Author Topic: School me on trip fences...  (Read 3974 times)

0 Members and 1 Guest are viewing this topic.

Nick Flores

  • Guest
School me on trip fences...
« on: June 17, 2014, 02:27:44 PM »
Just curious to know/see what you guys are doing with trip fences. This is new territory for me and I'd like to know more about it. Pictures are always helpful! Thanks. Nick

Online SPARKY

  • Global Moderator
  • Hero Member
  • ***
  • Posts: 6707
  • Age: 77
  • Location: Phoenix
Re: School me on trip fences...
« Reply #1 on: June 17, 2014, 02:50:38 PM »
Nick,  As I understand it:  ie  I am not an engineer
1.  Laminar flow has less drag
2.   Laminar flow has a boundary layer next to the skin that moves   
      slower that the laminar flow air above it--the boundary layer gets thicker as the length from the front of the body increases to the point that SEPERATES and it breaks up into various forms of eddies and such---killing laminar flow.


 ---A trip fence is used to "trip" the boundary layer---ie it stops or prolongs the separation and hence the turbulent flow which has more drag.
Miss LIBERTY,  changing T.K.I.  to noise, dust and RUST!!!

The # 1 issue is: TO KEEP THE REPUBLIC      
           tncsg.org     mrspowell.org

ELECTION  INTEGRITY  PROJECT
eip-ca.com  EIPAz.org  eipnv.

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

Nick Flores

  • Guest
Re: School me on trip fences...
« Reply #2 on: June 17, 2014, 08:03:54 PM »
Thanks Sparky. I'm no engineer and this is really just a bench racing, so my thinking on this is most likely screwy...  Let me add a bit to this because I may or may not be on the losing end of a milkshake bet.

Lets say a guy has a 1932 Ford Coupe, and class rules dictate that streamlining ahead of and including the cowl is allowed, similar to say Competition Coupe and Sedan. Now this ole boy has him a nice flat, nearly vertical windshield and the top has not been chopped. Assume he has made an extended aerodynamic nose, new hood and sides, and removed front fenders.  Now he gets to thinking about this thingamajig called a trip fence as described on page 52 (4.CC.10) in the 2014 rule book, and thinks why can't I just put me a little "lip" (trip fence) on the trailing edge of my new hood to help direct the air up and over the windshield. Would our friend be adding a trip fence (which would be allowed by the rules) or is he pushing the limits of what could be defined as a spoiler?

Online SPARKY

  • Global Moderator
  • Hero Member
  • ***
  • Posts: 6707
  • Age: 77
  • Location: Phoenix
Re: School me on trip fences...
« Reply #3 on: June 17, 2014, 10:11:46 PM »
I would call it a deflector ---because it is in front of something you are trying to improve or help.  i.e.  you are trying to start the path of the air upward before it hits the vertical surface.
Miss LIBERTY,  changing T.K.I.  to noise, dust and RUST!!!

The # 1 issue is: TO KEEP THE REPUBLIC      
           tncsg.org     mrspowell.org

ELECTION  INTEGRITY  PROJECT
eip-ca.com  EIPAz.org  eipnv.

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

Offline Bratfink

  • Full Member
  • ***
  • Posts: 61
  • Location: Belleville, MI
Re: School me on trip fences...
« Reply #4 on: July 30, 2014, 08:12:28 AM »
Sparky, your description is correct in aeronautical terms, but in road vehicle terms it's not quite that simple. There is very little truly laminar flow on a car, Reynolds number is almost always too high.

The more relevant term here is attached flow, which has a combination of laminar and turbulent flow.
 
Attached flow can increase induced drag due to the boundary layer acting on large surface areas on the skin of the vehicle. The purpose of the trip is to define a detached flow and thus reduce this induced drag. This is one case where more turbulent flow is better, although it's not quite that straight forward.   

Offline hotrod

  • Hero Member
  • *****
  • Posts: 1231
    • Black Horse photo
Re: School me on trip fences...
« Reply #5 on: July 30, 2014, 11:58:16 AM »
A trip fence delays separation of flow by adding energy to the stagnant boundary layer which helps it stay attached to the surface.
The dimples on a golf ball does the same thing, they cause mixing of the turbulent flow to help the flow remain attached to the surface.


http://www.bakker.org/dartmouth06/engs150/11-bl.pdf

http://www.aerospaceweb.org/question/aerodynamics/q0215.shtml


Offline Bratfink

  • Full Member
  • ***
  • Posts: 61
  • Location: Belleville, MI
Re: School me on trip fences...
« Reply #6 on: July 30, 2014, 04:57:29 PM »
Hotrod is of course right.

