WO1998005868A1

WO1998005868A1 - Aeroacoustic optimisation process of an axial fan

Info

Publication number: WO1998005868A1
Application number: PCT/EP1997/004205
Authority: WO
Inventors: Dieter Lohmann
Original assignee: Deutsches Zentrum für Luft- und Raumfahrt e.V.
Priority date: 1996-08-01
Filing date: 1997-08-01
Publication date: 1998-02-12
Also published as: EP0917627A1; DE19631093A1; DE59704806D1; EP0917627B1

Abstract

A process is disclosed for optimising the aeroacoustics of an axial fan. During a first phase, based on a digital computer simulation, the blade geometry of the axial fan blades is aerodynamically optimised by applying a reverse digital design process in conjunction with a first optimisation algorithm. In a second phase, in conjunction with a second optimisation algorithm, the blade geometry is acoustically optimised a first time to reduce stochastic noise by means of another digital process based on turbulence models, then the blade geometry and distribution are acoustically optimised a second time to minimise the periodic noise fraction by means of a singularity calculation process.

Description

Process for the aeroacoustic optimization of an axial fan

The invention relates to a method for the aeroacoustic optimization of an axial fan, in which, based on a numerical computer simulation, an aerodynamic optimization of the blade geometry of the axial fan takes place in a first phase by using an inverse numerical design method in connection with a first optimization algorithm.

A corresponding method for an axial fan, in particular for a cooling fan of a motor vehicle engine, is known from DE 43 26 147 A1. Axial nuts are, in particular, those fans that are used to cool the engines of motor vehicles and are driven mechanically or electrically. In the case of an electric drive, a thermostat switches the drive on when the head wind is no longer sufficient for cooling. In operation, the axial fan generates noise in addition to the desired cooling. When driving slowly or standing still, the noise generated can exceed the ambient noise and thus become a significant source of noise. It is therefore desirable to reduce the noise without reducing the fan performance. However, such an aeroacoustic optimization is subject to a number of boundary conditions, such as e.g. B. a low installation depth, low manufacturing costs, a design that is as recyclable as possible and a certain noise spectrum generated. The installation depth in the axial direction is of particular importance since appropriate fans have to be installed in the vehicle between the motor vehicle engine and the radiator with condenser. In the motor vehicle industry, therefore, a maximum installation depth of 15% of the radius is required. With regard to the noise spectrum, broadband noise tones at low levels are desirable compared to undesired whistle tones.

The axial fan known from DE 43 26 147 A1 ensures, due to the design method described therein, an axial fan with three-dimensionally designed fan blades, each of which has a strong forward sweep and a strong reverse sweep in the manner of a bird's wing, whereby the aerodynamics and automatically the acoustics are largely optimized at the same time. That is, a blade geometry is known for an axial fan in which

CONFIRMATION COPY Roll-up swirls and induced detachments are influenced in such a way that the thrust and the efficiency are hardly influenced.

However, further and experimental investigations have shown that leaf geometries determined in this way also have zones of turbulent, detached or flow-entrained flow. Such flow phenomena are stochastic in nature and cannot be calculated using the design method known from DE 43 26 147 A1 alone and can thus be avoided entirely.

From the essay "Aeroacoustic Optimization of a Fan Geometry", by Dring Dieter Lohmann, DLR News, Issue 78 (May 1995) pages 15 to 22, a further development of the above-mentioned also known as LBS code (Lifting Body Surface) emerges. Design process. Thereafter, an additional semi-empirical approach based on turbulence models is required for the complete recalculation of the fan acoustics, which, for example, result in an optimal blade geometry with regard to the acoustics. Although the results achieved in this way show a theoretical noise reduction of a few dB (A), a, albeit slight, reduction in thrust and efficiency was observed, which occurred due to the now increasing signs of separation in the middle area of the suction side and in the leading edge area of the blade geometry. Regardless of the result of the calculation, in order to avoid the greatest detachment, due to physical convictions, the forward arrow was successfully withdrawn somewhat in the hub area, as a result of which the losses became smaller again, as measurement results in the wind tunnel have shown.

