2. PRINCIPLE
The Tesla turbine works on the boundary layer principle. It was defined by Ludwig Prandtl. According to this principle, when the fluid passes over the discs the fluid particles adhere to the disc thus causing a condition of no slip. Thus, the velocity of the fluid near to the disc will be equal to that of the disc. In case of the disc being stationary, its velocity will be zero. The fluid velocity gradually increases as we move away from the disc surface, thus a velocity gradient is observed. This gradient exists in a direction normal to the surface, in a narrow region in its vicinity. This region is called boundary layer.
The boundary layer consists of two major regions: Laminar boundary layer and turbulent boundary layer. If U is the velocity of the fluid, in the laminar boundary layer, the velocity gradient dU/dy exists. Thus flow in this region is perfectly laminar. In the region between the laminar and turbulent boundary layers, the flow transitions from laminar flow to turbulent flow and hence its name. In turbulent boundary layer, the flow is completely turbulent.