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Flow Simulation Vehicle on Road

Question asked by Marc Wolle on Mar 17, 2013
Latest reply on Mar 26, 2013 by Jared Conway

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Hello every body,

for my final year thesis I designed a car body which I now have to analyse in terms of aerodynamic aspects.

To not neglect the "ground effect" I put my vehilce on a flat plate plate 4m x 6m x 1mm.

My computational domain does not include the whole street but "cuts" trough it and goes a few mm below the bottom of the plate.

I also added "real wall"boundary conditions to the plate making it move the opposite direction of the vehicle travel direction.

I wonder now if the plate is part of the lift and drag analysis of the car or not, I obviously want the road to be excluded!

I made a few simple experiments with a cylinder and a plate, but still can't seem to figure out if the plate is part of it or not:

just the cylinder

lift: 21.6

drag: -573 (direction of travel is +)

plate and cylinder

lift: -35 (downforce)

drag: -1393

plate in motion and cylinder

lift: -29

drag: -1364

HELP is very welcome!

Thanks already!

Have a great Sunday!

Jared Conway Mar 18, 2013 12:28 AM

Correct Answer

How are you measuring? Goal? Surface parameter? If goal, global or surface?

You should be able to get what your want with a surface goal, select all surfaces of your body, not the plate. All the surfaces is important, open surfaces will result in erroneous results. Check the swx know for more info on this.

In the end, compare to a known solution for verification.

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Jared Conway Mar 18, 2013 12:28 AM

How are you measuring? Goal? Surface parameter? If goal, global or surface?

In the end, compare to a known solution for verification.

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Marc Wolle @ Jared Conway on Mar 25, 2013 6:46 PM

sorry for the late reply.... Thanks' Jared! I should hire you as my personal consultant, you are a great help!

I was using global goals before, now using surface goals, works great!

I do have another question though,

When showing the results of the goals; along with min, max and average values, there are also "Delta" and "Criteria".

What are these, how do they influence my results and how can these be changed?

The reason why I am asking is because I ran simulations with identical parts and settings with a few days in between and getting different results the second time.

When trying to find reasons for that, i found, that delta and criteria also change significantly between the simulations....

Thanks!

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Jared Conway @ Marc Wolle on Mar 26, 2013 12:40 AM

If you ever get stuck, we do offer consulting and mentoring through my group at Hawk Ridge Systems.

The help, technical reference and solidworks on will give you more detailed information on criteria and delta but what you need to know is that criteria is based on your result resolution level. It is set by the solver based on a lot of factors. You can override it in the calc control options but it is generally suggested to leave it alone.

Delta is the deviation of the value over the last analysis interval. Usually the last 0.5 travel. If delta is lower than the criteria the solution is considered complete.

It would be strange to see the same problem run over again get completely unique results. How different are they? How do the convergence plots look when you put them overtop of each other?

Sometimes you can't just go by the final value, sometimes you have to look at the average over the analysis interval or if you really want to tighten convergence, up the result resolution or set the criteria manually. But before you do that, make sure you are running identical problems and check the convergence plots. It may be possible that you had a different end condition or that the results hasn't converged in one vs the other. Ie, check all the goals convergence values and plots, number of iterations, travels cells...etc

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Jared Conway Mar 18, 2013 12:28 AM
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Correct Answer
How are you measuring? Goal? Surface parameter? If goal, global or surface?

You should be able to get what your want with a surface goal, select all surfaces of your body, not the plate. All the surfaces is important, open surfaces will result in erroneous results. Check the swx know for more info on this.

In the end, compare to a known solution for verification.
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- Marc Wolle @ Jared Conway on Mar 25, 2013 6:46 PM
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  Correct Answer
  sorry for the late reply.... Thanks' Jared! I should hire you as my personal consultant, you are a great help!
  I was using global goals before, now using surface goals, works great!
  
  I do have another question though,
  When showing the results of the goals; along with min, max and average values, there are also "Delta" and "Criteria".
  What are these, how do they influence my results and how can these be changed?
  The reason why I am asking is because I ran simulations with identical parts and settings with a few days in between and getting different results the second time.
  When trying to find reasons for that, i found, that delta and criteria also change significantly between the simulations....
  
  Thanks!
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  - Jared Conway @ Marc Wolle on Mar 26, 2013 12:40 AM
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    Correct Answer
    If you ever get stuck, we do offer consulting and mentoring through my group at Hawk Ridge Systems.
    
    The help, technical reference and solidworks on will give you more detailed information on criteria and delta but what you need to know is that criteria is based on your result resolution level. It is set by the solver based on a lot of factors. You can override it in the calc control options but it is generally suggested to leave it alone.
    
    Delta is the deviation of the value over the last analysis interval. Usually the last 0.5 travel. If delta is lower than the criteria the solution is considered complete.
    
    It would be strange to see the same problem run over again get completely unique results. How different are they? How do the convergence plots look when you put them overtop of each other?
    
    Sometimes you can't just go by the final value, sometimes you have to look at the average over the analysis interval or if you really want to tighten convergence, up the result resolution or set the criteria manually. But before you do that, make sure you are running identical problems and check the convergence plots. It may be possible that you had a different end condition or that the results hasn't converged in one vs the other. Ie, check all the goals convergence values and plots, number of iterations, travels cells...etc
    Like • Show 0 Likes0
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Flow Simulation Vehicle on Road

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