Regardless of this state of the art, fans are aerodynamically designed using 2-D grid methods or singularity methods and tested in the wind tunnel. At low flow velocities that occur on the fan, the blades are usually not profiled. Rather, simple circular sheet metal profiles are used in spite of a greater flow resistance in order to prevent the flow from breaking off. Even before trying the prior art mentioned above, it had been found that fans with sickle-shaped blades could be quieter than the aforementioned. Corresponding sickle-shaped fan Sheets are known for example from US 5,064,345, DE 31 37 1 14 C2 and WO 90/15253. In this prior art, the type of sweep has generally been determined in trial trials. A physical explanation or even a mathematical law that describes why these fans are quieter is not known from the latter documents. In addition, it was found that an asymmetrical division of the leaves changes the spectral distribution and thus the dB (A) weighted noise level can be reduced. However, the level of the corresponding noise reduction could not be determined.

With the known design methods mentioned, the three-dimensional effects (3-D effects), which are of great importance for an optimal design, could not be completely captured. In addition, the type of sickle-shaped sweep could not be fully determined by calculation, taking into account the aerodynamic coefficients, the profile shape, the blade depth and twisting. In particular, the methods known in the prior art generally cannot completely calculate the sound level of fans depending on the blade geometry, which is why the known design methods cannot satisfactorily solve the following task.

The object of the present invention is to provide a method for the aeroacoustic optimization of an axial fan or a blower, propeller or rotor, in particular in the area of its wing shape, with which it is possible to reduce the noise of axial fans by an aeroacoustic design based solely on an to significantly reduce numerical computer simulation for both aerodynamic and acoustic optimization without the performance of the axial fan deteriorating, that is to say it has a largely loss-free and interference-free flow.

According to the invention, this object is achieved in that in a second phase in combination with a second optimization algorithm - first a first acoustic optimization of the blade geometry to reduce the stochastic noise component with the aid of a further numerical method based on turbulence models, and - A second acoustic optimization of the blade geometry and blade division to minimize the periodic noise component is then calculated using a singularity method. Further developments of the invention are defined in the subclaims.

It is therefore advantageous to use the same potential function to carry out the aerodynamic and then the acoustic design of the fan, blower, etc. profile. The acoustic pressure results from the derivation of the potential function over time, the aerodynamics from the derivation according to the coordinates.

Surprisingly, it has been shown that on the basis of DE 43 26 147 A1 mentioned at the outset and the article, a draft method can be specified which makes it possible to improve the theoretical draft within the known method.

With the known design methods of the prior art, no designs or optimizations of small and large fans have been possible until now without having to experiment. According to the invention, however, an optimization of fans, etc. of a small and large type is now possible, which takes place computationally and only validates individual constants within the design code through an experiment, but otherwise allows purely computational optimization in aerodynamics and acoustics.

At this point, it is pointed out that the geometry of a rotor blade of a fan, propeller or rotor is always described with blade geometry and that the axial fan mentioned at the beginning is described with the term fan. However, it goes without saying that the proposed method can be applied in a similar form to other fans. In particular, it is also pointed out that the above-mentioned DE 43 26 147 A1 is intended to be a fully descriptive component of this application.

According to the invention, provision is advantageously made for the Larrabee method to be used as the inverse numerical design method in the first phase and for a suitable calculation specification depending on the Reynolds number Laminar profile or circular profile is selected and thus the blade depth and the twist β for an elliptical load distribution at the design point E is calculated with a design code DESI1, which is an aerodynamically inverse design process. In the Larrabee method, lift and drag coefficients of the profile (polar) are taken into account and a design is carried out at the optimum point of the characteristic. The dimensionless induced velocities in the axial (a _v) and azimuthal {a _n) in the direction of the propeller or the fan to result from

(4EsinΦcosΦ) ''

«* - + ι

^α "(4 sinΦ) ^

+ 1

(σc ^* )

where the division factor (solidity) c?

ß (- σ = ^• R)

(3)

(2π -) R '

with B: number of sheets

t: division

R: tip radius rr: local radius; -: relative radius

F: Prandtl's approximation function

Φ: Angle between effective inflow and rotation level

c-: Coefficient of force normal to the propeller axis

c _y : force coefficient tangential to the propeller axis.

The calculation is therefore carried out by iteration. Since the propeller or fan is determined by the number of blades, the course of the pitch, the blade angle ß and the coefficients of lift and resistance as a function of the relative radius, the angle of attack α is first specified, from which Φ _u = ß -α and lift and Resistance coefficients c _a , c "after

calculates, resulting in α _v and a.

