Hydrocode Simulations Research Articles

Experimental Simulations of Hypervelocity Impact Penetration of Asteroids Into the Terrestrial Ocean and Benthic Cratering

AbstractSeafloor cratering is an important process that records the impact history of the Earth, affects projectile survivability, and determines the mass of ejecta from benthic rock that is transported to the atmosphere. We report experimental hypervelocity impacts of chondrite and other projectiles (olivine, stainless‐steel, polycarbonate) on a water‐covered iron target to derive a scaling relationship for benthic cratering. In situ observations of 5‐km/s impacts quantify the deceleration of projectiles in the water column by shock‐induced deformation and fragmentation. The minimum water depths at which multiple craters appeared on the benthic target were two and four times the projectile diameter for chondrite and stainless steel, respectively. Based on the observed deceleration of projectiles in water, the cratering efficiency of a benthic target for a given impact velocity is predicted to follow an exponential decay law in terms of water depth normalized by projectile diameter (H/d), given by πv ∝ exp(−(H/d)/κ), when a projectile of original mass collides with the target. Comparing the volume of the largest crater in the experiments and that derived from the scaling relation, mass ratios of the largest projectile fragment to original projectile in the 5‐km/s impact were calculated to be 0.1–0.3 (H/d = 2–6) and 1.0 ± 0.3 (H/d = 5.5) for chondrite and stainless steel, respectively. Using the scaling relationship, the volume of the transient crater on oceanic crust by an asteroid impact is estimated to be smaller than previously predicted by hydrocode simulation when the asteroid fragmentation in the water column controls seafloor cratering.

Strength of porous α-SiO2 in a shock loaded environment: Calibration via Richtmyer–Meshkov instability and validation via Mach lens

The strength of brittle porous media is of concern in numerous applications, for example, earth penetration, crater formation, and blast loading. Thus, it is of importance to possess techniques that allow for constitutive model calibration within the laboratory setting. The goal of the current work is to demonstrate an experimental technique allowing for strength assessment of porous media subjected to shock loading, which can be implemented into pressure-dependent yield surfaces within numerical simulation schemes. As a case study, the deviatoric response of distended α-SiO2 has been captured in a tamped Richtmyer–Meshkov instability (RMI) environment at a pressure regime of 4–10 GPa. Hydrocode simulations were used to interpret RMI experimental data, and a resulting pressure-dependent yield surface akin to the often employed modified Drucker–Prager model was calibrated. Simulations indicate that the resulting jet length generated by the RMI is sensitive to the porous media strength, thereby providing a feasible experimental platform capable of capturing the pressurized granular deviatoric response. Furthermore, in efforts to validate the RMI-calibrated strength model, a set of Mach-lens experiments was performed and simulated with the calibrated pressure-dependent yield surface. Excellent agreement between the resulting Mach-lens length in experiment and simulation provides additional confidence to the RMI yield-surface calibration scheme.

Determination of the JWL Parameters of Detonation Products Using Adaptive Genetic Algorithm

AbstractThe determination of Jones‐Wilkins‐Lee(JWL) equation of state (EOS)parameters for considered explosives plays a dominant role in hydrocode simulations, which are commonly used in the field of contemporary explosion mechanics. A two‐point fitting method, combined with the excellent exploratory capabilities of Adaptive Genetic Algorithm (AGA) and the cylinder energy model was developed for the determination of JWL parameters of the detonation products for all ideal condensed explosives without conducting a cylinder test. The JWL parameters can be determined by the proposed method while only the initial density and the detonation velocity of the explosive can be obtained. JWL parameters of eight explosives were determined based on this method, and the results indicated that the P‐V curves of eight fitted JWL EOSs, quantitatively evaluated by the determination coefficient, were in high agreement with that of standard JWL EOSs for all considered explosives. Moreover, the validity of the two‐point fitting method was verified by corresponding numerical simulations. It was found that the numerical results based on the fitted parameters agreed well with that based on thedata from literature. The applicability of the two‐point fitting method to a certain propellant was also discussed and the research can provide a reference for other propellants.

