Bow Wave Research Articles

Numerical Study on the Motion and Added Resistance of a Trimaran in Stern Waves Using a Hybrid Method

Numerical Study on the Motion and Added Resistance of a Trimaran in Stern Waves Using a Hybrid Method

Drag and Heat Flux Reduction on Hypersonic Re-entry Vehicles in Combination with Different Aerospikes

Hypersonic re-entry space vehicles, reusable launch vehicles and intercontinental ballistic missiles all use a blunt body as its fore body to decelerate the re-entry vehicle with aerodynamic heating as the result. Blunt bodies produce more wave drag due to strong detached bow shock wave formation in front of the body. To overcome this, wave drag aerospike has been introduced in front of the blunt body. This aerospike weakens the strong detached bow shock into a weaker oblique shock. It also creates a re-circulation zone (due to the adverse pressure gradient) in front of the body which greatly reduces the high pressure on the front portion of the body. Hence, aerospike is used in front of the blunt body to reduce the wave drag. This aerospike concept was first conceived in the 1950s as a means of reducing the heat transfer rates and aerodynamic drag on axis-symmetric blunt bodies. Many researchers have carried experiments out on the aerospike concept. In this study, computational investigations were carried out on blunt bodies with and without spikes, to know the flow behaviour at different Mach numbers with the help of computational fluid dynamics software. Experimental validations were compared. The effects on the coefficient of drag due to changing the spike tips, such as different aerospikes and spikes, were studied. The effect of the heat flux of the spike was also studied. Finally, conclusions on the optimum tip portion spike were chosen.

A Review of Computational Simulation Methods for a Ship Advancing in Broken Ice

Apart from breaking level ice, polar ships can interact with broken ice in various scenarios. In recent years, computational simulation models have increasingly been used for the evaluation of ship operability under broken ice conditions, presenting some challenging issues. This paper reviews existing simulation methods used to estimate ship performance and ice loads for ships advancing continuously in broken ice fields. Models for different types of broken ice, including ice floes, ice ridges, brash ice, and sliding ice pieces, are reviewed separately. A ship’s response in broken ice is divided into two categories: resistance, which relates to the overall ship performance, and local loads, which relates to structural safety. This review shows that most existing models are proposed for unbreakable ice particles, which are only applicable to broken ice of small size; most models treat fluid flow with extensive simplification, which does not reflect the influence of a ship’s wake or bow waves, and most models are aimed at resistance estimation, adopting elastic or viscoelastic contact models which do not include ice crushing. As for future work, it is suggested that more effort should be assigned to simulating a ship’s interaction with ice ridges and sliding ice pieces, the modelling of breakable ice floes, and the coupling of the Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD). More attention to the local ice load estimation is also encouraged.

Detachment of strong shocks in confined granular flows

Granular flows are highly dissipative due to frictional resistance and inelasticity in collisions among grains. They are known to exhibit shock waves at velocities that are easily achieved in industrial and nature-driven flows such as avalanches and landslides. This experimental work investigates the formation of strong shock waves on triangular obstacles placed in a dry rapid granular stream in a confined two-dimensional set-up. Oblique attached shock waves are formed for mild turning angles and higher flow velocities, whereas strong bow shock waves are formed for higher turning angles and slower granular streams. A shadowgraph imaging technique elucidates interesting characteristics of the shock waves, especially in the vicinity of shock detachment. Velocity distributions in the form of scatter plots and probability distribution functions are calculated from the flow field data obtained by particle imaging velocimetry. The flow field around the granular shock wave region represents a bimodal distribution of velocities with two distinct peaks, one representing the supersonic flow within the free stream, and the other corresponding to the subsonic faction downstream of a shock wave. Connecting the two is a population that does not directly belong to either of the modes, constituting the non-equilibrium shock wave region. The effect of grain size and scaling, for fixed free-stream conditions and fixed channel width, on the shock detachment is presented. The mechanisms of the static heap formation and the shock detachment process in a confined environment are discussed.

