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Simulation about a 3D cylinder using HRLES

In order to visualize more clearly the differences between RANS modeling and LES / HRLES, a direct comparison is presented in this section [1]. The flow about a three-dimensional cylinder was computed using both the k − ω SST (RANS) model and an advanced Hybrid RANS/LES model, developed at Georgia Tech by Dr. Sánchez-Rocha and Dr. Menon [2]. Instantaneous flow fields are shown in Fig. 1↓ and some simulation results are presented in table 2↓. The Q-criterion is defined such that it is only positive near vortices. This property can therefore be used to filter out the regions of high vorticity (boundary layers, vortices, etc).
figure images/cylinder1.png figure images/cylinder2.png
Figure 1 
Iso-surfaces of Q criterion about a 3-D circular cylinder. On the left: k − ω SST, 4 diameters, 101 planes. On the right: HRLES, 4 diameters, 101 planes. Source: Lynch (2011)
Looking at Fig. 1↑, it appears that RANS modeling does not capture the three-dimensional nature of the flow. The vortex shedding can be observed in both cases, but over a much longer extent for HRLES. This is because RANS models are typically very dissipative. The turbulence fades away rapidly in the wake. The Strouhal number is captured correctly by all three-dimensional models. However, the drag coefficient is dramatically overpredited by the k − ω SST model. The drag predicted by the 3D HRLES is very close to the experimental value, though it is still slightly under-predicted on all grids. All HR-LES simulations predict the separation location within the experimental bounds, while k-ω SST predicts it over 10 deg farther aft.
figure images/tableHRLES.png
Figure 2 
Predicted circular cylinder characteristics for various turbulence methods and grids with number of spanwise planes Nz, and spanwise extent Z. Separation location is given in degrees of azimuth from the leading edge stagnation point. Experimental Strouhal number is from L. Ong (1996), and separation location is from J. Son (1969) at ReD = 5000. LES data is from A. Kravchenko (2000). Source: Lynch (2011) [1]

References

[1] C.E. Lynch. Advanced CFD methods for wind turbine analysis. PhD thesis, Georgia Institute of Technology, 2011.

[2] M. Sánchez-Rocha and S. Menon. The compressible hybrid RANS/LES formulation using an additive operator. Journal of Computational Physics, 228(6):2037--2062, 2009.