# Time-step for Implicit Unsteady solver in STARCCM+ and maximum number of steps?

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 April 24, 2020, 07:50 Time-step for Implicit Unsteady solver in STARCCM+ and maximum number of steps? #1 Senior Member   MA Join Date: Mar 2020 Posts: 163 Rep Power: 4 Hi, I am working on the CFD simulation for a wind turbine blade. I chose Implicit Unsteady Solver for this purpose. I want to associate the time step with rotational speed of the blade I am unable to figure out how should I do this and what should be the max. physical time and maximum steps for the stopping criteria? Any comments please.

 April 25, 2020, 08:38 #2 New Member     Onur Join Date: Dec 2018 Location: Turkey Posts: 25 Rep Power: 5 Hey there, I have done a case where I had a spring valve closing and opening. The way I determined time step and inner iters. were somewhat trial and error but my procedure was like this (Im sure there are some kind of theoretical calculations involving this matter but I do not have the knowledge): Initially, calculate how far ur blade travels for each time step and according to your mesh, either coarse or fine, choose your time step. For example, lets say your mesh is really fine and has a size of 1mm and in 0.001 second it rotates by a value greater than 1mm, say 10mm (calculations should ofcourse be in radians and all that just an example). You should reduce your time-step in this case to be close to 1mm. (The deformation of mesh should not be a huge, sudden jump) As for the inner iteration, start with 7, and if your residuals drop below 10^-3 or lower, you can leave it like that or you can increase inner iters if you would like. This worked pretty good for me but ofcourse this is an entirely different case. Hope this helps. =D

April 25, 2020, 08:57
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 Originally Posted by SimulatorMaster Hey there, I have done a case where I had a spring valve closing and opening. The way I determined time step and inner iters. were somewhat trial and error but my procedure was like this (Im sure there are some kind of theoretical calculations involving this matter but I do not have the knowledge): Initially, calculate how far ur blade travels for each time step and according to your mesh, either coarse or fine, choose your time step. For example, lets say your mesh is really fine and has a size of 1mm and in 0.001 second it rotates by a value greater than 1mm, say 10mm (calculations should ofcourse be in radians and all that just an example). You should reduce your time-step in this case to be close to 1mm. (The deformation of mesh should not be a huge, sudden jump) As for the inner iteration, start with 7, and if your residuals drop below 10^-3 or lower, you can leave it like that or you can increase inner iters if you would like. This worked pretty good for me but ofcourse this is an entirely different case. Hope this helps. =D
Thanks for your input. Your case also seems reasonable. But I don't understand you said ''calculate how far ur blade travels for each time step and according to your mesh'' because I can only see how a parameter for instance velocity behaves across the blade because I have given one input that is incoming wind velocity and the rotational speed of the blade. How would you examine if it rotates a value greater than a certain number? My mesh is 0.005 m on the blade.

April 25, 2020, 10:46
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Firstly, I only have the fluid (static region) around the blade. The domain is 3D in bird view (V-shaped) where blade is placed on extruded-hub. I am not using overset mesh. My purpose it to capture flow transition not the deformation. Also, I already have the rotational speed for the blade and the incoming flow velocity (which is obviously needed to create the report).

This is still confusing '' For a specific time step, x, your blade will rotate a specific amount, y. So if it turns at 1rad/s, for a 0.001second time step, with a radius of 2 meters lets say, the tip of your blade will travel 0.002m or 2mms''. As I have already provided the rotational speed in reference frame how will I know that at what rate the blade is turning in a specific time step?

 April 25, 2020, 16:29 #6 New Member     Onur Join Date: Dec 2018 Location: Turkey Posts: 25 Rep Power: 5 Seems like I made unnecessary comments based on your domain. If I understand correctly, your rotation is constant (no acceleration) which means its rotation rate will be constant for each time step, say 1rad/s. If you have rotational acceleration, you can create a report in reports tab and create a motion report and choose your function accordingly. Then monitor and plot this report to see what your rotation speed is at each physical time. Otherwise, for a constant acceleration, it wouldn't make sense to monitor the rotation rate. (Maybe displacement) =D If you are still confused, just give it a time-step and let it run and keep an eye on your residuals. Reduce it if they go crazy.

April 25, 2020, 16:41
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 Originally Posted by SimulatorMaster Seems like I made unnecessary comments based on your domain. If I understand correctly, your rotation is constant (no acceleration) which means its rotation rate will be constant for each time step, say 1rad/s. If you have rotational acceleration, you can create a report in reports tab and create a motion report and choose your function accordingly. Then monitor and plot this report to see what your rotation speed is at each physical time. Otherwise, for a constant acceleration, it wouldn't make sense to monitor the rotation rate. (Maybe displacement) =D If you are still confused, just give it a time-step and let it run and keep an eye on your residuals. Reduce it if they go crazy.
You're right about rotation rate it's constant.But for my study I want to give several time steps and see on which time step I get steadiness of the results.
For instance, I was assuming to make calculations by considering the specific rotation degree in one full rotation of turbine blade i.e. 360. For example, if I take 2 degree rotation and say 4 number of rotations and then using this information I calculate time-step size and number of time-steps. It would be a good idea but I am just concerned about the choice of ''degree rotation'' and ''number of rotations'' that on what basis I should choose them - I mean there must a reason for choosing a specific degree rotation and number of rotations. Would appreciate your feedback on this.

April 26, 2020, 10:23
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 Originally Posted by Steklo_Plastik Your path is as old as my life. Look for another way. It should be with natural accompaniment. Take DFBI and 6 dof. That is happiness. Otherwise, the wind will not loosen your screw. You will have to set the fan speed. This is a bad and false way. Specifically for your question: the movement of the blade should not exceed 0.5 cells. You can use the automatic control model CFL. Download from physical models. If the accuracy of the calculation is important to you, and not the beauty of the animation, then use it. Draft calculations can be done in large increments, but no more, when the calculation is destroyed. It is established experimentally. Good luck and fast growth. I warn you: the program is not yet perfect in terms of 0versetMesh . It is difficult when there are many bodies. https://youtu.be/PigTaG_GFoo https://youtu.be/4fk2vH2pm-E https://youtu.be/587oz1d41KU Auxiliary images: Attachment 76955 Attachment 76956 Attachment 76957 Attachment 76958 Attachment 76959 Problems only occur when the rotating 6 DOF bodies are connected to the main body. We need to gain experience. There are no lessons here. Insert 6 DOF - (DFBI) in scenes, not in surface meshes of bodies.

Thanks. But my case is related to onshore WT blade not offshore so I have nothing to deal with drafts etc.

April 26, 2020, 10:54
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Thanks for the comments. I did the first run but I see that the residuals started showing up from the right side of the plot rather left side which is uncommon for me. Any idea what could go wrong?

Last edited by mazhar16823; April 27, 2020 at 06:31.

 May 10, 2020, 11:27 #11 New Member     Onur Join Date: Dec 2018 Location: Turkey Posts: 25 Rep Power: 5 Huh thats wierd. Sorry I didnt see this were you able to overcome the issues? A reason for that would be the plot settings on residuals graph which can be editted by right clicking on the axis.

May 10, 2020, 11:29
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 Originally Posted by SimulatorMaster Huh thats wierd. Sorry I didnt see this were you able to overcome the issues? A reason for that would be the plot settings on residuals graph which can be editted by right clicking on the axis.
Thanks. I had resolved this issue.

 Tags implicit unsteady, starccm+, time step