The non-uniform unsteady loading over the rotor disk due to the atmospheric inflow increases with rotor size. Thus, the requirements to the BEM modeling capability have changed considerably from the 15–20 m diameter rotors in the 1980s to 100–200 m rotors today. This important effect of turbulence scales relative to rotor size was already described by , noticing the difference in impact on aerodynamic loads of turbulence scales above and below the rotor size. also very briefly presented how to use the BEM method in sheared inflow. This approach has some resemblance to the BEM implementation that will be presented here.

To illustrate how the upscaling of rotors leads to a more non-uniform inflow and thus non-uniform loading of the rotor when operating in turbulent inflow (no shear), we simulate two turbines with the aeroelastic code HAWC2 (Horizontal Axis Wind turbine simulation Code; ): the AVATAR rotor with a diameter of 205 m and a downscaled version of the AVATAR rotor with a diameter of 51.4 m. Both turbines were simulated without tilt, with a stiff structure, and both operate at the same tip speed of 74.7 m s-1 and in the same turbulent inflow with no shear. The turbulent inflow was generated with the Mann model using a box with vertical and horizontal side lengths of 240 and 5600 m, the latter corresponding to the traveling length of the turbulence over the simulation time of 700 s and a mean wind speed of 8 m s-1.

As the turbine blades rotate through the turbulent vortex structures, the spectrum of the free wind speed at the tip of the blades has energy concentrated on multiples of the rotational frequency 1P (Fig. ). Since the size of the turbulent vortex structures is absolute (given a certain turbulence length scale), the distribution of energy upon the individual frequency multiples is different for different turbine sizes. What can be noticed is that the small rotor has a significant amount of energy on the very low frequencies (≪1P), whereas the larger turbine experiences a higher ratio of the total energy on 1P and multiples. In other words, for the increasing size of turbines, a bigger and bigger part of the turbulent eddies have a size comparable to or below the rotor diameter.

The increasingly non-uniform rotor loading with turbine size is also caused by inflow with shear. The largest modern turbines with the blade tips at top positions around heights of 300 m now span most of atmospheric boundary layer containing the main part of the shear . This is in particular seen for stable flow situations.

Other challenging wind situations comprise non-stationary wind conditions containing trends, such as wind shear developing over time. For very large rotors, these situations are important for the extreme load levels during operation. Thus, they need attention in the modeling phase if turbine designers shall be able to counteract such events using either active or passive load alleviation techniques.

Besides the upscaling trend, turbine design has changed in the same time span of years, which results in new requirements for the aerodynamic modeling in the aeroelastic codes. Pitch control is now the common power regulation method; therefore, situations like pitch fault have to be simulated for certification. Such a situation with, e.g., one blade pitch differing from the pitch of the other blades with, e.g., 20∘, results in a non-uniform rotor loading and expected azimuthal variation of induction. The pitch control for power regulation has been extended to include cyclic pitch to alleviate 1P loads, and now full individual pitch is being implemented for even better load alleviation. An important question is thus how to handle individual pitch action in a BEM-type modeling.

The subject of sheared and turbulent inflow was part of the work in the EU-funded UpWind project (2006–2011) with the main objective to study upscaling of turbines to 8–10 MW. The aerodynamic flow mechanisms at high shear in the inflow were investigated by simulating the sheared inflow on the 5 MW reference wind turbine with a range of models from high-fidelity CFD codes to vortex codes and to more BEM-type codes . One major finding was that all high-fidelity codes and vortex codes showed that the induction does vary within an annular element for sheared inflow. Different BEM implementations to cope with this were discussed.

Similar work was continued in the EU-funded AVATAR project (2013–2017) with a focus on even bigger turbines (10 MW and higher) than in the UpWind project. A summary of the findings has been presented by . One major finding was that a comparison of aeroelastic simulations with a free vortex code and a BEM-based aeroelastic code showed an overprediction of fatigue loads in the range of 15 % by the BEM-based aeroelastic code. It is further mentioned and discussed that the results depend on the actual implementation of the BEM model.

The basic BEM formulation originates from Glauert and was developed for airplane propellers . Glauert points out that the two major components are the “momentum theory” and the “blade element theory” which for many years were developed separately and, e.g., the use of finite aspect blade data was considered in the blade element theories to fit experimental rotor data. However, the combination of the two theories finally led to the BEM approach where the induced velocities at the rotor disk are derived from the momentum theory and the blade sectional forces are found on the basis of infinite aspect ratio (2-D) airfoil characteristics. In the present paper, the focus is on the momentum part of the BEM approach, although many uncertainties in rotor computations are linked to the blade element analysis such as 3-D airfoil characteristics due to rotational effects.

When the momentum part of the BEM theory is used in aeroelastic simulations, the actual flow conditions violate most assumptions in the basic theory: (1) turbulent and sheared inflow compared with the assumption of uniform, steady inflow; (2) non-uniform load in contrast to the assumed uniform loading and (3) skewed inflow in contrast to assumed axial inflow, just to mention the most important violations. To compensate for this, a number of submodels are introduced like dynamic inflow and skewed wake models. However, there is no real consensus on how the different phenomena should be modeled and how the submodels should interact. Therefore, we often see considerable deviations for BEM simulations on complex inflow cases .

Many researchers have contributed over time to the development of the BEM theory for wind turbines but only a few will be mentioned here. made an important contribution at an early stage to describe the theory. They also proposed a method based on a Taylor expansion to look into the effect of wind shear. Another important contribution at an early stage to the development of the BEM approach is made by , who envisioned the challenges in implementing the BEM theory for turbulent inflow.

