260DET Posted October 16, 2006 Author Share Posted October 16, 2006 The Mitsubishi paper seems to indicate that their vortice generators increased pressure on the rear of the car as a whole, they did not merely tidy up the airflow for the wing. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 16, 2006 Share Posted October 16, 2006 When I read it the paper it says that separated air is essentially creating a vacuum which lifts the back of the car. As I understand it this is also why we put lips in front of vents on cars, because the resulting turbulence behind the lip will create a vacuum and suck the air out of the vent. By using the VGs the air sticks to the rear of the car which provides at least low pressure air over the surface of the car, so it does increase downforce. Again, I would think that in our case the benefit would be maximized because we're talking about putting a spoiler right in that airflow. So instead of catching just bits and pieces of the airflow, we would have a nicely directed stream of low pressure air headed straight at the spoiler. This could be compared to the hood maybe... the hood has high velocity low pressure air going over it, then that air hits the windshield and that obstruction like a spoiler causes the redirection of the air which creates a lot of pressure. The pressure is what gives us downforce. Quote Link to comment Share on other sites More sharing options...
bjhines Posted October 16, 2006 Share Posted October 16, 2006 well... If the air detaches... it will provide less lift... If the air stays attached it will provide more lift... this is right out of small plane aerodynamics... A stalled wing provides great drag and less lift... My statement earlier will make more sense in this light.. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 16, 2006 Share Posted October 16, 2006 I'm not sure that I'm right John, but I did find this: http://www.aerospaceweb.org/question/aerodynamics/q0228.shtml And the last paragraph says this: The majority of the wing modifications we have described are typically seen on military aircraft, especially high-performance fighters. The leading edge extension, dogtooth, and fences are particularly common on these planes since these devices increase the stall angle of attack and improve maneuverability. These modifications are rarely seen on commercial airliners where excellent maneuverability is unnecessary. However, conventional vortex generators are quite common on airliners to increase the effectiveness of flaps and other control surfaces during takeoff and landing. And that seems to say to me that trying to use the flaps in separated airflow doesn't do much, so the vortex generators are used to keep the airflow stuck to the wing so that the flaps work more effectively. Since moving the flaps provides a lot of the lift you're talking about, I think that is where the VGs provide and advantage. I'm not totally sold on my own argument though. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 16, 2006 Share Posted October 16, 2006 As always, you can go a view the original research on these aerodynamic topics here: http://ntrs.nasa.gov/ Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 16, 2006 Share Posted October 16, 2006 Looks like they have a lot of info that you can ORDER John. Do they have any that you don't have to pay for? Quote Link to comment Share on other sites More sharing options...
bjhines Posted October 16, 2006 Share Posted October 16, 2006 quote from my post earlier... ""As far as small stick-on vortex generators go... the more you do to ensure air stays attached to the rear deck... the more LIFT(upforce) you will generate there... they(VGs) may increase the effectiveness of a shorter rear-lip-spoiler... but that might not actually achieve anything useful considering there is even more lift on the rear hatch window..."" The whole idea of the VGs is to keep airflow smooth and attached to the upper surface of the wing... Laminar flow is low drag and can provide the most lift... for an airplane... To agree with you JM... the VGs would increase the effectiveness of a short rear lip spoiler... just like they increase the effectiveness of the flaps on a plane... by keeping air STUCK to the surface... thereby increasing lift... WE WANT DOWNFORCE ON OUR CARS... SO the VGs are a double edged sword.. they increase LIFT(upforce) on the hatch window by sticking airflow to it.... but they make the rear lip spoiler more effective and increase DOWNFORCE there... My argument is that there is no net gain in DOWNFORCE... VGs may reduce drag.. as long as some other factor doesn't negate that as well... I will argue that VGs may not even help reduce drag... because of the 2 huge vortextes running down the left and right edges of the hatch and sails... FENCES along the roofline and hatchline might really improve the overall performance of any aerodynamic modifications... The problem with perspective comes from the fact that these devices are used to INCREASE LIFT on airplanes... the roof of our car is the same shape as the top of an airplane wing... but WE DONT WANT MORE LIFT... OUR analog to airplane FLAPS would be our rear lip spoiler... BUT IT TURNS THE WRONG WAY (UP!!)... so again anything you do to improve airlfow behind the car is going to be really turbulent due to the use of that rear lip to increase downforce... The more effective the rear lip.. the more drag it will induce... Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 16, 2006 Share Posted October 16, 2006 The whole idea of the VGs is to keep airflow smooth and attached to the upper surface of the wing... Laminar flow is low drag and can provide the most lift... for an airplane... Laminar? Laminar flow is smooth and has no vortices, right? I think that the VG trades a bit of drag to reduce the boundary separation layer, but is not laminar AFAIK. To agree with you JM... the VGs would increase the effectiveness of a short rear lip spoiler... just like they increase the effectiveness of the flaps on a plane... by keeping air STUCK to the surface... thereby increasing lift... OK, so what this means is that it makes the spoiler effective, and in the plane example that means more lift WHEN THE FLAPS ARE USED. On a car it does not mean more lift, it mean less boundary separation layer, and then beyond that it would make a spoiler or wing more efficient by giving it a larger amount of airflow. We at least agree that it makes whatever is at the end of the hatch more important. WE WANT DOWNFORCE ON OUR CARS... SO the VGs are a double edged sword.. they increase LIFT(upforce) on the hatch window by sticking airflow to it.... but they make the rear lip spoiler more effective and increase DOWNFORCE there... My argument is that there is no net gain in DOWNFORCE... VGs may reduce drag.. as long as some other factor doesn't negate that as well... I don't think this is right (again not sure). If I'm correct than the boundary separation layer creates vacuum right above the hatch. The question then is what has a lower pressure? The boundary layer coming over the hatch which is turbulating without the VGs or the higher velocity low pressure air more closely following the hatch contour with the VGs. I think that the VG's themselves increase drag, although when everything else is taken into consideration there may be a net reduction in drag. I don't see how you can argue that there is no net gain in downforce though. We'd need a test to figure that out. I will argue that VGs may not even help reduce drag... because of the 2 huge vortextes running down the left and right edges of the hatch and sails... FENCES along the roofline and hatchline might really improve the overall performance of any aerodynamic modifications... Fences definitely work. I've been tempted to try that myself (hood fences too, just like NASCAR, just because they only turn left doesn't mean they're idiots). The problem with perspective comes from the fact that these devices are used to INCREASE LIFT on airplanes... the roof of our car is the same shape as the top of an airplane wing... but WE DONT WANT MORE LIFT... OUR analog to airplane FLAPS would be our rear lip spoiler... BUT IT TURNS THE WRONG WAY (UP!!)... so again anything you do to improve airlfow behind the car is going to be really turbulent due to the use of that rear lip to increase downforce... I have done more searching and read more articles, and I have yet to find one article that says that the VGs increase lift. Quite a few that say that it will keep the wing from stalling even with a more aggressive angle of attack, and they ALL say that they make the flaps more effective and the FLAPS can provide more lift, especially at low speeds. Our top speed is extremely low speed for a plane, so I am still thinking that the benefit to us is that we keep the airflow attached to the hatch and use the wing/spoiler/whatever to it's best advantage. Without the spoiler I see the VGs reducing net drag and increasing downforce by eliminating the vacuum from on top of the hatch. I haven't done any testing, and I may be wrong... The more effective the rear lip.. the more drag it will induce... Agreed, but you still may find a net reduction in drag by installing parts that reduce the turbulence behind the car, along with a wing/spoiler/etc. That's what the C&D article John quoted shows. Quote Link to comment Share on other sites More sharing options...
johnc Posted October 16, 2006 Share Posted October 16, 2006 Most research documents done before 1985 are free. Sometimes you can't find an exact match to what you're looking for but you can find information from studies that are close: Experimental study of delta wing leading-edge devices for drag reduction at high lift Wind-tunnel development of underwing leading-edge vortex generators on a NASA supercritical-wing research airplane configuration Quote Link to comment Share on other sites More sharing options...
260DET Posted October 17, 2006 Author Share Posted October 17, 2006 quote from my post earlier...""As far as small stick-on vortex generators go... the more you do to ensure air stays attached to the rear deck... the more LIFT(upforce) you will generate there... they(VGs) may increase the effectiveness of a shorter rear-lip-spoiler... but that might not actually achieve anything useful considering there is even more lift on the rear hatch window..."" ..................................................................... WE WANT DOWNFORCE ON OUR CARS... SO the VGs are a double edged sword.. they increase LIFT(upforce) on the hatch window by sticking airflow to it.... but they make the rear lip spoiler more effective and increase DOWNFORCE there... My argument is that there is no net gain in DOWNFORCE... VGs may reduce drag.. as long as some other factor doesn't negate that as well... ...................................... If VG's help stick airflow to the hatch window, doesn't that increase air pressure there? Compared with the situation where no VG's are used? Putting it another way, the presence of air sticking to the hatch window creates more pressure than where the air becomes detached? If its detached the result must be lower pressure against the window surely. Assuming air speed is the same in both situations. Quote Link to comment Share on other sites More sharing options...
73ZCAR Posted October 17, 2006 Share Posted October 17, 2006 260DET, the airspeed is NOT the same. And higher airspeed over a surface generally creates a lower pressure at that surface. Mike. Quote Link to comment Share on other sites More sharing options...
