, the small scales of turbulence are found to be spatially organised in the form of sheets or layers. Upon straining, these sheets are found to get aligned with the mean flow. Further, they get elongated in streamwise and compressed in the radial direction. It is observed that the small scales are more severely distorted than the large scales upon staining inside the contraction. At the Reynolds number (Re) range employed in this thesis, there is no substantial difference in which turbulence is strained inside the contraction at disparate Re. Downstream of the contraction, at the axis of the pipe, the anisotropy of Reynolds stress is found to recover slowly. Further, this relaxation is seen to be Re dependent with higher Re turbulence relaxing slightly faster.