Asymmetric Collapse Research Articles

Effects of axial launch spacing on cavitation interference and load characteristics during underwater salvo

This paper analyzes the effect of launch interval on cavitation flow interference and load characteristics during underwater salvo. The study employs the Improved Delayed Detached Eddy Simulation and the Schnerr-Sauer cavitation model, Volume of Fluid (VOF) multiphase flow model, and overlapping grid. Additionally, decompression experiment systems are designed, and numerical simulations are found to be in good agreement with experimental results, thus verifying the effectiveness of the simulation. Detailed discussions are provided on multiphase flow field and load distribution. The results reveal a top-down collapse process of the cavity, with collapse shrinking to an isolated bubble at the end. Synchronized collapse pressure is characterized by short pulse widths at the peaks, all located at the lowermost part of the cavity. During the underwater stage, when the axial launch spacing ranges between 0.5 times and 1.0 times the length of the projectile, the head of the second projectile acts on the area below the center of mass of the first. This leads to gradual stabilization of the initial cavity and a decrease in deviation of the center of mass toward the inside. Despite experiencing large-scale fracture and detachment due to interference from the wake of the first engine, the motion stability of the inside cavity of the second projectile remains intact. In the water exit stage, when the axial launch spacing ranges between 0.75 times and 1 time the length of the projectile, it causes expansion and contraction of the inside cavity of the second projectile. However, asymmetric synchronous collapse loads may occur, leading to unstable motion posture.

Impact force-damage depth model for ship-bridge collision

The impact force-damage depth model, i.e., P-a model, provides a feasible approach to design and evaluate bridge resistance under ship collisions, which is inadequately addressed in the existing specifications and literature. In the present study, based on an 18500 DWT (deadweight tonnage) bulbous bow bulk carrier and the main tower of cable-stayed bridges, the P-a model for ship-bridge collision is established with consideration of pile cap, bridge pylon and striking ship. Firstly, the refined finite element (FE) model of prototype ship was established with detailed modeling of both the main and stiffening components. The material model parameters and FE analysis approach were validated through the existing tensile tests on marine steel coupons and crush tests on ship bow specimens. It is found that the modeling of stiffening components has rationality in predicting the impact force and asymmetrical folding collapse of ship bow. Then, concerning two collision scenarios with the main tower of cable-stayed bridges, i.e., only bulbous bow-pile cap collision and sequential bulbous bow-pile cap and bow overhang-bridge pylon collisions, the influences of nine critical parameters from pile cap (height, curvature and water level), bridge pylon (width, curvature, angle and bow overhang-bridge pylon distance) and striking ship (mass and velocity) on the impact forces and failure patterns of ship bow were comprehensively discussed. It indicates that: (i) the larger impact forces of bulbous bow and bow overhang appear with increased pile cap height, decreased bridge pylon curvature and inward inclined pylon angle; (ii) the loading paths of P-a relationships are approximately identical with different striking ship masses and velocities. Finally, the P-a model for ship-bridge collision was established by response surface methodology, and the corresponding calculation procedure was further given and validated.

Experimental research into the dynamics of underwater explosion bubbles near mutually perpendicular walls

It is of great significance to characterize the dynamics of underwater explosive bubbles in close proximity to mutually perpendicular walls for ensuring the safety of important underwater structures. In this paper, a dynamic experiment on underwater explosion bubbles was carried out near constructed mutually perpendicular walls. High-speed cameras were utilized to capture high-resolution images, while pressure sensors recorded pressure–time history curves. The main focus was on studying the evolution process of bubble morphology and pulse characteristics. When the position of the charge's center relative to the horizontal wall remained fixed, decreasing the distance between the charge's center and the vertical wall resulted in a reduction in the equivalent maximum radius of bubbles and an increase in its pulsation period. Additionally, the asymmetric collapse of bubbles on a single wall transformed into asynchronous collapse on two walls, with most collapsed bubbles tending to migrate and expand toward the corner formed by mutually perpendicular walls. The resulting jet from the collapse of bubbles exhibited deflection toward the vertical wall, with an inclination angle increasing approximately proportionally with dimensionless distance ratio γh/γv. Moreover, it became more difficult for achieving effective focusing of bubble energy as the jet approached the corners formed by mutually perpendicular walls. The experiments also implied that reducing the dead weight of the vertical wall weakened its contact with the horizontal wall, causing an increase in the equivalent maximum radius of bubbles and jet inclination, as well as a decrease in the bubble pulsation period, under the same dimensionless distance γv.

