The water-exit course of a submerged vehicle launched underwater that moves into the atmosphere is one of the most important considerations in naval applications. In the present work, a large-eddy ...simulation with a uniform filter of an octree-based isotropic mesh and a high-resolution interface-capturing algorithm is employed to investigate this issue, with a special emphasis on the flow structures and pressure features. Various launch parameters, including the immersion depth, navigation speed, and angle of attack are discussed. The flow structures sensitive to adverse pressure relevant to the water-exit angle are captured and discussed. The flow experiences a laminar-turbulent transition and forms coherent turbulence structures, which develop a series of primary and secondary hairpin vortices. The distribution of pressure is more monotonic and regular on the pressure side of the vehicle as the angle increases, while a propagation of disturbance and a transition to turbulence can occur across the lateral side. Based on the clearly captured evolution of the water surface and cavitation, the cavity shedding induced by a pinch-off effect of the travelling liquid-vapor contact lines is found and investigated. The structural features of turbulence are greatly affected in this case, which contrasts with the noncavitating features. The structures are tuned due to the existence of cavitation, which blocks the hairpin-vortex generation mechanism. The cavity surface is full of Tollmien-Schlichting wave-like vortical structures caused by liquid-vapor interface shearing. Hundreds of monitors are intensively arranged on the vehicle's surface to capture the transient pressure signals, through which the crucial pressure features induced by the growth and collapse of the cavitation are captured.
•LES with uniform filter of octree-based grids is used to study water-exit cavitating flow.•High-resolution laminar-turbulent transition and hairpin vortex packets are resolved.•Cavity pinch-off effect of travelling liquid-vapor contact lines is found and investigated.•Vortical flow structures are affected by cavitation blocking hairpin-vortex generation.•Crucial pressure features of cavitation collapse are captured using hundreds of monitors.
The free surface and pycnocline wake characteristics of SUBOFF were studied by simulations and experiments in a two-layer ocean. The experimental results showed that the simulation method accurately ...calculated the wake characteristics of the scaling model, and the simulation method was expanded to full-scale prototypes. When typical Froude numbers of flow is from 0.05 to 2.7, the submerged body velocity can influence the wake characteristics of the free surface and pycnocline interface. The wake system of the free surface would transform into a Kelvin wake, and the angles of the V-shaped divergent waves of the pycnocline interface would be reduced with the increasing Froude number.
Influence of forces on vehicle’s instability in floodwaters Shah, Syed Muzzamil Hussain; Mustaffa, Zahiraniza; Yusof, Khamaruzaman Wan ...
Ain Shams Engineering Journal,
December 2018, 2018-12-00, 2018-12-01, Volume:
9, Issue:
4
Journal Article
Peer reviewed
Open access
Flood hazards to vehicle have become more frequent and noticeable in the recent years. Therefore, this paper aims at estimating the forces observed on a partially submerged vehicle in floodwater, ...namely frictional, buoyancy, lift and drag forces. An understanding of the relevant forces involved is necessary to attempt to characterize the instability thresholds of vehicles in floodwater flows. With that regards, a model vehicle with the scale ratio of 1:24 was used and a series of flume experiments were conducted. While determining the vertical pushing force, the influence of lift force was found insignificant due to sub-critical state of the flow. Moreover, the critical water depth at which the up-thrust force governed was observed when the water depth exceeded 0.042 m. Below critical depth, the vehicle stability governed unless the drag force imposed by the flowing water was lower than the frictional force between the tires and floodplain surface.
The self-propulsion test of underwater vehicles is the key technique for predicting and evaluating the navigation performance of these submersibles. In this study, the numerical simulation of a ...standard propeller JD7704+Ka4-70 is first presented and the results are compared with experiments to validate the numerical approaches. The reason why the propulsion efficiency of the ducted propeller is higher than that of the conventional propeller is explored. Then, the paper proposes a series of numerical simulations conducted to test the performance of the ducted propeller designed according to the JD7704+Ka4-70 in order to match with the unmanned semi-submerged vehicle (USSV), and the propeller’s open water characteristic curves are obtained. The results show a reasonable agreement with the regression analysis. Afterwards, the numerical simulations focus on a self-propulsion test of the USSV with the designed ducted propeller and the self-propulsion point is obtained. The streamlines through the hull as well as the ducted propellers are clearly obtained, together with the velocity distributions of the propeller plane. The results vividly demonstrate the hydrodynamic performance of the USSV with the designed propellers. In this paper, all the CFD simulations are based on the numerical software, Star-CCM+, and use the Reynolds-averaged Navier‒Stokes (RANS) equations with the shear stress transport (SST) k-omega turbulence model.
The paper focuses on presenting the advantages of context aware cyber-physical systems through an experimental platform capable of assessing its surroundings and self-performing decisions. The ...context aware paradigm is present in the control law implementation with various advantages such as energy efficiency as well as in the environmental measurements that trigger the robot to perform context-relevant decisions. The platform provides high versatility and the results presented throughout the study can be adapted to a manifold of multidisciplinary fields.
To apply the measurements of model experiment in water tunnel to the actual sailing condition, it is necessary to know accurately the strut effect and its rule. In the present work, the corresponding ...interferences of one-side strut and two-side strut on the natural cavitating flows around a submerged vehicle in water tunnel were investigated numerically, using the homogeneous equilibrium two-phase model coupled with a natural cavitation model. The numerical simulation results show that the strut types have distinct effects on the hydrodynamic properties. For the same given upstream velocity and downstream pressure, the existence of the strut leads to an increment of natural cavitation number, reduces the low-pressure region and depresses the pressure on the vehicle surface near the sides of strut. In the case of given cavitaiton number, the influences of the two-side strut on the drag and lift coefficients are both enhanced along with the increment of attack angle, however the influence of the one-side strut gradually gets stronger on the drag coefficient but weaker on the lift coefficient contrarily. In addition, based on the present numerical results, a correction method by introducing the sigmoidal logistic function is proposed to eliminate the interference from the foil-shaped strut.
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