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Scaling effect on friction drag of car


In fluid dynamics drag is the resistance force acting on a solid body moving in a fluid (liquid or gas). Drag is constituted off frictional forces acting in a direction parallel to the body plus the pressure forces acting perpendicularly on the surface of the body. For any solid object moving through any fluid, the drag is said to be the sum of all hydrodynamic forces acting in a direction of fluid flow. To understand the drag force a simple example can be considered. If someone holds his hand out of the window of a moving car, the drag (i.e. the wind force) will try to push the hand behind.

Determination of Boundary layer thickness

(refer page 640-656 of Mechanics of Fluids by Shames) Boundary layer thickness is defined as the distance above a surface where the flow velocity is 99% of free stream velocity.


From the calculated results, it is evident that flow in case of wind tunnel case is laminar and so the boundary layer thickness is also more. The drag forces are small in magnitude due to the fact that, the velocity is less and so are the areas at different cross-sections. However, in full scale, the flow is turbulent and so; the boundary layer thickness also decreases.

In both the cases, the side plates experience the same drag forces at different cross-sections. The bottom plate experiences more drag forces due to roughness of the plate and it is considered to be thrice the values of the top plate in this particular case.

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