Convective Flow Velocity Research Articles

Detonation Studies in Dispersed Solid Particulate Explosives Using High Speed Time-Resolved Holography

IntroductionClouds of dispersed explosive or combustible solid particles are detonable and such systems may exhibit self-sustained detonation [1–8]. However, the method by which individual particles in the explosive cloud interact to sustain detonation is not well understood. The similar case of liquid fuel/air explosives was investigated in detail during the 1960's and 1970's. For these systems, it was established that incident shock waves serve to shatter large liquid droplets into a mist of micro-droplets. These microdroplets are almost instantaneously accelerated to the convective flow velocity behind the shock wave. The energy released upon ignition of the micromist supports the shock wave and selfsustained detonation results [9–11].

Implications and assumptions in using the ‘total counts’ and convection-dispersion equations for tracer flow measurements — With particular reference to transpiration measurements in trees

The application of two methods of analysis to the movement of a tracer through trees, involving ‘total counts’ or convection-dispersion models, is discussed, particularly in relation to fluctuating flows. A rigorous definition of the ‘measured’ flow rate obtained with the total counts method is developed. It is shown, from the principle of conservation of mass, that the total counts method provides an estimate of the weighted mean flow over the time period that the tracer is present at the sampling point. The weighting factor is the relative concentration of the tracer as measured at the sampling point. Using results from a tracing experiment on Eucalyptus teretecornis it is shown that the system parameters, diffusivity, porosity and convective flow velocity, are such that the time scale of the weighted mean flow-rate determination is a period of the order of 2 to 3 days. It is also shown that diurnal fluctuations in flow do not introduce significant errors in the estimate of the weighted mean flow estimate. Application of the convection-dispersion model allows these system parameters to be calculated. Although the implications of the use of these methods are discussed in relation to the flow of tracer through tree stems, they are equally valid in any situation in which the ‘total counts’ method is used.

Internal consistency of the Tsyganenko Magnetic Field Model and the Heppner‐Maynard Empirical Model of the ionospheric electric field distribution

We use the Tsyganenko [1987; 1989] magnetic field models to map the electric field distribution of Heppner and Maynard [1987] into the geomagnetic tail and infer the velocity of the CPS convective flow. We obtain qualitatively different convection patterns from the two Tsyganenko models. Results obtained with the 1987 model predict a narrow jet of earthward moving plasma in the center of the CPS, while the 1989 model predicts relatively uniform earthward convection throughout the CPS. Based on in situ observations of CPS convection, we conclude that the 1989 model provides the most realistic mapping. We also suggest that the effects of field‐aligned currents should be incorporated into the magnetic field models to improve the mapping.

LDA fiber-optic probe with adapter for two-point spatial velocity correlations.

A simple adapter for a two-dimensional, backscatter, fiber-optic laser Droppler anemometer probe consisting of two glass plates has been developed to measure the spatial correlation of velocity fluctuations. A conventional two-color, four-beam LDA probe has been modified by reversing the placement of two of the fibers so that both the green and the blue control volumes measure the same velocity component. An adapter for beam separation is ten mounted behind the front lens to separate the two control volumes. In the present work, the measuring point separation was varied up to 3 mm in the lateral and vertical directions. By using only one glass plate 10 mm thick, it was also possible to achieve in-line (Axial) displacements of the control volumes of up to 3.5 mm. spatial correlations were performed in an intermittent jet flow in which the convective flow velocity could be deduced from the offset of the cross-correlation function.

Sedimentation and convective flow in suspensions of spherical particles

We consider spherical particles immersed in an incompressible, viscous fluid and subject to applied forces. Averaging over a statistical distribution function, which is assumed known, we derive macroscopic equations for the mean convective flow velocity of the suspension and the mean particle current density. The two equations are coupled and involve four response kernels of short range. We show that two of the kernels are related by symmetry.

Gas-Phase Transport of Carbon-14 Released from Nuclear Waste into the Unsaturated Zone

AbstractGas-phase carbon-14 dioxide released from spent fuel buried in unsaturated rocks will move upward by molecular diffusion and advection. The flux resulting from molecular diffusion will be determined by the concentration gradient of gaseous carbon-14 dioxide. Advection will be caused by density-driven flow of the pore gas. Such flow occurs under present conditions as a result of annual and diurnal temperature changes and differences in chemical composition between pore gas and air, and it will be enhanced when the spent fuel heats the rock. The advective flux will be determined by the gas-phase concentration of carbon-14 and by the velocity of convective flow of the pore gas. Gas-phase concentrations of carbon-14 dioxide will be strongly influenced by isotopic exchange with bicarbonate dissolved in water that is retained in the unsaturated zone by capillary forces. Isotopic equilibrium between these two phases will be attained very rapidly on repository time scales. If the liquid phase is saturated with calcium carbonate, precipitation of calcite may provide a sink for removal of carbon-14 from the mobile phases.

