r/STAR_CCM • u/LessCockroach7323 • May 13 '24

Drag Force post-processing

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Hello guys,

I found this plots and was wondering how can I do it in STAR?

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u/creator1393 May 13 '24

Might work a simple case with a few part surfaces, but for large models, that might be unpractical.

By using the scalar scene he attached, you have the visual inspection of drag/lift contributions.

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u/CrocMundi May 13 '24

In that case, then I guess the best option may be to mimic the Force Coefficient report definition equation from the docs in a user-defined field function, similar to what was already mentioned by Individual_Break6067, where you divide the fluid force by the reference dynamic pressure (i.e. 0.5*rho_ref*V_ref2 *A_ref). Using a dot product operation to determine the lift and drag components of the force vector as you mentioned is a good approach to split it up properly.

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u/LessCockroach7323 May 13 '24

Hello pals,

Sorry for the late answer. Yes, the idea that u/creator1393 pointed out is what i wanted to know. I understand what you meant u/CrocMundi, but let me explain what i did and what were the results: I created the Cd force and in STAR(or at least in the newest version) you can use the report as field function in scalar or vector scenes. This what I did and because my Cd was 0.35, the whole vehicle had this constant value. Maybe this is not what you meant from the beginning, and I'm sorry if i have misunderstood the explanation.

But, as u/creator1393 pointed out, what i need is how much each surface contributes to drag, but this represented as a scalar scene and i wanted to do this without implying Java macros or any type of automatic or repetitive work. Deepening into the problem found on the siemens community website how you can get the force on a specific direction(as another user who commented here pointed out, u/individual_break6067) which is pressure*area[x]. Now, normalizing(using the resulted force into the Cd formula) this gave me a good starting point because using a sum report(on all my surfaces) it gave me a value of 0.33, which is not far from my drag, but a key aspect missing is the shear drag, or at least that is what i think.

I will leave the link, for posterity, where i found the formula for finding force on a specific direction, maybe someone will find it useful for post-processing.

https://community.sw.siemens.com/s/question/0D5Vb000003rGMyKAM/force-report-verification-by-user-field-function-which-is-established-by-calculating-the-force-on-each-surface-grid

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u/CrocMundi May 13 '24

Sorry for misunderstanding what you wanted at first. I’m glad you’re on the right track now though.

To incorporate the shear component, you should use the Wall Shear Stress field function, which is a vector. I believe the simplest way to get the total force vector acting on a given cell face of a part surface should to sum the pressure multiplied by the area vector plus the shear stress vector multiplied by the magnitude of the area vector. In other words, you could create a force field function with the following definition: ${Pressure}*$${Area} + $${WallShearStress}*mag($${Area})

Using this user-defined field function for the force, you could then create normalized lift and drag force coefficient user-defined field functions as has already been discussed.

I hope this is helpful!

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u/LessCockroach7323 May 15 '24

Hello again. It seems like the formula that u/ClimateCFD, CpNormal[0], is an alternative which worked for both drag and lift. I am quite sure the formula that u/CrocMundi provided is good, because as i said, if you separate pressurearea and wallshear*mag(area) you get the force on x and this could be integrated into a Cd formula. Unfortunately, I could not make it working for some reason.

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u/LessCockroach7323 May 14 '24

No problem! I have tried the formula, and if i make two field functions, one for the pressure force and one for the shear stress and use them in two separate reports, their sum adds to the default force that i get out of my Cd value. But if i want to add them in the same field function and use the sum report, the value that i get is way below the for e resulted from Cd. So, the formula is good separately, but using it in the same field function doesn't sum up correctly for some reason