Aerodynamic Damping Forces Research Articles

Across-wind vibrations of structures of circular cross-section. Part I. Development of a mathematical model for two-dimensional conditions

A model is presented for predicting the across-wind response of constant-diameter circular cylinders vibrating in a mode of uniform amplitude and subject to uniform flow. A key feature of the model is the representation of all motion-dependent phenomena by a nonlinear aerodynamic damping force. This force coexists with the fluctuating force which arises from vortex shedding on a stationary cylinder, and the two forces are assumed to be uncorrelated. The ability of the device used in representing the motion-induced force to model certain aeroelastic characteristics associated with vibrating cylinders is demonstrated. The device is shown to be capable of successfully reproducing two effects; namely, the increase of the spanwise correlation of forces with increasing amplitude, and the phenomenon of “lock in” where the shedding frequency is apparently dictated by the vibration frequency. The model is developed within the framework of random-vibration theory, and a number of simplifying assumptions are necessary to incorporate the nonlinear aerodynamic damping force and also to account for the influence of turbulence. Numerical experiments, undertaken to examine the nature of the approximations involved in the assumptions adopted, are described. The results of the numerical experiments are very encouraging and justify the simplifications made in the modelling process.

On Unsteady Aerodynamic Forces and Moments of Radial Impeller Blades

By method of singularities, the theoretical treatments of two-dimensional centrifugal impeller blades vibrating harmonically with the arbitrary frequencies and the phases are described for the forces and the moments under the assumptions of an incompressible, inviscid, nonstalled flow. Some computed results on the case of radial impeller blades are presented. The results show that the aerodynamic damping forces and moments about rotational axis vary significantly near the integer values of the ratio of vibrating frequency to rotational speed, so that the instability of single degree of freedom becomes possible at the frequencies depending upon the vibrating phase angle.

Aerodynamic damping of the vibrations of the vanes in turbines as a function of the spacing and the marginal angle of the grid

The density and the marginal angle of a grid are the dimensionless numbers which determine the overall characteristics of grids exposed to a nonstationary flow, which is of practical importance in the determination of the aerodynamic damping forces. The GD-1 equipment at the Academy of Sciences of the Ukrainian SSR was used. The system for generating vibrations in the equipment is described. The investigations show that the dependence of the aerodynamic damping on the Strouhal number is qualitatively the same for the flow in grids with different spacings and for the vibrations of a single vane in a current. The logarithmic decrement of the vibrations, which characterizes the aerodynamic damping, is proportional to the dimensionless ratio between the mass criterion and the Strouhal number. The character of the effect of the Strouhal number on the aerodynamic damping does not depend on the relative spacing, and the effect of the latter on the aerodynamic damping does not depend on the Strouhal number at k > l. The causes of the aerodynamic damping increasing as the spacing of the grid decreases are discussed.

Experimental Study on Flutter in Cascading Blades : 1st Report, Measurement of the Aerodynamic Damping Force Acting on Cascading Blades Vibrating in Translatory Mode

For the purpose of investigating the aerodynamic damping effects on vibrating cascade blades, the authors made a water tunnel with a linear cascade in which five blades vibrate in a translatory mode with arbitrary frequency and phase angle between the adjacent blades. A series of experiments were carried out on an isolated blade and on the cascades with the following conditions : Blades : NACA 65-(12) 10 (fixed : 8, vibrating : 5) chord : 30 mm span : 60 mm solidity : 1 stagger angle : 0°, 30° amplitude of vibration : 2 mm The experimental results were analyzed by comparing with the theoretical ones hitherto obtained and with the aid of the considerations about the quasi-steady force of vibrating blades.

Experimental Study on Flutter in Cascading Blades : 1st Report, Measurement of the Aerodynamic Damping Force Acting on Cascading Blades Vibrating in Translatory Mode

Experimental Study on Flutter in Cascading Blades : 1st Report, Measurement of the Aerodynamic Damping Force Acting on Cascading Blades Vibrating in Translatory Mode