CPCE Fluid Flow & Heat Transfer Analysis 5 — Questions and Answers
Question 1: What is the role of the Boussinesq approximation in natural convection CFD simulations?
- Linearizes turbulent viscosity variations with temperature
- Treats density as constant except in the buoyancy term of the momentum equation (Correct answer)
- Neglects viscous dissipation in the energy equation
- Assumes incompressible flow with constant thermal conductivity
Correct answer: Treats density as constant except in the buoyancy term of the momentum equation
The Boussinesq approximation treats density as constant throughout except in the gravitational body force term, where density variations due to temperature drive the buoyancy force.
Question 2: In CFD residual monitoring, what do normalized residuals below 10⁻⁴ typically indicate?
- The simulation has diverged
- Adequate convergence for most engineering applications (Correct answer)
- First-order accuracy has been achieved
- The mesh requires further refinement
Correct answer: Adequate convergence for most engineering applications
Normalized residuals below 10⁻⁴ are generally accepted as indicating adequate convergence for engineering-level accuracy in steady-state CFD simulations.
Question 3: Which phenomenon is captured by the turbulent Prandtl number in CFD heat transfer modeling?
- The ratio of molecular to turbulent viscosity
- The ratio of turbulent momentum diffusivity to turbulent thermal diffusivity (Correct answer)
- The enhancement of heat transfer due to turbulence intensity
- The relationship between wall temperature and bulk temperature
Correct answer: The ratio of turbulent momentum diffusivity to turbulent thermal diffusivity
The turbulent Prandtl number (Pr_t) represents the ratio of eddy viscosity to eddy thermal diffusivity, typically assumed near 0.85-0.9 for gases in wall-bounded flows.
Question 4: In CFD simulation of compressible flow, what occurs at a normal shock wave within the computational domain?
- Gradual deceleration from supersonic to subsonic velocity
- Abrupt discontinuity in velocity, pressure, and temperature (Correct answer)
- Smooth isentropic pressure recovery
- Boundary layer separation without pressure change
Correct answer: Abrupt discontinuity in velocity, pressure, and temperature
A normal shock represents an abrupt discontinuity where flow transitions from supersonic to subsonic with sudden increases in pressure, temperature, and density and decrease in velocity.
Question 5: What is the primary advantage of Detached Eddy Simulation (DES) over standard RANS turbulence models?
- DES requires fewer mesh cells near solid walls
- DES resolves large-scale turbulent structures in separated flow regions (Correct answer)
- DES eliminates the need for any turbulence modeling constants
- DES automatically switches to DNS when Reynolds number is low
Correct answer: DES resolves large-scale turbulent structures in separated flow regions
DES uses RANS near walls and switches to LES-like treatment in separated regions, better capturing large-scale unsteady turbulent structures in wakes and separated shear layers.
Question 6: For a fin array CFD simulation, which parameter most significantly affects the overall thermal resistance?
- Fin material density
- Fin spacing and height determining flow passage and surface area (Correct answer)
- Fin base thickness relative to height
- Inlet flow turbulence intensity
Correct answer: Fin spacing and height determining flow passage and surface area
Fin spacing controls flow velocity and pressure drop while fin height determines surface area, making their combined effect the dominant factor in thermal resistance optimization.
Question 7: In CFD analysis of a centrifugal pump, what reference frame technique is commonly used to simulate the rotating impeller?
- Sliding mesh with full transient rotation
- Multiple Reference Frame (MRF) or rotating reference frame approach (Correct answer)
- Overset mesh with dynamic remeshing
- Steady-state body force correction method
Correct answer: Multiple Reference Frame (MRF) or rotating reference frame approach
The Multiple Reference Frame (MRF) approach models the impeller region in a rotating frame and the volute in a stationary frame, allowing steady-state solution of rotating machinery with reasonable accuracy.
CPCE Fluid Flow & Heat Transfer Analysis 5
What is the role of the Boussinesq approximation in natural convection CFD simulations?