Bow Flow Research Articles

Research on the bubble sweep-down mechanism and suppression scheme of the research vessel

Vessels will encounter the phenomenon of bubble sweep-down when sailing in the ocean, which seriously affects the accuracy of underwater detection and resource exploration. In addition, the installation of defoaming attachments will affect the resistance performance of the ship. It is significant for a scientific research vessel to study the mechanism of bubble sweep-down and propose a bubble suppression scheme considering the ship's resistance characteristics. Detached-Eddy Simulation (DES) Shear Stress Transfer (SST) k-ω model combined with the Discrete Element Method (DEM) is used to simulate the process of microbubbles sweeping down to the ship's bottom under the influence of the flow at the bow of the scientific research vessel, and the deflection effect of the fairing on the bubbles, to carry out a mechanical analysis of the bubble sweep-down phenomenon. The results show that the bubble movement is closely related to the characteristics of the bow flow, the bubble's trajectory was related to the vessel speed and the bubble starting position, and the bubble at the bottom of the bow was easier to sweep down to the bottom of the ship. The fairing was a defoaming attachment with excellent resistance performance, which can physically block the air bubbles and strengthen the bottom vortex structure to change the bubbles' trajectory. Exploring the bubble sweep-down mechanism and the suppression scheme can better serve the design of the hull shape and defoaming attachment of scientific research ships.

Experimental study on bow flow field of scientific research ship based on flow visualisation

To investigate the bubble sweep-down phenomenon of a scientific research ship from the perspective of its flow mechanism, the flow-field characteristics of the bow were measured in a towing-tank environment. First, the silk-thread method was used to visualise the streamline characteristics of the bow. Next, the flow field of the bow was measured using multiple two-section 2-dimensional computed tomography scans with stereoscopic particle image velocimetry. The flow-field information of different sections at different speeds (e.g., velocity distribution, vector characteristics, and streamline distribution) were then obtained. A 3-dimensional (3D) three-component (3C) space reconstruction of the time-averaged flow field in the bow was thus realised. From the visualisation results, the flow field in the bow generally exhibits a downward movement trend. The flow structure of the bow flow field was found to be closely related to the wave-making change of the bow. On the other hand, the cross-sectional flow characteristics and 3D reconstruction results showed that with the bubble sweep-down, some bubbles flowed along the bilge and amassed with other bubbles, breaking through the geometric inflection point of the bilge. Compressed by the bilge vortex, they slid backward and affected the operation of acoustic equipment.

An Approximation for General Transit Time Function

People have unpleasant experience of driving through congested road network of urban areas, where transit time is not constant but flow-dependent. Network flow over time with flow-dependent transit times is more realistic than flow over time with fixed transit time on arcs which is an active area of research from last two decades. In traffic assignment problem, transmission time of the vehicles directly depends on the number of vehicles entering on the road. So, general transit time function is non-decreasing, convex and left-continuous. In this paper we discuss how a general transit time function can be approximated by the step function. It relaxes the inflow-dependent transit times to a bow graph. We calculate maximum bow flow by using temporally repeated flow for the given time horizon. Our aim is to show how flow value in step function converges to the flow value of general transit time function by increasing number of points in the domain of the step function.

Prediction of the Local Scour at the Bridge Square Pier Using a 3D Numerical Model

In this paper, the problem on local scour around a single square pier was studied by using both the numerical and physical models. The numerical model for the study is FSUM based on a finite-difference method to solve the Reynolds averaged Navier-Stokes equations (RANS) and the equations for suspended sediment concentration and bed morphology. The computed result was verified through data measured in the experimental flume with a sand bed. In general, the typical features of local scour around the pier were successfully simulated by FSUM, such as stream flow, bow flow, down flow, horseshoe vortex. The comparison between the computation and experiment data shows a quite good fitness. Both numerical model and experiment results show that the maximum scour depth occurs at two front edges of the pier. Although the computed result shows a little bigger scour depth in comparison with the measurement in the physical model, it still confirms the reliability of numerical model in some measure.

