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Old 12-11-2019, 12:07 AM   #1 (permalink)
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Help me evaluate CFD results

I have been playing around with a model of my car and putting it in a cfd program. The model is not that great, however it is pretty accurate (shape wise) from the center line of the car. Based on this I have been messing with the angle of the hood to windshield and the nose. I have a limited understanding of how the numbers should be interpreted and would like more opinions on the subject. Rather than posting a bunch of screen shots directly here I have inserted them in a google document.

(quick sample)



https://docs.google.com/document/d/1...it?usp=sharing

The program I used for modeling is Autodesk Inventor and the CFD is Autodesk Flow Design.

The wind speed is set to 10 m/s (I forgot to increase it) unless otherwise noted.

The 2d plane the pressure is mapped on is in the same spot for all screen shots.
  • The car is not sitting on the ground plane, would this effect how to interpret the front and top of the car?
  • I had two options for mapping, pressure or velocity. Is one better to look at or is it a combination of the two that makes the data interpretable?
  • Is there anything that you guys suggest I try and mock up and test?
  • The first thing I wanted to test was the simple hood height (because of visibility and simplicity), the only differences I see is a lower high pressure (on the scale) and a more localized section of low pressure where the windshield and the roof meet. Also something different is going on with the underside, not sure if its because of the adjusted scale or it somehow affected the undercarriage.

Thank you in advance,
Matt

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Old 12-11-2019, 12:32 AM   #2 (permalink)
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•The car is not sitting on the ground plane, would this effect how to interpret the front and top of the car?

Is the ground plane moving? Are the wheels represented?

•I had two options for mapping, pressure or velocity. Is one better to look at or is it a combination of the two that makes the data interpretable?

They interoperate, or something. There should be some algorithm to combine the results.

•Is there anything that you guys suggest I try and mock up and test?

Is this analyzing a 2D plane though the centerline of the vehicle? If so it's meaningless. The entire 3D form is necessary.

•The first thing I wanted to test was the simple hood height (because of visibility and simplicity), the only differences I see is a lower high pressure (on the scale) and a more localized section of low pressure where the windshield and the roof meet.

The only difference I see is the speed and pressure. What hood height difference is there? Once again, the 3D form is significant. A Jeep and a VW Beetle have flat windshields at a similar angle. But the Jeep's is square while the Beetle has a teardrop that has been truncated at the front. It's body is widest at the C-pillar.


edit:
I looked around a bit. You may find answers at www.cfd-online.com/Forums/autodesk-simulation-cfd/
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Old 12-11-2019, 01:39 AM   #3 (permalink)
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Quote:
Originally Posted by freebeard View Post
Is the ground plane moving? Are the wheels represented?


They interoperate, or something. There should be some algorithm to combine the results.

Is this analyzing a 2D plane though the centerline of the vehicle? If so it's meaningless. The entire 3D form is necessary.


The only difference I see is the speed and pressure. What hood height difference is there? Once again, the 3D form is significant. A Jeep and a VW Beetle have flat windshields at a similar angle. But the Jeep's is square while the Beetle has a teardrop that has been truncated at the front. It's body is widest at the C-pillar.


edit:
I looked around a bit. You may find answers at www.cfd-online.com/Forums/autodesk-simulation-cfd/
1.Ground plane is stationary as well as the wheels. Same as in a wind tunnel.

1.5 Would that algorithm be the cD number or similar?

2. Can you expand on that? Other threads look at 2d profiles, which would be basically the same as a smoke overlay (like in the first post of the wind tunnel thread) and we still can make assumptions based on that data such as how small the vortex at the back of the car is. I just want to try and figure out how much difference it makes and have a plan before trying to make something on the car.
https://ecomodder.com/forum/showthre...ead-26678.html

3. Its relatively small. Like 3-4 inches up the windshield. I only put it in the center of the model because thats where the plane is. Jeep and beetles have different approaches though. Jeeps hoods are pretty much parallel to the ground. Ignoring the part at the base of the windshield the beetles is more obtuse.