This process is repeated until the difference jΦ _fl - Φ „| is small enough, resulting in the twist ß from Φ _α .

The leaf depth can then be determined using the floor plan equation

^C V «_f ^4π ) ^λG (5) determine, where the form factor ζ is calculated from the thrust specification, i.e. the desired static pressure difference, and G is the Goldstein function.

The Goldstein function is defined as

Fx ² ^{G =} T77 ⁽⁶⁾

Ωr with = - (7)

where F: Prandlian approximation function

x: reciprocal local level of progress

Ω: angular velocity

r: local radius

V: axial flow velocity.

The first optimization algorithm thus determines an optimum for load distribution. The following formulas are used to determine the optimum:

The ratio of the displacement speed in the axial direction to the axial flow velocity is identified by ζ and is as

41, TV 21, 1- l- Are defined.

I and I ₂ result from this

and the shear coefficient T _c too

T _c = ζ -I, -ζ ^{2 *} I ₂ (11)

where D / L is the ratio of the radial displacement of the blade and

Stellen the places on the sheet where the speed is determined

as a relationship

(12) R

indicates.

The blade angle ß at each radial location results from the sum of the torsion angle Φ and the angle of attack α, where the torsion angle Φ results in J λ 1 ^" φ = tan ^ (~) (l + - ζ) (13)

where - ζ indicates the slip.

The efficiency of the fan or rotor then results

where P. is the coefficient of performance and η- is calculated from

With

ε = tan ^"1 (D /) (16)

In the case of an axial fan, the optimization process is modified as follows:

The blade tip angle calculation is omitted, the induced axial velocity a _v is set to zero and the volume number φ is calculated according to v

Bernoulli changed to: φ, where - is the degree of progress and is defined

, π π φ V to (with v: hub ratio) or to - - - (with V

1 - V iiR _γ

Inflow velocity, Ω: rotational speed, R _τ : tip radius).

Since known field methods are very computationally intensive, particularly with regard to optimization calculations, a singularity method or a method such as Larrabee is recommended for examining the aeroacoustics of the fans and their design, which, as tests have shown, then approximates to an optimization algorithm using an optimization algorithm aerodynamically optimal blade geometry leads. In the case of fixed interfaces in a flow, sound-generating forces can be replaced by a distribution of dipole singularities on the surface of a fan (blade) profile. From the local lift coefficients, the integral aerodynamic coefficients of the pressure difference upstream and downstream of the blower and the power follow as the cause of the air flow generated. The efficiency is then direct to the thrust and inversely proportional to the power. As comparisons between theory and experiment have shown, the aerodynamics at design point E near the maximum efficiency can be theoretically predicted with sufficient accuracy.

It is also advantageously provided that the geometry or aerodynamic coefficients resulting from the optimization of the first phase are used as the default for the calculation and optimization in the second phase. For the first acoustic optimization, an optimal sweep of the leaf geometry for the decorrelation of sound-producing vortex structures on the leaf trailing edge and the leaf depth at design point E is calculated with a design code DESI2, which is a semi-empirical method for calculating stochastic noise. Decorrelation means influencing macroscopic vortex structures in such a way that smaller vortex structures arise. Suction tips, which lead to unpleasant whistling, can also lead to an early change from laminar to turbulent flow, which creates additional sound sources. The suction tip can be calculated using the LBS code. The size of the sound source field due to turbulence can thus be estimated. LBS means lifting body surface method, which is a computer-based singularity or panel method for calculating aerodynamics and acoustics (LBS code AERO or AKU). A certain section of the area on the fan blade is called a panel or singularity. For this particular section, a coordinate system can be defined with ξ, η, ζ, where ζ represents the normal on the panel surface. An observer control point, for example in the form of a microphone, is formed outside the sheet surface for calculating the acoustic pressure - for calculating the aerodynamics thereon. The distance to the panel surface is denoted by d. The propeller or fan and the microphone move relative to each other, whereby by definition the microphone in the LBS code AKU moves once by 2π during an overall calculation.

The acoustic pressure p is calculated by deriving the

Potential function according to time (by deriving the same potential function according to the coordinates, the corresponding formulas for

Calculation of aerodynamics using the LBS code AERO) following formula:

^{1 3} dffa „p„ V + ApcosΘ άS

A- & 5 d (l - M_)

⁺ Hfe r ^{rfS (17)}

in which

Θ the angle between the distance vector d and the unit vector normal to the panel ή

o "the speed of sound 12

p "the density of the undisturbed flow

V is the normal velocity component of the inflow

M _{d is} the Mach number in the direction of the panel (= source) observer

/ ^** , / ^** "aerodynamic pressure on the panel

S the panel area

δ is the boundary layer thickness.

