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1 Machine Dynamics Problems

1 2004

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A Nonlinear Oscillator Model in Bluff Body Aerodynamics

Grzędziński J., Nowak M.

Machine Dynamics Problems

2004

Vol. 28, No. 1

67-78

The aim of this paper is the construction of a semi-empirical model to provide a phenomenological description of motion of a bluff (non-streamlined) body in a streaming fluid. This case differs significantly from the well known case of an airfoil, when the separation occurs only at high angles of attack. Perhaps the best known difference is the capture of the vortex-shedding frequency by the body frequency over a certain range of reduced velocity. This is usually known as "lock in" phenomenon. Similar synchronization behavior can be also observed among solutions of van der Pol equation, and most of models used in practice are based on equations of this type. The results of calculations of aerodynamic forces based on Navier Stokes equations, for a rotationally oscillating thick plate (with chord to thickness ratio equal to 8), for different amplitudes and a wide range of frequency are presented. It was shown that there exist also "lock in" phenomena for frequencies which are odd multipliers of the body frequency. The known semi-empirical models are able to describe the "lock-in" phenomenon only for the forcing frequency. The "lock-in" phenomenon for higher harmonics may be also described by the van der Pol equation, but until now there are no methods to derive such models.