CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Main CFD Forum

Reynolds number in wind tunne-sports car modelling

Register Blogs Members List Search Today's Posts Mark Forums Read

Like Tree1Likes
  • 1 Post By Martin Hegedus

Reply
 
LinkBack Thread Tools Display Modes
Old   March 4, 2012, 03:04
Default Reynolds number in wind tunne-sports car modelling
  #1
Senior Member
 
Ke Wu
Join Date: Jan 2012
Posts: 179
Rep Power: 5
itsme_kit is on a distinguished road
I have set different wind speed
Finally I wanna get a graph which drag coefficient against Reynolds number
However, how do I know the value of Reynolds number in this case?
Thanks
itsme_kit is offline   Reply With Quote

Old   March 5, 2012, 00:51
Default
  #2
Senior Member
 
Martin Hegedus
Join Date: Feb 2011
Posts: 464
Rep Power: 9
Martin Hegedus is on a distinguished road
Here is my two cents.

There isn't an easy answer. A race car can have a lot of different physics going on, and the Reynolds is a metric for those physics. For example, you could have a Reynolds number for the entire length of the car representing skin friction (drag buckets) and separation bubbles on the car. Or, you could have a Reynolds number for a wing which could be a representation of leading and trailing edge separation. Or you could have a Reynolds number for an exterior rear/side view mirror representing the type of separation from it. So, first, you need to identify the physics, and then the Reynolds number(s) can be determined. In other words, if you have a strong variation of Cd with velocity, you must identify where it is coming from. Unfortunately, understanding the physics can be very challenging for something with a lot of interactions, interferences, and complex geometries, such as some race cars.
shreyasr likes this.
Martin Hegedus is offline   Reply With Quote

Old   March 7, 2012, 06:42
Default
  #3
Member
 
Join Date: Mar 2011
Posts: 50
Rep Power: 6
cdf_user is on a distinguished road
You can get a rough approximation by using Re = rho*V*D/mu
where D is the height of the car
cdf_user is offline   Reply With Quote

Old   March 7, 2012, 07:24
Default
  #4
Senior Member
 
Ke Wu
Join Date: Jan 2012
Posts: 179
Rep Power: 5
itsme_kit is on a distinguished road
Quote:
Originally Posted by cdf_user View Post
You can get a rough approximation by using Re = rho*V*D/mu
where D is the height of the car

Any reason to use height of car?
itsme_kit is offline   Reply With Quote

Old   March 7, 2012, 07:27
Default
  #5
Senior Member
 
Ke Wu
Join Date: Jan 2012
Posts: 179
Rep Power: 5
itsme_kit is on a distinguished road
Quote:
Originally Posted by Martin Hegedus View Post
Here is my two cents.

There isn't an easy answer. A race car can have a lot of different physics going on, and the Reynolds is a metric for those physics. For example, you could have a Reynolds number for the entire length of the car representing skin friction (drag buckets) and separation bubbles on the car. Or, you could have a Reynolds number for a wing which could be a representation of leading and trailing edge separation. Or you could have a Reynolds number for an exterior rear/side view mirror representing the type of separation from it. So, first, you need to identify the physics, and then the Reynolds number(s) can be determined. In other words, if you have a strong variation of Cd with velocity, you must identify where it is coming from. Unfortunately, understanding the physics can be very challenging for something with a lot of interactions, interferences, and complex geometries, such as some race cars.

I want to have drag coefficient and lift coefficient against Reynolds number. So what characteristic length I should use?
Thanks
itsme_kit is offline   Reply With Quote

Old   March 8, 2012, 07:50
Default
  #6
Member
 
Shreyas Ragavan
Join Date: Feb 2012
Location: India
Posts: 35
Rep Power: 5
shreyasr is on a distinguished road
Quote:
Originally Posted by itsme_kit View Post
I want to have drag coefficient and lift coefficient against Reynolds number. So what characteristic length I should use?
Thanks
HI,
I think , since you are looking at the drag and lift coefficient values of the entire car, the Length of the car may be more appropriate.

The remaining kinds of Re numbers seem focussed on the effects of a particular Part of the car, whereas your focus is probably on the cumulative effect.

Fluid properties change with height.
The length is also, usually parallel to the velocity vector's direction and Drag is the resistance to the flow, in a simplistic way. Using the length seems to make more sense.

Besides, the frontal projected area comes into the Cd equation, and that means the main effect of the width and height are being accounted for.
shreyasr is offline   Reply With Quote

Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
Cell Reynolds number in Multiphase Zaktatir CFX 9 February 10, 2011 18:07
Reynolds number help Mech FLUENT 0 March 15, 2006 11:28
Reynolds Number Again Ogbeni CFX 2 June 2, 2005 23:34
Please help with flow around car modelling! Tudor Miron CFX 17 March 19, 2004 20:23
Wind Turbine Modelling Neil Campbell Main CFD Forum 1 November 10, 1998 20:03


All times are GMT -4. The time now is 23:05.