Theoretical and numerical study of choking mechanism of fluid flow in Hyperloop system

The Hyperloop is a proposed transportation system in which a high-speed vehicle travels in a near-vacuum tube. High-speed motion of an object (near Mach 1) in a confined space generates choked flow, endangering operational safety. This study investigates the physics and behaviour of choked flow in the Hyperloop system, and the corresponding law and influencing factors are theoretically analysed. A CFD method is used to simulate the compressible flow in the system. The different flow states caused by a pod travelling in the Hyperloop tube are numerically obtained, and the results concurred with the theoretical prediction. The results show that the choking strength gradually increases as the vehicle accelerates, reaching its maximum at Mach 1 and then gradually decreasing. When the blockage ratio reaches 0.4, nearly one-third of air hardly passes through the throat at most owing to the choking. The inflow velocity, choked flow density, and total pressure loss caused by the shock wave jointly affect the mass-flow limitation. In addition, the normal shock wave is the main characteristic of choked flow. The flow transitions from choked to unchoked regimes owing to the supersonic pod and forms a bow shock wave. These findings, including the equations describing the choking mechanism, can benefit various design aspects of the Hyperloop system.

Full-scale ship stern wave with the modelled and resolved turbulence including the hull roughness effect

Stern wave flow phenomena are investigated in a full-scale Kriso container ship. The hull roughness effects are studied with and without propulsion. Reynolds-averaged Navier–Stokes (RANS) and detached eddy simulations (DES) are utilized with the ghost-fluid method (GFM). The DES is carried out with a submodel where a stationary RANS solution is used as a boundary condition. The surface roughness effect on the stern is significant due to the increased boundary layer thickness. The differences in the stern wave shape between RANS and DES become more pronounced with propulsion as DES resolves the turbulence and wave breaking. With the smooth hull, DES indicates transom wetting which RANS does not. For the heavy fouling condition RANS and DES predict a wetted transom. For the smooth hull, the DES results indicate 6.8% higher pressure (2.8% in total) resistance at the transom compared to the corresponding RANS. The resistance of the wetted transom correlates with the velocity change at the transom location. Heavy fouling does not cause pressure resistance although RANS and DES predict a wetted transom. We propose that the increased boundary layer should be taken into account in the after body design.

Effect of hull displacement on hydro- & aerodynamics of a planing trimaran

The tunnel hydrodynamic and aerodynamic forces are the key to improve the overall resistance of planing trimaran. The variation of hull displacement has different effect on the tunnel flow characteristics at diverse sailing states, resulting in various change trend of resistance per displacement (R / △) of trimaran at various hull displacement. Combined with model tests, this paper conducts numerical investigation for the detailed analysis of hull displacement effect on the hydrodynamics and aerodynamics of a planing trimaran in various speed ranges. It is found that tunnel air cushion occurs when this trimaran running at low speed. The critical Froude Number of air cushion appearance is strongly influenced by the wave interaction in tunnel, where it may occur earlier at heavier hull load. At semi-planing speed, the difference of hull trim angle mainly accounts for the obvious discrepancy of R / △ of trimaran with different hull loads. Apart from the dominant effect of main hull, the high-pressure region at front tunnel, caused by bow wave washing, also plays a boosting role on enlarging the difference of bow-up trim angle between various load conditions. The heavier hull load leads to the stronger interaction between bow waves of main hull and tunnel at semi-planing speed. After entering planing speed, the tunnel wave crest moves astern more quickly at light-load where tunnel ventilation thus occurs earlier. The air flows through tunnel more smoothly at light load, which weaken the tunnel ram pressure effect in turn. At planing state, the variations of tunnel aero-lift and hydro-lift dominantly influence the different growth trend of trimaran R / △ at various hull loads. This work is helpful for the design and performance prediction of planing trimaran which designed for multi working conditions.