Dynamics of a bubble-pair between two parallel rigid walls

The dynamics of two gas bubbles placed side-by-side in liquid between parallel rigid plates is studied numerically using the multiphase volume of fluid (VOF) model in OpenFOAM. This model solves a single set of Navier–Stokes equations for both the gas in the bubbles and the surrounding liquid. Being initially stationary, the bubbles first expand to their maximum volume due to the high gas pressure inside them and then collapse. Both phases are treated as viscous and immiscible without any phase change. The model is validated against available experimental data of a single bubble oscillation between parallel plates and a bubble-pair oscillation near a single rigid wall. Parametric studies are then performed, with the main variables being the initial sizes of the bubbles and their standoff distances from the rigid plates. The bubbles exhibit symmetric or asymmetric behavior depending on these two variables. Three distinct scenarios are identified in the symmetric case based on the relative influence of bubble-bubble and bubble-wall interactions. The jet strength and direction, bubble oscillation period, coalescence, and early asymmetric bubble collapse are characterized by initial distances and radii. This study provides valuable insights into intricate hydrodynamic interactions and behaviors of bubble pairs bounded by rigid surfaces.

Above-Tg Annealing Benefits in Nanoparticle-Stabilized Carbon Molecular Sieve Membrane Pyrolysis for Improved Gas Separation.

Nanoparticles can suppress asymmetric precursor support collapse during pyrolysis to create carbon molecular sieve (CMS) membranes. This advance allows elimination of standard sol-gel support stabilization steps. Here we report a simple but surprisingly important thermal soaking step at 400 °C in the pyrolysis process to obtain high performance CMS membranes. The composite CMS membranes show CO2 /CH4 (50 : 50) mixed gas feed with an attractive CO2 /CH4 selectivity of 134.2 and CO2 permeance of 71 GPU at 35 °C. Furthermore, a H2 /CH4 selectivity of 663 with H2 permeance of 240 GPU was achieved for promising green energy resource-H2 separation processes.

Above‐Tg Annealing Benefits in Nanoparticle‐Stabilized Carbon Molecular Sieve Membrane Pyrolysis for Improved Gas Separation

AbstractNanoparticles can suppress asymmetric precursor support collapse during pyrolysis to create carbon molecular sieve (CMS) membranes. This advance allows elimination of standard sol‐gel support stabilization steps. Here we report a simple but surprisingly important thermal soaking step at 400 °C in the pyrolysis process to obtain high performance CMS membranes. The composite CMS membranes show CO2/CH4 (50 : 50) mixed gas feed with an attractive CO2/CH4 selectivity of 134.2 and CO2 permeance of 71 GPU at 35 °C. Furthermore, a H2/CH4 selectivity of 663 with H2 permeance of 240 GPU was achieved for promising green energy resource‐H2 separation processes.

Using ultrasound to increase copper and nickel dissolution and prevent passivation using concentrated ionic fluid.

This paper uses targeted ultrasound on a surface undergoing anodic dissolution. The aim of these experiments was to etch metals that would normally passivate. The study was carried out in deep eutectic solvents (DESs) as these are technologically useful for metal processing but suffer the disadvantages of being viscous and hence have slow mass transport which results in metal passivation. Linear sweep voltammetry showed a linear current-voltage response at potentials anodic (more positive) of the oxidation potential under sonication. Passivation was observed in silent conditions. It was also shown that the dissolution current was roughly 14 times larger under sonication. High speed imaging showed asymmetric bubble collapse leading to enhanced removal of material from the surface with etch rates as high as 3.8 μm.min–1 under ultrasonic conditions.