First observation of spalling in tantalum at high temperatures induced by high energy proton beam impacts

Spalling in tantalum has been observed in an experiment aiming at testing an antiproton production target prototype at CERN’s HiRadMat Facility. The experiment consisted in impacting 47 intense and high-energy proton beams onto a target equipped with ten cylindrical cores made of tantalum of Ø8 mm by 16 mm. Each of these proton beam impacts induced a sudden rise of temperature in the bulk of the Ta cores around 1800 °C in 0.9 μs leading to the excitation of a vibration mode which exposed their material to compressive-to-tensile pressures ranging from 2 GPa to 9 GPa, with pressure rates up to 20 GPa/μs. Post-irradiation analyses such as neutron tomography and metallographic examination of the cores, revealed the creation of voids in the bulk of the tantalum cores ranging from to 2 μm to 1 mm in diameter. These voids present a non-uniform size and density distribution within the cores, with limited growth and coalescence in areas subjected to higher temperatures and tensile pressures. Grain-growth due to a fast, thermally-induced, recrystallization has been also observed in some zones. In this work, we present a detailed characterization of the unique thermal and mechanical load that has induced this spall process by means of Finite Element and hydrocode simulations, together with post-experiment microscope and EBSD observations of the Ta rods. This analysis suggests that spall-induced void growth and coalescence is enhanced in the temperature and pressure window of 1300-1800 °C and 3-6.5 GPa, whereas is restrained at temperatures and tensile pressures above 2000 °C and 6.5 GPa. In addition, the analysis suggests that full thermal-recrystallization in tantalum can take place when exposed to temperatures above 2000 °C for less than 2 s. Four different hypotheses to explain the observed void size distribution trends are presented.

Understanding Asteroid 16 Psyche’s composition through 3D impact crater modeling

Asteroid 16 Psyche is the largest M-type (metallic) Main Belt Asteroid (MBA). Radar albedo data indicate Psyche’s surface is rich in metallic content, but estimates for Psyche’s bulk structure vary widely, with bulk density estimates ranging from 1.4 ± 0.3 g/cm3 to 4.5 ± 1.4 g/cm3 and porosity estimates ranging from 30% to 70%. A forthcoming NASA discovery mission will visit Psyche. Narrowing the range of estimated compositions of Psyche prior to mission launch can help to determine the necessary tools for asteroid analysis. Psyche has two large impact structures in its Southern hemisphere, with estimated diameters from 50 km to 70 km and estimated depths up to 6.4 km. In this work, we present results from 2D and 3D hydrocode simulations of the largest of these impact structures. Through quantitative comparison of the simulated crater dimensions with measured values, our models suggest that Psyche is largely composed of porous, metallic material. In addition, to match the crater dimensions, our simulations indicate that the angle of impact was significantly altered from vertical by 45–60 degrees.

Thermomechanical response of thickly tamped targets and diamond anvil cells under pulsed hard x-ray irradiation

In the laboratory study of extreme conditions of temperature and density, the exposure of matter to high intensity radiation sources has been of central importance. Here, we interrogate the performance of multi-layered targets in experiments involving high intensity, hard x-ray irradiation, motivated by the advent of extremely high brightness hard x-ray sources, such as free electron lasers and 4th-generation synchrotron facilities. Intense hard x-ray beams can deliver significant energy in targets having thick x-ray transparent layers (tampers) around samples of interest for the study of novel states of matter and materials’ dynamics. Heated-state lifetimes in such targets can approach the microsecond level, regardless of radiation pulse duration, enabling the exploration of conditions of local thermal and thermodynamic equilibrium at extreme temperature in solid density matter. The thermal and mechanical responses of such thick layered targets following x-ray heating, including hydrodynamic relaxation and heat flow on picosecond to millisecond timescales, are modeled using radiation hydrocode simulation, finite element analysis, and thermodynamic calculations. Assessing the potential for target survival over one or more exposures and resistance to damage arising from heating and resulting mechanical stresses, this study doubles as an investigation into the performance of diamond anvil high pressure cells under high x-ray fluences. Long used in conjunction with synchrotron x-ray radiation and high power optical lasers, the strong confinement afforded by such cells suggests novel applications at emerging high intensity x-ray facilities and new routes to studying thermodynamic equilibrium states of warm, very dense matter.

