文档介绍：Fluent Software Training
TRN-99-003
Modeling Turbulent Flows
D1 © Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
What is Turbulence?
u Unsteady, aperiodic motion in which all three ponents
fluctuate Õ mixing matter, momentum, and energy.
u pose velocity into mean and fluctuating parts:
Ui(t) º Ui + ui(t)
ui(t)
U i (t)
Ui
Time
u Similar fluctuations for pressure, temperature, and species
concentration values.
D2 © Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Why Model Turbulence?
u Direct numerical simulation of governing equations is only possible for
simple low-Re flows.
u Instead, we solve Reynolds Averaged Navier-Stokes (RANS)
equations:
2
¶Ui ¶p ¶ Ui ¶Rij (steady, pressible flow
ρUk =  + µ + w/o body forces)
¶xk ¶xi ¶x j¶x j ¶x j
where R ij =  ρ u i u j (Reynolds stresses)
u Time-averaged statistics of turbulent velocity fluctuations are modeled
using functions containing empirical constants and information about
the mean flow.
u Large Eddy Simulation numerically resolves large eddies and models
small eddies.
D3 © Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
Is the Flow Turbulent?
External Flows ρUL
where ReL º
5 µ
Rex ³5´10 along a surface
L = x, D, Dh, etc.
Re ³ 20,000 around an obstacle
D Other factors such as free-stream
turbulence, surface conditions, and
Internal Flows disturbances may cause earlier
Re ³ 2,300 transition to turbulent flow.
Dh
Natural Convection
gβDTL3ρ
Ra ³108 1010 where Raº
µα
D4 © Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
plex is the Flow?
u Extra strain rates
l Streamline curvature
l Lateral divergence
l Acceleration or deceleration
l Swirl
l Recirculation (or separation)
l Secondary flow
u 3D perturbations
u Transpiration (blowing/suction)
u Free-stream turbulence
u Interacting shear layers
D5 © Fluent Inc. 2/20/01
Fluent Software Training
TRN-99-003
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