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Ga-doped germanium was grown in a vertical Bridgman-Stockbarger system with melt stabilization by axial magnetic fields of 30 kG. It was found that radial segregation of gallium is negligible and that the initial transient of axial macro-segregation is in apparent compliance with the theory of diffusion controlled plane front solidification.
Laminar methane and propane gas-jet diffusion flames have been theoretically and experimentally studied at NASA-Lewis under microgravity conditions. It is noted that laminar diffusion flames are strongly affected by the combined effects of kinetics, radiation, and such transient phenomena as flame ignition, stabilization, and extinction. Observations on the distinctive nature of the flame color and luminosity parameters in microgravity are presented, and flame behavior under transient, high-deceleration rates is discussed. Test hardware and instrumentation design are described.
The dissipation mechanism of magnetic reconnection remains a subject of intense scientific interest. On one hand, one set of recent studies have shown that particle inertia-based processes, which include thermal and bulk inertial effects, provide the reconnection electric field in the diffusion region. On the other hand, a second set of studies emphasizes the role of wave-particle interactions in providing anomalous resistivity in the diffusion region. In this presentation, we present analytical theory results, as well as PIC simulations of guide-field magnetic reconnection. We will show that the thermal electron inertia-based dissipation mechanism, expressed through nongyrotropic electron pressure tensors, remains viable in three dimensions. We will demonstrate the thermal inertia effect through studies of electron distribution functions. Furthermore, we will show that the reconnection electric field provides a transient acceleration on particles traversing the inner reconnection region. This inertial effect can be described as a diffusion-like term of the current density, which matches key features of electron distribution functions.
The dissipation mechanism of magnetic reconnection remains a subject of intense scientific interest. On one hand, one set of recent studies have shown that particle inertia-based processes, which include thermal and bulk inertial effects, provide the reconnection electric field in the diffusion region. On the other hand, a second set of studies emphasizes the role of wave-particle interactions in providing anomalous resistivity in the diffusion region. In this presentation, we present analytical theory results, as well as PIC simulations of guide-field magnetic reconnection. We will show that the thermal electron inertia-based dissipation mechanism, expressed through nongyrotropic electron pressure tensors, remains viable in three dimensions. We will demonstrate the thermal inertia effect through studies of electron distribution functions. Furthermore, we will show that the reconnection electric field provides a transient acceleration on particles traversing the inner reconnection region. This inertia1 effect can be described as a diffusion-like term of the current density, which matches key features of electron distribution functions.
The dissipation mechanism of magnetic reconnection remains a subject of intense scientific interest. On one hand, one set of recent studies have shown that particle inertia-based processes, which include thermal and bulk inertial effects, provide the reconnection electric field in the diffusion region. On the other hand, a second set of studies emphasizes the role of wave-particle interactions in providing anomalous resistivity in the diffusion region. In this presentation, we present analytical theory results, as well as PIC simulations of guide-field magnetic reconnection. We will show that the thermal electron inertia-based dissipation mechanism, expressed through nongyrotropic electron pressure tensors, remains viable in three dimensions. We will demonstrate the thermal inertia effect through studies of electron distribution functions. Furthermore, we will show that the reconnection electric field provides a transient acceleration on particles traversing the inner reconnection region. This inertial effect can be described as a diffusion-like term of the current density. which matches key features of electron distribution functions.
The dissipation mechanism of magnetic reconnection remains a subject of intense scientific interest. On one hand, one set of recent studies have shown that particle inertia-based processes, which include thermal and bulk inertial effects, provide the reconnection electric field in the diffusion region. On the other hand, a second set of studies emphasizes the role of wave-particle interactions in providing anomalous resistivity in the diffusion region. In this presentation, we present analytical theory results, as well as PIC simulations of guide-field magnetic reconnection. We will show that the thermal electron inertia-based dissipation mechanism, expressed through nongyrotropic electron pressure tensors, remains viable in three dimensions. We will demonstrate the thermal inertia effect through studies of electron distribution functions. Furthermore, we will show that the reconnection electric field provides a transient acceleration on particles traversing the inner reconnection region. This inertial effect can be described as a diffusion-like term of the current density, which matches key features of electron distribution functions.
A computational study of turbulent evaporating sprays is reported. The major focus is to examine the sensitivity of the vaporization behavior of turbulent sprays to the transient liquid-phase processes. Three models considered to represent these processes are the thin skin, infinite diffusion, and diffusion limit models. Favre-averaged equations with k-epsilon-g turbulence model are employed for the gas phase. The Lagrangian approach with a stochastic separated flow method is used for the liquid phase where the effects of gas turbulence on droplet trajectories and interphase transport rates are considered using random-walk computations. Also the variable-property effects are considered in detail. Results indicate that, depending upon the boiling temperature and heat of vaporization of the fuel considered, the vaporization behavior of turbulent sprays may be quite sensitive to the modeling of transient liquid-phase processes. Thus, it is important that for most hydrocarbon fuels these processes be adequately represented in any comprehensive spray computations. The present results also provide further support to the conclusions of earlier studies which have been based on simplified spray configurations.