NUMERICAL SIMULATION OF FLOW AND LOCAL SCOUR AT TWO SUBMERGED-EMERGENT TANDEM CYLINDRICAL PIERS

In this paper, the flow and local scour variation around two submerged and un-submerged tandem piers are studied using 3D flow model where the upstream pier is submerged while the downstream pier is emergent. The model uses a finite-volume method to solve the non-transi ent Navier-Stokes equations for three dimensions on a general non-orthogonal grid. The   k turbulence model is used to solve the Reynolds-stress term. The numerical model solves the sediment continuity equation in conjunction with van-Rijn’s bed-load sediment transport formula to simulate the bed evolution. The 3D flow model is verified through experimental study in a non cohesive bed material in an experimental flume. The different causes of local scour around two submerged and unsubmerged piers are simulated well, such as bow flow, down flow, horseshoe vortex, pressure variation and lee-wake vortex. It is found from this study that the maximum local scour depth by interaction between two tandem submerged unsubmerged piers depends on submersion ratio of upstream pier, the densimetric Froude number, the longitudinal distance between piers and the ratio of pier diameter to channel bed width. The maximum scour depth decreases by increasing the submerged pier height then begin to increase by increasing the submerged pier to a height larger than half the water depth and in general the maximum scour depth is less than that of two unsubmerged piers. The results show good agreement between simulation and experimental results. Also, empirical equations are developed for computing the maximum scour depth due to the interaction between two submerged unsubmerged piers with circular shapes as a function of submergence ratio, piers spacing, densimetric Froude number and channel width to pier diameter ratio.

The fluid mechanics of splashing bow waves on ships: A hybrid BEM–SPH analysis

The free surface bow flow around a fast and fine ship in calm water is studied with an emphasis on generation and evolution of the breaking and splashing bow wave using the 2D+t theory. A temporal domain-decomposition strategy is adopted which combines sequentially two Lagrangian methods: a potential-flow solution, given by a boundary element method (BEM), follows the jet evolution up to the breaking and then initiates a rotational solution, provided by a smoothed particle hydrodynamics (SPH) technique. The latter describes the splashing and the evolution of rotational flow structures generated during the evolution. The compound method is very efficient and proved to be able to capture the breaking features. A validation has been performed through comparison with experimental data and other numerical solutions available in the literature. The DDG51 vessel is used to carry on a parameter investigation in terms of the ship speed (Froude number) and surface tension. The analysis confirms a substantial influence of the Froude number on the patterned morphology and highlights the importance of surface tension for small-scale ships (e.g. small-scale models tested in towing tanks) in inhibiting the jet formation and evolution.

3-D NUMERICAL SIMULATION OF FLOW AND CLEAR WATER SCOUR BY INTERACTION BETWEEN BRIDGE PIERS

In this paper, the flow and local scour variation a round single pier and introducing interaction effect between bridge piers were studie d using 3D flow model. The model used a finite-volume method to solve the non-transi ent Navier-Stokes equations for three dimensions on a general non-orthogonal grid. The e k turbulence model is used to solve the Reynolds-stress term. The numeric al model solves the sediment continuity equation in conjunction with van Rijn’s bed-load sediment transport formula to simulate the bed evolution. The 3D flow model was verified through experimental study in the non cohesive bed material in an experimental flume. The different causes of local scour around the pier wer e simulated well, such as bow flow, down flow, horseshoe vortex, pressure variation and lee-wake vortex. It was found from this research study that the local scour depth by interaction process between bridge piers depends on the Froude number, the dist ance between piers and the diameter of piers. The maximum scour depth for double piers is higher than that for single pier. Furthermore, the effect of pier shape on the scour process was studied and it was found that the maximum scour depth for circu lar pier is less than that for rectangular one for both single and double piers ca ses. The results show good agreement between simulation and experimental results. Also, empirical equation was developed from the experimental data for computing the maximum scour depth due to the interaction between bridge piers.

