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The atmospheric subsystem
Published in Stephen A. Thompson, Hydrology for Water Management, 2017
where uz is wind speed at height z, k is the von Karman constant (k ≈ 0.4), u* is the friction velocity, and z0 is the roughness length of the surface. The roughness length is the height above the surface where the wind speed is assumed to go to zero and depends upon the roughness of the surface (Table 4.3). If measured values are not available the roughness length can be approximated as 110 the height of the vegetation. The rapid increase in wind speed with height means gages should not be installed more than a few feet above the ground. Protecting the gage behind trees or buildings lowers wind velocities, but the barriers should not be so close as to cause turbulent eddies that may influence the catch of the gage. A guideline is that the distance from the gage to the barrier be approximately twice the height of the barrier. This translates into an angle of approximately 27° from the gage to the top of the barrier (Fig. 4.13).
Sources for loads on a wind turbine
Published in Martin O. L. Hansen, Aerodynamics of Wind Turbines, 2015
V10min(x)is the time averaged value for a period of 10 minutes at a height xabove the ground. V10min(h)is the time averaged value at a fixed height h, and zois the so-called roughness length. Alternatively the wind shear can be given by an exponent as in Equation 9.33. The roughness length depends on the surface characteristics and varies from 10-4m over water to approximately 1 m in cities. Values of zo can be found in Troen and Petersen (1989) and is summarized in Table13.1.
Using Meso-Scale Modelling for the Prediction of Wind Resources in Portugal
Published in Naim Hamdia Afgan, Maria da Graça Carvalho, New and Renewable Energy Technologies for Sustainable Development, 2020
The roughness length of land surfaces must also be specified to the model. This information is derived from the CORINE (Coordination of Information on the Environment) Land Cover maps available from Instituto Geográfico Portugués (IGP; formely CNIG). The vectorial maps correspond to the 1:100,000 scale map series from the former Instituto Português de Cartografia e Cadastro (IPCC; presently IGP), thus providing enough resolution for this study.
Near-wall modeling of forests for atmosphere boundary layers using lattice Boltzmann method on GPU
Published in Engineering Applications of Computational Fluid Mechanics, 2022
Xinyuan Shao, Marta Camps Santasmasas, Xiao Xue, Jiqiang Niu, Lars Davidson, Alistair J. Revell, Hua-Dong Yao
The effect of forests may be considered as a kind of surface roughness which is modeled by introducing a roughness length (Lo, 1990). However, this method is only applicable to short vegetation. The permeability of the foliage should also be taken into account for relatively high forests. Shaw and Schumann (1992) presented the effect of forests in their simulations as a drag force applied by the trees on the flow and the drag force varies according to the varying leaf area density (LAD). Their method has been accepted by many researchers and has been used to investigate many aspects of canopy flows, for example, coherent structures in canopy flow (Dupont & Brunet, 2009; Finnigan et al., 2009; Nezu & Sanjou, 2008), the influence of LAD on canopy flow (Dupont & Brunet, 2008b) and edge flow due to heterogeneity in forest structure (Dupont & Brunet, 2008a).
Comparative analysis of three numerical methods for estimating the onshore wind power in a coastal area
Published in International Journal of Ambient Energy, 2018
Mojtaba Nedaei, Abtin Ataei, Muyiwa Samuel Adaramola, Alireza Hajiseyed Mirzahosseini, Morteza Khalaji Assadi, Ehsanolah Assareh
The surface roughness (sometimes called surface roughness length or just roughness length) is defined as an effective height above the surface where wind speed reduces to zero. It is a parameter in the logarithmic law, which states that the wind speed varies logarithmically with the height above ground according to the following equation (Gualtieri 2015): where U (z) is the wind speed [m/s] at some height above ground z [m], U* is the friction velocity [m/s], k is von Karman’s constant (0.4), z0 is the surface roughness [m], ln is the natural logarithm.