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

The nonuniformity flow in the meridional section of a multi-blade sirocco fan has been represented as two domains. In the one domain, the air accompanied with the wake flows out to outside of impeller, and the vortex flow exists in another domain. In this study, the slip factor and the four pressure losses have been estimated. A pressure loss arose at the upstream side rather than the measurement point, and other losses were produced by the expansion of the scroll casing, mixing of the wake and the mixing of the vortex flow. As a result of analysis of the internal flow of the fan, it became clear that aerodynamic characteristics of the fan are influenced of the pressure loss produced in the process in which substantial channel reduced by the vortex flow carries out sudden expansion. When the effect on the mixing losses proposed in this study was applied to the estimate of the total pressure coefficient of the fan, the coefficient was able to be estimated within 6% errors.

https://www.jstage.jst.go.jp/article/kikaib1979/71/701/71_701_72/_pdf

Influence of the Mixing Losses on the Aerodynamic Characteristics of a Multi-Blade Sirocco Fan

Sound generation from tapered cylinder

The mechanism of the discrete frequency noise generation from the symmetric airfoils was investigated in transition Reynolds numbers. The following results are obtained. At the transition Reynolds numbers the boundary layer separate from the airfoil surface upstream of the trailing edge and the wake vortices form just at the trailing edge with inducing the shear flow fluctuation when the discrete frequency noise(DFN) generates. But the boundary layer flow attached up to the trailing edge and the wake vortices form at a downstream of the trailing edge. Then the shear flow do not fluctuate at the trailing edge and the discrete frequency noise little generate. that is, the formation of wake vortices does not always generate the DFN and the location of the wake vortices formation is closely relating to the discrete frequency noise at transition Reynolds numbers.

Relationship between the Discrete Frequency Noise and Wake Vortices on Symmetrical Airfoils

It is experimentally made clear in this paper that discrete tones appear in the frequency band lower than the blade passing frequency, when a rotor has manufacturing errors of stagger angle, pitch, camber or chord length, which is at variance with the general understanding that the frequency of discrete noise corresponds to the blade passing frequency and its harmonics. The relations between the magnitude of the deviation of the blading parameter from a design value and the induced sound pressure level of discrete tone and the resulted change of fluid dynamic characteristics of the fan are also examined. analytic considerations of measured wave patterns of pressure fluctuation indicate that this type of discrete noise originates in a distortion of circumferential distribution of mean flow rates through the rotor from the uniform distribution and the consequent circumferential variation of mean pressure level, arising from the unequal blade loading of each blade of a rotor due to the error in manufacturing the rotor.

Discrete Frequency Noise Due to Irregularity in Blade Row of Axial Fan Rotor

For the analysis of the laminar separated flow around the NACA symmetrical airfoil in transition region of Reynolds number, it is a problem that the flow separation and the development of the shear layer are sensitive to the flow conditions very much. Hence, we made the matching of laminar boundary layer theory with vortex method. We also proposed a new scheme that the vorticity field in the shear layer is decided from the Gaussian vorticity distribution and the no-slip condition was adopted by setting diffusive and productive vortices. We made clear that the flow separation is estimated with laminar boundary layer theory. And Computational results around the body showed good agreement with the experimental ones.

Yoshio Kodama

Papers

Influence of the Mixing Losses on the Aerodynamic Characteristics of a Multi-Blade Sirocco Fan

Sound generation from tapered cylinder

Relationship between the Discrete Frequency Noise and Wake Vortices on Symmetrical Airfoils

Discrete Frequency Noise Due to Irregularity in Blade Row of Axial Fan Rotor

Vortex Method for Laminar Separation Flow of the Airfoil