Effect Of Density Gradient Research Articles

The Resistance of Sandwich Cylinders With Density-Graded Foam Cores to Internal Blast Loadings

Abstract The resistance of sandwich cylinders with density-graded foam cores to internal blast loadings is investigated theoretically and numerically. Four kinds of density-graded foam core are designed, such as positive, negative, middle–high, and middle–low gradients. The deformation process of the sandwich cylinders is assumed to be split into three phases, i.e., fluid–structure interaction, combined deformation of core and inner wall, and sandwich stage of motion. Employing a rigid perfectly-plastic locking model of density-graded foam core, analytical models are proposed to predict the dynamic response of gradient sandwich cylinders. Finite element simulations of gradient sandwich cylinders subjected to internal blast loadings are carried out and agree well with the analytical predictions. Furthermore, the effects of the core density gradients and the wall thickness distributions on the blast resistance are explored and identified. It is shown that the thicker inner wall design can enhance the blast resistance of sandwich cylinders with the same mass.

Influence of the density gradient on turbulent heat transport at ion-scales: an inter-machine study with the gyrokinetic code stella

Abstract Efficient control of turbulent heat transport is crucial for magnetic confinement fusion reactors. This work discusses the complex interplay between density gradients and microinstabilities, shedding light on their impact on turbulent heat transport in different fusion devices. In particular, the influence of density gradients on turbulent heat transport is investigated through an extensive inter-machine study, including various stellarators such as W7-X, LHD, TJ-II and NCSX, along with the Asdex Upgrade tokamak (AUG) and the tokamak geometry of the Cyclone Base Case (CBC). Linear and nonlinear simulations are performed employing the δf-gyrokinetic code stella across a wide range of parameters to explore the effects of density gradients, temperature gradients, and kinetic electrons. A strong reduction in ion heat flux with increasing density gradients is found in NCSX and W7-X due to the stabilization of temperature-gradient-driven modes without significantly destabilizing density-gradient-driven modes. In contrast, the tokamaks exhibit an increase in ion heat flux with density gradients. Notably, the behavior of ion heat fluxes in stellarators does not align with that of linear growth rates, if only the fastest-growing mode is taken into account. Additionally, this study provides physical insights into the microinstabilities, emphasizing the dominance of trapped-electron-modes (TEMs) in CBC, AUG, TJ-II, LHD and NCSX, while both the TEM and the passing-particle-driven universal instability contribute significantly in W7-X.

Design of continuously radial density-graded aluminum foam with unidirectional and bidirectional and study on blast resistance of sandwich tube

This work focuses on investigation of the internal blast resistance of metal sandwich tubes with continuously radial density-graded aluminum foam cores. Based on the 3D-Voronoi technology in polar coordinates, unidirectionally and bidirectionally continuously density-graded aluminum foam were developed for the cores of sandwich tubes in Finite Element (FE) model. And the correctness of core gradient was verified by comparing theoretical design with the actual value generated by FE model. Effects of core density distribution and core density gradient on blast resistance of sandwich tubes were explored and identified. The results indicate that, when the core density gradient is constant, the sandwich tube with negative-gradient core exhibits the smallest maximum deformation of the outer tube, while the negative-middle-low-gradient core tube shows the highest specific energy absorption ( SEA). For negative-gradient core tube, as the core density gradient increases, the maximum deformation of the outer tube decreases significantly, with only slight reduction of the structural SEA. For negative-middle-low-gradient core tube, as the core density gradient increases, the maximum deformation of the outer tube decreases, while the SEA increases.

Influence of plasma density gradient on the tearing mode with the poloidal shear flow

The influence of the plasma density gradient on the m/n = 2/1 and m/n = 4/1 (m is the poloidal mode number and n is the toroidal mode number) tearing mode instability with poloidal flow and flow shear is investigated. Using the analytical solution that we obtained in a previous work and mainly focused on the factors of plasma density and poloidal shear flow, we found that the plasma density has a stabilizing effect on the classical tearing mode, and the poloidal equilibrium flow can intensify this beneficial effect. The density gradient was detrimental to the stability of the tearing mode. The effects of both density and density gradient are slight, but the effect of poloidal flow on the plasma density is significant. Considering that the plasma density changes with the poloidal flow, the values of the tearing mode stability index ∆′ clearly change. Our investigation also found that compared with the negative flow shear, the positive flow shear is beneficial to the stability of the tearing mode, and a larger poloidal flow shear has a better stabilizing effect on the classical tearing mode.