Later, a comprehensive description of the BEM modeling is presented in the handbook of with a detailed discussion of inflow models to handle dynamic and skewed loading as will be discussed later. Also, the handbook of gives a fundamental introduction to the BEM modeling approach as well as the doctoral thesis by .

In Sect. 3, we present a detailed description of the implementation of the grid BEM approach. However, first, in that section, we give a short introduction to the origin of the CFD simulations of the actuator disk flow used heavily in developing and tuning the submodel for yawed flow, the dynamic induction model and a submodel for radial induction. The mechanism of induction in turbulent and sheared flow is explored in Sect. 4, and we present the load and power impact for two turbines for design load case (DLC) 1.2 load cases. In Sect. 5, a selection of validation cases is presented, followed by conclusions in Sect. 6.

The general purpose CFD code FIDAP (Fluid Dynamics Analysis Program), based on the finite element method, is used for the AD computations. It was one of the first commercially available CFD codes and has an unstructured mesh capability which reduces the requirements to the total number of nodes.

In the past, the code has been used for several studies of the flow through an actuator disk model. In a first setup from 1996, the computations show good correlation with the momentum theory with one-third induction at the rotor disk and two-thirds in the far field for a prescribed uniform loading corresponding to a thrust coefficient of 0.89 . It should be mentioned that FIDAP has an option of using a discrete pressure formulation from element to element which was found important for AD simulations of the pressure jump at the disk. Typically, two cells with a total axial distance of 0.05R are used to model the disk in axial direction.

Later, in 1999, the AD model was coupled to the aeroelastic code HAWC , so that the computation of the induction could be shifted between BEM and the CFD-AD model . Several yawed flow cases for a 100 kW turbine were investigated with that model setup and a good correlation with experimental data was found, e.g., for the electrical power and flapwise moment . A further comparison was made using the data set for 45∘ yaw error from the NREL phase VI 10 m wind turbine tested in the NASA Ames 80 ft × 120 ft wind tunnel . The computed angle of attack variation at a radial position of 83 % showed good correlation with the measured inflow angle when corrected for the influence of upwash.

The CFD mesh and model from this setup is used for the present simulations with a prescribed uniform loading on the disk (Fig. ). The mesh extends 10 rotor diameters (D) in the z direction, which is the main flow direction for zero yaw, 12D in the x direction and 4D in the y direction. The inflow plane is 4D upstream the actuator disk, and yawed flow is simulated by changing the inflow direction with an x-velocity component. In total, about 25 000 nodes are used for the meshing.

The fundamental part of the BEM model is the relation between thrust on the rotor and the induced velocities. For a stream tube enclosing the AD, a 1-D momentum balance between axial forces on the turbine and the flow within a stream tube is T=m˙ΔU. Following classical literature like , and , this leads to the relationship between the thrust coefficient CT and the induction factor a: 1CT=4a(1-a), where a=U0-UrU0 and CT=T12ρAU02 with the rotor thrust T, the free-stream velocity U0, the air density ρ and the rotor area A.

For thrust values causing higher induced velocities than a=0.5, Eq. () breaks down, since the flow velocity in the wake far downstream according to the momentum theory is 1-2a, which in these cases is equal to or smaller than zero. This results in an infinite expansion of the flow behind the rotor and the flow can no longer be approximated by simple momentum theory. More complex flow models are needed, such as CFD, or an empirically based relation can be used.

For different reasons explained below, we use a BEM implementation where the induction in the whole operational range from negative CT to a high positive CT is expressed through the following third-order polynomial shown in Fig. : 2a=k3CT3+k2CT2+k1CT, where the coefficients k1 … k3 are defined as k1=0.2460, k2=0.0586 and k3=0.0883.

For CT<0.89, the polynomial fits well the momentum equation. At high loading, the curve was determined to fall between the Glauert empirical relation fitted to experimental results for a model rotor (see, e.g., ) and results from actuator disk simulations at high loading . At high values of CT>2.5, the gradient of the a(CT) curve is kept constant. Thus, a is determined as a(CT>2.5)=a(CT=2.5)+(CT-2.5)(da/dCT)|CT=2.5.

One important reason for using a polynomial fit to Eq. () is that we find that it is a more robust and fast method to compute the induction instead of solving Eq. () using a non-linear Newton–Raphson iteration solver combined with an empirical relation at high loading. Another reason is that it makes it easily possible to modify this CT(a) relation in order to simulate, e.g., a coned rotor as illustrated in , using AD-CFD simulations for the coned rotor. In this case, the CT(a) polynomials will be a function of radial position.

A next step in implementing the BEM model is to couple the momentum theory to the blade element theory where the forces on a blade section are derived by means of two-dimensional airfoil characteristics. We apply Eq. () on a ring element of the rotor with the radial extension dr, as illustrated in Fig. a: 3CT=dT12ρU022πrdr=Urel2CycNBU022πr, where Urel is the relative velocity to the blade section, c is the blade chord, NB is the number of blades, and Cy is the projection of the lift coefficient Cl and the drag coefficient Cd on a line perpendicular to the rotor plane.

Besides the elemental thrust dT on the ring element, there is also a torque dQ, and we can define a torque coefficient CQ by 4CQ=dQ12ρU02r2πrdr=Urel2CxcNBU022πr, where Cx is the projection of the lift coefficient Cl and the drag coefficient Cd on a line tangential to the rotor plane.

