Initial Velocity Conditions Research Articles

Analysis of the influence of different initial velocities on dynamic performance of multi-layer hard target penetration process

To determine the overload characteristics of the internal system of a fuze that penetrates multilayer hard targets using different fixed-link structures, a finite element model consisting of two fixed-link structures (a compression screw and a body screw) is adopted in this paper to simulate the penetration process of a three-layer concrete target plate with corresponding initial velocities. The peak amplification coefficient and vibration coefficient are used to analyze the time-domain characteristics of the penetration process signal during segmented analysis. The extracted acceleration signals of the projectile and sensor are processed by fast Fourier transform to obtain the frequency spectrum analysis results. The simulation results show that under the same working conditions, the sensor’s ability to amplify the peak acceleration of the projectile is 17.77% higher for the body screw fixed-link structure, and the average vibration coefficient is also 9.55% higher. Compared with that of the body screw fixed-link structure, the performance of the compression screw fixed-link structure is better under different initial velocity conditions. The initial penetration velocity affects mainly the amplitude of each frequency corresponding to the acceleration signals of the two fixed-link fuze structure projectiles and sensors while having a relatively small influence on the frequency distribution position.

Are the oscillations found in magnetized collisional electronegative warm plasma artifact during plasma-surface interaction?

The formation of the sheath and its characteristics are investigated in an electronegative plasma under the impact of an oblique and constant magnetic field. Specifically, the role of the magnetic field and initial conditions of positive ion velocity on the sheath characteristics are uncovered by adopting the Sagdeev potential approach for the determination of the modified Bohm’s criterion. At the sheath edge, the upper and lower components of the positive ion velocity perpendicular to the probe/wall are determined and examined in greater detail. The sheath structure is found to be modified considerably by the initial conditions. If the magnetic field is strong near the sheath edge, then the profile of the positive ion density shows a pulse-like structure for the case when we neglect the x-component (parallel to the wall) of positive ion velocity at the sheath edge. This pulse-like structure disappears when all the components of the positive ion velocity at the sheath edge are included, i.e. the ions are considered to move in any direction but towards the probe. The occurrence or disappearance of pulse-like structures or oscillations completely depend upon the initial conditions. Since in real situations the ions are expected to move in any direction (carrying all their velocity components) and are not aligned only in the direction perpendicular to the wall, these oscillations are artifact in nature. The sheath thickness is observed to be reduced with increasing magnetic field strength and its inclination. In order to better understand the phenomenon, isothermal and adiabatic cases are also discussed.

Space-time variational material modeling: a new paradigm demonstrated for thermo-mechanically coupled wave propagation, visco-elasticity, elasto-plasticity with hardening, and gradient-enhanced damage

We formulate variational material modeling in a space-time context. The starting point is the description of the space-time cylinder and the definition of a thermodynamically consistent Hamilton functional which accounts for all boundary conditions on the cylinder surface. From the mechanical perspective, the Hamilton principle then yields thermo-mechanically coupled models by evaluation of the stationarity conditions for all thermodynamic state variables which are displacements, internal variables, and temperature. Exemplary, we investigate in this contribution elastic wave propagation, visco-elasticity, elasto-plasticity with hardening, and gradient-enhanced damage. Therein, one key novel aspect are initial and end time velocity conditions for the wave equation, replacing classical initial conditions for the displacements and the velocities. The motivation is intensively discussed and illustrated with the help of a prototype numerical simulation. From the mathematical perspective, the space-time formulations are formulated within suitable function spaces and convex sets. The unified presentation merges engineering and applied mathematics due to their mutual interactions. Specifically, the chosen models are of high interest in many state-of-the art developments in modeling and we show the impact of this holistic physical description on space-time Galerkin finite element discretization schemes. Finally, we study a specific discrete realization and show that the resulting system using initial and end time conditions is well-posed.

