Thank you, it's very kind of you to let us know the video was helpful - good luck with your Thesis!

Thank you very much Oussama!

Thanks Eduardo, yeah OpenFOAM is really impressive - best of luck!

Thank you very much Doc!

Thanks a lot Glig

Thank you, you're most welcome!

The total force on a surge is the sum of the perpendicular force (pressure) and tangential force (friction. That is why you need to sum them. The minus sign is because of conventions on how to define force direction, has no specific meaning.

Hello, sorry how do I do this?

You're most welcome!

In OpenFOAM, the pressure values are relative to a reference pressure we set ourselves (p-infinity). And we set p_infinity to 0 in our simulations, so it drops from the equation.

Thanks Nikhil, I hope it's useful!

We don't have a video on this we have some very old OpenFOAM course material: airshaper.com/courses

@@AirShaper thanks for the link, ill at least check that out haha.

Dear Saverio, good question - I don't know! If I look purely at the friction forces and integrate them across the car, then I get a negative value in the driving direction. That is physically not possible. When I apply the minus sign, it matches perfectly with the forces we calculate in OpenFOAM. My guess is that perhaps this has something to do with the normals, which can be inside-out on the surface (because the surface you see is actually the boundary of the air volume, not really the surface of the car). If you find out more, thanks for letting me know!

Sometimes it could be because you need to go to the last time step, or select the decomposed case data

You need to run a separate post processing command in Open foam to get that field

Thank you very much Hess!

Thanks Diego!

Thank you Jaydeep!

It depends on how they store the pressure field. In some calculations, the pressure field p is the real pressure. In other calculations, it is the normalized pressure (divided by the density). It depends on which solver you use, if you use a reference pressure, compressible flow, etc.

Thanks :)@@AirShaper

Ow haven't had that before yet - surfaceLIC perhaps only works on surfaces (a slice, object surface, ...). Perhaps that is the reason?

This sim would typically take around 3 hours on a machine with around 170 virtual CPUs and a few 100gb of memory. But you can run it on smaller machines as well.

Yes, the forces and moments relate to 0 0 0. If you want to change that, you can do a translate operation on the entire internal mesh, to reposition the origin.

Thanks Michiel!

Hi Sylvain, for the AirShaper report, we first cut away the cells that are touching the surface of the object (in OpenFOAM). This avoids having the entire object surrounded in a "pressure bubble". We haven't found a way to do that in Paraview, but we stopped looking after we solved it in OpenFOAM directly. I would guess that with some creativity this should be possible in Paraview as well.

Ok Wouter 👍 The result is perfect! Great work.

Dear Nilma the Drivaer data is not available. But currently, you get a coupon for a free simulation when you create an AirShaper account. And that one comes with the OpenFOAM result data you can use for this tutorial!

it's a minus sign! --> ...kHat-coords)

Hi Braeden, the field files of OpenFOAM don't provide you with that information I'm afraid. The best way is to check, if you have the case files, in the constant properties & controlDict files which density & solver were used. Then, checking OpenFOAM documentation, you can deduct if the density was normalized. I'm not sure if it's always normalize by the density, it could be!

Hi! I don't think I completely understand the question - what do you mean with the single integrated vector (force & torque are not combined, I think, in our video?)?

@@AirShaper No no I meant using i.e. resulting (so integrated) force vector instead of force field. But I can see that after IntegratingVariables it already uses the single vector in further calculations. Thanks again

@@AirShaper Do you maybe have some hints on calculating the center of pressure? I already calculated the arm of the force and not sure how to proceed

Hope this all makes you go faster! I saw you race motorbikes --> be sure to check our interviews with Mv Agusta: airshaper.com/videos/race-talk-with-mv-agusta-top-aerodynamic-priorities-for-moto-gp-bikes/video/BUkaejkHRn8 airshaper.com/videos/race-talk-with-mv-agusta-how-to-improve-moto-gp-aerodynamics/video/fGNdMaVft_w

I haven't heard of Xsim before. In OpenFOAM, the wallShearStress is generated during post processing with a separate command.

@@AirShaper that's great to know, thanks. Xsim is a nice web based (runs locally) GUI tool that prepares the scene for openfoam, which then you'll run on your machine. But for some reason they didn't include the wallshearstress in the post processing, but the calculation of drag and lift coefficients is there. But I guess it's wrong

@@piereligiodisante Interesting, I hadn't heard of it before, thanks a lot! Hm, then best to double check all the calculation settings if the values are wrong :)

@@AirShaper i assume it’s wrong because there is no wallshearstress output, so that isn’t used for sure

@@piereligiodisante It could still be correct, the OpenFOAM "forces" and "forceCoeffs" might take wallShearStress into account (without writing that field)

By default, OpenFOAM (at least for the simpleFOAM solver) will write the normalized pressure, which means it is divided by the density. To obtain the real pressure (and thus the correct force) you indeed need to multiply it with the density. You can indeed calculate the pressure coefficient as well (which can be done using both the static and the total pressure) to get a "relative" distribution. When we show "pressure clouds" in our reports, we use an isosurface for the total pressure coefficient for a value of 0 (which nicely shows you the wake!). There are commands in openfoam to calculate the total pressure as a new field - those are pretty useful as well.

@@AirShaper okay, makes sense. Thank you very much!

Hi Pedro, I use Cp-total in tje tutorial. That is the pressure coefficient calculated using the total pressure. If you make a 3D isosurface of that in the air (so not at the outlet boundary) you get a 3D surface that denotes the split between zones with a total pressure coefficient above and below zero. Everything contained within those clouds has a Cp-total lower than zero (so lower than the total pressure of the free stream). These low pressure 'bubbles' hold the vehicle back, as this negative pressure creates a suction effect on faces pointing towards the rear of the car in many areas.

@@AirShaper Thank you! I found the channel today and it has been very educational. The chats with real world designers are a great touch. Please keep producing content.

Thanks Pedro, will do, good luck to you!

@@pedrobrito8080 Thanks a lot! And good luck with your projects!

Hi Willy, not for this model directly (although the Drivaer car can be requested at the TUM - Technical University of Munchen). I recommend using the simulation data of this public project instead: app.airshaper.com/projects/sample/sports-car Just click on the button "Simulation data" to download it - the structure is entirely the same as for this tutorial!

Dear Ahsok, yes, I believe it is possible to make animations in paraview (changing camera positions and even having particles move along a trajectory?). We don't have enough experience on that ourselves to make a video, however. There is quite a large paraview community (with forum), perhaps you can find answers there!

Hi Idrees, such a video would be too specific / detailed for our channel. But I've exported data from curves for example to csv, to then plot in excel!

@@AirShaper where is that vedio?

@@idreeskhan1629 There is no video on that topic - it's just how I did it in the past. For example, you can first plot the surface data on an object and then make a 2D slice through it. That will give you a curve. Then, you can click that slice and go to "File - Save data" and save it as a csv file. You can then process that data in excel. You may need to first add coordinates to the results (using a calculator) so X, Y and Z are exported as well.

@@AirShaperthank you, kindly if you send me your email i will send you a paper whose graphs i need if you guide me how draw those graphs please,

@@idreeskhan1629 Hi Idrees, I can't go as far as to provide custom assistance for free for everyone. If you need assistance, you can contact me at wouter@airshaper.com for regular (paid) consultancy.

This is a public model available for download here: www.mw.tum.de/en/aer/research-groups/automotive/drivaer/ Please note there are many different combinations possible (estate, fasteback, detailed or simple underfloor, ...). Good luck!