Bow Wave Research Articles

Research on the Motion and Dynamic Characteristics of the Hose-and-Drogue System under Bow Wave

To study the hose-and-drogue system’s motion under bow waves, this paper established a dynamic model of the hose-and-drogue system based on the multibody dynamics theory and the rigid ball-and-rod model. The wake of a tanker aircraft was taken into account in the simulation. The simulation results conformed to the general laws and verified the model’s accuracy. The equilibrium positions of the hose-and-drogue system were computed by the linear superposition of the bow waves and wake. The motion of the hose-and-drogue system was simulated and analyzed when a receiver aircraft moved at a constant speed or accelerated relative to the tanker aircraft. Since the receiver aircraft would not immediately stop after docking, the pulling force changes on the hose with and without a reel were compared. The present results are essential for improving the success rate of aerial refueling and ensuring the safety and stability of the hose-and-drogue system.

Experimental and numerical investigation of bow wave and wake flows around towed surface-piercing cylinders

An investigation was conducted to study the flow characteristics around towed surface-piercing cylinders and understand the mechanisms behind the bow wave and wake region formation. The experiments took place in a towing tank using three different cylinder models, covering a range of Froude numbers based on cylinder diameter from 0.28 to 1.73. Three-cylinder models are considered M1 is circular, M2 is semicircular ellipse and M3 is elliptical. Numerical simulations were performed using the OpenFOAM framework, considering turbulence effects through the unsteady Reynolds-Averaged Navier Stokes equations with the Shear Stress Transport k-ω turbulence model. The numerical results matched well with the experimental data. The bow wave's general features were similar across all cylinder models, with the height increasing as the Froude number rose. Froude numbers between 0.28 and 0.58 displayed a smooth surge surface, transitioning to a rough surface with roll-up initiation between 0.87 and 1.15. At Froude numbers 1.44 to 1.73, a fountain-like bow wave formed. Models M1 and M2 had the same average bow wave height, approximately 15% higher than that of Model M3. The depression depth or air ventilation at the trailing edge of the cylinders increased with the Froude number. Model M1 exhibited a greater depression depth than Models M2 and M3 at Froude number 1.15, and this difference further increased at Froude number 1.73 due to high vorticity and adverse pressure gradient generated by Model M1. Vortex coherent structures visualization revealed that at a low Froude number of 0.58, only the Karman vortex at the trailing edge was observed. At Froude number 1.15, the deep wake region maintained predominantly two-dimensional vortex structures, while the free surface displayed pronounced three-dimensional vortex structures. Model M1 had a drag coefficient approximately 40% higher than Model M3. These findings are valuable for engineering applications, particularly in the design of submarine masts.

Simulation of supersonic jet flow past a blunt body in a laboratory experiment using computer vision

The complex flow of a supersonic jet (Mach number 2.4–2.5) around the blunt aerodynamic body (consisting of a cylinder with a diameter of 16 mm and a truncated cone) was studied using the high-speed shadowgraph technique and computer vision-based visual data processing. The problem considered is directly related to the rocket and space systems stages safe separation problem in a sufficiently dense atmosphere. For systems with a sequential arrangement of stages, their separation often occurs under the rocket engine jet action. The occurrence of lateral forces in violation of axial symmetry can lead to overturning of the discarded stage and undesirable interaction with the outgoing stage. The goals of the paper were to study the pulsation characteristics of the incoming supersonic jet streamlining the body, as well as the pulsations of the bow shock wave in both axisymmetric and asymmetric flows. The recording frame rate in the experiments was up to 775 000 fps. Computer vision and various machine learning approaches were used to process a large number of shadowgraph images. Canny edge detection and Hough transform, as well as a convolutional neural network (CNN) based on YOLOv2 architecture, were used to automatically track the position of the bow shock relative to the body. Jet sound generation was studied - the amplitude dependence on the distance from the nozzle and the evolution in time. The position of the bow shock relative to the streamlined body and flow pulsations over time were analyzed. Bow shock pulsation spectra at zero angle of attack were compared to those at a small angle of attack (α = 2.5°–3°). The power spectral density was also estimated. The slope of the spectra for the zero angle of attack case was found to be horizontal and the spectra for the small angle of attack case had a slope close to −5/3. The strongest of the measured bow shock frequencies were found to be close to those of the jet mixing layer pulsations. The hardware signal spectrum was also measured and this signal can be considered as noise. The described approach makes it possible to obtain the physical characteristics of the flow at any point of interest. It was also shown that the automation of flow visualization experiments using computer vision techniques allows to increase the speed of data processing and obtaining new physical information, which may be important for engineers developing aircraft and spacecraft technologies.

