Cross Wind Influence on Noise Emission and Computed Vibrational Noise of an Axial Fan

Abstract

The total noise emissions of two commercial axial fans were measured in a semi-anechoic fan test rig in comparison. The total sound pressure levels and the respective spectra were found to change with the fans’ operating points. Increasing fan flow rates lowered the total acoustic pressure, with a broadband shift towards higher frequencies, keeping perceived (A-weighted) sound pressure levels approximately constant over a wide range of operating points. In a second step, Laser Scanning Vibrometry measurements of the fan blades’ axial motion were conducted in comparison inside a wind tunnel fan test rig. Rotating blade surface vibration data was used as sole input to a Ffowcs Williams and Hawkings algorithm, to estimate noise emission from vibration. The computed noise from surface vibration was found to be hardly affected by the change of fan flow rate. In the application of an axial fan subject to natural wind or induced cross flow at its inlet, the flow field and possible noise emission of the fan changes. Microphone measurements of the cross flow influence inside a semi-anechoic wind tunnel revealed increasing broadband noise with ambient flow field velocity, and an amplification of the sound at the blade passing frequency harmonics. Similar excitations of the blade passing frequency harmonics under cross flow influence were also found in sound pressure spectra computations based on the Laser Scanning Vibrometry measurement data captured in the wind tunnel fan test rig. Blade vibration is considered to contribute to the low frequency tonal noise emission of axial fans operating under cross flow conditions.

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“…These structural effects are also valid for a smaller scale objects, such as industrial fans. It is confirmed experimentally 19,20 that flow distortions due to crosswinds lead to increase in levels of structural vibrations of fan blades. It was found, in particular, that both acoustic and vibration structural responses of blades due to crosswind are similar and contain multiple superharmonics of the rotation frequency as well as first eigenfrequency of the blade.…”

“…Present methods of experimental detection and diagnostics of flow separation events at different surfaces are rather laboratory based and rely on use of wind tunnels, instrumented with flow visualization tools: smoke or oil complemented by sensor arrays for measurement of pressure distributions, for example, Pitot and Kiel probes and piezoresistive pressure sensors . Instruments for measurement of the local flow velocities, such as hot wires, hot films, and laser Doppler anemometers, are also widely used …”

“…For example, work used an autospectrum of wall pressure values for analysis of flow separation effects; study implemented power spectrum and coherence of pressure measurements for analysis of a separated flow; research use power spectra of hot film signals to show the difference between them before and after flow separation. Other authors applied spectral analysis to blade vibration signals from laser scanning vibrometer to investigate effects of local flow instabilities due to crosswind on the structural vibration of an axial fan blade. Some studies implement more complex and sensitive techniques: Γ functions with entity connection graphs applied to velocity and pressure measurements and autocorrelation based on the continuous wavelet, transform of pressure measurements for flow separation detection, diagnostics, and analysis.…”

“…11 Instruments for measurement of the local flow velocities, such as hot wires, hot films, and laser Doppler anemometers, are also widely used. 11,12,[19][20][21][22][25][26][27][28] In contrast to the computationally complex methods of the CFD involved into numerical studies of the flow separation, experimental detection and diagnostics of the flow separation are predominantly based on simple signal processing techniques.…”

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“…However, this method not only resulted in long design cycles but also incurred high production costs. In recent times, Fluent, a widely adopted commercial CFD software, has gained popularity due to its extensive range of physical models, advanced numerical methods, and robust pre-and post-processing capabilities (Heinemann et al, 2015). Fluent is capable of handling diverse problems related to fluid dynamics, heat transfer, and chemical reactions (Zhao et al, 2014).…”