Experimental and numerical investigations on hydrodynamic and aerodynamic characteristics of the tunnel of planing trimaran

The planing trimaran possesses distinctive hybrid hydrodynamic and aerodynamic performance due to the presence of tunnel. The research described in this paper was carried out based on the observation of wave characteristics of a planing trimaran model in towing tests, in which the resistance drops as soon as the wave surface separates from tunnel roof. In order to gain a deeper insight into the relationship between wave flow and forces in tunnel region, a comprehensive series of viscous CFD simulations considering free-surface and 2-DOF motion of the hull (heave and pitch) have been performed for the tested model at the volume based Froude numbers ranging from 3.16 to 5.87. The calculated results were validated by comparison with experimental data and showed good agreement. Numerical results of wave contours, longitudinal wave cuts and lifting force distributions at the calculated speeds were presented for the analysis of ventilation process in tunnel region and the corresponding variation of tunnel forces. It is found that, for the speeds higher than Froude number of 4.52, the aerodynamic forces provide major tunnel lift and mainly act on the straight section of the tunnel. And, therefore, numerical simulations of two modified models have also been performed for the analysis of influence of straight section length on the hydrodynamic and aerodynamic performance of planing trimaran.     

A combined method for the hydrodynamic characteristics of planing crafts

The prediction of the total resistance of planing crafts at high speeds is very important. In this paper, a combined method is investigated for determining the hydrodynamic characteristics of planing crafts in the calm water. The study consists of a potential-based boundary element method (BEM) for the induced pressure resistance, the boundary layer theory for the frictional resistance and practical method for the spray resistance. The planing surface is represented by a number of elements with constant velocity potential at each element. The unknown-induced pressure is obtained by using the free surface elevation condition and the Kutta condition at the transom stern. Hydrodynamic-induced resistance and lift are determined by the calculated dynamic pressure distributions. The boundary layer analysis method is based on calculations of the momentum integral equation applied to obtain the frictional resistance. A particular practical approach is introduced to present the region of the upwash geometry for the spray. A numerical program has been developed for the present research and applied to the hull form of the craft. Four different hull forms of Series 62 model 4666 planing craft are presented. It is shown that the present combined method is efficient and the results are in good agreement with the experimental measurements over a wide range of volumetric Froude numbers.     

Nonlinear wave resistance of a two-dimensional pressure patch moving on a free surface

A model problem of the flow under an air-cushion vessel is studied. Two different numerical techniques are used to determine the solution of the free-surface elevation and the wave resistance for a range of Froude number, Reynolds number, value of the pressure applied in the cushion, and depth of the water. The first numerical technique uses a velocity potential that satisfies linearized free-surface boundary conditions, whereas the second employs a finite-volume method to find a solution that satisfies the fully nonlinear free-surface boundary conditions. The results clearly show that for high Froude number and practical values of the cushion pressure, the linear-theory solution is in excellent agreement with the more exact nonlinear prediction. For lower Froude number the solution becomes unsteady, and the disagreement between the two methods is larger.     

On the shape of the wake behind a planing boat

Recent developments in slender planing theory permit rather precise estimates of forces and moments to be made. So using a methodology first suggested by Epshtein for flat plate wakes, the modern theory is employed to find the longitudinal shape of a flat plate's wake. The result is in better agreement with experiment than previous formulations. The same approach is then applied (for the first time) to a prismatic wake, and the agreement with experiment is found to be excellent. Finally, the equations developed are used to deduce the wave energy developed by the passage of the hull, and from this, its wave drag. Although there are no experimental data available for comparison, the ratio (wave drag)/(total pressure drag) follows the same trend with Froude number as earlier two-dimensional solutions by Sedov and Squire, and tends to zero as Froude number tends to infinity.     

Computation of wave-making resistance of a catamaran in deep water using a potential-based panel method

This paper presents a potential-based boundary element method for solving a nonlinear free-surface flow problem for a Wigley catamaran moving with a uniform speed in deep water. Since the interior flow of each monohull of the catamaran is different from the exterior flow, both monohulls must be considered as lifting bodies. The pressure Kutta condition is imposed at the trailing-edge of the lifting body by determining the dipole distribution, which generates required circulation on the lifting part. The effects of wave interference and hull separation on the hydrodynamic characteristics of the catamaran hull are analyzed and the validity of the computer scheme is examined by comparing the wave resistance with the numerical results of others. The present method could be a useful design tool for screening the suitable combinations of hull parameters and hull spacing at the preliminary design stage of catamaran hull.     