Aerodinamička analiza metka AK-47 koji se kreće brzinom od 2.0 maha u neposrednoj blizini zemlje

The bullet is shot near the wall or the ground during urban warfare. This nearness leads to a pressure distribution on the bullet's entire body, which is asymmetric. For every case of difference in the height of the ground/wall proximity, the shock reflection angle changes, and a particular shock wave's effect on the bullet varies. In the present study, the flowfield around a 7.82-mm bullet of an AK-47 Rifle moving at a supersonic speed of Mach 2 near the ground is studied using Computational Fluid Dynamics. Computational simulations have been carried out for the bullet moving at different heights from the nearby wall. The level of impact on the projectile is illustrated from the region of nearest influence to a ground distance which is five times the bullet's diameter. Ground effects from height to diameter ratios (h/D) of 0.25 to 5 are showcased, describing the influence on overall flowfield, pressure coefficient distributions, lift drag, and moment coefficients. The wake region flow field is also analyzed. It has been observed that a detached bow shock wave is generated at the tip of the bullet, which increases the drag coefficient experienced by the bullet. Generation of lift is the maximum for certain h/D cases like 1.0, 1.5, and 2.0 as the shock reflections from the ground hit the bullet. Further increasing the altitude from the near-wall reduces the lift and drag forces acting on the bullet due to the lesser ground effect. From the present investigation, the heights above the h/D ratio of 2.0 are suitable for firing the shot.

Numerical Investigation of Aeroelastic Characteristics of Grid Fin

Grid fin is widely used in the field of aerospace because of its unique advantages in the aerodynamic and structural performance. Meanwhile, the aerodynamic and structural characteristics of the grid fin are quite different from those of the conventional rudder. Deep understanding of the aeroelastic characteristics and mechanism of the grid fin is of great reference value for the fine design of grid fin. In this paper, an efficient reduction-order-method-based aeroelastic analysis method is established, and the method is verified by the flutter case of AGARD 445.6 wing. Finally, based on this method, the flowfield characteristics and the variation of flutter characteristics with Mach number are studied in detail. The results show that in the subsonic region, the transonic flutter “dip” of the grid fin occurs earlier than that of the conventional wing due to the acceleration effect of the grid fin on the airflow. In the transonic region, when the flowfield is characterized by a bow shock wave, the critical flutter speed increases slightly with the increase of Mach number. However, in the supersonic region, when the flowfield is characterized by an oblique shock wave, the flutter critical velocity increases substantially with the increase of Mach number. It is found that the variation law of the flutter boundary with Mach number is very similar to that of the normal force and the pitching moment with Mach number, and there is a close relationship between them. The ratio of flutter speed of the grid fin to the freestream velocity will reach the minimum value near . At the same time, it is discovered that the flutter instability of the grid fin is mainly caused by the coupling of the bending mode and torsional mode of the grid fin, and the instability mode is the torsional mode. Meanwhile, it should be noted that the flutter coupling characteristics of the grid fin differ greatly from those of the conventional rudder surface due to the great difference in structural characteristics and unsteady aerodynamic characteristics.

VESSEL WAVE WAKE CHARACTERISATION USING WAVELET ANALYSIS


 
 
 This work focuses on characterising vessel wave wake (wash) using wavelet analysis when a vessel is operating in the sub-critical and critical zone. Such characterisation complements other wash characteristics: Froude depth number, bow wave angle, solitons and decay coefficient. The examination of experimental results indicates that differences in characteristics with respect to water depth, Froude depth number, vessel displacement, hull form and soliton generation can be identified through wavelet analysis. The results demonstrate “proof of concept” that wavelet analysis is a powerful tool for characterising vessel wash and captures the effects of key operational and vessel changes.
 