I will check that out, thanks.


Wind speed set to 25 m/s. velocity instead of pressure.


Wind speed at 25 m/s velocity instead of pressure. It also seems like flow stays attached longer at the back.
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Old 12-11-2019, 02:34 AM   #4 (permalink)
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It appears you have a 3D shape that's being analyzed in a single plane. Do you have an isometric view? The single plane reduces the computation needed. How long does it take to generate a single view?

Further into the Wind Tunnel and Smoke thread:

Permalink #817

IIRC aerohead found a blister on the hood had no appreciable effect on his T-100 pickup.
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Old 12-11-2019, 10:20 AM   #5 (permalink)
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From memory the software is not that slow. And it does it in 3d.

It is sometimes less confusing to view it in 2d.
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Old 12-11-2019, 10:56 AM   #6 (permalink)
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CFD model

I have low confidence in all desktop CFD simulations.Optimization of shape involves details which can only be evaluated in 3-D,in ground contact,at 'full-scale,' above critical Reynolds number,on software we can't afford,running on a supercomputer we also can't afford.
From your model,the nose is okay.The nose slope would be at saturation )no additional steeper angle would affect drag),and the windshield angle would also be near saturation.So far so good!
The aft-body ought to follow the 'template.'The last portion of your roof-line cannot support attached flow. The diffuser angle should be relaxed down to 2.8-4.0 degrees otherwise the flow won't follow that either.
The VW XL1 would be a good example to follow.It's dead-nuts on as far as shape goes.
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Old 12-11-2019, 01:09 PM   #7 (permalink)
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Quote:
Originally Posted by freebeard View Post
It appears you have a 3D shape that's being analyzed in a single plane. Do you have an isometric view? The single plane reduces the computation needed. How long does it take to generate a single view?

Further into the Wind Tunnel and Smoke thread:
Permalink #817[/URL]

IIRC aerohead found a blister on the hood had no appreciable effect on his T-100 pickup.
Its not technically an isometric because of the angle, but there is a different view in the google doc.

a single view in the CFD is fairly quick. less than a minute for the 2d plane. About 45 seconds for the 3d and then time to settle down I guess.

The reason I am avoiding a 3d evaluation is because my model is lacking. I created it based on pictures of the car (from a distance zoomed in), but I don't have one from above to get the taper of the tail and such. I can post one if you think it would provide more information.

I wanted to test the hood to windshield because the air appeared to be separating at the back of the roof. I was hoping that it would indirectly keep flow more attached at the back by changing how it transitioned from the front roofline.

Current car @ 25 m/s


After letting it sit and run it changed to this:


The red box is where the flow lines are originating from.





Quote:
Originally Posted by aerohead View Post
I have low confidence in all desktop CFD simulations.Optimization of shape involves details which can only be evaluated in 3-D,in ground contact,at 'full-scale,' above critical Reynolds number,on software we can't afford,running on a supercomputer we also can't afford.
From your model,the nose is okay.The nose slope would be at saturation )no additional steeper angle would affect drag),and the windshield angle would also be near saturation.So far so good!
The aft-body ought to follow the 'template.'The last portion of your roof-line cannot support attached flow. The diffuser angle should be relaxed down to 2.8-4.0 degrees otherwise the flow won't follow that either.
The VW XL1 would be a good example to follow.It's dead-nuts on as far as shape goes.
Do you mean the addition to the car or the stock bumper as modeled in my last post?

At the very end where it takes a sharp turn or where the roof ends and meets the tail?

I had to compromise on the bottom of the car.

Do you think the CFD can be used as a data point? Not necessarily accurate for numbers, but just the flow pattern around the car.
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Old 12-11-2019, 02:08 PM   #8 (permalink)
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CFD

Quote:
Originally Posted by M_a_t_t View Post
Its not technically an isometric because of the angle, but there is a different view in the google doc.

a single view in the CFD is fairly quick. less than a minute for the 2d plane. About 45 seconds for the 3d and then time to settle down I guess.