The index of refraction can considerably influence the temperature distribution and radiative heat flow in semitransparent materials such as some ceramics. For external radiant heating, the refractive index influences the amount of energy transmitted into the interior of the material. Emission within a material depends on the square of its refractive index, and hence this emission can be many times that for a biackbody radiating into a vacuum. Since radiation exiting through an interface into a vacuum cannot exceed that of a blackbody, there is extensive reflection at the internal surface of an interface, mostly by total internal reflection. This redistributes energy within the layer and tends to make its temperature distribution more uniform. The purpose of the present analysis is to show that, for radiative equilibrium in a gray layer with diffuse interfaces, the temperature distribution and radiative heat flux for any index of refraction can be obtained very simply from the results for an index of refraction of unity. For the situation studied here, the layer is subjected to external radiative heating incident on each of its surfaces. The material emits, absorbs, and isotropically scatters radiation. For simplicity the index of refraction is unity in the medium surrounding the layer. The surfaces of the layer are assumed diffuse. This is probably a reasonable approximation for a ceramic layer that has not been polished. When transmitted radiation or radiation emitted from the interior reaches the inner surface of an interface, the radiation is diffused and some of it thereby placed into angular directions for which there is total internal reflection. This provides a trapping effect for retaining energy within the layer and tends to equalize its temperature distribution. An analysis of temperature distributions in absorbing-emitting layers, including index of refraction effects, was developed by Gardon (1958) to predict cooling and heat treating of glass plates. The interfaces were optically smooth; the resulting specular reflections were computed from the Fresnel reflection laws. This provides a somewhat different behavior than for diffuse interfaces. A similar application was for heating that occurs in a window of a re-entry vehicle (Fowle et al., 1969). A number of recent papers (Rokhsaz and Dougherty, 1989; Ping and Lallemand, 1989; Crosbie and Shieh, 1990) further examined the effects of Fresnel boundary reflections and nonunity refractive index. Other examples of analyses of both steady and transient heat transfer to single or multiple plane layers (Amlin and Korpela, 1979; Tarshis et al., 1969) have used diffuse assumptions at the interfaces as in the present study
An analysis is developed for obtaining transient temperatures in a two-layer semitransparent composite with spectrally dependent properties. Each external boundary of the composite is subjected to radiation and convection. The two-flux radiative transfer equations are solved by deriving a Green's function. This yields the local radiative heat source needed to numerically solve the transient energy equation. An advantage of the two-flux method is that isotropic scattering is included without added complexity. The layer refractive indices are larger than one. This produces internal reflections at the boundaries and the internal interface; the reflections are assumed diffuse. Spectral results using the Green's function method are verified by comparing with numerical solutions using the exact radiative transfer equations. Transient temperature distributions are given to illustrate the effect of radiative heating on one side of a composite with external convective cooling. The protection of a material from incident radiation is illustrated by adding scattering to the layer adjacent to the radiative source.
Hybrid organic-inorganic lead-halide perovskites (LHPs) are a class of semiconductors with remarkable properties relevant to optoelectronic applications. The structure-property-function relationship in LHPs, however, is poorly understood, leading to incomplete descriptions of optoelectronic properties and a persistent problem of device degradation due to ion migration. Here, we reveal the true structure to contain an assembly of dynamic, two-dimensional short-range structural correlations in the lead-halide octahedral sublattice, with additional correlations between organic molecules. Here, we propose that these correlations are the origin of large-amplitude halide displacements, which govern charge carrier mobility and the sharpness of the absorption edge. Correlations between organic molecules may introduce regions of transient ferroelectricity or antiferroelectricity, which increase charge carrier lifetime. Finally, the correlations introduce an ion diffusion barrier that is static on the timescale of diffusive jumps.
Single radiation effects can be transient and static. The charge collected from radiation has a prompt (drift) and a slow (diffusion) component. This current spike causes disparate results: SEU, SET, SEB, MBU, and more. A system level SEE mitigation technique, used to overcome, is the triple module redundancy (TMR). TMR for system Verilog (TMR4SV), written in Python, is the first step towards design IP triplication. It allows various triplication methods for flip flops, clocks, and logic.
A study is conducted of the limits of time-dependent differential thermal analysis (DTA). Applications of DTA to the hysteresis phenomena of glasses are also reported. The computation of DTA sample temperatures is discussed, giving attention to feasible heating rates, transient effects to be measured, and the simulation of DTA responses to changes in thermal diffusivity similar to hysteresis phenomena in glasses. The reproducibility of temperature recording is checked with zone-refined organic chemicals.