Hydrodynamic assessment of planing hulls using overset grids

In conjunction with high performance computers, recent developments in computational science paved the path to more accurate representation of body motions inside fluids. Small motions inside the flow can be computationally approximated by using rigid body motion but it is incapable of accurately predicting the large motions of a planing vessel. The implementation of overset grid has made it possible to better approximate the complex fluid-structure interaction problem of the planing regime. The focus of this study was to evaluate the opportunity of using an overset grid system to numerically solve the flow around a planing hull and to understand the planing regime with this invaluable tool. It was shown in this study that the overset grid better captures the large motions of the planing hull at high Froude numbers. Then, the results obtained by overset grid were used to calculate the resistance components of a planing hull in a wide Froude number range. The resistance components were discussed with respect to values generated by Savitsky approach. Using the benefits that the computational science brings, the flow was visualized to explain some underlying physics relevant to the planing regime.     

A numerical modeling of hydrodynamic characteristics of various planing hull forms

A numerical algorithm based on the boundary element method (BEM) is presented for predicting the hydrodynamic characteristics of the various planing hull forms. The boundary integral equation is derived using Green's theorem on the wetted body surface and the free surface. The ventilation function at the transom is estimated with Doctor's empirical formula. This function is defined as the transom zone free surface boundary condition. The combined boundary integral equation and modified free surface boundary condition are simultaneously solved to determine the dipole on the wetted hull surface and the source on the free surface. The method is applied to investigate three examples of planing hulls, which include flat-plates, as well as wedge-shaped and variable deadrise planing hulls. Their hydrodynamic characteristics are calculated for different speeds. Computational results are presented and compared with existing theories and experiments. On the whole, the agreement between the present method and the selected experimental and numerical data is satisfactory.     

The effect of appendages on the hydrodynamic characteristics of an underwater vehicle near the free surface

An underwater vehicle typically has various appendages such as sail, rudders and hydroplanes. These appendages affect the hull hydrodynamic characteristics, including the resistance components and the form of the generated wave due to the motion of the vehicle near the free surface. The effect of the appendages on the hydrodynamic characteristics of an underwater vehicle near the free surface is studied. Initially the DARPPA SUBOFF submarine without the appendages is selected and hydrodynamic characteristics, including the friction resistance, viscous pressure resistance, wave resistance and shape of the created wave on the free surface are calculated for Froude numbers in the range of 0.128–0.84 and non-dimensional submergence depths 1.3, 2.2, 3.3 & 4.4. Then, by adding the appendages and comparing these two conditions, the effect of appendages is obtained. The results of computations indicate that the appendages cause a mean increase of about 16% in the total resistance. This increment is due to viscosity of fluid and also the interaction of the main hull with the appendages. There are no significant changes in the wave pattern and wave making resistance due to the presence of appendages.     

A seakeeping analysis method for an air-lifted vessel

A seakeeping analysis in the frequency domain is presented to predict the motion response of an air-lifted vessel (ALV) in waves. The ALV is supported by pressurised air in two separate cushion chambers; the pressure variation in the cushions has a significant effect on the motions of the vessel. The adiabatic gas law is used to couple cushion pressure and the free-surface elevation of water inside the chamber. Attention is focused on the waves generated by the pressure, and a method is presented to compute the corresponding free-surface elevation. New numerical schemes are proposed for calculating the three-dimensional free-surface elevation for the four wave numbers. Numerical results of the free-surface elevation, escape area, escape volume and motion responses of the ALV are provided.     