Applying the angular momentum equation across the disk, we get 5dQ=ρ(2πrdr)rU0(1-a)(2ra′ω). Combining Eqs. () and (), we find 6a′=Urel2Cx(α)cNB8πr2(1-a)U0ω, where a′ is the tangential induction coefficient. To avoid division by zero, the 1-a term is limited to a minimum of 0.1 for a>0.9.

The relation between thrust and induced velocities (Eqs. –) is changed due to the presence of tip effects, caused by a finite number of blades. Within the wind turbine research community, the tip correction method has for a long time been the subject of numerous investigations and development. In a recent work, presents a comprehensive review of studies on the tip correction and contributes with full derivation of the commonly used Prandtl tip correction which, however, was further slightly modified by to be used in the BEM theory. The Prandtl tip correction factor F is presented by : 7F=2πcos⁡-1exp⁡-NB2R-rrsin⁡φ. We insert it into the momentum equation (Eq. ) as 8CTF=4a(1-a), where CTF has to be inserted instead of CT in Eq. (). The term CTF can reach very large values close to the tip where F becomes very small. In the code, the term is limited to a maximum value of 4. How to incorporate the tip correction factor is also discussed by , concluding that it can either be used to modify the circulation (loading) as done here or through a modification of the induced velocities.

Even though the BEM relationship is originally derived for a full rotor, it is generally implemented on an annular element form as proposed by . In such an annular BEM implementation, it is assumed that the loading and induction within each annular element are constant and that the annular elements are independent of each other. The CT coefficient now represents the average axial loading of the blades on an annular ring element.

In order to model azimuthal variations of induction due to azimuthal variations of blade loading as discussed above, we propose to expand the annular BEM approach. Dividing the annular elements into azimuthal sub-elements leads to a polar grid BEM approach; see Fig. b.

The induced velocity is found in each grid point using the a(CT) relationship in Eq. (). For a uniform inflow and loading, this leads to the exact same induction as the classic annular element approach, whereas differences are seen for non-uniform wind loading over the rotor. An important part of this azimuthal annular element approach is the definition of the local induction factor, where the local instantaneous induced velocity at a point in the grid is normalized with the local free wind speed (the wind speed without rotor induction) at the exact same point. 9a≡-ui,y|U0,l| As seen in Fig. , a question arises about how to find the local load in grid points that are not at the location of a blade. For the classic annular element, the blade loads are averaged and the resulting blade load is assumed to be constant over the annular element. The solution for the azimuthally divided annular element (grid point) is to compute two different thrust coefficients and torque coefficients. These coefficients use the pitch and velocities of the two neighboring blades combined with the local wind speed and induction at the grid point. The coefficients will be weighted by the azimuthal distance of the respective blades. For the corresponding radial position on these two blades, the transformation matrix from sectional to global coordinates is rotated by the azimuthal distance between the blade and the grid point. This corresponds to virtually rotating the blade position to the position of the grid point. The blade velocities are rotated as well such that, for example, the velocity in the direction of rotation at the blade location is applied as a velocity in the direction of rotation at the grid point. Then, the flow at the grid point can be computed as the sum of the free wind speed UgridG, the induced velocity Ui,gridG and the rotated velocity of the blade section x˙bG; the latter has a negative sign because the flow will be experienced in the opposite direction of the blade movement. 10Ugrid,bS=TG→SUgridG+Ui,gridG-x˙bG The superscripts “S” and “G” denote sectional and global coordinates, and the subscript b=1, 2 denotes the two closest blades. The angle of attack αb is computed by 11αb=arctan⁡2Ugrid,b,yS-Ugrid,b,xS, and the relative velocity Ur by 12Ur,b=Ugrid,b,xS2+Ugrid,b,yS2, and the local thrust in the grid points are calculated as 13dTl,b=12ρUr,b2Cy,bαbcNB, where Cy(α) is the lift and drag coefficient projected into the axial direction.

The computation of the local torque is done in the same manner. Then, the two resulting thrust and torque coefficients are interpolated based on the azimuth angle Ψ of the two blades b=1, 2 and the azimuth angle of the grid point: 14CT/Q,grid=CT/Q,1+Ψgrid-Ψ1CT/Q,2-CT/Q,1Ψ2-Ψ1.

It is evident that skewed inflow to the disk violates the conditions for the basic momentum equation (Eq. ) so that the momentum considerations used for derivation of the model are no longer valid. When the rotor operates in yaw, there are two main effects on the induced velocities, as described by . One effect is the change in the mean level of the induced velocities and the other effect is an azimuth variation of the induced velocities, as the wake vortex system is relatively closer to the rotor on one side compared to the other side.

A comprehensive investigation of yaw and dynamic inflow models for wind turbines and dynamic inflow modeling was carried out in the EU-funded project “Joint Investigation of Dynamic Inflow Effects and Implementation of an Engineering Method” . Here, also a short summary of yaw models for helicopters is presented, as these classical yaw models have been the basis for yaw models for wind turbines. The derivation and tuning of the present yaw model deviates slightly in the way that AD simulations of a uniformly loaded disk are used where cylindrical vortex models were a main source in the project . However, as the AD and vortex models should give almost the same results, we will see that the present yaw model is close to some of the models from the abovementioned dynamic inflow EU project.

A time-varying loading of the AD will cause a time delay of the velocities at the disk as the whole wake flow has to adapt to the new loading. This phenomenon, the dynamic inflow effect, was also part of the abovementioned EU-funded project “Joint Investigation of Dynamic Inflow Effects and Implementation of an Engineering Method” , where details about different modeling approaches can be found.

