ADVANCED FLUID MECHANICS CLASS PROJECT

FLOW SEPARATION Gathered by: FARAJOLLAH FARAHNAK JAHROMI

Supervisor: Dr. ALIZADE

WHAT IS FLOW SEPARATION?

When the fluid flow becomes detached from the

surface of the object , and instead take the form of eddies and vortices called flow separation occurred.Hint:However It make lot of problems in application , it could be helpful in several cases .

WHY FLOW SEPARATION OCCUR?

PRANDTL shows the separation caused by excessive momentum loss near the wall in a boundary layer trying to move downstream against increasing pressure which is called adverse pressure gradient

IN AN OTHER WORD NEGATIVE PRESSURE GRADIENT AS FLOW MOVE DOWNSTREAM IS DESIRE CONDITION WHICH NO FLOW

SEPARATION OCCUR

WHY FLOW SEPARATION OCCUR?

-PRANDTL derivation: first use momentum and mass conservation formula for two dimensions

Assumptions:

PRANDTL’S DERIVATION

- PRANDTL show applying these assumptions to second equation of momentum relations result:

So P ≈P(x)Using BERNOULLIES EQUATION:

Momentum along wall

PRANDYL’S DERIVATION

-at the wall , where u & v Are zero we obtain:

or

Thus in an adverse gradient the second derivative of velocity is positive at the wall , yet it must be negative at the outer layer to merge smoothly with the mainstream flow.It follows that the second derivative must pass through zero somewhere , in between at the point of

inflection .

WHER DOES IT HAPPEN?

-In wings of airplanes at high angle of attack

-in play balls-diffusers

-automobiles-wind turbines

…

LIFT FORCE GENERATION IN AIRFOIL

-Airfoil expose to wind at low angle of attack

FLOW SEPARATION AT HIGH ANGLE OF ATTACK

-stall phenomena-Substantially decrease lift force and

increase drag force

REMEDIAL ACTION

-passive method (example : shape ,surface roughness &

)…-active method

suction blowing

Using electromagnetic force …

REMEDIAL ACTION

-dimples on golf ball cause flow separation suppressed

it is because of turbulent boundary layer formation

REMEDIAL ACTION

generic blowing actuator: The actuator used to control separation consists

microjets blowing continuously normal to the wall. they are located close to the separation points .

It substantially suppress flow separation .

in a particular experiment the angle of attack with separation occur in changed 2 degree

REMEDIAL ACTION

This is tested by PIV in wind tunnel

REFERENCES

a first course in fluid mechanics , sabersky h.ralf,1989

fluid mechanics , frank m. white , 1988 by mcgraw-hill , inc

THANKS FOR YOUR ATTENTIONWITH BEST REGARDS