Lawrence J. Doctors

NONLINEAR MOTION OF AN AIR-CUSHION VEHICLE OVER WAVES

The nonlinear behavior of an air-cushion vehicle (ACV) during coupled pitch and heave motion over a sinusoidal wave is studied. The vehicle is of the plenum-chamber type with a transverse stability skirt, which is a contributor to the nonlinearily. Further nonlinearity results from the contact of the flexible skirt with the surface. The airflow is considered incompressible, and water compliance is neglected. Numerical computations have been made for the craft response in head seas as a function of encounter frequency for a set of Froude numbers and skirt configurations. The motion is found to be highly damped, and strongly dependent on the depth of the stability skirt, though not on its longitudinal position. Weak peaks in the response curves are displayed, their strength and location being a function of the wave height. The effect of skirt contact forces is found to be negligible on the craft motion. The predicted pressure fluctuations can be large and, at sufficiently high frequencies, result in subatmospheric pressures in the cushion.

Transom-stern Flow for High-speed Craft

Abstract Two series of experiments have been conducted, one at the University of Michigan (U-M), and one by The University of New South Wales (UNSW), with a focus to characterize the flow in the transom region of a high-speed vessel. At U-M, we have tested a destroyer type model, with and without a stern flap, while measuring pressures in the aft region of the hull and on the flap. The model was tested in both the free-to-sinkand-trim condition and the fixed condition. At UNSW, a series of geosimilar models was tested while measuring the free-surface elevation behind the vessel. The non-dimensional free-surface elevation was found to be primarily a function of the calm-water-transom-draft Froude number. To this end, an empirical formula that estimates the unwetting of the transom has been developed. This formula can be employed in a resistance prediction computer program which will provide an accurate calculation of the hydrostatic force on the transom. As a consequence, the total resistance of the vessel can now be computed accurately, even in the low-Froude-number region.

Comparison of nonlinear wave-resistance theories for a two-dimensional pressure distribution

The wave resistance of a two-dimensional pressure distribution which moves steadily over water of finite depth is computed with the aid of four approximate methods: (i) consistent small-amplitude perturbation expansion up to third order; (ii) continuous mapping by Guillotons displacements; (iii) small-Froude-number Baba & Takekumas approximation; and (iv) Ursells theory of wave propagation as applied by Inui & Kajitani (1977). The results are compared, for three fixed Froude numbers, with the numerical computations of von Kerczek & Salvesen for a given smooth pressure patch. Nonlinear effects are quite large and it is found that (i) yields accurate results, that (ii) acts in the right direction, but quantitatively is not entirely satisfactory, that (iii) yields poor results and (iv) is quite accurate. The wave resistance is subsequently computed by (i)-(iv) for a broad range of Froude numbers. The perturbation theory is shown to break down at low Froude numbers for a blunter pressure profile. The Inui-Kajitani method is shown to be equivalent to a continuous mapping with a horizontal displacement roughly twice Guillotons. The free-surface nonlinear effect results in an apparent shift of the first-order resistance curve, i.e. in a systematic change of the effective Froude number.

A Numerical Study of the Resistance of Transom-stern Monohulls

Abstract Experimental data for the wave profiles behind a series of transom-stern ship models has been re-analyzed to provide convenient and accurate regression formulas for the ventilation of the stern and the length of the transom hollow. Predictions of the wave resistance and, hence, the total resistance can now be improved by using this approach, together with the traditional thin-ship formulation for resistance. However, the accuracy of these predictions is improved further if one also considers that there must be a minimal effective hollow length based on the flow behind the backward-facing step. These theories, together with the use of form factors, are applied to a series of fourteen high-speed monohull vessels. It is shown that these methods provide a high degree of predictive accuracy.

A case study: theoretical and experimental analysis of motion characteristics of a trimaran hull form

Abstract Vessel motion is an aspect of design that requires a high degree of consideration with regard to passenger comfort. Within the last two decades, extensive research work has resulted in development of numerical and analytical methods for the prediction of heave and pitch motions of catamaran hull forms. However, in the recent past, there appears to be a strong interest in the development of trimaran hull forms. Investigations have shown that little research has been conducted on such hull forms to reduce their motions in heave and pitch. In this article, we investigate the effects of the (longitudinal) stagger of the sidehulls on the motions in heave and pitch of a representative trimaran hull. To quantify the effects of longitudinal stagger of the sidehulls (outriggers) with respect to the centrehull, experimental investigations were undertaken at the Australian Maritime College Ship Hydrodynamic Centre. A round-bilge high-speed hull form model of the Australian Maritime Engineering CRC systematic series was constructed and subjected to extensive experimental analysis as well as computer simulations (HYDROS) for four different longitudinal stagger positions. The investigations demonstrated that this variation and the resulting variation in the radius of gyration could have a significant effect on the heave and pitch motions. The literature survey indicated that, to date, investigations on trimaran hull forms have been confined to determining the effects of transverse and longitudinal positions of the sidehulls only on the resistance characteristics. The investigations undertaken within the scope of this article provide a starting point to investigate the effect of the trimarans sidehull position on the motions of the vessel.

