Session: 10-03 Design and optimization

Paper Number: 80190

80190 - A Comprehensive Analytical Model for Vortex Shedding From Low-Speed Axial Fan Blades 

Vortex shedding (VS) from low-speed axial fan blades is a potential cause for fan noise and blade vibration. A practically important feature of VS, to be considered in preliminary fan design and analysis, is the VS frequency. As the literature reveals, e.g. [1], a universal Strouhal number can be defined and used for estimation of the frequency of shedding of vortices from the blade profile. The universal Strouhal number is to be scaled with the transversal distance between the shed vortex rows, and such transversal distance is being determined from measurements. Establishment of an experimental database relevant to the fluid mechanics details of the VS phenomenon related to low-speed axial fan blades is therefore of engineering importance. Among others, such database serves as background for extending the applicability of the literature-based universal Strouhal number to realistic axial fan blades.

The authors formerly carried out a detailed survey on the engineering aspects of the VS phenomenon related to low-speed axial fan blades [2]. They supplemented the literature with wind tunnel experimental data on VS detected in the case of realistic asymmetric low-speed axial fan blade profiles. They elaborated an analytical model for VS as an aid to evaluating the aforementioned measurement results [3].

As further development of former modelling, this paper aims at presenting a new, comprehensive analytical model for the VS phenomenon. The model is based on the Reynolds-averaged Navier-Stokes equation as well as on the continuity equation. A thorough experimental validation of the model is reported in the paper. For validation purposes, hot-wire measurements were carried out in a wind tunnel on representative models of low-speed axial fan blades, incorporating asymmetric airfoil sections. The processing of measurement data incorporated a concerted evaluation of transversal profiles of time-averaged velocity as well as velocity RMS, at various streamwise positions downstream of the blade trailing edges. Turbomachinery specialists can benefit from the validated new analytical model from the following perspectives. a) A comprehensive understanding of the underlying physics related to the flow field featuring VS in the near-wake region. Expression of correlations among the various flow characteristics affected by the shed vortices. b) An advanced methodology for experimental determination of the transversal distance between the shed vortex rows, being used as scaling parameter for the universal Strouhal number in VS frequency calculation. c) Modelling the behavior of rows of shed vortices farther away from the trailing edge. Such behavior may influence the acoustic signature of VS, and, as such, it is to be considered in fan noise modelling.        

References

[1] Yarusevych, S., Boutilier, M. S. H., 2011, “Vortex Shedding of an Airfoil at Low Reynolds Numbers,” AIAA Journal, 49(10), pp. 2221-2227.

[2]  Daku, G., Vad, J., 2021, „Profile Vortex Shedding from Low-Speed Axial Fan Rotor Blades: a Modelling Overview,” Proc IMechE, Part A: Journal of Power and Energy (OnlineFirst)

[3] Daku, G., Vad, J., 2021, „Experiment-Based Preliminary Design Guidelines for Consideration of Profile Vortex Shedding from Low-Speed Axial Fan Blades,” Journal of Turbomachinery - Trans ASME, 143(6), 10 p.

Presenting Author: Janos Vad Budapest University of Technology and Economics

Presenting Author Biography: 1997 PhD in Mechanical Engineering: LDA measurements on axial fans<br/>2013 Academic Doctorate: Design of low-speed axial fans

Authors:

Gabor Daku Budapest University of Technology and Economics
Janos Vad Budapest University of Technology and Economics