Tutorial: CFD simulation of a Buoy Motions in Waves Using DFBI and Overset Mesh (STAR-CCM+)

[@kedielanora]

Thank you very much Mujteba. I've learned STARCCM from your tutorials. Greetings from Turkiye!

[@CFD_Mojtaba]

You are welcome @Kedi Elanora. I am really happy that I could help you.

[@ahmedsh.8817]

Thank you; important note at 45:34 for damping function

[@designsimulate3384]

Hi, just a quick question about gravity value. The laboratory coordinate z-axis is positive upwards, then why you used the +9.81 when defining gravity? should it be -9.81 ? @42:32 time in the video

[@CFD_Mojtaba]

The gravity should be pointing downward toward the earth, I think I have respected this in the tutorial.

[@markstephen6677]

Well-structured and well-made!!

[@mortezamaaliamiri4342]

Thanks for this video!! I enjoyed learning all these materials in it!!

[@TheHumanNexus]

Thank u for such a detailed tutorial…

[@MohammedKhantheacademician]

Why did you not specify the velocity of current and wind in the VOF? Is the Buoy stationary? I have a boat moving at certain speed, where should I specify its velocity?

[@designsimulate3384]

Hi, what if I want to give a specific velocity to the air, then how should I model it and what changes to make. please comment.

[@CFD_Mojtaba]

Hello, I think it is possible by using the air volume of fraction value at the inlet. About the weight, it has been calculated to guarantee the floating of the body by putting weight equal to buoyancy.

[@designsimulate3384]

@@CFD_Mojtaba Thank you :)

[@designsimulate3384]

@@CFD_Mojtaba Thank you for your reply. Just for my understanding, if flow is from left to right, then the left most surface of the background mesh block is defined as the volume fraction of air? I am confused in the point is that this same surface is shared with water too, so would that effect the wave generation?

[@CFD_Mojtaba]

@@designsimulate3384 The volume fraction of water and air changes in the direction of gravity, so at the bottom we have water (heavier) then going up we get air. It can affect surface waves, but it's something you need to evaluate. Have a look at this video for more information: kzbin.info/www/bejne/bYibe6SGZrGYaas

[@RaghuVarierSIESGSTMECHANICAL]

Are you the same Mojtaba who solves CFD doubts on forums? I think I may have read your name....Nice video.... Very helpful .... Thanks a lot

[@mmnyako]

Thank you for sharing! quick question pls, Can we employ symmetry plane boundary conditions at the XZ plane and YZ plane of the buoy? I am guessing it will reduce some computational demand, BTW I am using ANSYS fluent

[@basmahabeeb3485]

Hello! Could you please illustrate the value you used in the time step and you says"However, note that the estimation of the time - step size should also be in such a manner that in each time - step, the overset mesh moves a distance smaller than the size of the smallest cell in the background mesh. Therefore, considering this point, the time step size that we are going to use here is 0.001 sec. " from where we can see 0.001 sec in the simulation? please.

[@CFD_Mojtaba]

Hello, To calculate the time-step size, you can use the Courant number defined according to the minimum cell size within the background mesh, along with the maximum vertical displacement speed of the overset mesh. I hope this helps. Respectfully, Mojtab Maali Amiri.

[@basmahabeeb3485]

@@CFD_Mojtaba Hello! I still have a problem in the wave generation shape, I tried more than one example but still not normal shape, could you please let me know after calculating the courant number how we can use it in the simulation? I looked at papers says ::"with a second-order temporal discretization scheme. This results in a Courant number (C=Cg. Δt/ Δx, where Cg is the wave group velocity) less than 0.5; hence, the High-Resolution Interface Capturing (HRIC) scheme that is suited for tracking sharp interfaces is guaranteed to be used during the simulations. My question is after calculating the courant number (C=Cg. Δt/ Δx, how to use the result value in the simulation? Do you have an example, please?