As for the yaw modeling, we use the AD-CFD model results again to develop and tune an engineering submodel for the dynamic inflow. The AD simulations are carried out with a uniform loading and a step change in CT from 0.0 to 0.89 and another case with opposite loading sequence from 0.89 to 0.0. The computed axial velocities u at the disk for different radial positions are shown in Fig.  as function of non-dimensional time t (time divided by R/U0). It should be mentioned that the step size response was normalized to the BEM result of 1 to 0.666 for a change in CT of 0.89 for the different radial positions. This is because the AD-CFD results even for a constant loading typically show a non-uniform flow profile with the lowest velocity of 0.655 at the tip and a higher velocity of 0.695 at the center. This non-uniform velocity profile for a constant loading has been found and discussed by several authors: , , and .

Comparing the decay in velocity for the different radial positions in Fig. a, it can be seen that the decay is slightly faster towards the tip than at the center. Likewise, the increase in velocity for decreasing step loading is also slightly faster at the tip, as seen in Fig. b. The physical mechanism for this small difference in flow response along the radius is that the change in the constant loading sheds a vortex at the edge of the AD with strongest and fastest induction response in the edge region.

Approximating the response with an engineering model led to the conclusion that two time constants are necessary to obtain an accurate fitting to the AD data. We use the following expression for the two first-order filters: 24uav(t,r)=u(t,r)-Δu(t,r)A1exp⁡-tf1(a)τ1(r)+A2exp⁡-tf2(a)τ2(r). Here, u(t, r)`is the flow speed at time t at radius r, uav is the corresponding filtered flow speed, A1 and A2 are weighting constants of the two filters, τ1 and τ2 are the two time constants, and finally f1(a) and f2(a) are functions that adapt the time constants to the local flow speed depending on the induction factor a. The functions take the form 25f1(a)=1-kf1aandf2(a)=1-kf2a, where kf1 and kf2 are constants.

We use a numerical optimization routine to find the set of parameters that minimizes the difference between the AD-CFD step response curves in Fig.  and the results of the model in Eq. (). The variations of the two time constants along the radius are approximated with second-order polynomials in a non-dimensional radius.

The optimization gave the following polynomials for the time constants: 26τ1(r)=-0.7048r2+0.1819r+0.7329andτ2(r)=-0.1667r2+0.0881r+2.0214.

The τ functions are shown in Fig. a. It can be seen that while the highest time constant shows almost no variation along the radial distance, the lowest time constant decreases towards the tip, which corresponds to the faster flow response towards the tip as described above.

A further result of the optimization is the weighting constants of the two filters which gave the following result: 27A1=0.5847andA2=0.4153.

Finally, the functions for the local flow speed to adjust the time constants were determined as 28f1(a)=(1-0.50802a)andf2(a)=(1-1.9266a).

This result shows that the highest time constant (τ2) has to be scaled with a velocity very close to the wake flow velocity of 1-2a, whereas τ1 is scaled with a flow velocity that is between the flow velocity at the rotor disk (1-a) and the free-stream velocity. The induction factor a is limited to a maximum value of 0.4 in Eq. () to avoid unphysical behavior in the turbulent wake state.

As a test case of the implementation of the above-described dynamic inflow model implemented in the HAWC2 model, we run the same prescribed variation of CT as used above to derive the time constants. The comparison of the AD and HAWC2 model results in Fig.  shows a very good correlation, as should be expected. This is in good agreement with the work by , who derived a two-time-constant dynamic inflow model based on vortex models of an actuator disk.

In a time-marching formulation with non-dimensional time step δt (time step divided by R/U0), the dynamic inflow filter at each grid point can be implemented as follows: 29ui,yt=A1ui,y,1t+A2ui,y,2t30ui,y,1t=ui,y,1t-1e-Δtf1(a)τ1(r)-1+ui,y,QSt1-e-Δtf1(a)τ1(r)-131ui,y,2t=ui,y,2t-1e-Δtf2(a)τ2(r)-1+ui,y,QSt1-e-Δtf2(a)τ2(r)-1, where the superscripts t and t-1 denote the present and previous time steps, and ui,y,QS is the quasi-steady-induced velocity including the yaw correction (Eq. ).

For non-straight blades with sweep/prebend or in-plane and out-of-plane deflection, the radial distance between adjacent grid points is not equal to the distance along the curved blade. Therefore, both CT (Eq. ) and CQ (Eq. ) have to be multiplied with ds/dr, the derivative of the blade span with respect to the radius.

The calculation of ds/dr is demonstrated in the rotor coordinate system; see Fig. . The y axis is pointing downwind and the theoretical BEM rotor disk is in the x–z plane. The curved blade is represented by the half-chord line. The vector a is tangent to the half-chord line at this section point S. The vector r is pointing from the root point O to the section point S. The projection of vector r to the rotor plane (x–z plane) is b. It is equivalent to setting the y component of r to zero.

The curved length s is increasing in the direction of a, and the radius is increasing in the direction of b. Thus, ds/dr can be calculated as 32ds/dr=|a||b||a⋅b|.

The standard BEM theory does not give information about the radial induction component, and for plane rotors this induction component will only have minor influence on the loading. However, for rotors with out-of-plane bending blades or rotors with coning, the radial induction component will have an impact on the angle of attack (AOA) and thus also on the loading. An analytical expression for the lateral induction for a 2-D actuator disk is presented in and is adopted for an axis-symmetric AD in . The expression is 33v(r)=12.24CTav(r)4πln⁡0.042+(r+1)20.042+(r-1)2, where CTav(r) is the mean thrust coefficient as function of radial position defined as 34CTav(r)=∫0rCT(r)2πrdrπr2=2∫0rCT(r)drr, where the local thrust coefficient CT(r) is given in Eq. (). The use of CTav(r) instead of the total thrust coefficient is important only when CT shows a strong variation as function of radial position.

