10042008, 05:14 AM

#31 (permalink)

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11212008, 11:00 PM

#32 (permalink)

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Front wheel skirts why not just put a motorcycle styled fender that covers almost the entire wheel/tire. The cover could be mounted into where the hub attaches to the spindle.
I have a quick drawing of how it would mount to the spindle:
This motorcycle fender would have to be quite light also.
I have a question, has anyone had success with a rear body diffuser? From what I have seen, it looks like it would smooth out the air at the back of the car.



12042008, 04:13 PM

#33 (permalink)

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Looking for materials? I found a good site for my project. I'm going to be using ABS plastic like this guy did with his Prius.
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Check out mcmastercarr.com Search abs plastic, go to sheet, and you can get a 12x12 square less than 10$ at 1/8in.



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12042008, 04:49 PM

#34 (permalink)

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I added the diffuser first to the back of my car and didn't notice any smoothness change. The car did get quieter because of all the road noise in the hatchback i drive. The other aero threads state that sealing up the underside of the engine (front of car) is more important than the rear. Most rear bumpers are total parachutes so they do contribute to hurting aerodynamics, but you'll see more results with the front attack. Start with cardboard or coroplast as a stepping stone/template, to map out underside mounting points of your more high dollar materials. My 3 color aspire only deserves coroplast
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12132008, 04:31 PM

#35 (permalink)

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Air Density (rho) defining and using
Hello all.As we move into the colder weather,and later as we move back into warmer times,the density of the air will be affecting our air drag. For those who'd like to contemplate the implications of changing air density,the following is offered. Air density,designated by the Greek letter rho,and used in aerodynamic drag calculations is defined as the ratio of the air's specific weight,divided by the local acceleration due to gravity (g). A slingpsychrometer can be used to measure accurate dry and wetbulb temperatures,and using a Psychrometric chart (available from air conditioning contractors) one can determine current specific weight of air. Standard air is measured around 59degrees F and has a specific weight of 0.07651 pounds per cubic foot. (g) as measured at sealevel,is 32.2 ft/second/second. running the numbers,for standard air, 0.07651/ 32.2 = 0.002376. Rho is commonly stated at 0.00238 at standard conditions. Anyone with a psychrometer,or access to local weather conditions,a chart,and a calculator can calculate current air density to determine what cold winter /or hot summer air means to there drag.
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12132008, 05:17 PM

#36 (permalink)

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Reynolds Number: origin and calculation
Hello all.There is oftentimes confusion over laminar flow,turbulent flow,boundary layers,separation,etc.,and all have to do with Reynolds Number effects,so I thought it would serve us to look at the Reynolds Number and have it available to all members and lurkers to use as a computational tool. The number was a necessary invention,as air has mass and viscosity, and a tool was necessary for prediction of the air's effects when designing everything from a vacuum cleaner to an airplane. Reynolds Number = force (inertia)/ force (viscous)  The development of the formula begins with: N = [ ( velocity)x ( length)x (specific weight)]/ [ ( viscosity )x(acceleration due to gravity) ]. specific weight/ gravitational constant = rho,so formula becomes  [(velocity)x(length)x(rho)]/ viscosity. rho/viscosity is the inverse of kinematic viscosity(v),so formula can be simplified to N= [(velocity)x(length)x(inverse of kinematic viscosity)]. For" standard air"and U.S.Standard units,v= 0.0001567 seconds/ftsq,and it's inverse is 6380. So here at the end,N= velocity x length x 6380. An example for use would be my T100 at 70mph.Multiplying 70mph by 5280ft/mile gets me into feet,then dividing by 3,600 seconds per hour gets me into feet per second(my unit of length for calculations. At 16.5 feet Length(without the boattail), and 102.667 ft/sec Velocity,multiplying by 6380 gives me a Reynolds Number of 10,807,720. When comparing various shapes and their Cds as a function of Reynolds Number,one can calculate crucial data,especially if you intend to model,so that appropriate "scaling factors" can be accounted for. That's it,hope it helps dispel some of the mystery of aero noodling.
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12132008, 09:52 PM

#37 (permalink)

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Hi,
Can you distill it down to an approximate percentage change in drag from say 75F down to 30F? Is it linear as the temperature drops, or is the change accelerated as the air temp drops?



12152008, 03:16 PM

#38 (permalink)

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Is it linear?
Quote:
Originally Posted by NeilBlanchard
Hi,
Can you distill it down to an approximate percentage change in drag from say 75F down to 30F? Is it linear as the temperature drops, or is the change accelerated as the air temp drops?

Neil,since the density is in the numerator of the force and power formula,I think the change in density affects the results arithmetically,as a percentage of change. The density of air at 30degrees F,under standard conditions is 0.002520 slugs/cubic foot.Dividing by 0.00238 gives a density 5.8% higher than at 59degrees.The drag force according to the 1/2 rho Cd A V(squared) would increase 5.8% so it looks like a clean arithmetic relationship for us.'should be same in power calculation. I hope to post a table for those who do not have fluids text,which will list some data in the range we might expect to encounter over the course of a year. P.S. A lot of other variables will also be changing with cooler temps and it is customary for us to expect at least a 3mpg loss in winter driving.
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12152008, 05:14 PM

#39 (permalink)