The mechanisms of jetting, vortex sheet, and vortex ring development in asymmetric bubble dynamics

Bubble dynamics near a rigid boundary at Reynolds numbers of O(10–100) exhibit significant viscous effect, associated with ultrasonic cavitation and cavitation damage. We study this phenomenon experimentally using high-speed photography of spark-generated bubble oscillation in silicone oils, whose viscosity is about three orders larger than water. Comparing to bubbles in water, bubble surfaces in silicone oil are more stable and thus more cycles of oscillations may be observed and studied. Additionally, we investigate this phenomenon numerically using the volume of fluid method. We propose a non-reflective boundary condition, reducing the computational domain's dimensions tenfold based on the far-field asymptotic behavior. This paper pays particular attention in the mechanism for the bubble jetting, the vortex sheet, and the vortex ring development. Initially, a stagnation point at the bubble center moves away from the wall owing to asymmetric bubble expansion, leaving the bubble around the moment the bubble reaches its maximum volume. During this process, a vortex sheet forms inside the bubble. As the vortex sheet approaches the bubble interface, it transfers momentum to the gas–liquid interface, influencing the flow near the bubble wall. The high-pressure zone at the stagnation point drives the distal bubble surface to collapse first and fastest subsequently. This asymmetric collapse generates circulation around the bubble's side cross section, leading to the development of a vortex ring within the bubble gas at the outer rim of the decaying vortex sheet. The vortex ring, with its core inside the bubble gas, functions like a bearing system in accelerating the jet.

Flow and temporal effects on the sonolytic defluorination of perfluorooctane sulfonic acid

The removal of per- and polyfluoroalkyl substance (PFAS) pollution from the environment is a globally pressing issue, due to some PFAS’ recalcitrant, bioaccumulative, and carcinogenic nature. Destruction via ultrasonic waves (sonolysis) is a promising contender for industrialisation due to; moderate power consumption, applicability to several PFAS and sample types, and limited by-products. Liquid flow rate through an ultrasonic reactor can affect the size, shape, and spatial distribution of ultrasonic cavities and hence their chemical activity. Such effects have not been studied during PFAS sonolysis, and temporal effects have not been studied much beyond the reactant concentration. Here, the effects of varying recirculating flow rate on the ultrasonic defluorination of perfluorooctane sulfonic acid (PFOS) and implications for industrial scale up are presented. Under the ultrasonic power (200 W L−1, 2.27 W cm−2) and frequency (410 kHz) used, flow rates of 79 and 214 ml min−1 enhanced defluorination up to 14 % during 30 min of treatment. However, these effects were temporal and most significant in the initial minutes of treatment. This indicated a dynamic bubble size distribution which stabilised after around 15 min. Defluorination rates of PFOS were compared with measured potassium iodide dosimetry, calorimetry, sonoluminescence (SL), and sonochemiluminescence (SCL). Flow rates which enhanced defluorination correlated moderately with enhanced SCL and negatively impacted SL, calorimetry, and dosimetry. Effects were attributed to perturbed cavity surfaces, leading to asymmetric cavity collapse, and the possibility of enhanced solvated electron production/interaction. SL, SCL, dosimetry, and calorimetric measurements were also temporal, and each showed different times to equilibrate. Flow rates of 439 and 889 ml min−1 returned all sonochemical measurements to the levels without flow, likely due to continued collapse temperature quenching by furthered bubble asymmetry. Flow also enhanced reactor cooling, which is significant for industrial temperature control. The pump energy consumed was small (≈1.9 %) compared to that of the amplifier and chiller, hence, PFOS defluorination was more cost-effective using flow. However, the effect may be limited for the longer treatment times needed for environmental remediation.

Numerical investigation of ultrasound focusing and bubble collapse

Ultrasound focusing and microbubble collapse are numerically investigated using a level-set interface tracking method for two-phase flows with multiple interfaces. The computations for ultrasound propagating through a spherical lens demonstrate the ultrasound refraction and pressure intensification at the rear of the lens. The focusing of the initial negative pressure wave through the lens induces a converging flow and the focusing of the subsequent positive pressure wave further intensifies the pressure at the lens. Computations are extended to bubble oscillations near the focusing lens and compared with the no-lens case. The lens not only amplifies the bubble expansion and contraction rates significantly but also generates a larger pressure gradient across the bubble. This ultrasound focusing effect contributes to the asymmetric collapse of the bubble and the formation of a liquid jet that penetrates the bubble. The effects of lens size, initial bubble radius and bubble-lens distance on bubble expansion and liquid jet are further investigated.

