Fluid-structure interaction of a rolling restrained body of revolution at high angles of attack (original) (raw)
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Fluid-Structure Interaction of a Rolling Cylinder with Offset Centre-of-Mass
Notes on Numerical Fluid Mechanics and Multidisciplinary Design, 2016
With the aim of understanding discrepancies between experimental observations and numerical simulations for a cylinder rolling down an inclined plane, this study investigates the effect that offsetting the centre-of-mass from the cylinder centroid has on body forces, velocity and wake structures. The numerical cases considered focus on the same parameters as the referenced experiment: cylinder-to-fluid density ratio and wall inclination angle, for Reynolds numbers in a range around the critical value for the transition from stationary flow to periodic vortex shedding. The centre-of-mass is placed at a distance of up to 2 % of the diameter from the geometrical centre of the cylinder. It is found that the main features of the predicted wake flow are in good agreement with those observed experimentally. They include the inception of small-scale shear-layer vortices in the near wake, locked to the cylinder rotational frequency, as well as large-scale vortices further downstream. This is further confirmed through force and velocity histories, where two oscillations are found to operate at significantly different frequencies. While the amplitudes of the lift, drag and cylinder velocity oscillations see an increase with offset distance, the Strouhal numbers of the small-and large-scale structures remain unaffected and agree well with those measured in experiments at similar Reynolds numbers. Keywords Bluff-body wake • Fluid-structure interaction • Rolling cylinder •
Flow dynamics and forces associated with a cylinder rolling along a wall
Physics of Fluids, 2006
The wake flow structures and the drag force for a cylinder rolling along a wall without slipping were calculated for the Reynolds number range 20<Re<200, covering the two-dimensional shedding regime. Time-dependent numerical computations show the wake undergoes a steady to periodic shedding transition between 85<Re<90. The Strouhal number varies only weakly at higher Reynolds number, and is a factor of 3–4 lower than for an isolated rotating or nonrotating body. Also, within this shedding regime, the wake is characterized by counter-rotating vortex pairs, which propagate away from the wall via mutual induction. These pairs are formed as compact vortex structures from the top separating shear layer induce secondary vorticity at the wall, which is pulled up from the boundary to form the semidiscrete flow structures. Over both the steady and unsteady regimes, the (time-mean) recirculation length and drag are quantified.
A Numerical Study of the Roll Damping for Double-Symmetric Bodies
Procedia Manufacturing, 2020
Roll damping is essential for describing properly the motions of a ship, particularly when operating in rough sea conditions, being determinant for parametric or synchronous roll phenomena. Roll damping is a complex process of energy transfer from the hull to the water, which affects the amplitude of motion. Roll damping is dominated by viscous effects as well as by the interaction of the ship with the free surface. Numerical simulations of the free roll decay are carried out in this paper for a double-symmetric floating hull with one or two bilge keels based on an unsteady viscous flow solver. The numerical solutions reported in here are computed with the ISIS-CFD viscous flow solver, part of the Numeca FineTM/Marine suite. Turbulent flow is simulated by solving the unsteady equations of flow. Closure to the turbulence is achieved through the Shear Stress Transport (SST hereafter) based Detached Eddy Simulation (DES), which provides the accuracy of LES for highly separated flow reg...
Dynamics and stability of a fluid filled cylinder rolling on an inclined plane
The dynamics and stability of a fluid-filled hollow cylindrical shell rolling on an inclined plane are analyzed. We study the motion in two dimensions by analyzing the interaction between the fluid and the cylindrical shell. An analytical solution is presented to describe the unsteady fluid velocity field as well as the cylindrical shell motion. From this solution, we show that the terminal state is associated with a constant acceleration. We also show that this state is independent of the liquid viscosity and only depends on the ratio of the shell mass to the fluid mass. We then analyze the stability of this unsteady flow field by employing a quasi-steady frozentime framework. The stability of the instantaneous flow field is studied and transition from a stable to an unstable state is characterized by the noting the time when the eigenvalue crosses the imaginary axis. It is observed that the flow becomes unstable due to long wavelength axial waves. We find a critical Reynolds number (≈ 5.6) based on the shell angular velocity at neutral stability with the corresponding Taylor number being ≈ 125.4. Remarkably, we find that this critical value is independent of the dimensionless groups governing the problem. We show that this value of the critical Reynolds number can be explained from a comparison of time scales of motion and momentum diffusion, which predicts a value near 2π.
2016
Among all ship motions, roll motion is the most important response of a ship to calculate, because large amplitude roll motions may lead to capsize, cargo shift, loss of deck cargo and other undesirable consequences. However, the accuracy of the calculated results by using linear potential flow theory, such as strip method, for roll motion lag behind the other degrees of freedom. This is because; viscosity plays an important role in roll, especially near resonance. Computational methods based on potential flow theory do not capture these viscous effects such as effective creation of vortices in the boundary layer, flow separation at appendages and vortex shedding. The vortex shedding is the main physical phenomena involved in the viscous damping of the roll motion and it affects the flow velocity around the body that may lead to pressure increase or decrease. In this study, roll damping of a forced rolling hull with bilge keel for large amplitude roll motion with free surface is cal...
Numerical and Experimental Calculation of Roll Amplitude Effect on Roll Damping
Brodogradnja
Roll motion is still a challenging problem in naval architecture and an adequate prediction of this physical phenomenon is important because of its undesirable effects such as capsizing. There are several methods using linear potential theory to predict roll motion, such as strip method, however, the accuracy of the calculated results lag behind the accuracy of other degrees of freedom due to viscosity. Viscosity have an important effect on roll damping, especially near resonance, and as it is known, it is not included in potential flow methods. Vortex shedding is the main physical phenomena in viscous damping of the roll motion and it affects the flow velocity around the bilge. This may lead to pressure increase or decrease on the hull. In the present study, roll damping of a forced rolling hull with bilge keels at different roll amplitudes was calculated numerically by using an Unsteady Reynolds-averaged Navier-Stokes (URANS) solver. For the purpose of validation, forced roll experiments were carried out and the results were plotted next to numerical results. The generated vortices around the hull and bilge keel were observed in the URANS calculations. In the case of large roll amplitude motion, the vortex shedding from the bilge keel interacts with the free surface and leads to decrease on roll damping.
2011
The CFD calculations of the rolling moment coefficient by the FLUENT software package and the wind tunnel measurements were performed for two missile models. One missile model is with wrap-around fins and the other missile model is with flat fins. The purpose of this paper is to compare the calculated rolling moment coefficients of the selected missile models by computational fluid dynamics (CFD) to the experimental data. The influence of the turbulence model on the accuracy of the calculated rolling moment is also analyzed. There is better agreement between the calculated and measured rolling moment coefficients in the supersonic regions than in the subsonic regions of Mach numbers. It is proved that the rolling moment coefficient of the missile with wrap-around fins can be written as the sum of the moment due to the curvature of the fins and the moment due to the cant angle of the equivalent flat fins.