The Character of Turbulence in the Presence of Smooth and Highly-Irregular Rough Walls

 

Kenneth T. Christensen

Department of Mechanical Science and Engineering

University of Illinois at Urbana-Champaign

 

Most practical wall-bounded turbulent flows of interest occur at high Reynolds numbers (Re) and/or in the presence of non-canonical influences, like roughness, free-stream turbulence, strong pressure gradients, etc.   These influences can have a profound impact on the flow and therefore on the overall efficiency and lifetime of a flow system.  However, detailed experimental and computational investigations of these complex flows can be extremely difficult.  Large-eddy simulation (LES) is a promising computational methodology specifically tailored to high-Re studies wherein the evolution of the larger scales is computed directly while the smaller scales are modeled in some fashion.  The first portion of this talk will focus on recent high-resolution PIV measurements in the streamwise–wall-normal plane of a smooth-wall turbulent boundary layer that are used to assess the contributions of underlying vortical structures to the subgrid-scale physics that must be modeled for successful implementation of LES.  Substantial evidence exists supporting the presence of hairpin-like vortices in smooth-wall turbulence that serve as the building-blocks for larger-scale vortex packets.  The  important role that these vortices and vortex packets play in inter-scale energy transfer within the log layer is highlighted using conditional averaging techniques.    The second portion of this talk will be devoted to the discussion of on-going PIV measurements of wall turbulence in the presence of a technologically-relevant surface topology–the surface of a damaged turbine blade.  The topology considered contains a broad range of spatial scales and its character is quite representative of the surfaces encountered in many practical applications.  The statistical and structural features of this rough-wall flow are contrasted with the canonical, smooth-wall case as well as past roughness studies over more classical surface topologies (wire mesh, sand grain, etc.).  Of interest is how well classical roughness topologies mimic the impact of this more realistic surface topology, particularly with respect to its influence on the physics of the outer layer.

 

 

Short Bio.

 

Professor Christensen received an M.S. in Mechanical Engineering in 1996 from Caltech and a Ph.D. in Theoretical and Applied Mechanics in 2001 from the University of Illinois at Urbana-Champaign.  He was an assistant professor of Mechanical Engineering at the University of New Mexico prior to joining the faculty of the Mechanical Science and Engineering Department at the University of Illinois at Urbana-Champaign as an assistant professor in 2004.  He is a recipient of an AFOSR Young Investigator Award and an NSF CAREER Award.  His research interests include turbulence (particularly wall-bounded flows), microfluidics, thermal transport at the microscale and advanced experimental methods.