THz Radiation from Subwavelength Volumes on Water Surfaces and in Air: Effects of Mass Density Gradients and External Magnetic Fields

THz Radiation from Subwavelength Volumes on Water Surfaces and in Air: Effects of Mass Density Gradients and External Magnetic Fields

Density gradient effect on circulation patterns and mixing processes: Observations at the convergence front of a salt wedge, microtidal estuary

The convergence front in estuaries is a crucial area influenced by dynamic mixing processes which vary from one tidal cycle to another, with density gradients playing a key role in shaping circulation patterns. This significance is particularly pronounced in salt wedge estuaries, where discharge holds greater influence than tides. Despite this, there is a limited amount of research based on high-resolution in-situ data to analyze circulation and mixing processes affected by density gradients in the convergence front of a salt wedge estuary. In this study, continuous measurements of current velocities and CTD profiles were conducted over three tidal cycles to analyze the intratidal variability of circulation patterns (u, v, w) and mixing parameters in the convergence front. Stratification parameter (ηs) and Richardson numbers (RL and Ri) were calculated to analyze the mixing/stratification variability within a tidal cycle. The observations revealed a persistent Subsurface Velocity Maximum (SVM) in the longitudinal component, coinciding with density gradient increases > 4 kg m−3 along the water column. Similarly, lateral circulation patterns were influenced by those increases in density gradient, resulting in eastward-directed currents and the formation of two circulations cells. As expected in a salt wedge estuary, stratification occurs during flood and High Water (HW), when the density gradient increases due to intrusion of the salt wedge. Notably, in the Magdalena River estuary, when the discharge is below 3800 m3 s−1, the salt wedge remains consistently present irrespective of the tidal phase, resulting in a permanently stratified water column with ηs values between 1.60 – 1.95. It is expected that the steady presence of stratified conditions during periods of low discharge leads to an increased accumulation of sediments. These findings provide valuable quantitative insights into the complex interplay of density gradients, circulation patterns, and stratification dynamics in salt wedge estuaries.

Dynamic characteristics of density-graded cellular materials for impact mitigation

Due to their exceptional impact resistance capabilities, density-graded cellular materials have immense potential in applications where crashworthiness requirements are of prime importance. Under impact loading, the deformation in these materials is characterized by compaction front propagation. Previous studies have utilized numerical techniques to solve the equations governing compaction shock propagation for cellular materials with density gradation. In this study, analytical solutions are formulated using compaction front position as the independent variable. The expressions for the velocity of the impinging rigid mass, energy absorption, incident stress, and transmitted stress are determined. The analytical solutions are shown to be in excellent agreement with the cell-based finite element solutions. The effect of density gradient on energy absorption and stresses is studied. The impact resistance factor is employed to assess different density-graded cellular materials for their effectiveness in impact mitigation. This study demonstrates that density-graded cellular materials can offer superior impact protection to objects at both the incident and transmitted ends. Lower density toward the incident end enhances impact resistance to objects located there, and likewise, lower density at the transmitted end offers more impact resistance to objects at that end.

Enhancing ion transport in pressure-driven nanofluidic systems for energy harvesting

In this work, we propose a new design to enhance ion transport in pressure-driven nanofluidic systems for energy harvesting. The proposed system uses two counter-charged nanochannels, i.e., one of the channels is negatively charged while the other one is positively charged. Under a pressure gradient, cations and anions move through negatively and positively charged channels, respectively, in different directions and contribute to the streaming current. Molecular dynamics simulations are employed to study the effects of surface charge density, channel height, and pressure gradient on the streaming current. Compared with the traditional system, where the nanochannel is negatively charged, the streaming current in the proposed system can be enhanced by a factor up to 6.6. In addition to the involvement of both cations and anions, the enhanced current in the proposed system is caused by the strengthened ionic fluxes due to relatively low-energy barriers for ions entering the channels.

