Vortex Generation Behind the Cylinder Cascade of Fan Grill

F1 - Noise Prediction and Investigation for Axial Fans

The fan guard grill is one of the most important components that contribute to the increased sound power level and dissipation of fluid kinetic energy. Sound power measurements of axial fan operation with and without guard grill have shown differences in frequency spectra between 800 Hz and 2000 Hz, presumably caused by Karman vortex generation. To analyse Karman vortex generation air flow around cascade of cylinder was investigated. Experimental methods included visualization with oil vapour as a polutant and measurements of sound pressure levels in the far field.
Experimental work was performed on the standardized measuring station for aerodynamic and acoustic measurements. The cylinder cascade was mounted on the opening of reverberation room. During the measurement, air adjustment and quantity of oil vapour was altered, accordingly. Experimental installation enabled us the possibility to have similar working and ambient conditions for sound pressure level measurements and experimental visualization with oil vapour.
In addition, numerical simulation of air flow around cylinders was performed. Because of increased turbulence and dissipation of fluid kinetic energy, LES turbulent model was used. Boundary conditions of numerical model were adopted from the measurement results. The numerical model was also used for sound pressure level calculation.
Analysis focused on Karman frequency peaks behind cascade of cylinders and -5/3 dissipation law of fully developed isotropic turbulence. Results confirm that Karman vortex generation is responsible for dissipation of fluid kinetic energy, while results of theoretical, experimental and LES methods agree fairly well. The most advantage of experimental visualisation, sound pressure level measurements and LES numerical model calculation is that they do not disturb observed fluid motion. This fact and good correlation between mentioned technics and theoretical law is base for further work on fluid kinetic energy dissipation into rotating region of axial fans.