Air Flow Seperation Question... (where to mount roof VG's on Nissan Cube)

[donee]

Hi Chops...,

I think there actually may be a place on your car where they may help drag.

The windshield of the Cube is so vertical, that air has to be bleading sideways off the upper corners. The Air is going to have a bunch of momentum, and will be highly compressed. It will also be flowing significantly off the direction of the flow of the air around the car. When it flows off the windshield the momentum and misdirection will cause all sorts of turbulence, and make the aerodynamic size of the vehicle much bigger.

Try placing two of those about 6 inches in from the oustide edge of upper third of the wind shield. Before you place them do a yarn test to find out the wind direction, and make a marker mark from the inside of the windishield along the length of the yarn. Now align the blade of the vortex generators off that line a little bit. Experiment with either side of the line. Have somebody follow along the side of your car and take video, while you mist the windshield washer while travelling into the sun. It should become apparent which direction causes the mist to trail more closely down the side of the vehicle. That is the orientation you want.

[ChazInMT]

Sorry, gotta chime in here lest someone read that putting Vortex Generators on your FRONT WINDSHIELD may be good for fuel economy. It's not. It can't be. Plain and simple. If it was, we'd all have them on there now wouldn't we....seriously. Think about it. ENuf Said. I won't even entertain a debate or discussion about it beyond this post.

[donee]

Originally Posted by ChazInMT

Sorry, gotta chime in here lest someone read that putting Vortex Generators on your FRONT WINDSHIELD may be good for fuel economy. It's not. It can't be. Plain and simple. If it was, we'd all have them on there now wouldn't we....seriously. Think about it. ENuf Said. I won't even entertain a debate or discussion about it beyond this post.

Hi Chaz...,

Your quote is hardly any kind of reasoned response, ie text that describes your reasoning to support your position. Its a simple opinion , without any thought process described at all. As such its useless.

If you feel my idea is totally worthless, then PLEASE describe why you think this is so. Just saying it is totally insufficient.

I reapeat - The CUBE has a very vertical windshield. The OP needs to put yarn tell-tails on the outside edges of it to see which way the air flows. Its my hypothesis that in the outside upper areas of the windshield he will find air flowing with considerable force in a direction that is well off the line of the air flowing around the car. Its only there where VGs may be applied to good effect. This air has momentum. When it shoots off the windshield sideways, it wont instally conform to the air flow around the car. It will continue to flow in the direction it had on the windshield for a short period of time, and and then bend and rejoin the flow around the car. This phenomena will result in the effective cross-area of the car to be larger than it mechanically is.

Introducing vorticity in the flow along the windshield may help the flow rejoin the flow around the vehicle at the edges of the windshield. The vorticity will rob the air of momentum in its direction of travel. When it flows off the windshield, it will quickly conform the air flow around the car. This will decrease the effective area of disturbed air. And it may reduce drag.

To achieve this, the direction of introduced vorticity may be important. The OP will need to experiment with the angle of the VG from the apparent flow, and numbers of VGs to determine the best effect. My guess is no more than two at each windshield upper corner (four altogeter). It very well may be just one VG in each corner.

I think we all here have been satisfied that VGs do not reduce drag when applied to the rear of a box like vehicle, based on experiments that have been performed and documented on Ecomodder. In such a case, there is simply no flow for the air over the top of a box like vehicle to rejoin. This is how my suggestion is different from previous investigation.

[ChazInMT]

OK Donee.

You seem like a nice enough guy, (certainly nicer than me) I’ll try my best here to explain.


First off, it appears you have bought into pseudo science created by the people who market Vortex Generator gimmicks. It is apparent in your statement:

This phenomena will result in the effective cross-area of the car to be larger than it mechanically is.

The cross area of a car is what it is. What you mean to say is:

“The form drag created by the shape of the car is significant and it affects a larger volume of air than a car with a better aerodynamic shape for a given cross section.”

A huge volume of air is displaced when any vehicle moves through it, all of this affected air has an effect on the fuel mileage in the form of pressures created on the surface of the vehicle. These pressures are largely determined by the overall form, or shape of the vehicle, think in 3D out 3 to 5 feet away from the shell of the car, 60 to 80 feet back. When you put something on the layer of air which is closest to the skin of vehicle such as a Vortex Generator gimmick, it is little more than a small bump on the overall form and shape which really matters.

