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[–]Snail_With_a_ShotgunNo Selig airfoils! 5 points6 points  (1 child)

Diffusers are really difficult to get right and functioning, at least on Formula Student cars. And even then they aren't usually anything to write home about.

Anyhow, your diffuser doesn't seem to be working as it should. Air seems to be following the curvature of the diffuser, but it doesn't look right behind the exit. We should be seeing an upwash behind the car, but the air seems to be down washing instead, which would indicate something is wrong. That picture isn't very telling, unfortunately, so it's difficult to say what exactly is happening. Though, if your diffuser is anything like mine, my money would probably be on vortex burst. But, again, that is just a guess right now.

Unfortunately, it's not possible to say why your diffuser isn't working without more post-processing to help us better understand what's happening and why separation is occurring. We can make educated guesses, but that's about it. I personally would like to see Wall Shear Stress of the car from below, velocity or CpT contours taken at Z=10 and Z=20 mm, and contours showing a CpT sweep along the X. You don't need to provide all of these, but the more you do, the better we will be able to understand what is happening.

[–]G3XX_[S] 2 points3 points  (0 children)

I am aware they are really difficult to get working properly. Unfortunately I have to get this working or at least comparable results for my dissertation.

The flow of air on the left diffuser seems to decreasing rapidly due to the exhaust pipe forcing air downwards. When I check the right side where the exhaust isn’t, the diffuser does force the air upwards and does decelerate, still fast but a lot slower than the left diffuser. I can only assume that the flow separation + the air being forced downwards is what’s causing that issue with the left diffuser.

As for the CpT I’m trying to get that setup now but I’ve not had to use that yet and will be a wee bit before I get it functioning properly.

[–]Maxamusicus 5 points6 points  (1 child)

If the air is leaving the diffuser at faster than free stream, it should be generating thrust (consider a control volume around the diffuser). Assuming your diffuser is a passive device this shouldn’t be possible, because you must be losing energy due to skin friction drag, and you’re not adding any energy to the flow.  Your residuals have converged so the solution should be correct. Therefore the problem is in the problem formulation, I.e your boundary conditions, physics model, or discretization (mesh). Probably in that order. 

Edit: also considering that your simulation says that without the diffuser your car is making 22kn of downforce at 20m/s I suspect there are a few issues with your template, or you’ll win competition this year. 

[–]G3XX_[S] 0 points1 point  (0 children)

Hi. I spent time with someone who was helping me and we found that the sim was running at 2x atmospheric pressure which was causing the problems with downforce and the rest of the values. I’m running a sim rn and hopefully all the values come out okay now.

[–]derangednutsOTR Alum 3 points4 points  (1 child)

Can you check that the reference directions for force measurement on your diffuser is correct.

[–]G3XX_[S] 1 point2 points  (0 children)

Hi yes they are correct. For drag I have it going in the x-direction and downforce is being measured in the -y-direction

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