Michel Guilbaud

Simple analytical relations for ship bow waves

Simple analytical relations for the bow wave generated by a ship in steady motion are given. Specifically, simple expressions that define the height of a ship bow wave, the distance between the ship stem and the crest of the bow wave, the rise of water at the stem, and the bow wave profile, explicitly and without calculations, in terms of the ship speed, draught, and waterline entrance angle, are given. Another result is a simple criterion that predicts, also directly and without calculations, when a ship in steady motion cannot generate a steady bow wave. This unsteady-flow criterion predicts that a ship with a sufficiently fine waterline, specifically with waterline entrance angle 2α E smaller than approximately 25°, may generate a steady bow wave at any speed. However, a ship with a fuller waterline (25° E ) can only generate a steady bow wave if the ship speed is higher than a critical speed, defined in terms of α E by a simple relation. No alternative criterion for predicting when a ship in steady motion does not generate a steady bow wave appears to exist. A simple expression for the height of an unsteady ship bow wave is also given. In spite of their remarkable simplicity, the relations for ship bow waves obtained in the study (using only rudimentary physical and mathematical considerations) are consistent with experimental measurements for a number of hull forms having non-bulbous wedge-shaped bows with small flare angle, and with the authors measurements and observations for a rectangular flat plate towed at a yaw angle.

Boundary between unsteady and overturning ship bow wave regimes

Measurements of the bow waves generated by a rectangular flat plate, immersed at a draught D = 0.2 m, towed at constant speed U = 1.75 m s −1 in calm water and held at a heel angle 10° and a series of nine yaw angles α = 10°, 15°, 20°, 25°, 30°, 45°, 60°, 75° and 90° are reported. The measurements show that bow wave unsteadiness is significantly larger for the flat plate towed at yaw angles 30° ≤ α ≤ 90° than at 10° ≤ α ≤ 20°, which are associated with the unsteady and overturning bow wave regimes, respectively, separated by the boundary with g ≡ acceleration of gravity. These measurements of bow wave unsteadiness provide preliminary experimental validation of the foregoing simple theoretical relation for the boundary between the unsteady and overturning bow wave regimes for non-bulbous wedge-shaped ship bows with insignificant rake and flare. Extension of this relation to more complicated ship bows, notably bows with rake and flare, is also considered.

Sailing Boat Performance Prediction

Abstract The inviscid, steady, hydro-aerodynamic flow around a sailing boat fully equipped with sails and crew was investigated using two different solvers. The hydrodynamic problem was solved based on Green functions for the linearised wave resistance problem, using a simple plane wake model. The aerodynamic problem is fully non-linear with the sails wakes discretised by means of vortex carrying particles. The boat speed was obtained as a result of the balance between hydro- and aerodynamic forces. Only heel and drift angles were free. The balance problem was solved for two cases: crew position or heel angle prescribed.

Calcul de la propagation acoustique en milieux non homogènes infinis par la DRBEM

Resume On expose ici une evolution de la methode des elements de frontiere, dite DRBEM ( Dual Reciprocity Boundary Element Method ), a la resolution de lequation de Helmholtz gouvernant la propagation dondes acoustiques lineaires dans un milieu au repos non borne, a temperature moyenne variable. Loriginalite de cette methode est quelle permet de limiter la discretisation et lintegration aux seules frontieres du domaine, sans maillage du volume interne. La methode est appliquee a letude de la propagation des ondes dans une cavite non isotherme bafflee, et a travers un panache thermique sans vitesse se developpant dans un milieu infini a temperature exterieure constante.

Méthode couplée CVFE/BE pour les calculs acoustiques en domaine non borné

Resume On presente une methode de resolution nouvelle basee sur la conservation des flux pour des problemes acoustiques en milieu non borne. Le domaine detude initialement infini est tronque en introduisant une frontiere fictive definissant deux domaines complementaires. Une methode delements finis basee sur une formulation en volume de controle est utilisee pour le calcul dans le domaine interieur, alors que le champ exterieur est decrit par une methode delements de frontiere. Le couplage est effectue sur la frontiere virtuelle a laide dune expression appropriee des flux respectant la condition de Sommerfeld. Une configuration bidimensionnelle utilisant une interface circulaire montre la validite de lapproche au travers dune confrontation a la solution analytique ainsi qua des resultats de la litterature.

