Hi Everybody! I am simulating a circular cylinder by using 3D LES. It puzzled me that I can not get a perfact harmonic time sequence of lift coefficent. However I can do it by using other methods such as k-e model. Considering LES can get more information of fluctuating velocity, Is it a nessary of reasonable result by using 3D LES? Thanks!

Why do you expect a perfect harmonic time sequence? Have you compared with measurments or LES simulations of other researchers at the flow conditions you are studying?

Vortex shedding does not always produce a pure tone and this may well be the case for the flow you are studying. On the other hand, you may have numerical, modelling or bug issues with the code.

LES is a substantially more reasonable modelling assumption for large scale vortical motions than eddy viscosity models like ke which represent all the turbulent transport with a big diffusion coefficient. However, this is not necessarily the case near the surface of the cylinder where the modelling assumptions in typical LES codes can be relatively poor.

If you should expect a perfect periodic solution mainly depends on the Reynolds number. In the 2D laminar case, yes, it should be perfectly periodic. In your 3D turbulent case I wouldn't expect that. The easy way to find out: Look at other researcher's results (experiments and computations). You will find that a lot of people did this with RANS and LES already, so you should be able to verify your results.

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