Prius Coefficient of Drag

SureValla · 07-31-2007, 06:31 PM

So the prius Cd is given as .26.

Does anyone know is this the static coefficient of drag?

Also does anyone know if this number is determined experimentally or if they calculate a frontal area?

PriusTouring07 · 07-31-2007, 07:24 PM

As far as I know its tested in a wind tunnel.

David Beale · 07-31-2007, 09:39 PM

You're confusing frontal area with Cd. Measure the frontal area by measuring the vehicle height and width and multiplying. Multiply that figure by the Cd. This is the rectangular plate (bluff shape) that will produce the same drag as the car. Don't remove the space under the car. The air there is considered moving with the car, so that space is considered part of the car.

You must take the maximum height and maximum width. Don't worry that there are curves, the Cd figure takes that into consideration.

I don't know of any computer program that can accurately calculate Cd, I believe it still has to be measured in a wind tunnel.

Cd is, in effect, the aerodynamic design reducing the effect of the frontal area. In the Prius, it is even helped by the "rear diffuser" on the rear underside (black plastic part curled down on each side).

SureValla · 07-31-2007, 09:59 PM

<div class='quotetop'>QUOTE(David Beale @ Jul 31 2007, 09:39 PM) [snapback]488425[/snapback]</div>

Quote:

Measure the frontal area by measuring the vehicle height and width and multiplying. Multiply that figure by the Cd. This is the rectangular plate (bluff shape) that will produce the same drag as the car. Don't remove the space under the car. The air there is considered moving with the car, so that space is considered part of the car.

[/b]

in a rude approximation this is how it is done however complex CFD (computational fluid dynamics) codes can take every change in geometry into account not just the box the front fits inside.

I bet they just use wind tunnels now to provide an experimental look at the computational methods. (So to answer your question computers can do the work.)

These must apply a known force and the Cd is then backed out.

brick · 07-31-2007, 10:34 PM

Modern CFD codes are incredibly powerful but you still need to go full-scale in a wind tunnel to get real numbers. CFD is very good at giving the engineers an idea of what is better than something else but not necessarily profficient at telling you exactly what the numbers will be. Close, but not as good as a well-controlled experimental data.

The drag coefficient CD is backed out using the measured force, fluid velocity, fluid density, and the reference area. Here's a page worth looking at:
http://www.grc.nasa.gov/WWW/K-12/airplane/drageq.html

SureValla · 08-01-2007, 08:07 AM

just for fun: anyone know the prius's Cl (coefficient of lift)?

sharkmeister · 08-01-2007, 09:16 AM

I remember seeing somewhere that it was 0.26 and was 2nd lowest, with something like the Honda Insight (no longer in production) being the lowest. It's out there on the web somewhere.

Reminds me that I'm curious as to the drag coefficient of the Buggatti Veyron. Not that I'm thinking of getting one.

PS Excellent wikipeadia article, has a table with drag coefficients of vehicles:

http://en.wikipedia.org/wiki/Drag_coefficient

sharkmeister · 08-01-2007, 09:18 AM

PS
Good article with a table of vehicle drag coefficients:

http://en.wikipedia.org/wiki/Drag_coefficient

qbee42 · 08-01-2007, 03:00 PM

<div class='quotetop'>QUOTE(sharkmeister @ Aug 1 2007, 09:16 AM) [snapback]488571[/snapback]</div>

Quote:

I remember seeing somewhere that it was 0.26 and was 2nd lowest, with something like the Honda Insight (no longer in production) being the lowest. It's out there on the web somewhere.

Reminds me that I'm curious as to the drag coefficient of the Buggatti Veyron. Not that I'm thinking of getting one.

PS Excellent wikipeadia article, has a table with drag coefficients of vehicles:

http://en.wikipedia.org/wiki/Drag_coefficient
[/b]

He said Cl (coefficient of lift), not Cd.

Tom

alexstarfire · 08-01-2007, 04:17 PM

I was reading in Wired yesterday that there is a car that is designed from a boxfish, no joke, and has a Cd of like .19. It looks like a freaking Element so I'm not sure how that works. I believe it's called the Wow. I could be wrong since they were talking about 6-7 cars that have yet to come out.

