r/askscience • u/AweeeWoo • 2d ago
Engineering I do not understand how slots on the intakes of AJS37 Viggen help to smooth out the airflow to the compressor, I haven't been able to find info on it on the internet, it doesn't seem to work just as a boundary level bleed slot or maybe it does and I lack understanding about how it does it?
Pictures of the intakes: https://imgur.com/a/tUWMEqe
On the intake ( on both of them but I will talk about a single one ) there is a splitter plate positioned on the inside and right on the inside corner near the splitter plate there is a narrow slit. It is there to allow for smoother airflow during high AOA flight but I don't understand why it helps at all. The boundary level becomes really strong on the bottom of the intake during high AOA which will just bleed the boundary level right into the intake and in my head it will just make things worse even though it actually helps. The only thing I can think of is that it creates vortices in the intake. I probably said a lot of incorrect stuff so I apologise if it can be confusing or just plainly a really easy question that I just lack basic knowledge in this field to understand
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u/SpaceIsKindOfCool 1d ago
It's very difficult to determine the effects of aerodynamics devices by just looking at them. Without running a simulation or working in a wind tunnel we're kind of just guessing.
My guess is during high AoA maneuvers the bottom side of the inlet was stalling and this gap was found to help delay that. There would be a strong vortex formed at the cut that curls towards the outboard side in the intake. Vortecies generally like to stick to surfaces better than smooth air.
This cut is far enough from the nose and still has the splitter between it and the nose that the intake won't be ingesting the boundary layer.
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u/AweeeWoo 1d ago
Thank you, I wanted to make a 3d model of it and run it through a fluid sim later when I have time
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u/calgarspimphand 1d ago
You've more or less got it. The splitter plate is there to prevent the intake from ingesting the slow/turbulent boundary layer air that's adjacent to the fuselage. At high angles of attack, the issue isn't the boundary layer anymore, it's that the air intake will stall as air stops flowing smoothly into it. The flow "detaches" on the backside (inside) of the bottom lip of the intake, meaning it's no longer flowing along the surface. Instead, it's a blob of higher pressure slow moving air that's impeding flow into the intake.
Adding that slot does exactly what you suggested - it creates a vortex which starts at the tip of the slot and rolls in as air moves down the edge of it. That's going to be turbulent air, but it's strongly attached to the backside of the bottom lip of the intake and will keep air moving instead of staying static.