Exhaust Flow and Aero

[7800ebs]

Interesting subject.... which I know Very Little...

Although, some things are not on You tube... which doesn't mean they didn't happen.... OR WERE PUBLICIZED..

Like the effect Bob Herda's un-louvered ( NO vents ) under Body on his Liner created...

Speaking with those who were there, the spring rates had to be increased , due to the car "Sucking Down" at speed.

If you take the time... Note in the story below, about the "negative pressure" under the car...

http://bryantfamilyauto.com/my_friends.htm     

  scroll down...

http://picasaweb.google.com/psychocross/BonnevilleChromeYellow360More# 

 some pics mixed in with the Markley cars


Bob passed away in his car. But he was a very good aero engineer. Yes in fact that was his day job.

All of this took place ... before most anyone else knew ... .What Bob Herda knew..  in the 1960's....


just because it is not on You Tube... doesn't mean it did not happen..


403 mph .... Single engine car..... First SINGLE engine car over 400 mph ...

Where is that on you tube? and who was it?   I know ... as I watched it....   or was I dreaming..

bob dalton

[tortoise]

If we press up on an immoveable object, there is no work being done.  However the air is not immoveable, it is a fluid.  The first law of motion is that for every action there is an equal and opposite reaction.  When we create downforce by moving through a fluid (air) that force equals moving a mass of air up.  

F = m x a

So for a given downforce (F), we must accelerate (a) a mass of air (m) up.  This is power, and over time, work.

Consider the suction cup. It exerts downforce without moving a mass of air up. Now consider a perimeter skirted vehicle with a frictionless, airtight seal. Same thing. Of course, the frictionless, airtight seal does not exist. But to the extent you approach it, you get downforce without drag. It is not apparent to me how one would calculate the theoretical lower limits of friction and leakage.

[hotrod]

You forget that the suction cup had work done on it to force the air out in the first place and create the suction -- no free lunch here.

The energy needed to create the lower pressure comes from some where, it is not free, although it might be lower cost than other forms of equivalent down force.

Larry

[tortoise]

Hotrod, you support my point. There are other ways of making downforce than moving air upward, and aero drag is not a necessary result. The best example is, of course, ballast.

[A2WindTunnel]

I'm getting in this discussion way late but here’s something to think about.  Note: I have not read the last 16 pages in this thread but just the last few about downforce and the drag resultant.  When you tape a radiator grill inlet you reduce drag and gain downforce... (More than a free lunch)

[Saltfever]

The air under the car is attached to two surfaces. I think the effect is simple capillary action. Anyway, the velocity of the air attached to the salt is near zero. The velocity of the air attached to underbody parts is also near zero. This is a very old phenomena and is well documented with porting studies over the years. I agree these stagnant zones of air can be minimal and as little as  .002-.015” thick. But air velocity is near zero in these areas. Somewhere between the car body and the salt, turbulence develops. Whether it is at atmospheric pressure or less than atmospheric, the shearing action of the air or turbulence causes drag. You get down force but it does have a cost.

[hotrod]

I'm getting in this discussion way late but here’s something to think about.  Note: I have not read the last 16 pages in this thread but just the last few about downforce and the drag resultant.  When you tape a radiator grill inlet you reduce drag and gain downforce... (More than a free lunch)

I would submit that you gain no down force at all, but you do reduce lift. Just because the car got lower on the springs and quit darting around in the traps, does not mean you gained down force. By blocking the front grill opening you reduced pressure under the hood (lift force trying to lift the hood), you also reduced positive pressure on the fire wall which reduced the pressure drag.

Reducing lift and increasing down force are two very different things, but easy to mix up and ascribe one to the other.

If you reduce lift you naturally will also reduce the induced drag caused by the work performed by that lift. In that case you are getting exactly what you are paying for not a free lunch.

Larry

[Jon]

If the car is lower than it used to be at the same speed because you have reduced lift its better than a free lunch, you have stopped paying for lunch for someone you didn't like.

jon

[Saltfever]

. . .  By blocking the front grill opening you reduced pressure under the hood (lift force trying to lift the hood), you also reduced positive pressure on the fire wall which reduced the pressure drag.Larry

Haven't you just relocated it to the new blocked off section at the front of the radiator?

[Rex Schimmer]

Saltfever,
Just a comment on your statement regarding the velocities of the boundary layers next to the ground plan and also the surface of a moving body above the ground plane. The velocity of the boundary layer is zero relative to the surface that it is next to so the boundary of the ground plane is zero relative to the ground but it is at the speed of the moving body at the surface of the moving body. Now if they are relatively close to each other then if you look at a velocity profile of the air between the ground and the moving body it is zero at ground level but it is traveling at the moving body velocity at the surface of the moving body.

