The low-frequency emissions from a generic

Renewable sources of energy and especially wind power have seen a strong expansion in the last years. Even though the construction of large offshore wind farms is currently a strong focus, the potential of onshore wind turbines by opening up new, previously unused areas and repowering of existing sites is still significant. With regard to the acceptance and the fulfillment of stricter legal requirements concerning noise and vibrations, the research on low-frequency emissions from wind turbines gains importance.

As wind turbines are counted among the tallest machines on the planet that
work in an uncontrolled outside environment, noise and vibration emissions
occur in a broad frequency range. While sources of acoustic wind turbine
emission in the audible range are widely researched and understood and
different methods are applied to reduce aerodynamic and mechanical noise

Acoustic measurements in the low-frequency range

The scope of research on low-frequency noise from wind turbines is often its
impact on human beings.

For an optimization of the structure and foundations of future wind turbines
as well as for the assessment of the impact of low-frequency noise and
low-frequency seismic vibrations on the environment, reliable methods for the
prediction of emissions are of great importance.

There are few studies on the modeling of aeroacoustic low-frequency emission
from wind turbines. In the 1980s, NASA developed a code for predicting
low-frequency wind turbine noise based on Lowson's acoustic equation applied
to rotor forces

In recent years, CFD-based fluid–structure coupling has been applied
frequently for the investigation of wind turbines.

A totally revised FLOWer–SIMPACK coupling is revealed in the present
paper with the potential to take into account more degrees of freedom, like
tower deformation or changes in rotational speed in the structural model and
their impact on aerodynamics and aeroacoustics, respectively. Together with
the already existing process chain, fully coupled CFD simulations under
realistic turbulent inflow conditions can be conducted, providing both
airborne and structure-borne emissions simultaneously. A FW-H in-house code
is applied to calculate acoustic pressure at distant observers while
tower base loads represent the structure-borne emission. The aim of the
present paper is to identify the sources of low-frequency emissions and to
investigate the impact of the complexity of the numerical model on the
calculated low-frequency emissions from a generic

A high-fidelity process chain based on multiple solvers is established for the investigation of low-frequency emissions from wind turbines. It consists of the CFD solver FLOWer, the MBS solver SIMPACK and the FW-H solver ACCO. A coupling between FLOWer and SIMPACK was developed to generate high-fidelity time series of surface pressure distribution on the turbine and structural loads (forces and moments) acting on the foundation of the turbine. Using the CFD results, the aeroacoustic signal at distant, predefined observer positions is computed by means of ACCO.

FLOWer is a compressible, dual time-stepping, block structured
Reynolds-averaged Navier–Stokes (RANS) solver developed by the German Aerospace
Center (DLR)

SIMPACK is a commercial nonlinear MBS solver that can be applied to
simulate dynamic systems consisting of rigid and flexible bodies. Flexible
turbine components like tower and blades are modeled with linear or nonlinear
beam theory. The kinematics between the components are defined by joint
elements and internal forces can be considered. There are two ways to apply
external forces such as aerodynamic forces: either by built-in interfaces or
by programmable user routines. Controllers can also be integrated.
SIMPACK has been recently applied by industry and research groups for
the simulation of wind turbines. Examples can be found
in

To take the influence of unsteady structural deformation on the aerodynamics into account, a revised coupling between FLOWer and SIMPACK is implemented. The new approach generally allows the coupling of slender beam-like structures and is not limited to rotor blades or even wind turbines. Combined coupling of rotating and non-rotating parts can be applied and the deformation of adjacent structures is considered. Furthermore, coupling is not restricted to flexible deformations but rigid-body motions (rotations and translations) can also be realized. In the application to wind turbines, pitch motions and changes in rotational speed of the rotor can be transferred from the MBS solver to the CFD solver.

For the technical realization, an existing interface that was developed to
couple SIMPACK with the fluid solver ANSYS CFX for the
investigation of a tidal current turbine

Explicit coupling scheme of the FLOWer–SIMPACK coupling.