[@kedielanora]

Dear Mujteba could you please upload the 1080p version of this tutorial? Regards from ITU

[@CFD_Mojtaba]

Thank you very much for your comment; unfortunately, this is the only version that I have.

[@norisato3171]

Thanks for this video. Very helpful!! Could you please tell me why the side and top boundaries are velocity inlet?

[@CFD_Mojtaba]

Hello Nori Sato, Thank you for writing! The main reason to consider the side and top boundaries as velocity inlet is to prevent the flow from reflecting back to the domain and also to represent an open sea, which indicates deep water and infinite air conditions. I hope this was helpful. Respectfully, Mojtaba.

[@norisato3171]

Thanks to your explanation, I understood very well. Thank you.

[@SuperArundhathi]

Hi, first of all thank you very much for the video . I am doing free surface simulation of a floating body under waves in starccm+. As the first step i have generated waves without a body but getting reversed flow . my mesh refinement is fine . can you anyhow explain the reversed flow . And is there any other boundary conditions I can use instead of pressure outlet to absorb my waves like we do in in experiments in starccm+. Thanks in advance.

[@z-fb4qq]

Hi! Thanks for the video, I have a question, why are you set the gravity to [0,0,9.81], which means the direction of the gravity is upward(Z+)? Shouldn't the direction of gravity be downward? And set to [0,0,-9.81]. Please answer for me, thank you so much!

[@CFD_Mojtaba]

Dear zyh, The definition of the gravity vector direction depends on the system of coordinates that you are using. The point is that, this vector must be pointing downward toward the ground.

[@z-fb4qq]

@@CFD_Mojtaba thank you！

[@abdazisazrin5784]

ive tried this 4 times. but it always have reversed flow. but it just between +- 500 surfaces. I think we need to do more refinement isn it.

[@designsimulate3384]

Hi, If I want to give the floating body an initial pitching angle like 8 degrees, then where should i do that? I am struggling with it. I tried to transform and give rotation of overset block and then initialize the solution but the pitch plot starts from 0 degree, not 8 degrees. can you suggest something about it. Thanks

[@CFD_Mojtaba]

You may be able to do this under Rigid body configurations. I hope this helps! Good luck.

[@designsimulate3384]

@@CFD_Mojtaba under the DFBI option?

[@CFD_Mojtaba]

@@designsimulate3384 Yes. I think so.

[@designsimulate3384]

@@CFD_Mojtaba Thank you

[@ridoykarmoker8747]

Hello, Can I calculate added mass and damping coefficient for heave motion from this simulation?

[@CFD_Mojtaba]

Dear Ridoy, The calculation of the added-mass and damping coefficients are performed through forced harmonic motion.

[@ridoykarmoker8747]

@@CFD_Mojtaba Do I need to define forced harmonic motion in physics? All the other process shown in video will remain same?

[@CFD_Mojtaba]

@@ridoykarmoker8747 you need to specify this motion under the Rigid Body Motion

[@rohanverma4145]

Thank you for making this vedio... its very informative.

[@rachidtayeb1465]

Thanks for this video, but a have problems when I run simulation I see this messages 'enabling DFBI during time step unsupported. Thank you

[@CFD_Mojtaba]

Hi Rachid, Thank you for writing. Apparently, there is an error in your set-up, and you need to compare closely your CFD configuration to the set-up presented in the video. I hope this was helpful. Respectfully, Mojtaba Maali.

[@vanduyenvimaru6835]

Thanks for your good tutorial. Can you please tell me how to check the quality of mesh in StarCCM?

[@CFD_Mojtaba]

@Văn Duyền vimaru thank you for writing. The quality of the generated mesh can be measured through the evaluation of the cell skewness angle. For this purpose, right click on plots> New plot> select Histogram Plot. Click on Histogram Plot 1 and set the parts to your computational domain. Then, under axis type click on Bin, and set the field function to Skewness Angle. In the resultant plot, you can see the number of cells with a specific skewness angle. In order to reduce the influence of the skewness angle on the results it is recommended to keep it under 85 degrees. I hope this was helpful. Respectfully, Mojtaba.

