Lecture Objectives Project 1 Boundary Conditions Meshing Unsteady

Lecture Objectives - Project 1 - Boundary Conditions - Meshing - Unsteady State Flow

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 T 1=30 C inlet T 2=20 C outlet inlet outlet T 1 T 2 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

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

AIRPAK Software

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

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

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

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

Grid type and resolution Tetra Structured Unstructured

Grid type and resolution Polyhedral mesh

Steady vs. Unsteady – State Flow Example of velocity at specific point for different activities walking fan 1 0. 9 0. 8 0. 7 0. 6 0. 5 0. 4 0. 3 0. 2 0. 1 0 60 s recuperation block 20 s recuperation Velocity (m/s) door 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 250 200 150 100 50 0 V_low Time (s)

Steady vs. Unsteady – State Flow Example of temperature profile at two specific points for different activities

Unsteady-state (Transient) CFD simulations Computationally very expensive Steps • Identify the problem – Many problems do not require unsteady-state sim. – Identify equations which should be unsteady-state • Define the simulation period • Define the required time steps • Adjust other simulation parameters – turbulence model, mesh, convergence criteria, number of required iterations, etc. – Require substantial investigation for each problem

Computationally very expensive Change of in volume dxdydz In Time Discretize equation System of equation for each time step ap and f are function of Dt f is function of previous value for x = 1) Solve the system using the simple algorithm 2) Change the boundary conditions 3) Update the coefficient 4) Solve the new system of equations A F

Simulation period and time step • Simulation period – Depends on the boundary condition of considered phenomenon • Time step – Depends on the time scale – With too large time step quasi-steady-state simulation Set of steady state simulations (there is no link in-between previous and next time step)

Time period T and time step Dt • Uniform • Variable – Linear – Piecewise • User defined

Transient boundaries • For unsteady-state airflow created by transient boundaries

Transient Calculation Iterations in different time steps Change of the variable in time

Steady-state, unsteady-state or quasi-steady-state Examples • • • Airflow around the airplane Airflow in the room Airflow around the building Injection of pollutant in experiment Flow in the automobile engine cylinder DNS simulation of flow in the boundary layer