Question

2. In turbulent flow what determines the largest and the smallest eddy sizes? What name is used for this smallest scale? Give the expression used. . Sometimes CFD codes can face severe problems at low Mach number. Describe these problems when a standard explicit compressible CFD method is used for such a flow.

Answer 1

2.

The size of largest eddies (length scales) in the turbulent flow will be given by L. And the size of smallest eddies will be given by $\eta$ . The size of these length scales will only be constrained by the physical boundaries of the flow.

The size of smallest scale of the flow will be determined by viscosity.

It is given by the expression :

$\eta = (\frac{\nu^3}{\epsilon})^{1/4}$

Where , $\nu$ = viscosity (m²/s)

$\epsilon$ = dissipation rate per unit mass (m²/s³)

The smallest scale is called as Kolmogorov length scale.

3/4 Re

Where Re is the Reynold's number.