Characterization of Shallow Ground in Railway Embankments Using Surface Waves Measured by Dark Fiber Optics Sensors: A Case Study

For the maintenance of railways on soft soils, accurate knowledge of the subsoil conditions is essential. Soft soils at shallow depths have high variability; thus, high spatial resolution is required. Spare telecommunication fiber-optic cables, known as dark fiber, can be used as an array of sensors to measure waves generated by running trains, which offers a unique opportunity to characterize shallow soils at high spatial resolution. We used dark fiber to measure seismic waves generated by running trains and implemented a seismic interferometry technique to retrieve surface waves. We evaluated the reliability of selected parts of the recorded signals split as bow waves (the train approaching the fiber), train waves (the train passing alongside the fiber), and tail waves (the train leaving the fiber) to retrieve broad-band surface waves. The analysis was performed in two distinctive zones. Zone I consists of a thick–soft (2.0–6.0 m thickness) layer, and Zone II consists of a thin–soft (less than 2.0 m thickness) layer, both overlaying a “stiffer” sand layer. At Zone I, train waves yielded the best results in revealing the thick–soft layer. At Zone II, the bow waves yielded clear high-frequency energy, revealing the overall soil structure but without identifying the shallow thin–soft layer.

MTU2-Net: Extracting Internal Solitary Waves from SAR Images

Internal Solitary Waves (ISWs) play a pivotal role in transporting energy and matter within the ocean and also pose substantial risks to ocean engineering, navigation, and underwater communication systems. Consequently, measures need to be adopted to alleviate their negative effects and minimize linked risks. An effective method entails extracting ISW positions from Synthetic Aperture Radar (SAR) data for precise trajectory prediction and efficient avoidance strategies. However, manual extraction of ISWs from SAR data is time-consuming and prone to inaccuracies. Hence, it is imperative to develop a high-precision, rapid, and automated ISW-extraction algorithm. In this paper, we introduce Middle Transformer U2-net (MTU2-net), an innovative model that integrates a distinctive loss function and Transformer to improve the accuracy of ISWs’ extraction. The novel loss function enhances the model’s capacity to extract bow waves, whereas the Transformer ensures coherence in ISW’s patterns. By conducting experiments involving 762 image scenes, incorporating ISWs, from the South China Sea, we established a standardized dataset. The Mean Intersection over Union (MIoU) achieved on this dataset was 71.57%, surpassing the performance of other compared methods. The experimental outcomes showcase the remarkable performance of our proposed model in precisely extracting bow wave attributes from SAR data.

Integrating k-means Clustering and LSTM for Enhanced Ship Heading Prediction in Oblique Stern Wave

The stability of navigation in waves is crucial for ships, and the effect of the waves on navigation stability is complicated. Hence, the LSTM neural network technique is applied to predict the course changing of a ship in different wave conditions, where K-means clustering analysis is used for the category of the ship’s navigation data to improve the quality of the database. In this paper, the effect of the initial database obtained by the K-means clustering analysis on prediction accuracy is studied first. Then, different input features are used to establish the database to train the neural network, and the influence of the database by different input features on the accuracy of the navigation prediction is discussed and analyzed. Finally, multi-task learning is used to make the neural network better predict the navigation in various wave conditions. Using the improved neural network model, the course of an autopilot ship in waves is predicted, and the results show that the current database and the neural network model are accurate enough for the course prediction of the autopilot ship in waves.

