Hydrodynamic Force Model Research Articles

Restructuring of colloidal aggregates in shear flows and limitations of the free-draining approximation

We investigated the restructuring behavior of colloidal aggregates by means of the discrete element method. We used a recently proposed model [V. Becker, H. Briesen, Physical Review E 78 (6) (2008) 061404] for tangential inter-particle forces, capable of supporting bending moments. We extended this model by the capability of supporting torsional moments. The time evolution of the aggregates’ radius of gyration was tracked and a power law relation between the number of primary particles and the final radius of gyration was found. For the hydrodynamic drag forces the free-draining approximation is employed. We investigated the quality of the free-draining approximation by fully resolved finite element simulations for small aggregates. We found that the free-draining approximation overestimates the drag forces and we identified the usage of effective shear rates as a possible ansatz for reduced modeling of hydrodynamic forces.

A harmonic model of hydrodynamic forces produced by a flapping fin

The hydrodynamic control laws of unsteady fins inspired by swimming and flying animals are considered. A controller based on cycle-averaged forces requires a bandwidth lower than the flapping frequency, with correspondingly slow reactions to disturbances or commands in order to avoid undesirable feedback from the oscillating fins. A harmonic model of the periodic thruster forces was empirically found using a mechanical fin flapping in roll and pitch in hover, in uniform flow, and under various kinematic conditions. A multi-fin vehicle could use this model to account for the dominant non-linearities and minimize undesirable motions through coordinated control of individual fins.

Numerical Modeling of Flow and Hydrodynamic Forces around a Piggyback Pipeline near the Seabed

Steady boundary layer flows around a two-cylinder configuration near a plane wall is investigated numerically. The Reynolds-averaged Navier-Stokes equations are solved using an upwind finite-element method with a k-ω turbulence model closure. The numerical model is validated against independent experimental data for flow past a single cylinder near a plane wall. The two cylinders investigated in the present study are structured in the so-called piggyback configuration, in which the small cylinder is placed directly above the large cylinder. The diameter ratio of the small cylinder (d) to the large cylinder (D) is set at a constant value of 0.2. Different values of the gap (e) between the large cylinder and the plane wall and the spacing (G) between the two cylinders are investigated. The effects of the gap ratio e∕D and the spacing ratio G∕D on the flow around and the hydrodynamic forces on the cylinders are investigated. Four vortex shedding modes are found around the two-cylinder system. It is found tha...

Review of Seismic Codes on Liquid-Containing Tanks

Liquid storage tanks generally possess lower energy-dissipating capacity than conventional buildings. During lateral seismic excitation, tanks are subjected to hydrodynamic forces. These two aspects are recognized by most seismic codes on liquid storage tanks and, accordingly, provisions specify higher seismic forces than buildings and require modeling of hydrodynamic forces in analysis. In this paper, provisions of ten seismic codes on tanks are reviewed and compared. This review has revealed that there are significant differences among these codes on design seismic forces for various types of tanks. Reasons for these differences are critically examined and the need for a unified approach for seismic design of tanks is highlighted.

A method for estimating casualties due to the tsunami inundation flow

This study develops a method for estimating the number of casualties that may occur while people evacuate from an inundation zone when a tsunami has inundated an area. The method is based on a simple model of hydrodynamic forces as they affect the human body. The method uses a Tsunami casualty index (TCI) computed at each grid point of a numerical tsunami model to determine locations and times within the tsunami inundation zone where evacuation during the tsunami inundation is not possible and therefore where casualties are likely to occur. The locations and times can be combined with information about population density to compute the potential number of casualties. This information is useful in developing tsunami evacuation routes that avoid such locations. To illustrate the method, it is applied to the Seattle waterfront in Washington State, USA, that is under the threat of possible tsunami disasters due to Seattle Fault earthquakes. Preliminary results suggest that the tsunami casualties may occur within the Seattle waterfront for 15 min, during the time interval from 3 to 18 min after a large Seattle Fault tsunami is generated when the background tide level is mean high water.

