Boundary Layer Flow in Water Turbine Blading and its Effect on Model Similitude

The boundary layer flow on the blades of a water turbine are investigated and it is shown that at model Reynolds numbers the boundary layer is largely laminar. The effect of this on scale-up formulae is pointed out. It is shown that the model blades are hydraulically smooth while the full size blades are hydraulically rough. The use of laminar profiles in water turbines is discussed and their susceptibility to cavitation pointed out. The possibility of laminar separation in model water turbines causing an increase in loss is discussed. The influence of Reynolds number on the cavitation performance of the blades and on aerodynamic cascade tests is briefly indicated.