波浪对透空式结构物冲击作用的光滑粒子流体动力学数值模拟

基于光滑粒子流体动力学(SPH)方法的二维数值波浪水槽模拟了规则波对透空式结构物的冲击作用。通过黎曼解和CSPM相结合的方法对连续方程和动量方程进行了修正。对造波边界采用虚粒子法模拟。提出了一种耦合计算方法来消除造波边界附近的压力波动现象,在结构物边界处设置了适合的冲击边界条件。应用修正的SPH模型模拟了规则波对浪溅区结构物的冲击作用,给出了结构物附近流场和压力场的变化特征,并应用物理模型试验结果对数值模型进行了验证。     

Enhanced predictions of wave impact pressure by improved incompressible SPH methods

A new criterion is proposed for a more efficient assessment of free-surface particles in a particle-based simulation. Enhanced wave impact simulations are carried out by improved Incompressible SPH (ISPH) methods. The first improvement is the same as that in the Corrected ISPH (CISPH; [Khayyer A, Gotoh, H, Shao SD. Corrected incompressible SPH method for accurate water-surface tracking in breaking waves, Coast Eng 2008; 55 (3): 236–250]) method and is proposed for the improvement of momentum conservation. The second improvement is achieved by deriving and employing a higher order source term based on a more accurate differentiation to obtain a less fluctuating and more accurate pressure field. The enhanced performance of improved ISPH methods is demonstrated through the simulation of several fluid impact simulations in comparison with the experimental data and simulation results by other numerical methods.     

Numerical Prediction of Induced Pressure and Lift of the Planing Surfaces

This paper discusses the numerical prediction of the induced pressure and lift of the planing surfaces in a steady motion based on the potential flow solver as well as the spray drag by use of the practical method.The numerical method for computation of the induced pressure and lift is potential-based boundary element method.Special technique is identified to present upwash geometry and to determine the spray drag.Numerical results of a planing flat plate and planing craft model 4666 are presented.It is shown that the method is robust and efficient and the results agree well with the experimental measurements with various Froude humors.     

Numerical calculation of free-surface potential flow around a ship using the modified Rankine source panel method

A modified Rankine source panel method is presented for solving a linearized free-surface flow problem with respect to the double-body potential. The method of solution is based on the distribution of Rankine sources on the hull as well as its image and on the free surface. An iterative algorithm is used for determining the free surface and wave resistance using upstream finite difference operator. A verification of numerical modeling is made using the Wigley hull and the validity of the computer program is examined by comparing the details of wave profiles and wave making resistance with Series 60 model.     

Hydrodynamic analysis techniques for high-speed planing hulls

A planing hull is a marine vessel whose weight is mostly supported by hydrodynamic pressures at high-speed forward motion. Its high-speed character has made it popular and thus the interest for planing hulls for military, recreational and racing applications is progressively rising. The design and analysis procedure for high-speed planing hulls, due to their performance and speed requirements, is very important. Access to a fast, accurate technique for predicting the motion of these hulls plays a significant role in improvement in this field. Over the past several decades, numerous investigations have been done on hydrodynamic analysis of high-speed planing hulls. In this study, the existing techniques for analysis of these hulls are reviewed. Understanding the strengths and limitations of these techniques will help researchers and engineers select the most appropriate method for optimal design and analysis of a hull. To present a comprehensive study on the existing techniques, they are classified into two major categories: analytical–experimental and numerical techniques. The numerical techniques are further divided into methods for boundary value problems and domain-dependent problems. Each technique is applicable only for a limited range of cases.     

万米级深海陶瓷耐压结构水下内爆流场数值模拟

耐压结构是深海潜器的重要组成部分,但在深海的高压环境中却存在内爆的风险。为研究陶瓷耐压结构水下内爆的流场特性,使用针对可压缩多相流问题开发的开源代码,采用直接数值模拟,应用自适应直角网格,对两种压力条件下的耐压结构水下内爆进行了数值模拟。通过低压模拟结果与理论解和试验值比较,验证了模拟方法的有效性,进而开展万米级深海陶瓷耐压结构水下内爆模拟。分析发现：陶瓷耐压结构发生内爆后,其内部气腔存在多次压缩—反弹现象,深海环境压力越大则反弹越不明显;气腔反弹阶段,在结构外部将产生数倍于深海环境压力的冲击波,且传播速度接近声速;冲击波压力峰值与到球心距离呈负指数幂函数关系;在相同深海环境压力下,耐压结构外部监测点的冲击波压力与球体半径呈正比例关系。     

Design pressure distributions on the hull of the FLOW wave energy converter

This paper presents a procedure to calculate the design pressure distributions on the hull of a wave energy converter (WEC). Design pressures are the maximum pressure values that the device is expected to experience during its operational life time. The procedure is applied to the prototype under development by Martifer Energy (FLOW—Future Life in Ocean Waves).A boundary integral method is used to solve the hydrodynamic problem. The hydrodynamic pressures are combined with the hydrostatic ones and the internal pressures of the large ballast tanks. The first step consists of validating the numerical results of motions by comparison with measured experimental data obtained with a scaled model of the WEC. The numerical model is tuned by adjusting the damping of the device rotational motions and the equivalent damping and stiffness of the power take-off system. The pressure distributions are calculated for all irregular sea states representative of the Portuguese Pilot Zone where the prototype will be installed and a long term distribution method is used to calculate the expected maximum pressures on the hull corresponding to the 100-year return period.     

