© Fluent Inc. 9/8/2015M1 Fluids Review TRN-98-004 CFD Modeling of Turbulent Flows.
-
Upload
patience-webb -
Category
Documents
-
view
231 -
download
10
Transcript of © Fluent Inc. 9/8/2015M1 Fluids Review TRN-98-004 CFD Modeling of Turbulent Flows.
© Fluent Inc. 04/19/23M1
Fluids Review TRN-98-004
CFD Modeling of Turbulent Flows
© Fluent Inc. 04/19/23M2
Fluids Review TRN-98-004
Overview
Properties of turbulence
Predicting turbulent flows
DNS LES RANS Models
Summary
© Fluent Inc. 04/19/23M3
Fluids Review TRN-98-004
Properties of Turbulence
Most flows encountered in industrial processes are turbulent.
Turbulent flows exhibit three-dimensional, unsteady, aperiodic motion.
Turbulence increases mixing of momentum, heat and species.
Turbulence mixing acts to dissipate momentum and the kinetic energy in the flow by viscosity acting to reduce velocity gradients.
Turbulent flows contain coherent structures that are deterministic events.
Time
u tuuu ' tu'
u
© Fluent Inc. 04/19/23M4
Fluids Review TRN-98-004
Role of Numerical Turbulence Modeling
An understanding of turbulence and the ability to predict turbulence for any
given application is invaluable for the engineer.
Examples:
Increased turbulence is needed in chemical mixing or heat transfer when fluids with dissimilar properties are brought together.
turbulence increases drag due to increased frictional forces.
Historically, experimental measurement of the system was the only option
available. This makes design optimization incredibly tedious.
© Fluent Inc. 04/19/23M5
Fluids Review TRN-98-004
Characteristics of the Engineering Turbulence Model
Numerical modeling of turbulence can serve to improve the engineers ability to analyze turbulent flow in design particularly when precise measurements cannot be obtained and when extensive experimentation is costly and time-consuming.
The ideal turbulence model should introduce minimal complexity while capturing the essence of the relevant physics.
© Fluent Inc. 04/19/23M6
Fluids Review TRN-98-004
Modeling Turbulent Flows
Turbulent flows can be modeled in a variety of ways. With increasing levels of complexity they are:
Correlations Moody chart, Nusselt number correlations
Integral equations Derive ODE’s from the equations of motion
Reynolds Averaged Navier Stokes or RANS equations Average the equations of motion over time Requires closure
Large Eddy Simulation or LES Solve Navier-Stokes equations for large scale motions of the flow. Model only the
small scale motions DNS
Navier-Stokes equations solved for all motions in the turbulent flow
© Fluent Inc. 04/19/23M7
Fluids Review TRN-98-004
Turbulence Modeling Approaches
Zero-Equation Models
One-Equation Models
Two-Equation Models Standard k- RNG k-
Second-order closureReynolds-Stress Model
Large-Eddy Simulation
Direct Numerical Simulation
IncludeMorePhysics
IncreaseComputationalCostPer Iteration
© Fluent Inc. 04/19/23M8
Fluids Review TRN-98-004
Direct Numerical Simulation (DNS)
Currently DNS is the most exact approach to modeling turbulence since no averaging is done or approximations are made
Since the smallest scales of turbulence are modeled, called the Kolmogoroff scale, the size of the grid must be scaled accordingly.
A DNS simulation scales with ReL3 (u´L/) where ReL ~ 0.01Re. Turbulent
flow past a cylinder would require at least (0.01 x 20,000)3 or 8 million cells
© Fluent Inc. 04/19/23M9
Fluids Review TRN-98-004
Direct Numerical Simulation (DNS)
Given the current processing speed and memory of the largest computers, only very modest Reynolds number flows with simple geometries are possible.
Advantages: DNS can be used as numerical flow visualization and can provide more information than experimental measurements; DNS can be used to understand the mechanisms of turbulent production and dissipation.
Disadvantages: Requires supercomputers; limited to simple geometries.
