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Who uses vortex generators?
I work in the Aircraft industry, and the planes I build have specifically located VG's along the upper, leading edge of the wings...as do many Aircraft.
In our specific use, it increases lift, decreases drag and ultimately lowers fuel consumption and/increases speed... Where might these be useful on a production car I wonder? |
Been posted...
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I have a few.
They come stock on the 07 CR-V. :p Honda molded "edges" into the tops of the headlight to adjust airflow over the mirrors, and then molded more tabs into the mirrors to further adjust the airflow. The newer CR-V does away with them, though. The body has inherently better aero than the old one. Some of the best examples, though, have got to be on the Prius C. It has vortex generators on both the headlights and rear lights. |
Wungun - I work in the aircraft industry as well: Flight Test Engineer. I'm glad to see you here...
I'm interested in testing vehicles to determine vortex generator placement. I have a partner and friend who is also a Flight Test Engineer. Our first vehicle is the Prius with initial tuft testing already underway. Our goal is to produce a kit and map to place strakes and vortex generators on your vehicle. Do a quick search for Prius Tuft Testing in the Aerodynamics section and check out some videos I've posted. -Ryan |
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Very cool!!!
I'll have to look up your posts... Looking forward to seeing what you come up with! Quote:
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A neat instance of VGs in the aviation world that I've witnessed is the Joint Cargo Aircraft (C-27) which was supposed to be a twin engine Army/Airforce cargo plane. The neat thing is that the engines didn't counter-rotate (I'm guessing for logicistics and replacement purposes). Because of this, one side of the rudder had VGs placed up it to keep the flow attached. -Ryan |
Vortex generators can be used to improve the performance of aircraft in a variety of ways. If properly sized and positioned vortex generators can be used to lower the stall speed of an aircraft, improve stability and control during maneuvering, decrease turning radius, decrease takeoff distance and increase takeoff weight. For commercial aircraft this can mean higher payloads and increased safety. While the general aviation pilot will enjoy better maneuverability as well as the increase in safety.
Straight from this site: Aeronautical Testing Service, Inc. Vortex Generators explained. They do not increase speed or save fuel. Added lift and stability at the cost of aero. Thats the simple version for airplanes. For Vehicles. Vortex generator work like this: You increase drag at the point of vortex generation. You create turbulent air to flow past a trouble spot such as protruding mirror only to retattach as laminar flow. Thus actually creating less drag than the vortex generator actually caused. I see these on the trailing edges of the roofs on some vehicles such as the Lancer. You need computer simultation or wind tunnel testing to properly place these. I would not try it because you would probably do more harm than good unless you have access to proper testing. |
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News flash....cars and airplanes operate in very different environments aerowise. Are they both moving through air....why yes, but that is where the similarity ends. Cars are in ground effect and have zero laminar flow going across their skins, so vortex generators would only serve to make a turbulent layer more turbulentier....which makes no sense, which is why they're crap when it comes to auto aerodynamics.
http://i48.tinypic.com/24xnx8m.jpg The "Vortex Generator" on the CRV Honda headlight assembly is an optical reflector designed to make the turn signal more visible from a greater angle on the opposite side of the car. I got em on my 04 Civic too. |
I've used vortex generators on my Aspire with very positive results, but not quite as good as a kammback.