Gradient doping in Cu2ZnSnSe4 by temperature and potential induced defect steering

Kesterite materials are among the most promising emerging photovoltaic absorbers, despite the number of challenging issues this technology presents. The use of soft thermal post-deposition treatments is key to improving the CdS/kesterite interface quality. Thermal treatments can result in a low-temperature phase transition which affects the optoelectronic properties. In this work, the effects of applied voltage during a full device thermal post-deposition treatments above the critical temperature of the phase transition are explored. The applied voltage modifies the formation energy and drives in-depth migration of ionized defects, which can generate a shallow doping density gradient. Supporting the experimental findings, the effects of a shallow doping density gradient on the current–voltage curves and the external quantum efficiency are modelled using drift–diffusion calculations. The presence of bulk recombination centers in the modelling is a key aspect to precisely reproduce the experimental results. The shallow doping density gradient in opposite directions precisely matches the experimental results for opposite voltage polarizations. The effects on the band structure of the device are presented proving this as a promising strategy for improving charge carrier selectivity.This is the first time that a defect engineering approach has been proposed and successfully proven to control shallow doping density profiling and improve the charge carrier selectivity of kesterite devices. In this sense, the results and their thorough physical interpretation presented in this work will potentially open new perspectives in other photovoltaic technologies as well as in the field of materials engineering.

Dust density effects on electron density gradient driven lower hybrid waves in magnetized plasma

AbstractElectrostatic lower hybrid waves propagating transverse to the external magnetic field in magnetized dusty plasma are investigated, using the fluid equations and also including the effect of density gradient in plasma. Lower hybrid modes are considered important in laboratory, astrophysical as well as space plasmas in interpreting low frequency noises. Unlike other electrostatic waves, lower hybrid waves can resonantly interact with electrons as well as ions in plasma. The dust and gradient terms lead to novel growth of these waves in dusty plasma. A theoretical model is developed to investigate density gradient effects on lower hybrid waves in plasma comprised of electrons, ions and dust particles. The dependence of plasma components' density, strength of magnetic field, temperature and charge of electrons, ions and dust grains on the frequency and growth rate of electrostatic lower hybrid waves has been investigated and presented graphically. It has been observed that the number density of each plasma components directly affects the growth rate. The electrostatic waves are found to grow along the positive electron gradient and the dust effect depends on the gradient length chosen. Above a certain threshold value of gradient length, the waves are found to stabilize. A correlation of the obtained theoretical results with the experimental observations is also discussed.

In-Flight Plume Control and Thrust Tuning in Magnetic Nozzle Using Tapered-Coils System Under the Effect of Density Gradient

A three-thick tapered-coils system is developed and the role of spacing and relative size of the coils along with their tapering in controlling the plume geometry through a magnetic nozzle in-flight is revealed under the effect of the density gradient. Empirical relations for the plume radius as a function of the axial distance are obtained for a deeper understanding of the plume geometry after establishing a concept of plasma detachment. For the same current, the translation of the middle coil has a weaker impact on the plume geometry (size and slope profile) compared to the middle-to-outer coil diameter ratio. The combined effect of the tailorable plume geometry and the gradient of the plasma density, occurring because of the varying magnetic field, on the axial thrust is numerically evaluated. As the middle coil is shifted toward the rightmost or the leftmost coil in the system, a significant enhancement in the thrust is noticed, which rises further on tapering the coils and more than <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3~\mathrm{mN}$</tex-math> </inline-formula> thrust is achieved for the taper angle of 22 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^{\circ}$</tex-math> </inline-formula> . The results indicate that the plume structure can be changed and optimized in-flight for tuning the thrust without varying the input power.

Study of drop mobility over a surface having electric charge gradient

Though different methodologies like thermal and magnetic fields have been tried to control a droplet's mobility in biochemical and medical applications, charge imprinted super-amphiphobic surface is the recent and promising development in which droplets can be transported through any predefined path over the surface. Such surfaces are electrified by impacting water droplets from varying release heights through triboelectrification. Impacting droplets on pillared surface dynamics of the same have been analyzed to understand the nature of electrostatic charge distribution generated. Further to this, droplet actuation on such surfaces with surface charge gradient has been simulated. Charge separation inside the liquid droplet and charge accumulation near the interface has been visualized to understand the phenomenon. Asymmetry in droplet shape and charge distribution has been analyzed as the reason behind the droplet actuation. The effect of different charge density gradients and surface wettability over droplet mobility has also been studied.

