Fluid Mechanics

It's a question of definition of terms.

• A smooth aerofoil passing at speed through still air can be likened to the above. Turbulence appears towards the trailing edge of the aerofoil when the angle of attack becomes too steep; the aerofoil is said to be in "stall".

If the flow has a velocity high enough the laminar boundary layer becomes turbulent at a given length from the plate edge. The mainar layer grows over the length (about ≈ L^0.5) and when it becomes too thick it looses its stability thus the apparition of turbulence.

will the (upstream) flow be then called laminar or turbulant when the laminar boundary layer converts into turbulant boundary layer for a laminar flow?

I don't think so. Below a certain velocity (and Reynold's no.) turbulent flow cannot exist as there is insufficient energy. In laminar flow the velocity is zero at the solid surface, so in the case of pipe flow the friction loss is independent of pipe roughness.

At higher Reynold's no. when the flow becomes turbulent there is still a boundary layer, I believe theories of friction loss in turbulent flow are based on this.

There is always a boundary laminar layer but its thickness depends on velocity and viscosity (Reynolds).

At the entry of a pipe, for instance, flow is not laminar from the very start on full section. It becomes fully laminar after a "transitional length" due to the laminar boundary layer growing thickness. If the pipe is short the core cannot reach the laminar conditions.

If the requirements are not fulfilled the core stays turbulent and, as mentioned above, after a length the boundary layer becomes turbulent as well.

consider a lamniar flow. As u said after a certain length the boundary layer will become turbulant. Will the upstream flow(flow outside the boundary layer) will also turn turbulant or is it just the boundary layer that will turn turbulant and the upstream flow(flow outside the buondary layer) will still remain laminar ?

A flow is at pipe entry turbulent more or less due to changes in the flow pattern.

The friction to the wall leads to the apparition of a boundary layer which is laminar. The core is still turbulent.

If the conditions are fulfilled then the laminar layer grows so much that the core becomes also laminar. If not after a length the layer is unstable and it goes from laminar to turbulent. You cannot have a situation where the core is laminar and the boundary layer turbulent. You can have a laminar jet in a fluid volume where the fluid is turbulent and due to the mixing of layers progressively the jet goes as well turbulent.

Shameless, shameless plug... Turbulent: A Golf Game