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Airflow over car/spoiler

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Old   November 27, 2009, 12:44
Default Airflow over car/spoiler
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Rich
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Hi,

i want to set up my first Fluent Project. It's an analysis of flow over a car and spoiler.

i will list what exactly i am doing and i would be nice if someone could point out errors and mistakes, or suggestions to make it better.

i basically have a car body without spoiler and the spoiler as mesh (and then inlet, outlet, symmetry plane etc.)

  • i want to get the pressure distribution over the car and spoiler.
1. under residual monitors i keep the standard values 0.001
2. solution initialize. there i pick the inlet and initialize it.
3. under iterations i type in 200
4. let fluent calculate and wait until it converges
5. under post processing i plot the static pressures over the cars surface and under xy plot the cp over the carbody and spoiler.

  • cL of the spoiler
1. under results-reports i project the area of the spoiler onto the xy plane.
2. that area size i use under reference values-area.
3. i disable the converge option under residuals and just let fluent calculate 500 iterations
4. what i noticed: e.g. for 200 iterations i get an cL= -0.042 and for 500 iterations cL= -0.027.
i am not sure now how to interpret the difference in cL values, i guess the solution didn't converge yet? and cL decreases until it does? does anyone have experience how many iterations i should use?

  • cL of the car body without spoiler
1. same way as the spoiler except that i project the cars area onto the xy-plane.

  • how could i calculate cL of the whole car?
let fluent display the force acting in z direction of the car and spoiler, add them, and then use them in

cL = (Lcar + Lspoiler) / (0.5*density*speed^2*Area)

with Area = projected Area of the car onto the xy plane?
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Old   December 2, 2009, 13:18
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Joel
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What I've been told in the past is that for a converged solution you usually want to change the convergence criteria from .001 to .000001. This may be the reason your Cl value is still changing. This also will eliminate needing to guess how many iterations you need since it will stop once it reaches that convergence criteria.
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Old   December 2, 2009, 13:47
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Rich
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yep thanks. i let it run for 1500 iterations and cl became quite constant.

my only problem left now is how to calculate the lift of the whole car in fluent.
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