 

Getting the ducks in a row

Vessels – in the widest sense – travelling on a water surface continuously do work the water surrounding it, causing energy to be radiated in the form of surface waves. The concomitant resistance force, the wave resistance, can account for as much as half the total drag on the vessel, so reducing it to a minimum has been a major part of ship design research for many decades. Whether the ‘vessel’ is an ocean-going ship or a swimming duckling, the physics governing the V-shaped pattern of radiated waves behind it is in essence the same, and just as fuel economy is important for commercial vessels, it is reasonable to assume that also swimming waterfowl seek to minimise their energy expenditure. Using theory and methods from classic marine hydrodynamics, Yuan et al. (J. Fluid Mech., vol. 928, 2021, R2) consider whether, by organising themselves optimally, ducklings in a row behind a mother duck can reduce, eliminate or even reverse their individual wave resistance. They describe two mechanisms which they term ‘wave riding’ and ‘wave passing.’ The former is intuitive: the ducklings closest to the mother can receive a forward push by riding its mother's stern waves. The latter is perhaps a more striking phenomenon: when the interduckling distance is precisely right, every duckling in the row can, in principle, swim without wave resistance due to destructive wave interference. The phenomenon appears to be the same as motivates the recent US military research project Sea Train, a row of unmanned vehicles travelling in row formation.

Numerical investigation of flow structure and air entrainment of breaking bow wave generated by a rectangular plate

Breaking bow waves entrain massive gas that generate ambient noise and produce spray and bubbly wake with whitecap. This study aims to give a quantitative description of the flow structures and bubble formation during the breaking process. We consider the breaking bow waves induced by a surface-piercing flat plate and perform simulations based on an in-house code. We employ a conservative coupled level-set and volume of fluid method to capture violent variation of the liquid–gas interface. A robust immersed boundary method is adopted to model the motion of the plate. To resolve very small flow structures associated with the wave breaking process with the available computational resources, a block-structured adaptive mesh refinement strategy is used. It is found that the predicted wave characteristics, such as wave height, wave crest location, and wave profile, are consistent with the experiment. A wide range of flow phenomena, including the thin liquid sheet, jet overturning, and splash-ups are well reproduced by the present simulation. In addition, we implement a bubble-droplet detection program to track single bubbles, and the characteristics of bubble cloud (entrained air volume, spatial distribution, and penetration depth) can be analyzed quantitatively. Three typical bubble creation mechanisms for the air entrainment process of the breaking bow wave are reported, and ensemble-averaged statistics of the bubble size distribution are presented. We also quantify the evolution of the bubble distribution and discuss the power-law scaling during the bow wave breaking process.

Modeling of a Reduced Hybrid H2–Air Kinetic Scheme Integrating the Effect of Hypersonic Reactive Air with Supersonic Combustion

A hybrid H2–air kinetic scheme of 11 species and 15 reactions is developed, which is capable of simulating the high-temperature air reaction flows and H2–O2 combustion flows respectively or simultaneously. Based on the Gupta scheme, the mole fraction varying with a Mach number at specific conditions is analyzed, and the weakly-ionized 7-species 7-reaction scheme is selected. The effect of nitrogenous species on the H2–O2 combustion is analyzed by a zero-dimensional simulation of steady-state and unsteady-state combustion under specified conditions, and the selected dominant nitrogenous reaction N + OH = NO + H is distinguished by the production rate of the nitrogenous species. The thermodynamic properties are verified by comparison using the NIST–JANAF database. The reaction rate coefficients of the dominant reaction of the hybrid kinetic scheme distinguished by a sensitivity analysis are corrected. The proposed kinetic scheme is validated by a zero-dimensional calculation of the ignition delay time and two-dimensional computational fluid dynamics (CFD) simulation with finite-rate chemistry on the shock-induced sub-detonative and super-detonative combustion. The ignition delay time of the hybrid kinetic scheme is almost in the middle between the Shang scheme and Jachimowski scheme, and all the calculated ignition delay times are acceptably greater than the experiments due to the errors of the experiments and numerical models. The clearly captured bow shock wave and combustion front using the hybrid kinetic scheme and Shang scheme are almost the same, which is strongly consistent with the schlieren image. In addition, a good agreement of the flow characteristics and mass fraction of the species along the stagnation line is also obtained, which indicates the accuracy and reasonableness of the hybrid kinetic scheme to simulate hybrid H2–air reactive flows.