We test the radial induction model by a comparison with the AD-CFD solution for a constant loading of CT=0.89. As seen in Fig. , the radial induction computed with the engineering submodel correlates very well with the AD-CFD result.

An overview of the complete aerodynamic model is shown in Algorithm 1. The algorithm includes references to the relevant equations in this article and can be used as a manual for implementation of the grid BEM algorithm. It is crucial that the dynamic inflow filter is applied at the very end of the algorithm to prevent nonphysical rapid induction changes due to any of the submodels. Otherwise, for example, a change in yaw angle at one time instant at the rotor disk in turbulent inflow would lead to an immediate change of the induced velocities, even though the wake did not have time to deflect. For a typical setup, we use 16 azimuthal grid points. The number of radial grid points are somewhat dependent on the planform and tip shape but typically 30–50.

The aerodynamic model as described here is the aerodynamic model in HAWC2. However, it is also found in a stand-alone version HAWC2_Aero which can run the same type of simulations with turbulent inflow, pitch actions and rpm variations as HAWC2 but for a stiff structure. In this version, the simulation speed with all input/output operations is on the order of 7–10 times real time on a 2016 workstation laptop. This means that the computational time for the aerodynamic part is still small (10 %–20 %) relative to the total computational time for the aeroelastic simulations although we, in this BEM implementation, update the induction over the whole disk at each time step. One reason for this is that no sub-iterations in the induction modeling are necessary.

At very low rotor speeds below 0.1 rad s-1, the induction model is deactivated. At these rotor speeds, the rotor can no longer be modeled as a disk and the BEM model is reduced to a blade element theory (BET) model that computes the velocity triangles without induced velocities. Another option in HAWC2 is to use a near-wake trailed vorticity model to model induction in stand still and idling cases .

Unsteady airfoil aerodynamics effects (dynamic stall and Theodorsen effects in attached flow) are not included in the computation of the induced velocities. This is possible because unsteady airfoil aerodynamics occur at much faster timescales with time constants that depend on the half chord divided by the relative speed. For comparison, the dynamic inflow time constants scale with the rotor diameter divided by the free wind speed. After the induced velocities are computed, the unsteady airfoil aerodynamics are determined using the Beddoes–Leishman-type model described by , which was recently extended by .

The induction mechanism simulated with the grid BEM implementation for turbulent inflow is illustrated in Fig. . The simulations were carried out on the AVATAR turbine with a 205 m diameter rotor at 10 m s-1 and a turbulence intensity of 15 %, no shear and constant rotor speed. In the left panel of Fig. , we show the induced velocities at four grid points on the stationary rotor grid at a radius of 42.5 m with the monitoring points shown on the grid to the right. The induced velocities can be seen to vary slowly in time. They can be quite different in some periods due to the large turbulence scales causing different inflow velocities over the rotor. However, the induction seen from the rotating blade varies considerably faster, as it samples the induced velocities at the different azimuth grid positions. This rotational sampling of the induction field is thus basically the same mechanism as the rotational sampling of turbulence.

An important mechanism of the induction of the presented BEM implementation on a polar grid is that each grid point has a memory effect incorporated. Thus, past loading changes at a grid point (e.g., due to a pitch action in this region, a local gust, an instantaneous shear, a blade passing with another pitch angle offset) will influence the induction of the blade passing that grid point. The weighting of the impact of these past events is controlled by the dynamic inflow filter.

To illustrate further the characteristics of the induced velocities from the AVATAR rotor case mentioned above, the time trace of the induced velocity at a radius of 43 m is shown in Fig. a. Further is shown for comparison the induced velocity simulated with the annular mean BEM method. The dynamic characteristics are clearly completely different which is further explored by the power spectral density (PSD) spectra shown in Fig. b. The spectra of the induced velocity computed with the grid BEM model have distinct peaks at 1P, 2P, etc. and can be seen to have close resemblance with the spectrum of the axial free wind speed component relative to the blade at same radial position. As the rotational sampling of both the inflow and the induced velocity field has the same characteristics, it indicates that the induced velocity field over the rotor also has the same overall characteristics as the turbulent inflow, although considerably lower wind speeds.

As expected, the PSD of the induced velocity computed with the annular mean method has no peaks and has some resemblance with the PSD of the hub wind speed.

We will now illustrate the mechanism behind the load impact of using a mean annular BEM approach and a grid BEM model, respectively. Again, it is a simulation example for the AVATAR rotor.

A simulation was run with a ramp in wind speed from 4 to 20 m s-1 for uniform inflow. The induced velocities at three radial positions on the blade are shown in Fig. . As the inflow is uniform, both BEM implementations give the same result.

Now, a simulation is performed for sheared inflow with an exponent of 0.5 and at a wind speed of 8 and 14.5 m s-1, respectively. We show the induced velocity for the same radial positions on the blade as function of the local inflow velocity at that point.

For the mean annular BEM, the constant induced velocity as function of the local wind speed on the blade is obvious. The mean value might be slightly different from the value at the same wind speed for the turbine operating in uniform inflow due to non-linear effects from computation of the mean loading.

