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RichFox
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« Reply #735 on: October 24, 2011, 11:07:13 PM » |
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I am told the greatest drag on an open tire is at the top, as it is showing the air twice the speed of the car. That seems to indicate that a close fitting motorcycle type fender would give substantial gain without substantial work or increased frontal area. I don't know if street roadsters can run front fenders. They don't. So maybe this is stupid.
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38flattie
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« Reply #736 on: October 25, 2011, 06:01:50 PM » |
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Thanks guys- pretty sure we'll have some sort of fenders and axle cover or air dam.
While I've been pondering aero, Dale painted the block!
Yea, I know, but it's the little things....
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WOODY@DDLLC
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« Reply #737 on: October 26, 2011, 03:03:45 PM » |
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Buddy your build is top-shelf man! Loved seeing it at Speed Week and meeting you as well!! Attached are some wheel shots for everyone's continued mental anguish.  Air speed is 300 mph [L=>R], the wheels are rotating and the ground is moving, too. The plot is a 2D slice of the left front tire just about 1.38" off the center towards the outside. I probed areas around the tire so there is some 3D info at these 2D spots. Air velocity -Z is L=>R, +Y is up and +X is out of the picture. (-X going around the tire back towards center.) The 3D plot shows the air being pulled around the tires near the surface - sometimes in the opposite direction, hmmmm? Relative pressure zero is atmospheric so any low pressure on top is lift and on the bottom is down force. Conversely any high pressure on the bottom is lift and on the top is down force. The graph is the pressure on the tire surface so the numbers don't match the probes exactly. Enjoy!  |
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38flattie
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« Reply #738 on: October 27, 2011, 07:27:42 AM » |
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Woody, the charts are a neat visual, but I still don't know what I'm looking at! Looks like there is an easy 3X the amount of lift that there is down force, but I don't understand how much 'drag' is there. I may be the only one on the board that doesn't understand this info, but could you please explain it a little?  I've got some block/water info headed your way today, also. |
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Tman
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« Reply #739 on: October 27, 2011, 09:56:01 AM » |
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Buddy, looks like a tye dye to me as well! |
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38flattie
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« Reply #740 on: October 27, 2011, 06:43:21 PM » |
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I dropped the car off at Pinkees today, to get the side panels made.
Here is the 'finish' room. I'll get pics of the fab room next time- it's a real cool place!
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Glen
Global Moderator
Hero Member
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Age: 78
Location: South West Utah
Posts: 5942
SCTA/BNI timer 1983 to 2004,Semi Retired,.
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« Reply #741 on: October 27, 2011, 08:00:22 PM » |
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Nice clean shop.
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Glen  South West, Utah |
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4-barrel Mike
Hero Member
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Age: 65
Location: Portland, OR
Posts: 1885
Any fool can drive a V8
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« Reply #742 on: October 27, 2011, 09:00:00 PM » |
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This isn't a fly-by-night shop: http://pinkeesrodshop.com/  Mike |
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Mike Kelly - PROUD owner of the V4F that powered the #1931 VGC to a 82.803 mph record in 2008!
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Rex Schimmer
Hero Member
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Age: 70
Location: Fulton, CA
Posts: 1479
Only time and money prevent completion!
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« Reply #743 on: October 28, 2011, 12:01:17 AM » |
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Woody,
It took a long time but I think I finally figured out why your CFD does not look like I thought it should. You do not have the tire in contact with the ground, the blue area below the tire, which is the lowest pressure is also the highest velocity, which makes sense if the tire is not in contact with the ground. The air is flowing at 300 mph and this velocity is added to the air that is drawn down by the tire surface and this gives the 430 mph velocity under the tire. As usual Bernoulli is right, high velocity = low pressure. If the tire were in contact with the ground then the air flow would be blocked, i.e. low velocity, and the area right in front of the tire would be what I would expect, high pressure. I think you need to take another try at it with the tire in contact with the ground, which is the usual case with most Bonneville cars. (if we are lucky!)
Rex
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Rex
Not much matters and the rest doesn't matter at all.