Delayed detached eddy simulation method for breaking bow waves of a surface combatant model with different trim angle

Sailing attitudes vary with different loading conditions for ships sailing in open sea, and the trim angle, either by bow or by stern, exerts considerable impact on the flow field around the ship. Currently, attention has primarily been focused on hull resistance and wave patterns while few studies have touched upon breaking bow wave under different trim conditions. This study aims to analyze the breaking bow wave of a surface combatant model with a length of 5.72 m DTMB (name of a surface combatant model) under different sailing attitudes at Fr = 0.35. The Delayed Detached Eddy Simulation (DDES) approach was adopted to study the breaking bow wave features such as plunging jet and air entrainment. The interface-compression algebraic Volume Of Fluid (VOF) method in OpenFOAM was used to capture the free surface. The Computational Fluid Dynamics (CFD) approach was firstly validated by comparing hull resistance and wave contour with the available experimental data. The study then delves into the influence of trim angles on breaking bow waves through simulations of the different conditions, i.e., 1 deg trim by bow, test condition and 1 deg trim by stern. The wave contour, breaking wave profile, vorticity field and the wake field at several transverse sections are then compared in three trim conditions, followed by an analysis of the initial evolution of bow wave breaking based on the turbulent kinetic energy and the vorticity field at four cross sections. The results suggest that trim by bow makes free surface sharper and wave amplitude larger in the breaking bow wave region. The reason is that trim by bow enlarges the attack angle of the bow, thus energizing the bow wave and generating a more violent free surface of bow wave breaking.

Impact of High Inertia Particles on the Shock Layer and Heat Transfer in a Heterogeneous Supersonic Flow around a Blunt Body

One of the most important and complex effects associated with the presence of particles in the flow is the gas-dynamic interaction of particles with the shock layer. Of particular interest is the intensification of heat transfer by high inertia particles rebounding from the surface or by the products of erosion destruction, which reach the front of the bow shock wave and violate the gas-dynamic structure of the flow. In this case, according to experimental data, the increase in heat fluxes is much greater than it could be predicted based on the combined action of the kinetic energy of particles and a high-speed flow. The problem is related to the destruction of the flow structure. In this paper, the problem is studied with numerical simulation. We show that the key role in the intensification of heat transfer is played by the formation of an impact jet flowing onto the surface. An area of increased pressure and heat flux is formed in the zone of action of the impact jet. This effect is maintained over time by the successive action of particles.

Multi-Component MHD Model of Hot Jupiter Envelopes

A numerical model description of a hot Jupiter extended envelope based on the approximation of multi-component magnetic hydrodynamics is presented. The main attention is focused on the problem of implementing the completed MHD stellar wind model. As a result, the numerical model becomes applicable for calculating the structure of the extended envelope of hot Jupiters not only in the super-Alfvén and sub-Alfvén regimes of the stellar wind flow around and in the trans-Alfvén regime. The multi-component MHD approximation allows the consideration of changes in the chemical composition of hydrogen–helium envelopes of hot Jupiters. The results of calculations show that, in the case of a super-Alfvén flow regime, all the previously discovered types of extended gas-dynamic envelopes are realized in the new numerical model. With an increase in magnitude of the wind magnetic field, the extended envelope tends to become more closed. Under the influence of a strong magnetic field of the stellar wind, the envelope matter does not move along the ballistic trajectory but along the magnetic field lines of the wind toward the host star. This corresponds to an additional (sub-Alfvénic) envelope type of hot Jupiters, which has specific observational features. In the transient (trans-Alfvén) mode, a bow shock wave has a fragmentary nature. In the fully sub-Alfvén regime, the bow shock wave is not formed, and the flow structure is shock-less.

