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Turbulence Model
Turbulence Model in ENVI-met
ENVI-met uses the so-called 2-equation Turbulence Kinetic Energy (TKE) Model to predict the turbulence in the air.
The first equation describes the distribution of the kinetic energy (E) in the air depending on production, advection, diffusion and destruction:
The second equation is similar but describes the Dissipation Rate of TKE (ε or eps):
Finally, the Turbulent Exchange Coefficient Km is a result of both:
As the result of the TKE-ε equation Km (via Ke and directly) is used as input data for the next calculation cycle, there is a non-linear feedback loop between the turbulent exchange coefficient and the TKE-ε equation. Once Km begins to be unstable, the TKE-ε equation system will get unstable too, producing even more unstable Km's in the next loop.
To avoid numerical problems, several mechanisms have been introduced in ENVI-met making the turbulence model more reliable:
In previous ENVI-met versions, it was suggested to calculate the turbulence field using the “Update” method (Turbulence mode=1, see Turbulence Model-Section). In this mode, the TKE-e equation system was only solved in fixed time intervals until a stationary solution was found. This method was only introduced to keep the model executable on older computers.
Today, we do not suggest to use the “Update” mode any longer!
Recent computers are strong enough to use the “Continuous Mode” (2)!
As the old update method was a huge source of error and frustration, we have decided not to support it any longer in ENVI-met!
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