Body aerodynamics analysis?

[Broncobowsher]

I once had plans to make a board with a dozen or so U-tube manometers. https://en.wikipedia.org/wiki/Pressure_measurement#Liquid_column
Buy a couple hundred feet of aquarium tubing and get the smallest drill bit I can find to drill some copper tubing to make some static probes. https://en.wikipedia.org/wiki/Pitot-static_system (cross drilling multiple locations to get pressure without pitot tube effect)
Place them in various places and go for a drive. Checking pressures compared to ambient and to either side of the panel.

That was the plan, never did it.

 

[chuzie]

I have a guy that is willing to do a full CFD for $70 but needs decent CAD files. Does anyone have a pretty decent set?

 

[AZ73]

Curious if removing the PS fresh air vent (because of putting in a Vintage A/C) would allow some of the windshield pressure to dissipate through the hood vent and out the fender.

 

[snipes243]

I have a guy that is willing to do a full CFD for $70 but needs decent CAD files. Does anyone have a pretty decent set?

$70 bucks should buy you 15 mins in the wind tunnel up the street.

 

[MarsChariot]

I think snipes243 and bmc69 gave us the basic answer. And snipes243's plot shows that drag alone is about a factor of two higher at 75 than 55. The actual shape of the curve is probably a function of the power vs rpm curve for your engine and gearing. If mileage is a concern, it seems that a safe bet, short of having a wind tunnel up the street, is to stay at or below 60 mph.

 

[Broncobowsher]

$70 bucks should buy you 15 mins in the wind tunnel up the street.

That is some really cheap wind tunnel time. I normally see $1000+ per hour for third shift work.

This isn't gas mileage chasing. Read through the posts again. Trying to get engine cooling done right. Venting the engine compartment correctly will get rid of hot air and allow cool air through the radiator much better. You have to get hot air out of the engine bay so there is room for fresh air to enter.

 

[roundhouse]

I'd look and measure factory production vehicles with functional hood vents
Like the Cherokee after they stuffed the V-8 in it

I have installed louvers in my hood. And I just guessed at it
They are a few inches back from the front edge

But taping short lengths of yarn to the hood every couple of inches back from the front should reveal where the lowest pressure is

The low pressure areas should have the least movement of the yarn

I also added some louvers at the center of the hood right at the wind shield
To get some cool air in

My engine ran cool But the heat on the floor was cooking my feet

Literally

It cooked the paint off a section of the passenger floor

Sent from my iPhone using Tapatalk

 

[DirtDonk]

One more thing that changes the aero on a Bronco (or any truck) is the relative height (stock vs lifted in other words) and the changing amount of open space beneath. This becomes yet another very turbulent high pressure area to contend with, and will literally fight cooling air trying to exit the radiator and out under the vehicle.
Not sure how it effects, if at all, any venting ability towards the back of the engine compartment and hood, but it's probably something worth knowing.

The lower the rig and the smoother it is underneath, the better the flow underneath is. Better the aerodynamics and drag coefficients and all that fun stuff.
I added an air dam beneath the bumper of my F350 to see if I could squeak another mpg out of it. Unfortunately it didn't seem to help with the mileage, but the engine temps sure went down!
The little 3" tall factory air dam under the front panel on my '66 Corvair was good for 3 mpg at 65 mph. Consistently checked and documented. That was my very first experience with aerodynamics under a car and boy was I surprised.

Paul

 

[sykanr0ng]

This is close to how I'm thinking of:

http://classicbroncos.com/forums/showpost.php?p=2353099&postcount=27

But probably no hood scoop.

 

[metal1]

Attaching yarn tufts on the body is a very simple way of seeing air flow at speed we have done it on land speed cars and airplanes looking for high pressure areas for naca ducts for cooling oil coolers

 

[TAC71]

Interesting topic. When you drive a Bronco when it just starts to rain you can watch the water on the hood. On the centre line it rolls back to about centre and stops. The outside foot or so comes back about 2/3 of the hood then swings outboard. From the centre of the hood to the top centre of the glass airflow is basically dead.
I reconfigured the vents in my hood and fenders to work with this airflow. You can feel the heat blowing out of them even when its not moving just from the airflow of the cooling fans. My air cleaner is sealed to the bottom of the scoop which has its opening at the base of the windshield to take advantage of the high pressure there.
I found the speed vs fuel consumption point to be about 60 mph on my truck. So fuel consumption is always bad in my Bronco......

 

[snipes243]

That is some really cheap wind tunnel time. I normally see $1000+ per hour for third shift work.

This isn't gas mileage chasing. Read through the posts again. Trying to get engine cooling done right. Venting the engine compartment correctly will get rid of hot air and allow cool air through the radiator much better. You have to get hot air out of the engine bay so there is room for fresh air to enter.

It's generally a 4 hour min. At least it was the last time I was there to use it. The other reason is its a smaller older cup car tunnel. So once they built one next door this's one doesn't get used as much.

 

[chuzie]

Perhaps testing with yarn is the best initial step. My cooling is great on the highway; adding louvers for crawling. Geez, even my crawling temps never go above 220 so there is not a problem albeit I do manage temps via gear and torque converter lockup selection.

I want to manage temps less on the trail and feel louvers are the final addition to a great cooling system.

Louvers can be installed pretty much anywhere and get positive results for crawling, but I risk degradation of my highway cooling if I choose incorrectly.

