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FSI Mesh Motion Trouble

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Old   April 6, 2015, 18:36
Default FSI Mesh Motion Trouble
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I am trying to carry out a 2D FSI simulation using a rigid body solver in water. I have attached the picture of the mesh. I have used no expression so far in my simulation. I have specified a spring with its equilibrium position at the Center of the hole in the center. I am running the simulation with a laminar flow at a Re of 120.I am not getting any Mesh Displacement in Y(I know this as I have set a monitor point at the circumference of the hole). Is it a problem with my flow (Is the force too small to move the mesh) or is it a problem with my setup ?


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Flow is in the x direction and im monitoring motion in Y direction. I have used a velocity Inlet and Pressure outlet (reference pressure=0). I have set the mesh motion of all boundaries to unspecified except the interface which uses the rigid body solver.
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File Type: jpg 2D mesh.jpg (39.4 KB, 39 views)
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Old   April 8, 2015, 04:24
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Matthias Voß
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Did you get any vortex shedding behind the cylinder? How did you set/model the displacement?
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Old   April 8, 2015, 05:55
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This is a classic case of where you should make sure you can model the simple problem accurately before you consider the complex one.

I strongly recommend you model a stationary cylinder in cross flow and make sure you can model the vortex street accurately enough for your purposes. There are extensive data available on the web for this flow so you have high accuracy solutions to compare against.

Once you can model the simple stationary cylinder model then you activate FSI and try to model the more complex combined simulation.
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Old   April 10, 2015, 12:59
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Thank you Matthias,
Yes I get vortices, but i have not modeled the displacement . I expect cfx solver to internally solve for mesh displacement. Am i wrong in that aspect?

(Just to check if the mesh moves at all, I simulated the flow for 10 sec and gave it a large spring force to move it 2*diameter distance in that time. As i was trying this the mesh always failed due to negative element so i set the time step down to 0.01 sec and yet got results only up-to 8 sec, beyond that the mesh folds. Does this mean my mesh is faulty. Now as i was simulating this I noticed that I got a mesh displacement in Y = 0 at every time-step but some increasing value for the Total mesh displacement in Y. How do i visualize this displacement in CFX post? )

Thank you Glenn
I have already simulated the stationary cylinder case. I got laminar vortex street at Re 120. To validate stationary cylinder I wanted to match my results to the St Vs Re plot. I am not aware how to measure the shredding frequency and hence could not complete the validation. How do i measure the vortex shredding frequency in CFX post?
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Old   April 12, 2015, 07:48
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The best way to measure the vortex shedding frequency is to put a monitor point in the cylinder wake and monitor the pressure or velocity. Rerun the simulation and you should get a periodic trace, and measure the frequency.
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Old   April 13, 2015, 07:32
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I would recommend to do as Glenn said and model this step by step. For the displacement just model the displacement w.r.t. e.g. the time-step (via sin(f(Timestep))*1[cm] e.g.) and run the simulation without solving for the fluid to check if the displacement-formulation (convergence, tightness of displacement residuals) is capable to run more than let´s say 4 or 5 periods of motion. So you can check for the mesh to be appropriate over the whole expected movement with the expected vortex shedding.
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Old   October 26, 2016, 07:08
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Quote:
Originally Posted by mvoss View Post
I would recommend to do as Glenn said and model this step by step. For the displacement just model the displacement w.r.t. e.g. the time-step (via sin(f(Timestep))*1[cm] e.g.) and run the simulation without solving for the fluid to check if the displacement-formulation (convergence, tightness of displacement residuals) is capable to run more than let´s say 4 or 5 periods of motion. So you can check for the mesh to be appropriate over the whole expected movement with the expected vortex shedding.
Hello Mr. Matthias,

I want to plot the displacement over time curve to validate a FSI solver in foam-extend-3.2. Can you please explain how to plot displacement over time? Thank you
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Old   October 26, 2016, 07:29
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Try the open foam forum. This is the CFX forum, we know nothing about Openfoam here.
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fsi 2-way coupling, mesh displacement, rigid body motion

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