Numerical assessment of cavitation erosion risk on the Delft twisted hydrofoil using a hybrid Eulerian-Lagrangian strategy

Cavitation damage is a major threat to the life span of fluid machinery and has been a hot research issue in engineering for a long time. The current work aims to show the ability to assess cavitation erosion risk on the twisted hydrofoil using a hybrid Eulerian-Lagrangian solver. The volume of fluid method is used for the liquid-vapor interface of resolved vapor structures, while the discrete bubble model is utilized to track micro-scale unresolvable bubbles. A two-way coupling approach is introduced to enable the transition between resolved cavities and bubbles based on their volume, relative location, and shape. A Lagrangian erosion model considering the effect of asymmetric bubble collapse is proposed to predict cavitation erosion risk. Compared with the experimental result, the current model can accurately identify the high erosion risk regions on the hydrofoil surface. The distribution and intensity of the high impact pressures emitted from bubbles is quantitatively evaluated. In addition, the erosion sensitive zones at different stages of the one cloud cavitation cycle are determined and the hydrodynamic mechanisms of aggressive collapse events are analyzed in detail. The results reveal that the potential erosion risk is highest in areas where primary shedding occurs, which causes the macroscopic cavity to roll up. Bubbles with high impact pressures are mainly focused around the edges of the shedding cavity. The secondary shedding contributes to erosive structure in a limited middle angle of attack area, such that when the primary shedding U-shaped structure evolves and begins to collapse, it becomes less erosive, and only isolated points of erosion is found. Further investigation demonstrates that this is due to a transformation from U-shaped vortex to O-shaped vortex, moving the bubbles gradually away from the hydrofoil surface, thereby reducing the cavitation erosion risk.

Study on the effect of off-axis initiation on the jet forming process and armour-breaking performance of shaped charges

To study the effect of off-axis initiation on shaped charge performance. The nonlinear finite element software LS-DYNA is used to simulate the jet forming and armour-breaking process under different initiation offsets (0.025~0.125Dk, Dk is shaped charge diameter). A theoretical model is established to analyse the lateral velocity distribution of jet. The results show that the degree of asymmetric collapse and lateral velocity of jet increase with the offset. When the initiation offset exceeds 0.025Dk, the jet penetration capability of shaped charge jet will be seriously weakened.

Dissimilar cavitation dynamics and damage patterns produced by parallel fiber alignment to the stone surface in holmium:yttrium aluminum garnet laser lithotripsy.

Recent studies indicate that cavitation may play a vital role in laser lithotripsy. However, the underlying bubble dynamics and associated damage mechanisms are largely unknown. In this study, we use ultra-high-speed shadowgraph imaging, hydrophone measurements, three-dimensional passive cavitation mapping (3D-PCM), and phantom test to investigate the transient dynamics of vapor bubbles induced by a holmium:yttrium aluminum garnet laser and their correlation with solid damage. We vary the standoff distance (SD) between the fiber tip and solid boundary under parallel fiber alignment and observe several distinctive features in bubble dynamics. First, long pulsed laser irradiation and solid boundary interaction create an elongated "pear-shaped" bubble that collapses asymmetrically and forms multiple jets in sequence. Second, unlike nanosecond laser-induced cavitation bubbles, jet impact on solid boundary generates negligible pressure transients and causes no direct damage. A non-circular toroidal bubble forms, particularly following the primary and secondary bubble collapses at SD = 1.0 and 3.0 mm, respectively. We observe three intensified bubble collapses with strong shock wave emissions: the intensified bubble collapse by shock wave, the ensuing reflected shock wave from the solid boundary, and self-intensified collapse of an inverted "triangle-shaped" or "horseshoe-shaped" bubble. Third, high-speed shadowgraph imaging and 3D-PCM confirm that the shock origins from the distinctive bubble collapse form either two discrete spots or a "smiling-face" shape. The spatial collapse pattern is consistent with the similar BegoStone surface damage, suggesting that the shockwave emissions during the intensified asymmetric collapse of the pear-shaped bubble are decisive for the solid damage.

Numerical modeling of high-temperature melt droplet interaction with water

Three-dimensional numerical simulations are performed for the interaction of a hot melt droplet with water. Three-phase melt-water–vapor flows are described by the Volume-of-Fluid (VOF) model implemented in OpenFOAM software. In the calculations, initially non-moving and moving melt droplets were considered. Interaction was triggered by sharp increase in water pressure, leading to collapse of the vapor film. It is shown that melt droplet motion with respect to water affects the fragmentation mechanism. For a non-moving droplet, symmetrical collapse occurs, followed by melt surface perturbation and ejection of small melt jets. The estimated diameter of vapor bubble agrees with experimental observations on single droplet steam explosions. For moving droplets, asymmetric vapor bubble collapse generates a high-speed cumulative water jet penetrating into the droplet and fragmenting it. The effect of initial droplet velocity is investigated, and comparison of melt droplet shapes and surface areas for moving and non-moving droplets is presented.

