CPCE Fluid Flow & Heat Transfer Analysis 4 — Questions and Answers
Question 1: In CFD, what is the primary purpose of the wall y+ value in turbulent boundary layer modeling?
- To determine the mesh skewness near walls
- To ensure appropriate mesh resolution for the chosen near-wall treatment (Correct answer)
- To calculate the turbulent Prandtl number
- To set the turbulent intensity at wall boundaries
Correct answer: To ensure appropriate mesh resolution for the chosen near-wall treatment
The dimensionless wall distance y+ determines whether the near-wall mesh is fine enough to resolve the viscous sublayer or whether wall functions should be used.
Question 2: What is the physical significance of the thermal entry length in duct flow?
- The length required for fully turbulent flow to develop
- The duct length over which the thermal boundary layer grows to fill the duct (Correct answer)
- The distance at which maximum wall temperature occurs
- The length needed for pressure drop to stabilize
Correct answer: The duct length over which the thermal boundary layer grows to fill the duct
The thermal entry length is the downstream distance from the inlet where the thermal boundary layer fully develops and heat transfer transitions to the fully-developed profile.
Question 3: Which CFD boundary condition is most appropriate for a fully-developed flow outlet in an internal flow simulation?
- Constant pressure outlet with specified value
- Zero-gradient (outflow) condition for all variables (Correct answer)
- Symmetric boundary condition
- Periodic boundary condition
Correct answer: Zero-gradient (outflow) condition for all variables
A zero-gradient (outflow) condition assumes that flow variables do not change in the streamwise direction at the outlet, appropriate for fully-developed exit conditions.
Question 4: In the energy equation for CFD, what does the viscous dissipation term represent?
- Thermal radiation absorption
- Conversion of kinetic energy to heat through viscous stresses (Correct answer)
- Thermal energy stored in the fluid
- Heat exchanged with solid boundaries
Correct answer: Conversion of kinetic energy to heat through viscous stresses
Viscous dissipation represents the irreversible conversion of mechanical energy to thermal energy due to viscous stresses within the fluid, becoming important at high velocities or in highly viscous flows.
Question 5: What is the key characteristic of a pressure-based CFD solver compared to a density-based solver?
- Better suited for supersonic and hypersonic flow
- Uses pressure as the primary variable for incompressible and low-speed flows (Correct answer)
- Solves only the momentum equations
- Cannot handle heat transfer problems
Correct answer: Uses pressure as the primary variable for incompressible and low-speed flows
Pressure-based solvers use the pressure field as the primary variable and are well-suited for incompressible and mildly compressible flows where density variations are small.
Question 6: In heat exchanger CFD analysis, what does the NTU (Number of Transfer Units) method help predict?
- Turbulence intensity distribution
- Heat exchanger effectiveness from inlet conditions (Correct answer)
- Pressure drop across tube bundles
- Wall temperature distribution on tube surfaces
Correct answer: Heat exchanger effectiveness from inlet conditions
The NTU-effectiveness method calculates heat exchanger performance using the number of transfer units and capacity rate ratio without requiring outlet temperatures.
Question 7: For multiphase flow CFD, what distinguishes the Volume of Fluid (VOF) method?
- It tracks individual particle trajectories
- It uses a volume fraction to capture sharp interfaces between phases (Correct answer)
- It models dispersed bubbles with Lagrangian approach
- It solves separate momentum equations for each phase
Correct answer: It uses a volume fraction to capture sharp interfaces between phases
The VOF method uses a volume fraction field (0 to 1) to represent the interface between immiscible phases, making it effective for capturing sharp free-surface flows.
CPCE Fluid Flow & Heat Transfer Analysis 4
In CFD, what is the primary purpose of the wall y+ value in turbulent boundary layer modeling?