Fluid Mechanics: Navier-Stokes, Boundary Layer Theory & Moody Chart
Fluid dynamics principles, pipe friction losses, boundary layer separation, and Pelton/Francis/Kaplan turbine selection.
In-Depth Interview Questions & Model Solutions
Q1Explain Boundary Layer Separation over a curved surface and methods used to prevent it in aerodynamic/hydraulic design.
When a viscous fluid flows past a body experiencing an adverse pressure gradient (dp/dx > 0, where pressure increases in the flow direction), fluid particles in the boundary layer decelerate due to both fluid friction and opposing pressure. At the point where wall shear stress becomes zero `(∂u/∂y)|_y=0 = 0`, the boundary layer separates from the wall, creating a recirculating wake region with high form/pressure drag. Prevention methods: (1) Streamlining body geometry, (2) Boundary layer suction (removing low-energy fluid), (3) Boundary layer blowing (injecting high-velocity fluid), (4) Vortex generators to energize the turbulent boundary layer.
- Separation occurs under adverse pressure gradient (dp/dx > 0).
- Turbulent boundary layers resist separation better than laminar due to momentum mixing.
- Cavitation occurs when local static pressure drops below saturation vapor pressure of the liquid.
Q2How do you select between Pelton Wheel, Francis Turbine, and Kaplan Turbine based on Head and Specific Speed?
Selection criteria based on Net Head (H) and Specific Speed (Ns): (1) Pelton Wheel (Impulse Turbine): High head (H > 250 m), low discharge, low specific speed (Ns = 10 to 50). (2) Francis Turbine (Mixed/Radial Inward Reaction): Medium head (H = 60 to 250 m), medium discharge, medium specific speed (Ns = 60 to 300). (3) Kaplan / Propeller Turbine (Axial Flow Reaction with adjustable blades): Low head (H < 60 m), high discharge, high specific speed (Ns = 300 to 1000).
- Pelton: Impulse, atmospheric casing, bucket splitter.
- Francis: Reaction, spiral casing, draft tube converts kinetic energy into pressure.
- Kaplan: Axial flow with adjustable runner blades to maintain high efficiency across variable load.
Technical Panel Interview Strategy Tips
- Always mention the function of the Draft Tube in reaction turbines to recover kinetic energy at runner exit.