Trebuchet03's JettKamm (2000 VW Jetta)

[trebuchet03]

Best Mod So Far.... Put a Hole Through Your Car



I did a quick 2 minute solve (like the original stuff posted) - just for fun


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What I've found so far.... Be doing the whole refinement take 2 hour solves, I get much better results. BUT, what I have discovered is that things that were better before get even better or stay the same and things that were worse before get even worse or stay the same. As such, I'm not going to do some of the things we already know to be bad - to save on cycles.

That means I can quickly test something to see if it's worth doing a long test

So you too can see some of the prettier pictures I'm getting....


Weather or not that is "correct" is another story... But, I feel that it's closer to it compared to what I was getting before My roomate gave the quick reality check on it - you always get dirt and crap on top of your bumper - as that model would suggest

[Deezler]

Hmmm...! That is certainly different. I like the look of the flow patterns over the top of the vehicle, but I am a little surprised at the rear... perhaps I just got too used to the earlier images!

So the novel spoiler design still works in this new flow pattern? Your earlier result %s listed were total horizontal force calculations, correct?

Would it make sense to give the model a flat ground level beneath it?

[trebuchet03]

Originally Posted by Deezler

Hmmm...! That is certainly different. I like the look of the flow patterns over the top of the vehicle, but I am a little surprised at the rear... perhaps I just got too used to the earlier images!

So the novel spoiler design still works in this new flow pattern? Your earlier result %s listed were total horizontal force calculations, correct?

Would it make sense to give the model a flat ground level beneath it?

I keep throwing errors when I try and add a ground plane... I need to work on it...

BUT, I went to the library today and got a few new aero books that I haven't read (including Hucho's). In one of the books, it gives an approximation model for inviscid flow from the ground plane. Basically, take your model, and mirror it upside down offset twice the ground clearance about an invisible ground plane

Drawbacks for car application - it doesn't take into consideration viscosity which becomes more important at lower ground clearances. I'm going to implement and see what happens

%tages from earlier were from the "Z" axis - which would be my horizontal

New images look closer to Mitsubishi's, which gives me some confidence that it's better

[trebuchet03]

Following the information a book titled "Road Vehicle Aerodynamics" by Scibor-Rylski - I modeled a ground effect approximation. Downside is, it doesn't take into account viscous losses and I can no longer measure lift. What you don't see in that image is the bottom half of the model, which is a mirror image. Plane 3is the virtual ground - 14 inches below the vehicle.

Why? I don't have enough memory to run even a 2D ground plane... Bugger... At low resolution, I can make it

So I've added an engine approximation thing and a radiator. The radiator is a bunch of squares (40, I think) offset from each other by about a half inch. I added my two grille openings and an outlet vent on the top (which is a bit too big methinks).

According to Scibor-Rylski - internal losses account for about 12% of total aerodrag with gains to be had up to 6%. Mind you, this book is a bit dated
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Is this model right? I lean towards "no." My tuft testing showed fairly steady flow the entire length of the hood. BUT, light yarn doesn't need much velocity to show a flow direction. This could very well be what's happening, but I'm not entirely convinced just yet. I'm going to measure my grille's and modify.

[lunarhighway]

am i interpreting the information correctly when i say it seems no air is entering trough the top grill? would be interesting to see what blocking either intake would do the the internal and external airflow

[trebuchet03]

Originally Posted by lunarhighway

am i interpreting the information correctly when i say it seems no air is entering trough the top grill? would be interesting to see what blocking either intake would do the the internal and external airflow

I redid the model a bit, added the stylistic wings on the top grille and added the radiator cowling/outlet duct.

So flow seems to slowly enter at the bottom - and it looks like it's exiting at the top... That doesn't fly so well with me - but I don't really have a method of testing this....



What also doesn't fly for me is the hood... That big blue zone is all pointing the wrong direction - forwards.... Tuft testing really didn't show that. I think I'm going to need to do some flying tufts to see what's happening a couple inches above the hood....


Here's the overall setup


So why am I modeling the internals now? Am I getting too far ahead of myself? Well, the books I've been reading lately have been stressing that internal flow interaction will change external flow. Thus far, that is obvious - but we still look good for the JettKamm.... I'm essentially narrowing in on the 18.5 degree design for ease of manufacturing and easy of assembly/use.

I don't think I can make the model much more complicated - even these relatively "simple"/"low cost" runs are taking significant chunks of time and memory.

If anyone has some SODIMM DDR1 memory (1GB sticks up to 2 sticks) they'd like to donate

Next up - high resolution of the model above - for baseline

[Deezler]

Wait a sec dude!

The Jetta has a full engine belly pan (and side skirts). The engine is not open to the ground like that. I believe the flow exits out the back of the bay under the firewall, though its been a while since I've been under there and I have a diesel geek skidplate anyway.
Furthermore anyone with a Jetta might as well block off nearly the entire lower grill and especially the whole top grill. Actually one interesting note here is that I did my grill blocks sequentially, and the lower block made a huge difference on warm-up time, while the upper grill block appeared to do little if anything. I've probably got about 20% lower frontal flow area into the engine bay vs stock. I started with my lower block like this but trimmed a tiny slot open on the bottom during 90 deg summer days. Wish I hadn't now, as I need to remake the full block for better winter warm ups.


But I suppose this doesn't matter as much for flow towards the rear of the car. As long as we get that hood flow figured out.....?

I appreciate your continued efforts! I really like the improved flow detail and vectors, too.

[trebuchet03]

Mine is completely open - it's how I bought the car I know, it's terrible - when I open my hood and look down, I see ground (I should be ashamed ).

I will, eventually, block the grille's and add the stock under tray.... But it's important to get a baseline. Should someone else want to recreate, I want them to be successful.

I don't want a situation where the "base" is highly modified from stock whereas someone building for themselves doesn't get the claimed results. This is my beef with HHO peoples - "baseline" is nowhere near "stock" condition. Any failure just means you forgot X by those that want to sell you kits (and refuse to get/allow independent testing).

But I suppose this doesn't matter as much for flow towards the rear of the car. As long as we get that hood flow figured out.....?

The flow behind a car is dependent on the "history". I'm getting somewhat different results with the engine bay approximation added in.... Flow under the vehicle ultimately ends up behind the car, and capable of interacting with flow that originated from above.... Although, I did add the ground effect at the same time, so I don't know which one is causing the difference


And UGGGH.... I need (rather, want) better hardware

[trebuchet03]

Deezler, I forgot to mention the side skirts thing....

Keep in mind that this is a 2D cross section - just about right down the center of the vehicle. So features such as wheels, wheel wells, skirts, etc. will not show up. I am, however, ignoring things like the VW emblem (notice I just put the fins instead) and the antennae whip (next time I take my headliner down, that thing is getting removed).

[Deezler]

Wait, now you're planning with commercialization in mind?

Man, its like every change you make opens up a whole new can of worms. Now that the engine bay is adjusted, do you re-run all the rear mods to double check their effect? The number of simulations is growing exponentially. hehe.

Moar pretty pictures!