Specification of turbulence quantities at an inlet
- Use verified quantities from experiments as inlet boundary conditions for turbulent kinetic energy k
, and dissipation ε
, if they are available, as the magnitude can significantly influence the results. If there are no experimental data available, the values need to be specified using sensible engineering assumptions (see below), and the influence of the choice should be examined by sensitivity tests with different simulations.
- Specify values of the turbulent kinetic energy k
that are appropriate to the application. These values are specified through a turbulence intensity level TIL, which is defined by the ratio of the fluctuating component of the velocity to the mean velocity (k
= TIL·V2
). In external flows over airfoils the turbulence level is typically TIL = 0.003 (0.3%). In atmospheric boundary layer flows the level can be two orders of magnitude higher (TIL = 0.30 (30%)) and details of the actual boundary layer profiles are needed. In internal flows the turbulence level of TIL = 0.05 to 0.10 (5 to 10%) is usually appropriate.
- Specify values of the turbulent length scale, as an equivalent parameter for the dissipation ε
, that are appropriate to the application. For external flows remote from boundary layers, a value determined from the assumption that the ratio of turbulent and molecular viscosity μ
T
/μ
is between 1 and 10 is a reasonable guess. For internal flows a constant value of length scale derived from a characteristic geometrical feature can be used (e.g. 1 to 10% of the hydraulic diameter for internal flows).
- If more sophisticated distributions of turbulence variables are used, check their consistency with the velocity profile. An inconsistent formulation may lead to an immediate unrealistic reduction of the turbulence quantities after the inlet.
- For inlets that represent fully developed pipe or channel flows, algebraic profiles might be used.
- Check the consistency of the definitions of k
and ε
by making a plot of the ratio of turbulent to molecular viscosity μ
T
/μ
. Note that the ratio of turbulent to laminar viscosity depends on TIL, L, k
and ε
as μ
T
/μ
~ k
2
·ε
or μ
T
/μ
~ TIL·L.
- In cases where problems arise, move the inflow boundary sufficiently far from the region of interest so that a natural inlet boundary layer can develop.
- Note that ILES and Smagorinsky models don't need any imposition of boundary conditions for the turbulent variables.