# Key Issues on the Design of an Offshore Wind Farm Layout and Its Equivalent Model
## 2 Wake Models  
Determined by the layout and the wind conditions of wind farms, the power loss of a downstream wind turbine easily reaches up to 50% under full-wake conditions. Thus, modeling of the wake effect is important. Numerous wind-farm design tools that apply different analytical wake models [12; 13; 14] or computational fluid dynamic (CFD) flow solvers [15; 16] have been developed to calculate the power losses of wind farms. Generally, analytical wake models, such as [PERSON]'s model or [PERSON]'s model, are simpler than CFD flow solvers and require less computational time to estimate power generation by wind farms and power losses due to wakes. Such models involve algebraic equations for turbine-induced wake velocities and superpositions of multiple turbine wakes. When these models are used to determine wind farm configuration, the wake intensity for one wind turbine (also called a single wake) is calculated first, and then the superposition of the single wakes of several wind turbines in the wind farm (multiple wake) is calculated.  
One of the most widely used wake models (now called the Park model) was proposed by [PERSON], and subsequently modified by [PERSON]. It is a relatively simple wake model, which assumes a linearly expanding wake with a velocity deficit that depends only on the distance of the wake behind the rotor. The [PERSON] wake model was thus derived by conserving the momentum downstream of the wind turbine. The velocity of the wake is a function of the downstream distance from the turbine hub and the wake is assumed to expand linearly downstream. Accordingly, the [PERSON] wake model is strictly applicable only in the far wake region. Numerous comparisons of wake models have concluded that all models have highly uncertain performance [16]. However, many researchers recommend the use of the [PERSON] model to predict the power output of offshore wind farms in the past.  
The wake effect is more severe in larger wind farms. A particular turbine may be influenced by the wakes of more than one turbine. Such a situation is called the multiple wake effect [17]. Detailed models that consider the areas in the shadow of upstream wind turbines exist. The corresponding effect is called partial shadowing. Moreover, many investigations provided schematic wake models that involve wind turbines with different hub heights [18]. Some investigations have found that the use of various hub heights increases the power output of a wind farm even if the total number of wind turbines is constant [19].  
Some analytical wake models, such as the Park Model, have been implemented using industry-standard software, including Wind Atlas Analysis and Application Program (WAsP), WindPro and Garrad Hassan (GH) WindFarmer to assess wind resources and for the micro-siting of wind turbines and wind farms [20; 21]. A modified version of WAsP allows [PERSON]'s model to be used with turbulence models. Alternatively, CFD solvers are based on the solution to the averaged or filtered Navier Stoke equations. In current years, numerous works [22; 23; 24] have presented models for simulating the turbine wakes and associated power losses of actual wind farms.