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Noise generation in axial flow fans by C. G. van Niekerk
J. Sozuzd Vib. (1968) 7 (2), 310-311
LETTERS NOISE GENERATION
TO THE EDITOR
IN AXIAL
FLOW FANS BY C. G. VAN NIEKERK
Our interest in noise generation mechanisms in axial flow fans has prompted us to study the paper by C. G. Van Niekerk entitled “Noise generation in axial flow fans” [I]. In this it is assumed that the acoustic power radiated from an axial flow fan is a fraction of the losses occurring in the annulus less the losses associated with downstream swirl. An extensive analysis has been made of in-duct noise measurements for a family of 30 in. diameter fans. From this and the plane wave assumption, it has been possible to express the spectral distribution of sound pressure as a complicated function of fan flow parameters and fan geometry; this is given by Van Niekerk’s equation (10). The equation is derived in such a way that it is difficult to perceive the underlying physical mechanisms and nothing can be added to the author’s interpretation of the significance of the terms. From a practical point of view such an expression could be a useful tool for the prediction of fan noise levels particularly if it were valid over an extended range of parameters. To explore this possibility the expression has been evaluated, using a computer programme, for a number of cases, the initial ones being those given in the paper. Lack of basic data and poor agreement with the paper necessitated correspondence with Van Niekerk. A number of misprints were identified together with some small corrections which had been made to the spectrum peaks to allow for filter characteristics. These corrections are not referred to in the paper. Our first point of interest was an investigation of the predicted variation of total acoustic power with tip speed; this velocity, although it is not the most suitable, was chosen for ease of comparison with existing data. The fan chosen for the comparison was Van Niekerk’s configuration 7 which was operated at a constant efficiency, 7 = O-66. The results were found to agree well with those published by Bragg and Bridge [2] and Sharland [3], within the velocity range considered by Van Niekerk. A comparison is shown in the figure below. At higher velocities there appears to be a completely unexpected decrease in gradient and one might conclude that here the expression is not applicable.
II.
7o
_
Niekerk equation within specified mnge
____
Niekerk eqwtion for extended mnge
_._
Sixth power law (arbitmry zero level)
1
200
loo
BMe
I
300
tip velocity (ft eec-‘1
Figure 1. 310
I
400
t
5006
%0
311
LETTERSTO THEEDITOR
Second, for dimensionally similar fans operating at identical flow coefficients and speed there appears to be a decrease in acoustic power with increase in diameter. This seems to exclude the application of the equation, without additional terms, to fans of different diameter. Finally, a direct comparison was made with design data tabulated by Woods of Colchester Ltd. [4] for an almost identical fan without stators. Although qualitatively reasonable agreement was obtained, in absolute terms the Van Niekerk predictions were found to be as much as 12 dB less than the data given by Woods Ltd. These investigations lead us to the conclusion that though the expression may accurately predict noise levels for the particular fan diameter and geometry considered by Van Niekerk, operating within his restricted range of flow parameters, the predictions are of less value once the range of fan geometries and operating conditions is broadened. Whilst appreciating these limitations we feel that such an expression could still be useful if it were to give an indication of the relative importance of various parameters such as, say, blade Reynolds number or relative blade velocity; the present expression does not allow such an insight to be extracted. The experimental evidence we have used for our investigation is limited and though our comparisons are discouraging they are not conclusive; we hope Van Niekerk’s approach can be developed to yield a more widely applicable expression. B. BARRY B. TESTER
Rolls-Royce Limited, Littleover, Derby DE3 6FJ, England Received 13 December 1967
REFERENCES I. C. G. VAN NIEKERK 1966 J. Sound Vib. 3,46. Noise generation in axial flow fans. 2. S. L. BRAGG and R. BRIDGE1964 JI R. aeronaut. Sot. 68, 1. Noise from turbojet compressors. 3. I. J. SHARLAND1964 J. Sound Vib. 1, 302. Sources of noise in axial flow fans. 4. Woods aerofoil fans (Sales publication).
