Introduction to CFX -Transient Simulations

•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

•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

•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)

     Get free full Tutorial!
Please send me email:
cadxneter@hotmail.com