Shipping of water on a two-dimensional structure. Part 2

The water-shipping problem is modelled in a two-dimensional framework and studied experimentally and numerically for the case of a fixed barge-shaped structure. The analysis represents the second step of the research discussed in Grecoet al. (J. Fluid Mech., vol. 525, 2005, p. 309). The numerical investigation is performed by using both a boundary element method and a domain-decomposition strategy. The model tests highlight the occurrence of dam-breaking-type water on deck, (a) with and (b) without an initial plunging phase, and (c) an unusual type of water shipping connected with blunt water–deck impacts here called a hammer-fist type event never documented before. Cases (a) and (c) are connected with the most severe events and the related features and green-water loads are discussed in detail. A parametric analysis of water-on-deck phenomena has also been carried out in terms of the local incoming waves and bow flow features. We classify such phenomena in a systematic way to provide a basis for further investigations of water-on-deck events. The severity of (a)-type water-on-deck events is analysed in terms of initial cavity area and water-front velocity along the deck. The former increases as the square power of the modified incoming-wave (front-crest) steepness while the latter scales with its square-root. The two-dimensional investigation gives useful quantitative information in terms of water-front velocity for comparison with three-dimensional water-on-deck experiments on fixed bow models interacting with wave packets.

Smoothly attaching bow flows with constant vorticity

AbstractThe free surface flow of a finite depth fluid past a semi-infinite body is considered. The fluid is assumed to have constant vorticity throughout and the free surface is assumed to attach smoothly to the front face of the body. Numerical solutions are found using a boundary integral method in the physical plane and it is shown that solutions exist for all supercritical Froude numbers. The related problem of the cusp-like flow due to a submerged sink in a comer is also considered. Vorticity is included in the flow and it is shown that the behaviour of the solutions is qualitatively the same as that found in the problem described above.

Bow–like free surfaces under gravity

The nonlinear free–surface flow near the extreme bow of a ship is a classical hydrodynamic example of a violent surface motion. The nature of these bow flows is discussed, and previous studies on them are reviewed, with emphasis on splashes and their elimination. Some new examples are provided of bow–like flows without splashes that can be computed by inverse means. In effect, the shape of the free surface is given, and the bow shape that generates this free surface is determined. This exact solution-generating procedure, which dates at least from 1901, is in principle capable of being turned into a practical design tool. Although this point has not been yet reached, some of the examples given here are not unlike the flow at the bow of a bluff ship.

Viscosity and Surface-Tension Effects on Wave Generation by a Translating Body

Various theoretical and experimental studies have been carried out to examine the generation of waves ahead of a translating body. Not all issues pertaining to this wave-motion problem are, however, fully resolved. In particular, mechanisms pertaining to generation of white-water instability and inception of vortices in the bow region are not fully understood. In this paper, the two-dimensional, unsteady, nonlinear, viscous-flow problem associated with a translating surface-piercing body is solved by means of finite-difference algorithm based on boundary-fitted coordinates. Effects of surface tension and surfactants are examined. Results of this work resolve certain classic issues pertaining to bow flows. A continuous generation of short and steepening bow waves is observed at low (draft) Froude number, a nonlinear phenomenon uncovered recently in the case of inviscid fluid also. This indicates that, steady-state nonlinear bow-flow solution may not exist, even at low speed. It is postulated that these short bow waves are responsible for the white-water instability commonly observed ahead of a full-scale ship. The amplitudes of these short bow waves are suppressed by surface tension, which is, possibly, the reason why white-water instability is not distinctly observed in laboratory-scale experiments. The presence of surfactants on the free surface is found to intensify the generation of free-surface vorticity, thus resulting in the formation of bow vortices. The accumulation of surface-active contaminants at the bow is hence responsible for the generation of bow vortices observed in laboratory experiments at low Froude number. At high Froude number, an impulsive starting motion of the body results in the generation of a jet-like splash at the bow and a gentle start an overturning bow wave, as previously observed in the case of inviscid bow flow.

Bow flows with surface tension

Two-dimensional free surface flow past a semi-infinite plate is considered. Surface tension is included in the free surface condition, and the effect of gravity is neglected. The configuration models a bow flow, i.e. the flow occurring at the front of an object moving at a constant velocity at the surface of a fluid. It is shown analytically and numerically that flows with waves do not meet the plate tangentially: there is an angle $\delta \neq $ 0 between the plane of the plate and the free surface at the separation point. The shape of the free surface is computed by a boundary integral equation method as a function of $\delta $. The exact form of the wave in the far field is derived as a function of $\delta $ from the principle of conservation of momentum. In particular it is found that the dimensionless wavelength $\lambda $ = $\pi $(1 + cos $\delta $). The numerical results are shown to be consistent with the exact results.