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Coefficient of Friction( Cf): calculation and use
From time to time,members have asked about automobile surface roughness/skinfriction related issues.For those without fluid mechanics text is the following.A scientificcalculator is handy for this,as the exponents are "unconventional",and also,the memory functions are a real help. It's necessary to first calculate the Reynolds Number for your car in order to use the formulas.This has been covered above in the aero sticky. For Reynolds Numbers greater than 10(to the 7th power) Cf =[ 0.455/( log Nr)to the 2.58 power]. For Reynolds Numbers less than 10(to the 7th power) Cf =[ 0.0775/ ( Nr ) to the 1/5 power]. EXAMPLE: My T100 at 70mph ( 102.667 feet per second) and length of 16.5 feet,has Reynolds Number:[( 16.5 ) x ( 102.667 ) x ( 6380 )]= 10,807,720. Since this value is more than 10(7), then I use Cf= 0.455/[( log 10807720 ) to the 2.58 power] or 0.455/[ ( 7.0337) 2.58 power] or, 0.455/ 153.369 = 0.002966686513. So my Cf = 0.002967(rounded).******************************** To use the friction coefficient is much like the drag force calculation,although you are concerned with the surface area of your car instead of frontal area. If you have a simple boxshaped vehicle like a van,you can use the height,width,and length to calculate the area.In my fluids class,the underside of the vehicle was ignored,as the flow there was unlike that in the freestream,being dragged along the ground by the "dirty"undercarriage. For passenger cars,the shapes vary so much that S.F.Hoerner recommends we simply multiply the frontal area by 10,to get an approximation of surfacearea. In that case,the formula becomes Drag(friction) = [Cf x 1/2 x rho x Vsquared x 10 Af ]. EXAMPLE: T100 at aprox. 29.6 feetsquare and 102.667 feet/sec,and standard air, Drag(f) = 0.002967 x 1/2 x 0.00238 x 102.667 (squared) x 296 feet squared = 11.017 pounds resistance.^^^^^^^^^^^^^^^^^^^^^^^ Plugging Cd 0.44(the original drag coefficient) into the drag force equation gives me 163.362pounds resistance total for the truck.Subtracting the friction force leaves 152.345pounds aero drag force. The drag, do to skinfriction constitutes 6.74 % of the overall drag. If skin friction could be eliminated,at a steady 55mph,mpg would gain 3.3%.However,do to the nature of typical surfaceroughness and Reynolds Numbers effects,any additional "smoothing" to the body of a car will not reduce it's skin friction at all.It's basically a "DEADEND" for ecomodding.And as I mentioned in my "DIMPLING" thread,any surface roughness added near the leadingedge of a vehicle(golfballing) will only aggravate drag and lead to lower mpg.For cars,the transition to turbulent boundarylayer occurs at around Nr=500,000 (impossible for a car to avoid at or above 20mph).Bugs stuck to the leadingedge of an aircraft wing are enough to significantly compromise the performance of some laminarflow wings.
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12152008, 06:23 PM

#40 (permalink)

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Coefficient of aero.drag(Cd):origin and use
For those without access to fluid mechanics text is the following development of the coefficient of aerodynamic drag(Cd),and it's use in the drag force and aerodynamic power calculations.This is the most important tool any aeromodder can use to predict potential performance based on theoretical changes to an automobile. The aerodynamic drag coefficient (Cd) = Drag force/ dynamic pressure(q) x area(S). Dynamic pressure(q)=(rho) x (v)squared /2 or (1/2)x( rho )x(v)squared. Therefore, Cd = D/[ 1/2 rho (vsquared) S]. Substituting A for S(which is used more in aeronautical work),formula becomes Cd =D/ [1/2 rho A (vsquared)]. The Cd is calculated from measurements in windtunnels,and also determined from coastdown tests.Windtunnel results are preferred although can vary from tunnel to tunnel.+++++++++++++++++++++++++++++++++++ For examining drag, based on theoretical changes to the shape of a vehicle, drag force calculations can serve as barometer for potential results of modifications.The frontal area is known,or estimated,and"standard air "values for density can be plugged into the formula along with any speed.By making the Cd the only variable,one can estimate drag reduction or gain as a function of the changing Cd.************************** EXAMPLE: The T100 pickup. With original Cd0.44,and Af @ approx. 29.6feetsquare,and a velocity of 70mph(102.667 ft/sec)and standard air,the drag force D=[1/2 x 0.00238 x 0.44 x 29.6 x 102.667(squared).= 163.362 pounds resistance at 70mph.^^^^^^^^^^^^^^^^^^^^^^^^ To calculate the power requirement to overcome the aero drag force,then P = DV/(force per unit of time per power unit) or [163.362 pounds x 102.667 ft/sec/ 550 poundfeet persec per horsepower= 30.494 Horsepower,necessary to overcome aero drag at 70mph.If you know the mechanical efficiency of your driveline,your power requirements for rolling resistance,and the BSFC of your engine,you can predict your mpg for different values of Cd.***************************** A relationship between mpg and Cd is firmly established which offers a "backdoor" in which to calculate Cd based on a vehicles "baseline" performance at a constant 55mph driving. If you do a reliable test of your car's mpg,at a constant 55mpg before modifications,the following relationship may be used to estimate your new Cd. A 2% drag reduction will provide a 1% improvement in mpg,at a constant 55mph. If you were to modify your car,and after testing,realize a 5% improvement at 55mpg,then,it follows that you've reduced your Cd by 10%. So ,for say the T100,if I put some sillylooking device on it,and after testing,the mpg goes from 23.3mpg,to 24.465mpg,then from the formula,the Cd has dropped from 0.44,to 0.396. That's all there is to it,and no expensive windtunnel needed! For extensive modifications,it is claimed that a gear change may be necessary to wring out all the MPG from drag reduction.That is a very complicated situation and for a discussion on that I would refer you to Hucho's for a full explanation.
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