1. Lecture Objectives: Start using CFD Software Class project 1
Learn about Implementation of Boundary Conditions

2. CFD Software How to Define in Airpark (Fluent): Simulation domain
Boundary conditions
Turbulence Model parameters
Numerical parameters
Control the simulation process
Show the resuts ….

3. Project 1 Pat a) Numerical diffusion
The purpose of this project part is to analyze how mesh size and orientation affects
the accuracy of result.
outlet
inlet T1 T2
T1=30C
T2=20C
outlet
inlet
Pat b) Learn how to:
1) Model: geometry, heat sources, concentration sources, diffusers,
2) Select important simulation parameters
3) Generate appropriate mesh
4) Check the results
5) Present the results

4. AIRPAK Software

5. Example Modeling Problem
Office ventilation (tutorial 1 in handouts posted on the website)
Boundaries:
Geometry:

6. Surface boundaries wall functions
Wall surface
Introduce velocity
temperature and concentration
Use wall functions to model the micro-flow in the vicinity of surface
Using relatively large mesh (cell) size.

7. Surface boundaries wall functions
Course mesh distribution in the vicinity of surface Y
Wall surface
Velocity in the first cell will depend on the distance y.

8. Surface boundary conditions and log-wall functions
E is the integration constant and
y* is a length scale
Friction velocity
u+=V/Vt
y*=(n/Vt)
y+=y/y*
k- von Karman's constant
The assumption of ‘constant shear stress’ is used here.
Constants k = 0.41 and E = 8.43 fit well to a range of boundary layer flows.
Surface cells
Turbulent profile
Laminar sub-layer

9. K-e turbulence model in boundary layer
Wall shear stress
Eddy viscosity V
Wall function for e
Wall function for k

10. Modeling of Turbulent Viscosity in boundary layer
forced convection
natural convection

11. Temperature and concentration gradient in boundary layer
Depend on velocity field
Temperature
q=h(Ts-Tair)
Concentration
F=hc(Cs-Cair/m)
m=Dair/Ds
m- segregation coefficient
h = f(V) = f(k,e)
Tair Ts
Into source term
of energy equation
hC = f(V, material prop.)
Cair Cs

12. Example of BC: Inlets or Diffusers (Various types)
Valve diffuser
swirl diffusers
ceiling diffuser
wall or ceiling
floor