•Nearly
all flows in nature are transient!
–Steady-state assumption is possible if we:
•Ignore transient
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
–transient Heat Transfer – transient heating and cooling
–Many more.
•Naturally occurring
transients
–transient 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 transients
–Time-dependent
boundary conditions, source terms drive the transient flow field
–Examples:
pulsing flow in a nozzle, rotor-stator interaction in a turbine stage
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