Showing papers on "Shell balance published in 2009"

Cause of Bimodal Distribution in the Shape of a Terrestrial Gastropod

Abstract: The distribution of a phenotypic state is often discontinuous and dispersed. An example of such a distribution can be found in the shell shapes of terrestrial gastropods, which exhibit a bimodal distribution whereby species possess either a tall shell or a flat shell. Here we propose a simple model to test the hypothesis that the bimodal distribution relates to the optimum shape for shell balance on the substrates. This model calculates the theoretical shell balance by moment and obtains empirical distribution of shell shape by compiling published data and performing a new analysis. The solution of the model supports one part of the hypothesis, showing that a low-spired shell is the best balanced and is better suited for locomotion on horizontal surface. Additionally, the model shows that both high- and low-spired shells are well balanced and suited on vertical surfaces. The shell with a spire index (shell height divided by diameter) of 1.4 is the least well balanced as a whole. Thus, spire index is expected to show a bimodal distribution with a valley at 1.4. This expectation was supported by empirical distribution of a spire index, suggesting that the bimodality of shell shape in terrestrial gastropods is related to shell balance.

Nonlinear parametric vibrations of orthotropic cylindrical shells interacting with a pulsating fluid flow

Abstract: The paper addresses the dynamic interaction of an orthotropic cylindrical shell with the fluid flowing inside. Its velocity has a constant component and low-amplitude pulsations. A method to calculate the characteristics of the parametric vibrations of the shell when the velocity of the fluid is close to critical is proposed. The amplitude–frequency characteristics of the shell–fluid system at fundamental parametric resonance are determined

Balance of material momentum at a shock wave

Abstract: The balance of material momentum is obtained by a pull-back of the balance of physical momentum to the reference configuration. The usual formulation of these balances assumes the fields involved to be continuously differentiable. At a surface of discontinuity, the mechanical balance laws are represented by jump conditions rather than differential equations. The jump condition representing the balance of material momentum is derived in an analogous manner to the continuous case. In general, there remains an imbalance term in the discontinuous version of the balance of material momentum representing a concentrated source of material momentum carried along with a shock wave. The imbalance remains nonzero in a purely hyperelastic material without any thermal properties. It is shown that the imbalance vanishes if the material is assumed thermoelastic and heat conduction is omitted. The relevance of this result for the validity of a global balance of material momentum is discussed.

Forced oscillations of shells of revolution in a viscous fluid flow

Abstract: The influence of mechanical properties of a large-sized shell on its forced oscillations in a flow of a viscous fluid is considered. As is known [1], the flow past a shell can be controlled by generating a wave that runs over the shell surface. Hydrodynamic calculations performed for only transverse running waves [2–6] show the potential possibility of significantly decreasing the drag, but conditions under which these running waves can be maintained in a shell of finite length had not been studied until now. The present paper considers the laws that govern a running wave generated in the front part of a shell of revolution occurring in a flow of a viscous fluid.

Critical Velocity of Potential Flow in Interaction With a System of Plates

Abstract: The elastic structures subjected to flowing flow can undergo the excessive vibrations and consequently a considerable change in their dynamic behavior, and they may lose their stability. A fluid-solid finite element model is developed to model a set of plates subjected to flowing fluid under various boundary conditions, fluid level, and fluid velocity that strongly influence the dynamic behavior of the plates. A hybrid method, which combines the finite element approach with the classical theory of plates, is used to derive the dynamic equations of the coupled fluid-structure system. The membrane and the transversal displacement fields are modeled, respectively, using the bilinear polynomials and the exponential function. The structural mass and rigidity matrices are derived by exact integration of developed displacement field. The fluid pressure is expressed by inertial, Coriolis and centrifugal fluid forces written, respectively, as a function of acceleration, velocity and transversal displacement. The fluid dynamic pressure is determined using the potential flow equation. Integrating this dynamic pressure in conjunction with the structural element results in the flow-induced mass, damping, and stiffness matrices, hence, one can establish the dynamic equations of coupled fluid-structure system. The impermeability condition that ensures the permanent contact between the shell and the fluid is applied at the contact surface. A parametric study has been performed to investigate the effect of physical and geometrical parameters (e.g. boundary conditions, fluid level, and flow velocity) on the dynamic response of the coupled system. The results are in satisfactorily agreement with those of experiments and other theories.Copyright © 2009 by ASME