Walllayer models for largeeddy simulations.
Personnel: Profs. Ugo
Piomelli, Elias Balaras
Largeeddy simulations (LES) have become increasingly popular as a tool to compute turbulent flows with more accuracy and at higher levels of detail than can be achieved by turbulence models for the ReynoldsAveraged NavierStokes (RANS) equations, at a fraction of the cost of Direct Numerical Simulations (DNS). LES have been applied successfully to a considerable variety of flows of engineering and geophysical interest, and have contributed to significant improvements in the understanding of the physics of turbulent flows.
One area in which the cost advantages of LES over DNS are less clear is in the calculation of wallbounded flows: the grid requirements of DNS and LES in this case are comparable, due to the need to resolve the momentumproducing eddies, whose size depends on the Reynolds number. For highReynoldsnumber flows, one must instead bypass the walllayer and determine the wall stress as a function of the velocity in the outer layer, an approach analogous to the wall functions commonly used in RANS methods. Various approaches exist; recently, the hybridization of LES
with RANS for the simulation of highReynolds number wallbounded flows is receiving intense attention.
The quasisteady RANS equations are solved in the nearwall region with shallow grid cells, while an LES is performed away from the wall with nearly cubic cells. This technique, however, creates a transition layer between the RANS and LES regions, in which the shear
stres
s is fully modeled and fully resolved, respectively. This may result in inaccurate velocity profiles, typically involving an upward shift in the LES logarithmic region, and errors of up to 15% in skin friction.
The present study compares methods to couple the inner, RANS, region to the
outer, LES, one; in particular, the location of the interface between the two regions, and the type of model used in each are examined. Calculations of turbulent channel flow show that accurate predictions of length and timescales of the turbulent eddies in the RANS region are important, but are not the only factors determining accuracy. Modeling errors in the LES region also influence the mean flow profiles. Recent investigations have focused on the addition of stochastic forcing to the momentum equations, an effect that can eliminate the error in the skin friction prediction.
Sponsor: Office of Naval Research

Relevant publications:
 U Piomelli and E. Balaras
Walllayer models for largeeddy simulations
Annu. Rev. Fluid Mech. 34, pp. 349374 (2002).
View pdf file
 G.V. Diurno, E. Balaras & U.
Piomelli
Walllayer models for LES of separated flows.
In Modern simulation strategies for turbulent flows, ed.
B. Geurts, (Philadelphia, Edwards), pp.157174
(2001).
View pdf file
 U. Piomelli.
Largeeddy simulation: achievements and challenges.
Progress in Aerospace Sciences 35, pp. 335362 (1999).
View pdf file
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Interaction
between a cylinder wake and a boundary layer
Personnel: Ugo Piomelli
Direct and largeeddy simulations of the interaction between the wake of a circular cylinder and a flatplate boundary layer
are conducted. Two Reynolds numbers are examined. The simulations indicate that at the lower Reynolds number the boundary layer is buffeted by the unsteady
Kármán vortex street shed by the cylinder. The fluctuations, however, cannot be selfsustained due to the low Reynoldsnumber, and the flow does not reach a turbulent state within the computational domain. In contrast, in the higher Reynoldsnumber case, boundarylayer fluctuations persist after the
wake has decayed (due, in part, to the higher values of the local Reynolds number
Re_{q} achieved in this case); some evidence could be observed that a selfsustaining turbulence generation cycle was beginning to be established.
Sponsor: NASA Langley Research Center
Relevant publications:
 U. Piomelli, M. M. Choudhari, V.
Ovchinnikov & E. Balaras
Numerical simulations of wake/boundarylayer interactions
AIAA Paper 20030975 (2003).
View pdf file
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Visualization of the coherent eddies
(in gray) in the cylinder wake, and the streamwise velocity
fluctuations on the wall. Click on the figure to see an enlargement. 