Analytical Solutions of the Riccati Differential Equation: Particle Deposition in a Viscous Stagnant Fluid

In this communication, the solution of the differential Riccati equation is shown to provide a closed analytical expression for the transient settling velocity of arbitrary non-spherical particles in a still, unbounded viscous fluid. Such a solution is verified against the numerical results of the integrated differential equation, establishing its accuracy, and validated against previous experimental, theoretical and numerical studies, illustrating the effect of particle sphericity. The developed closed analytical formulae are simple and applicable to general initial velocity conditions in the Stokes, transitional and Newtonian regimes, extending the range of application of former published analytical approximate solutions on this subject.

Analysis of Flow Characteristics of a Debris Filter in a Condenser Tube Cleaning System

In a power plant that uses seawater as a coolant, a debris filter (DF) is required to remove foreign substances from the seawater, and differential pressure leads to a decrease in the coolant flow rate, leading to a decrease in the power generation efficiency. In this study, an analysis was performed for the cases wherein the initial flow velocity conditions of the DF used in the condenser tube cleaning system (CTCS) were 1.5 m/s, 2.0 m/s, and 2.5 m/s using Ansys Fluent 2021, and the flow characteristics were identified. The flow and differential pressure characteristics of a CTCS with an installed DF were considered in a comparative analysis of the velocity, pressure, and turbulence kinetic energy (TKE) distributions. The results confirmed that a vortex was generated in the pipe with the DF, apparently due to the collision of the flow with the bracket of the DF. As the flow rate increased, the range of the vortex increased, causing a loss in flow.

The Role of Inertia in the Onset of Turbulence in a Vortex Filament

The decay of the kinetic energy of a turbulent flow with time is not necessarily monotonic. This is revealed by simulations performed in the framework of discrete mechanics, where the kinetic energy can be transformed into pressure energy or vice versa; this persistent phenomenon is also observed for inviscid fluids. Different types of viscous vortex filaments generated by initial velocity conditions show that vortex stretching phenomena precede an abrupt onset of vortex bursting in high-shear regions. In all cases, the kinetic energy starts to grow by borrowing energy from the pressure before the transfer phase to the small turbulent structures. The result observed on the vortex filament is also found for the Taylor–Green vortex, which significantly differs from the previous results on this same case simulated from the Navier–Stokes equations. This disagreement is attributed to the physical model used, that of discrete mechanics, where the formulation is based on the conservation of acceleration. The reasons for this divergence are analyzed in depth; however, a spectral analysis allows finding the established laws on the decay of kinetic energy as a function of the wave number.

Numerical Simulation of Penetration Process of Depleted Uranium Alloy Based on an FEM-SPH Coupling Algorithm

In order to quantitatively study the penetration capability of depleted uranium alloy, a simulation model of bullet impact on target plate with FEM-SPH coupling algorithm was established by using LS-DYNA software, which was combined with Johnson-Cook intrinsic model, Johnson-Cook fracture criterion, and equation of state to conduct a simulation study of alloy bullets made of depleted uranium alloy, tungsten alloy, and high-strength steel to penetrate target plate at 1400 m/s initial velocity. The results show that under the same conditions of initial kinetic energy, initial velocity, and initial volume, the residual kinetic energy of the depleted uranium alloy bullet is 1.14 times that of tungsten alloy and 1.20 times that of high-strength steel, and the residual velocity is 1.14 times that of tungsten alloy and 1.18 times that of steel, and the residual volume is 1.13 times that of tungsten alloy and 1.23 times that of steel after the penetration is completed. The shape of the bullet after penetrating the target plate is relatively sharp, and the diameter of the target hole formed is about 1.70 times the diameter of the projectile, which is significantly larger than 1.54 times that of tungsten alloy and 1.39 times that of high-strength steel, indicating the excellent penetration performance of depleted uranium alloy.

Is The Probability of Tossing a Coin Really 50–50%? Part 2: Dynamic Model with Rebounds

In the first part of this paper, we considered several theoretical models, a static and four dynamic models without rebounds, of the throw of a fair coin landing on its edge, to demonstrate that the probability of heads or tails is less than 50%, depending on the initial toss conditions, the coin geometry and conditions of the coin and landing surfaces. For the dynamic model with rebounds that is the subject of this second part of the paper, it is found that the probability that a 50 Euro cent coin thrown from a normal height with common initial velocity conditions and appropriate surface conditions will end up on its edge is in the order of one against several thousand.

