Added mass

About: Added mass is a research topic. Over the lifetime, 2849 publications have been published within this topic receiving 47899 citations.

An Empirical Correlation of Drag Coefficient for a Single Bubble Rising in Non-Newtonian Liquids

Abstract: The motion of a single bubble rising unsteadily and steadily in a quiescent non-Newtonian liquid was investigated experimentally. By using a charged coupled device camera to follow the rising bubble, the sequences of the recorded frames were digitized and analyzed using image analysis software, and the measurements of the acceleration and steady motion of bubbles were obtained. Using the experimental data, we proposed an empirical correlation to predict the total drag coefficient calculated from the accelerating motion to the steady motion with the added mass force and history force included. This correlation is an extension of our previous work with non-Newtonian fluids. This new correlation represents very well the experimental data of the total drag force in a wide range covering both unsteady accelerating motion and steady motion in non-Newtonian fluids.

Hydrodynamic Optimization of a Wave Energy Converter Using a Heave Motion Buoy

Abstract: This paper reports a numerical study to optimize the buoy of a wave energy converter which uses the heave motion to absorb wave energy. To proceed to the buoy evaluation the equation of motion in the frequency domain is expressed as a function of the complex amplitude of the displacement, which can be determined from the amplitude of the excitation force and the hydrodynamic coefficients of added mass and damping. From the stroke and the PTO characterisation the mean power absorption by the device is computed as well as the capture width. In the study it was assumed a PTO connected to the ground which can be described through two terms: one is proportional to the strock and the other proportional to the velocity. If the stroke is lower than a prescribed maximum, the (optimal) mechanical spring and damping coefficients are computed from maximum power absorption. If larger, the mechanical spring and damping coefficients are set to limit the stroke to the prescribed maximum value. The methodology developed to optimize the buoy consists in the definition of a cylinder with the adequate length to tune the resonance at the desired frequency. Then, the specified cylinder is transformed into two buoy components, keeping the same hydrostatic coefficient as the original cylinder. The upper component (the 'surface buoy'), which crosses the free surface, may have a high hydrodynamic damping. Its volume, as low as possible, is set up according to the stroke required. The other component (the 'submerged mass') carries the remaining mass (cylinder mass minus the surface buoy mass) and is placed deeper in the water, at a depth dependent on its volume, to avoid the interference with the hydrodynamic damping of the surface buoy. A non-dimensional analysis has been performed allowing the establishment of relations between the maximum power increment (as the cylinder is progressively transformed into a more suitable shape) and the fraction of the cylinder volume reduction. It was also possible to relate the buoy centre of gravity with the absorbed power reduction if the submerged mass position is not deep enough to avoid any interference with the surface buoy.

Mechanisms of non-modal energy amplification in channel flow between compliant walls

Abstract: The mechanisms leading to large transient growth of disturbances for the flow in a channel with compliant walls are investigated. The walls are modelled as thin spring-backed plates, and the flow dynamics is modelled using the Navier–Stokes equations linearized round the Poiseuille profile. Analysis for streamwise invariant perturbations show that this fluid-structure system can sustain oscillatory energy evolution of large amplitude, in the form of spanwise standing waves. Such waves are related to the travelling waves which a free wall can support, modified to account for an ‘added mass’ effect. Simple scaling arguments are found to provide results in excellent agreement with computations of optimal disturbances, for low-to-moderate values of the stiffness parameter characterizing the compliant surface.

Vibration-Based Techniques for Measuring the Elastic Properties of Ropes and the Added Mass of Submerged Objects

Abstract: The magnitude of peak tensions during the deployment of subsurface moorings is dependent on the elastic stretch of the mooring line and the added masses of the mooring buoyancy and instruments. Measured transient tensions, which are twice the mooring anchor weight, highlight the importance of being able to quantify these critical parameters so that these dynamic tensions can be predicted. In this paper, the theory of the mass-spring oscillator is applied in two simple experiments to demonstrate practical techniques for measuring the elastic properties of ropes and the added mass of submerged objects. In the first procedure, a known mass is suspended from a rope, and the tension is monitored when the “mass-spring” system is set into free longitudinal oscillation. The spring constant is calculated from the measured frequency of this oscillation. Trials with ¼" diameter synthetic and wire ropes demonstrate that satisfactory results are attainable with rope lengths of less than 2 m. Results using dif...

Momentum evolution of ejected and entrained fluid during laminar vortex ring formation

Abstract: The evolution of total circulation and entrainment of ambient fluid during laminar vortex ring formation has been addressed in a number of previous investigations. Motivated by applications involving propulsion and fluid transport, the present interest is in the momentum evolution of entrained and ejected fluid and momentum exchange among the ejected, entrained fluid and added mass during vortex ring formation. To this end, vortex rings are generated numerically by transient jet ejection for fluid slug length-to-diameter (L/D) ratios of 0.5–3.0 using three different velocity programs [trapezoidal, triangular negative slope (NS), and positive slope (PS)] at a jet Reynolds number of 1,000. Lagrangian coherent structures (LCS) were utilized to identify ejected and entrained fluid boundaries, and a Runge-Kutta fourth order scheme was used for advecting these boundaries with the numerical velocity data. By monitoring the center of mass of these fluid boundaries, momentum of each component was calculated and related to the total impulse provided by the vortex ring generator. The results demonstrate that ejected fluid exchanges its momentum mostly with added mass during jet ejection and that the momentum of the entrained fluid at jet termination was < 11% of the total ring impulse in all cases except for the triangular NS case. Following jet termination, momentum exchange was observed between ejected and entrained fluid yielding significant increase in entrained fluid’s momentum. A performance metric was defined relating the impulse from over-pressure developed at the nozzle exit plane during jet ejection to the flow evolution, which increased preferentially with L/D over the range considered. An additional benefit of this study was the identification of the initial (i.e., before jet initiation) location of the fluid to be entrained into the vortex ring.