Rigid Bodies (Immersed Solids) Boundary Interaction & FSI
 

Good Day to all,

[Apologies for my ignorance; still a beginner.]

Need your help as you can see in the attachments (extremely basic & raw model, just to establish the idea), the intention is to observe a fluid induced moment/ rotation in the disc/ plate. The disc here is modeled as Rigid Body (Immersed Solid), the housing/ pipe as immersed solid. IDEALLY, the extrusion on the ID of the pipe is meant to stop the disc during clockwise motion, on the other hand the inner surface of the pipe should act as a stop to the rigid body motion during excess anti clockwise motion.

I do realize & have learnt from the forum/ CFD sources that immersed solids are actually only scaling momentum (not solid domain bodies), hence cannot be interfaced or interact with other immersed solids (as observed the disc clearly does not respect the pipe ID and boundaries of the stop).

I would like to request ideas on the fact that, is there no way then at all in CFX to model the rigid body motion specially considering the interaction with other solids/ immersed solids/ rigid bodies..?? And if at all the rigid bodies can interact, then would FSI still remain applicable (tried importing pressure in Static Structural but does not show any CFD surface)??

To summarize,
- If i attempt to model the system using Solid domain, that would compromise the rigid body modelling/ flow induced motion
- If rigid body domain is employed, FSI and interface is compromised

Could really use some ideas here fellow users.

P.S. CCL file (plate.txt) is attached for reference.

Cheers.

Some options include:
* Using CEL functions to constrain the immersed solid motion. For instance do not let the angle of swing go beyond a value because that is when it hits the stop.
* Doing a full FSI simulation and using an FEA model to handle the rigid body interactions

Hi Glenn,

It's been a while that I am working on the above methods you advised. Sorry if the below mentioned seems rudimentary (and lengthy).

I have reached a point where i would need to calculate the force that might not normally be considered in actuator sizing for Gave Valves (pretty close to my application). Normally considered forces are stem friction, seat friction, and hydrostatic. However, after some research specifically related to vertical channel flow valves, it seems there is another force that could be significant under such circumstances, e.g. higher viscosity and high velocity in the pipe. The hydrodynamic force that would vary from open to closed and vice versa.

A word of advise on how I could calculate this hydrodynamic force of pushing the gate into a high velocity stream (the force required to shear a fast moving stream)? It seems that viscosity, velocity, and Reynolds number are key to this value.
To reiterate, the application here is with high flow passing through small diameter pipe, a rotating disc with two circular openings (as it shows in the below picture). During rotation, the pipe undergoes pressure pulsation that can be used for another application.

How would i calculate the needed torque in CFX to rotate the disc without getting stuck due to high velocity stream? I have been trying to capitalize on Tutorial#22.5 however, any further help regarding the needed UDFs (if any) would do wonders.

http://i60.tinypic.com/2rnwgwg.png


One of the methods i tried was to use rigid body induced moving mesh in the fluid sub domain but issues such as specifying a constant angular velocity and calculating the resulting torque on the disc (specially near the flow shearing instance) are unsolved yet.

Cheers.

I do not understand your question. What rotates the disc? What constraints does it have? Can you show the full device?

Hi Glenn,

Apologies for the ambiguity. I took some time to simplify and to simulate a more of a generic version so I can come up with relatively tangible questions.
As can be seen in the attached picture, the cylindrical gate is modeled as a rigid body with a Y-Axis force (from the top) to push it down in to a high velocity fluid flow. The ultimate objective of this model would be to calculate the opposing force on the circumferential wall of the cylindrical gate/ obstruction, consequently to know the opposing torque against the motor that rotates the disc and variety of other components (I believe this also answers your query regarding disc rotation).
Modelling it as shown in the picture can simply provide me the Normal Force in Y direction on the Rigid Body which can then be visualized on the real model and torque could be computed. I believe this simulation could end up being fruitful towards the aim.

However, apart from the conceptual guidance and your word on whether the above would be the best way to proceed, another problem that i face during this simulation is the error#002100012 related to mesh folding & negative volume. The mesh is considerably fine and my extended question from you would be to know if this error is purely related to mesh modelling? or could it also incur due to setup inconsistencies CFX-Pre?


Thanks for your help.
Cheers.

Negative volume elements is an FAQ: http://www.cfd-online.com/Wiki/Ansys..._went_wrong.3F

Does the gate/disk follow a prescribed path or does its motion interact with the flow?

Thanks Glenn, going through the error help link.

The disc motion is induced by an External Force Definition in the Rigid Body configuration, using only a single Y-Axis degree of freedom. So we could say that the motion is prescribed, however i guess the driving force alters depending on the pressure profile on the gate from top..? (attaching a result picture here).

If the motion is prescribed then you should do this using moving mesh, not rigid bodies.

CFX rigid body simulation
 

Hi
Please, I will be very thankful if you help me to know where is the problem with this simulation. I used mass moments of inertia and I tried different things but the simulation progress stops after about 30 iterations.
I attached the CFX file I did. Thank you in advance..
http://fsend.net/?blndjn2ocge

I do not have time to look at your simulation in detail. Post a question describing your issue and we will try to help you.

Ansys CFX rigid body motion
 

Thank you very much for your reply.
My simulation includes the use of water force to rotate the wheel(the rigid body)without giving a rotational speed for the wheel. Because that the rotation is about Z-axis, I gave a zero value to all mass moments of inertia rather than Izz. For the first 30 iterations, the solution progress is going well and the wheel is rotating about 0.25 cycle due to the water drag force, but after that the simulaion progress stops with error and the CFX report shows the following message: "A negative element volume has been detected. This is a fatal error and execution will be terminated. The location of the first negative volume is reported bellow".
And sometimes this message be shown in the report: "Floating point exception:Overflow"
Even if I give the Ixx, Iyy, and Izz specific values larger than zero, I get also no result and the solution progress stops after some iterations.
I will be very thankful if you can help me. Regards.

Simulations like this are often MUCH more easily done by:
* Doing a series of simulations to get the torque versus rotational speed curve
* Then using the torque versus rotational speed curve in a ODE solver to do a reduced order model of the system speed.

Is this approach not valid in your case?

Thank you again for your help.
The simulation that I want to do is exactly similar to that shown in this animation:

https://www.youtube.com/watch?v=jk6veeHE8To

My comments regarding to this movie in the youtube and the comments of who published this movie explain what I want to do. I appreciate your help.. Best regards.

There is a FAQ on the negative volume element error: http://www.cfd-online.com/Wiki/Ansys..._went_wrong.3F

I tried several things in that document but unfortunatily I got nothing. I played with the mesh properties and even with the timesteps but the CFX report give the same error.

Let me quote the FAQ:

And it goes on to describe how to do it. Have you done as it suggests? This will allow you to identify where in your model the negative volume elements occur. That is the first step in fixing it. And post the images on the forum so we can help you.

The CFX report gives me the coordinates of the first negative element volume, but how can I fix this problem in CFX?
I am looking for a tutorial or geometry or mesh files similar to my simulation bu unfortunately I could't get it. I am confused and I don't know whether I had a problem with the geometry or the mesh or in the CFX-Pre..

Then why not post the image of the location on the forum and what you are doing and see if we can help you?

Hi.
this is the reults I got from CFX. You can see what is happened to the mesh near the blades, inside the red circles. I think that there is a folded mesh but I don't know how to solve this problem. I attached the error given by the report as well. Attachment 42797

Attachment 42798

[ATTACH]42799[/ATTA[ATTACH]TACH]CH]

Attachment 42800

Attachment 42801