DNS and High Reynolds Number Experiments Lead to New Insights for Overlap Region and Near-Wall Layer of Wall-Bounded Turbulence
Date and Time: July 21st at 14:15, LSTM Seminar Room
Speaker: Prof. Hassan M. Nagib, ILLINOIS TECH (IIT), Chicago, IL 60616, USA
Topic: DNS and High Reynolds Number Experiments Lead to New Insights for Overlap Region and Near-Wall Layer of Wall-Bounded Turbulence
The modeling, prediction and control of turbulent flows through ducts and over surfaces require accurate information on the overlap region between the near wall turbulence and the outer parts of the flow. After nearly a century of utilizing “classical” relations to represent the overlap region in mean flow and in turbulence, new developments could bring a fresh “movement” in a field impacting many applications including energy saving, efficient transportation and future of environment. With a fresh look at matched asymptotic approximations, together with employing the best available DNS data in channel and pipe flows, as well as experimental data in boundary layer and pipe flows, we develop new directions in our understanding of overlap regions. For the mean velocity profiles, a logarithmic plus linear overlap is found to be superior to the classical pure logarithmic one, and confirms the non-universality of the overlap parameters, including the Kármán coefficient, which are found to depend on flow geometry and pressure gradient. The self-similarity of the logarithmic plus linear overlap is demonstrated using several data sets, including those from the Superpipe, and to improve on the pure logarithmic overlap, because of the wider overlap region for the same Reynolds numbers. For the streamwise normal stress, the defect power trend, based on bounded dissipation is compared to the commonly used inverse logarithmic representation, developed by the wall-scaled eddy model. The evaluation of the two models is carried out using indicator functions for DNS results, in channel and pipe flows, and a new approach is found more suitable for experimental data. The method, which is simple and robust, is used to evaluate each model over a wide range of Reynolds number by applying it to two of the prominent experimental data sets in zero-pressure-gradient boundary layers (ZPG) and pipe flows. Ranges of validity for each of the two models are established both in the overlap region and in the near wall layer of the flows, revealing new understanding of dominant scales and viscous effects.
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Professor Nagib is the John T Rettaliata Endowed Professor of Mechanical and Aerospace Engineering at the ILLINOIS TECH (IIT), Chicago, Illinois, and was the Founding Director of the Institute’s Fluid Dynamics Research Center. His field of specialty is in fluid mechanics, turbulent flow and flow management and control. At ILLINOIS TECH, he served as MMAE Department Chair, Dean of Armour College, Academic Vice President, and Chief Scientist for IIT Research Institute (IITRI). Professor Nagib is the recipient of several prestigious honors including being a Fellow of the American Physical Society, the American Association of Advancement of Science, the American Institute of Aeronautics and Astronautics, and the American Society of Mechanical Engineers. From his base institute for over half a century, he has been a visiting faculty on several occasions at Stanford University, Caltech, KTH, EPFL, Friedrich-Alexander University, Erlangen, and Tewkesbury Fellow, in Department. of Mechanical Engineering at University of Melbourne.