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Ground effect with respect to aircraft is a "phenomenon" where aircraft are able to increase pressure under their wings due to proximity to the ground. If we are talking car ground effect, it is the same "phenomenon" but reversed where a spoiler or the vehicle itself is angled downward increasing the low pressure under the body due to Bernoulli's principle of incompressible flow. It has nothing to do with vortex generator efficiency or practicality on vehicles. Secondly, vortex generators re-energize boundary layers. Why would you think to energize a laminar boundary layer? Laminar skin friction is the least drag of any type and the least your worries. Never interrupt that flow with a vortex generator. Vortex generators are used to keep the turbulent boundary layer attached longer, thus reducing pressure drag and to give control surfaces authority. Quote:
AeroModder - Thanks for the prior testing, that was a good thread if I'm not mistaken, the same one you have your avatar picture of? Wungun - I'm assuming the VGs are part of a STOL kit? -Ryan |
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The Coanda effect will cause laminar flow to follow that surface, while turbulent flow will by and large move on in the same direction. Most cars have curved edges on their rear bumpers. In a straight headwind the airflow along its sides will be turbulent, but when hit by sidewind it will be more or less laminar, curve round the bumper and create low pressure on the edge pulling it sideways and back. I am planning to put zigzag tape on my 2011 Insight's rear bumper just ahead of the curve to break laminar flow. If that stops the jerking sensation when hit by sidewind then I will know that it works. |
Ryan,
I'm not talking about what happens when you're trying to land an airplane. And I don't mean shaping a car to increase down force. I'm talking about ground effect as opposed to free air, when your 10,000 feet up air acts differently than it does when you're 10 inches off the ground. Air does not behave the same around a bluff body when moving close to the ground. When looking at the aerodynamics of a car, you must remember that the air will move quite differently around the car when it is on the road as opposed to flying at 10,000 feet. So just like we don't design airplanes to fly at 2 feet off the ground, we don't design cars to fly at 10,00 feet. Get it? So what I'm really trying to say is, because it works on airplanes, it doesn't mean it's going to work on a car. Mitsubishi spent a crap load of time & effort dinking around with VG's with real engineers and real wind tunnels, they were able to gain 2%.....2%. Now how are you gonna top that? With good enthusiasm and a positive outlook? Really, answer me this, besides the Mitsubishi EVO from a few years ago, what car manufacturer has put these on a production car? If VGs were able to reduce drag, you'd see them everywhere on everything. I suppose you could claim that "Big Oil" is keeping the car makers from getting too carried away with fuel efficiency. I suppose you also believe there are 100MPG carburetors out there too that they won't install. Put this on your car, they claim it'll double you mileage. http://pesn.com/2005/03/08/6900067_R...ges/viggo2.JPG Fact is, there's crap out there and there's things that really work, VGs fall into the crap realm. Please post your positive results when you have them, you'd be the first to do so. ABA testing please, we all know that YMMV from hour to hour without any changes to the car. And long term tests have so many variables it isn't even funny. I suggest you find yourself a nice hill to coast down at 40-50mph or so and see what kind of speed you maintain both with, and without, however many VG gimmicks you want to place on your car. Unless you're driving a crate, you will only see a tiny bit worse Cd when the VG's are in place. Certainly Wunguns Fiesta will suffer from any application of VG's. Here's the Mitsubishi Thing I didn't just fall off a turnip wagon Mr. Flight Test Engineer, I'd be behind vortex generators 100% if there was a shred of hope that they'd work on cars. There may be some cars out there with very poor aero design that VGs could help, but they're few and far between, most modern cars are fairly well optimized for what they are. VG tweeks will yield very small positive results at best and smallish negative ones at worst. Waste your time if you want. Real gains can be made by blocking your grill, pumping up your tires, and lowering your front airdam among other things. Unless you want to radically reshape your car, you're stuck with what you got, VG's just won't do much. |
... My bad, I just realized this thread is about deliberately generating vortices instead of just turbulence. :o
I see no way how vortices could help reduce drag, I'm no expert though. |
Chaz - Something irritates me about the way you present your thoughts. I'm going to try and bandaid as much of your rash as I can.