Dynamics of Shock Structure and Frontal Drag Force in a Supersonic Flow Past a Blunt Cone under the Action of Plasma Formation

The paper is devoted to the experimental and CFD investigation of a plasma formation impact on the supersonic flow over a body “blunt cone-cylinder”. In the experiments, a series of schlieren pictures of bow shock wave–blast waves non-stationary interaction was obtained with the use of high speed shadowgraphy. The accompanying calculations are based on the system of Euler equations. The freestream Mach number is 3.1. The plasmoid is modeled by the instantaneous release of energy into a bounded volume of gas, increasing the pressure in the volume. The research of the dynamics of a shock wave structure caused by the bow shock wave and blast flow interaction has been conducted. The significant value of energy released to a supersonic flow (500J) allowed constructing a diagram of the generation and dynamics of the resulting shock waves and contact discontinuities, as well as obtaining a significant drop in the drag force and stagnation pressure (up to 80%). The dynamics of a low density and high gas temperature zone, which becomes the main factor reducing the frontal body drag force, was researched. The dynamics of the front surface drag forces have been studied for different values of the plasmoid energy as well. Qualitative agreement of the numerical flow patterns with the experiment ones has been obtained.

Unsteady RANS CFD simulations of ship manoeuvrability and course keeping control under various wave height conditions

Ships encounter different wave heights in real sea states. Various wave heights can lead to substantial changes in the behaviour of a ship during manoeuvring, and hence a ship's manoeuvrability in waves should be accurately evaluated to ensure navigational safety at sea. The aim of this study is to estimate wave height effects on the manoeuvring behaviour of the benchmarking KRISO Container Ship (KCS) model by performing a fully nonlinear unsteady RANS simulation. Three types of manoeuvres were simulated with the free-running KCS model appended with an actuator disk and a moving semi-balanced horn rudder using a dynamic overset technique: self-propulsion, course keeping control, and turning circle manoeuvres. Manoeuvring analyses were carried out in the bow quartering waves of a range of wave heights for constant wave length, as coupled with the fifth-order Stokes wave model. In order to control the rudder deflection based on manoeuvre mechanisms, a feedback controller was embedded in the present CFD model. The results clearly revealed that wave heights have a strong effect on the manoeuvring behaviour of the KCS, including ship's speeds, seakeeping behaviour, and critical turning indices through comparative analyses under different wave height conditions. One important finding is the standard manoeuvring parameters in waves, which would help navigation officers in decision-making for manoeuvring actions in waves.

Vertical acceleration control using LQG approach for a passenger ship

In this paper, vertical motion (heave, roll, and pitch) state estimation and control simulation are performed by using the LQG approach. Cummins’ equation is used to represent the mathematical model of 3DOF ship motions. A passenger ship in irregular bow quartering waves at the Froude number of 0.45 is considered during the simulations. Two pairs of foils which are located on the bow side of the passenger ship are used as actuators. First, the results of the uncontrolled simulation case are presented to show the reliability of the mathematical model. Then, all states are estimated by measuring only velocity signals based on LQE design. Finally, the LQG approach is applied to mitigate the vertical motions and accelerations by taking feedbacks from only easy-measurable velocity signals. LQG results are compared with LQR results as well. Results showed that the implemented LQG approach reduced the bow motions and accelerations up to approximately 40%.

A theoretical method for solving shock relations coupled with chemical equilibrium and its applications

In this study, a theoretical method is proposed to solve shock relations coupled with chemical equilibrium. Not only shock waves in dissociated flows but also detonation waves in combustive mixtures can be solved. The global iterative solving process is specially designed to mimic the physical and chemical process in reactive shock waves to ensure good stability and fast convergence in the proposed method. Within each global step, the single-variable equations of normal and oblique shock relations are derived and solved with the Newton iteration method to reduce the complexity of the problems, and the minimization of free energy method of NASA (National Aeronautics and Space Administration) is adopted to solve equilibrium compositions. It is demonstrated that the convergent process is stable and very close to the real chemical-kinetic process, and high accuracy is achieved in the solutions of normal and oblique reactive shock waves. Moreover, the proposed theoretical method has also been applied to many problems associated with reactive shocks, including the stability of oblique detonation wave, bow detonation over a sphere, and shock reflection in dissociated air. The great importance of using chemical equilibrium to theoretically predict the theoretical range of the wedge angle for a standing oblique detonation wave (the standing window of the oblique detonation wave), the stand-off distance of bow detonation wave and the transition criterion of shock reflection in dissociated air with high accuracy have been addressed.