The reason I am avoiding a 3d evaluation is because my model is lacking. I created it based on pictures of the car (from a distance zoomed in), but I don't have one from above to get the taper of the tail and such. I can post one if you think it would provide more information.

I wanted to test the hood to windshield because the air appeared to be separating at the back of the roof. I was hoping that it would indirectly keep flow more attached at the back by changing how it transitioned from the front roofline.

Current car @ 25 m/s


After letting it sit and run it changed to this:


The red box is where the flow lines are originating from.







Do you mean the addition to the car or the stock bumper as modeled in my last post?

At the very end where it takes a sharp turn or where the roof ends and meets the tail?

I had to compromise on the bottom of the car.

Do you think the CFD can be used as a data point? Not necessarily accurate for numbers, but just the flow pattern around the car.
Looking at the representation,I don't believe it's worth working with it.In orthogonal view,the model is failing to represent flow separation and vorticity from the square edges which would be present in the real world.I don't have any faith that it could properly represent any aspect of flow.It could lead you down a rabbit hole if you relied on it.
The bumper would be okay if it had adequate corner radii,but as depicted in plan-view,it has none at all,and guaranteed separation.Front fenders Dittoo.A-pillars ditto.
Nothing you can do at the front can solve the issue you have at the back.The present roofline is too aggressive towards the tail,and will absolutely trigger separation.No where can you exceed 23-degrees downslope.
I understand the diffuser issue,although there's no reason an active diffuser,which could deploy downwards on the highway wouldn't solve the issue.GM did it with their 'Epcot' concept of the early 1980s.
I think you'd be better off just looking at the wind tunnel flow images of real cars here on the Aero Forum.
I'm okay with CFD,just not the 'toy' kind.If you want to get serious,you better bee a millionaire!
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Last edited by aerohead; 12-11-2019 at 02:10 PM.. Reason: add data
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Old 12-11-2019, 04:34 PM   #9 (permalink)
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Old 12-11-2019, 07:39 PM   #10 (permalink)
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Quote:
Originally Posted by aerohead View Post
Looking at the representation,I don't believe it's worth working with it.In orthogonal view,the model is failing to represent flow separation and vorticity from the square edges which would be present in the real world.I don't have any faith that it could properly represent any aspect of flow.It could lead you down a rabbit hole if you relied on it.
The bumper would be okay if it had adequate corner radii,but as depicted in plan-view,it has none at all,and guaranteed separation.Front fenders Dittoo.A-pillars ditto.
Nothing you can do at the front can solve the issue you have at the back.

The present roofline is too aggressive towards the tail,and will absolutely trigger separation.No where can you exceed 23-degrees downslope.
I understand the diffuser issue,although there's no reason an active diffuser,which could deploy downwards on the highway wouldn't solve the issue.GM did it with their 'Epcot' concept of the early 1980s.
I think you'd be better off just looking at the wind tunnel flow images of real cars here on the Aero Forum.
I'm okay with CFD,just not the 'toy' kind.If you want to get serious,you better bee a millionaire!
I can't find the diagram I remember seeing, but I was hoping to try and get the airflow to be re-directed away from the roof at the windshield and have it come back down and reattach following the roofline. Similar to what seemed to happen with the hood to windshield model (post #3).

Are you eyeballing the slopes or calculating? I have been eyeballing. When I overlay the template (I understand has limitations) the tail pretty closely matches the upper shape. I would like to do some tuft testing, but don't really have the stuff required to record it.




Quote:
Originally Posted by freebeard View Post
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Thanks for finding that. They don't have the correct year escort though




Do you think a gurney flap at the tip of the tail would help flow? I remembered I have 2 wide angle dash cams I might be able to use to record from a chase vehicle. I will try to get something this weekend.

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Last edited by M_a_t_t; 12-11-2019 at 08:53 PM..
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