Introductory FLUENT Training-Transient Flow Modeling

Motivation

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.

Origins of Transient Flow

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

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 veriables 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)

Transient Flow Modeling Workflow

Enable the transient solver.

Set up physical models and boundary conditions as usual.

– Transient boundary conditions are possible – you can use either a UDF or profile to accomplish this.

Prescribe initial conditions

– Best to use a physically realistic initial condition, such as a steady solution.

Assign solver settings and configure solution monitors.

Configure animations and data output/sampling options

Select time step and max iterations per time step

Prescribe the number of time steps.

Run the calculations (Iterate)

Enabling the Transient Solver

To enable the transient solver, select the Transient button on the General problem setup form:

Before performing iterations, you will need to set some additional controls.

– Solver settings

– Animations

– Data export / Autosave options

Selecting the Transient Time Step Size

Time step size, Dt, is set in the Run Calculation form.

– Dt must be small enough to resolve time-dependent features; make sure the convergence is reached within the number of Max Iterations per Time Step

– The order or magnitude of an appropriate time step size can be estimated as:

– Time step size estimate can also be chosen so that the transient characteristics of the flow can be resolved (e.g. flow within a known period of fluctuations)

To iterate without advancing in time, specify zero time steps. This will instruct the solver to converge the current time step only.

The PISO scheme may aid in accelerating convergence for many transient flows (set in the Solution Methods form).

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