Quiz 6 Flashcards
In turbulent flow, is the inertial forces or the viscous forces the dominant force?
Inertial forces
Which are the characteristics of turbulence?
o Irregularity, randomness, chaos
▪ Deterministic methods fail, resort to statistical descriptions (but N-S equations still hold)
o Diffusivity
▪ Rapid mixing of heat, mass, momentum (increased spreading rates, delayed separation over bluff bodies)
▪ Random pattern without mixing not turbulent (aircraft contrails)
▪ Perhaps most important feature of turbulence
o Large Reynolds number
▪ Turbulent flows always at high Re
▪ Often arise due to instabilities growing with Re
o Three-dimensional vorticity fluctuations
▪ Turbulence is rotational and three dimensional
▪ Vortex stretching essential component
▪ Note, random water waves not turbulence
o Dissipation
▪ Turbulent flows are always dissipative
▪ Viscous shear stresses raise internal energy at expense of kinetic energy of turbulence. Without a supply of energy turbulence rapidly decays
▪ Energy cascade from large to small eddies to viscosity
o Continuum
▪ Turbulence is governed by the equations of fluid mechanics, even at the smallest scales
o Turbulence is a flow not a property of the fluid
Which is the smallest length scale in a turbulent flow?
Kolmogorov scale
What happens with the energy contained in a turbulent flow past the smallest scales?
Dissipates into heat
Is DNS (direct numerical simulation) currently feasible for a typical engineering problem like fire? Give a reason for your answer.
No, will take too long to do simulation with all scales represented (enough memory exists)
What is Philip’s rule number 2?
If you haven’t filled your computer, you are not trying hard enough.
What is the general approach of RANS (Reynolds Averaged Transport Equations)?
o Decompose instantaneous velocity into mean and fluctuation
o Rely on empirically based models to describe fluctuations
o Relatively computationally efficient
o Struggle to capture complex flow phenomena
What is the general approach of LES (Large Eddy Simulation)?
o Turbulent energy contained in largest scales
o Only the large scales of motion will be resolved
o Effect of smallest scales of motion modelled
What are the criteria performing proper LES?
o 80% of the dynamic energy is resolved accurately (Pope)
o Scales of motion resolved include the Taylor microscale (Ferziger)
o Subgrid scales are within the inertial subrange (Ferziger)
Why do we need to use a sub grid scale model when doing LES simulations and what is its purpose?
o Turbulent energy contained in largest scales
o Only the large scales of motion will be resolved in time and space
o Effect of smallest scales of motion modelled
o Assumes that the turbulent energy cascade is valid
o Need not model fluctuations, only energy dissipation
