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Old 12-19-2012, 07:50 PM   #91 (permalink)
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Quote:
Originally Posted by euromodder View Post
Check out recent cars - quite a few already have small deflectors in front of the rear (and front) wheels.
Like the ZR1 Corvette: Chevrolet Corvette ZR1 photos and wallpapers - tuningnews.net
and maybe the Viper ACR? Dodge Viper Production Car Leader at Nürburgring | Street Legal TV

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Old 12-19-2012, 07:54 PM   #92 (permalink)
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Quote:
Originally Posted by WesternStarSCR View Post
The Mercedes ML 350 has curved front air deflectors / spats / spoilers / whatever you want to call them in front of the front tires.

Looks to move some air to the side of the car and some under it.

I am attempting to mimic this using plastic PEX tube bend supports from Grainger as the mounting point for approx. 3" lawn edging coming down.

Grainger Item # 10A675

Seems it takes advantage of the air dam affect to the farthest sides of the car to route air around un aero things like the tires and suspension. But leaves the center of the air dam higher to not force ALL below bumper flow to the sides.

I will create thread with pics when I can. And let you know if it seems to help. I may then try same for rear tires.
I actually tried to do this once with some old mud flaps. But I didn't use strong enough hardware to hold the bottoms in place and they pulled out and rubbed the rear tires...
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Old 12-19-2012, 10:11 PM   #93 (permalink)
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Thumbs up Here's pics fyi

Link to pictures:
about to get real aero dynamic

@econoram:
They seem to work in terms of durability and keeping suspension area clean, and no scraping. I have not put any on rear yet. The pex supports really allowed me to put them exactly how I envisioned from what I saw on the Mercedes SUVs.
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Old 12-26-2012, 10:33 AM   #94 (permalink)
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Originally Posted by donee View Post
Hi Virg,

I am no expert, but the bow bulb on a ship has to do with the interface between water and air. The bulb lifts the water ahead of the ship, which then falls away from the above water bow. Apparently its takes less work to lift the water than to push it aside (air is more compressible than water, and one is pushing against air when lifting the bow water).

On a car, or submarine, or airplane there is no such interface involved, and hence no bulbs.
When a boat pushes through the water, it creates a bow wave (high pressure area in the front, just like cars). This bow wave lifts the front of the vessel and raises the angle of attack of the boat. Instead of the boat sitting flat in the water, it is now climbing a constant hill... the faster it tries to go, the steeper the hill (because the wave builds bigger). To try and reduce this constant hill, the bulb was introduced to build a wave in front of the boat hull. Every wave consists of two parts, a crest and a trough. The crest is above the average waterline and the trough is below the average waterline. So if you can build a crest in front of the hull and position the trough where the hull wave builds... the bulb trough (positioned at the hull) and the hull crest should cancel themselves. Cancelling the bow wave allows the boat to sit level in the water as it powers through it.

Yes, you have to pay for the bulb wave's energy, but less energy is required to build the bulb wave than to power the entire boat up a hill.

I hope this helps!

Ryan
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Old 12-30-2012, 11:33 AM   #95 (permalink)
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It is even more subtle than that. In fact both the high pressure and the air-water interface have a function here.

When a boat moves through the water it needs to put aside the same amount of of water that it displaces. A 10.000 ton ship has to move 10.000 tons of water aside.
It requires a lot of force to set that amount of water in motion. That force is pressure; it needs to be higher than its surroundings. Once the water is moving aside its momentum will cause it to make way for the steadily increasing cross section area of the ship's bow.
When the bow finally reaches full width and the sides bend straight the water is forced to stop moving aside, creating low pressure. You can see that for yourself on any ship in motion; at the point where the hull straightens the water level will be slightly below the water line.

The protruding bulb forces itself through the water. As the water around it is not yet moving aside the pressure rises sharply, which accelerates the mass of water flowing aside and up.
But what goes up must come down. While the water that moves aside keeps doing that, the water on top recedes and transfers into sideways motion. Just at the point where it becomes level again the sharp edge of the bow cuts in. As the water is already moving down and aside it meets less resistance than it would have without the bulb.

The bulb serves in making the transition from a body of water in rest to that same body moving sideways more gentle. There is a close relationship between the rate at which the cross section area of a body moving through water varies and the resistance it meets. As you see the variation of the cross section as a function, you need to keep the second order derivato of that function as close to zero as possible.

You cannot fully apply aquadynamic rules to aerodynamics. But the base principle that when you move something through the air, the momentum of the displaced air does influence resistance and you have to keep its transitions gradual, holds true.
The first fighter jets that were able to break the barrier of sound could only just do that, and not for long due to massive fuel consumption. The breaktrough tor really fast jets came with the realization that the wings of the planes displaced air, and at supersonic speeds that air had nowhere to go. This was solved by narrowing down the fuselage by a comparable amount; the so-called Coke contour. (other solutions were swept wings and the Delta wing, which made the wings themselves displace air gradually).

