Pop N Wood Posted December 7, 2006 Share Posted December 7, 2006 I always thought the flap on the back of a Blazer was to prevent the rear window from getting dirty. I thought it was so exhaust fumes wouldn't get sucked up into an open back window. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted December 8, 2006 Share Posted December 8, 2006 Probably works for both, but these threads on other forums backs my theory up: http://coloradok5.com/forums/showthread.php?t=37568 http://townhall.edmunds.com/direct/view/.ee9516f/560 Quote Link to comment Share on other sites More sharing options...
RedFive Posted December 8, 2006 Share Posted December 8, 2006 It keeps the back window clean too. --Edit-- It's just a side effect. Or you could consider the aerodynamic advantages to be the side effect. Whichever. Quote Link to comment Share on other sites More sharing options...
RedFive Posted December 8, 2006 Share Posted December 8, 2006 Dave, Didn't see your post right away. Yeah, they delay the flow from separating from the surface because they've created turbulent flow. But at the cost of what? A good amount of drag -- at least compared to the wing the WRX has. Okay, I was a little extreme saying the wing's mission was completely different...but I would opt for the wing option on a vehicle way before a vortex generator. Vortex generators are great for high performance aircraft that require maneuverability at very high angles of attack. It keeps the flow from separating from the surface, and the drag trade off is worth it. They're also used on commercial jets sometimes as a safety factor. If your wingtip stalls (when a wing stalls, the flow starts to separate in one spot and then spreads from there as AOA increases. Where it begins and ends depends on the design of the wing), your ailerons become useless and you risk losing control of the airplane. With the vortex generator, you can delay the stall, but again at the cost of drag. Is it worth it? Sure. If that plane stalls and starts going into a spin, nobody on that flight is going to want to fly with you again. But why do I think the Subaru guys made a good decision with the upper wing? Because the only drawback is a little bit of skin friction drag. The plus is reducing separation near the rear window, which not only reduces overall drag for the car, but makes the rear wing useful for downforce. It's a better trade off for the situation. Car designers work pretty hard to make sure flow doesn't separate on top of or near the roof of the car...because it would be loud as hell. In most cars, once they've accomplished that, they don't care too much about the rest of the car. Clearly Subaru does. Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted December 12, 2006 Author Share Posted December 12, 2006 Logan, That is not at all how I understand the function of vortex generators. A vortex generator does not create turbulent flow (when I say `turbulent flow' I am basically referring to sporatic reversion) it creates flow in the shape of a vortex/helix. Essentially the same shape as the groove in a drill bit. This simply adds energy to the flow stream by accelerating it. Ie, instead of taking the shortest path between two points, the flow takes a diagonal line, but it must cover the same longitudinal distance in the same time, hence it accelerates. This increased inertia can then be used to flow up and unfavourable pressure gradient, such as is found on the rear windscreen. We can discuss how the wing would compare with a vortex generator: Wing will induce much more drag/lift on its surface. Vortex generator does not induce any lift. Wing is much more likely to have a significant influence on separation point down the rear window. What I am also interested in is how the wing compares to not having the wing at all. Since its on a competition car (WRC) one would assume this is favourable, but lets have a ponder on it shall we. I don't believe you ever get anything for nothing, and I think that an important reason that wing is there is that its moment about CoG is much smaller than the main wing's. Thus although it is inducing lift in order that the rear wing can induce more negative lift, I believe that the rear wing's moment about CoG is larger, and hence it has a positive overall effect on downforce. As for drag, I am not familiar enough with CFD equations to be able to guestimate whether the larger drag pressure over the small wing area would be larger than the reduced drag pressure over the larger rear windscreen. Can anybody help on that one? Dave Quote Link to comment Share on other sites More sharing options...
RedFive Posted December 12, 2006 Share Posted December 12, 2006 Dave, A vortex, except under very specific circumstances, is pretty much always turbulent by definition. Don't confuse turbulent flow with flow separation -- read stall (in most cases). Maybe I can help you out with this concept. Golf balls... This is what "Ideal Flow" around a cylinder looks like. In other words, when I use the theoretical aerodynamic equation Cp=1-(4)(sin(x))^2 But that NEVER happens in real life. What happens in REAL life looks more like this: Note that the flow stays attached only until around the top and bottom of the cylinder. Then it separates from the surface and starts forming oscillating vorticies -- aka turbulent flow -- which create drag because they are LOW pressure. See the line going to the front of the circle in the middle and then stopping? That's the stagnation point, the highest point of pressure on that thing. If your wife is facing you, yelling at you, and putting a lot of pressure on you to do something, but behind you is an open door through which you could escape the conversation, you're going to go that way. Well maybe not, but if the cylinder had a wife.... Okay, so bad analogy. You catch my drift. Pressure differences are responsible for lift, drag, etc. In this case, drag. So why is a golf ball so much better? The dimples "trip" the boundary layer and make it turbulent. A turbulent boundary layer stays attached to the surface longer, thus creating less drag. Lets see if I can find a picture... I guess that one will do. It's a little big, but the best I could find. Anyways, you get the idea. As for the wing, I think you're wrong. If that wing is at the proper angle of attack (and it may not be), then it shouldn't be stalling out. The only real amount of drag you should see off of that thing would be skin friction drag (aka parasitic), and I think that would be negligible compared to the rest of the car. It will create some wake drag...but again, minimal compared to the whole car. It would be hard to find out without doing wind tunnel testing. I'd offer, but since I don't have a WRX, ehh...I'd need some convincing. If you wanted me to make a model of a Z and test some things on a force balance, that would be different. I will probably end up doing some of that this summer/spring anyways. You're wrong about vortex generators not generating lift. Since vorticies are low pressure, they have the potential to create lift. The wing doesn't have to create lift. It depends on the airfoil, the angle of attack, etc. For all I know, that wing could be a symmetric airfoil (maybe I should go back and look at the picture again to check). Based on all of this, there MUST be a reason Subaru decided to go with a wing. Not the least of which is that its mission is to prevent turbulent flow and separation which would render the rear "wing" useless for creating negative lift (read "downforce"). Last but not least, I'm a little fuzzy on your question about drag. Are you asking if the small wing is creating more drag compared to the same car without the small wing? Again, if they did it right, it should reduce drag considerably. If I am way off on what you're asking, help me out. I hope that helped a little bit. Logan Quote Link to comment Share on other sites More sharing options...