Realistic evaluation of hull performance for rowing shells, canoes, and kayaks in unsteady flow.

Abstract In this study, we investigated the effect of hull dynamics in shallow water on the hydrodynamic performance of rowing shells as well as canoes and kayaks. An approach was developed to generate data in a towing tank using a test rig capable of reproducing realistic speed profiles. The impact of unsteady shallow-water effects on wave-making resistance was examined via experimental measurements on a benchmark hull. The data generated were used to explore the validity of a computational approach developed to predict unsteady shallow-water wave resistance. Comparison of measured and predicted results showed that the computational approach correctly predicted complex unsteady wave-resistance phenomena at low oscillation frequency and speed, but that total resistance was substantially under-predicted at moderate oscillation frequency and speed. It was postulated that this discrepancy arose from unsteady viscous effects. This was investigated via hot-film measurements for a full-scale single scull in unsteady flow in both towing-tank and field-trial conditions. Results suggested a strong link between acceleration and turbulence and demonstrated that the measured real-world viscous-flow behaviour could be successfully reproduced in the tank. Thus a suitable tank-test approach could provide a reliable guide to hull performance characterization in unsteady flow.

The experimental wave resistance of an accelerating two-dimensional pressure distribution

A series of experiments designed to measure the wave resistance of an accelerating two-dimensional air-cushion vehicle is described. A model was towed at a constant acceleration from rest over water of various depths. In addition, the effects of different levels of acceleration and different cushion pressures were examined. The results are compared with linearized potential-flow theory and show particularly encouraging agreement. The predicted humps and hollows in the curve of wave resistance vs. time are verified, but with a small shift with respect to time. Also, the theoretical region of negative wave resistance in water of finite depth is demonstrated.

Scaling of Resistance From Surface-Effect Ship Model Tests

In the case of conventional (displacement) hulls, model testing is based on the assumption (with or without certain refinements) that the total resistance can be expressed as: RT = RF + RR where RT is measured in the towing tank, and the frictional resistance RF can be accurately estimated by the application of a friction line and the use of the calm-water wetted surface. It is assumed that the dimensionless residuary resistance RR is the same for the model and the prototype vessel. Our paper may be considered to be an extension of the classic paper by Wilson, Wells, and Heber (1978), to the more complex case of the surface-effect ship, as follows. Specifically, we opine that: RT = RF + RW + RH + RS + RM + RSPRAY Here, RW is the wave resistance of the vessel (caused by a combination of the actions of the cushion pressure and the two sidehulls), RH is the transom (hydrostatic) drag, RS is the seal drag, RM is the momentum drag, and RSPRAY is the spray drag. RT is the only one of these quantities that is measured during the model test. The other components require the use of a variety of estimates. In the paper, we present specific examples of our approach, as applied to a number of tests on SES models that we have studied in recent years.

Development of a large-scale surface effect ship bow seal testing platform

A large-scale surface effect ship (SES) bow seal testing platform has been constructed by the University of Michigan and is presently being commissioned at the U.S. Navy’s Large Cavitation Channel (LCC) in Memphis, TN. Utilizing a recently installed (2008) free-surface forming gate, the test platform is capable of investigating the physics of the two-dimensional planing seal and three-dimensional finger-type bow seal in calm water conditions and at scales relevant to SES designers and numerical modellers. The LCC environment permits extended run times at high Reynolds number and provides unfettered optical access to the seal geometry and flow field. This paper describes the development of the testing platform and presents some preliminary results. The test platform is nominally 7.9 m long, 1.52 m wide and 2.0 m tall and is of welded and bolted steel construction. The seals are nominally sized similarly to those currently used by the U.S. Navy’s Landing Craft Air Cushion (LCAC) class. An extensive measurement suite is integrated with the test platform. The goal is to provide numerical modelers a data set with sufficient spatial and temporal resolution to validate their models of the experiment, and where appropriate, to develop new analytic models. The results of this effort demonstrate a feasible system for investigating surface effect ship seal physics within a large free surface water channel.

The Waves Generated by a Trimaran

Abstract The question of the wave generation by river vessels is one that has been studied at length by many researchers, particularly since the advent of the RiverCat, a catamaran designed specifically for operation on the Parramatta River, which leads into Sydney Harbour. There was good correlation between the measured height of the generated waves and the theoretical predictions of the wave resistance, as was shown by Doctors, Renilson Parker, and Hornsby (1991). In the current work, we compare the wave system generated by a model trimaran with the theoretical predictions of a classic inviscid approach, called HYDROS, which has now been enhanced to include surface tension as well as viscosity, both of which are now understood to be important for small models at low speeds. As a consequence, experimental and theoretical results presented demonstrate the effectiveness of the stagger (longitudinal position) of the sidehulls of the trimaran, along with depth and Fn combinations, on its wave-generating characteristics.