Thread: Discussion boundary layer thickness
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Old Oct 01, 2012, 01:23 PM
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Frank, you're picking the worst possible place to put your fan unit and in the worst possible mount by inluding it on the rear of the wing and inset down flush like this with the surface of the wing. Then you add some sexy looking spines that lead up to the fan to further potentially trip the local airflow around the mouth of the inlet.

As you suspect you want to worry about turbulence either from boundry layer effects or trailing turbulence off the structure. The equipment hump just ahead of the inlet looks sexy but it could well become a source for turbelence off the rear face due to poor pressure gradients at high speeds leading to separation bubbles. And even if that doesn't occur there's going to be some amount of turbulent and accelerated wake behind the bump and due to the wing itself....

All in all this is why despite seeing lots of artist's conceptions for upper surface mounted engines and jets this idea is seldom seen in final designs. And even then on most designs the jet inlets are typically spaced up off the wing or fuselage surfaces to attempt to minimize or avoid ingesting that turbulence off the wing and any fuselage structure and any skin bound boundry layer.

Even on fighter jets with forward inlets you typically see "splitter plates" to avoid ingesting the tubulence and spiralling flow that is found attached to the skin and immediate zone above the skin. So inletting the EDF unit down into the surface as you have done is most certainly going against this design factor.

So how thick is the boundry layer? As I understand it you may as well ask "how long is a piece of string?". It depends on the speed and airflow and shape of the wing or structure. It's thin in some spots and thickens in areas of lower local pressure as it flows across the surface.

And lets not forget what slow speed is going to be like on your delta wing. As the angle of attack rises the flow and lift from a delta wing shifts to a strong rolling vortex coming up and over the leading edge and rolling inwards. The upper surface of your wing will transition from fairly smooth during max speed to almost washing machine like during low high AoA flying. And your EDF is trying to suck up and spit out such air.

All in all this is why pretty much any delta wing aircraft has the inlets either under the wing or positioned well forward and ducts the air to the engine. If you're looking to get the most out of your 3.5Kw of power you might want to consider doing the same. Instead of on top tunnel out the lower side similarly to the top. The top side could then be a little arched in the rear with the pointed "duck tail" swooping up and over the half tunnel exhaust area.

Again, asking how much the penalty for mounting it on top is like asking about the string. Without CFD or wind tunnel testing of the design it's impossible to guess at the effect of any boundry layer or trailing turbulence off the forward hump. But it is almost certain that there would be SOME effect. Mouting the EDF up on some mini pylons that lifts it about a half inch clear of the wing surface would aid by likely letting any turbulent boundry layer slip by at high speed. But there may be a bad suction bubble behind the forward hump that feeds the inlet. Again, without testing it's nearly impossible to say for sure.

And then there's the low speed behaviour. Trying to use the very turbulent air that WILL be found on the top side of a delta for low speed thrust is almost certainly going to ensure that accelerating out of the high alpha vortex flow and getting the wing back up "on to the step" where the air flows more or less straight over the wing is likely going to be delayed and require more power than if the EDF was mounted on the underside where it would be seeing cleaner and more direct inlet air.
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