The picture is quite different for the grid BEM method, as shown in Fig. b. For all radial positions, at both wind speeds, we see that the induced velocity increases in magnitude but with the steepest slope at high wind. The mechanism behind this is that as soon as the inflow velocity is different from the hub wind speed the local blade section operates in conditions where either the rotational speed and/or the pitch does not correspond to the equilibrium conditions for that wind speed.

At 8 m s-1, it is mainly the rpm that influences the variation in the local induction around the mean operational wind speed of 8 m s-1. When the local wind speed is lower than 8 m s-1, the local tip speed ratio is above the mean value and the blade section operates at a higher thrust coefficient. The opposite holds when the local wind speed is above the mean wind speed. The result is that the relation of the induction versus wind speed deviates from the induction curve for the turbine in uniform inflow. It also appears that the slope of this local relationship between induced wind speed ui,y and free local wind speed U0 decreases from root to tip. For local wind speeds below the operational point, the increased CT will increase the induced wind speed, whereas the decreased local wind speed (being a factor on the induction) will decrease the induced wind speed. In most conditions, the impact of the local wind speed multiplied on the induction factor is strongest but at high thrust coefficient regions towards the tip and for bigger deviations from the mean wind speed, we can see that the CT impact increases and the slope of the ui,y(U0) curve decreases. The slope can even be positive for very high thrust coefficients.

At 14.5 m s-1, it is mainly the pitch that influences the induction variation around the mean wind speed, as the rpm is constant for wind speeds above rated. So, when the blade section is in a region with a lower wind speed than the mean, the pitch is too high, which gives a lower thrust and a reduced induction. Opposite again when the local wind speed at the blade section is above the mean, the pitch is too low corresponding to that wind speed, which gives an increased induction. In this region, we can thus conclude that both the effect from the changes in CT and the variation in local wind speed when deviating from the mean operational point have the same sign, which means that we always will see a decreasing induction from a decreasing local wind speed and vice versa for a local wind speed above the mean operational point.

The important impact on the loads is that changes in the local wind speed will always be counteracted to some extent by the induced wind speed and thus reduce the variations in AOA and likewise variations of the aerodynamic loads. This will be further explored below for turbulent inflow and quantified for a few test cases.

The characteristics of the induced velocities for turbulent inflow are basically determined by the same mechanism as described above for sheared inflow. As discussed above, the turbulent inflow with dimension of structures less than one rotor diameter cause a non-uniform inflow over the rotor disk. It means as for sheared inflow that a point on the rotating blade will see a local wind speed different from the mean wind speed corresponding to the mean operational conditions of the turbine. In Fig. , the induced velocity at radius 64 m is shown as function of wind speed for uniform inflow. The induced velocity as function of local wind speed from the same position on the blade for simulation with turbulent inflow at a mean wind speed at 14.5 m s-1 and a turbulence intensity of 15 % with standard control is shown as dots. Figure a shows the mean annular BEM, and Fig. b shows the grid BEM results. As the mean wind speed changes continuously for turbulent inflow, the ui,y(U0) curves as discussed above are more difficult to see here. However, for the mean annular BEM, the horizontal patterns of the dots are visible. For the grid BEM, we have to imagine that the ui,y(U0) curves have the negative slope, as shown above for shear at 14.5 m s-1.

The impact of the grid BEM model on fatigue loads and power production according to DLC 1.2 has been investigated. Computations were performed for both the DTU 10 MW RWT and the AVATAR 10 MW turbine . To avoid seed dependency, 18 seeds at each wind speed were used: six seeds at 0∘ yaw error and six seeds at ±10∘ yaw error, respectively. The wind speeds range from 4 to 26 m s-1, with a 2 m s-1 spacing.

For brevity, this section focuses only on the 1 Hz equivalent load of the flapwise blade-root-bending moment and the mean power. All results are presented as percent relative difference compared to an annular BEM model that includes the yaw correction presented in Sect. . Results from a mean annular BEM model without yaw correction are also included so that the influence of grid versus mean annular BEM can be compared to the impact of a more widely used type of BEM model. To isolate the reaction of the induction model to shear and turbulence, additional runs of DLC 1.2 without shear and turbulence are shown. The runs without shear use the same 18 seeds per wind speed at 0∘ + 10∘ - 10∘ yaw error as the regular DLC 1.2 computations.

The results for the DTU 10 MW RWT are shown in Fig. . It can be seen that the difference of grid compared to annular BEM has a much larger impact on the results than the yaw correction. The yaw correction has some influence at wind speeds below rated, but above rated wind speeds, the influence is close to zero.

Overall, the grid BEM results in significant lower fatigue loads, up to 8 %, except in a narrow wind speed interval between 7 and 10 m s-1 with an increase of 1 %, as seen in Fig. a. When splitting up in contributions from turbulent inflow and shear, we can see in Fig. b that the fatigue from turbulence is reduced for all wind speeds for the grid BEM with a reduction of roughly 6 % at 16 m s-1 and above. However, for the impact from shear, the fatigue is increased up to 6 % at low wind speeds, which is due to the high thrust coefficient for that rotor, causing a positive slope of the ui,y(U0) curve as discussed above.

The influence of the grid-based BEM for the power production of the DTU 10 MW is very small at roughly ±0.3 % below rated. In the pure shear case at 4 m s-1, a large power increase by 6.5 % can be seen, but that increase almost disappears for combined turbulent and sheared inflow.

The results for the AVATAR turbine (Fig. ) show a much larger impact of the grid BEM approach, while the yaw correction only has very minor influence. Relative to the annular BEM, the fatigue loads predicted by the grid BEM in pure shear are reduced on average by roughly 12 %, the loads in pure turbulence by 7.5 % and in the combined case by roughly 10 %. At the same time, the power below rated is predicted to increase by roughly 0.5 %, which seems to be mainly due to better operation in turbulent inflow.

