BOUNDARY LAYER SEPARATION

BOUNDARY LAYER SEPARATION

When fluid flows past a body (cylinder or an airplane), fluid is accelerated due to pressure drop and ∂p/∂x is negative. Pressure decreases as distance x increases. When ∂p/∂x becomes zero, velocity decreases and separation of flow occurs and back flow is just to start. At the point of separation, the following are observed.

(i) It is a point of stagnation.

(ii) ∂p/∂x becomes zero at this point.

(iii) ∂u/∂y = 0

(iv) U = 0 and v = 0

(v) Velocity profile has a point of inflexion at the point of separation i.e. there will be change in direction of the velocity after the point of separation.

(vi) Beyond the point of separation fluid starts flowing in the back direction of the main stream.

(vii) Eddies will be formed and there will be energy loss which is highly un-desirable.

(viii) This is also found in the divergent portion of the convergent-divergent portion of nozzles used before the steam turbine.

(ix) Flow patterns and flow nets are useful in finding the points (or regions)of separation

(x) Design should be such that back flow does not occur.