Characteristics of wake morphology during debris flow when passing a cylindrical obstacle

Predicting wake morphology during debris flow when passing a cylindrical obstacle is vital for disaster assessment, early warning, evacuation planning, engineering design, and ecologic conservation. It can provide a scientific foundation for pertinent decision-making processes, diminishing the risks and impacts of debris flow disasters. This study extracts the morphological characteristics of debris flow cylindrical flow traces through the steady-state motion of debris flow observed in a flume during cyclical tests. It introduces a theoretical prediction formula and compares it to empirical data. The results indicated that the morphology of debris flow cylindrical flow traces can be described as a wall-jet-like bow wave (a bow wave formed by an upward wall jet on the obstacle upstream face). The primary upstream inflow is predominantly discharged through the wall and lateral jets. Formulas for three crucial parameters that determine the morphology of the traces are derived by combining the aerodynamics theory and extant literature. The predicted outcomes strongly align with the experimental data, underscoring their high predictive precision.

Path-following control problem for maritime autonomous surface ships (MASS) in adverse weather conditions at low speeds

With the increasing popularity of autonomous vehicles, the maritime industry has focused on the development of Maritime Autonomous Surface Ships (MASS). An essential aspect of autonomous ships is their ability to follow a predetermined path at sea, as deviations from this path can jeopardize navigational safety, leading to accidents like collisions and grounding incidents. Therefore, ensuring that autonomous ships can safely navigate by following predefined routes is of utmost importance. Simultaneously, the implementation of the Energy Efficiency Design Index (EEDI) by the International Maritime Organization (IMO) has raised interest in a ship's manoeuvring performance in adverse sea conditions, aiming to reduce greenhouse gas emissions. The Marine Environment Protection Committee (MEPC) has provided guidelines to determine the minimum propulsion power required to maintain ship manoeuvrability in adverse conditions. This study investigates the path-following performance of a ship operating at low forward speeds in adverse weather conditions using a free-running Computational Fluid Dynamics (CFD) model, enabling accurate prediction of manoeuvring behaviour. The numerical results emphasise the significance of low forward speeds in enabling a ship to follow a predetermined route, offering valuable insights into path-following performance with minimum propulsion power in adverse weather. The study reveals that increasing propulsive power reduces deviations from the predetermined route when the ship encounters bow and beam waves. However, the impact of propulsion power on deviation is negligible in quartering waves during path-following control. This research contributes to improving guidelines for minimum ship powering, ensuring safe autonomous navigation in adverse weather conditions.

Experimental and numerical studies on hydrodynamic effects of ice floe drift and impact process in uniform current

The safety of offshore structures is often threatened by the impact load of isolated ice floes. However, the accurate estimation of the impact load can be challenging due to the unquantified near-field hydrodynamic interaction. In this paper, the hydrodynamic interactions between ice floes and offshore structures are investigated by a model experiment and the numerical calculation. The model experiment is conducted to study the whole process of a model ice drifting and impacting a cylinder under the influence of uniform flow. Meanwhile, the interaction process is also numerically simulated by using the Computational Fluid Dynamics (CFD)-Finite Element Method (FEM) coupling method. The experimental and numerical results show good agreement in terms of velocity time histories, peak impact forces and peak accelerations. The influences of current velocity, ice floe size, and ice floe shape on various interactions are discussed based on numerical results, including the velocity attenuation effect before impact, the added mass effect during collision, the kinetic energy and motion attitude of the ice floe. The main findings of present research suggest that the viscous effect in near-field and the bow wave are noteworthy factors causing the velocity attenuation of the ice floe and ignoring these influences can lead to underestimation of the velocity attenuation effect to some extent. Additionally, the (effective) added mass coefficients calculated based on the ice floe motion equation and the momentum balance equation are found to be close to each other. However, with the decreasing of the ice floe size, the difference between the added mass coefficients calculated by the two methods gradually increases.

