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#21
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Dimples On Model Aircraft Could Greatly Extend Range
On Nov 5, 4:10*am, bbrought wrote:
pressure drag due to seperated flow and skin friction drag. First, on majority of the drag is pressure drag due to the flow seperating as it drag as a smaller region of flow eventually seperates. The drag Reynolds numbers you sometimes get what is called a "seperation bubble". While still laminar, the flow seperates, but then it trailing edge if properly designed. These seperation bubbles are the point where the flow would have seperated, to prevent the formation of the seperation bubble. If well designed and placed, you towards the rear but without the seperation bubble. The overall drag ============================================ Remember that separate has a rat in it. |
#22
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Dimples On Model Aircraft Could Greatly Extend Range
On Nov 5, 3:16*pm, BobG wrote:
On Nov 5, 4:10*am, bbrought wrote: pressure drag due to seperated flow and skin friction drag. First, on majority of the drag is pressure drag due to the flow seperating as it drag as a smaller region of flow eventually seperates. The drag Reynolds numbers you sometimes get what is called a "seperation bubble". While still laminar, the flow seperates, but then it trailing edge if properly designed. These seperation bubbles are the point where the flow would have seperated, to prevent the formation of the seperation bubble. If well designed and placed, you towards the rear but without the seperation bubble. The overall drag ============================================ Remember that separate has a rat in it. Dammit, I always get it wrong English is my second/third language and it seems you eventually reach an age where you just stop improving... |
#23
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Dimples On Model Aircraft Could Greatly Extend Range
Have they tried dimples on radio controlled aircraft? � The size and
speed could designed around the magic Reynolds number = 100,000 where the coefficient of drag drops precipitously. Dimpling could vastly extent the range of large and slow as well as small and fast radio controlled aircraft. A competitive cyclist is the right size and speed for Nre = 100,000 so dimple suits can work. �Same for golf balls. Nre = 100,000 for widebodies going 0.5 knots so dimples won't work except on the runway. From fluid mechanics the Reynolds number is the ratio of inertial forces/viscous forces. N re = Diameter X velocity X density of fluid/viscosity of fluid. Bret Cahill You have a fundamental misunderstanding of aerodynamics. There are several mechanics that produce drag, and the two involved here are pressure drag due to seperated flow and skin friction drag. First, on a bluff body, such as a golf ball (and a cyclist for that matter), the majority of the drag is pressure drag due to the flow seperating as it cannot negotiate the steep adverse pressure gradient towards the rear of the object. Pressure drag is much higher - sometimes one or more orders of magnitude - than skin friction drag. Skin friction drag comes from the shear inside the boundary layer, where the airspeed drops from approximately the free-stream velocity outside the boundary layer to zero where it actually touches the surface. This comes in two forms - laminar and turbulent. The skin friction drag due to a laminar boundary layer is once again much lower than that due to a turbulent boundary layer. The reason dimples work on a golf ball is due to the fact that a turbulent boundary layer, although having more drag than a laminar boundary layer, tends to stay attached through much steeper adverse pressure gradients than laminar boundary layers. The dimples force the flow to transition from laminar to turbulent, which means it stays attached for longer and you therefore end up reducing the pressure drag as a smaller region of flow eventually seperates. The drag savings therefore is because there is less seperated flow, not because a dimpled surface causes less skin friction than a smooth one. Many bluff bodies can benefit from this. When it comes to streamlined bodies, such as an airplane wing, the situation is very different. When an airfoil is well designed (I'll get back to low Reynolds number airfoils on which I have done quite a bit of work over the years) the flow is almost completely attached at the typical local angle of attack that the wing sees at speeds between loiter and maximum speed, which is of course where the low drag matters. Since there is virtually no seperated flow (there is usually a tiny bit right at the trailing edge), there is no extra benefit to be had from dimpling. In fact, if you dimple the whole wing you are going to transition to a turbulent boundary layer early and you are actually goint to increase the total drag. Low Reynolds number airfoils are slightly different. The Reynolds numbers of interest for small - not micro - UAVs is typically between about 40,000 on tail surfaces to about 500,000 on the wing. At these Reynolds numbers you sometimes get what is called a "seperation