Rex

[hotrod]

Haven't you just relocated it to the new blocked off section at the front of the radiator?

Yes you have but the peak stagnation pressure is only achieved in small area of the center of the radiator, you have significantly reduced the surface area that pressure is acting on.

Also by adding a smooth front surface you force the air to go around the radiator rather than through it, so you also eliminate a significant amount of "internal drag". Radiators by their very purpose are intentionally built to maximize air stream surface area contact and passage turbulence in between the fins and tubes of the radiator to maximize the heat transfer to the air flow. That means flow drag and pressure drop across the radiator, both of which essentially disappear when you keep the air from going through the radiator core in the first place. Much less flow drag for the air to move across a smooth body panel than to push its way through a nest of closely spaced fins.

Larry

[Saltfever]

Thanks, Rex. I didn’t make that point perfectly clear or maybe I even mis-stated it.  Obviously, if the air is stuck to a moving body its velocity will be the same.  I think the picture I was trying to paint was looking out at that vast, flat, 32 square miles of salt, the air is hardly moving in relation to the car. When we pass a body through that air mass we are imparting various degrees of acceleration (laminar or turbulent) to the entire mass under, around, and over the car. Accelerating air anywhere takes energy regardless of the dynamic effect on the car.

[A2WindTunnel]

I would submit that you gain no down force at all, but you do reduce lift. Just because the car got lower on the springs and quit darting around in the traps, does not mean you gained down force. By blocking the front grill opening you reduced pressure under the hood (lift force trying to lift the hood), you also reduced positive pressure on the fire wall which reduced the pressure drag.

Reducing lift and increasing down force are two very different things, but easy to mix up and ascribe one to the other.

If you reduce lift you naturally will also reduce the induced drag caused by the work performed by that lift. In that case you are getting exactly what you are paying for not a free lunch.

Larry

Larry, this is proven with track testing and wind tunnel testing.  By taping grill openings or reducing the inlet size you reduce the under hood pressure and create more downforce. Have you ever wondered why they tape off the nose in NASCAR for qualifying?  Because it reduces the drag and gives the cars more downforce (mostly front) and lets the drivers get better lap times then in race trim by providing a few more %front balance.  They can only run a few laps before engine cooling becomes an issue, but it is done by 100% of the teams for this reason for the 2 lap qualifying.   

 I am talking about a car that has -CL already. And IF you have lift this would decrease the lift (more on the nose) but is still the same "free lunch" you guys say can't exist. Lift is a bad thing and to counter act you need to either add ballast or reduce the lift.  If you reduce the lift and decrees drag, isn't that the same free lunch?

[hotrod]

Lift is a bad thing and to counter act you need to either add ballast or reduce the lift.  If you reduce the lift and decrees drag, isn't that the same free lunch?

Yes and no -- I am making an important but subtle distinction regarding first principles. Reducing lift is not the same as "creating down force" it is the elimination of an undesirable negative not the creation of a new force pointing in the opposite direction at no cost (ie violation several physical laws)

Cutting loose a boats anchor is not the same as putting up more sail while leaving the anchor out. They may both increase the speed of the boat but different things are going on. In the case of taping the nose you are cutting the anchor chain, reducing a negative that already exists, you are not getting any free energy you are reducing your losses.

I agree with you entirely, NASCAR is an excellent example of reducing drag by cutting internal flow drag by taping off the radiator.

The point several people here are trying to make is that the conception that "down force" in certain situations is some how "free" in terms of drag is a faulty understanding of the physics involved. If that faulty understanding is applied uncritically to other problems it leads to big mistakes both in understanding what is going on and why you should be doing it.

Some folks understand that distinction some do not. Yes the reduction in drag is "free" in the sense that the tape does not cost much and little effort is involved but physically it does not "create" anything new. Lowering front end lift by reducing underhood pressure is not "increasing down force" it is reducing lift. Two very different things that have the same net result in this case.

For example adding a gurney lip on an inverted airfoil substantially increases down force generated by that inverted airfoil at a relatively small cost in drag, but it is still not "free down force" you do pay a small price in additional drag. That penalty may be much smaller than the drag penalty you would pay for creating the same down force with a bigger wing or a wing with more camber or with the same wing at a higher angle of attack but it is still not free, it is just the least costly of the 4 alternatives to get to the same place.

Going back to your analogy, adding ballast actually creates down force and does not increase aero drag in any way, but it will increase rolling resistance, and bearing losses in the loaded wheel bearings. It also certainly slows down your acceleration to the 2. Again it appears free but it is not, you are just paying for it at a different window.

Larry

[tortoise]

Going back to your analogy, adding ballast . . . certainly slows down your acceleration to the 2.

Not certainly.  A powerful, nose-heavy car might be helped, no?