Zoom Meeting: https://kaust.zoom.us/j/92543986189

Abstract

Using time-resolved volumetric velocity measurements, we study the evolution of turbulent velocity, vorticity r.m.s, and coherent structures in turbulent flow advecting through a set of three 4:1 2-D contractions of different lengths, as well as within a much more rapid 16:1 3-D contraction. Visualization using vorticity magnitude criterion shows the emergence of long, stretched cylindrical vortices aligned with the mean flow near the exit of the 3-D contraction.  This alignment is characterized by the probability density function (PDFs) of the direction cosines.  We propose two measures to quantify the strength of alignment, the peak height in the probability and a coefficient from the moment of its PDF, both of which reaffirm the strong streamwise alignment of the coherent structures.  It is found that, for the 2-D contractions, the alignment of these structures with the mean flow direction is most prominent for the intermediate-length contraction. In the 3-D contraction, having the largest contraction ratio, we identify long, stretched vortical structures strongly aligned with the mean strain. The r.m.s. of the streamwise vorticty grows within the contraction to become 4.5 times larger than the transverse component, at the downstream location where the contraction ratio is C=11.  The characteristic vortices become as long as the measurement volume, or more than 4 times the integral scale of the turbulence at the entrance to the contraction.  We characterize the vorticity enhancement along individual coherent vortices, measuring 65% strengthening over the streamwise distance along the vortex, where C goes from 4 to 11.  The prevalence of these coherent structures is estimated from 700,000 measured volumes, showing that near the outlet it is more likely to have 1 or 2 of these structures present than none.

Bio

Abdullah Alhareth is from Najran, KSA. He obtained his BS in Mechanical Engineering from Purdue University, Indianapolis, USA in 2016. Following that, he earned his MS in Mechanical engineering from KAUST, KSA in May 2018. Abdullah is a Ph.D. candidate in Mechanical engineering in the PSE division under the supervision of Professor Sigurdur Thoroddsen. He is also a researcher at the Center of Excellence for Aeronautics and Astronautics (CEAA) at King Abdulaziz City for Science and Technology (KACST), Riyadh, KSA. He specializes in turbulence measurement using Lagrangian Particle Tracking technique with high-speed cameras. His research interest include turbulence measurement, droplet dynamics and PIV-based techniques.