wind energy resource evaluation in a site of central italy ... - WindSim

Figure 2.10: Vertical profiles for R=0.6 and 20 vertical cells.As can be easily seen, the two boundary conditions at the top level producedifferent results in the shape of the velocity profile. The most important effect is thevelocity variation during the flow development on the domain. According to theneutral ABL, the velocity profile should not change along the domain, an this is thecase of figure 2.8 where a low value of roughness was imposed. The effect of profilevariation is amplified by the growth of the roughness value. This effect can be due tonumerical models used to close the RANS equation (turbulence model) and tosimulate the terrain roughness. In all the cases showed before, the effect of thevelocity deficit is more evident in simulations with zero pressure condition than insimulations with the wall with no friction condition.The second effect is the deformation of the velocity profile near the geostrophicheight. The zero pressure boundary condition produces in the exit station a velocityprofile that shows a maximum velocity similar to that of the inlet station. This fact,together with the strong decrease of the axial velocity below the geostrophic heightleads to a diminution of the volume flow in the vertical stations along the domain.As a results, the vertical component of the velocity increases and parte of thevolume flow exits from the top boundary condition, in contrast with the definition ofneutral ABL.24