Is the Probability of Tossing a Coin Really 50–50%? Part 1: Static Model and Dynamic Models without Rebounds

Considering that a fair coin has two sides and a cylindrical edge, the probability that it would fall on its edge is calculated, yielding the probability of heads or tails of less than 50%. In this first part, the theoretical models for a static case and for five dynamic cases, without rebounds, show that there is a small probability that the coin does not fall on its head or tail, depending on the initial toss conditions, the coin geometry and conditions of the coin and landing surfaces. It is found that the probability that a 50 Eurocent coin thrown from a normal height with common initial velocity conditions and appropriate surface conditions will end up on its edge is in the order of one against several thousand. In the second part of the paper, the dynamic model with rebounds is investigated.

Time-Optimal Velocity Tracking Control for Consensus Formation of Multiple Nonholonomic Mobile Robots.

The problem of velocity tracking is considered essential in the consensus of multi-wheeled mobile robot systems to minimise the total operating time and enhance the system’s energy efficiency. This study presents a novel switched-system approach, consisting of bang-bang control and consensus formation algorithms, to address the problem of time-optimal velocity tracking of multiple wheeled mobile robots with nonholonomic constraints. This effort aims to achieve the desired velocity formation in the least time for any initial velocity conditions in a multiple mobile robot system. The main findings of this study are as follows: (i) by deriving the equation of motion along the specified path, the motor’s extremal conditions for a time-optimal trajectory are introduced; (ii) utilising a general consensus formation algorithm, the desired velocity formation is achieved; (iii) applying the Pontryagin Maximum Principle, the new switching formation matrix of weights is obtained. Using this new switching matrix of weights guarantees that at least one of the system’s motors, of either the followers or the leader, reaches its maximum or minimum value by using extremals, which enables the multi-robot system to reach the velocity formation in the least time. The proposed approach is verified in a theoretical analysis along with the numerical simulation process. The simulation results demonstrated that using the proposed switched system, the time-optimal consensus algorithm behaved very well in the networks with different numbers of robots and different topology conditions. The required time for the consensus formation is dramatically reduced, which is very promising. The findings of this work could be extended to and beneficial for any multi-wheeled mobile robot system.

Probabilistic Analysis of Debris Motion in Steady-State Currents for Varying Initial Debris Orientation and Flow Velocity Conditions

AbstractFloodborne debris impact loads depend on several factors, including debris orientation and velocity. Due to the random nature of turbulent flood flows, debris motions are inherently random....

Mechanical characterization of flat faced deformable AUV during water entry impact considering the hydroelastic effects

In this paper, the dynamic behavior of flat faced deformable autonomous underwater vehicle (AUV) subjected to hydroelastic water entry impact is numerically investigated by using Arbitrary Lagrangian Eulerian (ALE) method. The experiments on the flat faced AUV head section of 200 mm diameter are carried out in a water tank to validate the numerical model by comparing the impact accelerations and underwater cavity evolution by using accelerometers and high speed cameras. Then, the effect of hydroelasticity is numerically investigated by comparing the impact characteristics of rigid and deformable AUV with different rigidities. It is revealed that the hydroelastic effect increases with the decrease in the rigidity of the structure. Mechanical behavior of deformable AUV of various rigidities under the initial conditions of same velocity, same kinetic energy and same mass is examined. The influence of various initial conditions on the hydrodynamic load, impact pressure and effective strain for deformable AUV are discussed. The influence of the water entry angle and impact velocity on the semi-rigid and deformable AUVs are also studied. It is found that the effective strain increases exponentially with the increase in impact velocity and entry angle for deformable AUV, however, semi-rigid AUV exhibit linear strain behavior. Numerical conclusions drawn from this study are vital in selecting the rigidity of AUV and highlighted the influence of hydroelasticity for flat faced AUVs. Additionally, the results obtained in this study can be beneficial for the structural design of high speed air launched AUVs.

Temperature Measurement of a Bullet in Flight.

