The Acceleration of an Air Cushion Vehicle Under the Action of a Propulsor

1973 ◽  
Vol 17 (02) ◽  
pp. 121-128
Author(s):  
Lawrence J. Doctors ◽  
Som D. Sharma

This paper presents the solution for the motion of an air-cushion vehicle (ACV) starting from rest under the action of a propulsor of given thrust-speed characteristics. The wave resistance is based on linearized potential theory, while the aerodynamic drag components are assumed to be strictly quasi-steady. The problem is treated in two different ways: calculating the wave resistance in a truly unsteady manner, and on the simplified quasi-steady basis. The results show that the shape of the propeller characteristics has only a minor effect on the velocity pattern. However, the effect of overloading the ACV is shown to have crucial effects on its ability to surpass the critical depth hump. In this respect, the simpler quasi-steady calculations lead to unnecessarily pessimistic estimates of the acceleration margin. Under certain circumstances in relatively shallow water, the quasi-steady analysis would suggest that the ACV could not overcome the critical hump, while the more elaborate unsteady calculations show that it has indeed adequate power to reach its final cruising speed.

1978 ◽  
Vol 22 (03) ◽  
pp. 170-177
Author(s):  
H. J. Haussling ◽  
R. T. Van Eseltine

Wave patterns and wave resistance are computed for air-cushion vehicles with time-dependent cushion pressures moving at uniform speed over deep and shallow water. The effect of beam-to-length ratio, Froude number, and water depth on the resistance is investigated. The resistance is found to exhibit a distinctive behavior at a critical frequency. This behavior corresponds to a singularity in the resistance at the critical frequency. The importance of this behavior is found to diminish with decreasing beam-to-length ratio and increasing Froude number.


Author(s):  
В.М. Козин

При решении таких ледотехнических задач, как измельчение крупно-битого льда для облегчения его пропуска через гидротехнические сооружения при весенних ледоходах, разрушение ледяных заторов и зажоров в условиях мелководья, где не могут работать ледоколы из-за их осадки и пр. находят применение амфибийные суда на воздушной подушке (СВП). В работе поставлена задача оценить влияние сплоченности битого льда на скорость и длину волн в битом льду, возбуждаемых СВП при их движении в условиях мелководья со скоростями, соответствующими возникновению максимального волнового сопротивления. С этой целью в опытовом бассейне были проведены соответствующие эксперименты с использованием искусственного битого льда, изготовленного из полиэтилена высокого давления, плотность и коэффициент трения которого были близки к естественному льду. Выполненные исследования позволяют разработать наиболее эффективные приемы маневрирования СВП в условиях битого льда различной сплоченности с целью его измельчения, т.е. уменьшения размеров обломков. На основании выполненных экспериментов и анализа известных теоретических зависимостей показано, что при определении таких важных параметров гравитационных волн, как их длина и скорость распространения, возбуждаемых судном в битом льду, могут быть использованы соответствующие зависимости из теории волн на чистой воде. When solving such ice-technical problems as crushing coarse-crushed ice to facilitate its passage through hydraulic structures during spring ice drifts, the destruction of ice jams and blockages in shallow water, where icebreakers cannot work due to precipitation, etc. amphibious vessels air cushion (SVP). The objective of the work is to assess the influence of the cohesion of broken ice on the speed and wavelength in broken ice excited by SVPs when they move in shallow water with speeds, corresponding to the occurrence of maximum wave resistance. For this purpose, corresponding experiments were carried out in the experimental pool using artificial beaten ice made of high pressure polyethylene, the density and friction coefficient of which were close to natural ice. The performed studies allow us to develop the most effective methods for maneuvering SVPs in the conditions of broken ice of various cohesion with the aim of crushing it, i.e. debris reduction. Based on the performed experiments and analysis of known theoretical dependences, it was shown that in determining important parameters of gravitational waves, such as their length and propagation velocity, excited by a vessel in broken ice, the corresponding dependences from the theory of waves in pure water can be used.


1978 ◽  
Vol 22 (02) ◽  
pp. 67-79 ◽  
Author(s):  
Lawrence J. Doctors

The harmonic heave and pitch motion of an air-cushion vehicle traveling at a constant speed over water is studied here, with a view to determining the power radiated by the surrounding wave system. The planform of the particular craft considered is compartmented into forward and aft subcushions, and the fluctuations of pressure in these are utilized to represent the effect of the vehicle on the water. The usual linearized incompressible potential flow theory is used. The calculations show that at typical Froude numbers and encounter frequencies, considerable power can be radiated in this manner, and it is generally of similar magnitude to the power required to overcome the usual steady-state wave resistance. Surprisingly, the singularity in the linear theory that occurs at the critical speed-frequency condition was found to be extremely localized and is therefore only significant in the case of a two-dimensional pressure band, or in the case of a three-dimensional pressure patch, at low Froude numbers.


1972 ◽  
Vol 16 (04) ◽  
pp. 248-260 ◽  
Author(s):  
Lawrence J. Doctors ◽  
Som D. Sharma

This paper deals with the theoretical wave resistance of an air-cushion vehicle (ACV) traveling over water of uniform finite or infinite depth, in steady or unsteady motion. Referring first to steady motion, it is shown that the unrealistic oscillations in the wave resistance curve at low Froude numbers found by previous workers can be eliminated by using a smoothed-out pressure distribution rather than one with sharp edges studied exclusively in the past. The main result of unsteady motion calculations is that the peak wave resistance in shallow water, even in moderately accelerated motion, is appreciably less than the corresponding steady-state value. An interesting feature of unsteady motion is that besides wave resistance there is another mechanism transferring energy to the free surface, which is here called the dynamic sustention power. Contrary to intuition, the wave resistance in unsteady motion over finite depth sometimes becomes negative at supercritical Froude numbers before finally vanishing at infinite speed.


2006 ◽  
Vol 6 (1) ◽  
pp. 1-9
Author(s):  
V. Miska ◽  
J.H.J.M. van der Graaf ◽  
J. de Koning

Nowadays filtration processes are still monitored with conventional analyses like turbidity measurements and, in case of flocculation–filtration, with phosphorus analyses. Turbidity measurements have the disadvantage that breakthrough of small flocs cannot be displayed, because of the blindness regarding changes in the mass distributions. Additional particle volume distributions calculated from particle size distributions (PSDs) would provide a better assessment of filtration performance. Lab-scale experiments have been executed on a flocculation–filtration column fed with effluent from WWTP Beverwijk in The Netherlands. Besides particle counting at various sampling points, the effect of sample dilution on the accuracy of PSD measurements has been reflected. It was found that the dilution has a minor effect on PSD of low turbidity samples such as process filtrate. The correlation between total particle counts, total particle volume (TPV) and total particle surface is not high but is at least better for diluted measurements of particles in the range 2–10 μm. Furthermore, possible relations between floc-bound phosphorus and TPV removal had been investigated. A good correlation coefficient is found for TPV removal versus floc-bound phosphorus removal for the experiments with polyaluminiumchloride and the experiments with single denitrifying and blank filtration.


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