湍流及气动噪声仿真培训

Modelling
• Numerous developments: – Correlation based models – Low-Re models – en linear stability – … – DNS
Almost all industrial CFD simulations are calculated without a transition model
F1 = 1 F1 → 0
in the inner layer in the outler layer
Embedded LES • 嵌入式大涡模拟
– 也可以和DES/SAS模型联用
E-LES: Spatially decaying turbulence
E-LES: Fully developed channel flow RANS LES Re=395
RANS
Lt ≤ c∆
?
LES
• • •
Lt ≥ c∆
Overcomes threshold limit of LES Strong grid sensitivity in RANS region Open question concerning transition region between RANS and LES
– RANS equations in bl – LES „ detached “ regions
• Switch of model
– Based on ratio of turbulent length-scale to grid size – Different numerical treatment in RANS and LES regions
• SAS (Scale-Adaptive Simulation)
– – Extends URANS to many technical flows Provides “LES”-content in unsteady regions.
SAห้องสมุดไป่ตู้-URANS
Hybrid Models
• Hybrid Model:
主题
• • • • • ANSYS CFD ANSYS CFD ANSYS CFD ANSYS CFD ANSYS CFD 湍流模型新发展 湍流模型使用技巧 湍流模型验证案例 流动噪声模型简介 噪声计算案例
Industrial Turbulent Flows
large-scale unsteadiness
thick BL, mildly separated flows
RANS Reynolds-Stress Models:
1) LRR Reynolds Stress 2) QI Reynolds Stress 3) Speziale, Sarkar and Gatski Reynolds Stress 4) SMC-ω model 5) Baseline (BSL) Reynolds' Stress model
Eddy Simulation Models:
1) Large Eddy Simulation (LES) [transient] 2) Detached Eddy Simulation (DES)* [transient] 3) Scale Adaptive Simulation SST (SAS)* [transient]
Transition Modelling in Industrial CFD
• Low-Re models (only bypass transition)
– – –
Based on transport equations for e.g. k and ε (compatible with modern CFD codes) Cannot be calibrated independently of viscous sublayer model Poor accuracy and robustness – not used in industry
•
Correlation based model
– – –
Reasonably accurate Correlations can be found for many different transition mechanisms (e.g. FSTI, dp/dx, Roughness) Not compatible with 3D flows and unstructured/parallel CFD codes – non-local formulation
transitional flows
如何选择合适的湍流模型？
rotating & swirling flows crossflow/secondary flows thin shear flows separated & recirculating flows rapidly strained flows
URANS
• DES (Detached Eddy Simulation)
– First industrial-strength model for high-Re with LEScontent – Increased complexity (grid sensitivity) due to explicit mix of to modelling concepts
k ε= ε
4 F1 = tanh arg1
(
)
k 500ν 4 ρ σ ω 2 k arg1 = min max β *ω y , y 2 ω , CD y 2 kω 1 ∂k ∂ω , 10 − 20 CDkω = max 2 ρσ ω 2 ω ∂x j ∂x j
Mean velocity values inside LES zone.
Viscosity ratio on iso-surfaces of q-criterion (-500)
Turbulence Models in CFX
• A large number of turbulence models are available in CFX, some have very specific applications while others can be applied to a wider class of flows with a reasonable degree of confidence
Dk Dk F1 ρ + ⋅ ⋅ ⋅ + (1 − F1 ) ρ + ⋅ ⋅ ⋅ Dt inner Dt outer φ = F1 φ1 + (1 − F1 )φ2 where φ = β , σ k , σ ω , γ
k-ω model transformed from std. k-3 ε2 model Modified Wilcox kmodel Wilcox’ original kωω model Wall
Goal – correlation based model using transport equations
Transition Model Formulation in CFX
• 2 Transport Equations
– Intermittency (γ) Equation
• Fraction of turbulent vs laminar flow • Transition onset controlled by relation between vorticity Reynolds number and Reθt • Used to pass information about freestream conditions into b.l. e.g. impinging wakes
*
Not available in the ANSYS CFD-Flo product
Transition Modelling in Industrial CFD Effects
• • • • • Re number effects Heat transfer Wall shear stress Separation behaviour Efficiency of many technical devices
– Transition Onset Reynolds number Equation
Unsteady Models
• URANS
– URANS gives unphysical single mode unsteady behavior
• LES (Large Eddy Simulation)
– Too expensive for most industrial flows due to high resolution requirements in boundary layers
RANS Eddy-viscosity Models:
1) 2) 3) 4) 5) 6) 7) 8) Zero Equation model. SA model Standard k-ε model. RNG k-ε model. Standard k-ω model. Baseline (BSL) zonal k-ω based model. SST zonal k-ω based model. (k-ε)1E model.
Turbulence Models in FLUENT
One-Equation Model Spalart-Allmaras Two-Equation Models Standard k–ε RNG k–ε Realizable k–ε Standard k–ω SST k–ω k-kl-ω Transition Model (3 eq.) SST Transition Model (4 eq.) 4-Equation v2f Model Reynolds Stress Model Detached Eddy Simulation SAS Large Eddy Simulation
•
eN method (only natural transition)
– – – –
Very accurate predictions for 2D airfoils (low FSTI) N-S codes are not accurate enough to evaluate stability equations Extension to generic 3D flows very difficult (impossible?) Cannot account of non-linear effects (e.g. high FSTI, roughness)
ANSYS CFD 湍流模型 及流动噪声高级应用培训
姓名 刘伟 公司 安世亚太
目录
• • • • • ANSYS ANSYS ANSYS ANSYS ANSYS CFD CFD CFD CFD CFD 湍流模型新发展 湍流模型使用技巧 湍流模型验证案例 流动噪声模型简介 噪声计算案例
主题
• • • • • ANSYS CFD ANSYS CFD ANSYS CFD ANSYS CFD ANSYS CFD 湍流模型新发展 湍流模型使用技巧 湍流模型验证案例 流动噪声模型简介 噪声计算案例
Increase in computati onal cost
RANS Models
Menter’s SST k-ω Model
• The two sets of equations and the model constants are blended in such a way that the resulting equation set transitions smoothly from one equation to another.