Numerical simulation of cavitation characteristics in high speed water entry of head-jetting underwater vehicle

Autonomous underwater vehicle will be subjected to a huge impact load during high speed water entry, which will damage the structure and the internal instruments of the vehicle. Therefore, it is of great significance to study the buffer mechanism of the vehicle during the process of water-entry. In this paper, a kind of head-jetting device with disk cavitation is used. The complex cavitation forms, under the three-phase coupling of gas, liquid and solid, in the water entry process of the vehicle on which the device is installed. In this paper, the numerical simulation of high-speed water entry of the vehicle equipped with head jet device is carried out. Through the analysis of water entry cavitation under typical working conditions, the following conclusions are obtained. After the installation of head jet device, the water entry cavity of the vehicle changes gradually from cone to spindle shape. The air jet, compared with that without jet, can promote the formation of water inlet supercavitation, decrease the interaction area between the vehicle and water, and reduce the impact load during water entry. At the same water entry depth, the diameter of cavitation increases with the amount of air jet. The water entry velocity has a great influence on the difference of cavitation shape. The water entry depth closure phenomenon, when the water entry velocity is less than 100 m/s, can be observed in the depth of 3.5 times of the projectile length. The water entry angle has a significant effect on the cavitation shape. The cavity shows obvious asymmetry when the vehicle slants into the water, and the diameter and length of the bubbles decrease with the increase of the water entry angle. The research content of this paper provides technical support for the engineering practice of high-speed water entry and load reduction, and the conclusions are of great significance in related fields.

Extraterrestrial dust as a source of bioavailable Fe for the ocean productivity

Bioavailable Fe is an essential nutrient for phytoplankton that allows organisms to flourish and drawdown atmospheric CO2 affecting global climatic condition. In marine locales remote from the continents extraterrestrial-dust provides an important source of Fe and thus moderates primary productivity. Here we provide constraints on partitioning of extraterrestrial Fe between seawater and sediments from observations of dissolution and alteration cosmic spherules recovered from the deepsea sediments and Antarctica. Of the ~ 3000–6000 t/a extraterrestrial dust that reaches Earth surface, ~ 2–5 % material survives in marine sediments whilst the remainder is liberated into seawater. Both processes contributes ~ (3–10) × 10−8 molFe m−2 yr−1. Also, Fe contribution due to evaporation of survived particle is estimated to be ~ 10 % of Fe contribution to meteoric smoke. Changes in extraterrestrial-dust flux vary not only the amount of Fe by up to three orders of magnitude, but also the partitioning of Fe between surface and abyssal waters depending on entry velocity and evaporation.

Use of Modern Digital Software to Model the Motion Dynamics of Spacecraft during Landing

Landing systems for future space missions in Earth and Mars require trustable technologies capable of achieving their aim in the most accurate way possible. For this purpose, systems should go through rigorous dynamic simulations run by precise and efficient software. This study aims to approximately determine the dynamic motion of a landing vehicle using the modern digital software of Universal Mechanism and MATLAB. Universal Mechanism applies the classic mechanics theory on a model based on the geometry of the spacecraft, taking into account the environmental conditions that affect its motion and the properties of the ground to resist its impact [1]. The forces implied in the vehicle phase of descent are also included in MATLAB code to calculate the landing area of the vehicle according to its re-entry velocity [2-4]. Studies were conducted for different initial conditions and approaches to the surface. As a result, the values of the arising overloads and forces acting on the descent vehicle were obtained [1]. The data provided by the simulations conclude the safest landing options that should be taken into account for the success of future missions.

Experimental study on water entry of cylindrical projectiles with different nose shapes

Water entry experiments of projectiles with different nose shapes were performed under different entry angles and velocities using high-speed photography technology. The cavity flow characteristics of the near water surface, including splash jet, splash crown, surface seal of cavity, pull away, deep seal of cavity and cavity collapses, were systematically investigated using a high-speed camera. The emphasis of the study is paid on the effect of nose shape, water entry angle and velocity on the evolution of the air entraining cavity. The experimental results demonstrate that the nose shape of projectile has a significant influence on the jet flow, the cavity diameter and trajectory stability in the case of certain other conditions. On the other hand, the splash scale, cavity diameter increase gradually with the increasing of the water entry velocity, as well as the cavitation closed in advance. Furthermore, the water entry angle of the projectile plays an important role in the cavity evolution and the close type.