The first term indicates the thickness noise, calculated according to the profile thickness. The second term represents the dipole noise and the third term takes into account the quadrupole or boundary layer noise. Each type of sound source is therefore examined separately. In the case of steady flow, d and M _{d are} functions of time.

It is advantageously provided that the LBS code AKU then calculates the pressure time curve of the fan rotation. The theoretical values thus determined are in good agreement with experimentally determined ones. The design process therefore provides a good design basis for periodic signals from fans.

As experiments have shown, there are still zones of turbulent, detached or swirling currents that are stochastic in nature and cannot be calculated directly. For the complete recalculation or correction of the fan acoustics, an additional semi-empirical approach based on turbulence models is therefore required, as is possible with the design code DESI2. With this code, the aeroacoustics of a fan can be satisfactorily recalculated or corrected. According to the invention, if the recalculation method is coupled with an optimization code, this work can be carried out with the second optimization algorithm. It is thus for example after a 13

Geometry changes in small steps first the aerodynamics and then the acoustics, whereby a minimum of noise while maintaining the aerodynamic performance is selected as the criterion. The geometry can be changed, for example, in the depth of the sheet and the displacement of individual cuts, so that a crescent-shaped sheet outline results from a rectangular one. However, other optimizations are also conceivable, such as. B. the twisting of the leaves, the number of leaves together with the diameter or an asymmetrical distribution of the leaves etc.

With a stochastic time dependency of speeds as in turbulent boundary layers, the stochastic noises cannot be assigned to the aerodynamic sources (as with periodic sound according to equation (17)). Statistical breakdowns in space and time factors are required, which is why turbulence models are formed. There is a description with transfer functions that are based on experimentally determined auto and cross-correlation functions of the aerodynamic loads.

The DESI2 process is based on these turbulence models. The sound sources are formulated as dipoles, resulting in a range of services. The fan or rotor blade is paneled similarly to the DESI1 method to calculate the broadband noise using this method.

The acoustic power spectrum of the sideband spectrum is dimensionless

with ξ = vΔ; v: hub ratio; Δ: characteristic turbulence length; 4-: Angle between the rotor axis and R _p , i.e. the distance between the fan center and the observer control point. E, "is the interference of sound in the far field

(nB - /) sin μ cos μ cos Ψ - ₀ (/ / Ω) πB- ² (M ₀ sinΨ) (19)

E, is the modulation of the harmonic frequencies due to turbulence

E _Λrf , is the influence of turbulence according to Liepmann's turbulence model

(21)

and

E, _pα „= (l + A- _r c / π _α . (23)

describe the transfer functions or the behavior of the load on the sheet with regard to convected caster.

The acoustic level of the trailing edge and sideband noise can be calculated directly using the acoustic spectral density or power.

The power spectrum of the broadband spectrum is calculated using the Fourier transformation of the cross-correlation function of the pressure: (S (f, d, ä)) =] dx (p (d, a, t -τ I 2) p (d, a, t + τ / 2)) e ^{_Ω *} * (24)

Equation (24) takes into account all correlations in space and time, where

oT angle between R P „and the rear edge

a _" Speed of sound

τ time of emissions

t time

p pressure

p density

f frequency

d is the observer control point distance from the panel surface.

The first method implemented in code DESI 2 to calculate the trailing edge noise is Howe's method. A swirl correlation function is introduced as a transfer function, which comes from the measurement of stochastically fluctuating wall boundary layers. The sound sources are generated by incident and detached fluidized beds near a flat plate. The far field result of this calculation is

p (f-ä, Θ) = ^{Po {fc} > sin cos ³⁷² ß sin (Θ / 2) (25) d

(1-M ₀ + M _e )

(l + M ₀ cosΘr ² (l + (M ₀ -M.) cosΘ) 16

Where p _{0 describes} the transfer function. The frequency f follows from the Strouhal number

S _t = f ^~ (26)

Here is / the vortex diameter, V _c the vortex convection speed, Θ the angle between d and the tangent at the trailing edge,

