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High Reynolds Number Smooth/Rough-wall Turbulent Boundary Layers

Event Type: 
Date and Time: 
Friday, February 26, 2016 - 16:15
CTR Conference Room 103
Event Sponsor: 
Parviz Moin, Director of Center for Turbulence Research
Xiang I. A. Yang, Mechanical Engineering Department, Johns Hopkins University

We use tools including Large-eddy-simulations, wind tunnel experiments and the framework provided by the Townsend attached eddy hypothesis to study the flow physics in high Reynolds number turbulent boundary layers. We developed a hierarchical random additive process model (HRAP) for the cascading process in wall bounded flows. With this HRAP, a new family of two-point logarithmic scalings in the inertial region is discovered and confirmed using the experiment data from the Melbourne HRNBLWT. The scalings of single-point, two-point moment-generating-functions in high Reynolds number wall bounded flows are also investigated. The MGFs provide us with new insights into the near wall flow physics that are un-available in conventional moments. While the work mentioned above focuses on smooth-wall turbulent boundary layers, in this talk, we discuss the flow behaviors in the presence of ground roughness. LES are used for this purpose. By conducting LES of flow over various ground roughness, we find the mean flow behavior beneath the height of ground roughness follows a generic exponential profile. This exponential behavior, combined with the commonly accepted logarithmic behavior in the inertial layer, as well as a geometric sheltering model that accounts for the wake interactions among roughness elements, leads to a rough wall model that enables us to make rapid predictions on rough wall hydrodynamic properties solely based on roughness morphology. LES data are used to evaluate the model performance and we find reasonably good agreement between the LES measurements and the model predictions. Last, possibilities of using the recently gained knowledge of generalized log laws and the HRAP model to construct LES wall models are discussed. 

Xiang I. A. Yang is currently a Ph.D. candidate in Mechanical Engineering Department of the Johns Hopkins University. He received M.S. in Mechanical Engineering from the Johns Hopkins University. His graduate research topic is mainly on high Reynolds number turbulent boundary layers over smooth and rough surfaces. Some of his work also includes the development of numerical schemes.