AkumaNoZeta Posted October 12, 2009 Share Posted October 12, 2009 Can someone tell me why the raindrop shape is more drag-efficient than say a shape like a sewing needle? Quote Link to comment Share on other sites More sharing options...
Leon Posted October 12, 2009 Share Posted October 12, 2009 In a nutshell, the boundary layer does not separate from the surface as the taper angle of a long raindrop is not large. If you flip that raindrop front-to-back then the boundary layer will detach at the end where there is an abrupt drop-off. Flow separation results in increased drag. Quote Link to comment Share on other sites More sharing options...
AkumaNoZeta Posted October 13, 2009 Author Share Posted October 13, 2009 I understand that, but I mean why is o> a better shape than <>? Quote Link to comment Share on other sites More sharing options...
Leon Posted October 14, 2009 Share Posted October 14, 2009 I understand that, but I mean why is o> a better shape than <>? I would say it has to do with the raindrop shape promoting a laminar boundary layer as a fluid flows over it at the front. As the fluid moves down the shape its velocity increases and the flow becomes turbulent, thus more drag. That shape you showed looks like the initial boundary layer will be turbulent from the get go. Quote Link to comment Share on other sites More sharing options...
AkumaNoZeta Posted October 14, 2009 Author Share Posted October 14, 2009 But airplanes and salt lake cars seem to have the needle point noses, how come it works for them and not slower vehicles? The needle nose seems to make sense because it looks like it would cut through the air. Quote Link to comment Share on other sites More sharing options...
Administrators BRAAP Posted October 14, 2009 Administrators Share Posted October 14, 2009 These should help explain a little bit... http://www.answers.com/topic/aerodynamics http://www.tonyfoale.com/Articles/Aerodynamics/AERO.htm http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml Quote Link to comment Share on other sites More sharing options...
Flexicoker Posted October 14, 2009 Share Posted October 14, 2009 But airplanes and salt lake cars seem to have the needle point noses, how come it works for them and not slower vehicles? The needle nose seems to make sense because it looks like it would cut through the air. I'm not going to pretend to be an aerodynamicist, but from what I understand as you get close to the speed of sound, the ideal shape begins to change as you have to start taking into account shock waves and compressible flow. At supersonic speeds (or close to supersonic speeds) you want something like the pointy needle tip, but at lower speeds the rounder front is better. Quote Link to comment Share on other sites More sharing options...
AkumaNoZeta Posted October 14, 2009 Author Share Posted October 14, 2009 That makes sense, but if it works for the high speeds, why doesn't it work for the low speeds? Quote Link to comment Share on other sites More sharing options...
260DET Posted October 14, 2009 Share Posted October 14, 2009 Its my understanding that a pointed front end does not result in a problem, providing the rest of the object's shape is suitable. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 14, 2009 Share Posted October 14, 2009 http://en.wikipedia.org/wiki/Aerodynamics Supersonic flowMain article: Supersonic Supersonic aerodynamic problems are those involving flow speeds greater than the speed of sound. Calculating the lift on the Concorde during cruise can be an example of a supersonic aerodynamic problem. Supersonic flow behaves very differently from subsonic flow. Fluids react to differences in pressure; pressure changes are how a fluid is "told" to respond to its environment. Therefore, since sound is in fact an infinitesimal pressure difference propagating through a fluid, the speed of sound in that fluid can be considered the fastest speed that "information" can travel in the flow. This difference most obviously manifests itself in the case of a fluid striking an object. In front of that object, the fluid builds up a stagnation pressure as impact with the object brings the moving fluid to rest. In fluid traveling at subsonic speed, this pressure disturbance can propagate upstream, changing the flow pattern ahead of the object and giving the impression that the fluid "knows" the object is there and is avoiding it. However, in a supersonic flow, the pressure disturbance cannot propagate upstream. Thus, when the fluid finally does strike the object, it is forced to change its properties -- temperature, density, pressure, and Mach number -- in an extremely violent and irreversible fashion called a shock wave. The presence of shock waves, along with the compressibility effects of high-velocity (see Reynolds number) fluids, is the central difference between supersonic and subsonic aerodynamics problems. The purpose of the sharp tip of a supersonic shape is to minimize the shockwave. The purpose of the rounded nose of a subsonic shape is to cause the air to react to the shape and stick to it, reducing drag. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted October 15, 2009 Share Posted October 15, 2009 Can someone tell me why the raindrop shape is more drag-efficient than say a shape like a sewing needle? The raindrop shape is not more drag efficient. The needle shape minimizes frontal area for a given volume. Airliners, wooded arrows, NASCAR drafting packs are all long and skinny. If you want to move a lot of stuff through the air, you make it all travel with the least frontal area. That way you don't need to open up more air, everything follows behind and in trail. The raindrop curve works when you cannot control the frontal area as a parameter. If your stuck with a 5' wide passenger car then the only option is to shape the body as best you can to minimize turbulence. If you were unlimited in design and wanted minimum drag then you make your minivan with all 7 seats behind each other in a needle shape... Quote Link to comment Share on other sites More sharing options...