© Fluent Inc. 04/19/23M10
Fluids Review TRN-98-004
Large Eddy Simulation (LES)
LES is a three dimensional, time dependent and computationally expensive simulation, though less expensive than DNS.
LES solves the large scale motions and models the small scale motions of the turbulent flow.
The premise of LES is that the large scale motions or eddies contain the larger fraction of energy in the flow responsible for the transport of conserved properties while the small.
t
u
DNS
LES
DNS
LES
© Fluent Inc. 04/19/23M11
Fluids Review TRN-98-004
Large Eddy Simulation (LES)
The large scale components of the flow field are filtered from the small scale components using a wavelength criteria related to the size of the eddies
The filter produces the following equation used to model the small scale motions
where
The inequality is then modeled as
ij is called the subgrid scale Reynolds Stress. Different subgrid scale models are available to approximate ij .
i
j
j
i
jij
jii
x
u
x
u
xx
p
x
uu
t
u
jiji uuuu
jijiij uuuu
© Fluent Inc. 04/19/23M12
Fluids Review TRN-98-004
Large Eddy Simulation (LES)
Mixing plane between two streams with unequal scalar concentrations Unsteady vortex motions with growing length scale
© Fluent Inc. 04/19/23M13
Fluids Review TRN-98-004
Reynolds Averaged Navier-Stokes (RANS) Models
DNS and LES can produce an overwhelming quantity of detailed information about a flow structure. Generally, in engineering flows, such levels on instantaneous information is not required.
Typical engineering flows are focused on obtaining a few quantitative of the turbulent flow. For example, average wall shear stress, pressure and velocity field distribution, degree of mixedness in a stirred tank, etc.
The approach would be to model turbulence by averaging the unsteadiness of the turbulence.
This averaging process creates terms that cannot be solved analytically but must be modeled.
This modeling approach has been around for 30 years and is the basis of most engineering turbulence calculations.
© Fluent Inc. 04/19/23M14
Fluids Review TRN-98-004
Velocity or a scalar quantity can be represented as the sum of the mean value and the fluctuation about the mean value as:
Using the above relationship for velocity(let = u) in the Navier-Stokes equations gives (as momentum equation for incompressible flows with body forces).
The Reynolds Stresses cannot be represented uniquely in terms of mean quantities and the above equation is not closed. Closure involves modeling the Reynolds Stresses.
RANS Equations
t'
i
j
j
i
jijiji
j
i
x
u
x
u
xx
puuuu
xt
u
© Fluent Inc. 04/19/23M15
Fluids Review TRN-98-004
Closure of RANS equations
The RANS equations contain more unknowns than equations.
The unknowns are the Reynolds Stress terms.
Closure Models are:
zero-equation turbulence models Mixing length model no transport equation used
one-equation turbulence models transport equation modeled for turbulent kinetic energy k
two-equation models more complete by modeling transport equation for turbulent kinetic energy k and
eddy dissipation second-order closure
Reynolds Stress Model does not use Bousinesq approximation as first-order closure models
© Fluent Inc. 04/19/23M16
Fluids Review TRN-98-004
RANS equations require closure for Reynolds stresses and the effect of turbulence can be represented as an increased viscosity
The turbulent viscosity is correlated with turbulent kinetic energy k and the dissipation rate of turbulent kinetic energy
Modeling Turbulent Stresses in Two-Equation Models
2kCt Turbulent Viscosity:
Boussinesq Hypothesis:eddy viscosity model
i
j
j
itijji x
u
x
ukuu
3
2
© Fluent Inc. 04/19/23M17
Fluids Review TRN-98-004
Turbulent kinetic energy and dissipation
Transport equations for turbulent kinetic energy and dissipation rate are solved so that turbulent viscosity can be computed for RANS equations.
