Cfd modelling of h-darrieus vertical axis wind turbine

4.6.2 Turbulence Model 
Depending on the nature of the case to be studied, Fluent permits the use of various turbulence 
models. The Shear Stress Transport (SST) was used in this instance, as it was in most of the 
publications identified about VAWT. The SST turbulence model is a four-equation model that 
combines the two-equation models k-ε and k-ω. [84] shows that SST gives better values for the kind 
of flow expected in VAWT. The default values for Fluent have not been changed.
 
4.6.3 Time Step Calculation 
A transient simulation, as previously stated, needs the application of time-dependent calculations. In 
the case of a sliding mesh problem, this time step determines how many degrees the mesh revolves 
in each computation based on the turbine's imposed rotational velocity. Instead of dealing with the 
time step value, the relation between the degrees of rotation of the sliding mesh each time step was 
used because it proved to be a more straightforward and comprehensive parameter. The relationship 
between these two parameters is stated in the following paragraphs: 
This procedure begins with the turbine's rotational velocity being set. The next step is to calculate 
the time required to spin one degree, taking into consideration the radian-to-degree relationship: 

---

43 
𝜔 =
∆𝜃
∆𝑡
 
Eq (4.1) 
∆𝑡 =
∆𝜃
𝜔
 
Eq (4.2) 
∆𝑡|
1°
=
1
𝜔(𝑟𝑝𝑠). 360
 
Eq (4.3) 
The relationship between the time step and the rotational velocity is significant to observe from the 
previous equation. This means that the time step must be adjusted for the entire range of tip speed 
ratio values out of which turbine performance is calculated. 
In chapter 5, a time independence study explains how to choose the right time step size.