1,721,052 research outputs found
Nonlinear supersonic flutter of circular cylindrical shells
No full textThe aeroelastic stability of simply supported, circular cylindrical shells in supersonic Row is investigated. Non-linearities caused by large-amplitude shell motion are considered by using the Donnell nonlinear shallow-shell theory, and the effect of viscous structural damping is taken into account, Two different in-plane constraints are applied to the shell edges: zero axial force and zero axial displacement; the other boundary conditions are those for simply supported shells. Linear piston theory is applied to describe the fluid-structure interaction by using two different formulations, taking into account or neglecting the curvature correction term. The system is discretized by Galerkin projections and is investigated by using a model involving seven degrees of freedom, allowing for traveling-wave flutter of the shell and shell axisymmetric contraction. Results show that the system loses stability by standing-wave flutter through supercritical bifurcation; however, traveling-wave flutter appears with a very small increment of the freestream static pressure that is used as the bifurcation parameter, A very good agreement between theoretical and existing experimental data has been found for flutter amplitudes. The influence of internal static pressure has also been studied
A Method to Identify Gear Errors by Vibrations of a Spur Gear Pair
No full textA new method to identify modal parameters (natural frequency and damping) and the equivalent gear error of a spur
gear pair is introduced. The equivalent error is a function of the gear position and is related to the errors of the driving and the
driven gears and to the non-dimensional stiffness of the teeth. The method is based on the measurement of the gear torsional
vibrations. The test rig is modelled as a single-degree-of-freedom system and must be assembled by using stiff bearings and
torsionally compliant shafts. The solution of the equation of motion is obtained through the harmonic balance method. The
proposed approach has some advantages with respect to traditional metrological methods. The effect of noise on the accuracy
of the identification is also investigated and discussed. Applications of the method to the identification of natural frequency,
damping and profile errors are shown
Multimode approach to nonlinear supersonic flutter of imperfect circular cylindrical shells
No full textThe aeroelastic stability of simply supported, circular cylindrical shells in supersonic flow is investigated by using both linear aerodynamics (first-order piston theory) and nonlinear aerodynamics (third-order piston theory). Geometric nonlinearities, due to finite amplitude shell deformations, are considered by using the Donnell's nonlinear shallow-shell theory,, and the effect of viscous structural damping is taken into account. The system is discretized by, Galerkin method and is investigated by using a model involving lip to 22 degrees-of-freedom, allowing for travelling-wave flutter around the shell and axisymmetric contraction of the shell. Asymmetric and axisymmetric geometric imperfections of circular cylindrical shells are taken into account. Numerical calculations are carried out for a very thin circular shell affixed Mach number 3 tested at the NASA Ames Research Center. Results show that the system loses stability, by travelling-wave flutter around the shell through supercritical bifurcation. Nonsimple harmonic motion is observed for sufficiently high post-critical dynamic pressure. A very good agreement between theoretical and existing experimental data has been found for the onset of flutter flutter amplitude, and frequency. Results show that onset of flutter is very sensible to small initial imperfections of the shells. The influence of pressure differential across the shell skin has also been deeply investigated. The present study gives, for the first time, results in agreement with experimental data obtained at the NASA Ames Research Center more than three decades ago
Errori dinamici di trasmissione e correzioni di profilo in coppie di ingranaggi a denti dritti
No full textAn approach in the frequency-wavenumber domain able to determine a set of concentrated
forces acting on a structure through response measurements is presented. The
method allows the identification of both locations and intensities of the
applied forces even when they are applied
in points not amenable of measurements.
The sensitivity of the solution to the noise in the measurements is studied.