You state here:

Introducing vorticity in the flow along the windshield may help the flow rejoin the flow around the vehicle at the edges of the windshield. The vorticity will rob the air of momentum in its direction of travel. When it flows off the windshield, it will quickly conform the air flow around the car. This will decrease the effective area of disturbed air. And it may reduce drag.

You give your VG's unbelievable superpowers in this statement, they are simply too small to significantly change the airflow around the car.

Fact: While moving the air piles up in the front of the car. The amount of air affected is at least 3-5 feet out and more.
Fact: The Vortex Generator gimmicks stick out only 2 inches tops. The small effect it does have will quickly dissipate behind it, it will be simply overwhelmed by the huge amount of air, its effect will only maybe be felt 5 inches behind it and it will only be felt in .05% of the total air moving around the car.
Fact: By changing the airflow, however slight, the gimmicks WILL CREATE DRAG.
Fact: The gimmick creates DRAG.
FACT: DRAG is created when the gimmick is in the airstream.
Fact: When you create drag, it is ALWAYS less fuel efficient, although again, probably too small to notice.
Given: Sometimes, very rarely, the drag created by VG’s can be offset by some benefit downstream, this is not one of those times. You need a very particular set of circumstances for this to work, and it has to do with the air at the rear of the vehicle and maintaining attached flow for a little bit longer.

Conclusion: Only a small amount of air could be affected by these gimmick devices, certainly not enough to change the overall airflow to the benefit of fuel mileage. It is like trying to use a pencil to plug a hole the size of a basketball in a dam. The volume of air is simply too great.

As a matter of opinion, The VG’s would look stupid stuck to the windshield of the car.
As a matter of local law, placing them on the front windshield of the car could get you a ticket because they obstruct the view.

[AeroModder]

Fact: When properly designed and used, vortex generators CAN and DO REDUCE DRAG.

Calling them "gimmicks" is your personal opinion, not scientific fact. If they were "gimmicks," they would not be used in racing and aeronautical applications.

Fact: The Vortex Generator gimmicks stick out only 2 inches tops. The small effect it does have will quickly dissipate behind it, it will be simply overwhelmed by the huge amount of air, its effect will only maybe be felt 5 inches behind it and it will only be felt in .05% of the total air moving around the car.

False. It is well-known to professionals that even small changes upstream can drastically change airflow downstream. A small change as simple as a sharp rear edge as opposed to a rounded one can dramatically alter and smooth the airflow well past the car, resulting in lower drag.

Fact: By changing the airflow, however slight, the gimmicks WILL CREATE DRAG.

False. When properly used, they can and will REDUCE drag. Mitsubishi's testing verified this. What testing have you done?

Fact: The gimmick creates DRAG.

Only half true. By generating a small amount of drag in the proper location, you can reduce a body's overall Cd. Air dams and tire spats are perfect examples.

Try doing some actual research before you claim "facts." Given, vortex generators are much more effective at reducing lift than drag, but your assertions are NOT "facts."

Does anyone recall the testing of Airtab VGs on tractor trailers? Here's some reading material for those interested: http://www.marama.org/diesel/frieght...DCAeroOvw2.pdf

[ChazInMT]

I have done plenty of research on the Gimmicks and will continue to call them such as long as they are misapplied by people.

I give you that in rare situations these may have a net gain effect and I stated such. So besides the Mitsubishi test (Which I'm well aware of) on a shape that was ideally suited to the possibility of using the Gimmicks for net gain....Can you name me 5 other effective tests, not produced by the makers of the Gimmicks in question, where the Gimmicks create a net gain in fuel efficiency? Can you name 1 test where these were placed on the leading edge of a shape for net fuel efficiency gain? Put the links here I'd love to see them.

I am not talking about reducing lift anywhere here. I am saying these Gimmicks do not reduce drag unless properly applied in rare cases. Heck, ANYTHING will reduce lift! For Peets sake. At what cost do we reduce lift??? Drag & Fuel Efficiency.

And you bring Aeronautics into it....duh....airplanes are not cars, different sets of rules altogether. Of course there are a multitude of applications for Properly Applied Gimmicks, stall strips, spoilers, and air fences to be used on aircraft and racing, none of these have anything to do with net efficiency gains in fuel mileage, they are for control.

