Refinement of a semi-empirical method for the estimation of profile vortex shedding frequency from low-speed axial fan blade sections

Authors

Esztella Balla - Department of Fluid Mechanics, Faculty of Mechanical Engineering, Budapest University of Technology and Economics
János Vad - Department of Fluid Mechanics, Faculty of Mechanical Engineering, Budapest University of Technology and Economics

Abstract

The noise reduction of turbomachinery is a timely issue as it may have an effect on the well-being of humans, and additionally it may also be linked to the losses occurring on the machine. Low-speed axial fans often operate in the vicinity of humans, and as such its noise level is of significant importance. According to the literature, profile vortex shedding is one of the prominent noise sources of low-speed axial fans. This type of vortex shedding occurs at moderate angles of attack, and at a certain chord based Reynolds number range. It causes noise with a characteristic frequency peak, to which humans are more susceptible than to broadband noise, and vibration issues may also occur in the machine as a consequence to vortex shedding. Based on the above the prediction of the vortex shedding phenomenon, already in the design phase becomes necessary. A thorough literature review was carried out to create a database, including cases where frequency measurements were made, related to profile vortex shedding. The cases include symmetric and asymmetric NACA profiles. Based on the database an estimation method is proposed, for the frequency of profile vortex shedding. The method establishes a connection between the momentum thickness of the profiles, and the distance between the shed vortex rows. The momentum thickness is linked to the drag coefficient of the profile, and thus can be estimated from it. The frequency can be calculated from the distance between the shed vortex rows, using the universal Strouhal number, proposed in the literature. The authors’ method takes into account the relative thickness, and the angle of attack of the profiles. In order to be able to compare the symmetric and asymmetric profiles, the usage of a zero-lift based angle of attack definition is proposed. The historical database is supplemented by recently made hot-wire measurements, carried out on basic models of asymmetric axial fan blade sections: a cambered plate and a RAF6-E profile. The results of these measurements support the validity of the model on asymmetric profiles. The novelty of the authors’ method compared to previous estimations is that the vortex shedding frequency can be estimated by using an integral quantity, i.e. the drag coefficient. The precision of the estimation is higher compared to the former method, as it takes into account the relative thickness and the angle of attack. The extension to asymmetric profiles generalizes the method further.