Skin friction and heat transfer (St) predictions were made for a turbulent boundary layer over randomly rough surfaces at Reynolds number of . The rough surfaces are scaled models of actual gas turbine blade surfaces that have experienced degradation after service. Two different approximations are used to characterize the roughness in the computational model: the discrete element model and full 3D discretization of the surface. The discrete element method considers the total aerodynamic drag on a rough surface to be the sum of shear drag on the flat part of the surface and the form drag on the individual roughness elements. The total heat transfer from a rough surface is the sum of convection on the flat part of the surface and the convection from each of the roughness elements. Correlations are used to model the roughness element drag and heat transfer, thus avoiding the complexity of gridding the irregular rough surface. The discrete element roughness representation was incorporated into a two-dimensional, finite difference boundary layer code with a mixing length turbulence model. The second prediction method employs a viscous adaptive Cartesian grid approach to fully resolve the three-dimensional roughness geometry. This significantly reduces the grid requirement compared to a structured grid. The flow prediction is made using a finite-volume Navier-Stokes solver capable of handling arbitrary grids with the Spalart-Allmaras turbulence model. Comparisons are made to experimentally measured values of and St for two unique roughness characterizations. The two methods predict to within and St within , the RANS code yielding slightly better agreement. In both cases, agreement with the experimental data is less favorable for the surface with larger roughness features. The RANS simulation requires a two to three order of magnitude increase in computational time compared to the DEM method and is not as readily adapted to a wide variety of roughness characterizations. The RANS simulation is capable of analyzing surfaces composed primarily of roughness valleys (rather than peaks), a feature that DEM does not have in its present formulation. Several basic assumptions employed by the discrete element model are evaluated using the 3D RANS flow predictions, namely: establishment of the midheight for application of the smooth wall boundary condition; and Nu relations employed for roughness elements; and flow three dimensionality over and around roughness elements.
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e-mail: jbons@byu.edu
e-mail: smcclain@eng.uab.edu
e-mail: zjw@iastate.edu
e-mail: chixingk@egr.msu.edu
e-mail: tomshih@iastate.edu
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April 2008
Research Papers
A Comparison of Approximate Versus Exact Geometrical Representations of Roughness for CFD Calculations of and St
J. P. Bons,
J. P. Bons
Department of Mechanical Engineering,
e-mail: jbons@byu.edu
Brigham Young University
, Provo, UT 84602-4201
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S. T. McClain,
S. T. McClain
Department of Mechanical Engineering,
e-mail: smcclain@eng.uab.edu
University of Alabama-Birmingham
, Birmingham, AL 35294-4461
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Z. J. Wang,
Z. J. Wang
Department of Aerospace Engineering,
e-mail: zjw@iastate.edu
Iowa State University
, Ames, IA 50011-2271
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X. Chi,
X. Chi
Department of Aerospace Engineering,
e-mail: chixingk@egr.msu.edu
Iowa State University
, Ames, IA 50011-2271
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T. I. Shih
T. I. Shih
Department of Aerospace Engineering,
e-mail: tomshih@iastate.edu
Iowa State University
, Ames, IA 50011-2271
Search for other works by this author on:
J. P. Bons
Department of Mechanical Engineering,
Brigham Young University
, Provo, UT 84602-4201e-mail: jbons@byu.edu
S. T. McClain
Department of Mechanical Engineering,
University of Alabama-Birmingham
, Birmingham, AL 35294-4461e-mail: smcclain@eng.uab.edu
Z. J. Wang
Department of Aerospace Engineering,
Iowa State University
, Ames, IA 50011-2271e-mail: zjw@iastate.edu
X. Chi
Department of Aerospace Engineering,
Iowa State University
, Ames, IA 50011-2271e-mail: chixingk@egr.msu.edu
T. I. Shih
Department of Aerospace Engineering,
Iowa State University
, Ames, IA 50011-2271e-mail: tomshih@iastate.edu
J. Turbomach. Apr 2008, 130(2): 021024 (10 pages)
Published Online: March 25, 2008
Article history
Received:
November 30, 2005
Revised:
March 16, 2006
Published:
March 25, 2008
Citation
Bons, J. P., McClain, S. T., Wang, Z. J., Chi, X., and Shih, T. I. (March 25, 2008). "A Comparison of Approximate Versus Exact Geometrical Representations of Roughness for CFD Calculations of and St." ASME. J. Turbomach. April 2008; 130(2): 021024. https://doi.org/10.1115/1.2752190
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