Comparing the two cases, we can conclude that the impact of the grid BEM approach depends on the actual turbine design with an increasing reduction of fatigue loads for lower loaded (low-induction) rotors. For both turbine designs, the load reduction is considerable (8 % to 10 %) for wind speeds above rated power.

In , a validation study of both the HAWC2 model and the dynamic wake meandering (DWM) model was carried out on the basis of comparisons of model predictions with full-scale turbine measurements from the Dutch wind farm Egmond aan Zee consisting of 36 Vestas V90 turbines. In the paper, it is concluded that the measurements are of a remarkable high quality, enabling comparison of not only fatigue loads but also simple statistics in terms of maximum, minimum and mean values. It was found that when comparing the predicted power curves with measurements in both free and wake sectors, an excellent agreement is seen. Further, a very fine agreement was also seen between measured and simulated loads in both the free sector and a sector with wake effects from five turbines separated with seven diameters.

In the other validation publication by , the coupling of the HAWC2 structural model to EllipSys3D is presented. This provides an excellent basis for validation of the grid BEM aerodynamic model for simulations on the NREL 5 MW turbine , as a direct comparison with high-fidelity model results for the exact same input data and structural model is possible. Besides results for uniform inflow, a comparison of flapwise and edgewise tip deflection as function of azimuth is presented for 0, 30 and 60∘ yaw angles. In the paper, it is concluded that “both models still show a very good agreement”. Finally, a challenging case of an emergency shutdown is presented and also for that case it is concluded that the responses of the two models agree very well.

The first validation case is to demonstrate the model response to a considerable shear in the inflow for the NREL 5 MW turbine . We have chosen a case with shear in the horizontal plane because a vertical shear representing atmospheric inflow with shear can be considerably influenced by the interaction of the flow with the ground surface and thus disturb the direct impact of the induction modeling . An artificial shear inflow was created by changing the inflow velocity from 10 to 5 m s-1 over a narrow region around the hub center, according to the following analytical expression: 35U0=U0,max0.75-0.25tanh8.78044xR, where x is the horizontal distance from the rotor center, and R is the rotor radius. The resulting horizontal shear profile is shown in Fig. .

The CFD simulations were carried out with the 3-D incompressible Navier–Stokes solver EllipSys3D by and , with a surface-resolved representation of the rotor, omitting the nacelle and tower. The flow on the no-slip surface of the rotor was assumed fully turbulent and modeled using the k-ω–SST model by . In the computations, we used an overset grid approach in which a curvilinear rotor resolved mesh rotated together with two successively coarsened cylindrical meshes resolving the near field around the rotor, which were embedded in a larger stationary coarse semi-cylindrical mesh resolving the far wake with the far-field boundaries placed eight rotor diameters away from the surface and a ground boundary modeled using a symmetry boundary condition placed 90 m below the rotor center. The surface of each blade was resolved with 256 cells in the chordwise direction and 128 cells in the spanwise direction, and grown into a volume mesh with 64 cells normal to the surface using the in-house hyperbolic mesh generator HypGrid . The first cell height was set to 1×10-6 m, resulting in a y+ value below 2. The full grid assembly contained 15×106 cells. Figure  shows a front view and side view of the volume mesh.

To minimize the computational time, both grid sequencing and time step sequencing were used. To settle the overall induction field, the flow was simulated with a coarse time step of 2.2765×10-2 corresponding to 300 time steps per revolution for 20 revolutions on a mesh coarsened by a factor of 2 in each coordinate direction (Gr3). With the same mesh refinement level, the time step was subsequently refined by a factor of 5 to 4.553×10-3, yielding 1500 time steps per revolution for another 15 revolutions (Gr2). Finally, the mesh was refined to the finest grid level and the time step refined by a factor of 2 to Δt=2.2765×10-3 (Gr1). The resulting mean integral forces of the grid/time step sequence are shown in Table .

A user-defined shear flow can be input to a HAWC2 simulation so the case could be simulated by a default setup. When comparing the normal and tangential loading on the blade at azimuth positions of 90 and 270∘ (0∘ is vertical upwards), which are in the extreme low and high inflow regions, we find overall a good correlation as can be seen in Fig. . There are minor deviations in the tip region where the grid BEM overestimates the normal force loading. Also, the tangential loading is slightly underestimated on the central part of the blade for the 270∘ azimuth position.

The case is further analyzed by comparing the integrated normal and tangential blade forces as function of azimuth as shown in Fig. . Again, an overall good correlation between the high-fidelity CFD results and the grid BEM results is found. However, there is a time delay for HAWC2 in the rising of the loads from low to high wind inflow (high to low CT) around an azimuth of 180∘. However, the same is not seen at around 0∘, where the wind speed is changing from a high to low value.

Detailed aerodynamic measurements on full-scale turbines are very limited. However, in the DanAero project, such measurements were carried out in 2009 on a NM80 turbine with an 80 m diameter . The experimental setup comprised surface pressure measurements at four radial positions from which the aerodynamic forces normal and tangential to the local cord were derived. A validation exercise using these data was described and presented recently by , so we will only present a single set of results from that paper. The case is for a mean wind speed of 6.1 m s-1, a turbulence intensity of 6.8 % and minimal shear. For details of the experimental and modeling setup, the reader should see .