[@vanduyenvimaru6835]

@@CFD_Mojtaba thank you so much for your replies. It is so helpful!

[@vanduyenvimaru6835]

Could you please show me how to export the mesh from StarCCM?

[@CFD_Mojtaba]

@@vanduyenvimaru6835 you need to select File>Export . This menu item opens a dialog, in which you can specify the following items: 1. file type. 2. the name of the new file 3. the types of data to export to the file. You can find more information about this topic in "Exporting Mesh and Solution Data" in the user guide. I hope this was helpful! Happy new year! Respectfully, Mojtaba.

[@vanduyenvimaru6835]

@@CFD_Mojtaba thank you so much!

[@DevanarayananRoe22m010]

How did you calculate mass of the buoy as 780 kg?

[@CFD_Mojtaba]

You can calculate the mass based on its draft. To be in balance, weight must equal buoyancy.

[@DevanarayananRoe22m010]

yes but here for this cylinder if we are considering the draft as 2m , we are getting 1560 kg as weight right. Then why it is considered as 780 kg?

[@CFD_Mojtaba]

@@DevanarayananRoe22m010, because we are simulating only half of the cylinder.

[@DevanarayananRoe22m010]

thanks a lot @@CFD_Mojtaba

[@DevanarayananRoe22m010]

can we get linear added mass coefficient and radiation damping coefficient from star ccm+? If so, can you tell how to do that?

[@rameshmukkund3321]

How are you defining initial overset dimensions?

[@CFD_Mojtaba]

Dimensions are selected almost randomly here. However, it may be necessary to carry out a sensitivity study in relation to this parameter to ensure that it is not affecting the results obtained.

[@prothomontim575]

Thanks for this video!! Can u plz tell me how can I calculate the mass moment of inertia specifically?

[@CFD_Mojtaba]

@prothom ontim thank you for writing! In this tutorial, as the cylinder has no angular motion, no mass moment of inertia is necessary. But, if you want to include the pitch motion, the mass moment of intertia would be (1/12)*m*(3*r^2+h^2). I hope this is helpful. Respectfully, Mojtaba Maali.

[@CFD_Mojtaba]

@@jawadulislamchowdhury4813 thank you for writing. You can find the KVLCC2 characteristics, which you are looking for, in these two papers: www.mdpi.com/2077-1312/9/2/136, www.sciencedirect.com/science/article/pii/S2092678216305076. I hope this is helpful. Respectfully, Mojtaba.

[@xiudiren2787]

Hi Mojtaba, there is a message told me that "Please check your initial values for "Volume Fraction" in the following regions: [Backgrounds, Overset]", however, I check all settings which are the same to yours. Could you help me how can I fix this? Thanks very much.

[@CFD_Mojtaba]

Dear Xiudi Ren, If the settings are the same, I'd say you need to check the quality of your mesh on the free surface and at the interface between the background and overset regions. Most of the error in the CFD can be traced back to the mesh. I hope this was helpful. Respectfully, Mojtaba.

[@xiudiren2787]

@@CFD_Mojtaba Hi Mojtaba, I found that if I change the version of the Star CCM+, then it works. Thank you very much for the video and reply!

[@fouadfayez6247]

sorry if I want to change wave angle to 225 degree how can i do that ?

[@CFD_Mojtaba]

You can change the angle of the body under investigation.

[@lex-h7c]

can someone suggest how to do a free decay test for heave in this simulation

[@CFD_Mojtaba]

Hello Dear, Thank you for your comment. You just need to apply some initial displacement in the heave direction without considering any waves. In this video you can learn hoe to simulate the calm water case: kzbin.info/www/bejne/eIu0aamFr511qrc

[@lex-h7c]

@@CFD_Mojtaba Thank u sir

[@hassanahmadsheikh5130]

Need help....Why is it showing to me that rigid body "Bouy" has zero moment of inertia?

