https://acoustique.ec-lyon.fr/publi/roger_jsv05.pdf

Back-scattering correction and further extensions of amiet's trailing-edge noise model. Part 1: theory

An experimental study is described to investigate the negative effects of the tip clearance gap on the aerodynamic and acoustic performance of axial turbomachines. In addition to the increased broadband levels reported in the literature when the tip clearance is enlarged, significant level increases were observed within narrow frequency bands below the blade passing frequency. Measurements of the pressure and velocity fluctuations in the vicinity of the blade tips reveal that the tip clearance noise is associated with a rotating flow instability at the blade tip, which in turn is only present under reversed flow conditions in the tip clearance gap. A turbulence generator inserted into the tip clearance gap is found to be effective in eliminating the tip clearance noise and in improving the aerodynamic performance.

Experimental study of tip clearance losses and noise in axial turbomachines and their reduction

Fused Deposition Modeling is an Additive Manufacturing technology able to fabricate prototypes, tooling and functional parts without geometrical complexity limitations Despite of the potential advantages of this technology, a limiting aspect of its industrial diffusion is the obtainable accuracy The literature highlighted that significant deviations from the nominal values are observed: these deviations are not constant over all the part surfaces but strictly depend upon the process parameters, ie the layer thickness and the deposition angle This involves poor surface quality: the parts could not satisfy the design specifications nor assure the functionality and the assembly fit with other components The aim of this work is the development of a design for manufacturing methodology able to improve the dimensional accuracy obtainable by this technology It operates in the design model step performing a virtual model preprocessing: an anisotropic offset is applied to the surfaces, defined by a mathematical formulation, in order to compensate for the abovementioned dimensional deviations This way, without to eliminate the physical sources of the errors, it is possible to obtain a part with dimensional values very close to nominal ones This method does not require any additional resources for its application such as preliminary artifact construction and measurements A design for manufacturing to improve FDM part accuracy is proposedA redesign of the solid allows compensating for the dimensional deviationThe model surfaces are modified by means of an anisotropic offsetThe proposed method is applied to parts defined by a mathematical formulationThe method is validated through the application to three mechanical components

Design for manufacturing of surfaces to improve accuracy in Fused Deposition Modeling

Tip clearance noise of axial flow fans operating at design and off-design condition

The effects of tip clearance on the noise of low pressure axial and mixed flow fans

In the prediction theory for the broadband noise generated from a multiblade fan, the vortices in the Karman vortex street was divided into n pieces. The frequency distribution of the noise was estimated so that the Strouhal number could become constant even if the wake is spread by the diffusion. From the results of the measurement of the internal flow of the fan, it was found that the noise was related to the wake characteristics of the specific location in the scroll casing where the relative flow velocity was high. The noise operating in the vicinity of the maximum efficiency point of the fan was distributed over the domain from 500 Hz to 1250 Hz. It was experimentally proved that the influence of the Karman vortex street on the noise in the domains of high and low frequencies did not exist when the distribution of the estimated sound pressure level corresponded to the measured broadband noise.

Influence of the karman vortex street on the broadband noise generated from a multiblade fan

Noise generated by a fan consists of the rotating noise and the turbulent noise. These two kinds of noise have entirely different characters with respect to decay in an axisymmetric pipe. In this paper the generation and decay of these two kinds of noise are experimentally examined by changing various factors, such as the suction pipe geometry, the location and the thickness of struts, and the combination of the number of impeller blades and the number of struts. The rate of decay of the rotating noise is predictable and in most cases the noise quickly decreases in proportion to the distance from the rotor, providing that the number of impeller blades and the number of struts are properly chosen. Therefore, the noise observeda way from a low pressure axial flow fan is mostly the turbulent noise.

Noise Generated by Low Pressure Axial Fans

In the prediction theory for a broadband frequency noise generated from a multiblade fan, the vortices in Karman vortex street were divided into n pieces. The frequency distribution of the noise was estimated so that the Strouhal number could become constant even if the wake is spread by the diffusion. From the results of the measurement of the internal flow of the fan, it was found that the noise was related to the wake characteristics of the specific location in the scroll casing where the relative flow velocity was high. The noise operating in the vicinity of the maximum efficiency point of the fan was distributed over the domain from 500 Hz to 1 250 Hz. It was experimentally proved that when the distribution of the estimated sound pressure level corresponded to the measured broadband frequency noise, no influence of the vortices on the noise in the domains of high and low frequencies existed.

https://www.jstage.jst.go.jp/article/jsmeb/49/3/49_3_678/_pdf

Influence of Karman Vortex Street on Broadband Frequency Noise Generated from a Multiblade Fan

The effects of a volute angle of scroll casing and a groove on the suction surface of blades were investigated relating to the characteristics of aerodynamics and noise of a multi-blade centrifugal fan with forward swept blades. It was shown that in the aerodynamic characteristics, the optimal volute angle existed between 4.8 and 7.2 degrees, and in the characteristics of noise, that existed in the near 7.2 degrees. The wake width of grooved blade fan became narrower than that of the ordinary blade fan. Therefore, the grooved blade fan was superior in noise characteristics to the ordinary blade fan. Using the method of predicting width of wake and the formulae to estimate the turbulent noise level which we proposed in this paper, the sound pressure level can be predicted with the accuracy of about ±3 dB within these experiments.

Yoshio Kodama

Papers

The effects of tip clearance on the noise of low pressure axial and mixed flow fans

Influence of the karman vortex street on the broadband noise generated from a multiblade fan

Noise Generated by Low Pressure Axial Fans

Influence of Karman Vortex Street on Broadband Frequency Noise Generated from a Multiblade Fan

Aerodynamic and Noise Characteristics of a Multi-Blade Centrifugal Fan