Flow Turning Flashcards
Describe flow turning in a sentence.
Flow is turned downwards and due to conservation of momentum the aerofoil experiences lift.
What does the aerofoil experience due to flow turning?
A reaction force of lift.
Describe the vertical momentum balance of flow turning and how it varies.
Fluid receives downward momentum at a rate dependent on the magnitude of turning and the volume of fluid affected. The aerofoil receives upwards momentum at the same rate, creating lift.
Why does flow turning create a pressure difference at different points in a flowfield?
a. The higher the rate of flow turning, the larger the pressure change relative to freestream.
b. Near the LE of an aerofoil where there’s the maximum flow turning, is where the largest pressure difference relative to freestream is.
c. The TE of an aerofoil where there’s the minimum flow turning, is where the smallest pressure difference relative to freestream is.
d. The difference in flow turning creates the pressure gradient along the aerofoil surface.
What do streamlines show about a flowfield?
The pressure difference in a flowfield.
What causes a streamline to curve?
A pressure gradient acting normal to the aerofoil surface.
If there is higher p above a streamline than below it, and the streamline is above the aerofoil, what’s the effect on the fluid and aerofoil?
Creates a force downward on the fluid, and thus lift on the aerofoil.
Name a reason that DOESN’T cause streamlines to turn?
Viscosity - air is not viscous enough.
Use inviscid theory to explain why a vacuum cannot exist behind a (suction) surface with a straight streamline.
The streamline would be sucked in, and would curve.
Use inviscid theory to explain why stagnant air cannot exist behind a (suction) surface with a straight streamline.
There would be a pressure difference, and the streamline would curve.
Use inviscid theory to explain why a recirculation region cannot exist behind a (suction) surface with a straight streamline.
There would be rotation, which isn’t possible for inviscid flow.
Use inviscid theory to explain why an unbounded recirculation cannot exist behind a (suction) surface with a straight streamline.
There would be a shear layer, and hence rotation.
What does inviscid theory dictate must happen to a streamline near an aerofoil?
It must curve to follow its surface.
How many solutions does inviscid theory create for streamlines?
Infinite, with the positions of the stagnation points moving.
What causes a solution to be selected from the inviscid solutions of streamlines?
The Kutta Condition.
Define the Kutta Condition.
A body with a sharp trailing edge, which is moving through a fluid, will create about itself a circulation of sufficient strength to hold the rear stagnation point at the trailing edge.
What effect does the Kutta Condition have physically?
Flow can’t turn the TE due to the pressure gradient, so the rearward stagnation point must be aligned with the TE.
When does the solution of streamlines selected by the Kutta Condition not apply to real flows?
When then is early separation.
What are the points you need to hit to describe how lift is created?
Effect of flow turning on momentum and reaction of lift
How lift varies with flow turning
How flow turning is related to pressure difference
How pressure difference is represented by streamlines
Inviscid theory forces streamlines to follow wing surface
The Kutta Condition forces one solution
This applies to real flows unless there is early separation
