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Timestep for Implicit Unsteady solver in STARCCM+ and maximum number of steps? 

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April 24, 2020, 07:50 
Timestep for Implicit Unsteady solver in STARCCM+ and maximum number of steps?

#1 
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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 
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Onur
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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 timestep 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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April 25, 2020, 10:04 

#4 
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I assume that your turbine(your solid region) is in another region that is a fluid rotating(perhaps you used overset mesh?). What i mean by that line is that your turbine will rotate at a specific amount according to your timestep. 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. 2mm travel is pretty acceptable if your mesh size is around 1mm or so AT YOUR BACKGROUND MESH.
The mesh on your blade is important yes, but whats really important in this case is the mesh on your background region, which is your fluid region. So, what you need to do initially is to calculate the maximum rotational speed that your turbine will achieve with a given fluid velocity. Then, calculate your maximum displacement at max. rotation then proceed with the calculation I did above. Finally, find your timestep. You can also calculate the rotational acceleration of the turbine and find your maximum physical time needed to achieve max rotational speed. (There should be equations lying around=P) Furthermore, if you are going to analyze the deformation on your wing, (just throwing it out there), after you setup your morphing(a motion option), mapped interfaces and all that junk, adjust your mesh similar to your background mesh (your fluid region). The motion Tutorials on StarCCM really help. Hope I was clear. Though if you have more questions please ask. =D 

April 25, 2020, 10:46 

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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 
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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 timestep 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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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 timestep size and number of timesteps. 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, 09:44 

#8 
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Yea well said friend. I have dealt with overset mesh which was a pain in the a** but I managed somehow. You do need some experience through trial and error and, ofcourse, man hours.
Here is theh thing about timestep; I choose it accordingly after few trials while monitoring residuals. Again, you do not want a huge time step for better accurate results. Like my friend said "...the movement of the blade should not exceed 0.5 cells." You should not base your rotation rate on your timestep; should be the other way around. So, choose a rotation rate, and then choose your time step according to whatever mesh you have. If you would like to implement acceleration, find the max displacement over a second, and again choose your timestep. If you would like to analyze the change of increasing timestep on your domain, you can assign functions that will automatically do it for you. (Or just do it by hand after a full rotation. Since your flow is constntly flowing but your blade is rotating, you wont really reach a steady state. What important are you residuals. Assign a timestep and give your sim a go. First step is always important. =D Im sorry if Im confusing. Im not an expert Im just giving out my knowledge from my experince through overset mesh and motion sims. 

April 26, 2020, 10:23 

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

May 10, 2020, 11:27 

#11 
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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 

#12 
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Tags 
implicit unsteady, starccm+, time step 
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