Car designers have a lot of missions to fill, regulations to abide by, etc. Why do you think commercial jets have winglets? Do you think winglets are more efficient than just extending the wing longer? It isn't. They are doing this to get the maximum efficiency while staying inside regulation. It would really help aero to cut down on the fuselage but that's part of the aircraft's mission so it can't be changed. All of the low hanging fruit has already been picked. Also, there isn't much that separates someone sitting in the corporate office designing up the next car and you and I. Once they get what they think is good, I'm sure they sit through a design review and human factors engineers tweek it to less efficiencies to provide "better" curves. I'm sure you know this: they want to provide bad ass engineering, but that doesn't put food on their plate. They need to sell cars. If the Prius and Insight gets 51 mpg and 51 mpg is better than 28 mpg, why isn't every single car on the road a Prius or Insight? Because the rest of the world doesn't care. They're happy with their sexy Z4's (love the way those look), A5's, or maybe an older Civic. Fact is, looks sold that car, and I'm 99.99% sure they didn't have the Cd posted on the sticker. "Honey, look at this one, it has a flat plate drag of only 0.7!" said NO ONE EVER. RedDevil - unless you have introduced a new surface, I have my doubts that it is laminar at your rear bumper. Laminar flow is more likely to separate from the vehicle than turbulent flow (which I suspect is what you have). I'd like to hear what happens. Don't forget to post it for us! -Ryan |
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Apparently, Honda felt that "visibility" was unnecessary on the new CR-V... but then, the side mirror seems more aerodynamic, now, so the hard edge on the corner of the vehicle is no longer necessary. |
I believe VGs should be used in conjunction with other drag-reducing methods in order to be fully effective. They have to be implemented correctly though and that is where the challenge resides. Experiments based on sound studies such as Mitsubishi's are worth undertaking, IMO. I know I will, eventually. :thumbup:
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I was under the impression that the VG on the rear roofline of the Lancer was to aid rear down force? Surely that goes against this conversation of VG use for economy?
And in the game of fuel economy, is 2% not a substancial gain? |
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The vortex generators were brought in to push the separation point lower down the back glass which helped pull some freestream air towards the spoiler. I'd take 2% over nothing :D -Ryan |
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I don't know for sure as I have no way to see what actually happens. I just deduced what happens from the bits of information I have: Observation - The 2Gen Insight suffers from side wind effects; under some conditions I can feel a constant but erratical series of jerks, esp. when moving slightly with the wind instead of against it. - Fuel consumption is much higher on side wind impact than when driving straight against the wind. At 6 Bft I noticed the FC went up 30% compared to driving straight against it driving on a gently sweeping highway in a completely flat and almost treeless plane, steady pace etc. Theory - Airflow on a plate that leans into the wind tends to be laminar. - Airflow moving along a plate is unstable and tends to get turbulent. - Rounded edges can make an airflow passing along it follow the curve. This is the Coanda effect. A laminar flow may follow the curve completely. - The Coanda effect is inherently instable and tends to enlarge any instabilities in the airflow until the turbulence allows it to break loose. The radius of the bend is strongly related to the instability; if it is too sharp the flow cannot follow it and breaks loose. - Turbulent air won't follow an inward curve but rather move on in the same direction. - Vortices are more or less stable by themselves and folllow a curved edge, especially the part of the vortex that is already moving in that direction. That may explain why vortices help in the Mitsubishi case; it allows part of the air to curve an edge too sharp for laminar or turbulent air to follow. Deduction When hit by a strong sidewind the airflow curves around the edge of the rear bumper, creating a low pressure area at the side and rear of the bumper, pulling it back. Straight against the wind the air is turbulent and ignores the corner when it moves by. In between the air will sometimes curve round and sometimes go straight. Both conditions are unstable and likely to induce the other. This produces the shaking sensation. Remedy Breaking or preventing the Coanda effect on the corner should enhance stability and yield better MPG. One way is to sharpen the trailing edge (Prius II, Chevy Volt etc.) Another is inducing turbulence by adding a ridge or crease or sticking on zigzag tape just before the corner. The latter is my plan. Proof Nobody that I know of has done this, however Cobb has removed his rear bumper and claims that improves side wind stability and MPG. The uncovered bodywork falls back slightly with a sharp cliff and has sharp edges. This is no way as much proof as I would like, but it does fit. Testing Both my car and me are quite new to this game. There are some more mods that I want to do and some testing gear to get and use. Especially the rear wheel well covers, as I suspect that will decrease turbulence and so increase the effect of the edge curve cure. Once that is sorted out and the conditions are right (I need strong and stable southwestern wind, like 6 or 7 Bft) then sure I will get out the word. Meanwhile, if there's anyone out there with a wind tunnel and an Insight, I'd be over the moon if you could park it slightly askew in that tunnel and test it with and without zigzag tape, ridges and the like. Well, who knows? |