Influence of density gradient and hybrid effect on quasi-static axial crushing behavior of lattice cylindrical structures

Uniform/gradient lattice cylindrical structures (ULCS/GLCS) and hybrid uniform/gradient lattice cylindrical structures (HULCS/HGLCS) are innovatively designed and fabricated. The axial crushing behaviors of these four-type cylindrical structures are investigated by the quasi-static axial compression experiments and numerical simulations. The influence of the density gradient and the hybrid effect (metal skin+ABS core) on the deformation mode, loading capacity, and energy absorption of the lattice cylindrical structures are elaborated systematically. The results show that the gradient design realizes the function of regulation and controllability of the structural deformation. The softer layers (with smaller relative density) in GLCS begin to deform first and then gradually extend to the harder layers (with larger relative density). And it effectively reduces the initial peak compression force (PCF), and makes the load more stable on the premise of energy absorption. However, unstably catastrophic failures are observed before the final densification in both ULCS and GLCS during the compression due to the brittleness of the ABS material and the compression–expansion​ deformation of structure. For HULCS/HGLCS, it presents progressive crushing mode and overcome the unstably catastrophic failure due to the ductility and constraint of the metal skins. The effect of “1 ＋ 1 > 2” on energy absorption from hybrid effect is achieved in HULCS/HGLCS, in which the EA of HULCS and HGLCS increases by 74.62% and 100.78% comparing the sum of all components individually compressed. Moreover, the progressive peak with increasing trend of HGLCS demonstrated that the hybrid design enhances the advantages of gradient design in sandwich structures.

Experimental 3D concentration profiles along an electrodialysis channel reveal a strong effect of natural convection

Electrodialysis and electrodeionization separate ionic components from processed water solutions by applying a DC electric field on a stack of ion-exchange membranes. The ion separation indicates ionic concentration changes along the channels in the membrane modules. Simultaneously, concentration polarization at the ion-exchange membranes causes ionic concentration variations across the channels. We constructed a milifluidic cell with a diluate channel to measure spatial concentration profiles under single-path electrodialysis conditions. Desalination of 0.1 M NaCl solution showed that the concentration decreases linearly along the channel when connecting relatively low average current densities (< 30 A/m2), yielding <70 % desalination. Higher average current densities causing desalination close to 100 % produced nonlinear concentration profiles characterized by a steep linear drop followed by an almost constant and small concentration region. We show that at a current density of 50 A/m2, only one-third of the diluate channel is employed for desalination. Interestingly, uniform concentration profiles developed across the channels even at high polarization current densities. Unlike constant concentrations across the channel, we found substantial concentration variation in the vertical direction of the channel, indicating the effect of density gradients and natural convection. This effect was confirmed by optical microscopy and particle image velocimetry of this convection in stagnant solution layers. Natural convection, thus, can represent a mechanism intensifying the mass transfer from the solution bulk to the membrane surfaces.

On the mechanisms controlling near-coast circulation in the southern Colombian Pacific at tidal, seasonal, and interannual time scales

This research analyzed the near-coastal circulation along 300 km in the southern Colombian Pacific, a meso-tidal region formed by Tumaco Bay and the Mira River Delta. The interaction between tidal dynamics and the Mira River plume stratification and dispersion is not well known. Moreover, the combined effects of tide and density gradients on the circulation patterns in Tumaco Bay have rarely been studied. The region was investigated using the Delft3D hydrodynamic model, calibrated and validated using field data. The results show that Tumaco Bay is ebb-dominated, and the tide and bottom shape are the dominant forces in the circulation patterns inside the bay. The weak horizontal density gradient induced by Mira River did not show a substantial effect on the circulation of the bay. Near the mouth of the Mira Delta, the water column was predominantly partially mixed, but stratification changed within the tidal cycle, presenting tidal straining along the minor axis of the plume. This may generate strongly stratified conditions in the water column during flood periods. Although tidal straining is very common along the major axis of tide-dominated estuaries, it is not as common in coastal river plumes. The analyzed system provides additional evidence about this phenomenon in a tropical delta. • Hydrodynamics of Mira River and Tumaco Bay as an integrated system is studied for the first time using 3D modelling. • Tumaco Bay is ebb-dominated, and the bottom shape dominates the circulation patterns. • Tidal straining is observed along the minor axis of the Mira River plume. • Cross-shore tidal straining in the plume of tropical rivers has been barely reported previously.

Mesoscale simulation on the shock response of functionally graded Al-PTFE material

In this paper, the shock response of functionally graded Al-PTFE granular composites is firstly investigated by means of mesoscale simulations. A tailored arrangement of granular filler is infiltrated with matrix to study the effects of density gradient on mechanical and chemical characteristics of the material under impact loading. Based on the shock wave propagations, noticeable differences among pressure, temperature, and strain response are visualized at the grain-level. Results demonstrate that higher pressure is concentrated in the shock wave front and decrease over time. Moreover, a much greater energy-releasing and higher strain deformation exhibit along the grain/matrix interfaces. Compared with uniform reactive material, the functionally graded reactive material with decreased density gradient has a higher initial velocity in wave propagation, and the sample with increased density gradient has superior capability in wave attenuation, and a higher level of hot-spots concentration.