Sharp & rounded edges on cars are huge elements not 2 inch square things sticking up on the skin in 4 places. In particular, the gross airflow and positive pressures on the front of a car are really less inclined to be improved on by any sort of Gimmick application.

For someone who knows as much as you AeroModder, you should know that in most cases, the application of Vortex Generators is a waste of time and money. They are nothing more than Tornado Intake devices in a more visible location. Read the multitude of posts right here in EcoModder that state this exact thing. I'm just being point blank about it cause I'm tired of people who put up fairy tales as fact. Sorry if that offends you.

[AeroModder]

Can you name me 5 other effective tests, not produced by the makers of the Gimmicks in question, where the Gimmicks create a net gain in fuel efficiency?

Science Links Japan | Research on Aerodynamic Drag Reduction by Vortex Generator
NASA - Micro-Vortex Generators Enhance Aircraft Performance
Browser Warning
Low drag vortex generators - US Patent 5058837 Description
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http://www.mhest.com/spotlight/autom...rodynamics.pdf
http://www.marama.org/diesel/frieght...DCAeroOvw2.pdf
http://www.vortaflow.com/consulting/...0reduction.pdf

Can you name 1 test where these were placed on the leading edge of a shape for net fuel efficiency gain?

Development of Wing Leading Edge Vortex Generators for a Single Engine General Aviation Airplane
http://ecomodder.com/forum/showthrea...ors-12365.html

airplanes are not cars, different sets of rules altogether.

How so? Do planes move through a different fluid than cars? Aerodynamic principles hold on both planes and road vehicles, and they are very often intertwined. Racing spoilers are inverted airplane wings, for example. To use own words, "I'm just being point blank about it cause I'm tired of people who put up fairy tales as fact. Sorry if that offends you."

That's as far as I'm going here.

[donee]

Hi Chaz,

Boy are you wound up about this.

First off, I am not trying to sell these things. The OP has already spent his money, and anybody else watching should see if the OP applies my advice, and it results in any improvements before buying these things.


My words were "effective cross-area", which I meant to say "effective cross section area". Your reply comment to this is a thought-limiting semantic. As a degreed engineer , with patents, I let myself out of the box when brainstorming. Its a good thing.

You speak the aerodynmacist lingo well. I am not an aerodynamacist, and explain what I am trying to say in my simple intuitive understanding.

I have run tests with yarn tufts on the side of my Prius and turbulator strips in this area of the windshield. There is a very significant effect in windless conditions, and .06 inch thick Turbulator is an even smaller disturbance to the flow than a VG would be, and they do have a real effect. So, its not improbable that a VG on a car with a much more vertical windshield to also have an effect.

On the Cube windshield, the VG will be an almost horizontal thin blade. And my advice was to find the local flow direction, and orient the blade to be nearly parallel to that local flow direction. I found on my Prius, to my great supprise, that the flow was about 30 degrees off the axis of the car in the upper corners of the windshield! I am currious if the air will be much greater than the 30 degrees on the Prius (my guess) or less. My expectation is the air will be flowing almost sidways on the Cube, due to the nearly vertical windshield. Which represents a much greater flow momentum to disturbe the side-slip air.

One way to look at this, is there is a psuedo-pipe of high pressure air in front of the car, where the pipe is made of stretchy plastic. How easily the air flows down the side of the car, will effect how wide the stretchy pipe stretches. The wider the pipe, the more power it takes to hold it at pressure.


Again, we are commenting to the OP to try this, with no garuntees it will work. Just try it and see what happens.

[ChopsQube]

I do believe that I posted up there somewhere that the VG's did nothing for FE, or anything else for that matter. I just figured since I already had the darn things, I might as well try them out. Stupid idea?... Maybe. Does it matter?... No.


True Story....

However, the other morning there was a road crew along side the highway with those big concrete circular saws cutting up some pavement. There had to be a few of these guys going at once as there was a large white dust cloud rolling over the highway. As I was going through this cloud of dust, I just happen to look in my rear view mirror so I could get in the next lane when I noticed EIGHT very thin streamers coming from the top of my Cube that looked like they dropped down below the roof line by a few inches. So with that, they are in fact doing something back there.


Maybe what I need to do with my Cube to improve FE is throw a boat tail on the front of it!

[AeroModder]

Boattailing the front won't have nearly as much of an effect as boattailing the rear.