Motivation
Nearly all flows in nature are transient!
-Steady-state assumption is possible if we:
Ignore unsteady fluctuations
Employ ensemble/time-averaging to remove unsteadiness (this is what is done in modeling turbulence)
In CFD, steady-state methods are preferred
-Lower computational cost
-Easier to postprocess and analyze
Many applications require resolution of transient flow:
-Aerodynamics (aircraft, land vehicles,etc.) – vortex shedding
-Rotating Machinery – rotor/stator interaction, stall, surge
-Multiphase Flows – free surfaces, bubble dynamics
-Deforming Domains – in-cylinder combustion, store separation
-Unsteady Heat Transfer – transient heating and cooling
-Many more
Origins of Transient Flow
Natural unsteadiness
-Unsteady flow due to growth of instabilities within the fluid or a non-equilibrium initial fluid state
-Examples: natural convection flows, turbulent eddies of all scales, fluid waves (gravity waves, shock waves)
Forced unsteadiness
-Time-dependent boundary conditions, source terms drive the unsteady flow field
-Examples: pulsing flow in a nozzle, rotor-stator interaction in a turbine stage
Transient CFD Analysis
Simulate a transient flow field over a specified time period
-Solution may approach:
.Steady-state solution – Flow variables stop changing with time
.Time-periodic solution – Flow variables fluctuate with repeating pattern
-Your goal may also be simply to analyze the flow over a prescribed time interval.
.Free surface flows
.Moving shock waves
.Etc.
Extract quantities of interest
-Natural frequencies (e.g. Strouhal Number)
-Time-averaged and/or RMS values
-Time-related parameters (e.g. time required to cool a hot solid, residence time of a pollutant)
-Spectral data – fast Fourier transform (FFT)
How to Solve a Transient Case
Transient simulations are solved by computing a solution for many discrete points in time
At each time point we must iterate to the solution
Similar setup to steady state
The general workflow is
1.Set the Analysis Type to Transient
2.Specify the transient time duration to solve and the time step size
3.Set up physical models and boundary conditions as usual
-Boundary conditions may change with time
4.Prescribe initial conditions
-Best to use a physically realistic initial condition, such as a steady solution
5.Assign solver settings
6.Configure transient results files, transient statistics, monitors points
7.Run the solver
1. Analysis Type
Edit Analysis Type in the Outline tree and set the option to Transient
2. Time Duration and Time Step
Set the Time Duration
-This controls when the simulation will end
Options are:
-Total Time
When restarting this time carriers over
-Time Per Run
Ignores any time completed in previous runs
-Maximum Number of Timesteps
The number of timesteps to perform, including any completed in previous runs
-Number of Timesteps per Run
For this run only. Ignores previously completed timesteps
Set the Time Step size
-This controls the spacing in time between the solutions points
Options are:
-Timesteps / Timesteps for the Run
Various formats accepted, e.g.
0.001
0.001, 0.002, 0.002, 0.003
5*0.001, 10*0.05, 20*0.06
http://www.cadfamily.com/html/Article/Introduction%20to%20CFX%20-Transient%20Simulations_770_1.htm
http://www.cadfamily.com/html/Article/Introduction%20to%20CFX%20-Transient%20Simulations_770_2.htm
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