This study answers a primary question concerning how the temperature changes during the flight of a bullet. To answer the question, the authors performed unique research to measure the initial temperatures of bullet surfaces and applied it to four kinds of projectiles in a series of field experiments. The technique determines the temperature changes on metallic objects in flight that reach a velocity of 300 to 900 m/s. Until now, the tests of temperature change available in the literature include virtual points that are adopted to ideal laboratory conditions using classic thermomechanical equations. The authors conducted the first study of its kind, in which is considered four projectiles in field conditions in which a metallic bullet leaves a rifle barrel after a powder deflagration. During this process, heat is partly transferred to the bullet from the initial explosion of the powder and barrel-bullet friction. In this case, the temperature determination of a bullet is complex because it concerns different points on the external surface. Thus, for the first time the authors measured the temperatures at different position on the bullet surface. Moreover, the authors showed that basic thermodynamic equations allow for the credible prediction of such behavior if the initial conditions are identified correctly. This novel identification of the initial conditions of temperature and velocity of flying bullets was not presented anywhere else up to now.

DYNAMIC MODELLING FOR THE SECOND FLIGHT PHASE OF THE YURCHENKO LAYOUT VAULT BASED ON MSC. ADAMS

Gymnasts attempt to increase the angles of rotation about transversal and longitudinal axes during the post-flight of vaulting, and these angles are related to different mechanical properties. The present study uses a 3D angle-driven computer simulation model of a gymnast who performs Yurchenko layout vault using ADAMS software. Simulation initial conditions are horizontal and vertical velocities of gymnast’s pelvis center and angular velocities about the transversal and longitudinal axes which can be easily measured. The initial linear and angular velocity conditions of the simulation model are each changed in certain increments from measurement data collected from an elite woman gymnast. Increasing initial horizontal velocity results in an increased horizontal flight distance, but has no connection with the duration of flight and angle of twists. The overall angle of twists is concerned with initial vertical velocity and angular velocities about the transversal and longitudinal axes. Also, increasing initial vertical velocity and angular velocity about transverse axis leads to increase in touchdown angle between ground’s horizontal axis and gymnast’s longitudinal axis.

Formation of film cooling on the turbine blade back and pressure side in the case of using V-shaped dimples

Alongside the development of methods of intensifying convective heat transfer inside the blade, development of methods of local improvement of the efficiency of film cooling of the blades surface is still of immediate interest. The film is formed on the blade surface in conditions of high-camber shape and low initial velocity of the gas flow in the vicinity of the leading edge with its subsequent abrupt acceleration. The paper presents some data on the peculiarities of film formation on the back and pressure side of the blade in the vicinity of the leading edge. Experimental temperature distribution over the adiabatic wall was obtained with the use of a FLIR-E 64501 thermal imager. It was found that the conditions for the film formation on the blade back are more favorable than those on the pressure side. It manifests itself in the fact that optimal blowing parameters on the blade back are considerably lower than those on the pressure side. The use of V-shaped dimples located on the wall immediately behind the holes for blowing was suggested as a measure for local improvement of film cooling efficiency. The efficiencies of film cooling in the formation of a curtain, without the use and with the use of V-shaped dimples behind the holes for blowing were compared. Local improvement of efficiency and uniformity of film cooling distribution with the use of V-shaped dimples behind the holes for blowing was observed.

Consensus of Second-Order Multiagent Systems With Both Velocity and Input Constraints

This paper considers consensus control of second-order multiagent systems subject to both velocity and input constraints under general directed communication graphs. A new class of distributed consensus controller is proposed, which is shown that under proper initial velocity conditions, consensus can be achieved, while both the velocity and input constraints are satisfied during the consensus process. The proposed controller can be designed without relying on global information about the communication graph such as the eigenvalues of the Laplacian matrix. Extensions to dynamic formation control and leader-following consensus with both velocity and input constraints are also presented based on the consensus controller. Furthermore, the sampled-data leader-following case is also investigated when the communication rate is constrained. Simulations on the outer-loop coordination control of multiple quadrotors are used to illustrate the effectiveness of the proposed controllers.