A Numerical Study on the Hydrodynamic Performance of a Tanker in Bow Sea Conditions Depending on Restraint Conditions

The importance of accurate ship performance estimation is increasing for efficient ship operation. Ship performance has been evaluated through model tests in the past, but there are limitations in terms of facilities and costs. With the spread of high-performance computers, the method of evaluating the performance of a ship by numerical analysis, especially computational fluid dynamics (CFD), has become common. There have been many numerical studies on added resistance under various wave conditions for many years, showing a high reliability. Meanwhile, most of the studies were conducted under conditions where the degree of freedom (DOF) of the ship was limited due to computational complexity. In this study, we tried to compare the added resistance performance and fluid dynamics of S-VLCC with 6 DOFs in the regular wave conditions. One of the methods for utilizing the 6 DOFs is the soft-mooring system, which allows springs to be attached to the bow and stern to recover the non-restoring force of the hull. The second method considers the free-running condition. The virtual disk is used for the self-propulsion of the ship, and the rudder can be rotated to maintain its course. The propeller rotation speed and rudder angle are controlled through PID control. The bow wave (ψ = 180°) and oblique wave (ψ = 150°, 120°) conditions were considered, and various regular wave conditions from short to long wavelengths were regarded. The effects of restraint conditions on the added resistance and motion response amplitude operator (RAO), according to each wave condition, were compared. As a result, there was a difference in the roll motion for each restraint condition, and the y-direction force and yaw moment generated on the hull were compared to analyze the cause. In addition, we observed the change in flow characteristics by comparing the streamlines around the hull and the nominal wake on the propeller plane.

PROPAGATION OF CARGO SHIP WAKE INTO SECONDARY CHANNELS

Cargo ship wake can be a strong and frequent source of energy along shorelines in the vicinity of commercial shipping channels. Large vessels in narrow channels produce low-frequency wake that consists of a positive front wave at the bow of the vessel, a depression (drawdown) spanning the length of the vessel, and an upwards surge produced by the stern wave at the stern. This low-frequency wake is followed by high-frequency wedge-shaped Kelvin wake, as well as continued low-frequency oscillations that may persist in the channel more than 30 minutes after a vessel passage. Low-frequency cargo ship wake with wave heights up to 2.8 meters was identified as the dominant source of erosion at Bird/Long Islands in the Savannah River, Georgia, USA based on the analysis of one month of Aquadopp measurements that captured over 300 large vessel wake events. The islands divide the river into the Main Channel, which contains the shipping channel, and the South Channel, a shallow, sheltered secondary channel trafficked only by small craft. Cargo ship wake was the primary energy source in both channels. The dominance of cargo ship wake energy in the sheltered South Channel demonstrated that the effects of wake extend beyond the shipping channel shorelines. This finding motivated three follow-up studies to quantify the extent and significance of cargo ship wake in the far-field, secondary channels connected directly or indirectly to the shipping channel.

The effect of PID control scheme on the course-keeping of ship in oblique stern waves

Sailing in oblique stern waves causes a ship to make sharp turns and uncontrollable course deviation, which is accompanied by a large heel and sometimes leads to capsizing. Studying the control algorithm in oblique stern waves is imperative because an excellent controller scheme can improve the ship’s course-keeping stability. This paper uses the Maneuvering Modelling Group (MMG) method based on hydrodynamic derivatives and the Computational Fluid Dynamics (CFD)-based self-navigation simulation to simulate ship navigation in waves. This study examines the effect of proportion-integral-derivative (PID) controller schemes on the stability of course maintenance based on hydrodynamic derivatives and 3DOF MMG methods. Then, the optimized PID control parameters are used to simulate the ship’s 6DOF self-propulsion navigation in oblique waves using the CFD method. The nonlinear phenomena during the process, such as side-hull emergency, slamming, and green water, are considered. This study found that the range of the control bandwidth should be optimized based on the ship's heading and wave parameters.

