Free-fall Experiments Research Articles

Aerodynamic characterization of rod-like debris with application to firebrand transport

Aerodynamics of the firebrands' flight is fundamental to fire spotting; yet, this phenomenon is relatively poorly understood. Variants of Tachikawa's transport models have been developed for different shapes of debris but, only simplified versions are implemented in firebrand transport models. Failure to include lift and rotational forces reduces the problem to a two-dimensional one, in contrast to the 3D trajectories observed. As such, many studies may have under-/overestimated the flight distance and failed to capture the spotting distribution, accurately. Also, there is virtually no detailed quantitative experimental validation available for rod-like debris transport models. Hence, a set of free-fall experiments with non-combusting model firebrands were run in the absence of cross-wind. The results are employed for verification and performance evaluation of a 3D deterministic 6-Degrees-Of-Freedom (DOF) rod-like debris transport model. It is shown that a transport model must include the complete 6-DOF aerodynamics of the debris for estimating the flight characteristics, such that results are not statistically significantly different from the experimental data. The findings have applications beyond firebrand transport, as they represent the most comprehensive experimental data set and analysis of rod-like debris transport, currently in the literature.

Experimental assessment of buoyant cylinder impacts through high-speed image acquisition

In this paper, we experimentally study the water entry of a set of buoyant bodies with a circular cylindrical shape, which is relevant for several engineering applications, being related to aircraft fuselages in sea landing, solid rocket boosters, and bulbous bows of large ships. Free fall experiments are conducted for three different specimen weights and four different falling heights. The hydrodynamics of the interaction of the buoyant cylinders with the water free surface upon impact is analyzed through an accelerometer, a high-frequency potentiometer and a high-speed camera. Results are presented in terms of impact velocity, kinematics of the cylinder penetration and free surface dynamics, comprising the pile-up and water jet dynamics. Further, the air cavity development is analyzed after the cylinder is completely immersed below the undisturbed free surface.

Observation of the transition from primary dendrites to coupled growth induced by undercooling within Ni[sbnd]Zr hyperperitectic alloy

The liquid Ni-20at.% Zr hyperperitectic alloy was substantially undercooled by electromagnetic levitation technique up to 225K (0.13TL). A sudden drop for the measured growth velocity is observed around the critical undercooling of 155K. This is confirmed as a transition of growth mode from primary Ni7Zr2 dendritic growth to coupled growth of Ni7Zr2 and Ni5Zr phases. Similar interface attachment kinetics and the planar front growth of Ni7Zr2 and Ni5Zr phases are responsible for the formation of peritectic coupled growth. The eutectic-like lamellar structure also forms in a free fall experiment, which further clearly demonstrates the existence of peritectic coupled growth induced by undercooling.

The free-fall mode experiment on LISA Pathfinder: first results

The LISA Pathfinder space mission is testing the critical experimental challenge for LISA by measuring the differential acceleration between two free-falling test masses inside a single co-orbiting spacecraft at a level of sub-femto-g for frequencies down to 0.1mHz. In LPF it is necessary that one test mass (TM) is electrostatically forced to follow the orbit of the other TM. This force represents a noise source in differential acceleration at frequencies below 1mHz. The free-fall mode experiment has been performed in order to reduce this source of noise: the actuation is limited to short impulses on one TM, so that it is in free fall between two successive kicks, while the other TM is drag-free. The free-fall mode thus provides a different technique for measuring the differential TM acceleration without the added force noise and calibration issues introduced by the actuator. Data analysis challenge is related to the presence of the kicks: they represent a high-noise contribution and need to be removed, thus leaving short gaps in data. This article presents preliminary data of the LPF free-fall measurement campaign and describes the three data analysis techniques developed to mitigate the presence of gaps.

Was Galileo Right

An important scientific debate took place regarding falling bodies hundreds of years ago, and it still warrants close examination. Galileo argued that in a vacuum all bodies fall at the same rate relative to the earth, independent of their mass. As we shall see, the problem is more subtle than meets the eye ‐ even in vacuum. In principle the results of a free fall experiment depend on whether falling masses are sequential or concurrent, whether they fall side by side or diametrically opposed. In the current paper we will present both the classical mechanics treatment and the general relativity one. In the case of classical mechanics, we start from the basic equations of motion. On the other hand, the determination of particle equations of motion in gravitational fields in general relativity is done routinely via the use of covariant derivatives. Since the geodesic equations based on covariant derivatives are derived from the Euler-Lagrange equations and since the Euler-Lagrange formalism is very intuitive, easy to derive with no mistakes, there is every reason to use them even for the most complicated situations and this is exactly what we do in the second part of the current paper.

