Woodruff School of Mechanical Engineering
Mechanical Engineering Seminar
Inner-outer interactions in a rough-wall turbulent boundary layer
University of Illinois, Urbana-Champaign
Friday, March 17, 2017 at 1:00:00 PM
MRDC Building, Room 4211
Dr. Devesh Ranjan
The structure of a wall-bounded turbulent flow can be described broadly by dividing it into three regions: (i) the near-wall region, (ii) the wake region, (iii) and the inertial region in between that overlaps the two regions. The near-wall region is the dominant turbulence-generating region that is rich in viscous small scales, and that operates a seemingly autonomous, self-sustaining turbulent cycle. A cascade of scales in the inertial region, which embodies large- and very large scale motions (LSMs and VLSMs), enables a net energy transfer from the free stream to the near-wall region. With increasing Reynolds number (Re), the near-wall region is confined to a decreasingly small physical region, and the LSMs and VLSMs become increasingly stronger. This had long led to speculations suspecting that in high Reynolds number flows pertinent to many engineering applications, the near-wall cycle, while could still be autonomous (and Reynolds number independent), will face an increasing influence from the ever strengthening LSMs and VLSMs. Recent studies have found compelling evidence of these outer structure influences on the near-wall dynamics – seen as amplitude and frequency modulation of the latter by the large scales in smooth-wall turbulent flows. A high Re boundary layer flow over rough-walls is anatomically similar to smooth-wall boundary layer, except that the near-wall production cycle is replaced by a ‘roughness sublayer’. The current work experimentally investigates similar influences in rough-wall turbulent boundary layers. Hot-wire measurements on a complex roughness with a distribution of roughness scales have indicated the presence of similar interactions. Both amplitude and frequency modulations were observed similar to those in smooth-wall flow, indicating a robust nature of the phenomenon. Additionally, time-resolved PIV measurements on idealized hemispherical roughness provides a spatio-temporal description of the same, which can be used in concert with DNS simulations to form effective LES models.
Gokul Pathikonda is a PhD candidate working with Prof. Kenneth T Christensen in the Mechanical Science and Engineering department at the University of Illinois, Urbana-Champaign. He got his Bachelors in Technology in Mechanical Engineering from National Institute of Technology, Warangal, India in 2009. He worked as a Research Fellow at the Department of Aerospace Engineering at Indian Institute of Science, Bangalore, before joining University of Illinois, Urbana-Champaign to receive his MS in Aerospace Engineering in 2013. He is a recipient of RISE fellowship in 2012, from Indo-US Science and Technology Forum, under which he worked briefly at Jawarharlal Nehru Center for Advanced Scientific Research, Bangalore. His research interests are fluid mechanics, turbulence, and experimental techniques.
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