[@CFD_Mojtaba]

Dear hassan ahmad sheikh, This may be due to the fact that, in this simulation, the Buoy has a one-degree-of-freedom motion in heave direction. To have moment of inertia, you may need to activate the DOF of motion about a rotational axis. Respectfully, Mojtaba Maali.

[@hassanahmadsheikh5130]

@@CFD_Mojtaba Thanks dear I'll try this tomorrow in my lab.... As I'm simulating hull in sea condition so prior to that i found your tutorials very informative at this point I'll also request you to please create a series of videos regarding fluid theory concept in terms of practical application and in softwares.

[@basmahabeeb3485]

Could you please tell me how to change the language of the STAR CCM+ Program ? for example from Chinese to English？

[@CFD_Mojtaba]

Dear Basma Habeeb, For this purose, you need to right-click on the desktop icon and select Properties. Then under the Target add this sentence: -locale en. I hope this was helpful. Respectfully, Mojtaba.

[@basmahabeeb3485]

@@CFD_Mojtaba thank you very much ! I tried this but it was appear a message not correct. I need to send a screen shot to you , but I do not know how to send?

[@basmahabeeb3485]

@@CFD_Mojtaba My friend

[@basmahabeeb3485]

@@CFD_Mojtaba I tried again already changed thank you very much!!

[@DOE-c7w]

in continua am not geting mesh option please help

[@CFD_Mojtaba]

The new versions of STAR-CCM+ do not have this option. Please have a look at this video to know how to generate the grid: kzbin.info/www/bejne/oZKmoHWtbd6hnq8

[@crisppyy_bacon3784]

What would be the reason for reversed flow on the outlet faces? I am experiencing this error on a similar simulation

[@CFD_Mojtaba]

Hi @CL, Thank you for writing. If your simulation involves the propagation of waves, to dampen properly the waves close to the outlet, you can apply a linear damping function with coefficient equal to f1 = πω, where, ω is the wave frequency, over a distance equal to the length of the wave system. If your simulation doesn't involve the waves, the reversed flow on the outlet can be an indication of vortical flow strucutres. This signifies that the distance between your body of interest and the outlet boundary should be increased to resolve properly the flow characteristics. This problem can also be delt with by applying a fully developed boundary condition on the outlet, in which the normal gradient of variables is zero. I hope this was helpful. Sincerely, Mojtaba Maali Amiri.

[@crisppyy_bacon3784]

@@CFD_Mojtaba Thanks so much for your quick response! I will try the linear damping in my model.

[@CFD_Mojtaba]

@@crisppyy_bacon3784 good luck in your modeling, let me know if you have further questions.

[@crisppyy_bacon3784]

@@CFD_Mojtaba it appears to be working now! at first i got reversed flow even with damping but it eventually straightened out. I am using a 4-core laptop so my mesh is a bit more coarse. thanks for the help!

[@CFD_Mojtaba]

@@crisppyy_bacon3784 with a 4-core laptop you may have difficulty in performing your mesh convergence study. But, anyway, I am glad to know that it is working now.

[@anjalikrishnac3738]

Hello sir ,can I follow the same procedure for a stationary sphere in a freesurface !

[@CFD_Mojtaba]

Hello, Thank you for your comment. Yes, you can use the same procedure. Respectfully, Mojtaba

[@anjalikrishnac3738]

@@CFD_Mojtaba Thankyou for your reply sir😊

[@anjalikrishnac3738]

Sir ,In my starccm version, it is recommended to use part based meshing instead of region based, as a warning .So I tried with part based meshing ,but while executing the mesh ,a warning came like " trimmer cannot currently accomodate multiple regions in one continuum in starccm+". Sir, please help me to solve this one. Can you please explain the things I have to follow while doing part based meshing instead of region based meshing .

