Simple knife-edge (aerodynamics) question.

[5.0stang]

With components, why is it best to knife-edge the backside?

For example:

Knife-edge a crankshaft
Knife-edge the back of a throttle body blade

What principle is this following?

 

[hks85]

look at the shape of a rain drop...

 

[luben]

knife edging those components streamlines the structure along the flow direction, and thus reduces the chance of flow separations. Eventually, the flow induces least drag on the structure. Another reason can be to reduce mass, for example, of a crank, but i think this is a lesser reason.

 

[hxtasy]

Well IMO for the throttle plate example, if you had it knifed in the front, it could split the air, disturbing flow. If you don't knife it in the back, and it is square shape, there is a spot where the air can settle out of the parts of the moving air, this will create a bubble behind the throttle plate, a bubble of air molecules which will have an impact on any incoming air surrounding that area. think about a river and a rock sitting in the middle, and how right behind the rock there is a spot where the current doesn't flow, and it also may have a vortex type effect which will just interfere more.

 

[weiszed]

Same reason as a football. Not exactly a knife edge but the concept still applies. If the backside of a football were flat, the stagnated air behind the ball would be a lower pressure than the air in the front creating drag from the high-to-low force pushing back on the ball. For components, the same principle is true but on a lesser scale; hence it is just an efficiency modification.

 

[hxtasy]

You are correct about the shape of the football. But for the football's application, low drag is essential but i think lift is also important, although I don't see how the football is designed for greater lift. The texture of the skin is rough though, creating separation delay from the boundary layer of the ball. This could also reduce drag, but I'm not really sure because the skin texture on a football is not as defined in comparison with say, a golf ball, which would be less than a third efficient without its dimples.


One time we ripped apart a throttle body, bored and smoothed it, shaved down the screws that attach the butterfly to the rotating shaft, and knifed it as well. Although no air flow difference was actually measured, other than the placebo "think it works better effect", i think the main advantage was just taking it apart and cleaning it, although that was years ago when i was dumb. On my application of cars you will never reach a point in performance where the engine could draw in so much air that the throttle body would be a measurable drag resistance point.

 

[5.0stang]

Same reason as a football. Not exactly a knife edge but the concept still applies. If the backside of a football were flat, the stagnated air behind the ball would be a lower pressure than the air in the front creating drag from the high-to-low force pushing back on the ball. For components, the same principle is true but on a lesser scale; hence it is just an efficiency modification.

Would not the air behind the ball (stagnated) be of a higher pressure.

Thinking Bernoulli's principle here...

 

[h2oski1326]

The air behind isn't the stagnated. The air on the surface of the front would be stagnated, creating the high pressure. The flow behind would be a turbulent wake at a lower pressure. The more projected area there is at the front the higher form drag is created.

 

[luben]

yes agree. the bernoulli principle only applies when following one same streamline. Pressure can drop much if the referred streamlines have very different mechanical energy.