Generating metal-polluting debris in white dwarf planetary systems from small-impact crater ejecta

ABSTRACT Metal pollution in white dwarf photospheres originates from the accretion of some combination of planets, moons, asteroids, comets, boulders, pebbles, and dust. When large bodies reside in dynamically stagnant locations – unable themselves to pollute or even closely approach the white dwarf – then smaller reservoirs of impact debris may become a complementary or the primary source of metal pollutants. Here, we take a first step towards exploring this possibility by computing limits on the recoil mass that escapes the gravitational pull of the target object following a single impact onto an atmosphere-less surface. By considering vertical impacts only with the full-chain analytical prescription from Kurosawa & Takada, we provide lower bounds for the ejected mass for basalt, granite, iron, and water-rich target objects across the radii range 100–103 km. Our use of the full-chain prescription as opposed to physical experiments or hydrocode simulations allows us to quickly sample a wide range of parameter space appropriate to white dwarf planetary systems. Our numerical results could be used in future studies to constrain freshly generated small debris reservoirs around white dwarfs given a particular planetary system architecture, bombardment history, and impact geometries.

Response of a new structural ultra-high performance concrete barrier wall subjected to blast loading

ABSTRACTUltra-high performance concrete (UHPC) is a promising construction of newly developed material. In this study, a nonlinear three-dimensional hydro-code numerical simulation was carried out using autodyn-3d. In this simulation, a high explosive material (TNT) is blasted against two concrete panels. The model was first validated using experimental tests carried out by Jun Li and has shown good results. Two numerical models were carried out to investigate the effect of using ultra-high performance concrete in mitigating the effect of blast waves. The reflected overpressure and post-test damage were observed. The results showed that UHPC barrier walls performed a lot better than normal strength concrete (NSC) barrier walls in blast loading resistance. Furthermore, the damage that the concrete barrier wall experiences after the detonation decreases a lot as UHPC is used which affects the integrity of the concrete wall barrier and limits the amount of fragments ejected from the concrete barrier wall as it is one of the serious harmful effects that causes casualties among humans and structures.

Ballistic impact on fired clay masonry bricks

We perform an experimental and numerical study of ballistic impact on the German fired clay brick material VMz28. Parameters for the concrete material model applied in hydrocode simulations stem from material characterization experiments, especially planar plate impact experiments, engineering assumptions, and parameter studies. The performance of the numerical model is illustrated by a comparison of its results to ballistic data on penetration and perforation of spherical steel projectiles. Agreement is found between experiment and simulation for residual velocities and penetration depths. The experimental damage patterns are reproduced in the simulation with sufficient agreement. The application of the model to the impact of another differently shaped generic projectile reveals good predictive capabilities in a part of the impact velocity range. For the deviation found in the other part, limitations of the model concerning the full reproduction of the observed spall failure are discussed as a possible explanation. Overall, with moderate experimental effort, a numerical model with reasonably good predictive capabilities is obtained for engineering purposes.

Prediction of Probabilistic Detonation Threshold via Millimeter‐Scale Microstructure‐Explicit and Void‐Explicit Simulations

AbstractWe present an approach and relevant models for predicting the probabilistic shock‐to‐detonation transition (SDT) behavior and Pop plot (PP) of heterogeneous energetic materials (HEM) via mesoscopic microstructure‐explicit (ME) and void explicit (VE) simulations at the millimeter (mm) sample size scale. Although the framework here is general, the particular material considered in this paper is pressed Octahydro‐1,3,5,7‐tetranitro‐1,2,3,5‐tetrazocine (HMX). To systematically delineate the effects of material heterogeneities, four material cases are considered. These cases are homogeneous material, material with granular microstructure but no voids, homogeneous material with voids, and material with both granular microstructure and voids. Statistically equivalent microstructure sample sets (SEMSS) are generated and used. Eulerian hydrocode simulations explicitly resolve the material heterogeneities, voids, and the coupled mechanical‐thermal‐chemical processes. In particular, it is found that both microstructure and voids strongly influence the SDT behavior and PP. The effects of different combinations of microstructure heterogeneity and voids on the SDT process and PP are quantified and rank‐ordered. The overall framework uses the Mie–Grüneisen equation of state and a history variable reactive burn model (HVRB). A novel probabilistic representation for quantifying the PP is developed, allowing the calculation of (1) the probability of observing SDT at a given combination of shock pressure and run distance, (2) the run‐distance to detonation under a given combination of shock pressure and prescribed probability, and (3) the shock pressure required for achieving SDT at a given run distance with a prescribed probability. The results are in agreement with general trends in experimental data in the literature.