The developed coupling is a partitioned approach, where two independent solvers run simultaneously on different machines and exchange data via a Secure Shell (SSH) connection at discrete positions, so-called markers. The markers are positioned inside the bodies. While rigid bodies only have one marker, flexible bodies like rotor blades have several markers that are distributed along the beam. On the one hand, deflections and rotations of these markers relative to their non-deformed position are computed by SIMPACK. On the other hand, aerodynamic forces and moments acting on these markers are calculated in FLOWer. For each structure that is coupled, a communication coordinate system is defined that has to be in the same position and same orientation in both models at all times. It does not have to be fixed but can be rotating or translating in a predefined way. All data concerning the respective structure is communicated in this coordinate system.

The task of the deformation library implemented in FLOWer is to apply
the deformations of the markers on the corresponding CFD surfaces and to
deform the surrounding volume mesh accordingly. The surface is represented by
a point cloud which is generated from the CFD mesh. For rigid structures only
one marker is used and all surface cloud points perform a rigid-body motion
based on the translation and rotation of this marker. A cubic spline
interpolation is applied for the mapping of flexible structures (beams)
consisting of more than one marker. The deformation of each surface cloud
point is then realized as rigid-body motion based on the corresponding
positions along the beam. While a complete spline approach is used for the
deflections, taking the rotation at the end points into account, the
rotations and the non-deformed marker positions are interpolated using
natural splines. A similar approach has been presented by

The load library implemented in FLOWer enables the calculation of aerodynamic loads on grid surfaces by the integration of friction and pressure over the cell faces. This is also necessary for the coupling to SIMPACK, as there is no surface in the structural model and the aerodynamic forces have to be mapped to the discrete marker positions. For this purpose, the CFD surface is divided into segments based on the deformed marker positions. For each of these segments, loads are integrated and afterwards assigned to the respective markers. Moments are calculated with respect to the origin of the corresponding communication coordinate system. For structures with only one marker, loads are integrated over the whole CFD surface of the respective structure.

The communication is realized by means of files. Data files contain deformations or loads and status files indicate that the data file is ready to be read. While SIMPACK is running on a local Windows machine, FLOWer is usually executed in parallel mode on a high-performance computing (HPC) system running on Linux. A portable communication script in Windows' inherent scripting language PowerShell enables fast and reliable communication between the two solvers. The Linux machine is accessed using a SSH connection via the Windows Secure Copy (WinSCP) client.

In the presented work, an explicit coupling scheme is applied. The size of
the coupling time step is equal to the physical FLOWer time step and
remains constant throughout the simulation. Both solvers run in a
sequential way, waiting for the other solver to reach the next time step and
to send communication data. SIMPACK runs one time step ahead
doing time integration with the aerodynamic loads that FLOWer computed
at the end of the previous time step (Fig.

Acoustic pressure at arbitrary observer locations is calculated by means of
the in-house FW-H solver ACCO. Pressure and velocities on surfaces
enclosing the noise sources are evaluated at each time step of the transient
CFD solution, including velocities due to the deformation, translation and
rotation. For the present study, the surfaces used for the acoustic analysis
are identical with the physical surfaces of the turbine (rotor, tower, hub, etc.). Volume sources generated by free-flow turbulence are neglected, which
is justified for low Mach number flow because quadrupole volume noise is
proportional to Ma

The application of the FW-H analogy allows the evaluation of the contribution of selected components of the wind turbine to SPL by excluding surfaces of particular components (e.g., tower) from the analysis.

The examined turbine is based on the
generic 5 MW turbine developed by the National Renewable Energy Laboratory (NREL;

CFD surface mesh, showing the connection of hub, blades and nacelle with overlapping meshes.

The CFD model of the OFFWINDTECH turbine consists of 10 independent body meshes, which are embedded in a Cartesian hanging grid node
background mesh using the CHIMERA technique. Blades, hub, nacelle and
tower are considered in the simulation with fully resolved boundary layer
(

Concerning inflow, three different cases are regarded in the present study.
Uniform inflow, steady atmospheric boundary layer and turbulent atmospheric
boundary layer. An exponent of

The SIMPACK model of the OFFWINDTECH turbine was built
by

Details on the foundation of the wind turbine, similar to

Definition of simulation cases, ordered according to increasing complexity.

