Learning Forum

[Saeed]

Hi,

 I want do ANSYS fluent - Rocky DEM coupling and I want to use Dynamic mesh for my simulation , I want to know if dynamic mesh is compatible with the Rocky DEM. 

[Lucas Bertollo Baggio]

Hello Saeed,

Yes, it is compatible. You can check on Rocky Tutorial 16 for more information about this compatibility. 

 

Best regards,

Lucas

[Saeed]

Hi,

Thanks for your reply.

I have an academic account and dont have acces to it. is there any other way you can send me that tutorial ?

 

Regards 

[Rob]

Click on Help in the Rocky solver and you should have access to tutorials. There are also materials on Learning. 

[Saeed]

Dear Rob and Lucas, 

Thanks for your help.

I have been looking into tutorial 16 and 17 and I have some issues. in 16, the simulation is with water, air and the particle which is a comsidered multi phase , my simulation and case is just water and particle in a mixing tank, I tried to follow steps like in tutorial 16 and did not include air but i keep getting an error after some itterations. 

Also, I looked at tutorial 17 which is just air and then some particles are added in the rocky, now in my case which is a mixing tank but with just water and particle, do I have to simulate it as multi phase like 16 or i should do it like 17 which is single phase and then i add the particles i want, in rocky ? 

 

Thanks for your help. 

[Rob]

If you want to include the liquid free surface it's multiphase, if you just want particles in the water then 17 should be OK. As Rocky handles the particles we don't need to worry about them in Fluent (note, there are some data transfer methods that may use multiphase - I'm a little rusty on the "how" at present). 

[Saeed]

Hi 

 

I tried to run my simulation based on tut 16 but in single phase, but I get the attached error. the only differene my simulation has with tut 16 is that on page 14 step B, there is no option for rocky phase in my simulation since i did my fluent simulation single phase.

Can you please help me with it ?

[Rob]

Try Tutorial 14, I think that's single phase. Note, you may need to use elements of several examples to replicate your application. 

[Saeed]

Thank you for your reply, I tried using tut 13,16 and 17 to run my simulation , do you know what could be the reason for the error ? I tried running it in one way coupling and it worked but in 2-way im seeing that error. 

[Rob]

If one way is working then Rocky is seeing the Fluent session to read it. Are both transient? 

[Saeed]

yes they are both transient , I used the same case for both

[Rob]

Can you check Fluent runs OK from the initial conditions, and if there's a .trn file? That should contain whatever Fluent did/tried to do when launched from Rocky. 

[Saeed]

when i uploade the initial condition file which is the .dat file the fluent runs and a .trn file opens and starts to run, but after a it porgress for a few seconds it gives me the error. 

[Rob]

OK, and what's in the .trn?  

[Saeed]

these the .trn erros before it closes.

Thank you for your help. 

[Rob]

Odd, nothing about Fluent loading?

[Saeed]

No, the fluent loads with no problem, the mesh check does not give me any error , and I just run 20 sec one way of that same case and no error shows up. 

 

[Rob]

And it breaks at 20s? 

[Saeed]

no it does not break thats the end time i defined. 

[Vinicius]

Hello Saeed, did you try following tutorial 14? This is also single-phase. Compare your Fluent setup against tutorial 14, check the simulation log (located in project_folder/project_name.rocky.files/simulation/rocky_simulation.rocky20.log) for any error messages.

 

[Saeed]

Hello, 

Thank you for your reply, I did that this morning and attached is the errors I got.

first when I click start ansys opens since I am not putting -g in additional arg like tut 16. I have attached the commands in the console and the terminal. 

Best.

 

 

[Saeed]

Dear Ron and Vinicius , 

 

I wanted to thank you for your help. I figured out why I was getting that error.

Best

[Rob]

Please share, it's useful for others searching the Forum for solutions. 

[Saeed]

i was saving the simulation each 5 time step in calculation activities , but after I did not save it anymore error was solved. 

[Saeed]

Hello ,

I have a question about time steps.

for example if I define my time step 0.01 in fluent , when I go to Rocky and want to  2 way couple it with fluent what time step should I define in Rcoky ? should it be the same as fluent or it could be any value ?

