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pravinpardeshi December 11, 1998 04:09

turbulance and seperation
 
respt. sir,

i would be glad if you could explain me what happens to drag

if turbulance goes on increasing ?

what when seperation of flow occurs ?

pravin

Jonas Larsson December 11, 1998 08:15

Re: turbulance and seperation
 
I complex question. Very simply speaking you can say that a turbulent boundary layer gives a larger frictional drag and a lower wake drag compared to a laminar boundary layer. The lower wake drag is explained by the fact that a turbulent boundary layer separates later and thus gives a smaller wake. The larger frictional drag is explained by the fact the a turbulent boundary layer has a profile which gives a higher velocity gradient close to the wall. Which effect that is most important depends on the shape of the solid body.

I suggest you look in a good textbook on aerodynamics for more details.

Bipin Lokhande December 26, 1998 10:54

Re: turbulance and seperation
 
Dear Pravin,

There are two types of drag associated with the body immersed in a flowing stream of fluid. 1) pressure drag 2)friction drag. 1) Pressure Drag; is there because of wake generated behind the body when the separartion occurs. This drag depends on the width of the wake region. Higher the width higher is the drag.

2) Friction drag is there beacuse of the viscosity of the fluid. This is proportional to the surface area with which the fluid is in contact.

Turbulence increases the friction drag and reduces the pressure drag upto certain extent. As the Reynolds number increases the friction drag increases because the gradient of velocity is very high near the wall, while the pressure drag decreses upto a certain reynolds number as it prolong the separation and reduces the width of the wake.

The vortex generator used in planes are used for reducing pressure drag.


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