74Adam Posted October 17, 2006 Share Posted October 17, 2006 "If VG's help stick airflow to the hatch window, doesn't that increase air pressure there?" No. The greater the speed a fluid flows over a surface, the less pressure it exerts on that surface. Try this: Take a piece of paper and hold two corners such that the paper is taut. Hold the paper in such a way that the edge of the paper that you are holding on to is pointing up but then falls over forward in front and away from you. Then hold the paper in front of your mouth and blow hard over the "hump" of paper and watch what happens. This is a good and basic illustration of fluid flow and pressure. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 17, 2006 Share Posted October 17, 2006 260DET, the airspeed is NOT the same. And higher airspeed over a surface generally creates a lower pressure at that surface. This is exactly where I think you and bjhines have it wrong. My theory is that there is literally NO steady airflow at the back of the hatch on a Z. There is a vacuum, and whatever air does come in to fill the void isn't coming into the void in a uniform enough way to eliminate the vacuum. This is why we get exhaust fumes sucked into the car. What I think you guys are missing is that there is a great big hole or vacuum behind and ABOVE the car where the boundary layer is. So if you only have random turbulent airflow as the surrounding air trys to fill the void on the hatch and the back of the car as well, then you get vacuum or suction or lift or whatever you want to call on the back of the hatch. Here is a simply explained, common sense website with a fairly good explanation: http://selair.selkirk.bc.ca/aerodynamics1/drag/Page2.html Like the swimming pool example, we are pushing air out of the way in front and fighting the vacuum behind when we drive a car through the air. I think you guys are just underestimating the size and location of the vacuum created behind our cars. I don't know how bad the boundary separation layer is on a Z. All I know is it can suck fumes being expelled at pretty good velocity out of the tailpipe back towards the car and into any small leak in the gaskets or seals there. I think that the size and strength of that area of vacuum is what we really need to find out to resolve the argument, with and without the VG's of course. Regardless, if there is a vacuum above the back of the car that air is rushing into to fill, it seems logical to me that you would have more pressure (and downforce) on the back of the car when you eliminate the turbulent vacuum layer from the surface of the car, even if you only had a steady flow of high velocity low pressure after the fix was made. It's not the best case scenario for downforce, but I do still think it's better than having a huge vacuum there. I've also been looking at websites where they've alternately used vacuum inside a wing to suck the airflow to the wing removing the boundary layer into the wing itself, and others where they put high pressure air into the boundary layer fill the boundary layer and keep air attached to the rear section of a wing. I'm seeing more possibilities there, like drilling holes in a Lexan rear window to try and equalize the pressure. Seen that on enough racecars... Maybe even put some ducts from the side windows to the hatch and reroute some air into the boundary layer. Again, it seems like this would reduce lift and drag, even if no spoiler were used. Quote Link to comment Share on other sites More sharing options...
RedFive Posted October 17, 2006 Share Posted October 17, 2006 Vortices -- be it in the rear of a truck, car, or wingtips of an airplane -- are low pressure by definition. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 17, 2006 Share Posted October 17, 2006 Vortices -- be it in the rear of a truck, car, or wingtips of an airplane -- are low pressure by definition. I haven't seen anyone in this thread dispute that statement. Quote Link to comment Share on other sites More sharing options...
RedFive Posted October 17, 2006 Share Posted October 17, 2006 When I read your post, it sounded like there was some question somewhere... Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 17, 2006 Share Posted October 17, 2006 That's not what I intended. I'm saying that I think the low pressure from the VGs might be higher pressure than the turbulent boundary separation layer, and that this would explain why the Mitsu engineers showed increased downforce despite the increase of "low pressure" air over the rear window and trunklid. Quote Link to comment Share on other sites More sharing options...
Guest Mike Posted October 17, 2006 Share Posted October 17, 2006 This is why so many UFO's are cigar-shaped with tapered ends front and back:ugg: So, when they enter atmosphere at speed, the vacuum is well behind the rear taper. Of course, a car shaped like that would look similar to the Oscar Meyer weenie mobile:wink: NO I DON'T believe in UFO's... but I DO BELIEVE that HYBRIDZ ADMINS ARE ALIENS8-) Quote Link to comment Share on other sites More sharing options...
260DET Posted October 18, 2006 Author Share Posted October 18, 2006 That's not what I intended. I'm saying that I think the low pressure from the VGs might be higher pressure than the turbulent boundary separation layer, and that this would explain why the Mitsu engineers showed increased downforce despite the increase of "low pressure" air over the rear window and trunklid. Thats my understanding too. Quote Link to comment Share on other sites More sharing options...
bjhines Posted October 18, 2006 Share Posted October 18, 2006 Let talk in wing terms a minute.. even though it can confuse things. As soon as the smooth flowing air separates from the upper surface of an airplane wing.. it stalls... it looses lift... Sticking air to the upper surface would make the wing harder to stall and therefore IMPROVE LIFT... Stick the air to our Z-car upper surface and you increase LIFT... Quote Link to comment Share on other sites More sharing options...
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