bubble". While still laminar, the flow seperates, but then it transitions to turbulent off the surface and then re-attach as a turbulent boundary layer that remains attached all the way to the trailing edge if properly designed. These seperation bubbles are sometimes unavoidable, but good airfoil design can minimize their size and therefore their drag. In some instances, a small strip can be used to force the boundary layer to transition to turbulent just ahead of the point where the flow would have seperated, to prevent the formation of the seperation bubble. If well designed and placed, you end up with a nice low drag laminar boundary layer over the forward part of the airfoil, and then a higher drag turbulent boundary layer towards the rear but without the seperation bubble. The overall drag is usually only reduced over a small part of the flight envelope and only if designed and placed properly - and it only really works at Reynolds numbers below about 200,000. Again dimples would be too crude to lead to an overall improvement, as you will once again end up with a fully turbulent boundary layer while you could have benefitted from keeping some of the flow laminar. Finally, your equation: N re = Diameter X velocity X density of fluid/viscosity of fluid. That 100,000 you used is for Reynolds number based on diameter as in your equation above, which is indeed valid for a sphere or cylinder. However, a wing's Reynolds number is based on the local chord (it changes along the span if the wing is tapered): Re = chord X velocity X density of fluid / viscosity of fluid. Because we are talking about a completely different situation on a streamlined body such as a wing, that magic Reynolds number of 100,000 that you quoted for a sphere is simply not relevant. I caught that in a post yesterday but I'm glad someone gave a more detailed treatment. Usually I have to correct my errors myself. Still there may be some situation where an airfoil might conflict with a structure, either because of cost or other considerations. Bret Cahill |
#24
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Dimples On Model Aircraft Could Greatly Extend Range
On Wed, 05 Nov 2008 00:10:29 -0800, bbrought wrote:
On Nov 4, 9:04Â*pm, Bret Cahill wrote: Have they tried dimples on radio controlled aircraft? Â* The size and speed could designed around the magic Reynolds number = 100,000 where the coefficient of drag drops precipitously. Dimpling could vastly extent the range of large and slow as well as small and fast radio controlled aircraft. A competitive cyclist is the right size and speed for Nre = 100,000 so dimple suits can work. Â*Same for golf balls. Nre = 100,000 for widebodies going 0.5 knots so dimples won't work except on the runway. From fluid mechanics the Reynolds number is the ratio of inertial forces/viscous forces. N re = Diameter X velocity X density of fluid/viscosity of fluid. Bret Cahill You have a fundamental misunderstanding of aerodynamics. There are several mechanics that produce drag, and the two involved here are pressure drag due to seperated flow and skin friction drag. First, on a bluff body, such as a golf ball (and a cyclist for that matter), the majority of the drag is pressure drag due to the flow seperating as it cannot negotiate the steep adverse pressure gradient towards the rear of the object. Pressure drag is much higher - sometimes one or more orders of magnitude - than skin friction drag. Skin friction drag comes from the shear inside the boundary layer, where the airspeed drops from approximately the free-stream velocity outside the boundary layer to zero where it actually touches the surface. This comes in two forms - laminar and turbulent. The skin friction drag due to a laminar boundary layer is once again much lower than that due to a turbulent boundary layer. The reason dimples work on a golf ball is due to the fact that a turbulent boundary layer, although having more drag than a laminar boundary layer, tends to stay attached through much steeper adverse pressure gradients than laminar boundary layers. The dimples force the flow to transition from laminar to turbulent, which means it stays attached for longer and you therefore end up reducing the pressure drag as a smaller region of flow eventually seperates. The drag savings therefore is because there is less seperated flow, not because a dimpled surface causes less skin friction than a smooth one. Many bluff bodies can benefit from this. When it comes to streamlined bodies, such as an airplane wing, the situation is very different. When an airfoil is well designed (I'll get back to low Reynolds number airfoils on which I have done quite a bit of work over the years) the flow is almost completely attached at the typical local angle of attack that the wing sees at speeds between loiter and maximum speed, which is of course where the low drag matters. Since there is virtually no seperated flow (there is usually a tiny bit right at the trailing edge), there is no extra benefit to be had from dimpling. In fact, if you dimple the whole wing you are going to transition to a turbulent boundary layer early and you are actually goint to increase the total drag. Low Reynolds