Non-Gaussian Model for Ringing Phenomena in Offshore Structures

Significant interest has been shown in identifying the nonlinear mechanisms that induce a ringing type response in offshore structural systems. This high frequency transient type response has been observed in offshore systems, particularly in tension leg platforms (TLPs). Given the implications of this behavior on the fatigue life of TLP tendons, it is essential that ringing be considered in the overall response evaluation. This study presents two non-Gaussian probabilistic models of nonlinear viscous hydrodynamic wave forces that induce ringing. The response of a single-degree-of-freedom system exposed to these non-Gaussian wave force models is then evaluated using analytical and numerical studies based on the Ito⁁ differentiation rule and the Monte Carlo simulation procedure, respectively. The results demonstrate that the proposed models induce ringing type response in a simplified structure. This study provides a probabilistic framework for modeling ringing type phenomenon which will serve as a building block for more refined hydrodynamic load models.

On the Mathematical Model of Hydrodynamic Forces due to Cross Flow in Ship Turning Motion

On the Mathematical Model of Hydrodynamic Forces due to Cross Flow in Ship Turning Motion

A Component-type Mathematical Model of Hydrodynamic Forces in Steering Motion Derived from a Simplified Vortex Model (5)

A Component-type Mathematical Model of Hydrodynamic Forces in Steering Motion Derived from a Simplified Vortex Model (5)

Wake II model for hydrodynamic forces on marine pipelines including waves and currents

The Wake II model for the determination of the hydrodynamic forces on marine pipelines is extended to include currents and waves. There are two main differences between the Wake II and the traditional model. First, in the Wake II model the velocity is modified to include the pipe's encounter with the wake flow when the velocity reverses. Second, the model uses time dependent drag and lift coefficients. The flow field is assumed to be the linear superposition of regular waves and uniform current and is treated as wave only but in two different phases. The model requires eight empirical parameters that are obtained from comparisons with field data for various Keulegan–Carpenter numbers and current to wave ratios. The effective velocity and the force predictions are compared with field data from Exxon Production Research Company and with the conventional model. The model gives satisfactory results and predicts lift forces that in shape, magnitude and phase relative to the velocity are in very close agreement with measured forces. For the horizontal forces the results are very accurate. A substantial improvement is obtained over the predictions with the conventional model. This work is applicable to the design of submarine pipelines laying on the sea bottom in water depths where waves or waves and currents contribute to the hydrodynamic forces.

A Component-type Mathematical Model of Hydrodynamic Forces in Steering Motion Derived from a Simplified Vortex Model (4)

One of the authors presented the component-type mathematical model of hydrodynamic forces in steering motion on the basis of kinematic forces acting upon the both ends of the hull. The mathematical model can describe well the forces X, Y and N with large drift angles and tough turning motion. The rotative coefficients in the model are estimated from the static coefficients at oblique motion. The rotative coefficients thus derived are, however, somewhat different from the ones delivered from the experimental data of turning motion.In order to improve this point, the authors will now break up the hydrodynamic forces into the following components, i. e., ideal flow force, viscous force, induced drag, cross flow drag, cross flow lift and frictional resistance in the normal sense. Then a simplified vortex theory is called upon to evaluate the ideal flow force, viscous lift and induced drag.As the results, the present mathematical model can describe the hydrodynamic forces acting upon a hull in turning/drifting motion with a fair accuracy. The model also provides a sensible insight on the mechanism of hydrodynamic forces appearing in steering motion.

Prediction of hydrodynamic forces on submarine pipelines using an improved Wake II Model

The hydrodynamic force model for prediction of forces on submarine pipelines as described includes flow history effect (wake effects) and time dependence in the force coefficients. The wake velocity correction is derived by using a closed-form solution to the linearized Navier–Stokes equations for oscillatory flow. This is achieved by assuming that the eddy viscosity in the wake is only time dependent and of a harmonic sinusoidal form. The forces predicted by the new Wake (Wake II) Model have been compared to Exxon Production Research Company Wake Model in terms of time histories (force shape) and magnitudes of peak forces. Overall, the model predictions by the Wake II Model are satisfactory and represent a substantial improvement over the predictions of the conventional models. The conventional force models representing adaptations of Morison's equation with ambient velocity and constant coefficients give predictions that are in poor agreement with the measurements especially for the lift force component. The Wake II Force Model can be used for submarine pipeline on-bottom stability design calculations for regular waves with various pipe diameters.