Analysis of Steam Explosion under Conditions of Partially Flooded Cavity and Submerged Reactor Vessel

A steam explosion in a reactor cavity makes a mechanical load of the pressure pulse, which can result in a failure of the containment isolation. To prove the integrity of the containment during the ex-vessel steam explosion, the effects of water conditions on a steam explosion have to be identified, and the impulse of a steam explosion has to be exactly assessed. In this study, the analyses for steam explosions were performed for the conditions of a partially flooded cavity and a submerged-vessel in a pressurized water reactor. The entry velocity of a corium jet for the scale of the test facility was varied to simulate the two plant conditions. The TEXAS-V code was used for simulating the phases of premixing and explosion, and the load of a steam explosion was estimated based on the pressure variation. The impulse of a steam explosion under the condition of a corium jet falling into water without a free-fall height is bigger than that under a free-fall height. The fragmented mass of corium in an explosion phase and the distribution of steam fraction are the main parameters for the total load of the steam explosion. This study is expected to contribute to analyses of a steam explosion for a severe accident management strategy.

Investigation of Bottom Slamming on Ships in Irregular Waves

The bottom slamming of a chemical tanker and a LNG carrier advancing in irregular waves is investigated numerically and compared with experiments. The probability of slamming occurrence on longitudinal locations and the slamming induced pressures on the bottom of two ships are discussed. It is considered that slamming occurrence at a point is dependent upon two conditions: the relative vertical motion at the same longitudinal position of the ship being larger than the vertical distance from the still water to the concerned position, and the entry velocity exceeding some threshold velocity. Ship motions in irregular waves predicted by a time domain seakeeping code and measured from the model tests are used to calculate the slamming occurrence statistically based on the two conditions mentioned above. Only heave and pitch motions are considered in the calculations. The seakeeping code combines body linear radiation and diffraction forces with body nonlinear Froude-Krylov forces, hydrostatic forces and shipping of green water on the bow. The effects of body nonlinearity are considered by a simplified method: the memory functions, infinite frequency added masses and the radiation restoring coefficients are assessed at each time instant as function of the instantaneous wetted surface. A similar procedure is used to calculate the diffraction forces. The experimental data of the wave-induced loads on these two vessels in different sea states are analyzed statistically. Probability of exceedance of entry velocities and pressure peaks for the sections at the bow and stern are computed for various irregular sea states. The results of the slamming occurrence on longitudinal locations and wave induced loads on these two types of ships are discussed.

Experimental Study on Hydroelastic Impact of One Wedge With Stiffened Panels

In this paper, two wedges with deadrise angle 45 degrees made of steel and aluminum alloy respectively was designed and a series of free-drop model tests were carried out, we recorded time histories of water-entry velocity of models, slamming pressures on outer surface of stiffened panels and stress response of the structures. The time space distribution of slamming load and characteristic of structure dynamic response were studied. And through comparison analysis of slamming loads between two model’s corresponding measuring points, the effect of hydroelastic on slamming loads were discussed. Finally, an equivalent static analysis method to determine the structural response of hull structures under slamming loads was presented by analyzed the slamming loads and structure dynamic response that got from experiment combined with finite element method.

Experimental Reconstruction of the Hull Sectional Forces Under Slamming Events

An analysis of the slamming load distribution along a slender floating body on the basis of experimental measurements is presented. The dataset is provided by the (lumped) vertical forces measured on the hull portions of a segmented scaled model in seakeeping tests. Using the proper orthogonal decomposition the vector time-history of the hydrodynamic forces is first decomposed into the summation of few terms, each one retaining separately time and space information. This decomposition allows highlighting interesting features regarding variations in the load distribution under different test conditions. The analysis has exploited also the use of pre-filtering for separating the load components at different frequencies to be further decomposed with POD. Then, a polynomial spline approximation under integral constraints is used to approximate the shape functions and then to obtain the continuous distribution of the sectional force. The considered tests were carried out in both irregular sea and regular waves with a choice of model speed and wave parameters so as slamming occurs. The identified distribution of force per unit length over the impacting hull segments is then compared with a modified Von-Karman model (with 3D correction) which accounts for the water uprise in the expression of the slamming force and variable entry velocity.

Effect of Compressibility on Peak Impact Pressure and Pressure Distribution During Water Entry of a Wedge

Ship hull slamming and wave impact on wave energy converters (WEC) are an important problem in naval architecture and marine hydrokinetic for the survivability of vessels and WECs in adverse environmental conditions. An idealized canonical model for these problems can be traced back to the water entry of a wedge–shaped rigid object. Common asymptotic and empirical models are often based on the assumption that the fluid is incompressible and the initial wedge entry velocity to sound speed ratio (defined as the characterized Mach number) is low. In this paper, we present some initial results of a numerical study of the effect of fluid compressibility on the peak slamming impact pressure. The deadrise angle and entry velocity of the model are varied to examine the sensitivity of the peak and distribution of the pressure response with respect to these parameters. The fluid compressibility is controlled through the bulk modulus and manifested as various sound speeds. The normalization using this sound speed shows that the pressure coefficient changes consistently for a certain deadrise angle. The results indicates that despite a low Mach number, the fluid compressibility in the numerical model affects the peak pressure significantly, implying an overestimation in engineering application by the classical asymptotic incompressible theory.