Go Back   EcoModder Forum > EcoModding > Aerodynamics
Register Now
 Register Now
 

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 01-25-2020, 06:07 PM   #1 (permalink)
EcoModding Lurker
 
Join Date: May 2019
Location: Belgium
Posts: 5
Thanks: 1
Thanked 0 Times in 0 Posts
Cd constant... or speed dependant?

Hi guys,

I didn't do a full theoretical investigation on this subject, so please forgive me for this lazy topic / question.

I've read in at least one thread here, that Cd is said to be quasi-constant at legal road speeds.
However, this doesn't align with my gut-feeling. If I'd compare a relatively short teardrop shape with a very long one, I imagine they can perform equally well at low speeds.
Maybe at 30 mph or so, even the short teardrop could get fully attached, laminar flow. But at 60 mph, I guess you'd need a longer teardrop to achieve a fully attached flow...?

  Reply With Quote
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 01-25-2020, 09:50 PM   #2 (permalink)
Moderator
 
Vman455's Avatar
 
Join Date: Feb 2012
Location: Champaign, IL
Posts: 1,838

Pope Pious the Prius - '13 Toyota Prius Two
Team Toyota
SUV
90 day: 55.56 mpg (US)

Tycho the Truck - '91 Toyota Pickup DLX 4WD
Thanks: 196
Thanked 1,645 Times in 864 Posts
At 30 mph, no full-size car is operating in laminar flow; the Reynolds number at any point on the body except the very front is too high for that.

When flow trips from laminar to turbulent, drag suddenly drops and then continues to rise on a velocity-dependent curve.

In this turbulent-flow regime, Cd is proportional to the inverse square of velocity--but this variation is so small that it can be considered constant for road-vehicle speeds.
__________________

  Reply With Quote
The Following User Says Thank You to Vman455 For This Useful Post:
Xist (01-26-2020)
Old 01-26-2020, 04:00 PM   #3 (permalink)
EcoModding Lurker
 
Join Date: May 2019
Location: Belgium
Posts: 5
Thanks: 1
Thanked 0 Times in 0 Posts
Hi Vman, my 30 mph and 60 mph were just some random figures, just for some scale in my comparison.
I was comparing ideal teardrops / raindrops. Those do get laminar flow, don't they?

As far as I can project my comparison to full-size cars: I would imagine that two different shapes could have similar flow attachment at a low speed (thus similar Cd), but vastly different attachment at high speeds...?
  Reply With Quote
Old 01-26-2020, 09:11 PM   #4 (permalink)
Moderator
 
Vman455's Avatar
 
Join Date: Feb 2012
Location: Champaign, IL
Posts: 1,838

Pope Pious the Prius - '13 Toyota Prius Two
Team Toyota
SUV
90 day: 55.56 mpg (US)

Tycho the Truck - '91 Toyota Pickup DLX 4WD
Thanks: 196
Thanked 1,645 Times in 864 Posts
Quote:
Originally Posted by stefaantje View Post
Hi Vman, my 30 mph and 60 mph were just some random figures, just for some scale in my comparison.
I was comparing ideal teardrops / raindrops. Those do get laminar flow, don't they?
It all depends on Reynolds number (Re), which is the ratio of inertial force to viscous force of the fluid. At sub-critical Re, flow is laminar; above sub-critical Re, flow is turbulent. Shape doesn't come into that; it's purely a function of fluid viscosity, fluid velocity, and test length.

You're probably thinking of attached versus unattached flow, a common mistake. Yes, an ideal teardrop should have attached flow at the speeds we talk about here.
__________________

  Reply With Quote
Old 01-26-2020, 09:54 PM   #5 (permalink)
Master EcoModder
 
freebeard's Avatar
 
Join Date: Aug 2012
Location: northwest of normal
Posts: 19,625
Thanks: 5,502
Thanked 6,464 Times in 5,196 Posts
__________________
.
Conclussion: a realization that smacks you upside the head
_________________

乁(⌐■‿■)ㄏ https://www.fontspace.com/lenny-face
_________________
  Reply With Quote
The Following 2 Users Say Thank You to freebeard For This Useful Post:
stefaantje (01-27-2020), Xist (01-26-2020)
Old 01-27-2020, 05:46 AM   #6 (permalink)
EcoModding Lurker
 
Join Date: May 2019
Location: Belgium
Posts: 5
Thanks: 1
Thanked 0 Times in 0 Posts
that's a nice graph, it's rather tangible for me.