Impact force of roll waves against obstacles

The roll waves in open-channel flow on steep slopes can strike an obstacle with great force. We conducted two-dimensional shallow-water simulations to study the impact force of the waves against structures of various shapes and orientations. The focus is on the front runner of a wave packet developed from spatial instability. The numerical results include the stand-off distance of the bow shock wave, the front face's run-up height and the wave force on the obstacle. The strength of the impact depends on the Froude number of the undisturbed flow and the obstacle's distance from the local disturbance but not much on the form of the perturbation that initiates the instability. The wave force could reach a peak of more than an order of magnitude greater than the force on the structure without the roll waves. However, an obstacle with a sharp and pointy front can deflect the incident waves, significantly reducing the impact force.

Flow Field Simulations Over Reentry Modules at High Speed

Reentry capsule configurations significantly differ from each other due to entry conditions and mission requirements. This paper describes numerical simulations of the viscous flow over the Beagle and the OREX (Orbital Reentry EXperiments) configurations for freestream Mach numbers in the range of 1.2 - 5.0. The flow fields over the reentry modules are obtained by solving time-dependent axisymmetric compressible laminar Navier-Stokes equations. The fluid mechanics equations are discretized in spatial coordinates employing a finite volume method, which reduces the governing equations to semi-discretized ordinary differential equations. Temporal integration is carried out using a two-stage Runge-Kutta time-stepping scheme. A local time-stepping is employed to get the steady state solution. The numerical simulation is performed on a single-block structured grid. The flow field features around the reentry capsules such as bow shock wave, sonic line, expansion fan and recirculation flow in the base-shell region are well captured by the present numerical computation. The effects of the geometrical parameters of the module, such as aspect ratio, frontal segment bluntness, fore body cone angle and shoulder rounding radius on the wake throat width, distance from the wake throat to the base of the model, flow departure angle and aerodynamic drag are analyzed using the numerically simulated flow field.

Numerical investigation of unsteady aerodynamic characteristics induced by the interaction of oblique/bow shock waves during rotation

The interactions between oblique and bow shock waves are significant problems related to the aerodynamic force and thermal performance of hypersonic vehicles, but few studies have considered the dynamic effect of the body's motion on the phenomena. In this work, a numerical study on the oblique and bow shock waves ahead of an elliptic cylinder rotating with a forced-oscillation approach was conducted at Mach 5 by solving the unsteady, two-dimensional Navier–Stokes equations in a non-inertial coordinate system. The hysteresis loops of aerodynamic coefficients were analyzed first, and it was found that the moment is sensitive to rotation. Then, two different hysteresis forms were found at positive and negative angles of rotation (AOR), corresponding to cases with the interference point above or below the wall, respectively. When AOR is positive, the rate-dependent transition hysteresis among various shock interaction types causes the movement of strong flow structures (reflected shock wave, Mach stem, and jet) to always lag behind the body's motion. When AOR is negative, besides the evolution hysteresis of flow structures, two unusual patterns between Edney Types III and VI were observed on different transition paths, which led to very different peak pressures. Also discussed are the driving mechanisms associated with the effect of the subsonic region and the downstream boundary of the interaction zone, as well as the modulating action of the formed virtual Laval flow channel. Additional simulations were performed to study the effect of rotation speed on the transition boundary and the transition structures between Types III and VI.

Scale effects on bow wave breaking of KCS ship model: Insights from DDES investigations

Scale effects on bow wave breaking of KCS ship model: Insights from DDES investigations

Numerical Simulation of Flow Field Over a Sharp-Tipped Double Cone at High Speed

The flowfield over a sharp-tipped double cone axisymmetric configuration is carried out by solving time dependent axisymmetric laminar compressible Navier-Stokes equations for freestream Mach number range of 2.0 - 6.0. The fluid dynamics equations are discretized in spatial coordinates using integral formulation in conjunction with finite volume method which reduces the governing equations to semi-discretized ordinary differential equations. Temporal integration is performed employing multistage Runge-Kutta time-stepping scheme. A local time step is used to achieve steady-state solution. The numerical computation is carried out on a mono-block with structured grids. The flowfield features over the sharp-tipped double cone configuration such as conical shock wave, separation region, separation and reattachment shock wave and bow shock wave in the cone region, expansion fan, and recirculation flow in the base region are well captured at Mach number 3 and 6 which are identical to the Edney VI type shock interaction. The velocity vector, Mach contours plots and variations of surface pressure coefficient along the sharp-tipped double cone configuration are analyzed at various Mach numbers. The fore-body aerodynamic drag is calculated employing computed pressure distribution. The paper presents the influence of the freestream Mach number on the flows with shock interactions over the sharp-tipped double cone geometry.