(goldfish) Posted October 17, 2009 Share Posted October 17, 2009 Frontal area is only one element. the shape determines the Coefficient of Drag. Even stranger is near supersonic speeds the object should have a narrowing near the middle, kinda wasp like. Quote Link to comment Share on other sites More sharing options...
Leon Posted October 17, 2009 Share Posted October 17, 2009 The raindrop shape is not more drag efficient.The needle shape minimizes frontal area for a given volume. Airliners, wooded arrows, NASCAR drafting packs are all long and skinny. If you want to move a lot of stuff through the air, you make it all travel with the least frontal area. That way you don't need to open up more air, everything follows behind and in trail. The raindrop curve works when you cannot control the frontal area as a parameter. If your stuck with a 5' wide passenger car then the only option is to shape the body as best you can to minimize turbulence. If you were unlimited in design and wanted minimum drag then you make your minivan with all 7 seats behind each other in a needle shape... Not more efficient in incompressible flow? So would you want a brick shaped skinny thing or a wing shaped skinny thing? Would you want your needle minivan to have a square front, or the blunt curve and taper of the wing shape? Can you give me your reference that says that the raindrop shape is not more efficient, I'd be surprised and interested. Quote Link to comment Share on other sites More sharing options...
Leon Posted October 17, 2009 Share Posted October 17, 2009 Airliners, wooded arrows, NASCAR drafting packs are all long and skinny. Take a closer look at an airliner; they are raindrop shaped! NASCAR drafting is a whole other phenomenon, if the cars themselves were long and tapered they'd be more "drag efficient." As for arrows, I'd say their pointed nature is for another reason besides keeping flow laminar and decreasing separation! If you want to move a lot of stuff through the air, you make it all travel with the least frontal area. That way you don't need to open up more air, everything follows behind and in trail. That aspect is minimal. As goldfish said, that is merely a small part of the equation. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted October 17, 2009 Share Posted October 17, 2009 Take a closer look at an airliner; they are raindrop shaped! . I don't see any airliners that are only twice as long as they are wide (like raindrops). Maybe that's why airliners don't fall??? Have a look at: http://www.aerioncorp.com/ And here is NOT AN AIRLINER: Quote Link to comment Share on other sites More sharing options...
Leon Posted October 18, 2009 Share Posted October 18, 2009 This is what I'm talking about when I say "raindrop shape." Notice, the shapes are blunt in front, slowly tapering to a point. That is a drag efficient shape. Also notice that the plane you showed was meant for supersonic speeds, meaning that it's optimal geometry will be way different when compared to aircraft engineered for lower velocities. Here is a subsonic aircraft: Subsonic airliner: As you can see, this airliner has a blunt nose and slowly tapers to a point. Of course there are other design considerations for airliners, such as fitting in many passengers and tons of cargo so the shape isn't as efficient as a smaller plane. However, it follows the general trend and utilizes the "raindrop shape." Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted October 18, 2009 Share Posted October 18, 2009 This is what I'm talking about when I say "raindrop shape." Well, That in not what raindrops look like. Raindrops are almost spherical, will go to concave, and then break apart to smaller pieces. No way in hell that raindrops look like airfoils or fuselages. Just because YOU make up meanings for words does not mean much... Quote Link to comment Share on other sites More sharing options...
Leon Posted October 18, 2009 Share Posted October 18, 2009 Dude, calm down. I don't think people look at the physics when they think of raindrops. It is generally referred to a raindrop shape since when you say raindrop most people see this: I don't see why you have to be so argumentative when the OP was asking why the raindrop shape is more drag efficient. You're just arguing semantics here. Quote Link to comment Share on other sites More sharing options...
Guest Rolling Parts Posted October 18, 2009 Share Posted October 18, 2009 Dude, calm down. I don't think people look at the physics when they think of raindrops. . This is a Technical Forum. If you are arguing PHYSICS then we look at PHYSICS. I'm being calm and VERY clear. Raindrops do NOT look like that. If you say raindrop shape you mean raindrop shape. Raindrops are spherical to concave. Needles are cylindrical and long. Needles have less drag. Words have meanings; use them correctly on a technical forum or your just making stuff up in your own head. Quote Link to comment Share on other sites More sharing options...
JMortensen Posted October 18, 2009 Share Posted October 18, 2009 FWIW I was thinking of the shape in the picture. Words have meanings; use them correctly on a technical forum or your just making stuff up in your own head. Uh... you... uh... you have a little typo in this sentence that makes it technically not make any sense. Ironic... Quote Link to comment Share on other sites More sharing options...
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