M _{0 is} the Mach Mach number and M _{c is} the Mach Mach number

^M - = f ^{v →}

According to Grosveld and Schlinker, the theoretical results are scaled on the measured acoustic data of an overflowed wing segment. After that is acoustic spectral density

(27)

with the directional characteristic

sin ² (Θ / 2) cos ß ³ sin ² (d)

D = (28) (l + M ₀ cosΘ) [l + (M _o - M cosΘ:

which is expanded by the rear edge arrow angle factor cosß for code DESI 2. When introducing the arrow angle, the factor K ₂ is rescaled. It means:

K ₂ empirically determined constant; after validation by experiment

customized

U flow velocity

B number of sheets

δ boundary layer density.

The center frequency is defined too

with 5 _m . _x = 0.1. The propeller blade is also paneled for calculating the broadband noise with code DESI 2 in a similar way as in the case of code DESI 1. Here, the sound sources are arranged near the Kutta panel (arranged on the rear edge, only tangential speed by definition effective there).

According to a special embodiment it is provided that in the second phase the second acoustic optimization of the blade geometry or blade division is calculated in the form of an asymmetrical division of the blades in order to minimize noise. The noise is redistributed spectrally or partially eliminated by interference. If the spectral distribution changes, the noise level of a particular frequency is redistributed to areas of adjacent frequencies in such a way that overall smaller noise levels are generated. This variation possibility of the blade division or change of the blade geometry integrated in the second optimization algorithm enables further noise reduction by means of the one method.

It is advantageously provided that in the second phase to the second acoustic

Optimization of the blade pitch or blade geometry a calculation taking into account three microphones arranged immediately before the fan runs in at angles of ψ = 60 °, 70 ° and 75 ° to the fan center also done with the panel method for calculating the acoustics, called LBS code AKU, at design point E.

The initial equation of the LBS code is an integro differential equation, which describes both the acoustic and the aerodynamic fields of stationary / unsteady, compressible, sub-shell currents subject to friction. The solution of the aeroacoustic potential equation is obtained by differentiating a speed and acceleration potential. The result is the velocity field as a derivative of the potentials based on the coordinates. The third term, which describes the influence of the shear layer in the form of a volume integral, can be used to calculate the friction. This involves converting the integral into a surface integral for description of blow-out speed, which simulate a boundary layer and with which the boundary conditions on the leaf surface can be modified accordingly (perspiration method). The acoustic pressure follows directly from the equation of potential by differentiation according to the time instead of the coordinates

An exemplary embodiment is explained in more detail below with reference to a drawing. It shows:

1 shows a rectangular sheet outline to be optimized, FIG. 2 shows a section through the sheet according to FIG. 1,

Fig. 3 shows an efficiency η volume flow V diagram

4 is a plan view of a sickle-shaped sheet according to the invention,

5 is a schematic view in the axial direction of an axial fan,

6 shows a schematic side view of an axial fan, and FIG. 7 shows a schematic view of four optimization steps.

Fig. 1 shows schematically a Biatt floor plan 10, which in the illustrated axial view has a rectangular projection surface and is attached on the one hand to a hub 11 and on the other hand to a jacket element 12. The arrow A indicates the direction of rotation. Line 13 shows a section line of the laminar profile of sheet 10.

Fig. 2 shows a section of Fig. 1 along the section line 13. The depth of the sheet 14 can be seen on the laminar profile as well as the twist angle β.

3 shows an η-V diagram in which the design point E is entered close to the maximum efficiency.

Fig. 4 shows, similar to Fig. 1, a sheet outline 10, which is connected to a casing element 12 and a hub 1 1 and has a crescent-shaped configuration. The egg-shaped blade 10 has a strong forward arrow 17 and a strong reverse arrow 18 on its front edge 15 and on its rear edge 16. 5 shows in the axial direction a schematic representation of an axial fan 19 with seven blades 10. The different blade pitch t can be clearly seen.

6 shows a side view of a schematically represented axial fan 19 with blades 10 and a jacket element 12, on which one side of the microphone M1, M2 and M3 at an angle ψ = 60 °, 70 ° and 75 °, based on the air axis 20, assigned.