Large amplitude oscillatory shear (LAOS) and Fourier transform rheology of ultrasonically extruded epoxidized soybean oil colloids containing organically modified nanoclay

LAOS studies were carried out on untreated and ultrasonically treated ESO/nanoclay 30B colloids obtained by means of an ultrasonic twin-screw extruder at 100 and 400 rpm and ultrasonic amplitudes of 5, 10 and 13 μm. Obtained results were used to perform Fourier transform. Waveform, 3D stress response, elastic and viscous Lissajous curves show significant differences between the untreated colloids and colloids treated at 10 and 13 μm. G1′ and G1″ of colloids are increased with increase of ultrasonic amplitude, as a result of better nanoclay dispersion at higher ultrasonic amplitudes. A higher screw rotation speed also improved nanoclay dispersion for untreated colloid and colloid treated at 5 μm. However, for colloid treated at 10 and 13 μm, a lower screw rotation speed resulted in a better dispersion. This is due to less asymmetric bubble collapse and more jet flow in ultrasonic zone at lower screw speed, which promotes dispersion. Colloids treated at 10 and 13 μm showed yielding and then a non-linearity at a higher strain amplitude compared with untreated colloids. The values of GM′ and ηL′ of colloids calculated from intracycle data exhibited the densification and filler network breakup much more observable than those seen on values of G1′ and G1″. This means GM′ and ηL′ are more suitable for description of colloids with a two-step yielding behavior. Parameters S and T calculated based on intracycle data showed enormous differences in the strain hardening/softening and shear thickening/thinning behaviors between the untreated colloid and the colloids treated at 10 and 13 μm.

Asymmetric Compassion Collapse, Collateral Consequences, and Reintegration: An Experiment

Public opinion is doubly important for reintegration, as it shapes both the policy and the stigma environments that people with criminal records must face. Nowhere are the policy and stigma environments bleaker than for record holders convicted of sex crimes. Drawing on the theory of compassion collapse (or psychic numbing) and using experimental data from a national survey, we examine the effects of informing members of the public about the hardships faced by record holders convicted of sex crimes, and we compare those effects (or the lack thereof) to the effects of victim discourse. We also randomize the information format: aggregate/statistical versus personal narratives. We find that narratives about crime victims’ suffering matter to the public—increasing aversive emotions, support for collateral consequences, and stigmatization—but narratives about record holders’ suffering do not. We conclude by discussing alternative communication strategies that public criminologists may use to garner public support for progressive criminal justice reforms.

Spinal Metastases without Pedicle Signs on Radiograph and their Associated Clinical and Radiological Features.

The pedicle sign is a radiographic indicator of spinal metastases. However, it is not only the pedicle sign that is important in radiographic diagnosis of bone metastases. In the present study, the radiological features of symptomatic spinal metastases in patients without the pedicle sign were retrospectively examined. Among 186 patients with symptomatic spinal metastases who visited our department between January 1, 2011, and December 31, 2017, 64 without the pedicle sign and with available computed tomography (CT) and magnetic resonance imaging (MRI) data in the first visit were enrolled and their data were analyzed. One author evaluated radiographs for findings suggestive of spinal metastases, CT to assess bone destruction, and MRI to evaluate the extent of lesions. Clinical variables were also examined and compared between patients with and without bone changes on radiography. Bone changes strongly suggesting bone metastasis, other than the pedicle sign, were observed in 31 out of 64 patients: bone cortical disappearance in 20, increased radiolucency of the central area in the vertebral body in 8, an irregular osteoblastic change in 5, and asymmetrical vertebral collapse in 10. An analysis of CT data revealed that intertrabecular, mildly osteolytic, and mildly osteoblastic types were more frequent in patients without any changes suggestive of bone metastases on radiographs. Radiographic findings other than the pedicle sign are useful for diagnosing bone metastases. The key to a radiographic diagnosis of spinal metastases is to pay attention to changes in the bone cortex of all vertebral components on radiographs in addition to the pedicle.