The comparison of PSD spectra of the measured and simulated aerodynamic forces perpendicular to the chord is shown in Fig. . Besides the grid BEM results, we have also added the mean annular results for comparison with the measurements. Overall, the correlation between simulations and measurements is good. In particular, the 1P, 2P and 3P peaks are captured well, and both simulation and experiment show the increasing size of the peaks towards the tip of the blade due to the rotation sampling effect of the turbulent inflow.

There is a clear tendency for the simulated spectra to fall below the measured one at higher frequencies, in particular for the outboard stations, which might be due to the resolution in the turbulence box which is 1.28 m in the vertical and horizontal directions. Finally, it can be seen that in this case the difference between the two BEM implementations is quite small. This can be explained by the above considerations in Sect. 4: if the local thrust coefficient is high, the slope of the ui,y(U0) curve in the grid BEM becomes almost horizontal and thus equal to the annular mean BEM. However, a light tendency of the annular mean BEM to overestimate the 1P and 2P on the two inboard stations with a lower thrust coefficient confirms the expected trend.

In the New Mexico experiments , the aerodynamic loading on a 4.5 m diameter model turbine in uniform inflow and yawed inflow was measured. These measurements have been compared to results from many aerodynamic codes of different fidelities in . For a specific evaluation of the yaw modeling (Sect. ), we look at the differences between the aerodynamic forces between the uniform and 30∘ yawed flow cases at roughly 15 m s-1 wind tunnel speed. In both cases, the turbine had a rotor speed of 425.1 rpm, the blades were pitched at -2.3∘, and the tunnel speed was very similar at 15.06 m s-1 (axial flow) and 15.01 m s-1 (yawed flow). As such, these measurements provide an excellent opportunity to validate the effect of yaw on both the mean and azimuthally varying load levels. Figure  shows the differences in normal (perpendicular to local chord) and tangential (parallel to local chord) forces at three measured sections at 25 %, 60 % and 82 % radius. These differences are computed as ΔFn/t=Fn/t,yaw-Fn/t,axial. Such a comparison involving both axial and yawed flow measurements and computations together was not included in .

It can be seen that there is a phase shift in the azimuthal force variation between the normal forces at the inboard section (Fig. a) and the section further outboard (Fig. c). This phase shift is due to the dominance of either the root vortex (at the inboard section) or the tip vortex (outboard section). The root vortex is not accounted for in the present model, and thus the HAWC2 computations do not agree well with the measured normal force at the inboard section. A recent engineering model that is based on high-fidelity simulations and includes a correction for the root vortex is described by .

For the sections further outboard, the influence of the tip vortex becomes more important and the phases of the azimuthal force variation agree well. There is a slight overprediction of the mean loading, especially in the tangential direction. Comparing the integrated out-of-plane and in-plane blade-root-bending moments in Fig.  shows that the phase difference seen in the inboard loads is not significant for the blade root moments. HAWC2 predicts a smaller reduction of the mean out-of-plane and in-plane moments, but the phases compare well to the measurements.

The NREL/NASA Ames phase VI experiments , performed in the NASA Ames open-loop wind tunnel, include runs targeting dynamic inflow effects at 5.1 m s-1 wind tunnel speed (denoted as case Q0500000). The rotor speed was constant at 71.62 rpm and the pitch was varied 20 times at a rate of roughly 66∘ per second between -5.9∘ pitch (heavily loaded rotor, induction factor a≈0.5) and 10.02∘ pitch (unloaded rotor, a≈0). Averaging the responses of these 20 pitch steps shows pronounced dynamic inflow effects at all the instrumented blade sections at 30 %, 47 %, 63 %, 80 % and 95 % radius.

The measured data have been analyzed by using a BEM code and a free wake code and by using a BEM code, a computational fluid dynamics code and a near-wake model. More recently, this case was also used for comparison of various research codes in IEA Task 29, phase III . An investigation of the radial dependency of the time constants in the force response, which seemed to reverse when the pitching direction was reversed, was conducted by .

A comparison of measurements with the dynamic inflow model described in Sect.  is shown in Fig. . All the forces are scaled such that the pitch steps are between 0 and 1. This approach makes it possible to compare the dynamic response at different sections on the blade easily and was also used by . The computations assume a stiff turbine. It can be seen that the force overshoots at the 30 % section are generally larger than at the 80 % section. This is due to the slower response inboard due to the larger distance from the tip vortex. The dynamic inflow model takes this radial dependency of the time constants into account and the predicted overshoots of the forces are generally in good agreement with the measurements. An exception is that the overshoot of the tangential force at the inboard section (solid lines in Fig. b) is underestimated by HAWC2. The behavior of the tangential force for the pitching down case (Fig. d) can be explained by a zero crossing of the angle of attack at roughly 0.4 s. For both positive and negative AOAs close to 0∘, the lift force has a component that is pointing towards the leading edge. Therefore, the forces when pitching to lower loading are decreasing until the AOA reaches roughly zero. Then, the tangential forces increase as the AOA undershoots and then decrease again as the induced velocities decrease (causing AOA to increase and move closer to zero) towards the equilibrium state at low loading.

The comparison shows good agreement; however, some disagreement is to be expected due to inherent limitations of the actuator-disk-based model. Specifically, the root vortex dynamics are missing and the disk model also assumes an infinite number of blades. Therefore, differences are expected close to the root and the tip of the blade, where the induction from a helical wake deviates most from the induction due to a cylindrical wake. An option to address these limitations is to couple a vortex-based near-wake model to the BEM code . However, the work in IEA Task 29 has shown that care has to be taken when coupling the induction dynamics .