Modelling of Fall-cone Tests with Strain-rate Effects

Material Point Method (MPM) is a numerical method, which is well suited for large displacement simulations. Large displacements problems are relatively common in geotechnics, including post-failure behaviour of landslides as well as a wide range of problems involving penetration into the soil body. One of those problems is the fall-cone test, commonly used to establish the undrained shear strength and the sensitivity of saturated fine grained soils.This paper shows a Generalized Interpolation Material Point Method (GIMP) simulation replicating published free-fall cone experiment performed on a kaolin clay. In the fall-cone tests, the penetration characteristics of the cone, such as velocity and total penetration depth depend on the soil properties. Those properties are affected greatly by the strain-rate which must be accounted for in a numerical simulation. Hence, the simulations shown uses a Mohr-Coulomb / Tresca material extended with strain-rate effects.The presentednumerical simulations are compared with the published fall-cone experiment in whichdisplacement and force were measured. The comparison indicates that Generalized Interpolation Material Point Method and Mohr-Coulomb / Tresca model extended with strain-rate effects areableto replicate the fall-cone penetration test very well.

Nanoaluminum/Nitrocellulose microparticle additive for burn enhancement of liquid fuels

Addition of metal and metal oxide nanoparticles to hydrocarbon fuels has shown the ability to increase the volumetric energy density, decrease ignition delay, increase heat of combustion, and catalyze fuel decomposition in recent research. However, energetic metal nanoparticles are prone to aggregation, which occurs at an increased rate near the regressing surface of a burning liquid droplet where local concentrations increase and can form a transport-inhibiting shell, ultimately decreasing the droplet burning rate. Alternatively, gas ejections from the droplet can disrupt shell formation and transport nanoparticles from the droplet to the flame zone. This study quantifies up to a 12.1% decrease in the burning rate constant of Kerosene droplets when 6.1wt% nanoaluminum (nAl) particles are added (the maximum stable loading) with a hydrocarbon-based surfactant in a free-falling single droplet combustion experiment. Addition of nitrocellulose (NC) particles to the nanofuel diminishes or fully counteracts the burning rate decreases and provides a means of tuning the burning rate constant higher than that of pure Kerosene (maximum 13.8% increase over control with 2.3wt% nAl and 0.6wt% NC added). To reach stable nanofuels at higher particle loadings up to 15.0wt% solid additives, nAl and NC were electrosprayed into composite mesoparticles (MP) before suspending with surfactant in Kerosene. These MP-based nanofuels boast increased dispersibility and additive loadings and thus higher achievable burning rates (maximum 26.5% increase over control) than physically mixed analogs. A mechanism is proposed in which droplet disruptions influenced by NC addition include cyclical inflations, during which the liquid gasification rate increases, e.g. by expanding the outer surface area of the droplet.

A new approach to the experiment intended to test the weak equivalence principle for the neutron

A new approach to the free fall experiment with UCN is proposed. The idea is that the experiment is performed with neutrons from different equidistant lines of a discrete energy spectrum with a precisely known distance between the lines. Such a spectrum may be formed by neutron diffraction from a moving phase grating. Time-of-flight values of neutrons from different lines of the spectrum must be measured. Neither the initial neutron energy, nor the geometry parameters of the installation are required to be known in this method.

Crucial tests of macrorealist and semiclassical gravity models with freely falling mesoscopic nanospheres

Recently, several proposals have been made to test the quantum superposition principle in the mesoscopic regime. Most of these tests consist of a careful measurement of the loss of interference due to decoherence. Here we consider, instead, the spread in position of a freely falling nanosphere. We study in depth the dependence of this spread on self-gravity in the presence of decoherence (exotic and non-exotic). We show that the influence of self-gravity is robust in the presence of weak decoherence, and quantify this robustness by introducing a new parameter, the critical decoherence, aimed at estimating the critical value above which self-gravity is overwhelmed by decoherence. We also emphasise the crucial role played by the spread of the initial wave packet for the sensitivity of free-fall experiments to decoherence.

충격파 풍동에서의 자유 낙하 장치를 활용한 힘 측정

In this paper, acceleration and pressure exerted on a human model were measured under a supersonic condition in a shock tunnel. In order to measure these in an interference-free environment, free-fall technique with an electromagnet and a three-dimensional iron-powdered human model was used. Free-fall experiment was conducted at Mach 4 and the force acting on the model was obtained by calculating the displacement from the flow visualization images.