Effect of pressure-dilatation on energy spectrum evolution in compressible turbulence

The effect of internal-kinetic energy exchange on transient spectral energy transfer in compressible turbulence is investigated. We derive the spectral evolution equations for kinetic energy and pressure fields to highlight the key mechanisms that affect the turbulence spectral evolution. Direct numerical simulations of decaying isotropic turbulence are performed from solenoidal, dilatational, and mixed velocity initial conditions. It is shown that internal-kinetic energy exchange arising due to pressure-dilatation renders the dilatational kinetic energy amplitudes at large scales of motion to be oscillatory. The oscillatory behavior of amplitude diminishes with increasing wavenumber. The dilatational energy spectrum also exhibits a wider range of scales due to its inherent tendency to form shocks. The findings are expected to lead to an improved understanding of energy dynamics in high-speed compressible flows.

Optimal guidance and collision avoidance for docking with the rotating target spacecraft

The guidance and control strategy for spacecraft rendezvous and docking are of vital importance, especially for a chaser spacecraft docking with a rotating target spacecraft. Approach guidance for docking maneuver in planar is studied in this paper. Approach maneuver includes two processes: optimal energy approach and the following flying-around approach. Flying-around approach method is presented to maintain a fixed relative distance and attitude for chaser spacecraft docking with target spacecraft. Due to the disadvantage of energy consumption and initial velocity condition, optimal energy guidance is presented and can be used for providing an initial state of flying-around approach process. The analytical expression of optimal energy guidance is obtained based on the Pontryagin minimum principle which can be used in real time. A couple of solar panels on the target spacecraft are considered as obstacles during proximity maneuvers, so secure docking region is discussed. A two-phase optimal guidance method is adopted for collision avoidance with solar panels. Simulation demonstrates that the closed-loop optimal energy guidance satisfies the ending docking constraints, avoids collision with time-varying rotating target, and provides the initial velocity conditions of flying-around approach maneuver. Flying-around approach maneuver can maintain fixed relative position and attitude for docking.

The formation of multi-steady-states of buoyancy ventilation in underground building

The study of buoyancy ventilation in underground buildings is crucial for the purpose of temperature control and smoke extraction. Some underground spaces have unique properties such as their deep location, with installed vertical channels of several hundred meters in length, raising the potential to rely on buoyancy driven ventilation only. This paper studies a particular phenomenon in such deep spaces with high intensity heat sources, e.g. power generating turbines. The phenomenon studied in this paper concerns the way that different steady states of flow and temperature can establish themselves. They are studied through an experimental set up and numerical simulations. Multiple factors, like the initial condition, the intensity and location of the heat source, are investigated, in particular to find how the final steady states in different situations are reached. It is well understood that the dominant flow tends to follow the expansion of the local thermal plume. An initial velocity may however alter this trend and an alternative steady state can be reached but this depends on the magnitude of the initial velocity. The critical initial velocity for the transition among these steady states is therefore studied in detail. However, once the steady solution is reached, this is not dependent on the initial velocity or other conditions. Thermal buoyancy pressure of fluid flow is generated by gravity forces due to density difference. Buoyancy forces work together with other forces (such as mechanical fans which have a given location and direction, and wind pressures over openings) to result in natural ventilation flows in spaces with openings that connect with outside air. The multi-steady-states of fluid flow in an underground space is determined by the fundamental characteristic of thermal buoyancy pressure and its understanding and predictability is crucial to be able to predict temperatures and natural ventilation flows correctly.

Use of Genetic Algorithms to solve Inverse Problems in Relativistic Hydrodynamics

We present the use of Genetic Algorithms (GAs) as a strategy to solve inverse problems associated with models of relativistic hydrodynamics. The signal we consider to emulate an observation is the density of a relativistic gas, measured at a point where a shock is traveling. This shock is generated numerically out of a Riemann problem with mildly relativistic conditions. The inverse problem we propose is the prediction of the initial conditions of density, velocity and pressure of the Riemann problem that gave origin to that signal. For this we use the density, velocity and pressure of the gas at both sides of the discontinuity, as the six genes of an organism, initially with random values within a tolerance. We then prepare an initial population of N of these organisms and evolve them using methods based on GAs. In the end, the organism with the best fitness of each generation is compared to the signal and the process ends when the set of initial conditions of the organisms of a later generation fit the Signal within a tolerance.