Annnd the radiator and water pump conspired to put me in the local Pep Boys lobby for a good portion of today.
Which means you get early CFD results! (Colors For Dollars according to HARA's new president....)
I followed Boatgeek's lead and set the flow for 2 degrees off axis. Did 80, 60, and 30 m/s based on the velocity profile predicted for a D12.
Square: 60m/s 25.4" (almost an exact match of my OR model)
Ellipse: Cabernut's 26" OR model
dot: 80m/s 27.36"
The pics below show pressure on top and velocity on bottom. I put the legends at pretty near the same values for all three cases.
The 30m/s gave really screwy results, so I just kept the P-v distributions
Careful, you're getting awfully close to the murky waters I've been treading in recently.:dark:
First off, I'm assuming (and hoping), that this thing will never be flying fast enough for the following issues to be .....issues.
If it was a pitot-tube, you want the static pressure port to be 4 diameters away from the tube tip to allow the boundary layer to steady itself.
The tricky thing here, is that the airflow is still kind of unsteady at the payload tube. (there's a difference of... .4 (?) kPa (.06 psi) across the length of the fairing at the ~max speed.). Taking my above assumption (and the fact that I can easily stick an altimeter in the fairing, I honestly have no problem placing 3-4 holes at just above the transition shoulder. If you wanted to be REally rigorous, you could put the holes on the body tube 5-6" down from the transition (the internal volume is all connected after all. It appears that the flow disturbance evens out pretty quickly once the transition meets the body tube, and the ambient pressure near it is pretty much the same in all three velocity cases.
Now this has been excellent thought exercise for me because I'm still developing how to make an deployment AV bay in the transition of my unbuilt Ventris and Argent. If you're just logging data or getting the max alt/vel, then the little perturbations of a poorly placed vent hole aren't that consequential. But if I'm relying on it for reliable apogee deployment, then I want to be much more thoughtful.
Sorry, ignore the colors to get what you're after. The marks on the tube near the fins are the results (also summarized in the paragraph above. Square, ellipse, dot etc...)
In the case of the Falcon 9, the range is 25.4 to 27.4 inches from the tip of the nose (since CP moves with speed) [all results are approximations and not legally binding lol] I think its cool that at 60 meters/second, my results matched my OpenRocket position.
Comp. Fluid Dynamic software calculates each fluid element and then checks the calculations against itself again and again until the answer doesn't change (convergence).
The program can calculate things like Torque about the origin and the overall forces in each principle direction. I've been practicing getting the CP by dividing the Torque at one axis (say, Z) by the other resultant Force (Y). Torque divided by Force is a distance between the force and the origin which in this case has the origin at the tip for convenience (leaving the Force acting at the CP).
That was the important info. But since the calculations had been completed, I made a couple displays of the pressure and velocity distributions around it for fun.
In each picture, Pressure is the top half, and velocity is the bottom. Its just a cool way to represent whats physically going on with the air. You can see that wherever the velocity increases (more reddish), the pressure in the same region decreases (shifts more towards blue). This matches what fluid mechanics laws predict, so I can at least know that the model isn't doing anything TOo crazy.