Turbulence Models Flashcards
What are the three types of viscous flows?
Laminar
Turbulent
Transitional
What are characteristics of turbulent flow?
Highly fluctuating velocity field both in space and time
Nearly all macroscopic flows are turbulent
Typically manifest themselves as eddies, leading to the concept of energy cascade
What is Kolmogorov Hypothesis?
At sufficiently high Reynolds Number, small scale turbulent motions are statistically isotropic
Energy cascade transfer KE from largest to smallest scales
What are three approaches for turbulence modelling?
Direct Numerical Simulation
Large Eddy Simulation
Reynolds Averaged Navier Stokes Simulation
What is a key quantity for characterization of viscous effects and what is its formula?
Reynolds Number; Re = (u_infty l)/v
Where are viscous effects especially significant in aerospace applications?
Sheer layers, especially in the boundary layer
What are the characteristics of Laminar boundary layer?
- Low skin friction
- Low resistance to separation
- Low mixing rates
What are the characteristics of turbulent boundary layer?
- High skin friction
- Higher resistance to separation
- Higher mixing rates
What is the Energy Cascade conceptualization of Turbulence?
- Turbulence is composed of eddies of different sizes
- Large eddies break up and transfer energy to smaller eddies
- At the smallest scale molecular viscosity dissipates eddies energy
What is Direct Numerical Simulation?
- Resolution of all scales within the energy cascade
- Basic Research, no modelling
What is Large Eddy Simulation?
- Resolution of the energy cascade (>80% turbulent KE)
- Interface between research and engineering application
What is Reynolds-Averaged Navier Stokes Simulation?
The equations are only solved for the temporal mean of the flow quantities.
Turbulence is entirely modeled
What additional term does the averaging process of the RANS produce?
The Reynold Stress tensor
What are two approaches for solving the closure problem for RANS?
- Eddy-viscosity models
- Reynolds Stress Transport models
How does the Eddy viscosity model solve the closure problem?
By coupling the turbulent stress tensor to the mean rate of strain leading to the eddy viscosity variable
What are the limiting cases of the eddy viscosity concept?
- Sudden changes in mean strain rate
- Flow with significant streamline curvature
- Flows with rotation
What is Prandtl’s Mixing Concept?
- Closure relation for eddy viscosity
What are the properties of Algebraic Models?
- Efficient and numerically robust
- Applicability is limited
What are the Pros and Cons of One-Equation Models?
Pros:
- Simple mathematical formulation
- Numerically robust
Cons:
- Modest accuracy improvement to mixing length models
- Length scale still needs to be defined
- Lack of generalization for complex flow
What are the three ranges of eddies in turbulent flow?
- Energy containing range
- Inertial Subrange
- Dissipation Range
Name two Algebraic Models for Eddy viscosity approach of solving the closure problem
- Prandtl mixing length
- Baldwin lomax
Name a two equation model for Eddy viscosity approach of solving the closure problem
- Spalart Allmaras model
Name two two equation Models for Eddy viscosity approach of solving the closure problem
k - E model
k - omega model
