wilmywood8455 -

SS, something you might find interesting, speaking of vortices sealing tunnels......

On the GTP car I worked on in the late 80's, early 90's, we took it to Daytona for the first time, and experienced 'porpoising' going down the backstraight. After trying a few things to determine the cause, we discovered that a bit less rear wing or a bit more rear spring or rear ride height solved it. Turned out that the reduction in rear ride height at high speed (160mph plus or so) was just enough to reduce or eliminate the effectiveness of the vortices that ran down the side pods, sealing the undertray......... the seal would go away, the car would rise and the seal was reestablished, the car would quickly suck back down to where the seal would go away again, over and over and over......

I also believe that the front wheel covers in F1 these days contribute to this side floor sealing effect, and is their primary purpose. Of course, I could be completely wrong.

Porpoising in ground-effects cars (I'm going to call them what they were -- Venturi Effect cars) was a special case of what happened when designers took the idea too far.

The first Venturi cars, like the Lotus 78 and Lotus 79 created a great deal of downforce from their side pods, but they still had front and rear wings to trim the car. The next generation Lotus and the Arrows of the same year did away with front wings entirely, relying on even bigger sidepods for all their downforce; the rear wing became not much more than a trim tab.

What happened was this:

The faster the car went, the more air went through the side pods, the faster the air traveled and the more downforce was generated. So far, so good.

But there came a point when the venturi pods were developing so much downforce that they compressed the springs and lowered the car beyond a critical point. When that happened, the bottoms of the side pods were too close to the ground. Airflow was choked off, downforce was reduced and the car rose on its springs.

As the car rose, air started flowing through the sidepods again, causing more downforce, which pulled the car down on its springs, choking off the airflow again and so on.

From that point, it became a cycle: Downforce -- too much downforce -- choked-off downforce -- too little downforce -- downforce, etc. until the car slowed down below the critical speed.

As you discovered with your GTP car, the solutions are to either balance the car better, aerodynamically, or stiffen the springs to prevent the cycle from starting. In Formula One cars of the early 80s, the front springs were around 900 pounds per inch of compression -- each, mind you -- and the rears, 1200 pounds per inch. That totals up to 1800 pounds per inch in front, 2400 pounds per inch at the rear, and 4,200 pounds per inch, over all. That explains the "coal cart" ride cars had in that era. It also means that when a 150 pound driver stepped aboard, the springs settled about 1/28th inch, which is less than a millimeter (there are 25.4 mm per inch.)

GreyWolf74 - 23 October 2009 10:40 PM

wilmywood8455 -

SS, something you might find interesting, speaking of vortices sealing tunnels......

On the GTP car I worked on in the late 80's, early 90's, we took it to Daytona for the first time, and experienced 'porpoising' going down the backstraight. After trying a few things to determine the cause, we discovered that a bit less rear wing or a bit more rear spring or rear ride height solved it. Turned out that the reduction in rear ride height at high speed (160mph plus or so) was just enough to reduce or eliminate the effectiveness of the vortices that ran down the side pods, sealing the undertray......... the seal would go away, the car would rise and the seal was reestablished, the car would quickly suck back down to where the seal would go away again, over and over and over......

I also believe that the front wheel covers in F1 these days contribute to this side floor sealing effect, and is their primary purpose. Of course, I could be completely wrong.

Porpoising in ground-effects cars (I'm going to call them what they were -- Venturi Effect cars) was a special case of what happened when designers took the idea too far.

The first Venturi cars, like the Lotus 78 and Lotus 79 created a great deal of downforce from their side pods, but they still had front and rear wings to trim the car. The next generation Lotus and the Arrows of the same year did away with front wings entirely, relying on even bigger sidepods for all their downforce; the rear wing became not much more than a trim tab.

What happened was this:

The faster the car went, the more air went through the side pods, the faster the air traveled and the more downforce was generated. So far, so good.

But there came a point when the venturi pods were developing so much downforce that they compressed the springs and lowered the car beyond a critical point. When that happened, the bottoms of the side pods were too close to the ground. Airflow was choked off, downforce was reduced and the car rose on its springs.

As the car rose, air started flowing through the sidepods again, causing more downforce, which pulled the car down on its springs, choking off the airflow again and so on.

From that point, it became a cycle: Downforce -- too much downforce -- choked-off downforce -- too little downforce -- downforce, etc. until the car slowed down below the critical speed.