07-31-2007, 09:39 PM	#3
David Beale Senior Member Join Date: Jul 2006 Location: Edmonton Alberta Posts: 2,116 My Car: 2007 Prius Model: Package: B Thanks: 3 Thanked 77 Times in 58 Posts Friends: 0	You're confusing frontal area with Cd. Measure the frontal area by measuring the vehicle height and width and multiplying. Multiply that figure by the Cd. This is the rectangular plate (bluff shape) that will produce the same drag as the car. Don't remove the space under the car. The air there is considered moving with the car, so that space is considered part of the car. You must take the maximum height and maximum width. Don't worry that there are curves, the Cd figure takes that into consideration. I don't know of any computer program that can accurately calculate Cd, I believe it still has to be measured in a wind tunnel. Cd is, in effect, the aerodynamic design reducing the effect of the frontal area. In the Prius, it is even helped by the "rear diffuser" on the rear underside (black plastic part curled down on each side). __________________ Edmonton Alberta "Pearl" is a 2007 Driftwood Pearl Prius.

07-31-2007, 10:34 PM	#5
brick Senior Member Join Date: Dec 2006 Location: Columbia, SC Posts: 895 My Car: 2007 Prius Model: Package: #2 Thanks: 10 Thanked 21 Times in 15 Posts Friends: 0	Modern CFD codes are incredibly powerful but you still need to go full-scale in a wind tunnel to get real numbers. CFD is very good at giving the engineers an idea of what is better than something else but not necessarily profficient at telling you exactly what the numbers will be. Close, but not as good as a well-controlled experimental data. The drag coefficient CD is backed out using the measured force, fluid velocity, fluid density, and the reference area. Here's a page worth looking at: http://www.grc.nasa.gov/WWW/K-12/airplane/drageq.html __________________ Improvements: Analog Tach, ScanGauge II, RadioShack EV Button, Turn Signal DRLs, dogfriend Seat Mod, some bits of pipe insulation stuck in my grill, and whatever I'm forgetting. 53.4mpg @ 54,000mi

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07-31-2007, 06:31 PM	#1
SureValla Senior Member Join Date: Jul 2007 Location: White Plains, NY Posts: 505 My Car: 2007 Prius Model: Package: #3 Thanks: 17 Thanked 9 Times in 7 Posts Friends: 0	So the prius Cd is given as .26. Does anyone know is this the static coefficient of drag? Also does anyone know if this number is determined experimentally or if they calculate a frontal area?

07-31-2007, 07:24 PM	#2
PriusTouring07 Join Date: Jul 2007 Posts: 177 My Car: Model: Package: Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	As far as I know its tested in a wind tunnel.

08-01-2007, 08:07 AM	#6
SureValla Senior Member Join Date: Jul 2007 Location: White Plains, NY Posts: 505 My Car: 2007 Prius Model: Package: #3 Thanks: 17 Thanked 9 Times in 7 Posts Friends: 0	just for fun: anyone know the prius's Cl (coefficient of lift)?

08-01-2007, 09:16 AM	#7
sharkmeister Join Date: Apr 2006 Posts: 56 My Car: Model: Package: Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	I remember seeing somewhere that it was 0.26 and was 2nd lowest, with something like the Honda Insight (no longer in production) being the lowest. It's out there on the web somewhere. Reminds me that I'm curious as to the drag coefficient of the Buggatti Veyron. Not that I'm thinking of getting one. PS Excellent wikipeadia article, has a table with drag coefficients of vehicles: http://en.wikipedia.org/wiki/Drag_coefficient

08-01-2007, 09:18 AM	#8
sharkmeister Join Date: Apr 2006 Posts: 56 My Car: Model: Package: Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	PS Good article with a table of vehicle drag coefficients: http://en.wikipedia.org/wiki/Drag_coefficient

08-01-2007, 04:17 PM	#10
alexstarfire Join Date: Feb 2007 Posts: 218 My Car: Model: Package: Thanks: 0 Thanked 0 Times in 0 Posts Friends: 0	I was reading in Wired yesterday that there is a car that is designed from a boxfish, no joke, and has a Cd of like .19. It looks like a freaking Element so I'm not sure how that works. I believe it's called the Wow. I could be wrong since they were talking about 6-7 cars that have yet to come out.