Lecture Objectives

• Meshing

• Unsteady State CFD

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

boundary-fitted, structured grid

Uniform

Nonuniform (2-D)

Unstructured hexa (2-D)

Grid type and resolutionTetra

Structured

Unstructured

Grid type and resolutionPolyhedral mesh

Computational resource saving by mesh type

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

SgradΓVdiv ρτ

ρ eff,

Change of in volume dxdydz In Time

System of equation for each time step

fΦaΦaΦaΦaΦaΦaΦa LLHHNNSSWWEEPP

Discretize equation

x =

FΦA

ap and f are function of f is function of previous value for

1) Solve the system using the simple algorithm 2) Change the boundary conditions3) Update the coefficient4) Solve the new system of equations

τ

)(ρ

τρ

τ

Steady-state, unsteady-stateor quasi-steady-state

Examples• Airflow around the airplane• Airflow in the room• Airflow around the building• Injection of pollutant in the chamber experiment• Flow in the automobile engine cylinder• DNS simulation of flow in the boundary layer

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

0~τ

ρτ

ρ

Set of steady state simulations

(there is no link in-between previous and next time step)

Time step

• Uniform

• Variable– Linear– Piecewise

• User defined

Transient boundaries

• For unsteady-state

airflow created

by transient

boundaries