ABL, atmospheric boundary layer; SD, steady deformation.

Overall,

CFD surface of turbine including markers for coupling with SIMPACK. Rotating hub coordinate system is shown in blue and tower base coordinate system in red.

In Table

The aim of the simulation chain is to model airborne and
structure-borne emissions simultaneously by evaluating SLPs at
distant observers and load fluctuations at the tower base. In the
fluid–structure coupled simulations tower base loads are evaluated directly
in the structural model at the interface between tower and foundation,
whereas in the non-coupled simulations aerodynamic loads are computed from
CFD results. In both cases the tower base loads are presented with respect to
the tower base coordinate system which is shown in
Fig.

Observer positions for the evaluation of aeroacoustic emissions. Tower base coordinate system shown in red. View from above; turbine in the center; wind from left.

Acoustic simulations using ACCO are conducted to calculate the
acoustic pressure at a carpet of observers on the ground surrounding the turbine.
Figure

In this section three non-fluid–structure coupled cases are compared at
uniform inflow conditions. As reference the rotor only case (LC1) is regarded
where unsteady effects on the loads only result from the tilt of the rotor,
the proximity to the ground and unsteady flow separation. In a second case,
the tower is considered (LC2), and in a third case steady deformation is
applied to the blades (LC2_FSC1SD). The CFD surfaces of all three
cases are shown in Fig.

CFD turbine surfaces of cases LC1

Spectra of aerodynamic loads with respect to tower base (moment reference point) for cases LC1, LC2 and LC2_FSC1SD.

In the non-fluid–structure coupled cases, no unsteady structural forces occur
as all structures are rigid. Thus, load fluctuation only arises from
aerodynamics. Figure

The composition of the aerodynamic loads is investigated in detail for case
LC2_FSC1SD. Therefore, aerodynamic loads on rotor and tower were
evaluated separately with respect to the tower base coordinate system (moment
reference point). Figure

The surface pressure amplitudes on the tower are displayed in
Fig.

Spectra of aerodynamic loads with respect to tower base (moment reference point) for case LC2_FSC1SD. Comparison of loads on rotor, tower and all surfaces.

Pressure amplitudes on CFD tower surface of case LC2_FSC1SD
at

Figure

Spectra of unweighted SPL (reference sound pressure of

To examine the aeroacoustic noise emission in detail, the noise emission
originating from tower and rotor surfaces are evaluated separately for case
LC2_FSC1SD. Figure

Spectra of unweighted SPL (reference sound pressure of

Unweighted SPL (reference sound pressure of

In the second study, the cases LC2_FSC1SD, LC2_FSC1 and LC2_FSC3 are regarded. The aim is to evaluate the influence of the degrees of freedom of the structural model on the low-frequency emissions from the wind turbine. Case LC2_FSC1SD has zero degrees of freedom but considers the mean blade deformation of case LC2_FSC1 where only the rotor blades are flexible; thus, it is chosen as reference case for this study.

The spectra of the tower base loads for all three cases are plotted in
Fig.

Spectra of tower base bending moments for the cases LC2_FSC1SD, LC2_FSC1 and LC2_FSC3.

There are two effects which go hand in hand, both having an influence on the
tower base loads. When considering the flexibility of the blades, on the one hand,
gravitational forces and inertial forces start acting and, on the other hand,
aerodynamic forces change due to unsteady deflection of the blades. The mean
blade tip deflection applied in case LC2_FSC1SD is

The enabled flexibility of the tower in case LC2_FSC3 shows a much
stronger impact on the tower base loads compared to case LC2_FSC1 as
it significantly changes the structural eigenmodes of the turbine. In

The increase in degrees of freedom in the structural model only marginally influences the SPL at the observers. The spectrum at observer position C shows a small decrease in the amplitude at BPF while there is a small increase at second to sixth harmonics of BPF. However, observer D shows a small increase at BPF while amplitudes of higher harmonics are almost unchanged. Generally, the effect is a bit stronger for case LC2_FSC3. These small changes might be an impact of the slightly reduced blade–tower distance and the increased blade tip velocity when the blade passes the tower, which is reported in the previous section. For frequencies below BPF, the maximum amplitude increases slightly, which could be induced by the structural eigenmodes of the turbine as well as by the impact of vortex shedding at the tower.