RedFive Posted December 12, 2006 Share Posted December 12, 2006 Just checked out the photo again. Can't tell from that picture if there's any camber in that airfoil or not. *shrug* Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted December 12, 2006 Author Share Posted December 12, 2006 Logan, A typical vortex generator creates vorticies (two) about the longitudinal (X) axis. The turbulent (as they are include a forwards component, constituting reversion) vorticies you mention are about the vertical (Z) axis. They are worlds apart. Longitudinal vorticies do not involve any reversion, hence I don't think of them as being turbulent. All they are is a flow that has a rotational component. As for the wing, I am not talking about the wing stalling, I am talking about as it is on a relatively high angle of attack, its resultant force vector will be back and up. The backwards component is drag, and the upwards component is lift. The question is whether this force vector has a larger or smaller impact on the vehicle as a whole, than the resulting increase in the rear wing's downwards and backwards vector, and decrease in the rear window's backwards vector. Hope that makes sense. Also, what size wind-tunnel do you have access to? Full-scale? Dave Quote Link to comment Share on other sites More sharing options...
geordieggg Posted December 13, 2006 Share Posted December 13, 2006 Ok, I hate jumping into debates such as this one, where obviously everyone here has a hell of a lot more knowledge than myself. I have been following it with interest, but now I have to ask a question about the following: I am talking about as it is on a relatively high angle of attack, its resultant force vector will be back and up. The backwards component is drag, and the upwards component is lift. Isn't the AOA of any wing relative to the direction of the air, making the force vectors you're referring to negligible at best? Or should I shut my dumb mouth and go sit back in the corner? Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted December 13, 2006 Author Share Posted December 13, 2006 Geordie, Yes the angle of attack is relative to the airstream direction. No this does not make the force vectors negligible. It will change the path of the air over a wing (the point at which it seperates/stalls etc) but it will not affect the direction of the force the wing generates which I think(?) is generally approximated as perpendicular to the chord line. There are no dumb questions. Dave Quote Link to comment Share on other sites More sharing options...
proxlamus© Posted December 14, 2006 Share Posted December 14, 2006 Dave and Logan... take a look at this PDF file from Mitsubishi.. this describes how and why the vortex generators were implemented on the roof of the EVO http://www.mitsubishi-motors.com/cor...004/16E_03.pdf Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted December 14, 2006 Author Share Posted December 14, 2006 You mean this paper: http://www.mitsubishi-motors.com/corporate/about_us/technology/review/e/pdf/2004/16E_03.pdf I've already read it. I believe I understand how and why vortex generators work, not only in the evo's application, but in general. Dave Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted December 14, 2006 Author Share Posted December 14, 2006 Urgh, I am sorry, I wrote that in a hurry on a break from work. I apologise for sounding so arrogant. That paper has loads of great information, thanks for the link. And apparently I didn't know that another term for `longitudinal vorticies' is `streamwise vorticies'. Dave Quote Link to comment Share on other sites More sharing options...
RedFive Posted December 15, 2006 Share Posted December 15, 2006 I wish I had time for a more detailed reply. I've been really busy writing my own research paper (much like the Mitsubishi one) on wing tip sails. But anyways...really quick... They are not worlds apart...I don't know where you picked that up. A key point that the Mitsubishi guys make in their abstract is that the vortex generators create drag and you MUST look at their placement and application and decide if the net outcome is good for your vehicle. Obviously, Subaru decided a wing was the better decision. Anyways, I will try and comment some more on this in maybe a few days or so. Sorry, I'm just swamped right now. Take care! Logan Quote Link to comment Share on other sites More sharing options...
thehelix112 Posted December 15, 2006 Author Share Posted December 15, 2006 I know what you mean about busy. Product delivery is in less than 4 weeks and we don't have a prototype yet. Eep. They ARE worlds apart. A z-axis vortex is produced BY flow separation. A x-axis (streamwise) vortex is produced on purpose to DELAY flow separation. Sure they both induce drag, but a x-axis vortex produces drag because it a, has a protruding generator, and b, reduces the static pressure downstream. Given the example we are talking about, this will reduce the pressure immediately after the generator from what it would be without the generator. But x-axis vorticies have an overall reduction in drag as seen by their effect in delaying z-axis vorticies (aka, flow separation). As I said, worlds apart. Regarding the wing, the question becomes whether the wing will have a larger increase in drag than a row of vortex generators, and if so, is the additional benefit of having non-vortex flow running over your rear wing element worth the drag penalty. I would guess it is. It wouldn't surprise me if (as was mentioned somewhere) the wing was developed for the WRC car, then added to the STI for homoglation purposes. Dave Quote Link to comment Share on other sites More sharing options...
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