LETTERS NOISE GENERATION
TO THE EDITOR
IN AXIAL
FLOW FANS BY C. G. VAN NIEKERK
Our interest in noise generation mechanisms in axial flow fans has prompted us to study the paper by C. G. Van Niekerk entitled “Noise generation in axial flow fans” [I]. In this it is assumed that the acoustic power radiated from an axial flow fan is a fraction of the losses occurring in the annulus less the losses associated with downstream swirl. An extensive analysis has been made of in-duct noise measurements for a family of 30 in. diameter fans. From this and the plane wave assumption, it has been possible to express the spectral distribution of sound pressure as a complicated function of fan flow parameters and fan geometry; this is given by Van Niekerk’s equation (10). The equation is derived in such a way that it is difficult to perceive the underlying physical mechanisms and nothing can be added to the author’s interpretation of the significance of the terms. From a practical point of view such an expression could be a useful tool for the prediction of fan noise levels particularly if it were valid over an extended range of parameters. To explore this possibility the expression has been evaluated, using a computer programme, for a number of cases, the initial ones being those given in the paper. Lack of basic data and poor agreement with the paper necessitated correspondence with Van Niekerk. A number of misprints were identified together with some small corrections which had been made to the spectrum peaks to allow for filter characteristics. These corrections are not referred to in the paper. Our first point of interest was an investigation of the predicted variation of total acoustic power with tip speed; this velocity, although it is not the most suitable, was chosen for ease of comparison with existing data. The fan chosen for the comparison was Van Niekerk’s configuration 7 which was operated at a constant efficiency, 7 = O-66. The results were found to agree well with those published by Bragg and Bridge [2] and Sharland [3], within the velocity range considered by Van Niekerk. A comparison is shown in the figure below. At higher velocities there appears to be a completely unexpected decrease in gradient and one might conclude that here the expression is not applicable.
II.
7o
_
Niekerk equation within specified mnge
____
Niekerk eqwtion for extended mnge
_._
Sixth power law (arbitmry zero level)
1
200
loo
BMe
I
300
tip velocity (ft eec-‘1
Figure 1. 310
I
400
t
5006
%0
311
LETTERSTO THEEDITOR
Second, for dimensionally similar fans operating at identical flow coefficients and speed there appears to be a decrease in acoustic power with increase in diameter. This seems to exclude the application of the equation, without additional terms, to fans of different diameter. Finally, a direct comparison was made with design data tabulated by Woods of Colchester Ltd. [4] for an almost identical fan without stators. Although qualitatively reasonable agreement was obtained, in absolute terms the Van Niekerk predictions were found to be as much as 12 dB less than the data given by Woods Ltd. These investigations lead us to the conclusion that though the expression may accurately predict noise levels for the particular fan diameter and geometry considered by Van Niekerk, operating within his restricted range of flow parameters, the predictions are of less value once the range of fan geometries and operating conditions is broadened. Whilst appreciating these limitations we feel that such an expression could still be useful if it were to give an indication of the relative importance of various parameters such as, say, blade Reynolds number or relative blade velocity; the present expression does not allow such an insight to be extracted. The experimental evidence we have used for our investigation is limited and though our comparisons are discouraging they are not conclusive; we hope Van Niekerk’s approach can be developed to yield a more widely applicable expression. B. BARRY B. TESTER
Rolls-Royce Limited, Littleover, Derby DE3 6FJ, England Received 13 December 1967
REFERENCES I. C. G. VAN NIEKERK 1966 J. Sound Vib. 3,46. Noise generation in axial flow fans. 2. S. L. BRAGG and R. BRIDGE1964 JI R. aeronaut. Sot. 68, 1. Noise from turbojet compressors. 3. I. J. SHARLAND1964 J. Sound Vib. 1, 302. Sources of noise in axial flow fans. 4. Woods aerofoil fans (Sales publication).