In the last study the influence of inflow conditions on the tower base loads and on the aeroacoustic emission is investigated. While uniform inflow is applied for the previous studies, more realistic inflow is considered in this study. Two cases – one with vertically sheared inflow (LC3_FSC3) and one with turbulent vertically sheared inflow (LC4_FSC3) – are compared to the uniform inflow case (LC2_FSC3). For the turbulent inflow case, a longer time series is evaluated in order to obtain more representative results.

The spectra of tower base loads in Fig.

Spectra of tower base bending moments for the cases LC2_FSC3, LC3_FSC3 and LC4_FSC3.

By superimposing turbulence on the vertically sheared flow in case
LC4_FSC3, the character of the spectra changes as the amplitudes at
BPF harmonics become much less prominent. There are some clear peaks
remaining, but the broadband load level massively increases. The global
maximum now arises for

Figure

Spectra of unweighted SPL (reference sound pressure of

Taking the turbulent inflow into account (case LC4_FSC3) leads to an increase in the broadband noise level due to turbulent inflow noise, generated by the interaction of the rotor blade with the turbulence. The inflow noise is emitted from the rotor and predominantly directed in upstream and downstream direction, leading to higher broadband noise levels at observer C compared to observer D. Since the rotor blades encounter the turbulence at considerably higher relative velocity than the tower, the emission from the tower hardly increases compared to case LC3_FSC3. However, despite the increased broadband noise level, the peaks at BPF harmonics are still dominant at both observer positions.

In the first study the influence of the presence of the tower and of steady
blade deformation on low-frequency emissions is evaluated at uniform inflow
conditions in stand-alone CFD simulations. Concerning the aerodynamic loads,
the presence of the tower leads to an increase in amplitudes at BPF and its
higher harmonics. Applying a steady deformation to the rotor blades further
increases the amplitudes especially for higher harmonics due to the stronger
blade–tower interaction. Splitting the loads up into rotor and tower loads
shows that the major part of the fluctuations originates from the tower and
is caused by blade–tower interaction. Load oscillations induced by vortex
shedding can be observed but do not play an important role. Evaluating the
SPL on the ground at a distance of

In a second study, the influence of degrees of freedom in the structural
model is investigated using three cases: one with steady blade deformation
already regarded in the first study, another with flexible blades, and a
third with additionally flexible tower and foundation. Flexible blades have
only a minor impact on the calculated tower base loads. Structural eigenmodes
play a more significant role in the third case when tower and foundation are
flexible too. The peaks at BPF harmonics are still prominent, but the
amplitudes change and the maxima are shifted towards BPF harmonics close to
structural eigenfrequencies. Additionally, peaks corresponding to the first
bending modes of the tower (

The third study deals with the influence of the inflow condition on the emissions. Uniform inflow is compared to vertically sheared inflow with and without turbulence. For vertical shear inflow, tower base loads tend to increase at BPF and decrease at higher harmonics of BPF. With superimposed turbulence, the peaks become much less prominent since the broadband load level rises. Amplitudes at frequencies close to structural eigenmodes rise, and BPF harmonics become less dominant in the spectra. The tonal noise level of the aeroacoustic emission tends to reduce slightly with the vertical shear and increase again due to the superimposed turbulence. The broadband noise level strongly increases, especially for observers upstream and downstream of the turbine, which is mainly caused by turbulent inflow noise emitted by the rotor. Thus, the BPF harmonics become less prominent but are still dominant in the spectra.

As a generic wind turbine is investigated, no measurements for validation are
available. Nevertheless, a qualitative comparison between the presented
results and two studies found in the literature is drawn.

