Abstracts - KTH Mechanics

Prediction of wall bounded flows using LES, DES and RANS
T. Persson a, M. Liefvendahl b, R.E. Bensow a and C. Fureby a,b
In the marine industry Computational Fluid Dynamics (CFD) is an important tool
for resistance prediction and hull optimization, while methods for the simulation of
propulsion, manoeuvring and cavitation are under development. These applications
result in complicated unsteady flow fields and the codes used to solve these problems
are often based on the Reynolds Average Navier-Stokes (RANS) equations 1. Although
RANS correctly models the mean flow in many cases, it often fails when facing more
complex flows, or when applied to flows dominated by unsteady effects. Therefore it
is important to investigate alternatives to RANS. The main candidates are Detached
Eddy Simulation (DES) 2, and Large Eddy Simulation (LES) 3. Both DES and LES are
based on the idea of separating scales, splitting the flow into two regimes by which all
scales larger than the grid are resolved using a space/time accurate algorithm, and only
the effects of the subgrid scales on the large scales are modeled. The objective of this
paper is to provide a systematic comparative study of DES, LES and RANS for three
selected cases of particular interest. The first case consists of the fully developed turbulent
channel flow at the friction velocity based Reynolds number (Re) of Re=395,
595, 1800 and 10,000, respectively. The second test case consists of the flow over a
surface mounted axisymmetric hill at Re=130,000 4. The final test case of interest to
this study consists of the flow past an axisymmetric hull with an elliptic forebody and
a smoothly tapered stern – the DARPA Suboff model AFF-1 5. The computational
results, in particularly for the axisymmetric hill, show that the LES and DES models
give better agreement, with the experiments for the time-averaged data, than RANS.
a
Chalmers, Shipping and Marine Technology, SE-412 96 Göteborg, Sweden.
b
FOI, Div. of Weapons & Protection, Warheads & Propulsion, SE-147 25 Tumba, Sweden.
1 Wilcox, D.C. (1993) DCW Industries
2 Spalart P.R. et al. (1997) 1st AFSOR Int. Conf. On DNS/LES, Greyden Press, Columbus Oh.
3 Sagaut P. (2001) Springer Verlag.
4 Simpson R.L., Long C.H. & Byun G. (2002) Int. J. Heat & Fluid Flow, 23, p 582.
5 Huang T.T., Liu H-L., Groves N.C., Forlini T.J., Blanton J. & Gowing S. (1992) Proc. 19th Symp.
on Naval Hydrodynamics, Seoul, Korea.
(a) (b)
Figure 1: (a) Cross-stream velocity profiles in the wake of the axisymmetric hill and (b)
stream-wise velocity components in the DARPA Suboff AFF-1 case.
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