number airfoils are slightly different. The Reynolds numbers of interest for small - not micro - UAVs is typically between about 40,000 on tail surfaces to about 500,000 on the wing. At these Reynolds numbers you sometimes get what is called a "seperation bubble". While still laminar, the flow seperates, but then it transitions to turbulent off the surface and then re-attach as a turbulent boundary layer that remains attached all the way to the trailing edge if properly designed. These seperation bubbles are sometimes unavoidable, but good airfoil design can minimize their size and therefore their drag. In some instances, a small strip can be used to force the boundary layer to transition to turbulent just ahead of the point where the flow would have seperated, to prevent the formation of the seperation bubble. If well designed and placed, you end up with a nice low drag laminar boundary layer over the forward part of the airfoil, and then a higher drag turbulent boundary layer towards the rear but without the seperation bubble. The overall drag is usually only reduced over a small part of the flight envelope and only if designed and placed properly - and it only really works at Reynolds numbers below about 200,000. Again dimples would be too crude to lead to an overall improvement, as you will once again end up with a fully turbulent boundary layer while you could have benefitted from keeping some of the flow laminar. Finally, your equation: N re = Diameter X velocity X density of fluid/viscosity of fluid. That 100,000 you used is for Reynolds number based on diameter as in your equation above, which is indeed valid for a sphere or cylinder. However, a wing's Reynolds number is based on the local chord (it changes along the span if the wing is tapered): Re = chord X velocity X density of fluid / viscosity of fluid. Because we are talking about a completely different situation on a streamlined body such as a wing, that magic Reynolds number of 100,000 that you quoted for a sphere is simply not relevant. Now THATS what Usenet should be like! Thanks for an interesting, informative post. |
#25
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Dimples On Model Aircraft Could Greatly Extend Range
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#26
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Dimples On Model Aircraft Could Greatly Extend Range
Hi Bret
On Nov 4, 11:04 am, Bret Cahill wrote: Have they tried dimples on radio controlled aircraft? The size and speed could designed around the magic Reynolds number = 100,000 where the coefficient of drag drops precipitously. Dimpling could vastly extent the range of large and slow as well as small and fast radio controlled aircraft. A competitive cyclist is the right size and speed for Nre = 100,000 so dimple suits can work. Same for golf balls. Nre = 100,000 for widebodies going 0.5 knots so dimples won't work except on the runway. From fluid mechanics the Reynolds number is the ratio of inertial forces/viscous forces. N re = Diameter X velocity X density of fluid/viscosity of fluid. Bret Cahill I design/build model gliders as a hobby, (not an expert :-). The dimples on golf balls are primarily to operate aero- dynamically when spinning, like baseball seams, http://en.wikipedia.org/wiki/Baseball_(ball) As Mr Brought posted so well, they react with turbulent flow. I think the Reynolds number is actually a quantum effect because air is particles, so flying insect wing design is quite different from an average birds, due to scaling. I should mention some mysteries, such as the roughness of shark skin and the unusual nature of feathers that are rough at a smaller size, that have quite different Reynolds, that seem to contribute to improved gliding performance, thus supporting your suggestion. Interesting subject. Ken |
#27
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Dimples On Model Aircraft Could Greatly Extend Range
On Nov 5, 1:08 pm, wrote:
On Wed, 5 Nov 2008 12:52:12 -0800 (PST), in sci.engr.mech "Ken S. Tucker" wrote: I should mention some mysteries, such as the roughness of shark skin and the unusual nature of feathers that are rough at a smaller size, that have quite different Reynolds, that seem to contribute to improved gliding performance, thus supporting your suggestion. These are not mysteries and have been studied and initially implemented as riblets. My understanding is the theory is NOT _well_ understood, but is evolving, along with applications, by experimental feed-back, aka trial & error, (I'm using SM board). The turbulent air very near the surface, where laminar air flow interfaces with the surface, uses "riblets" to convert the turbulence into the equivalent of a microscopic "ball bearings" type phenomena. Apparently it works, so who knows maybe cars of the future will feel rough to the touch, and need a special wax for a reduction of 10% on Cd. Ken |
#28
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Dimples On Model Aircraft Could Greatly Extend Range
On Nov 5, 2:40 pm, wrote:
On Wed, 5 Nov 2008 13:59:27 -0800 (PST), in sci.engr.mech "Ken S. Tucker" wrote: On Nov 5, 1:08 pm, wrote: On Wed, 5 Nov 2008 12:52:12 -0800 (PST), in sci.engr.mech "Ken S. Tucker" wrote: I should mention some mysteries, such as the roughness of shark skin and the unusual nature of feathers that are rough at a smaller size, that have quite different Reynolds, that seem to contribute to improved gliding performance, thus supporting your suggestion. These are not mysteries and have been studied and initially implemented as riblets. My understanding is the theory is NOT _well_ understood, but is evolving, along with applications, by experimental feed-back, aka trial & error, (I'm using SM board). Forgive me, I have no idea what SM board is. styrofoam sm board, the blue stuff, it has a very interesting texture. The turbulent air very near the surface, where laminar air flow interfaces with the surface, uses "riblets" to convert the turbulence into the equivalent of a microscopic "ball bearings" type phenomena. Apparently it works, so who knows maybe cars of the future will feel rough to the touch, and need a special wax for a reduction of 10% on Cd. Not quite. As far as riblets go, it is my understanding that the height and spacing of the riblets is specified according to the boundary thickness such as to prevent the growth of turbulent bursts which causes an exchange of low momentum fluid near the surface with higher momentum fluid from above. This momentum exchange being a loss/drag mechanism. The other point of importance is the orientation of the riblets along streamlines. Yes, that's seems clear...but NOT simple :-). The fundamental studies were done at NASA Langley Research Center in the late 70s/early 80s. The first open use of riblets was on the boat Dennis Connor (sp?) used to win back the America's Cup, being applied via a special 3M tape. The wind tunnel test articles from that study reside in the basement of the building next door to the one the branch I work resides. I my be wrong but It's possible the same the facility used in the study was also used for the work leading to the Speedo LZR Racer suit. Nifty, Ken |
#29
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Dimples On Model Aircraft Could Greatly Extend Range
On Nov 6, 1:44 pm, wrote:
On Thu, 6 Nov 2008 01:54:20 -0800 (PST), in sci.engr.mech "Ken S. Tucker" wrote: On Nov 5, 2:40 pm, wrote: On Wed, 5 Nov 2008 13:59:27 -0800 (PST), in sci.engr.mech "Ken S. Tucker" Not quite. As far as riblets go, it is my understanding that the height and spacing of the riblets is specified according to the boundary thickness such as to prevent the growth of turbulent bursts which causes an exchange of low momentum fluid near the surface with higher momentum fluid from above. This momentum exchange being a loss/drag mechanism. The other point of importance is the orientation of the riblets along streamlines. Yes, that's seems clear...but NOT simple :-). That's a lot different that your original point that it was not well understood. Well I think we're nit-picking sematics, my quote, "My understanding is the theory is NOT _well_ understood, but is evolving, along with applications, by experimental feed-back, aka trial & error, (I'm using SM board). " Note the word "theory" Also after the original riblet research was performed similarities to shark scales/skin were observed. http://ntrs.larc.nasa.gov/search.jsp...de%20matchall&... Yes! Thanks for those links. Those papers are experimental results and testing, AFAIK, there is NO generally accepted theory of the "riblets effect", though it appears to be evolving. (If you have a ref to a General Theory of Riblets, I'd would appreciate a link). I'm guessing: At a molecular level the riblets control the turbulent interfacing between fluid and surface and inhibit the integrated formation of macroscopic turbulence, such as Eddy's. That micro control is certainly a quantum relation between molecules in the fluid and the interacting solid surface, whereby the micro turbulences are quantized. Setting aside sharks skin, we may want to have a look at penguin swimming, that also has very low resistance. Regards Ken |
#30
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Dimples On Model Aircraft Could Greatly Extend Range
On Nov 6, 9:40*am, wrote:
On Wed, 5 Nov 2008 13:59:27 -0800 (PST), in sci.engr.mech "Ken S. Tucker" wrote: On Nov 5, 1:08 pm, wrote: On Wed, 5 Nov 2008 12:52:12 -0800 (PST), in sci.engr.mech "Ken S. Tucker" wrote: The fundamental studies were done at NASA Langley Research Center *in the late 70s/early 80s. The first open use of riblets was on *the boat Dennis Connor (sp?) used to win back the America's Cup, being applied via a special 3M tape. The wind tunnel test articles from that study reside in the basement of the building next door to the one the branch I work resides. I my be wrong but It's possible the same the facility used in the study was also used for the work leading to the Speedo LZR Racer suit. German researchers concurrently came up with the same conclusions from studying shark skin, they've continued with considerable efforts in 'printing' them into UV cured paints and other techniques. In any case the riblet effect has been known for over 50 years. http://researchnews.osu.edu/archive/riblets.htm |
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