Modelling the Fluctuations of Reactive Shock Waves in Heterogeneous Solid Explosives as Stochastic Processes

AbstractWhile current phenomenological burn models are useful for describing the average or bulk reactive flow behaviour of heterogeneous explosives, one fundamental weakness inherent to these models is the loss of detailed microstructural information at the scale of the calculation. In order to include the effects of the microstructure, and in particular the underlying material heterogeneities that influence the build‐up to detonation, a new paradigm is put forth for modelling sub‐grid, reaction‐induced fluctuations (i. e. “hot spots”) at the continuum level. This modelling approach assumes that the reaction rate is stochastic, rather than deterministic, and it uses Langevin‐type equations with a mathematical framework built upon Itô calculus and Lambourn's CIM model for shocked heterogeneous explosives. This approach follows directly from our previous letter, Ref. [1], and is inspired by the probability density function (pdf) methods used in turbulent reactive flows. Here, the stochastic burn model is derived, implemented, and exercised far beyond what has been shown in previous work. New hydrocode simulation results demonstrate the role of stochastic fluctuations during shock initiation; these fluctuations are approximated by collections of discrete particles, that evolve with drift (i. e. deterministic) and diffusion (i. e. stochastic) coefficients. Additionally, the particle values are propagated and averaged to calculate the heat release, yield strength, and material impedance in each computational cell. Hydrocode simulation results further show how the fluctuating hot spot energy may or may not be transmitted to the wave front, and result in a detonation wave‐like structure. The fundamental stochastic nature of this model permits simulations to have varying outcomes with the same initial conditions; this allows for go/no‐go estimation (e. g., marginal or failed detonations), which might possibly be calibrated using the statistical distributions from real materials. Mesoscale calculations of shocked heterogeneous explosives also show that these fluctuations are physically justified (i. e., Ref. [2]), and it is hypothesized that the pdf functions provide a link between the meso(grain) and continuum scales for practical engineering calculations. Thus, our approach is a paradigm shift for building efficient, reduced order continuum burn models, that represent the sub‐grid stochastic behaviour of shocked heterogeneous solid explosives.

Constraints on terrestrial planet formation timescales and equilibration processes in the Grand Tack scenario from Hf-W isotopic evolution

We examine 141 N-body simulations of terrestrial planet late-stage accretion that use the Grand Tack scenario, coupling the collisional results with a hafnium-tungsten (Hf-W) isotopic evolution model. Accretion in the Grand Tack scenario results in faster planet formation than classical accretion models because of higher planetesimal surface density induced by a migrating Jupiter. Planetary embryos that grow rapidly experience radiogenic ingrowth of mantle 182W that is inconsistent with the measured terrestrial composition, unless much of the tungsten is removed by an impactor core that mixes thoroughly with the target mantle. For physically Earth-like surviving planets, we find that the fraction of equilibrating impactor core kcore≥ 0.6 is required to produce results agreeing with observed terrestrial tungsten anomalies (assuming equilibration with relatively large volumes of target mantle material; smaller equilibrating mantle volumes would require even larger kcore). This requirement of substantial core re-equilibration may be difficult to reconcile with fluid dynamical predictions and hydrocode simulations of mixing during large impacts, and hence this result does not favor the rapid planet building that results from Grand Tack accretion.

Dynamic response of aluminum honeycomb sandwich panels subjected to hypervelocity impact by porous volcanic rock projectile

The dynamic response and damage behavior of aluminum honeycomb sandwich panels (HC/SPs) subjected to hypervelocity impact by volcanic rock projectiles were investigated by hypervelocity impact tests and hydrocode simulations. The experiments were conducted using a two stage light gas gun and the results showed that the failure modes in HC/SPs subjected to hypervelocity impact by volcanic rock projectiles mainly took forms of front-face denting and circular perforation, honeycomb core collapsing and rapture, rear-face petal-ling and perforation etc. A 3D discrete configuration of the porous volcanic rock projectiles was set up. The hypervelocity impact behavior of the HC/SPs was investigated through hydrocode modeling, within a Lagrange-SPH coupling method in LS-DYNA solver. It was found that the dynamic response and failure modes in the HC/SPs were significantly influenced by the impact location and the impact velocity of the volcanic rock projectile.