Experimental and simulated tests are performed on a onedimensional structure
Identification of gear errors by vibrations of a spur gearpair
No full textA new method of identifying modal parameters (natural frequency and damping) and the equivalent gear error of a spur gear pair is developed. The equivalent error is a function of the gear position and is related to the errors of the driving and the driven gears and to the non-dimensional stiffness of the teeth. The method is based on the measurement of the gear torsional vibrations. The test bench is modelled as a single degree of freedom system and must be assembled by using stiff bearings and torsionally compliant shafts
Non-linear dynamics and stability of circular cylindrical shells containing flowing fluid. Part IV: Large-amplitude vibrations with flow
No full textThe response of a shell conveying fluid to harmonic excitation, in the spectral neighbourhood of one of the lowest natural frequencies, is investigated for different flow velocities. The theoretical model has already been presented in Part I of the present study. Non-linearities due to moderately large-amplitude shell motion are considered by using Donnell ́s non-linear shallow-shell theory. Linear potential flow theory is applied to describe the fluid-structure interaction by using the model proposed by Paidoussis and Denise. For different amplitudes and frequencies of the excitation and for different flow velocities, the following are investigated numerically: (1) periodic response of the system; (2) unsteady and stochastic motion; (3) loss of stability by jumps to bifurcated branches. The effect of the flow velocity on the non-linear periodic response of the system has also been investigated. Poincare maps and bifurcation diagrams are used to study the unsteady and stochastic dynamics of the system. Amplitude modulated motions, multi-periodic solutions, chaotic responses, cascades of bifurcations as the route to chaos and the so-called blue sky catastrophe phenomenon have all been observed for different values of the system parameters; the latter two have been predicted here probably for the first time for the dynamics of circular cylindrical shells
Nonlinear stability of circular cylindrical shells in annular and unbounded axial flow
No full textThe stability of circular cylindrical shells with supported ends in compressible, inviscid axial flow is investigated. Nonlinearities due to finite-amplitude shell motion are considered by using Donnell's nonlinear shallow-shell theory; the effect of viscous structural damping is taken into account. Two different in-plane constraints are applied at the shell edges: zero a-vial force and Zero axial displacement; the other boundary conditions are those for simply supported shells. Linear potential flow theory, is applied to describe the fluid-stricture interaction. Both annular and unbounded external flow are considered by using two different sets of boundary conditions for the flow beyond the shell length: (i) a flexible wall of infinite extent in the longitudinal direction, and (ii) rigid extensions of the shell (baffles). The system is discretized by the Galerkin method and is investigated by using a model involving seven degrees-of-freedom, allowing for traveling-wave response of the shell and shell axisymmetric contraction. Results for both annular and unbounded external flow show that the system loses stability by divergence through strongly subcritical bifurcations. Jumps to bifurcated states can occur well before the onset of instability predicted by linear theory, showing that a linear study of shell stability is not sufficient for engineering applications
Non-linear dynamics and stability of circular cylindrical shells conveying flowing fluid
No full textThe non-linear dynamics and stability of simply supported, circular cylindrical shells containing inviscid, incompressible fluid flow is analyzed. Geometric non-linearities of the shell are considered by using the Donnell's non-linear shallow shell theory. A viscous damping mechanism is considered in order to take into account structural and fluid dissipation. Linear potential flow theory is applied to describe the fluid-structure interaction. The system is discretized by Galerkin's method and is investigated by using two models: (i) a simpler model obtained by using a base of seven modes for the shell deflection, and (ii) a relatively high-dimensional dynamic model with IS modes. Both models allow travelling-wave response of the shell and shell axisymmetric contraction. Boundary conditions on radial displacement and the continuity of circumferential displacement are exactly satisfied. Stability, bifurcation and periodic responses are analyzed by means of the computer code AUTO for the continuation of the solution of ordinary differential equations. Non-stationary motions are analyzed with direct integration techniques. An accurate analysis of the shell response is performed by means of phase space representation, Fourier spectra, Poincare sections and their bifurcation diagrams. A complex dynamical behaviour has been found. The shell bifurcates statically (divergence) in absence of external dynamic loads by using the flow velocity as bifurcation parameter. Under harmonic load a shell conveying flow can give rise to periodic, quasi-periodic and chaotic responses, depending on flow velocity, amplitude and frequency of harmonic excitation. (C) 2002 Elsevier Science Ltd. All rights reserved
Nonlinear vibrations and multiple resonances of fluid-filled, circular shells, part 1: Equations of motion and numerical results
No full textThe response-frequency relationship in the vicinity of a resonant frequency, the occurrenceof travelling wave response and the presence of internal resonances are investigatedfor simply supported, circular cylindrical shells. Donnell’s nonlinear shallow-shelltheory is used. The boundary conditions on radial displacement and the continuity ofcircumferential displacement are exactly satisfied. The problem is reduced to a system offour ordinary differential equations by means of the Galerkin method. The radial deflectionof the shell is expanded by using a basis of four linear modes. The effect of internalfluid is also investigated. The equations of motion are studied by using a code based onthe Collocation Method. The present model is validated by comparison of some resultswith others available. A water-filled shell presenting the phenomenon of 1:1:1:2 internalresonances is investigated for the first time; it shows intricate and interesting dynamics
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