Despite the advanced modeling approach applied in the presented study, there are still several limitations that have to be mentioned. In the applied FW-H calculations, effects of unsteady flow field, refraction and reflection of acoustic waves and atmospheric layering are not taken into account for the propagation. On the other hand, this makes the method very suitable for the investigation of the aeroacoustic emission of the turbine, as the SPL at the observer positions is not influenced by the effects mentioned above. Due to the computationally expensive CFD approach, there are limitations concerning the length of the time series and temporal resolution and consequently the statistical convergence of the results and the resolved frequency range. Although the flexibility of rotor blades, tower and foundation is considered in the simulations, further degrees of freedom are neglected. The drive train is kept totally rigid and at fixed rotational speed. As SIMPACK is a MBS solver and only deformations of points along a beam are transferred, eigenmodes of the shell cannot be considered in the presented approach. However, the shortcomings mentioned do not change the general findings of this paper.

In the present paper the low-frequency emissions from a generic

A major advantage compared to lower-fidelity approaches is that, as all
geometries of the turbine are fully resolved, the unsteady pressure
distributions on all surfaces, and thus all aerodynamic loads, are a direct
outcome of the simulations. Regarding the aeroacoustic emission it is found
that the blade–tower interaction plays a key role and the noise emitted from
the tower is higher compared to the noise emitted from the rotor. Only an
indirect impact of fluid–structure coupling on the aeroacoustics could be
observed. Elastic blades reduce the distance between blade and tower and thus
increase the strength of the blade–tower interaction. Turbulent inflow on
the other side mainly influences the broadband noise level of the rotor. For
the regarded turbulence level of

Blade–tower interaction also has a great influence on the tower base loads; however, with increasing degrees of freedom structural eigenmodes play a much stronger role than for the aeroacoustic emission and amplitudes at eigenfrequencies become more dominant when turbulent inflow is applied. Nevertheless, blade-passing frequency harmonics can still be identified in the spectra. For aerodynamic load fluctuations at uniform inflow, it is found that the contribution of the tower exceeds the contribution of the rotor.

Several conclusions for the modeling of low-frequency emissions using CFD simulations can be drawn from the conducted studies. The blade–tower interaction is found to be the main source of aeroacoustic noise and triggers a major part of the aerodynamic load fluctuations. The tower itself as well as a realistic blade–tower distance has to be considered in the simulation to capture the blade–tower interaction properly. Fluid–structure coupling is the most appropriate way to a realistic blade–tower distance and is mandatory if structural emission shall be regarded. Moreover, the acoustic emission from the tower has to be considered in the noise evaluation and the loads on the tower have to be included in the fluid–structure coupling. Concerning the structural emission, it is not only the flexibility of the rotor blades but also that of the tower and foundation that have to be taken into account as they change the character of the tower base load spectra. Turbulent inflow should also be taken into account because it enhances the excitation of structural eigenmodes.

The findings can be transferred to any modeling method of low-frequency emissions from wind turbines. The method has to be capable of capturing the impact of blade passing not only on the blades but also on the tower and its effect, on the one hand, on the aerodynamic load fluctuations and, on the other hand, on the aeroacoustic noise emission.

Future work will deal with several of the listed limitations. A slightly smaller commercial wind turbine will be investigated numerically with the presented approach and field measurements will be available for comparison. Subsequently, the turbine will be simulated taking into account the operational conditions of the measurements. The influence of full shell coupling on the low-frequency emission will be investigated in a future study. Based on the presented findings, constructional measures such as lattice towers, increased blade tower distance or swept blades are likely to reduce low-frequency emissions and should be taken into account for future research.

Data of the NREL

LK implemented the coupling, performed the CFD-MBS simulations and wrote most of the paper. JG was responsible for the acoustic simulations and the turbulent inflow and contributed parts of the manuscript. FW contributed parts of the manuscript. TL and EK initiated the research, supervised the work and revised the manuscript.

The authors declare that they have no conflict of interest.

The studies were conducted as part of the joint research project “Objective Criteria for Seismic and Acoustic Emission of Inland Wind Turbines (TremAc), FKZ 0325839A”, funded by the German Federal Ministry for Economic Affairs and Energy (BMWi). The authors are grateful for the financial support. The authors gratefully acknowledge the High Performance Computing Center Stuttgart for providing computational resources within the project WEALoads. Edited by: Alessandro Bianchini Reviewed by: three anonymous referees