A

I was present at very interesting test in a windtunnel in the early 90's. The car was a 92h Reynard (Atlantic). As you described a venturi shaped tunnel, instead of the typical bottom of a wing shape like the Swift DB4. This was before the rules were rewritten in Atlantic, to include 50lb weights that were hung on the outside of the tunnels and measured for flex.
These tunnels were designed to flex and the sides would flex so the rear of the tunnels would make contact with the ground, greatly increased the tunnels ability.
During this test we did a couple of things, one was we sealed the sides like the F1 cars of the Lotus era (late 70's) The numbers recorded were close to a 15% increase over typical running conditions (air gap to the ground).
Next we removed the front wings and recorded an approx 4% increase just from removing the turbulent air.
The last test we did was the most significant and left all of us, wondering what happened. We sealed the in board side of the tunnel to the ground. The downforce numbers declined by almost 8%... ??? Thinking we made a mistake, took them off, the normal numbers returned. So we added the inboard strakes again and bingo lost downforce.... yet all of us thought it should increase even more.
(BTW) forgot to mention the front wings were back on the car, throughout this test.
After adding some more aero taps to the bottom of the car, it was discovered a rise in higher pressure areas in between the inboard strakes. So we shorten the inboard ones, leaving a small gap to the ground, 1% improvement. cut it in half, 1-2.5% improvement, shortened it again but still below the bottom floor, 3% total improvement over being fully on the ground?
It was not until we shortened it to beyond the floor level that the numbers jumped back to respectable, though fully removing it put the numbers in a big leap back.
This is interesting because up to this point I firmly believed that it was the tunnel itself causing the downforce, when infact it was the surrounding areas of the floor and removal of high pressure that planted the car.
After this test I no longer believed that a tunnel is solely a downforce producer, but an anti lift device that scavenges the air around it in it's operation. It can be considered as semantics to look at it this way, and can lead to interesting agruments as the differences in thinking of devices as anti lift or down force producers.
In my mind, one device on a race car produces downforce (the opposite of lift or negative lift device), the wing. The rest are downward force allowance items either by reducing lift in a high pressure scavenging role (tunnels and diffusers) or a downward force role (spoilers) or a air flow blockage role (air dams) or a redirection or guide role (barge boards, fins, chin spoilers and splitters)
As a note on the porpoising issue. When the side strakes are in contact with the ground, the car becomes much less pitch sensitive, then when they are removed and small pitch changes throw the balance of the car out of whack in a hurry in comparison.... a lot of it has to do with the "invitation" of high pressure areas that form in the air gaps and under the car....IMHO

speedsense - 24 October 2009 12:32 AM

“an anti lift device that scavenges the air around it in it's operation”.

Classic personal input. Thank you.

So the point of focus (mentally) is not 3 inches behind the rear diffuser, where the low pressure is, or some wing-like downforce on the diffuser body itself – but the entire underneath of the car – where air has been “scavenged” from.

I’m thinking of Daniel Day-Lewis in the movie There Will Be Blood:

“If you have a milkshake, and I have a milkshake, and I have a straw and my straw reaches across the room and starts to drink your milkshake. I drink your milkshake! I drink it all up!”

ipso - 24 October 2009 01:05 AM

speedsense - 24 October 2009 12:32 AM

“an anti lift device that scavenges the air around it in it's operation”.

Classic personal input. Thank you.

So the point of focus (mentally) is not 3 inches behind the rear diffuser, where the low pressure is, or some wing-like downforce on the diffuser body itself – but the entire underneath of the car – where air has been “scavenged” from.

I’m thinking of Daniel Day-Lewis in the movie There Will Be Blood:

“If you have a milkshake, and I have a milkshake, and I have a straw and my straw reaches across the room and starts to drink your milkshake. I drink your milkshake! I drink it all up!”

Notice on the "colored" drawings of the the bottom of the car, where the diffuser starts, it's dark blue..indicating low pressure/high velocity..notice it starts before the diffuser starts and the areas around the "mouth" of the diffuser...
What's happening out the back of the diffuser is increasing this area in the front of the diffuser....
so yes, the straw effect, LOL..

ipso - 24 October 2009 01:05 AM

So the point of focus (mentally) is not 3 inches behind the rear diffuser, where the low pressure is, or some wing-like downforce on the diffuser body itself – but the entire underneath of the car – where air has been “scavenged” from.

Exactly; the tunnels/diffuser/whatever ...... accelerate the airflow and create low pressure on the entire floor. This is also true on the hood or decklid of a sedan; lower the pressure on the underside by extracting air, and it acts on the entire underside. Lower the pressure under a 50 inch by 50 inch hood by 1/4psi, that's 0.25psi times 2500sq.in. of hood = 625 lbs of downforce, or of less lift. It all adds up fast.