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Apocolapse February 19, 2012 12:38

Cylinder vortex shedding (3D)
Hi all,

I am currently doing a project that involves analysing the effects of flow around the free ends of a cylinder on the vortex shedding that follows.

I am using a cylinder with diameter of 0.016m and velocity of 0.0075m/s and the fluid considered is water, giving me a reynolds number of approximately 120.

I have simulated the system successfully in 2D in both a steady and unsteady state (as as 'infinite' length cylinder). The physics models used for steady were:
  • constant density
  • segregated flow
  • liquid
  • laminar
  • steady
  • two dimensional

This appeared to give me a very good von Karman vortex street, as was intended.
And for unsteady physics model I only changed the steady to implicit unsteady.
I measured the frequency of the shedding by creating a report measuring the lift on the cylinder, and it was close to the value given by strouhals number, so I THINK this simulation is correct.

However, now when I try to run the simulation within a 3D environment (with free ends), the simulation doesnt output any vortex shedding when implicit unsteady is used. With the following models, there is shedding (affect by the flow around the free ends of course), but unsteady changes all my data.
  • constant density
  • segregated flow
  • liquid
  • laminar
  • steady
  • two dimensional

The reason I am trying unsteady is to get an output plot with time-step so I can compare the results of frequency shedding etc, is there another way to get a time/frequency reading for the models?

Also, am I correct in assuming I should use a laminar model (as I have a low Re)? Or should it be transient, moving into a turbulent model after the cylinder?

Sorry for the questions! I am very new to Star CCM and do not do any fluids lessons so my knowledge is very limited, I have to try and learn it all in my spare time. If any of you could also point me to a good learning resource that would be great too haha.

Furthermore, If any of you have done or seen any work on finite cylinder flow at low Re numbers, then please could you give me a shout! Would be great to use for references and a knowledge base.

Many thanks to you all

Kind regards

abdul099 March 4, 2012 16:24

You should run it unsteady when you want to see unsteady effects. So you already did it right.
You should run a turbulent case when you expect turbulent vortex shedding (sorry, I don't know without looking it up above which Re that will happen).

When no vortex shedding occurs, check your mesh resolution, especially around the ends of the cylinder. Also choose an appropriate time step. When it still doesn't occur, try to put in some instability for a few time steps. Maybe change the flow direction for a few time steps. This should initiate the vortex shedding.

racerdude777 April 3, 2014 20:33

Were you ever able to get the shedding to occur in your 3D simulation? I am having similar difficulties.

More specifically, how long did you run your simulation for before you started to experience the shedding? My simulation is identical to yours except my reynold's number is 76 and is being run in air instead of water, which shouldn't matter given the closeness of the Reynold's Numbers.

I don't want to leave the simulation running for unnecessary time waiting for shedding to occur if by all other accounts (x, y force plots and residuals) the simulation has converged.

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