Learning Forum

[haebi]

Hi Community,

I am doing a CFD analysis for my major project in Mechanical Engineering.

I am supposed to improve a water driven motor that is build like a lobe pump.

I want to simulate a 3D domain using Fluent. The first problem to deal with was the limit of mesh cells in the student version of Ansys, while providing an acceptable orthogonal quality and skewness.

After adjusting the mesh, I ended up with 400k cells and a mesh quality which Fluent is not complaining about.

I am using an UDF for defining the rotation of the rotors in dynamic mesh and applying

the k-epsilon-model. I also set remeshing and smoothing.

However, after every iteration Fluent is remeshing and the total amount of mesh cells slightly increases, until the mesh cell limit is exceeded. Then the calculation stops and Fluent ends.

In the forum I read, that this issue can not be solved.

So, to decrease the initial number of cells, I cut down the domain more and more ending up with the following mesh.

 

[Karthik R]

Hi,nFirstly, I like your idea of cutting down your domain further to give yourself an additional cushion. nRegarding your negative cell volume issue - your mesh quality is still very high. The max skewness in your domain is 0.98. I'd try to reduce that further.nAlso, what are your dynamic mesh settings and which boundaries are you applying these on? Please provide the necessary screenshots.nThank you.nKarthikn

[YasserSelima]

I understand that having a 3D simulation is important if you are taking the casing effect into consideration. However, I can't see this. So, why don't you start by a 2D simulation that reduces the number of cells and enables you to have smaller time step?n

[Karthik R]

- great point. n

[YasserSelima]

Thank you!n

[haebi]

nGood afternoon, nFirst I want to thank you for your ideas. I think I got your point. I have to speak to my supervisor about this.nHowever, I also tried to improve the skewness of the model, as suggested.nnTherefor I divided the geometry into three zones like in the picture below.nnAfter setting the parameters for the mesh in collaboration with my supervisor, I obtained a mesh that definitly should fit the requirements.nThe statistics of this mesh are: nnFLUENT:nI launched fluent with the following configuration.nThe Project-Tree is shown in the pictures below:nAs fluid, I am using water-liquid from the fluent database.nDYNAMIC MESH:nThe settings were used as follows:nnThe UDF for motion of the paddles was compiled with built in compiler in fluent, that was launchend with MS visual studio 2019.nWhen I started the motion preview, everything looks alright.nThe code of the UDF is shown in the picture below:nnI wanted to try this model, but when I run the calculation I receive a floating point exception now. nnSo this is the next problem that needs to solved. nI hope I provided the necessary information in this comment and am looking forward to get some help for my problem(s).nThanks a lot for your help and time!nnBest regards, Floria n

[YasserSelima]

Go to Controls and decrease the under-relaxation factors ... at least for few time steps.nIn methods, use first order whenever available.nDecrease the time step to a very low value ... increase the number of iterations in every time step .. and decrease the residuals convergence criteria. nRun few time steps like this until you find the residuals converge (reach straight line) ... now increase the under-relaxation and time steps. n

[haebi]

Hello again, nI want to finally update my discussion. nFirst of all thanks for your help and the quick response.nI troubleshooted the problem a lot together with my supervisor. One outcome was, that decreasing the time step size just delays the error with negative cell volume, because the motion of the paddle just starts later and so the error ocurrs on a later time step. nThe problem that causes the negative cell volume error might be the mesh within the tiny gaps in the fluid domain.nFor example between the two rotors.nzoom to this section:nThe error could be caused because there is only one cell across the gap.nTo refine the mesh in this areas I enabled capture proximity in the mesh settings as shown in the picture below.nThe mesh that was obtained looked way better.nThis mesh might probably work for the simulation, but as I am using the student version, the number of elements is way too much now. nThe limit of mesh cells and the comment of Arrayfrom above finally led to switch from 3D to 2D, because considering the fluid layer on the front side would just increase the number of elements further.nnThe update is just an assumption what might solve the problem but maybe it is necessary for anyone else.nThanks again for the support!nnBest regards, Floriann

[YasserSelima]

Thanks for the update!nGood Luck!n