In order to noticeably reduce the noise from fans 19 by the aeroacoustically optimal shaping according to the invention, without the performance of the fan 19 being impaired, the following procedure can now be carried out in an advantageous manner

The sheet 10, which has a laminar or circular profile, is predetermined according to Fig 1/2. The blade depth and twist ß is calculated using a design code DESI1 (using the formulas above) taking into account the profile polares, an optimum being calculated for an elliptical load distribution at design point E (cf. FIG. 3). The DES11 method is a singular method with which the aerodynamics on or on the singularities or panels on the surface of the air sheet is calculated

The geometric data and aerodynamic coefficients resulting from the aforementioned calculation are now the basis for the calculation of the optimal sweeps 17, 18 of the leaf profile 10 for the decorrelation of sound-producing vertebral structures at the trailing edge of the leaf 16, i.e. for influencing macroscopic vertebral structures so that smaller vertebral structures result from this calculation also the blade depth 14, calculated at draft point E with draft code DESI2 and a microphone position about 1 m from the inlet on the rotor axis with an angle ψ = 0 ° (see Fig. 6). The DESI2 method is used to calculate only stochatically generated acoustic signals. The change the Biattgeometne and the asymmetrical division t of the rotor blades 10 for sound reduction due to changed spectral distribution and Ausioschung is based on a calculation in design point E with LBS code AERO and AKU, the lifting body surge described above face method, using three microphone positions M1, M2 and M3 immediately before the fan 19 runs in at the angle ψ = 60 °, 70 ° and 75 °, based on the fan axis 20 (cf. FIG. 6). The result of such an optimization is shown in FIG. 7, the individual optimization steps I-IV being outlined.

Reference list

10 leaf plan

1 1 hub

12 jacket element

13 Cutting line of the laminar profile

14 page depth

15 leading edge

16 trailing edge

17 Forward arrow

18 Backward arrow

19 axial fan

20 fan axis

A arrow (for direction of rotation) ß torsion angle

E draft point t sheet division

M1 microphone

M2 microphone

M3 microphone

Ψ Angle microphone fan axis

Claims

Patent claims

1. Aeroacoustic optimization method for an axial fan, in which, based on a numerical computer simulation, an aerodynamic optimization of the blade geometry of the axial fan takes place in a first phase by using an inverse numerical design method in connection with a first optimization algorithm, characterized in that in a second phase in combination with a second optimization algorithm

- first a first acoustic optimization of the blade geometry to reduce the stochastic noise with the help of a further numerical method based on turbulence models, and - then a second acoustic optimization of the blade geometry and

Sheet division for minimizing the periodic noise component is calculated using a singularity method.

2. The method according to claim 1, characterized in that in the first phase as an inverse numerical design method, the method according to Larrabee in modified form or a panel method LBS AERO for coated propellers is used and as calculation specifications the polar of a profile (10) and thus the blade depth (14) and a twist β for an elliptical load distribution at the design point E is calculated with a design code DESI1, so that the first optimization algorithm determines an optimum for the load distribution.

3. The method according to claim 2, characterized in that for the calculation and optimization in the second phase, the geometries (10) and aerodynamic coefficients resulting from the optimization of the first phase are used as specifications and an optimal sweep for the first acoustic optimization ( 17, 18) of the blade geometry for the decorrelation of sound-producing vortex structures on the blade trailing edge (16) and in connection therewith the blade depth (14) is again calculated at draft point E with draft code DESI2.

4. The method according to claim 3, characterized in that the draft code DESI2 calculates the sound level at a distance of approximately 1 m from the inlet of the fan axis (20) at an angle ψ = 0 °.

5. The method according to claim 4, characterized in that in the second phase there is a second acoustic optimization of the leaf geometry and leaf division t with the aid of a singularity or panel process, in particular the LBS code.

6. The method according to claim 5, characterized in that using the panel method LBS AERO to the present geometry

(10, 12) the aerodynamic coefficients and thus in the second phase the second acoustic optimization of the blade pitch t or blade geometry (10) to change the spectral distribution or interference a noise reduction is calculated with the LBS code AKU.

7. The method according to claim 6, characterized in that in the second phase for the second acoustic optimization of the blade line and blade geometry (10, 12) a noise calculation taking into account three immediately before the inlet of the fan (19) with angles of ψ = Microphone positions M1, M2, M3 arranged at 60 °, 70 ° and 75 ° to the fan center with LBS code at design point E.

8. The method according to any one of the preceding claims, characterized in that blowers, propellers and / or rotors, in particular of wind turbines, are optimized by means of the method.