A Component-type Mathematical Model of Hydrodynamic Forces in Steering Motion Derived from a Simplified Vortex Model (2)

A Component-type Mathematical Model of Hydrodynamic Forces in Steering Motion Derived from a Simplified Vortex Model (2)

A Component-type Mathematical Model of Hydrodynamic Forces in Steering Motion Derived by Simplified Vortex Model

One of the authors presented the component-type mathematical model of hydrodynamic forces in steering motion on the basis of kinematic forces acting upon the both ends of the hull. The mathematical model can describe well the forces X, Y and N with large drift angles and tough turning motion. The rotative coefficients in the model are estimated from the static coefficients at oblique motion. The rotative coefficients thus derived are, however, somewhat different from the ones delivered from the experimental data of turning motion.In order to improve this point, the authors will now break up the hydrodynamic forces into the following components, i. e., ideal flow force, viscous force, induced drag, cross flow drag, cross flow lift and frictional resistance in the normal sense. Then a simplified vortex theory is called upon to evaluate the ideal flow force, viscous lift and induced drag.As the results, the present mathematical model can describe the hydrodynamic forces acting upon a hull in turning/drifting motion with a fair accuracy. The model also provides a sensible insight on the mechanism of hydrodynamic forces appearing in steering motion.

Hydrodynamic Forces on Seabed Pipelines

A new hydrodynamic force model for the prediction of forces on seabed pipelines has been verified. Comparisons of forces predicted by the new model with those measured in full‐scale laboratory and field investigations demonstrate the accuracy of the model, in terms of general force characteristics, peak force values, and spectral energy content, for both regular and irregular waves, with or without superimposed currents. Pipeline motions calculated from predicted forces are in good agreement with motions calculated from measured forces. Similar comparisons using the conventional Morison equation show its deficiencies and demonstrate the considerable improvements attained by the new model. Two effects included explicitly in the new force model, related to the wake behind the pipeline and the time dependence of the force coefficients, are supported by measurements.

Wake model of hydrodynamic forces on pipelines

The hydrodynamic force model for pipelines presented includes flow history effects (wake effects) and time dependence in the force coefficients. These two features in the model were necessary to obtain satisfactory agreement between model predictions and full scale field measurements of pipeline forces. Conventional force models which represent adaptations of Morison's equation with ambient velocity and constant coefficients give predictions which for the lift component of the force in particular are in very poor agreement with the measurements. The parameters in the new model have been estimated on the basis of the full scale measurements and reflect a wide range of flow conditions. The model can be used in pipeline on-bottom stability design calculations for regular or irregular waves.

On a New Mathematical Model of Manoeuvring Motions of a Ship

In this paper, a new mathematical model describing manoeuvring motions of a ship is presented according to the detailed investigation of captive model tests. This model, based on the method proposed by the subcommittee of JTTC named Group-MMG, is composed of individual characteristics of hull, propeller and rudder and their interactions.First, the model for longitudinal hydrodynamic forces is dealt with.Second, the method to describe the hydrodynamic forces concerning to rudder is treated. A simple model consisting of open characteristics of rudder, inflow angle and velocity at rudder is proposed.Two problems as to lateral hydrodynamic forces acting on hull are then argued. One of them is the matter of nonlinear terms and, from the practical point of view, third order terms with rate of turn and swaying velocity are recommended. The other factor is the fin effect of propeller, and it is verified not to be negligible for manoeuvrability.Last, comparing the free-running results with estimated manoeuvres calculated by the present model, satisfactory coincidence is obtained. Consequently, the proposed model describing manoeuvring motions of a ship is found promising.