Shock wave propagation in layered planetary interiors: Revisited

While major impacts during late accretion of a Mars type planet occur on a differentiated body, the characteristics of the shockwave propagation are poorly known within these layered objects. Here, we use iSALE-2D hydrocode simulations to calculate shock pressure in a differentiated Mars type body for impact velocities ranging from 5 to 20 km/s, impactor radii ranging from 50 to 200 km, and different rheologies. To better represent the distribution of shock pressure as a function of distance from the impact site at the surface, we propose two distinct regions in the mantle: a near field region that extends to 7–15 times the projectile radius into the target, where the peak shock pressure decays exponentially with increasing the distance from the impact site, and a far field region where the pressure decays strongly with the distance following a power law. At the core-mantle boundary, the peak shock pressure increases from the mantle side to the core side. The refracted shockwave travels within the core where the shock pressure decreases following a second power law. In this study, we fit the output obtained from iSALE hydrocode simulations to determine scaling laws that illustrate the influence of the distance from the impact site, the ray angle, the target rheology, the impactor size and the impact velocity. Finally we combine these shock-pressure scaling laws with the formalism proposed by Watters et al. [2009] to determine the impact heating induced by large impacts within a differentiated Mars.

Observation of the Velocity Variation of an Explosively-Driven Flat Flyer Depending on the Flyer Width

Lim proposed a theoretical study on the velocity profile of an explosively-driven flat flyer affected by the rarefaction (or release wave) intrusion during the metal’s projection. This work shows somewhat reasonable agreement in a given range. However, this work is limited only in the early stage of detonation (~3 µs), and the larger scaled flyer projection (or extended time duration) behavior is needed for an engineering perspective. As continued work originating from this investigation, the velocity profile of explosively-driven flyers with different widths is studied based on multiple different approaches which include hydrocode simulation, the Gurney model, and Baum’s side loss correction (or effective charge mass approach), followed by a series of field experiments. In this study, the focus is on the observation of the flat flyer velocity (or terminal velocity) variation, depending on the width of the flyers which vary from 12, 25, 50, and 75 mm (or 100 mm). The terminal velocity profile variation, depending on the flyer width, is observed, and a general trend is identified.

Bayesian calibration of strength parameters using hydrocode simulations of symmetric impact shock experiments of Al-5083

Predictive modeling of materials requires accurately parameterized constitutive models. Parameterizing models that describe dynamic strength and plasticity require experimentally probing materials in a variety of strain rate regimes. Some experimental protocols (e.g., plate impact) probe the constitutive response of a material using indirect measures such as free surface velocimetry. Manual efforts to parameterize constitutive models using indirect experimental measures often lead to non-unique optimizations without quantification of parameter uncertainty. This study uses a Bayesian statistical approach to find model parameters and to quantify the uncertainty of the resulting parameters. The technique is demonstrated by parameterizing the Johnson-Cook strength model for aluminum alloy 5083 by coupling hydrocode simulations and velocimetry measurements of a series of plate impact experiments. Simulation inputs and outputs are used to calibrate an emulator that mimics the outputs of the computationally intensive simulations. Varying the amount of experimental data available for emulator calibration showed clear differences in the degree of uncertainty and uniqueness of the resulting optimized Johnson-Cook parameters for Al-5083. The results of the optimization provided a numerical evaluation of the degree of confidence in model parameters and model performance. Given an understanding of the physical effects of certain model parameters, individual parameter uncertainty can be leveraged to quickly identify gaps in the physical domains covered by completed experiments.

Hydrocode simulations of asteroid airbursts and constraints for Tunguska

A wide range of meteors were simulated impacting Earth's atmosphere using the ALE3D hydrocode. The size, density, strength curve, entry angle, and velocity of the meteors were varied to cover the parameter space of airbursts of interest to planetary defense in general which encompasses the Tunguska event in particular. The hydrocode simulations were used to calibrate a simple analytical model that can be used to quickly estimate the burst height of an incoming asteroid or comet.The hydrocode simulations show both rocky asteroids and icy comets are plausible Tunguska meteors, over a wide range of sizes, speeds, entry angles, at least in terms of the energy delivered and height of burst required to produce the tree-fall, and the lack of an obvious impact crater on the ground. This agrees with previous analytical estimates and hydrocode simulations of hypothetical Tunguska meteors, and provides a complete range of possibilities.Modelling of the tree wind resistance predicts windspeeds of 40–50 m/s are required to fell most of the trees. This is slower than previous estimates from nuclear test data, but faster than lower bound estimates from a 3 Mt blast. It suggests an energy of 10 Mt is most likely for Tunguska, but requires missing tree size distribution and wind response data to better narrow the probability distribution of potential Tunguska impactors.