Effect of density gradients on the generation of a highly energetic and strongly collimated proton beam from a laser-irradiated Gaussian-shaped hydrogen microsphere

An investigation is made on the influence of the sharpness of the density gradients on the generation of energetic protons in a radially Gaussian density profile of a spherical hydrogen plasma. It is possible to create such density gradients by impinging a solid density target with a secondary lower intensity pulse, which ionizes the target and explodes it to create an expanded plasma target of lower effective density for the high-intensity main pulse to hit on. The density gradients are scanned in the near-critical regime, and separate regimes of proton motion are identified based on the density sharpness. An intermediate-density gradient [npeak≈(1.5–2.5)γnc] favors the generation of high energetic protons with narrow energy spectra that are emitted with better collimation from the target rear surface. Protons with energies exceeding 100 MeVs could be achieved using such modified plasma targets with circularly polarized lasers of peak intensities I0∼1020 W cm−2 and peak energy ∼10 J.

Impacts of short-term wind events on Chukchi hydrography and sea-ice retreat

We seek a better understanding of the summer ice retreat over the Chukchi shelf in the presence of a warm background inflow by investigating mechanisms through which wind forcing mediates changes in sea-ice cover, water-column hydrography, and their inter-related evolution in time. The problem is addressed with observations and an idealized representation of the Chukchi Sea's Central Channel within the Regional Ocean Modeling System (ROMS). An inflow from the south, surface heat fluxes, and wind forcing drives the numerical model. Due to strong atmosphere-ice coupling, direct wind-forced advection of the ice edge emerges as an important factor that controls the oceanic response, including corresponding alterations of the sea surface height. Surprisingly, we find no significant wind control of the cross-frontal eddy transport, due to counterbalancing effects of the frontal density gradient and the pycnocline depth. We propose that wind direction along with differences in the strength of atmosphere-ocean and atmosphere-ice-ocean coupling (drag coefficients) regulate the wind influence on the Chukchi Shelf summer ice retreat and hydrographic structure. Advection of ice away from the inflow reduces the ice melt rate and increases the salinity of the meltwater plume. Advection of ice toward the inflow increases the ice melt rate and decreases the meltwater plume salinity. The identified mechanisms represent steps toward a more complete understanding of the summer conditions in the Chukchi Sea and will help future investigations of seasonally ice-covered shelf ecosystems.

Drift ion-acoustic waves in a nonuniform rotating magnetoplasma with two-temperature superthermal electrons

The theory of low-frequency (in comparison with the ion cyclotron frequency), long wavelength, electrostatic drift ion-acoustic waves (IAWs) is studied in a nonuniform rotating magnetoplasma with two temperature superthermal electrons. In the linear limit, the coupling of IAWs and drift waves by the density inhomogeneity is shown to produce a new wave mode which typically depends on the density gradient, the rotational frequency and the spectral indexes of superthermal electrons. In the nonlinear regime, an evolution equation for the drift IAWs is derived by the dispersion approach, and using the Jacobi elliptic function expansion technique its exact solitary and periodic wave solutions (namely, cnoidal and dnoidal) are also obtained. The properties of these solutions are numerically examined and it is found that they are significantly modified by the effects of the background density gradient, the superthermality of electrons and the Coriolis force associated with the rotational motion of ions.

Effect of electron density gradient on power absorption during gigahertz electromagnetic wave propagating in cold plasma

Considering the effect of electron density gradient, an analytical, parameter adjustable density distribution function is presented, and a multislab plasma model is used to investigate power absorption of gigahertz electromagnetic waves between 0.20 and 30 GHz in a partially ionized cold plasma layer. The effects of plasma parameters on the absorbed power during electromagnetic wave propagation are investigated and compared with corresponding uniform cases. An optimized asymmetric electron density gradient profile is designed by calculating the corresponding absorption spectrum with selected parameters to enhance the absorption rate near original peak frequencies. The possibility of theoretically designing electron density gradient profiles is important to understand how to enhance the plasma cloaking in some specific electromagnetic wave frequency bands.