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WOODY@DDLLC
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« Reply #744 on: October 28, 2011, 07:57:16 AM » |
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Buddy, I will elaborate some later. This was just a teaser! Rex, you and Sr Bernoulli are both right and wrong. You other lurkers chime in too or PM me and I will try to respond to all! Headed to Maxton this morning for the grand finale this weekend! |
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38flattie
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« Reply #745 on: November 02, 2011, 06:02:29 PM » |
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Buddy, I will elaborate some later. This was just a teaser! Rex, you and Sr Bernoulli are both right and wrong. Ok Woody, you're going to have to explain the above comment! On the fenders, how much of the tire do I want to cover-as much as possible? How much 'clearance' do I want from the fender to the tire? |
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« Last Edit: November 02, 2011, 09:04:41 PM by 38flattie »
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WOODY@DDLLC
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« Reply #746 on: November 03, 2011, 08:14:30 PM » |
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Rex, the tire is embedded in the surface. This is a 2D slice where the air can get around the tire but not under it. So you were wrong about the tire in the surface but right about the flow around the tire! Good eye! The 'jets' and vortices around the bottom were first identified by Fackrell and Harvey in 1973. For an up to date study see: SAE 2007-01-0107, Fig 12 & 13 (SP-2066). Regarding Sr. Bernoulli, he discovered that there is a relationship between pressure and velocity. (This is conditional but for our uses and understanding we'll let the PhD's worry about that!) The problem is the way we are taught his principle. All any fluid ever wants is to not be disturbed in any way. A bunch of Utah air molecules are banging off each other in a random, happy manner and reach this steady state condition that we call pressure. And then along comes a bunch of land speed racers to stick a lot of funny looking objects in the breeze and upset their randomness!  All the velocity changes we see are a result of the pressure differentials that result from an object moving in a fluid or a fluid moving around an object. Force, velocity and acceleration are all vectors which just means they have magnitude and (3D) direction. The force on the now not so random air molecules tells them which direction they are going to go, how fast they will go and how quickly they will get there. But it is a one-way relationship - pressure can change velocities but velocity does not change the pressure! Pressure is a force - velocity is not! However, for simple shapes like a venturi we can calculate the pressure if we know the velocity because they are joined at Sr Bernoulli's hip!  Not everyone buys this idea but it's my story and I'm sticking to it! BTW: These guys helped me confirm my suspicions: Understanding Flight, Second Edition by David Anderson and Scott Eberhardt. If you start to think about fluid flow strictly in terms of pressure differential little lights will start coming on. When I ask what do you measure on a flow bench the usual answer is CFM. Actually you measure pressure differential and you calculate CFM or velocity or density or .... Buddy, the fender question is an old one. Flow around an open tire is very complex and is being studied and restudied all the time. If the tire is open the first thing is to make it smaller, then put discs on it and lastly maybe get the tire guys to make it more aero in cross section otherwise they are just in the wind! I would put a fixed aero fender around it. The fender may have slightly more frontal area but it will be cleaner and more slippery. Remember the pressures on the body will talk to the pressures on the tire or fender, too. A partial fender behind it would give a partial improvement. In an open fender the ratio of the fender volume to tire volume is the key factor. In other words make the inner fender as close to the tire as you can. You old salt dogs got any usable comments on salt packing in the wheel wells? Here is a rotating wheels video: http://www.deexchange.com/video/cfd-for-dexMotorcycle liner aka long, round fender with two wheels. (Bonus link there to a scoop concept at 300 mph.) I will be doing some studies for Buddy and Sparky. Thank Sparky for the tires shots here! Wendy Jeffries has asked me to do some articles for Bonneville Racing News about what it is I do. Buddy and Sparky have agreed to share some of their results for the articles. I hope you will enjoy them! I feel like this was the first chapter. |
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38flattie
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« Reply #747 on: November 03, 2011, 09:06:16 PM » |
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Good info Woody! I'm going to have to 'study' this a while, and try and wrap my simple mind around it! I know this is an open ended question, but I'm looking for an answer. The first pic was posted by Rex, and I'm guessing it would be a very good aero design for a wheel cover. It may be hard for me to fabricate though. The second pic is what I had in mind, although I don't know if the benefit will match the effort. Basically, it's two Harley FLH style fenders, cut and welded together, with inner and outer covers welded in to cover the tire. Yes, I know I can't even draw in the paint program! Lol! Is my idea even worth the effort, aero wise, or do I need to do something similar to Rex's rendition, to see real benefits? Anyone got any real world experience, or theoretical data/ideas? |
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« Last Edit: November 03, 2011, 09:25:38 PM by 38flattie »
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RichFox
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« Reply #748 on: November 04, 2011, 08:28:57 AM » |
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Do you ever watch the telavision show. "Father and son fight and build very expensive motorcycles
"? They buy a sort of drum, full round and with sides on it. Then cut out what they want for a fender. I believe you can get them in several sizes. You might look into that.
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Tman
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« Reply #749 on: November 05, 2011, 10:19:10 AM » |
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