It doesn't compare the influence of speed however.

I guess skin friction in passenger cars is a few factors smaller than the drag caused by the wake?
  Reply With Quote
Old 01-27-2020, 10:41 AM   #7 (permalink)
Moderator
 
Vman455's Avatar
 
Join Date: Feb 2012
Location: Champaign, IL
Posts: 1,838

Pope Pious the Prius - '13 Toyota Prius Two
Team Toyota
SUV
90 day: 55.56 mpg (US)

Tycho the Truck - '91 Toyota Pickup DLX 4WD
Thanks: 196
Thanked 1,645 Times in 864 Posts
Quote:
Originally Posted by stefaantje View Post
I've read in at least one thread here, that Cd is said to be quasi-constant at legal road speeds.
However, this doesn't align with my gut-feeling. If I'd compare a relatively short teardrop shape with a very long one, I imagine they can perform equally well at low speeds.
Going back to your original question--

I think you're also confusing Cd with drag force, when these are not the same; Cd is a factor in drag force and so is the square of velocity.

A short teardrop and long teardrop will probably see similar drag force at a low speed, but not because Cd has changed or because the flow characteristics have changed. Assuming your "low speed" metric has Re large enough that the flow field is turbulent, there will be little difference between that and "high speed." But because of the v^2 factor, the velocity makes more difference than Cd anyway.

For example: Consider a short teardrop of cross-sectional area 1.0 m^2 and Cd 0.12, and another of the same area but Cd 0.06. At 60 mph (26.8 m/s) the teardrops will see 52.8 N and 26.3 N respectively. At 30 mph (13.4 m/s), the teardrops will see 13.2 N and 6.6 N respectively, a much smaller difference--but it isn't because their Cd has changed, it's because the velocity has.

Is that what you're asking?
__________________

  Reply With Quote
Old 01-27-2020, 11:51 AM   #8 (permalink)
Master EcoModder
 
freebeard's Avatar
 
Join Date: Aug 2012
Location: northwest of normal
Posts: 19,625
Thanks: 5,502
Thanked 6,464 Times in 5,196 Posts
Paraphrasing aerohead here: Reynolds number determines the low speed end of aerodynamic drag, depending on barometric pressure and temperature, at ~25mph for vehicle sized objects. Scale models need to be in water instead of air to compensate for scale.

At the high end, transonic shockwaves cap the range of speeds when the formulae apply.
__________________
.
Conclussion: a realization that smacks you upside the head
_________________

乁(⌐■‿■)ㄏ https://www.fontspace.com/lenny-face
_________________
  Reply With Quote
Old 01-27-2020, 12:47 PM   #9 (permalink)
Master EcoModder
 
NeilBlanchard's Avatar
 
Join Date: May 2008
Location: Maynard, MA Eaarth
Posts: 7,882

Mica Blue - '05 Scion xA RS 2.0
Team Toyota
90 day: 42.48 mpg (US)

Forest - '15 Nissan Leaf S
Team Nissan
90 day: 156.46 mpg (US)

Number 7 - '15 VW e-Golf SEL
TEAM VW AUDI Group
90 day: 155.81 mpg (US)
Thanks: 3,403
Thanked 2,901 Times in 1,819 Posts
Cd is constant, up to ~250MPH, all else being equal, as I understand it.
__________________
Sincerely, Neil

http://neilblanchard.blogspot.com/
  Reply With Quote
Old 01-27-2020, 01:05 PM   #10 (permalink)
Master EcoModder
 
Join Date: Feb 2014
Location: Missoula, MT
Posts: 2,334
Thanks: 228
Thanked 1,006 Times in 690 Posts
The problem as I understand comes to play when the size of the object is scaled down to fit in a wind tunnel. I still can't wrap my head around it but apparently you have to increase the wind speed on a scaled model to get an accurate Cd measurement. To me it would seem if you had say a 1/10th scale model, you would run it at 1/10h the speed or 6 mph vs 60 mph but it's not the case. If speeds had so little effect on Cd then it wouldn't matter and models would be just as good as the real thing without messing with speeds.

__________________

  Reply With Quote
Reply  Post New Thread


Thread Tools




Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2021, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com