Turbulent Kinetic Energy:
Dissipation Rate of Turbulent Kinetic Energy:
zzyyxxii uuuuuuuuk 2
1
2
1
i
j
j
i
j
i
x
u
x
u
x
u
© Fluent Inc. 04/19/23M18
Fluids Review TRN-98-004
Standard k- ModelTurbulent Kinetic Energy
Dissipation Rate
21, ,, CCk are empirical constants
(equations written for steady, incompressible flow w/o body forces)
Convection Generation DiffusionDissipation
ikt
ii
j
j
i
i
jt
ii x
k
xx
U
x
U
x
U
x
kU )(
DestructionConvection Generation Diffusion
kC
xxx
U
x
U
x
U
kC
xU
it
ii
j
j
i
i
jt
ii
2
21 )(
© Fluent Inc. 04/19/23M19
Fluids Review TRN-98-004
Deficiencies of the Boussinesq Approximation The two-equation models based on the eddy viscosity approximation provide
excellent predictions for many flows of engineering interest.
Applications for which the approximation is weak typically are flows with extra rate of strain (due to isotropic turbulent viscosity assumption). Examples of these are:
flows over boundaries with strong curvature flows in ducts with secondary motions flows with boundary layer separation flows in rotating and stratified fluid strongly three dimensional flows
The RNG k- model is an improvement over the standard k- for these classes
of flow by incorporating the influence of additional strains rates.
A higher-order closure approximation can be also applied to include wider class of problems including those with extra rates of strain.
© Fluent Inc. 04/19/23M20
Fluids Review TRN-98-004
RNG k- ModelTurbulent Kinetic Energy
Dissipation Rate
Convection DiffusionDissipation
ik
it
ii x
k
xS
x
kU eff
2
Generation
j
i
i
jijijij x
U
x
USSSS
2
1,2
where
are derived using RNG theory 21, ,, CCk
(equations written for steady, incompressible flow w/o body forces)
Additional termrelated to mean strain& turbulence quantities
Convection Generation Diffusion Destruction
RkC
xxS
kC
xU
iit
ii
2
2eff2
1
© Fluent Inc. 04/19/23M21
Fluids Review TRN-98-004
Second-Order Closure models
The Second-Order Closure Models include the effects of streamline curvature, sudden changes in strain rate, secondary motions, etc.
This class of models is more complex and computationally intensive than the RANS models
The Reynolds-Stress Model (RSM) is a second-order closure model and gives rise to 6 Reynolds stress equations and the dissipation equation
k
ijkijijij
k
ijk x
JP
x
RU
Reynolds Stress Transport Equation
GenerationPressure-Strain redistribution Dissipation
DiffusionConvection
© Fluent Inc. 04/19/23M22
Fluids Review TRN-98-004
Reynolds Stress Model
Generation
k
ikj
k
jkiij x
Uuu
x
UuuP
i
j
j
iij x
u
x
up
k
j
k
iij x
u
x
u
2
Pressure-StrainRedistribution
Dissipation
TurbulentDiffusion
(modeled)
(related to )
(modeled)
(computed)
(equations written for steady, incompressible flow w/o body forces)
Pressure/velocity fluctuations
Turbulenttransport
kjiikjjkiijk uuuupupJ
© Fluent Inc. 04/19/23M23
Fluids Review TRN-98-004
Near Wall Treatment
The RANS turbulence models require a special treatment of the mean and turbulence quantities at wall boundaries and will not predict correct near-wall behavior if integrated down to the wall
Special near-wall treatment is required
Standard wall functions
Nonequilibrium wall functions
Two-layer zonal model
© Fluent Inc. 04/19/23M24
Fluids Review TRN-98-004
Comparison: RNG k- vs. Standard k-
© Fluent Inc. 04/19/23M25
Fluids Review TRN-98-004
Summary
Turbulence modeling comes in varying degrees of complexity. Determining the right choice of turbulence model depends on the detail of results expected.
DNS and LES are still far from being engineering tools but in the near future this will be possible.
Two-equation models are widely used for their relatively simple overhead. However, increased complexity of the turbulent flow reduces the adequacy of the models.
Improvements to the two-equation models to incorporate extra strain rates, and the second-order closure RSM model provide the extra terms to model complex engineering turbulent flows.