Lecture Objectives Learn about Implementation of Boundary Conditions Meshing Project 1

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.

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.

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

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

Modeling of Turbulent Viscosity in boundary layer forced convection natural convection

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

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

Grid type and resolution Hexa Uniform hexa Nonuniform hexa Unstructured hexa Body-fitted coordinate hexa - Structured Unstructured Tetra mesh Structured Polyhedral mesh

Grid type and resolution hexa Unstructured hexa (2-D) Uniform boundary-fitted, structured grid Nonuniform (2-D)

Grid type and resolution Tetra Structured Unstructured

Grid type and resolution Polyhedral mesh

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

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

AIRPAK Software

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