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However, I think you have turbulent and laminar flow flip-flopped. Laminar flow will separate early while the turbulent air tends to 'stick' a little longer. Its hard to think about and conceptualize... [been sitting here backspacing for about 5 minutes trying to think of a good way to explain it] The usual example is the golf ball but I don't like that. With turbulent air, the momentum is broken and skin drag increases. The particles have vectors in all directions which means SOME of them have vectors in the direction we WANT it to go. In laminar flow, the air is saying "No sir, me and my buddies are going this way."... However since they are all going in that direction, there is less air bouncing off of the surface causing less skin friction. Finesse is needed to keep laminar flow as long as needed and then trip the flow to turbulent so it will stick to the converging tail. Vortices are such an interesting subject matter. The low pressure region in the center wants to suck the air inward, while the centrifugal force wants to pull it apart... while the whole time, the shear forces within caused by speed differentials cause eddies and reverse vortices (can you tell I geek over vortices? :D). Not sure why, and it's a high level subject, but the center of the vortex DOES have an affinity to a surface. I doubt this is evident in the dirty tails of our vehicles... Ok, sorry so long. Have a great night! -Ryan |
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I may as well have misjudged the role of laminar flow and turbulence. I can't be certain. But I find the Coanda effect hard to explain with anything other than laminar flow. The only thing that can make a stream of air or water alter its direction is a difference in pressure (and vice versa). I can visualize that happening in laminar flow. I cannot visualise that in turbulent flow. Parts of a turbulent flow will follow the bend but as it requires less energy to just move on, that's what most of the air will do. But I do admit that I worry about the effect of zigzag tape on the corner to break the Coanda effect. A sharp edge will break the effect but that does not depend on the properties of the airflow (being laminar or turbulent). The zigzag tape however only works if my argument is true and yours is not. Which makes my alarm bells ring. Or, scientifically; Hey, this might be interesting; what is really happening here? |
I think there's a vortex generating in this thread... :D
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RedDevil - I'm treading lightly around the Coanda effect. I believe the "Coanda effect" is really just a case of Bernoulli's principle without the duct. You can't have it without there being a jet present. Please don't believe 99% of what you read on the web about it, because it looks like it's coming from those "scientists" that "don't need a formal education". You know, the ones that think you can put a wind turbine on a car and drive for free. At least those were the sites Google pulled up for me.
Take a piece of paper and hold one end of it. Let the other end dangle. Blow strongly across the top surface of the paper and it will lift. This is because the static pressure in the jet is less than the static pressure of the ambient air. The air between the paper and your blown jet is sucked up into the jet and thus lifting the paper. This principle explains why the wake pressure behind flow separation is governed by the low pressure of the separation point. The air is being sucked back up to the low pressure point at the separation (flow reversal...). BACK TO YOUR CAR AND CROSSWIND... I'd suspect the influence to your vehicle's aerodynamic AoA is causing induced drag but the big culprit is probably the incidental lift. Although the lift is perpendicular to your path (so it shouldn't matter right?), you're having to compensate for that perpendicular force by correcting your steering. I bet your MPG losses are due to your now misaligned wheels rubbing the road to make up for your aerodynamic force vector. Bed time! Have a great night! -Ryan |
I think people tend to lose sight of the big picture when talking about VG's and Ziggy Tape. The air at the skin of the car is only a tiny fraction of the air that is being affected by the passing of the vehicle, certainly the air at 3 to 4 feet out from the sides of a vehicle is fully in play, and for many tens if not hundreds of feet behind. It is only by gross management of this air flow that we can expect significant gains, meaning changing the shape of our cars. Screwing around with the already "Turbulent Boundary Layer" is a piffle compared to the huge amounts of air that are important to what the overall Cd is on the car.
