CFX13_08_流体瞬态分析.pptVIP

Transient Simulations 8-* ANSYS, Inc. Proprietary ? 2010 ANSYS, Inc. All rights reserved. Release 13.0 December 2010 Training Manual 8-* ANSYS, Inc. Proprietary ? 2010 ANSYS, Inc. All rights reserved. Release 13.0 December 2010 Chapter 8CFX流体瞬态分析 CFX瞬态分析  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 Kelvin-Helmholtz Cloud Instability Rotor-Stator Interaction in an Axial Compressor 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 so