Sent from my GS3 on Cyanogenmod with the older / better version of Tapatalk!

 

[Broncobowsher]

One more thing that changes the aero on a Bronco (or any truck) is the relative height (stock vs lifted in other words) and the changing amount of open space beneath. This becomes yet another very turbulent high pressure area to contend with, and will literally fight cooling air trying to exit the radiator and out under the vehicle.
Not sure how it effects, if at all, any venting ability towards the back of the engine compartment and hood, but it's probably something worth knowing.

The lower the rig and the smoother it is underneath, the better the flow underneath is. Better the aerodynamics and drag coefficients and all that fun stuff.
I added an air dam beneath the bumper of my F350 to see if I could squeak another mpg out of it. Unfortunately it didn't seem to help with the mileage, but the engine temps sure went down!
The little 3" tall factory air dam under the front panel on my '66 Corvair was good for 3 mpg at 65 mph. Consistently checked and documented. That was my very first experience with aerodynamics under a car and boy was I surprised.

Paul

Lift and air packing under the truck is another factor. With 33" tires and no lift the truck was fairly stable at 100 MPH. Put a 3½" lift on and it started getting sketchy at 80 MPH and felt like the front wheels wanted to come off the ground at 100 MPH. That was another reason I never got around to doing my aero analysis, no good baseline to start with anymore.

I was also wanting to play around with aero drag pressures in the wheel wells. I know some fender vents that were allegedly used to relieve pressure at high speeds, and other fender vents that are air intakes. Those may be like hood vents where location is the difference between high and low pressures?

The strings are good at showing flow and delamination of airflow. But I don't see how they are going to show pressure very well. At the base of the windshield you are going to get the strings flapping wildly because the airflow isn't smooth but that is a high pressure area. But the leading edge of the hood the strings are going to lay down flat and straight and that is also a high pressure area. Difference is the leading edge will have air flowing over it smoothly.

Stuff I may eventually get around to if I ever get to retire and have time to play.

 

[chuzie]

Well since I'm more concerned with pressure rather than flow, perhaps yarn isn't the best route. Considering my bronco is highly modified, I'm also guessing using a general CAD drawing to have CFD done on it will be pointless. Lastly, if I stick it in a wind tunnel without probes, I am left with airflow once again.

Sadly i think my only pre louver option is running an array of static probes which will never happen due to lack of time and cost/benefit.

At this point i think i will take a shot in the dark and install the louver(s). Then I can incrementally remove sheet metal from under them and test flow effectiveness with yarn until i finally hit a high pressure spot.

Sent from my GS3 on Cyanogenmod with the older / better version of Tapatalk!

 

[DirtDonk]

Any way to extrapolate results of yarn tests without the hood? Sounds strange I know, but if the hood is off, all that air is exiting unimpeded except for perhaps over the cowl/windshield area. With the little yarn "flow indicators" and a few temperature sensors (wife's meat thermometers) carefully placed, maybe you can see some sort of pattern that will help you determine where to put the vents.

Kind of a long shot perhaps, but maybe something along those lines is worth the experiment.
Spitballing at it's finest!

And in the spirit of collating numbers from actual real world use, like TAC my best fuel economy was always between 55 and 60 mph. Any higher and it started dropping noticeably.
For comparison purposes, this was always with 32x11.50x15 tires, 4.11 gears, cut fenders, stock engine and carb.
Changes that did not seem to net any loss in fuel economy, but usually boosted performance (with one exception) were: small 4bbl carb (4 different models), electronic ignitions of different types, two different exhaust configurations, three different brands of tires but same size, and synthetic oils.
Don't remember any changes between wearing the hard top, soft top, or not top. Although there should have been something measurable there, I don't remember seeing anything.
The ONLY replacement item that ever gained me mpg's was a Holley 450cfm "Economaster" 4bbl carb. Stupid thing couldn't get out of it's own way performance-wise, but I consistently got 2 mpg better on the highway than with any other mod.
At that 55-60 range I could consistently get 17.5 mpg.

I know you didn't ask all that stuff, but seemed like a little bit of info to add to the mix.

Paul

 

[metal1]

Didn't read the hood was off but yarn tufts in a high pressure area just stick to the body with hardly any movement and low pressure they lift up ,but if you got access to a tunnel it would be pretty cool to see where all the air is going or not going .on the left coast they are very rare

 

[chuzie]

Hood is on. Paul was spit balling.

Sent from my GS3 on Cyanogenmod with the older / better version of Tapatalk!

 

[Rustytruck]

Street wind tunnel test circa 1980 conducted by me.
1974 Bronco 30-10.50 15 tires with no lift driving 55mph.
Aftermarket air dam mounted to the front of hood netted a 1 mpg increase at 55 mph.

Bugs use to slam into the windshield and air forced over the hood use to push up the windshield wipers. Windshield use to get very wet when driving in heavy rain.

After mounting the front air dam on the hood, air was forced over the windshield. No more lifting of the windshield wipers and at speed most of the bugs were now going over the windshield. less rain water coming off the front of the truck slamming into the windshield. Loss of the high pressure area in the windshield made the heater and drivers vent less effective. Under the 55 mph you didn't see the benefit.

I ran it for about 5 years while the fad lasted. No 70 mph back in those days. My air dam was clear blue plexiglass but I doubt the color made any difference.

After that I put on a 2.5" lift and with more air going under the truck I doubt that helped the aerodynamics.