Wave interaction with a concentric porous cylinder system

This is a theoretical investigation of wave interaction with a concentric surface-piercing two-cylinder system. The exterior cylinder is porous and considered to be thin in thickness and the interior cylinder is impermeable. Both cylinders are rigidly fixed at the sea bed. The fluid motion is idealized as a linearized potential flow. The free-surface elevation and the total net hydrodynamic forces acting on both cylinders are determined analytically. The wave-induced overturning moments are also evaluated. It is found that, with the existence of the exterior porous cylinder, the hydrodynamic force acting on the interior cylinder is reduced if compared to the force exerted on the interior cylinder by a direct wave impact. The reduction of the wave amplitude around the leeward side of the outer porous cylinder is shown from the free-surface computations. In this paper, results are also presented to illustrate the effects of wave parameter and structural porosity on this wave and cylinder interaction problem. The role played by the ratio of radii of the inner and outer cylinders is duscussed.     

压力公式的修正及其在波浪荷载计算中的应用

计入前进速度流动与非定常流动之间的干扰,修正了切片法中计算压力的公式,从而建立了计算波浪荷载的方法,开发了相应的计算机程序。计算实例同国内外有关资料的比较表明结果吻合良好,为船舶与海洋结构物结构有限元分析程序提供了有效的前处理模块     

Hull component interaction and scaling for TLP hydrodynamic coefficients

The purpose of this paper is to validate a new method that can be used by offshore platform designers to estimate the added mass and hydrodynamic damping coefficients of potential Tension Leg Platform hull configurations. These coefficients are critical to the determination of the platform response particularly to high frequency motions in heave caused by sum-frequency wave forcing i.e. “springing”. Previous research has developed the means by which offshore platform designers can extrapolate anticipated full-scale hydrodynamic coefficients based on the response of individual model scale component shapes. The work presented here further evaluates the component scaling laws for a single vertical cylinder and quantifies the effects due to hydrodynamic interaction. Hydrodynamic interaction effects are established through a direct comparison between the superposition of individual hull component coefficients and those evaluated directly from complete hull configuration models. The basis of this comparison is established by the experimental evaluation of the hydrodynamic coefficients for individual hull components as well as partial and complete platform models. The results indicate that hydrodynamic interaction effects between components are small in heave, and validate component scaling and superposition as an effective means for added mass and damping coefficient estimation of prototype platforms. It is found that the dependency of damping ratio with KC for a TLP is almost identical to that of a single column, thus offering a scaling methodology for prototype damping ratio values.     

Practical evaluation of sinkage and trim effects on the drag of a common generic freely floating monohull ship

A practical method to account for the influence of sinkage and trim on the drag of a freely floating (free to sink and trim) common monohull ship at a Froude number F ≤ 0.45 is considered. The sinkage and the trim are estimated via two alternative simple methods, considered previously. The drag is also estimated in a simple way, based on the classical Froude decomposition into viscous and wave components. Specifically, well-known semiempirical expressions for the friction drag, the viscous pressure drag and the drag due to hull roughness are used, and the wave drag is evaluated via a practical linear potential flow method. This simple approach can be used for ship models as well as full-scale ships with smooth or rough hull surfaces, and is well suited for early ship design and optimization. The method considered here to determine the sinkage and the trim, and their influence on the drag, yields theoretical predictions of the drag of the Wigley, S60 and DTMB5415 hulls that are much closer to experimental measurements than the corresponding predictions for the hull surfaces of the ships in equilibrium position at rest. These numerical results suggest that sinkage and trim effects, significant at Froude numbers 0.25 < F, on the drag of a typical freely floating monohull ship can be realistically accounted for in a practical manner that only requires simple potential flow computations without iterative computations for a sequence of hull positions.