Electrostatic Positioning System for a free fall test at drop tower Bremen and an overview of tests for the Weak Equivalence Principle in past, present and future

The Weak Equivalence Principle (WEP) is at the basis of General Relativity – the best theory for gravitation today. It has been and still is tested with different methods and accuracies. In this paper an overview of tests of the Weak Equivalence Principle done in the past, developed in the present and planned for the future is given. The best result up to now is derived from the data of torsion balance experiments by Schlamminger et al. (2008). An intuitive test of the WEP consists of the comparison of the accelerations of two free falling test masses of different composition. This has been carried through by Kuroda & Mio (1989, 1990) with the up to date most precise result for this setup. There is still more potential in this method, especially with a longer free fall time and sensors with a higher resolution. Providing a free fall time of 4.74s (9.3s using the catapult) the drop tower of the Center of Applied Space Technology and Microgravity (ZARM) at the University of Bremen is a perfect facility for further improvements. In 2001 a free fall experiment with high sensitive SQUID (Superconductive QUantum Interference Device) sensors tested the WEP with an accuracy of 10-7 (Nietzsche, 2001). For optimal conditions one could reach an accuracy of 10-13 with this setup (Vodel et al., 2001). A description of this experiment and its results is given in the next part of this paper. For the free fall of macroscopic test masses it is important to start with precisely defined starting conditions concerning the positions and velocities of the test masses. An Electrostatic Positioning System (EPS) has been developed to this purpose. It is described in the last part of this paper.

Proactive regulation of axial crushing behavior of thin-walled circular tube by gradient grooves

The progressive buckling behavior of a gradient grooved tube (GGT) designed for high temperature gas cooled reactor (HTR) is systematically studied in this paper. Based on the plastic hinge formation process, a sectionalized theoretical model is established to predict the quadratic upward trend of crushing force of GGT. The ordered and stable energy absorbing process of GGT is examined by low-speed axial impact using drop hammer test machine. The effect of proactive regulation parameters, including non-dimensional groove width W/(h0+h), groove depth h0/h and half wavelength (H+W−h)/Dh on buckling modes and force–displacement curve is determined by experiments and FE simulation. The non-dimensional groove width W/(h+h0) is determined as π/2 according to the geometry coordination of bending grooves. The non-dimensional groove depth h0/h influences local buckling behavior and the critical value 0.6 for regular bending has been determined by numerical simulation. The quadratic trend force–displacement curves of GGT can be actively controlled by (H+W−h)/Dh within a certain range less than 1.2. Free-fall impact experiments show that GGT has a better stability than perfect tube when used as an energy absorber to protect the graphite in HTR from slender control rod impact.

A perspective on motion sensors and free-fall

When an ultrasonic motion sensor is not aligned correctly with an object's motion, the geometry of the transmitted and reflected sonar signals leads to an underestimate of velocity. This is not a large effect under most teaching laboratory conditions, but compounding such an error in the production of acceleration data can produce surprisingly large overestimates. Presented here is an analysis of the general problem for free-fall experiments, followed by experimental data that show the relative importance of these errors.

Experiments on the Dynamics of Flexible Cylindrical Shells Impacting on a Water Surface

This work presents a comprehensive set of experimental results on the water entry of compliant cylindrical shells. Free fall experiments are conducted on a flexible thin cylinder varying the drop height. The problem studied here is not representative of a free cylinder as this is hold by a sledge, which acts as a concentrated mass. The impact dynamics is analyzed from accelerometers, linear position sensors, and through the analysis of high speed images. Further, an experimental methodology based on the modal decomposition method is developed and utilized to reconstruct the overall structural deformation and the distributed strain field on the base of local strain measurements. Fiber Bragg gratings are utilized for this purpose. Results show that the flexibility of the structure plays an important role on the impact dynamics, which is found to completely differ from the impact of rigid structures. The overall deformation of the shell follows the first mode shape of vibration of a free ring, while the stresses are influenced by the superposition of the higher mode shapes that are excited during the impact.

Was Galileo Right?

An important scientific debate took place regarding falling bodies hundreds of years ago, and it still warrants close examination. Galileo argued that in a vacuum all bodies fall at the same rate relative to the earth, independent of their mass. As we shall see, the problem is more subtle than meets the eye ‐ even in vacuum. In principle the results of a free fall experiment depend on whether falling masses are sequential or concurrent, whether they fall side by side or diametrically opposed. In the current paper we will present both the classical mechanics treatment and the general relativity one. In the case of classical mechanics, we start from the basic equations of motion. On the other hand, the determination of particle equations of motion in gravitational fields in general relativity is done routinely via the use of covariant derivatives. Since the geodesic equations based on covariant derivatives are derived from the Euler-Lagrange equations and since the Euler-Lagrange formalism is very intuitive, easy to derive with no mistakes, there is every reason to use them even for the most complicated situations and this is exactly what we do in the second part of the current paper.