Numerical analysis on the transom-stern wake with a horizontal plunging jet

The turbulent flow behind a dry-transom stern with a plunging water jet is numerically studied. High-resolution large eddy simulation (LES) is performed to predict the wave-elevation topology and vortex structure in the wake with varying jet height. The theoretical impact position between jet flow and stern wave is proposed. The turbulent wake under the stern-jet configuration can be divided as converging-corner-wave region (CCW), mixing region (MR), transition region (TR) and developed region (DR). High-speed plunging jet speeds up the averaged-axial velocity and enhances the shear effect near the free surface. The upward movement of the rooster tail is suppressed by the parabolic jet flow, and periodical large scale air pockets are clearly observed following the wake angle along the downstream direction. Air pocket vortices and jet vortices are merging in the near wake and forming U-shaped vortex structures. Reducing jet height accelerates the instability of coherent vortex structure and energy dissipation in the developed region. The main contributions in the power spectra density (PSD) spectrum of Turbulent Kinetic Energy appear in the frequency range of 10–20 Hz with the evolution of wake vortices.

The retrofitting of ships by applying retractable bow hydrofoils: a case study

Increasing environmental requirements and a relatively long ship life of 30 years mean more attention is needed to retrofit existing ships. One possibility is using hydrofoils to reduce the ship’s resistance and improve comfort and safety in rough sea conditions. This study investigates the influence of retractable bow hydrofoils on the seakeeping performance and operational conditions of a selected case study vessel (V-shaped bulbous bow). The methods used were full-scale computational fluid dynamics (CFD) simulations and towing tank experiments for validation. The analysis was conducted for bow waves of different lengths and a ship’s operating speed. The most beneficial effect of hydrofoils was observed for wavelengths from λ/LWL = 1.0 to λ/LWL = 1.2. For the wavelength λ/LWL = 1.2, the reduction of heave motion was equal to 33%, pitch motion was equivalent to 28%, and the reduction of wave-added resistance was equal to 25%. The analysis also showed unfavourable conditions for which hydrofoil folding is needed to avoid causing an excessive increase in resistance. A generalized procedure has been developed to assess the potential for resistance and motion reduction by retrofitting existing ships using hydrofoils.

Hydrodynamic performance of high-speed displacement vessel with hull vane

The hydrodynamic performance of high-speed displacement vessels can be enhanced by installing basic stern fittings such as stern wedges, stern flaps, interceptors, and hull vanes. The hydrodynamic performance of a high-speed displacement vessel with hull vane is analysed and compared to the bare hull condition in this study. The hull vane is a hydrofoil wing attached transversely to the bottom of the transom that focuses largely on minimising the wave-making resistance of the vessel by altering the flow at the stern. It increases flow on its upper surface, which interferes favourably with the stern wave system, hence lowering wave-making resistance. This research investigates the effects of installing the hull vane on a high-speed displacement vessel at various locations in the X and Z directions from the transom edge. The tests were carried out on model ship with and without a hull vane to verify the accuracy of the computational findings. The experiments were carried out in the towing tank facility available at Indian Institute of Technology Madras (IIT Madras), Department of Ocean Engineering (DOE). Computational fluid dynamics (CFD) simulations were carried out for model vessel with and without hull vane for a range of Froude numbers from 0.17 to 0.42 using RANSE-based commercial software. The research examines the resistance, trim, pressure distribution, and stern flow development of the vessel at various speeds and in calm water conditions. The results indicate that installing a stern hull vane reduces resistance and trim of the vessel. The effect of a hull vane is enhanced when it is positioned away from the stern of the vessel.