The transient cooldown of a gray, absorbing, isotropic scattering, emitting, and conducting medium bounded by gray, diffusely emitting and reflecting parallel plates is considered. Numerical solutions are obtained for the initial boundary-value problem with a discontinuous decrease in temperature at one boundary. The quasi-steady equation of radiative transfer is solved using Gaussian quadrature and a matrix eigenvector technique together with explicit numerical solution of the unsteady energy equation. Temperature and energy flux distributions are presented for variations of optical thickness, boundary emissivity, albedo, and conduction-radiation parameter.
The stability of stratified plane Couette flow in a rotating frame is investigated for a case in which the gravitational force is parallel to the rotation vector. Partial differential equations describing the behavior of disturbances in the linear regime are derived. Unstratified flow is stable as long as the angular momentum gradient is positive. If the gradient is negative, nonaxisymmetric disturbances grow as a power law in time, if the gradient is sufficiently steep. In flow which is unstable to convection, all perturbations asymptotically grow at the rate given by the Brunt-Vaisala frequency. If heat diffusion is included, all nonaxisymmetric perturbations now eventually decay as t exp -2, even if the flow is unstable to convection. If heat diffusion and viscosity are weak, nonaxisymmetric disturbances in convectively unstable flow will undergo a large transient growth before their eventual decay.
In this work, we present an approach for constructing thermodynamically consistent time-dependent models relevant to thin films of diblock copolymers in applied electric fields. The approach is based on the principles of linear irreversible thermodynamics, and, in this work, it is applied to study the effects of electric fields on thin films of incompressible diblock copolymers. Enforcement of local incompressibility constraint at all times leads to a local order parameter dependent transport coefficient in the model for the diblock copolymers. The dependence of the transport coefficient on the local order parameter is used to relate it with the diffusion constant of Rouse chains and leads to sensitivity of the model to initial conditions. In addition, transient behavior is found to be affected when compared with an ad hoc model assuming a constant transport coefficient. Numerical results such as electric field induced alignment of lamellae domains due to the field are found to be in qualitative agreement with experiments. This approach opens up a systematic way of developing kinetic models for simulating effects of electrolytes added to thin films containing diblock copolymers in the presence of applied electric fields.
A set of convenient thermoelectric device solutions have been derived in order to capture a number of factors which are previously only resolved with numerical techniques. The concise conversion efficiency equations derived from governing equations provide intuitive and straight-forward design guidelines. These guidelines allow for better device design without requiring detailed numerical modeling. The analytical modeling accounts for factors such as i) variable temperature boundary conditions, ii) lateral heat transfer, iii) temperature variable material properties, and iv) transient operation. New dimensionless parameters, similar to the figure of merit, are introduced including the device design factor, fin factor, thermal diffusivity factor, and inductance factor. These new device factors allow for the straight-forward description of phenomenon generally only captured with numerical work otherwise. As an example a device design factor of 0.38, which accounts for thermal resistance of the hot and cold shoes, can be used to calculate a conversion efficiency of 2.28 while the ideal conversion efficiency based on figure of merit alone would be 6.15. Likewise an ideal couple with efficiency of 6.15 will be reduced to 5.33 when lateral heat is accounted for with a fin factor of 1.0.
This paper presents the experimental and theoretical results for expanding methane and ethylene diffusion flames in microgravity. A small porous sphere made from a low-density and low-heat-capacity insulating material was used to uniformly supply fuel at a constant rate to the expanding diffusion flame. A theoretical model which includes soot and gas radiation is formulated but only the problem pertaining to the transient expansion of the flame is solved by assuming constant pressure infinitely fast one-step ideal gas reaction and unity Lewis number. This is a first step toward quantifying the effect of soot and gas radiation on these flames. The theoretically calculated expansion rate is in good agreement with the experimental results. Both experimental and theoretical results show that as the flame radius increases, the flame expansion process becomes diffusion controlled and the flame radius grows as gamma t. Theoretical calculations also show that for a constant fuel mass injection rate a quasi-steady state is developed in the region surrounded by the flame and the mass flow rate at any location inside this region equals the mass injection rate.
The combined effects of transient free stream velocity and turbulence at a stagnation point on a cylinder situated in a crossflow is investigated analytically, and a model is formulated for the eddy diffusivity induced by free-stream turbulence. The steepest descent method is used to integrate the governing momentum expression, and numerical solutions are given for the unsteady wall shear stress function for specific free-stream transients. It is found after correlation of the results by means of a new turbulence parameter that wall friction increases with increasing free-stream turbulence intensity, and that the friction factor increases with increasing reduced frequency of oscillation values.