Tdyn reference manual
- 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 μT/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 μT/μ. Note
that the ratio of turbulent to laminar viscosity depends on TIL, L, k
and ε as μT/μ ~ k2·ε
or μT/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.