Shells In Cross Flow Research Articles

Mechanism of ovalling vibrations of cylindrical shells in cross flow

The mechanism of wind-induced ovalling vibrations of cylindrical shells is numericallyrninvestigated by using a vortex method. The subject of this paper is limited to a two-dimensional structurernin the subcritical regime. The aerodynamic stability of the ovalling vibrations in the second to fourthrncircumferential modes is discussed, based on the results of a forced-vibration test. In the analysis, two modalrnconfigurations are considered; one is symmetric and the other is anti-symmetric with respect to a diameterrnparallel to the flow direction. The unsteady pressures acting on a vibrating cylinder are simulated and thernwork done by them for one cycle of a harmonic motion is computed. The effects of a splitter plate on thernflow around the cylinder as well as on the aerodynamic stability of the ovalling vibrations are also discussed.rnThe consideration on the mechanism of ovalling vibrations is verified by the results of a free-vibration test.

Ovalling Oscillations of Cantilevered Cylindrical Shells in Cross-Flow: New Experimental Data

This article deals with an experimental investigation of the ovalling phenomenon for clamped-free cylindrical shells in cross-flow. From the interpretation of the data, conditions for two-dimensional and three-dimensional flows independent of a gap at the free end of the shell are deduced. In two-dimensional flow conditions, the nondimensional ovalling onset fluid velocity is observed to vary linearly with the Scruton number, a nondimensional form of the modal damping. A hysteresis is also observed in the shell ovalling vibrations, two steady amplitudes being possible for a given fluid velocity. From the data it can be inferred that ovalling is triggered whether regular vortex shedding is present or not and that the shell ovalling motion disturbs regular vortex shedding.

An analytical model of the ovalling oscillations of clamped-free and clamped-clamped cylindrical shells in cross-flow

This paper deals with an analytical model of the ovalling phenomenon or the aeroelastic oscillations of clamped-free and clamped-clamped cylindrical shells. Shells of finite length are exposed to a steady two-dimensional incompressible and irrotational cross-flow, with their wall assumed homogeneous, isotropic and elastic. The governing equations of motion, based on Donnell's theory, are coupled to an aerodynamic force taking into account the Reynolds number. The model is tested with 18 shells (34 tests, of which 14 had clamped-free conditions and 20 had clamped-clamped conditions) and its predictions of the ovalling onset velocity are in good agreement with the experimental results.

An improved theory for flutter of cylindrical shells in cross-flow

A series of experiments with cylindrical shells in cross-flow has shown that making the windward side of the shell rigid has little effect on ovalling, but if the same is done to the lee side, ovalling is suppressed, thus establishing the importance of what goes on in the wake in determining the onset of ovalling. Hence, the hybrid viscous-potential analytical model developed earlier, in which the phenomenon is modelled as one of aeroelastic flutter, has been refined, taking into account empirically determined changes in the base pressure as the shell is oscillating in a given mode, as well as introducing other refinements. Base pressure variations were determined by measuring the pressure around statically deformed shells, in given circumferential modes, and also measuring the phase lag between shell motion and pressure in the wake. Excellent qualitative and good quantitative agreement is achieved between the improved analytical model and experiment, making it more certain than ever that the mechanism of ovalling is one of aeroelastic flutter and that it is not related to vortex shedding. This realization made possible a more direct approach for determining the onset of ovalling, namely by establishing the balance of energy gained by the shell from the flowing fluid to the energy lost by dissipation. Agreement achieved between this method and experiment is excellent.

Ovalling oscillations of thin circular cylindrical shells in cross flow—An experimental study

Experiments have been made to study the effect of a splitter-plate on the oscillation amplitudes, wake fluctuations and unsteady surface pressures of a shell undergoing ovalling oscillations in a cross flow in a wind tunnel. The results suggest that there is no major effect of the splitter-plate on these quantities and that periodic vortex shedding is probably the case of the ovalling.

Ovalling oscillations of cantilevered and clamped-clamped cylindrical shells in cross flow: An experimental study

Some experiments on cantilevered, thin cylindrical shells in cross flow are presented, as well as experiments with shells clamped at both ends and spanning the wind tunnel test section. Ovalling oscillations were found to occur mainly in the second, third, fourth and fifth circumferential modes of these shells, in both first and second axial modes, for both types of support condition, the critical mode numbers depending mainly on shell geometry. Contrary to previous tests, it was found that at the onset of ovalling, the vortex shedding frequency was not always an integral sub-multiple of the ovalling frequency of the shell—although an integral relationship (or nearly so) was found to exist more often than not. In the experiments with clamped-clamped shells, careful study was made of the power spectral densities of both the wake signal, measured by a hot wire anemometer, and the shell vibration signal, measured by a non-obtrusive fibre optics probe; the modal damping in various modes of the shell was also measured.

An analytical model for ovalling oscillation of clamped-clamped cylindrical shells in cross flow

This paper presents a quasi-static aeroelastic theory for predicting the ovalling oscillations of thin cylindrical shells in cross flow. The flow is treated as a superposition of the measured viscous mean flow and a potential flow associated with deformation of the shell. The existence of the wake is taken into account, but flow within it associated with the formation of vortices, is entirely ignored. The aerodynamic forces are formulated by means of strip theory and the dynamics of the shell are described by means of Flugge's equations. It is found that at sufficiently high flow velocities, the shell becomes subject to oscillatory instabilities associated with negative aerodynamic damping. The principal observed characteristics of ovalling oscillation are qualitatively predicted by theory, but critical flow velocities are overestimated by a factor ranging from 1·6 to 5.

Flutter of thin cylindrical shells in cross flow

This paper presents an analytical model for the aeroelastic instability of an infinitely long cylindrical shell in cross flow. The mean flow field is represented by a free-streamline model, and the perturbation flow field by a velocity potential associated with deformation of the shell cross-section; motions of the shell are described by Flugge's two-dimensional equations. It is shown that certain types of shell motions induce a negative aerodynamic damping, which increases with flow velocity; for sufficiently high flow, it overcomes the positive dissipative damping of the system, precipitating flutter, sequentially in the second, third and higher circumferential modes of the shell – each with specific orientation of the nodal pattern with respect to the free-stream vector. These analytical predictions are in agreement with observations in wind-tunnel experiments; quantitatively, predicted and measured flow-velocity instability thresholds are of the same order of magnitude.

On ovalling oscillations of cylindrical shells in cross-flow

This paper presents an experimental investigation into the nature of the ovalling oscillation of cantilevered cylindrical shells, induced by cross-flow—a phenomenon which has been observed to occur in thin metal chimney stacks when subjected to sufficiently strong cross winds. It is shown that the mechanism involved is not periodic vortex shedding, as had been believed heretofore, but that it is a more complex three-dimensional phenomenon in which the flow pattern in the wake plays an important role.

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Some experiments on cantilevered, thin cylindrical shells in cross flow are presented, as well as experiments with shells clamped at both ends and spanning the wind tunnel test section. Ovalling oscillations were found to occur mainly in the second, third, fourth and fifth circumferential modes of these shells, in both first and second axial modes, for both types of support condition, the critical mode numbers depending mainly on shell geometry. Contrary to previous tests, it was found that at the onset of ovalling, the vortex shedding frequency was not always an integral sub-multiple of the ovalling frequency of the shell—although an integral relationship (or nearly so) was found to exist more often than not. In the experiments with clamped-clamped shells, careful study was made of the power spectral densities of both the wake signal, measured by a hot wire anemometer, and the shell vibration signal, measured by a non-obtrusive fibre optics probe; the modal damping in various modes of the shell was also measured.