Surface-piercing Hydrofoil Research Articles

Inverse method for hydrodynamic load reconstruction on a flexible surface-piercing hydrofoil in multi-phase flow

An inverse method for reconstruction of the in situ steady-state hydrodynamic load distribution using limited strain measurements is developed and validated on a rectangular, flexible surface-piercing hydrofoil in fully-wetted (FW) and fully-ventilated (FV) flows. The hydrofoil is used as a canonical proxy to more complex hydrodynamic lifting surfaces such as marine propulsors and turbines. The approach involves using a forward fluid–structure interaction (FSI) model to predict the hydroelastic response for given operating conditions. The inverse problem is solved as an optimization problem to determine unknown operating conditions. The forward FSI model consists of a nonlinear lifting line (LL) fluid solver with considerations for free surface, ventilation, and viscous effects, and a solid finite element method (FEM) solver using 1-D beam elements representing the spanwise bending and twisting deformations.The coupled FSI model was validated using data collected during towing tank experiments at the University of Michigan. Predictions of the lift and moment coefficients, as well as spanwise bending and twisting deformations agreed well with experimental results in FW and in FV flows.The inverse problem is formulated as an optimization problem to determine the unknown operating conditions that will minimize the difference between the measured and predicted deformations. To avoid non-uniqueness problems often encountered by inverse problems, a dynamic constraint using the measured wetted natural frequencies was added to help regularize the problem and speed up the solution process. A sequential quadratic programming algorithm was used as the optimizer for the inverse problem. The experimental studies showed that the inverse FSI model accurately determined the unknown operating conditions (angle of attack and immersed aspect ratio) for a given a known flow speed and a limited number of strain measurements in both FW and FV conditions. The converged results were used to reconstruct a 3-D hydrodynamic load distribution on the foil and to predict the cavity shape for FV operating conditions, which were found to agree well with experimental measurements and observations.

Numerical Simulations of a Surface Piercing A- Class Catamaran Hydrofoil and Comparison Against Model Tests

Hydrofoil supported sailing vessels gained more and more importance within the last years. Due to new processes of manufacturing, it is possible to build slender section foils with low drag coefficients and heave stable hydrofoil geometries are becoming possible to construct. These surface piercing foils often tend to ventilate and cause cavitation at high speeds. The aim of this work is to define a setup to calculate the hydrodynamic forces on such foils with RANS CFD and to investigate whether the onset of ventilation and cavitation can be predicted with sufficient accuracy. Therefore, a surface piercing hydrofoil of an A-Class catamaran is simulated by using the RANS software FineMarine with its volume of fluid method. The C-shaped hydrofoil is analyzed for one speed at Froude Number 7.9 and various angles of attack (AoA) by varying rake and leeway angle in ranges actually used while sailing. In addition, model tests were carried out in the K27 cavitation tunnel of TU-Berlin, for the given hydrofoil and in the same conditions as simulated with CFD to provide data for validation. Based on the CFD calculations this paper presents how the rake and leeway angles influence the foil's lift to drag ratio. The simulations have been verified by extensive analyses, including domain size verification for unrestricted water, mesh refinement and y+ verification. The influence of the dimensions of the K27 (cavitation tunnel of the Technical University of Berlin) on the flow around the hydrofoil and the wave system is also considered, as the test section of the K27 significantly influences the flow around the foil, the forces and the wave elevation. Finally the CFD results are compared against the experiments conducted in the K27.

Ventilated cavities on a surface-piercing hydrofoil at moderate Froude numbers: cavity formation, elimination and stability

The atmospheric ventilation of a surface-piercing hydrofoil is examined in a series of towing-tank experiments, performed on a vertically cantilevered hydrofoil with an immersed free tip. The results of the experiments expand upon previous studies by contributing towards a comprehensive understanding of the topology, formation and elimination of ventilated flows at low-to-moderate Froude and Reynolds numbers. Fully wetted, fully ventilated and partially ventilated flow regimes are identified, and their stability regions are presented in parametric space. The stability of partially and fully ventilated regimes is related to the angle of the re-entrant jet, leading to a set of criteria for identifying flow regimes in a laboratory environment. The stability region of fully wetted flow overlaps those of partially and fully ventilated flows, forming bi-stable regions where hysteresis occurs. Ventilation transition mechanisms are classified as formation and elimination mechanisms, which separate the three steady flow regimes from one another. Ventilation formation requires air ingress into separated flow at sub-atmospheric pressure from a continuously available air source. Ventilation washout is caused by upstream flow of the re-entrant jet. The boundary denoting washout of fully ventilated flow is expressed as a semi-theoretical scaling relation, which captures past and present experimental data well across a wide range of Froude and Reynolds numbers.