Extension de la DRBEM à la propagation guidée en écoulement cisaillé

Resume Nous etendons ici le principe de la Dual Reciprocity Boundary Element Method a la resolution dans le domaine frequentiel des equations de propagation acoustique au sein dun ecoulement cisaille. Loriginalite de la demarche est decrire les equations dEuler linearisees sous la forme dune equation de propagation du second ordre pour la pression couplee a une equation du premier ordre pour la vitesse acoustique transversale. La comparaison des resultats numeriques avec des solutions analytiques approchees valables pour des ecoulements a faible nombre de Mach (M

Calcul des écoulements hydrodynamiques à surface libre par une méthode de singularités de Kelvin

Resume Lutilisation des fonctions de Green satisfaisant la condition de surface libre linearisee de Kelvin en hydrodynamique navale sest longtemps heurtee aux difficultes numeriques liees a cette condition, entrainant des temps de calcul tres eleves, et au calcul de lintegrale de ligne qui porte sur les derivees, des distributions de singularites sur la ligne de flottaison. On presente ici une methode de singularites avec calcul de la fonction de Green et de ses derivees premieres a partir dinterpolations dans des tables et une technique de calcul de lintegrale de ligne pour des navires avec ou sans effet portant. Des resultats pour un corps en derapage percant la surface libre, avec satisfaction de la condition de Kutta—Joukovsky par une procedure iterative et sur une carene de Wigley sont presentes.

Etude expérimentale et théorique de la vague d'étrave des navires

ABSTRACT An experimental study of the bow wave generated by a ship in steady motion performed in a towing tank on a flat plate at different locations (simulating the half entrance angle αE and the flare angle γ of a classical hull) is reported. The measurements obtained from digital photographs give the influence of the different parameters, particularly on the shape of the contact curve between the wave and the plate. The results have been compared with reasonable agreement with analytical expressions; it had been too shown that these expressions are also valid for real ship bows. Furthermore, as a ship in steady motion is shown to generate an unsteady bow wave if αE 12° and if the ship speed is smaller than a critical speed defined in terms of αE by a simple analytical expression, we verify by experimental observations that a ship in steady motion generate an unsteady bow wave.

On the Integration of the Diffraction-Radiation with Forward Speed Green Function

Abstract The article summarizes the difficulties involved in calculating the various integrals required to develop a first-order panel method to calculate the behavior of a ship in a seaway using the diffraction-radiation with forward speed Green function. This function automatically satisfies both a linearized form of the free surface boundary condition and the radiation condition. Use of the third Green identity leads, for lifting flows, to the calculation of boundary integrals of this Green function and its first and second derivatives on ship-hull panels, on waterline segments, and on semi-infinite strips extending from the trailing edges of the hull’s lifting parts to downstream infinity. These integrals are computed after having interchanged the boundary and Fourier integrals. The first integrals are calculated analytically, using the Stokes theorem. The last integrals are computed numerically using an Adaptive Simpson method of integration, which controls the numerical error. The level of difficulties decreases from the calculation of the function, to the integration on semi-infinite strips, on waterline segments or on panels respectively. The level of difficulty increases with the order of the derivatives. Difficulties also increase close to the free surface, particularly for the waterline integrals. A technique to calculate these various integrations is presented.

Lifting Effects for Wave Resistance or Sea-Keeping Calculations

We present the introduction of lifting effects in a code of calculation [1–3] based on a first order panel method using the diffraction-radiation with forward speed Green function satisfying a linearised free-surface condition and the radiation one. A mixed formulation has been used with a source distribution on the hull and a doublet one on the plane of symmetry and the wake of lifting parts of the body, leading to an integral equation derived from the 3 rd Green identity. The Green function and its derivatives are not computed but are directly integrated on elementary panels, segments or semi-infinite strips. Results are presented for semi-submerged ellipsoid, rectangular surface-piercing bodies, Wigley hull, Series 60 ship, sailing boat and military 5415 hull. Global forces, moments but also free surface elevations are compared with the results of other methods and with measurements, either in steady or in unsteady flows in the frequency domain.Copyright