 

[Rob]

Rocky should sort itself out as the time step is based on the collision time scale. It'll be covered in the tutorials. 

[Saeed]

Hello,

 

I have been facing an issue with the Rocky motion frame. when I want to add rotation of a rotating object around the z axis, some times the positive sign in angular velocity is counterclockwise and some time is clockwise. As I am aware , in ansys always positive sign is counterclockwise. Can you please help me with that ? 

As you can see in the image the angular velocity is -260 but its rotating counterclockwise  and before this simulation I put 100 RPM and its rotating counterclockwise.

[Lucas Bertollo Baggio]

Hi Saeed, 

 

by default, negative numbers result in clockwise rotation, and positive numbers in counter-clockwise rotation. Please review your project and Wall settings. Also check the rotations in the motion preview window.

[Saeed]

Dear Lucas,

Thank you for your reply. 

I actually did some trials about this issue.  when I increase the angular velocity which goes above 230 rpm , (like the 260 that I mentioned above ) the rotation would change to clockwise and if i change the angular velocity to 100 in the same case and check the rotaion the motion is counterclock wise. it is so confusing for me too because I do not change the case and I just change the value of the angular velocity and the rotation direction changes. 

[Rob]

To clarify, are you setting the rotation speed in Rocky for a coupled solution?  If not, how are you determining which way the blades are turning? 

[Saeed]

the first time, i faced this issue i was doing one way coupling which i set the rotation my self. 

i also some time face this when i do the 2 way coupling , which the rotation is set by fluent , when i run the simulation with 100 RPM which is counterclockwise . I tried 260 RPM which was set by the fluent but it was clockwise. so I defined another motion exactly like the one that was set by fluent , but i changed the rotation to -260 to have counterclockwise. 

 

I did my trials with a case that was not coupled I just imported the walls and defined the motion and assigned it. what i saw was that above 220 or 230 the motion rotation chnages. 

[Rob]

So the particle flow changed as you passed 220-230 rpm? I'm trying to understand if the direction is changing or if it's an optical illusion caused by the image refresh rate. 

[Saeed]

the rotation of my moving object changed when i passed 220-230 RPM. And I dont think its an optical illusion becasue the first time I faced this problem I ran the simulation and looked at the rotation with 0.01 s time step. 

I want to know since I need to have a counterclockwise rotation, if the motion frame imported from the fluent was clockwise (although it is in positive number ) if I define a another frame that is counterclockwise , does that effect the simulation or it does not matter if the frame is from fluent or defined by me?

 

Thanks 

[Lucas Bertollo Baggio]

I would suggest you to check the motion in the motion preview with a smaller time interval to be sure about the direction. Rocky follows the right-hand rule.  

[Saeed]

Hello,

I have a question regarding the time steps in a FLUENT-ROCKY 2 way coupled simulation. the time step size of DEM is deinfed as fluent output multiplier. my fluent simulation is with 0.01 s time step and the lowest number i can go in fluent output multiplier is 1 that defines the DEM time step as 0.01 s, same as fluent. As I saw in different literature, the DEM time step size should be smaller than CFD . I tried to change the output multiplier to 0.1 but Rocky changed all the numbers below 1 , to 1 and does now allow time step smaller than fluent. Can you please help me with that? 

Thanks

[Lucas Bertollo Baggio]

Hi Saeed, 

this "Output Frequencies | Simulation" (in CFD-DEM coupled simulations) or "Time Interval" (in DEM-only) is about the outputs you wanna save (to visualize or access data, for example).   

To process the simulations, Rocky automatically calculates the time step, that you can see in Solver | Summary tab. You can also manually define a time step by enabling advanced features (from Rocky Menu | Options | Preferences |  Additional Features) and defining a Fixed Timestep in Solver | Advanced tab.

 

Best regards,

Lucas

[Saeed]

Hello, 

I have doubled the size of my geometry and meshed with cartesian mesh method , I ran the fluent , then I used that case and dat file to couple with Rocky, when i load the dat or case file there is no errors . but when I start the simulation, after initialization, i recieve the floating point exception error. I ran the same simulation with the small geometry with no problem, their only difference is that I used the multizone mesh for the small one. 

Can you please help me with this? 

Thanks