A numerical nonlinear analysis of two-dimensional ventilating entry of surface-piercing hydrofoils with effects of gravity

The presence of the free surface adds an element of difficulty to the development of numerical and theoretical methods for the performance prediction of surface-piercing hydrofoils. Existing methods of analysis for two-dimensional surface-piercing hydrofoils or blade sections of a surface-piercing propeller solve either a linear problem, assuming a thin section and ventilated surface along with linear free-surface boundary conditions, or a nonlinear problem in a self-similar setting. Both these approaches cannot be used when the effects of gravity are important, which is the case when a craft is operating at low speeds. A two-dimensional boundary-element-method-based numerical scheme is presented here that overcomes these drawbacks by solving the fully ventilated flow past a surface-piercing hydrofoil of finite dimensions and includes the whole gamut of nonlinear free-surface interactions. The unique aspect of the numerical scheme is that fully nonlinear boundary conditions are applied on the free surface which allows for the accurate modelling of the jet generated on the wetted boundary and the ventilated surface formed on the suction side as a result of the passage of the hydrofoil through the free surface. Moreover, the effects of gravity can be considered to take into account the influence of the Froude number. Ventilated-surface shapes predicted by the present scheme are compared with existing experimental results and are shown to be in good agreement.

Multivariable Model Identification of Ship Dynamics

This paper deals with the problem of determining dynamic stochastic models for two different types of naval crafts, viz. conventional ships and surface-piercing hydrofoils. For conventional ships, a single-input singleoutput model describing the heading response to the rudder is identified from experimental data collected from a hydrograPhic vessel equipped with a minicomputer system. For surface-piercing hydrofoils, a multi variable model descri bing the roll and pitch response to the four flap actions is identified from experimental data recorded during special sea trials carried out by a microprocessor- based system. In both cases a number of identification techniques are used and their corresponding results are discussed and compared. In parti cular, maximum-likelihood identification is used in two different forms: first, by considering inputs and outputs as separate processes; and, next, as a single joint process. Moreover, preliminary identification of model order and structure is carried out by using several statistical and informational crite ria recently proposed in the literature.

Comment on "Canadian Advances in Surface-Piercing Hydrofoils"

Comment on "Canadian Advances in Surface-Piercing Hydrofoils"

Canadian Advances in Surface-Piercing Hydrofoils

The surface-piercing hydrofoil system exemplified by HMCS BRAS D'OR is a unique development which appears to combine some characteristics of other contemporary designs but, in fact, is derived from an early concept of F. W. Baldwin and Alexander Graham Bell. Essential features of this concept are compared with other surface piercing systems in a review of today's state of the art. Lessons learned from trials at sea have been incorporated in design studies of possible follow-on naval hydrofoil ships. Highlights from this continuing exploration indicate good prospects based on important advances in key areas of design for high performance with good seakeeping and cost benefits.

Experimental Determination of Steady and Unsteady Loads on a Surface Piercing, Ventilated Hydrofoil

Measured steady and unsteady section lift and moment coefficients at two spanwise locations on a surface-piercing ventilated hydrofoil are presented. The foil, of wedge cross section, was supported vertically, and submerged one chord length from the tip. Excitation in rigid-body rolling and pitching modes to the cantilevered foil produced the unsteady loads. All tests were made at a nominal angle of attack of 12 deg.

Control of the Hydrofoil Ship

Modern hydrofoil ships have evolved in response to the need for a fast reliable means of sea transportation which is capable of operating in all sea conditions. It is the purpose of this paper to describe how the application of modern stabilization and navigation technology to hydrofoil ships has contributed to the achievement of these goals.First, consider the subject of stability. In particular, consider the source of roll stability in three types of ships: a displacement ship; a surface piercing hydrofoil and a submerged-foil hydrofoil. A roll disturbance of each of the ships is depicted in Fig. 1. In both the displacement ship and in the surface piercing hydrofoil, the righting moment is produced entirely by the change in attitude of the ship relative to the water surface. In the case of the displacement ship, the righting moment is due to the shift of the centre of buoyancy. For the surface piercing hydrofoil, the moment occurs due to the shift of the hydrodynamic pressure centre of the foils. However, for the submerged-foil hydrofoil, no righting moment is produced by the change of the relative position of the water surface. The righting moment for this ship must be produced by underwater control surfaces in response to the change of ship attitude relative to inertial space, as sensed by a vertical gyro. It is this transfer of stability reference from the water surface to an inertial reference that leads to the superior seakeeping capabilities of the submerged foil hydrofoil.

Unified Lifting-Line Theory of Fully Wetted Hydrofoils

Unified lifting-line theory is developed for the hydrofoil of fully-submerged and surface-piercing type. The indirect and direct problems are discussed in some detail in relation to the effect of the Froude number. The disturbing velocity potential is derived from the linearized boundary condition on the free water surface. Then from the boundary condition on the hydrofoil, a basic integral equation is obtained for the distribution of circulation over the span, from which the lift and resistance can be computed readily. Some numerical examples of practical interest are shown for the characteristics and optimum condition in a specified condition of operation for the fully-submerged and surface-piercing hydrofoils.

Unified Lifting-line Theory of Fully Wetted Hydrofoils

Unified lifting-line theory is developed for the hydrofoil of fully submerged and surface piercing type. The indirect and direct problems are discussed in some details in relation with the effect of the Froude number.The disturbing velocity potential is derived from the linearized boundary condition on the free water surface. Then, from the boundary condition on the foil, a basic integral equation is obtained for the distribution of circulation over the span, from which the lift and resistance can be readily computed, and also the optimum condition is clarified.Some numerical examples of practical interest are shown for the characteristics, and optimum circulation and minimum resistance in a specified condition of operation for the fully submerged and surface piercing hydrofoils.