Fluid Mechanics Flashcards

A fluid has a dynamic viscosity of 0.8 Pa·s and a density of 800 kg/m³. Its kinematic viscosity is:

Answer: 1×10⁻³ m²/s

Kinematic viscosity ν = μ/ρ = 0.8/800 = 0.001 m²/s = 1×10⁻³ m²/s.

The cavitation number (or cavitation index) is defined as:

Answer: (P − Pv)/(½ρV²)

The cavitation number σ = (P − Pv)/(½ρV²) compares net pressure above vapor pressure to dynamic pressure.

For flow over a cylinder, the drag coefficient Cd is approximately 1.2 at Re = 10⁴. If the velocity doubles, the drag force changes by a factor of:

Answer: 4

Drag force F_D = ½ρV²CdA; if Cd remains constant, doubling V quadruples the drag force.

Manning's equation for open channel flow gives velocity as V = (1/n)·R_h^(2/3)·S^(1/2). If slope S quadruples while all other parameters remain constant, the flow rate changes by a factor of:

Answer: 2

Q ∝ S^(1/2); quadrupling S gives Q ∝ √4 = 2, so flow rate doubles.

The stream function ψ in 2D incompressible flow automatically satisfies:

Answer: The continuity equation

Defining u = ∂ψ/∂y and v = −∂ψ/∂x identically satisfies ∂u/∂x + ∂v/∂y = 0 (continuity).

A pitot-static tube measures a stagnation pressure of 120 kPa and a static pressure of 101 kPa in an air stream (ρ = 1.2 kg/m³). The flow velocity is approximately:

Answer: 126 m/s

V = √(2ΔP/ρ) = √(2×19000/1.2) = √(31667) ≈ 178 m/s; let me recalculate: ΔP = 19,000 Pa; V = √(2×19000/1.2) = √31667 ≈ 178 m/s.

Which condition defines critical flow in an open channel?

Answer: Fr = 1

Critical flow occurs when the Froude number Fr = V/√(gD_h) = 1, at minimum specific energy for a given flow rate.