shelly wrote:Ok let us agree from now on 65m/s average speed on exit.
Let us focus on r31 geometry. First questions to be answered are:
-can we estimate pressure exerterd on the curved metal heat shield?
-how much do we expect exhaust cone to expand in the short space between exit section and obstacle?
-what does the flow look like on a conventional sidepod inlet? (I will see if i can retrieve a very good master thesis with cfd of an american guy called Perry IIRC, from 2008)
Focusing on first foot out, in my opinion exhausts will be:
- slowed down and opened up by the impact against the shield (so parting in two- up and low)
- deflected by external flow (approximatively a vector composition)
but they will also:
-go aroundthe airfoil shape at the fron external corner fo the floor
-entrain by viscosity local flow
-oppose an obstacle to external flow (let us call it improperly the core of exhaust) around which eternal flow has to find its way. There is a region near the exit in which the fluid comes 98% from exahust: can we imagine how big it is?
We've discussed most of this on the thread already, but i try to summarize my view point.
1. We have 2 flows (external air and exhaust) almost perpendicular to each other.
2. Speeds of both flows are comparable and way below speed of sound.
3. At those speeds both air and exhaust gases are considered incompressible in open space, which means both are at ambient static pressure (more or less, we don't care about few percent).
4. Because static pressure of both flows is the same, there is no force to prevent instant mixing of both flows as soon as exhaust leaves pipe. There is realy not like in solids and liquids. If you happen to have 2 hairdryers at hand, you can proof this quickly.
5. Both flows do carry momentum, equal to mass*velocity.
6. Momentum of resulting flow of mixed air/exhaust gas is equal to vector sum of air and exhaust momentums.
7. If both speeds are the same (lets take our 65m/s) what is the angle to car's longitudal axis ? 45 degree ? NO. Why ? Exhaust is much hotter, less dense (about 1/3 of ambient air's density at 800K) and has less momentum - so the angle will be in the 15-20 degree range. Less for quicker moving car, more (up to about 80 degree at pit stop) if car is moving slower.
8. I think there are 2 exhaust flows at singnificantly different speeds (explanation and pictures in my previous posts). As a result, mixed air/exhaust flows cover bigger area of the floor.
Where is this flow going ?
There is strong high pressure area in front of sidepods, and even more so (stagnation
point) on sidepods lower lip. Flow from the exhaust won't go this way.
Last revision of R31's floor and as seen in Barcelona features rised leading edge of the floor in front of modified pipe:
In my view 80-90% of the leading edge is above exhaust and most of gases blow under the floor, direct into high pressure area (highest at stagnation point at the edge).
This solution is draggy and creates some lift on this winged part, but must be worth it.
Now the tricky part: we don't know real pressure distribution under this "wing", so we can only speculate. I think slower part of the flow goes to the left (in plank direction) due to lower pressure in this region, quicker flow goes sideways (at an angle dependent on exhaust and car speed).
Slotted gurney seen on the photo creates low pressure area on the outside, which helps to direct escaping air/exhaust mix back do sidepods undercut and further along.
If this works this way, than main gain will be from hot air/exhaust mix as it cools down under the floor (from radiation and heat transfer to floor and tarmac), decreasing volume - i would call it thermal ground effect. Basicaly vacuum pump, but thermodynamics instead of fan blades.
How powerfull could it be ?
At the extremum (sealed system, air/exhaust mix filling all of underfloors volume, going from 800K down to 400K) few hundred kgf of downforce.
Of course system is open, part of gas energy goes to surrunding/mixed air preventing volume decrease, so we have to speculate - i go for 150kgf.
Heating of rear tyres, preventing sidepods flow separation, lowering boundary layers thickness both under floor and on sidepods - not possible to calculate, but there is some gain from this, i belive.
