shelly wrote:@marekk: I have followed the thread, but I have seen that there no agreemente has been found, that's why I wanted to recap shared ideas and build on them.
Below I report my ideas on your view point, item by item, to see if we can convince each other about some new "bricks" in this reasoning.
1. external flow is strongly conditined by teatray-sidepod shape: so in exahust proximity it will be slower and bended some 60° to longitudinal direction.
It will be more longitudinal in the layer below, that goes under the floor.
So there is an attenuation of prependicular interaction, which is somehow limited to the lower zone of the jet.
2. agree that both are subsonic for each fluid temperature and that they are comparable (you know I think exahust are faster, but not that faster).
3. we do care for pressure variation coming form dynamic pressure
4. disagree on the two dryers example, in which two jets are intersecting. We are in a jet in crossflow situation, like in the cfd image you posted a while ago. No instantaneous 100% mixing, turbulent mixing instead
5. agree
6. agree
7. agree from 6, but slightly disagree basing on my 1: I think your point 7 is applicable on the part of exahsut going under the floor, not on the top part
8. I think it could be, but I do not understand why separating jet in two parts lead to greater area covered
I think flow from exaust will partly go on floor lip (heat shielded part) and then around it. External flow will be partly deviated by the presence of the jet itself and will squeeze under the floor, as part of the exhaust will do; mixing and entraining will take place.
Agree that most of exhaust will go under the floor; raising the lip goes probably in the direction of increasing percentage of under the floor exhaust.
Agree that cooling under the floor is beneficial(inward deflection of pathlines will call fluid from tha sides of the floor).
Effect estimate: 150kgf at 60m/s is huge, some 70 points of downforce. I think half this value is big enough to justify this effort; but if we see r31 making 1-2 in melbourne hands down I will agree on 150kgf. Do not forget that external air will be called inside in cooling.
I think we can discuss disagreement points further
1. Agree. hard to tell how much without CFD and tunnel data, but you are right - external flow comes slower and angled.
2. Exhaust speed is strongly related to pipes exit area (not that mauch to temp). Once wee have better pictures with some reference points, we can try to calculate it more precisely.
3. I still belive there is no dynamic pressure in free flowing stream - it's "potential" part of total pressure of gases, stored in flow's kinetic energy, and can be converted to significant dynamic pressure only on contact with solid/liquid (much more dense) surfaces. I will research it further.
4. This CFD image is a little misleading because of closed space, but i agree mixing will be not uniform and highly turbulent. I've used wrong wording - instant means for me first few molecules of both flows will mix as soon as exhaust leaves pipe (or even a little earlier).
7. Agree
8. 2 cone-shaped flows starting form pipe's exit, but at different angle, cover more area than single flow.
Because there is no vertical momentum in exhaust gases, i don't believe it will change external flow's vertical direction, but i try to research it further.
My 150 kgf of downforce is a litlle from the hat. I've made some calculations of thermal radiation and heat transfer rate to tarmac and floor, but lacking any real data except floors area i can be easily wrong by more then 50%.
We can try to estimate downforce gain related to increased mass flow, but at 0,45 kg/s and 50-150 m/s i doubt it will be THAT (as in all other explanations of FFE) significant.
