Cfd Simulation of Vertical Axis Wind Turbine

CFD Simulation of Vertical Axis Wind Turbine

Fundamentals of Vertical Axis Wind Turbine:

The Wind turbine consists of a number of straight blades vertically mounted on some struts, which are attached to the main shaft. The number of the blades depends on the solidity of the turbine. Solidity is the ratio of the turbine blade area to the turbine swept area, and can be calculated as,

                        [pic 1]

Where n is the number of the blades, c is the chord length of the blade, and D is the

Diameter of the turbine.

        The blades rotate around the main shaft relative to an axis, which is perpendicular to the wind direction. An azimuthal angle θ is defined by its orientation from the vertical position, and it varies from 0 to 180 degrees on the front-half cycle, and from 180 to 360 on the rear half-cycle. The front half-cycle corresponds to the part of the cycle in which the blade is traveling along the upwind portion of the turbine circumference, and the rear half-cycle corresponds to its downwind portion.

[pic 2]

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Figure-1: VAWT layout (Top view) .With a freestream velocity, U∞, coming from the left, is rotating at an angular velocity Ω.

For better understanding of aerodynamic forces the individual blade aerodynamics has been shown in below Figure-2.

[pic 8]

Figure-2: The velocity vectors and aerodynamic forces on a blade located at the front half-cycle.

Where,

      U∞ = Velocity vector of the free stream wind

      Uinduced = the velocity vector induced by the blade’s rotation around the main axis and is always anti-parallel to the blade’s velocity vector.

The magnitude of the   Uinduced   can be determined as,

                                      Uinduced = R×Ω

Where R is the turbine’s radius, and Ω is the turbine’s angular velocity.

Tip Speed Ratio: (TSR)

Tip Speed Ratio is defined as the “ratio of turbine induced velocity to the velocity of the free stream wind”. It is one of the most important factors in wind turbine design. It can be calculated as,

[pic 9]

The blade angle of attack depends on the Pitch angle. If the pitch angle is Zero, the induced velocity cannot provide the blade with an angle of attack. However, when the VAWT is spinning, the blades move forward through the air in a circular path. The resultant of the oncoming free-stream velocity vector and the blade’s induced velocity vector is called the relative velocity, which creates a varying angle of attack to the blade.