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004391

Dipl.-Ing. (BA) Andreas Niemeier, M.Eng.

2020-03-26

Using Boundary Layer Model in RWIND Simulation

Does RWIND Simulation apply a boundary layer model?

Answer:

In RWIND Simulation, each model surface in the wind flow is treated as a "smooth" wall. As of RWIND Simulation version 1.26, it is possible to define the roughness parameters. This definition results in a boundary layer in the areas around the flow close to the walls, which has an influence on the velocity profile perpendicular to the wall depending on the air viscosity. This boundary layer is created in RWIND Simulation according to the "wall law". This law describes the velocity profile perpendicular to the wall and can be represented by the dimensionless variables u⁺ and y⁺.

FAQ

FAQ 004391 | Does RWIND Simulation apply a boundary layer model?

Turbulent Boundary Layer

Dimensionless variable u⁺:

u^{+} = \frac{U}{u_{τ}}

U	Velocity on the wall
u_τ	Frictional velocity

1 Dimensionless Variable u+

Dimensionless variable y⁺:

y^{+} = \frac{u_{τ} \cdot y}{ν}

y	Wall distance
u_τ	Frictional velocity
ν	Kinematic viscosity of the air

2 Dimensionless Variable y+

Using the frictional velocity u_τ (Formula 03), the boundary layer model describes the following velocity distribution in the viscous partial layer directly next to the wall (Formula 04) and in the adjacent logarithmic layer (Formula 05).

u_{τ} = \sqrt{\frac{τ_{w}}{ρ}}

τ_w	Shear stress
ρ	Air density

3 Frictional Velocity

u^{+} = y^{+}

4 Wall

u^{+} = \frac{1}{κ} \cdot \ln y^{+} + C

κ	Kármán constant
C	Constant

5 Logarithmic Layer

Here is:

κ = 0.41 for the simulation of a smooth wall)
C = 5 for the simulation of a smooth wall

If the roughness parameters are defined, the nutkRoughWallFunction is used with the parameters K_s and C_s. The formula of the nutkRoughWallFunction can be found here:
Formula of nutkRoughWallFunction

K_S	Equivalent sand roughness
C_s	Roughness constant

FAQ 004391 | Does RWIND Simulation apply a boundary layer model?

FAQ 004391 | Roughness Parameters

To ensure that the solution process is relatively fast and robust, the program specifies the corresponding boundary layer model directly in the first cell next to the model surface. The remaining part of the boundary layer results from the solution of the globally applied Navier-Stokes equations.

Generating Boundary Layer in RWIND Simulation

Author

Mr. Niemeier is responsible for the development of RFEM, RSTAB, RWIND Simulation, and in the area of membrane structures. He is also responsible for quality assurance and customer support.

Downloads

FAQ Model | RWIND Simulation 1 File

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