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Sunday, February 28, 2016

Variation of wind velocity with building height

looking at moving fluid and a solid surface, viscosity manifests itself in the creation of shear force aligned opposite to the direction of fluid motion, a similar effects takes place between surface of earth and atmosphere. viscosity reduces the air velocity adjacent to the earths surface to almost zero.

the height at which the velocity ceases to increase is called the graient height, and the corresponding velocity, the gradient velocity. the height through which the wind speed is affected by the topography is called the atmospheric boundary layer.

wind speed variation is given by a simpler power-law expression as below:

Vz = Vg (Z/Zg)^a

Vz = main wind speed at height of Z
Vg = gradiant wind speed, assume constant above boundry layer
Z = heigh above ground
Zg = depth of boundary layer
a = power law coefficient

wind speed of Z can be calculated using aboive equation by knowing mean wind speed at gradient height and the value of exponent a. value of a ranges from low of 0.14 for open country to about 0.5 for built-up urban areas. the pressure and suction generated by wind are a function of the wind speed, and in general increase with the building height.

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