In car terms, this means that you can compensate for anything you move into a stream by making a dent in another part close by so the total area does not change much, and the momentum of the air at some distance to the car is not disturbed.
Or you can gradually build up and recede the area just before and after the intruding bit for the same effect; like the leading and trailing spoilers at the wheels of low Cd concept cars. They work because of this principle.

Phew!
Anyway, a ships bulb bow would not work on a car; there's no water to lift and fall back. That said, it is the very mod I'm planning to do to my car.
I will replace my lower grill block by a bulge that slightly protrudes the front bumper and license plate of my 2011 Insight. It should curve back towards the sides to make a flush transition with the 45 degree angle of the front bumpers edges. But will not really stick out like a ships bow.
Before I do so I wil do some other simple mods and get some data on the MPG that yields as a base for further testing. Hope I can give some results soon.
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Old 12-30-2012, 10:32 PM   #96 (permalink)
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Quote:
The bulb serves in making the transition from a body of water in rest to that same body moving sideways more gentle. There is a close relationship between the rate at which the cross section area of a body moving through water varies and the resistance it meets. As you see the variation of the cross section as a function, you need to keep the second order derivato of that function as close to zero as possible.
If this were the case, extending the bow to give a longer length to width ratio would help towards the Sears-Haack model you are referring to with the "Area Rule".

I wiki'd bulbous bow and found this. Excellent explanation of how they work (with pictures as well )

Bulbous bow - Wikipedia, the free encyclopedia

Quote:
In car terms, this means that you can compensate for anything you move into a stream by making a dent in another part close by so the total area does not change much, and the momentum of the air at some distance to the car is not disturbed.
Or you can gradually build up and recede the area just before and after the intruding bit for the same effect; like the leading and trailing spoilers at the wheels of low Cd concept cars. They work because of this principle.
I think I need to chew on it a little more. This principle is the Area Rule (referred to earlier). It's been used extensively by transonic aircraft to reduce the number of shock waves forming as the aircraft approaches mach. I'd be interested in seeing how effective it is at low speed vehicle design.

-Ryan
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Old 12-31-2012, 06:44 AM   #97 (permalink)
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Good catch, should have added that wiki link myself.
Extending the bow length would help, but has issues discussed in the wiki page.
The bulbous bow produces the same gain as a longer and sharper bow but at lower cost.

The area rule should be effectice at any speed, but at everyday driving speed disrespecting it won't destroy your vehicle nor cause total loss of control.

When calculating the area you will have to account for other effects.
The air in the wake of your car, just in front of the nose and at the lower part of the windscreen moves at pretty much the same speed as the car and should be added to the calculated area.
You can turn it around and use the area rule to estimate the size of the pockets of air.

The area rule cannot explain all. Vortices, laminar flow, the Coanda effect etc influence air flow in ways that may defeat the area rule. For instance, many cars have rounded edges at the rear. In the wind tunnel the airflow close to the sides of the car will be turbulent because of the wheel wells and panel gaps and such. But a heavy sidewind will flatten those out so the airflow will be laminar at the edge, and curl around it causing drag.
Slightly altering the direction of the airflow hardly influences the area rule but will sharply raise resistance due to altered airflow.
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Old 02-13-2013, 10:25 PM   #98 (permalink)
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I was curious in fabricating my own grill block for my 03 malibu. Would that result in warmer running temperatures?
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Old 02-13-2013, 10:35 PM   #99 (permalink)
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I think your Malibu feeds cooling from the bottom like most GM cars, my Century included. I have blocked the upper grill opening wihtout any noticeable temperature increase. Remember, the fans turn on around 228 and off again around 206, so the engine won't overheat as long as the fans can get air flow from somewhere. The blocked grill would be equated to sitting in traffic with no air through the grill induced by forward motion. See my profile pictures for the amount of cooling flow restirction you should be able to get away with. I ran under 215 during my commute both directions today and the air temp was above 50 on the way home. I do see temps run a bit higher when I have traffic lights to contend with....around 220 as long as I don't sit too long.
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Old 02-13-2013, 11:27 PM   #100 (permalink)
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Quote:
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I was curious in fabricating my own grill block for my 03 malibu. Would that result in warmer running temperatures?
It could, depending on how completely you block your grille. Recommend you have a good way of monitoring your temperature so you can keep track. A ScanGauge will do it, and a whole lot of other stuff.

When I blocked the grille on my Jimmy, it started running hot until I cut a hole for cooling air. I had to enlarge the hole once, and then that did the trick.

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