Near and far-field hazards of asteroid impacts in oceans

Hazards resulting from asteroid ocean impacts were modelled using hydrocode simulations to examine the near-field effects including the initial formation and subsequent long range propagation of tsunami waves that can transport potentially damaging energy far from the impact site.Three-dimensional simulations of oblique impacts into deep water, with trajectory angles ranging from 27° to 60° above the horizontal, were performed with the Los Alamos Rage hydrocode. The simulations include atmospheric effects such as ablation and airbursts. These oblique impact simulations are performed in order to help determine whether there are additional dangers due to the obliquity of impact not covered by previous studies. The energy transferred to both the air blast wave and the water are calculated as well as the amount of sea water lofted into the upper atmosphere. Water crater sizes and subsequent surface elevation profiles, surface pressures, and depth-averaged mass fluxes within the water are prepared for use in far-field propagation studies. Like previous three-dimensional simulations, these simulations show that except at exceedingly shallow entry angles below those simulated here the resulting waves are roughly circular and that the initial waves and central jet oscillation are highly turbulent and dissipate a lot of the energy.Two-dimensional axisymmetric simulations of long range propagation of impact tsunami were performed using the Lawrence Livermore ALE3D hydrocode on the NASA Pleiades supercomputer. These simulations showed that impacts under 1 gigaton TNT equivalent into the deep ocean basins will create deep-water waves that undergo dispersion, whereas impacts onto continental shelves will create shallow-water waves that do not suffer dispersion. The simulations also showed that on the order of 1% of the kinetic energy of the impact is converted into the tsunami wave. This is an order of magnitude less than previous semi-empirical estimates of ∼15% based on explosion test data and laboratory scale impacts.

Post-impact thermal structure and cooling timescales of Occator crater on asteroid 1 Ceres

Occator crater is perhaps the most distinct surface feature observed by NASA's Dawn spacecraft on the Cerean surface. Contained within the crater are the highest albedo features on the planet, Cerealia Facula and Vinalia Faculae, and relatively smooth lobate flow deposits. We present hydrocode simulations of the formation of Occator crater, varying the water to rock ratio of our pre-impact Cerean surface. We find that at water to rock mass ratios up to 0.3, sufficient volumes of Occator's post-impact subsurface would be above the melting point of water to allow for the deposition of faculae-like deposits via impact-heat driven hydrothermal effusion of brines. This reservoir of hydrothermally viable material beneath the crater is composed of a mixture of impactor material and material uplifted from 10′s of kilometers beneath the pre-impact surface, which could sample a deep subsurface volatile reservoir, if present. Using a conductive cooling model, we estimate that the lifetime of hydrothermal activity within such a system, depending on choice of material constants, is between 0.4 and 4 Myr. Our results suggest that impact heating from the Occator forming impact provides a viable mechanism for the creation of the observed faculae, with the proviso that the faculae formed within a relatively short time window after the crater itself formed.

Effect of peak stress and tensile strain-rate on spall in tantalum

Materials subjected to dynamic environments experience a complex and wide range of stress, strain, and strain-rate conditions. To have confidence in material models, an accurate, predictive capability is required. In this study, we present a series of flyer-plate impact tests on well characterized, high purity tantalum. The shock-waves generated at impact release from the free-surfaces, reflect, and interact to produce incipient spall fracture. By varying the flyer-plate material and impact velocity, both the peak stress and the strain-rate in the samples were controlled independently. Velocimetry was used on the rear free-surface of the samples to measure the shock-response and the spall strength. While this measurement provided the same spall strength for all cases, at approximately 5.1 GPa, when the samples were sectioned during post-mortem, the quantity and distribution of internal damage was markedly different. For the high-strain rate cases, voids remained small and isolated, whereas in the lower strain-rate experiments, the spall damage was far more localized, with a well-defined continuous spall plane. With the use of hydrocode simulations, this was discovered to result from how the different release rates affect the interaction volume inside the sample. These results highlight the importance of careful sample recovery, and the risks of relying solely on free-surface velocity measurements.