This is my primary reason for saying VG's & ZT's on cars are a waste of time when trying to achieve any real gains. Fun to think about I suppose, but in the end you need to visualize the thousands of cubic feet of air that are moving up, down, to and fro, and ask yourself if something sticking up a few inches tops is really gonna do anything. Clearly the VG's & stuff are great for airplanes, I understand that. But again it is because you're dealing with airflows on wings which is laminar by nature, car aero is seldom laminar. |
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Furthermore, the boundary layer has huge effects on flow separation. Reduction of flow separation in certain areas of a car can reduce the C pillar vortices strength. This reduction can greatly lessen the size of the wake of a car and thus reduces the induced drag. |
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What CFD software are you using? -Ryan |
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I'm done with argueing, I know what I know and I will put that in practice as soon as I'm done and happy with the easier mods.
Hard data however might be helpful, in any shape or form. PS (edit): Please don't take this as a personal attack. A devil with clogs is bound to hurt a few toes by nature, I fear. |
RedDevil - Hope that isn't aimed at me. I'm only here to help. Good luck!
-Ryan |
i have some more questions, but i saw a late model toyota camry up close today at rideout and martin road--the headlights have some sort of VG molded into the top outer corner that even has some sort of zigzag-stair-step pattern on the leading edge. Thanks to this thread i had a clue...lol
Ryan, When you say that your rear window has separated flow, do you mean there is a laminar flow boundary layer close to the glass and there is also a flow stream higher up above the surface of the glass that has detached from the upper edge of the boundary layer? And this separated flow layer makes the rear spoiler ineffective? Is the separated layer (always) turbulent or possibly laminar/free-stream in character? How far above the glass do you think this separated flow begins (at the top edge of a very thick boundary layer)? Maybe some redundancy in the questions here, so feel free to summarize it if you can explain all this flow stuff to some of us aero novi... |
I only state things that I've heard from others who know way more than me. Think that you have laminar flow over your car body all you want, facts are still that the flow is not laminar, unless you're driving a wing shaped car in the middle of nowhere with no other cars around.
I write this not to argue, but to make sure your fallacies are not taken as fact by others who look in on this discussion thread. Don't believe me. But you may wanna listen to this guy. I know yer gonna beat me to death with #7, read #13 before you get too carried away. |
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Posted late and without much consideration for the fact that what sounds straight in Dutch can sound blunt if literally translated into another language. Dutch conversation is usually very open and direct, even challenging without really meaning to. I apologize. I just wanted to state that I'll go ahead with my plan, but will not close myself for factitional info that may point me in a different direction. But as we don't agree on some matters let's not repeat ourselves. |
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As for the questions, I'll just go down through the para. Q1: When you say that your rear window has separated flow, do you mean there is a laminar flow boundary layer close to the glass and there is also a flow stream higher up above the surface of the glass that has detached from the upper edge of the boundary layer? A1: No, this isn't possible to my knowledge. -Boundary layers are a result of the shear stress of the object on the air. It starts smoothly, trips to turbulence, and then eventually if the object has adverse curvature (curving away from the flow), the flow detaches and there is a stagnation point... if the detached flow is large enough and an adverse pressure gradient is present, you'll have reversed flow. Q2: And this separated flow layer makes the rear spoiler ineffective? A2: Yes, the spoiler requires air to move across it to produce it's downforce. If the flow across the spoiler is moving very slowly as a result of being in the detached flow (the stagnant or reverse flow area), no air is really pushing across it rendering it ineffective. It doesn't produce drag! But it doesn't produce downforce... Q3: Is the separated layer (always) turbulent or possibly laminar/free-stream in character? A3: Separated flow is flow that is no longer part of the freestream. This is a hard question to answer because stagnant air has neither turbulent nor laminar characteristics. The separated "area" can changes characteristics based on your velocity. It can start out as just a stagnant spot with very small eddies and as you go faster, can result in the oscillating vortices that you experience behind the big rigs on the interstate. Q4: How far above the glass do you think this separated flow begins (at the top edge of a very thick boundary layer)? A4: On my Prius, it looks to separate just behind the antenna. It appears the separation caused by the antenna acts like a catalyst to break up the flow further down the back glass. -Ryan |
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