Phase Contrast X‐Ray Imaging of the Collapse of an Engineered Void in Single‐Crystal HMX

AbstractImaging the collapse of a single void that creates a hot spot initiation site in an otherwise defect‐free explosive is challenging given the spatial and temporal scales involved in explosive systems. This work presents our attempt to examine a single hot spot mode (void collapse) in single‐crystal octahydro‐l,3,5,7‐tetranitro‐l,3,5,7‐tetrazocine (HMX) embedded in Sylgard. The hot spot heating mechanisms involved with pore collapse include adiabatic heating, jetting, and viscoplastic dissipation. Quantifying the dynamics of a pore collapse is a crucial step to understanding which mechanisms dominate during ignition events. Our experiments were conducted with a single‐stage, light‐gas gun at Argonne National Laboratory's Advanced Photon Source, applying the phase contrast imaging technique while collecting high‐speed video. The details of HMX single crystal production, defect (pore) engineering, and sample construction, along with experimental results are presented here. These results demonstrate that detailed collapse dynamics can be obtained from homogeneous, single‐crystal explosives with this approach. Qualitative comparisons are made with simulation data which show good agreement in the transition between a quasi‐symmetric pore collapse and an asymmetric collapse with jetting across the pore as measured with normalized pore area and pore circularity.

Numerical prediction of cavitation erosion risk in an axisymmetric nozzle using a multi-scale approach

In the present study, a two-way coupling Eulerian–Lagrangian approach is developed to assess the cavitation erosion risk in an axisymmetric nozzle. Macroscopic cavitation structures are simulated using the large eddy simulation along with the volume of fluid method. The compressible Rayleigh–Plesset equation and the bubble motion equation are introduced to resolve the microscopic bubble dynamics. The calculated results agree favorably with the experimental data and can capture more flow details, which is associated with the potential erosion risk. Based on the bubble information in multi-scale cavitating flow, a new asymmetric bubble collapse model is proposed to calculate the impact pressure, which is then used to quantitatively assess the cavitation erosion risk in the nozzle. The results show that, compared with the traditional Euler method, the location and value of the potential maximum cavitation erosion risk predicted by this new method are closer to the experimental measurement. The advantages of the newly proposed method are further elaborated systematically. The study found that the high environmental pressure triggered by the collapse of shedding clouds can cause the near-wall bubbles to shrink and even collapse, releasing impulsive pressure, which directly damages the material surface. This phenomenon is considered to be closer to the actual cavitation erosion process. Finally, analyzing the relationship between multi-scale cavitation structures and erosion risk reveals that the high risk of cavitation erosion is mainly due to the oscillation and collapse of near-wall bubbles which are generated near the attached cavity closure line or surrounding the shedding clouds.

Effects of interlayer exchange on collapse mechanisms and stability of magnetic skyrmions

Theoretical calculations of thermally activated decay of skyrmions in systems comprising several magnetic monolayers are presented, with a special focus on bilayer systems. Mechanisms of skyrmion collapse are identified and corresponding energy barriers and thermal collapse rates are evaluated as functions of the interlayer exchange coupling and mutual stacking of the monolayers using transition state theory and an atomistic spin Hamiltonian. In order to contrast the results to monolayer systems, the magnetic interactions within each layer are chosen so as to mimic the well-established Pd/Fe/Ir(111) system. Even bilayer systems demonstrate a rich diversity of skyrmion collapse mechanisms that sometimes co-exist. For very weakly coupled layers, the skyrmions in each layer decay successively via radially-symmetric shrinking. Slightly larger coupling leads to an asymmetric chimera collapse stabilized by interlayer exchange. When the interlayer exchange coupling reaches a certain critical value, the skyrmions collapse simultaneously. Interestingly, the overall energy barrier for the skyrmion collapse does not always converge to a multiple of that for a monolayer system in the strongly coupled regime. For a certain stacking of the magnetic layers, the energy barrier as a function of the interlayer exchange coupling features a maximum and then decreases with the coupling strength in the strong coupling regime. Calculated mechanisms of skyrmion collapse are used to ultimately predict the skyrmion lifetime. Our results reveal a comprehensive picture of thermal stability of skyrmions in magnetic multilayers and provide a perspective for realizing skyrmions with controlled properties.