Body aerodynamics analysis?

[chuzie]

Have any of you engineering geeks taken the time to do any aerodynamics simulations for the Bronco? I am particularly interested in accurately narrowing down the high and low pressure areas of the hood at various speeds as well as plotting drag / resistance for the entire vehicle to determine its most efficient speed.

Since I am not trying to reinvent the wheel nor do I want to spend a week teaching myself how to do this, I was hoping someone here has either already done it or has the know how.

Cheers,
Mike

 

[snipes243]

Short answer its bad haha

 

[HoldMyPocket]

I'm no engineer but I've read a little about it and the efforts of hobbyists. It would seem that the flat back-end of the Bronco is a real issue (especially with the hardtop). Another one is all of the gaps in the body (hood and fender, cowl hinge, fender to door, etc).

In the end I decided that any efforts to increase my gas mileage would take many years to recover from financially so screw it

 

[pcf_mark]

Generally high pressure exists where air "piles up" because it can't make a smooth exit. In the case of our trucks the pressure directly in front of the windshield must be very high - air crashes into the flat windshield then piles up on the back of the hood before spilling off the sides.

The grill is big and flat. The aero problem with that is the air build up in front of the radiator and then spills off UNDER the truck which is really disturbed and causes a ton of drag. If you put a decent air dam under the grill I bet cooling would go up and so would gas mileage and top speed.

The rear is flat. The air exits the top then the sharp corner causes ti curl like a rip current back toward the rear. This is what draws in the exhaust fumes if you do a rear exit.

If it would not look so stupid I would do a spoiler. Might even handle a little better.

 

[snipes243]

Cause I'm a bit bored I will bit Chuzie. Based on a frontal area of 36 ft^2 ( if you can find the actual I can change it) the drag force vs. mph is..

Fyi this was done pretty quick and the numbers rounded to make it easier. So it's what you call ruff numbers

 

[chuzie]

Im actually looking at installing a hood louver and don't want to tap in to any high pressure areas. Broncobowsher hooked me up with a graphic analysis of a boxy car which helped for a general idea, but I am looking for more specific data.

http://www.buildyourownracecar.com/wp-content/uploads/Aerodynamics_DragLiftDownforceOverBodyFlow.png

The drag inquiry was more geared towards me selecting a long haul highway speed rather than finding ways to mechanically change the Bronco for fuel efficiency.

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[JAFO]

It is always the case that the faster something moves the more drag, Bronco or whatever. And the Bronco is a square brick rolling down the road, so it is going to have some drag. So, slow driving, gradual acceleration is going to get you the best mileage I would think.

 

[chuzie]

You're absolutely right. However, there will be a point at which consumption increases exponentially. My goal is to find the sweet spot just under it.

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[metal1]

Just picture a brick

 

[bmc69]

Years ago I wrote up a simplified analysis of the EB to predict, from first principles, what to expect for MPG and why it is what it is. I did not run a full CFD on the thing, though..I used handbook methods for calculating a suitable drag coefficient to use in the overall analysis. I'll see if I can find it. I came to the final result using specific fuel consumption (HP produced for gas burned) for modern gasoline engines...so the result was essentially "engine independent".

Couple things to always keep in mind: Rolling resistance and driveline losses are generally linearly increasing with speed. Aerodynamic drag, however, increases with speed squared. So, double the speed and get four times the aero drag.

 

[chuzie]

Got it. Problem solved.

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[JAFO]

Ha. Yes, that is the answer. Same discussion comes up from time to time on the FJ40 forum. That is an even taller brick going down the road and gets much worse MPG. Mine gets around 12 or 8 or something like that. I actually have never checked the mileage since it wouldn't stop me from driving it anyway, so I'd rather not know. Let's just say that I am helping use up the remaining crud oil which will expedite development of other more efficient propulsion systems.

 

[sykanr0ng]

The only reason I would check my MPG is for trip planning if I intended to travel into areas with very high gas prices.

The rest of the time?

Smiles per gallon matter, not miles per gallon.

 

[chuzie]

Just so I'm clear on this, much more interested in the pressure areas of hood for louver than the fuel question.

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[JAFO]

Well, a crap load of air blows in the drivers vent, so a lot of air is forced down that louver for sure.

 

[chuzie]

Exactly; mostly at the leading edge of the hood and at the windshield area. The big question is how far forward of the windshield do I need to go to stay out of the high pressure in order to keep the louver 100% effective.

Thus the need for the modeling.

If I really want to complicate stuff further, I'd like to know more about flow in terms of preventing hot louver air from being forced into the existing vents.

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[sykanr0ng]

Just guessing, but I would say that the closer to the middle of the hood the farther forward you would want the louvers, since the air has a path to spill away nearer to the sides.

How far forward is a question I would be interested in the answer to as well.

A year or two ago I saw a pic of a Bronco hood with two rows of louvers on the hood near each fender, looked good and probably worked pretty well.

 

[jw0747]

Just to be doubly sure next week we're going to test this in our wind tunnel. Results pending.

 

[chuzie]

I don't know you well enough to determine if you are being serious or not. Lol ;-)

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[AWS]

Years ago I remember someone (maybe Colby) taped strings or something like that on the hood to see where the high pressure zones were. I think some of the strings didn't even move at speed.
Steve
PS the Sweet spot for fuel mileage is in the driveway with the key off!