Detailed Bow-Flow Data and CFD for a Series 60 CB = 0.6 Ship Model for Froude Number 0.316

The bow flow of the Series 60 CB = 0.6 ship model is reinvestigated using both experiments and computational fluid dynamics (CFD). More detailed bow-flow data are obtained by taking measurements of the wave elevation with a point gauge very near the bow. In the CFD, the bow flow is resolved in more detail by using the real bow geometry instead of the simplified vertical zero-thickness bow used in previous studies, which requires a much finer grid in the bow region. The experiments and CFD are briefly described; results are presented; and discussions are made concerning comparisons of the new and old CFD solutions with the extended data regarding the wave profile and elevation at the bow, the thin film and beads (i.e. attached spray sheet and bow vortices), and the stagnation effects.

Nonlinear bow flows with spray

The steady flow past the bow of a two-dimensional ship in water of infinite depth is considered. The ship is assumed to be a semi-infinite flat-bottomed body terminated by a face inclined at an angle β with the horizontal. The spray is modelled by a layer of water rising along the bow and falling back as a jet. A series truncation method is used to solve the fully nonlinear problem numerically. It is shown that for a prescribed value of β, there is a one-parameter family of solutions. Values of the drag and of the jet thickness are presented for different values of β.

Bow flows with smooth separation in water of finite depth

AbstractThe bow flow generated by a wide flat-bottomed ship moving in water of finite depth is examined. Solutions obtained using an integral equation technique are presented for a range of different depths and for a range of angles of the front of the bow. The solution for the limiting case of infinite Froude number is obtained as an integral, and numerical solutions are found for the nonlinear problem in which the Froude number is finite. Solutions with smooth separation are shown to exist for all values of Froude number greater than unity, for any bow slope.

Bow Flow Field with Spray Phenomena around a Planing Plate

A perturbational study is made of the bow flow field with spray phenomena around a planing plate with a small dead rise angle and a small trim angle in order to obtain the basis on the configuration of the wetted surface area. A hypothesis is proposed whereby the plate planes on a water surface deformed as a parabolic cylinder with transverse curvature near the bow, the deformation seems to be caused by the displacement effect. The hypothesis allows the assumption that a similarity flow changing the scale of transverse section as a square root of longitudinal length. The flow field is divided into three regions for the analysis. By analysing the bow near field, the spray root line is obtained as forming a parabola on the plate, so that good coincidence with an experimental result of pressure contour is shown with the help of the same flow near the spray root line as Wagner's local flow. The spray flow developing on the plate outside the spray root line is analyzed. The flow rate and thickness of the spray and the spray drag show good coincidence with an experimental result. The spray edge line is shown by assuming a lower bound of spray thickness.

Bow flows in water of finite depth

Free-surface flow past the bow of a ship in water of finite depth is considered. The bow is assumed to be a semi-infinite two-dimensional, flat-bottomed body with a vertical front. The problem is solved numerically by series truncation for various values of the Froude number F. It is shown that there are splashless flows for 1.22<F<(2)1/2.

Bow Flow and Added Resistance of Slender Ships at High Froude Number and Low Wave Lengths

Flow around a slender ship bow at high Froude number and regular incident head sea waves is analyzed by matched asymptotic expansions. The near-field solution implies solving a two-dimensional Laplace equation with complete linear free-surface conditions. A solution technique with fundamental sources and dipoles is used. The solution technique is tested with good results for transient forced heave oscillation of a circular cylinder and for steady flow around a wedge. Comparisons with other numerical and experimental results for steady bow flow around a Wigley hull and a Series 60, Cb = 0.6 model show partly satisfactory results. The theoretical model for unsteady flow around a ship bow is used to calculate added resistance on a slender ship in high Froude number and incident regular head sea waves of low wave lengths. Experimental results for added resistance at low wave length for a cargo ship with Cb = 0.61 are not in complete agreement with the theoretical predictions.