The GEKO model is currently not published. It is based on a k-ω formulation and features four free coefficients that can be tuned/optimized within given limits without negative effect on the underlying calibration for wall boundary layers at zero pressure gradient as well as channel and pipe flows. The coefficients are:
- Parameter to optimize flow separation from smooth surfaces. 0.7<
<2.5 (default 
= 1.75)Increasing
leads to earlier and stronger separation.
mimics the standard 
model, 
is close in performance to the SST model.
- Parameter to optimize flow in non-equilibrium near wall regions (such
as heat transfer or 
).-2<
<2 (default 
=0.5)Increasing
leads to higher heat transfer rates at reattachment locations
(backstep case, for example).
is mostly used to adjust for flows with heat transfer in impingement
zones. It has a very strong default value and typically should not be changed except
if detailed experimental data are available.
- Parameter to optimize strength of mixing in free shear flows.-0.5<
<1 (default - correlation 
) where sgn is the sign function, returning -1, 0, or 1 depending on
the value of the argument (C_SEP - 1) being negative, zero, or positive,
respectively.Increasing
from its default value will lead to stronger mixing in free shear
flows (mixing layer). The correlation 
assures that, for changes in 
, the classical mixing layer spreading rate is preserved. However,
can also be given a value other than the correlation.
- Parameter to optimize free shear layer mixing (optimize free jets
independent of mixing layer).0<
<1 (default 
=0.9).Increasing
will also increase the spreading rates of free jet flows. This can
be undesirable, and therefore 
allows you to reduce the effect of 
on free jet flows. 
is only active when 
is non-zero and reduces the spreading rates of free jets. This parameter will only be required in a limited number of applications.
is a sub-model of 
– this means that when 
=0, 
will also not be active.
You also have the option to tune the model with the curvature correction parameter,
. This parameter is also available for other models and is described in Curvature Correction for the Spalart-Allmaras and Two-Equation Models.
There are parameter combinations that are of specific interest. For 
, 
, and 
, the model obtains an exact transformation to the standard 
model (albeit with the improved near wall treatment of the 
model). Note that 
is not relevant as 
.
The default values of 
, 
, 
, and 
give a close approximation of the SST model (especially in terms of
separation behavior not in terms of formulation). For free shear flows, this combination is
superior compared to SST. It gives similar spreading rates for mixing layers but improved
spreading for round jets.
By default, the GEKO model features a realizability limiter that ensures positive normal stresses in the entire domain:
 | (4–131) |
The function involving and is controlled by a blending function 
which deactivates these parameters inside boundary layers (
). For free flows, 
.
 | (4–132) |
This function requires wall-distance, which can be expensive to compute in some cases
(flows with moving geometries, changing meshes). For such applications, the model can be run
with Wall Distance Free enabled. In this mode, the parameters
and 
are de-activated and 
is automatically set to 1. To maintain free mixing layer spreading rates,
when using the wall distance free option, you should select the 
model variant (
, 
, 
). However, more aggressive settings for separation prediction are possible
(
) but would result in lower spreading rates for free shear flows. It is
important to understand that the wall distance can also be required by other models (for
example, transition models, hybrid RANS-LES models). If you select Wall Distance
Free, then all other model options using wall distance become unavailable. If you
first select a sub-model within the turbulence dialog box that requires wall-distance, then
the Wall Distance Free option becomes unavailable.
, 
, 
, and can also be specified as
an expression or via UDF (see DEFINE_KW_GEKO Coefficients and Blending Function in the Fluent Customization Manual).
Beside the four free coefficients and , four auxiliary
parameters are available which allow further fine-tuning of the model behavior by changing
their constant values. The majority of users need not modify the default values of these
coefficients:
: allows the adjustment of log-layer and 
(default = 1.7).
: allows fine-tuning of the parameter to optimize free jets (default = 2.0).
Higher values make the impact of more effective.
: Factor for main shielding function 
(default = 2.0). Decreasing this value will decrease the thickness of the
layer near walls, where 
and 
are deactivated. This will result in the activation of the 
and 
formulation closer to walls.
: Only active if you have enabled the Intermittency Transition Model. Factor
is a part of 
and shields the laminar boundary layer (default = 25.0). This parameter
provides additional shielding of the boundary layer from any impact of 
and 
in laminar regions. This parameter is required to avoid any influence of
these parameters when the GEKO model is combined with a laminar-turbulent transition model.
Setting a lower value will reduce this shielding.