But that isn't what I was getting at. I was trying to make the point that in a car there isn't much of a laminar boundary layer, thus transitioning from laminar to turbulent isn't going to give you anything. My suggestion is to try to detach the flow to reduce pressure drag on the surfaces. I guess "trip" is the wrong word from that, "separation feature" is probably better.

I can't think of many automotive situations where you would actively want to re-attach flow to reduce drag. And if I did I'd probably be using VG's or other separation style features (hard corners on tail lights etc).

Offline hotrod

  • Hero Member
  • *****
  • Posts: 1231
    • Black Horse photo
Re: School me on trip fences...
« Reply #7 on: July 30, 2014, 09:06:22 PM »
I can't think of many automotive situations where you would actively want to re-attach flow to reduce drag. And if I did I'd probably be using VG's or other separation style features (hard corners on tail lights etc).

Those are already being used on show room ready cars. The late model EVO's use vortex generators on the rear of the roof to help keep the air flow attached at the rear glass, and improve the effectiveness of the trunk spoiler and reduce drag for improved fuel mileage.


http://www.autospeed.com/A_3058/cms/article.html
http://www.autospeed.com/cms/A_3059/article.html

Offline Bratfink

  • Full Member
  • ***
  • Posts: 61
  • Location: Belleville, MI
Re: School me on trip fences...
« Reply #8 on: July 31, 2014, 08:41:56 AM »

Those are already being used on show room ready cars. The late model EVO's use vortex generators on the rear of the roof to help keep the air flow attached at the rear glass, and improve the effectiveness of the trunk spoiler and reduce drag for improved fuel mileage.


http://www.autospeed.com/A_3058/cms/article.html
http://www.autospeed.com/cms/A_3059/article.html

That was the case I was thinking of but it wasn't re-attaching flow, it was energizing separated flow to force flow down onto the wing. The road car application of this was a bit of a gimmick to be honest, the real purpose was to increase rear downforce on the WRC car, drag would have been a secondary concern. It was a very specific solution to a very specific problem with the design of the Lancer and the WRC rules for wing height. And as modeling and replicating rear window angle separation points is a common problem on road car aerodynamic testing, putting the same solution on the same car in a different tunnel could yield different results.

Online SPARKY

  • Global Moderator
  • Hero Member
  • ***
  • Posts: 6707
  • Age: 77
  • Location: Phoenix
Re: School me on trip fences...
« Reply #9 on: July 31, 2014, 08:57:20 AM »
Thanks guys--most of my aero comes from when I flew sailplanes  :oops:
Miss LIBERTY,  changing T.K.I.  to noise, dust and RUST!!!

The # 1 issue is: TO KEEP THE REPUBLIC      
           tncsg.org     mrspowell.org

ELECTION  INTEGRITY  PROJECT
eip-ca.com  EIPAz.org  eipnv.

"Security is mostly a superstition. It does not exist in nature, nor do the children of men as a whole experience it. Avoiding danger is no safer in the long run than outright exposure. Life is either a daring adventure or nothing."   Helen Keller

Offline Bratfink

  • Full Member
  • ***
  • Posts: 61
  • Location: Belleville, MI
Re: School me on trip fences...
« Reply #10 on: July 31, 2014, 09:03:37 AM »
http://www.autospeed.com/cms/A_3059/article.html

In reference to this article in particular, looking at the results it shows some pretty spurious conclusions. Presumably these are the road car test results because the WRC car showed much bigger changes (and ran a much more aggressive wing). A 3 count reduction in drag coefficient! That is so small it could almost be within the repeatability of the tunnel. If they were truly working and putting air onto the wing there would be a more significant increase in downforce than 4-5 counts which would likely end up increasing the drag. I suspect what is happening here is that the car makes rear lift (as most road cars do) and the reduction in drag is induced drag associated with the rear lift becoming more neutral.

The other thing to note here is that the VG's changing the local direction of flow would mean that the wing angle would have to be reset to keep a neutral level. i.e. rather than just getting more air onto the wing its actually changing the flow condition around the wing. so say 2deg ND is no longer really 2deg ND in relation to the flow. Doesn't say if they reset the wing angles to account for this or they are just recording the result of bolting on the VG's, in which case the individual effect could be greater or lesser depending on the combination of factors.

Like I said before, it just isn't that simple.