[@CFD_Mojtaba]

@@anjalikrishnac3738 part-based meshing is basically the same as region-based one. Also, to solve the problem you are facing, you need to associate a separate mesh continuum to each separate region. I hope this was helpful. Respectfully, Mojtaba.

[@anjalikrishnac3738]

@@CFD_Mojtaba yes sir it's helpful.

[@PVRADHKRISHNAN]

Hello sir, can you share me the step by step procedure in Star ccm+ to calculate the torque and rpm of a wind turbine?

[@CFD_Mojtaba]

Hello RADHKRISHNAN PV, Thank you for writing. The angular velocity (rpm) is defined by the user. On the other hand, torque at any blade section is defined as the net aerodynamic force in the plane of rotation times its distance from the center of the rotor. Thus, the moment in the direction normal to the rotor plane is exactly the torque value. To calculate the torque value, all you need to do is create a local coordinate system such that one of its axes is perpendicular to the rotor plane. The moment in the direction of this axis is equal to the torque value. I hope this was helpful. Respectfully, Mojtaba.

[@PVRADHKRISHNAN]

Thank you so much sir for your response, I will try it

[@ogulcanahi8093]

I have a question about StarCcm+, an advanced one. Can you contact me?

[@CFD_Mojtaba]

@Oğulcan AHI thank you for writing. You can ask your question here, or send your question with detail to my personal email: [e-mail]. I will get back to you as soon as possible, since I am traveling at the moment. Respectfully, Mojtaba.

[@ogulcanahi8093]

@@CFD_Mojtaba Okay thank you!

[@rylangomes9408]

Hi Mojtaba, thank you for your reply on the previous video. Wanted to know if I could use the same steps from the video to simulate a rotating bicycle wheel? Can I reach you through email or zoom if you can help me that would be great! Thanks.

[@CFD_Mojtaba]

Dear Rylan, Sure you can use the steps from the video to simulate a rotating bicycle wheel, but you need to implement the characteristics of your body instead of the buoy. You can reach me through the following email: [e-mail]. Respectfully, Mojtaba.

[@giuseppeocchipinti6875]

thank you

[@davidmestanza6158]

Muito obrigado pelo tutorial!

[@babakkhadivar6635]

Very good

[@Nature_Shorts116]

Wow!! Very well!!

[@designsimulate3384]

I am learning Star CCM from your tutorials. Much appreciated. Can you share star ccm file for this tutorial?

[@CFD_Mojtaba]

Hello, thank you for your comment. Unfortunately, I have lost the STAR file of this particular tutorial.

[@designsimulate3384]

@@CFD_Mojtaba no worries. Do you have geometry of 5 MW reference wind turbine?

[@CFD_Mojtaba]

@@designsimulate3384 I dont have this geometry, but you can find it several sites such as GrabCad. I have also found this link: www.stlfinder.com/model/nrel-3-blades-5mw-rotor-W0T1qhHP/2640361/

[@designsimulate3384]

@@CFD_Mojtaba Thank you

[@milad9546]

Thanks, Great Video, Please do a video on 3d water entry of sphere!

[@CFD_Mojtaba]

Dear Milad Farahi, Thanks for writing! I will definitely consider your comment about making a video on this topic. Respectfully, Mojtaba.

[@holliswilliams8426]

@@CFD_Mojtaba I would also find this very useful if you have time to make it.

[@CFD_Mojtaba]

@@holliswilliams8426Thanks for writing! I will definitely consider your comment. Respectfully, Mojtaba.

[@jasonthomasshajee]

Thanks for the video. Can you make a simple video for DFBI of a sphere? I am trapped in an error. Thank you

[@CFD_Mojtaba]

Dear Jason, thank you for your interest in the video. Please send me an email to the following address explaining the error that you are experiencing: [e-mail]

[@teacherjessicacaroline]

Excellent!

[@AJAYKUMAR-et4xy]

Nice video'

[@JamesVestal-dz5qm]

So ive figured out that mesh is the geometry you import from autocad.