Beam preparation for studying the gravitational behavior of antimatter at rest (GBAR)

The specific antiproton- and positron-beam requirements of the CERN AD-7 experiment, GBAR (Gravitational Behavior of Antimatter at Rest) are presented. GBAR will synthesize antihydrogen ions which will be sympathetically cooled before performing a free-fall experiment on the atom. Antiprotons delivered by CERN’s ELENA facility in 100-keV, 300-ns pulses will be electrostatically decelerated and transformed to keV energies using a pulsed drift tube. Positrons are created using a linear electron accelerator and collected into a Penning-Malmberg trap. Descriptions of these ion optical systems are given along with the status.

1P2-K03 自由落下実験による環境駆動型トルカの性能評価(宇宙ロボット)

1P2-K03 自由落下実験による環境駆動型トルカの性能評価(宇宙ロボット)

Constraints on gravitational properties of antimatter from cyclotron-frequency measurements

A fundamental question in physics that has yet to be addressed experimentally is whether particles of antimatter, such as the antiproton or positron, obey the weak equivalence principle (WEP). Several theoretical arguments have been put forward arguing limits for possible violations of WEP. No direct `classical' gravitational experiment, the measurement of the free fall of an antiparticle, has been performed to date to determine if a particle of antimatter would experience a force in the gravitational potential of a normal matter body that is different from normal gravity. 30 years ago we proposed a free fall experiment using protons and antiprotons, modeled after the experiment to measure the gravitational acceleration of a free electron. At that time we gave consideration to yet another possible observation of gravitational differences between matter and antimatter based on the gravitational red shift of clocks. I will recall the original arguments and make a number of comments pertaining to the technical problems and other issues that prevented the execution of the antiproton free fall measurement. Note that a different gravitational force on antimatter in the gravitational field of matter would not constitute a violation of CPT, as this is only concerned with the gravitational acceleration of antimatter in the gravitational field of an antimatter body.

Fruit walls and nut shells as an inspiration for the design of bio-inspired impact-resistant hierarchically structured materials

Until today the structuring of different types of fruit walls has been used only as an inspiration for packaging when seen from a biomimetic perspective. However, by a detailed investigation of the Macadamia nut with its tough testa, Citrus maxima, possessing a large spongy mesocarp and Cocos nucifera, having a combination of a fi brous mesocarp and a tough endocarp, it becomes evident that those structures also provide excellent biological role models for impact- and puncture-resistant materials. Both Citrus maxima and Cocos nucifera are relatively heavy, lack any aerodynamic adaptation and share the same challenge of having to withstand the impact from heights of >10 m. Conducting high-speed camera-controlled free fall experiments of Citrus maxima from 6 m height, we could demonstrate a deceleration of the fruits of 3100 m/s², which corresponds to 316 g, without any visible damage of the fruit. An analysis using cyclic quasi-static compression tests of the pericarp of Citrus maxima revealed that the material behaves constant in good approximation after the fi rst loading cycle. During the fi rst cycle, almost 75% of the energy is dissipated. The pericarp of Citrus maxima is highly visco-elastic, which causes the samples within 1 min to recover 30% of their initial deformation caused by loading to 40% strain. The mesocarp of Citrus maxima is best described as an open-pore foam with a gradual increase in the pore size. Understanding the principles of as to how combining the structure and material in biological constructions yields a fully functional protection layer will allow us to construct new lightweight bio-inspired materials of high impact and puncture resistance with a combination of high energy dissipation, benign failure and almost complete recovery from large deformations.

Revisiting Riccioli’s free-fall calculations

The article by Christopher Graney about the free-fall experiments of Giovanni Battista Riccioli opens a fascinating window on the work of one of the first modern physicists and his approach to devising a standard clock, finding its limitations, and using it to better determine fundamental laws of nature.Not much has changed in the clock business since 1651. Unfortunately, the data on a falling clay ball, taken in painstaking detail by Riccioli and coworkers, is slightly misrepresented in Graney’s figure 5. The time axis should be in units of seconds, not in pendulum strokes as stated. If taken seriously, the graph conveys that the ball would have fallen more than 250 Roman feet (about 75 m) in 4.5 strokes of the pendulum, less than 1 second (1 second corresponds to roughly 6 strokes of Riccioli’s pendulum).© 2013 American Institute of Physics.