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Contents:
  1. Wall-layer models for large-eddy simulations.
  2. Interaction between a cylinder wake and a boundary layer.
  3. Large-eddy and direct simulation of bypass transition.
  4. Flow in an S-duct
  5. MEMS Ultrasonic flowmeters.

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Wall-layer models for large-eddy simulations.

Personnel:  Profs. Ugo Piomelli, Elias Balaras


Large-eddy 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 Reynolds-Averaged Navier-Stokes (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 wall-bounded flows: the grid requirements of DNS and LES in this case are comparable, due to the need to resolve the momentum-producing eddies, whose size depends on the Reynolds number. For high-Reynolds-number flows, one must instead bypass the wall-layer 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 high-Reynolds number wall-bounded flows is receiving intense attention.

The quasi-steady RANS equations are solved in the near-wall 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 time-scales 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
    Wall-layer models for large-eddy simulations
    Annu. Rev. Fluid Mech. 34, pp. 349-374 (2002).
    View pdf file
  • G.V. Diurno, E. Balaras & U. Piomelli
    Wall-layer models for LES of separated flows.
    In  Modern simulation strategies for turbulent flows, ed. B. Geurts, (Philadelphia, Edwards), pp.
    157-174 (2001).
    View pdf file

  • U. Piomelli. 
    Large-eddy simulation: achievements and challenges.
    Progress in Aerospace Sciences 35, pp. 335-362 (1999). 
    View pdf file

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Interaction between a cylinder wake and a boundary layer

Personnel: Ugo Piomelli

Direct and large-eddy simulations of the interaction between the wake of a circular cylinder and a flat-plate 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 self-sustained due to the low Reynolds-number, and the flow does not reach a turbulent state within the computational domain. In contrast, in the higher Reynolds-number case, boundary-layer 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 self-sustaining 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/boundary-layer interactions
    AIAA Paper 2003-0975 (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.

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Large-eddy and direct simulation of bypass transition

Personnel: Prof. U. Piomelli and V. Ovchinnikov


Sponsor: NASA Langley Research Center
 

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Flow in an S-duct

Personnel: Profs. U. Piomelli and J. Laginha Palma (U. Porto, Portugal) and 
Dr. A. Silva-Lopes


The flow in an S-duct at a low Reynolds number was modeled by large-eddy simulation, The numerical method successfully predicted the stabilizing and destabilizing effects on turbulence due to convex and concave curvature. The boundary-layer features in the regions where curvature changed were found to be essentially dictated by the pressure gradient (favorable if it was going to a convex or leaving a concave region and adverse if it was going to a concave or leaving a convex region) and not by the curvature. At the simulated Reynolds number, the boundary-layer exhibited intermittent separation every time it was subjected to an adverse pressure gradient, which resulted in increased turbulence production. Visualization of streamwise turbulent fluctuations showed coherent structures along the concave walls; these well-ordered structures may be an indication of the presence of Taylor-Görtler vortices, or may be due to pressure-gradient effects.

Visualization of the low- and high-speed streaks near the walls through contours of the streamwise velocity fluctuations.  Click on the figure to see an enlargement.


Relevant publications:
  • A. Silva Lopes, U. Piomelli & J. L. Palma
    Large-eddy simulation of the flow in an S-duct
    AIAA Paper 2003-0964 (2003).
    View pdf file

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Last modified: February 18, 2003 07:20 AM