Convective stabilization of ionospheric plasma clouds

We derive a stability criterion for the large‐scale structuring of ionospheric plasma clouds due to the E × B gradient drift instability. For the equilibrium we consider a cylindrical two‐dimensional water bag cloud aligned along a uniform magnetic field that is polarized by a uniform neutral wind. We perform a stability analysis that allows three‐dimensional perturbations (in r, θ, and z), and we consider both local and global modes. We find that when the parallel wave number kz exceeds a threshold value, exponentially growing poloidal eigenmodes form which are localized on the “backside” of the cloud. This is in contrast to the kz = 0 limit in which there are no exponential solutions. As kz is increased further, the unstable modes localize at a finite angle away from the backside, at a point where the diamagnetic propagation velocity Vd balances the convective flow velocity of the background plasma around the cloud, Vb. We find that the E × B gradient drift instability is stable when Vd > Vb so that the cloud is no longer susceptible to large‐scale structuring. We apply these results to ionospheric barium clouds and estimate that they will cease structuring when L⊥ < (cT/eB)(M + 2)/2Vn where L⊥ is the transverse size of the cloud, T = Te + Ti is the total temperature, M = nc/nb (nc is the cloud density, and nb is the background density), and Vn is the neutral wind velocity. For mid‐latitude barium releases at ∼180 km we estimate L⊥ ∼ 160–480 m, which is consistent with observations.

Estimation of the Wavevector-Frequency Spectrum of Turbulent Boundary Layer Wall Pressure by Multiple Linear Regression

This paper examines the wavevector-frequency spectrum of the turbulent boundary layer wall pressure in the incompressive, inviscid domain in the intermediate and high frequencies range, i.e, ωδ*/U∞ >> 0.5. It is shown that the wavevector-frequency spectrum can be normalized by a factor so that it becomes simply a function of nondimensional Strouhal wavenumber Uck1/ω and Uck3/ω, where Uc is the convective flow velocity, and k1 and k3 are the wavenumbers in the plane of the wall along the streamwise and the crossflow directions, respectively. The normalization factor is the point pressure frequency spectrum times (Uc/ω)2. It follows that the normalized wavevector-frequency spectrum can be scaled with respect to the Strouhal wavenumbers Uck1/ω and Uck3/ω. The rationale of using a linear regression model for estimating the normalized wavevector-frequency spectrum with a set of measured response data from a wavevector filter is presented. The contention is that the actual spectrum can be obtained by the multiplication of a trial spectrum with a correction spectrum. The correction spectrum is approximated by a polynomial in Uck1/ω with a set of coefficients to be determined. The multiple linear regression model relates the response of a measuring system to these coefficients which are determined by least square minimization of a set of measured response data. The advantages of the regression approach are that it relaxes the requirements of the wavevector filter’s ability to discriminate against the spectral elements outside the wavenumber bandwidth of the filter, and this approach is capable of better estimating the entire wavevector spectrum as compared to the existing methods which are limited to measurements of the low-wavenumber spectra. Some preliminary numerical results are presented.

Diffusion coefficient measurement by the "stop-flow" method in a 5% collagen gel

We measured the translational bulk diffusion coefficient (D) of solute in a collagen gel column of 5% concentration (wt/wt) by a new, noninvasive method applicable to a wide range of solutes and gels. The system also enabled measurement of solute partition coefficients and convective flow velocity since the gel was contained within a chromatography column. The spread of diffusing solute in the gel column is measured during an interval of stopped flow in this method. Experimentally determined values of D/D degrees (free aqueous diffusion coefficient) ranged from 0.24 (3H2O) to 0.13 (ovalbumin) as anticipated by observations of other investigators from interstitium in heart and mesentery, but were significantly smaller than predicted by the widely used Ogston gel model with parameters extracted from partition coefficient data.

Convection Analysis of the Dendrite Remelting Rocket Experiment

T HE dendrite remelting rocket experiment (experiment 7421) was performed aboard a Black Brant VG sounding rocket during a period in which gravity levels of approximately 10 ~ 5 g prevailed. The objective of this experiment was to gain information on the origin of the equiaxed zone obtained in many metal castings. To observe the actual solidification process in lowg, however, the transparent metal-model material NH4C1-H2O (aqueous ammonium chloride solution) was used. Utilizing the crystallization of ammonium chloride from aqueous solutions as a model for the solidification behavior of metal alloys in any given circumstance requires a comparison of the convective flow velocities and patterns. Convective flow velocities can profoundly affect temperature and concentration distribution and, hence, cause various segregation phenomena. High convective velocities can also bend or break off delicate dendrite arms, thus possibly causing the equiaxed zone. The purpose of the present study was to assess the convective conditions in aqueous and metallic liquid systems under conditions of the flight experiment to help establish the relevance of the rocket experiment to metal casting phenomena. The results of the study indicate that aqueous or metallic convective velocities in the dendrite remelting rocket experiment ceil are of insignificant magnitudes at the 10~ to 10~ g levels of the experiment. The crystallization phenomena observed in the rocket experiment, therefore, may be indicative of how metals will solidify in low g.

BaO(X 1Σ +) vibrational relaxation

Vibrationally excited BaO(X 1Σ +) was produced by reacting Ba atoms with O 2 under “jet flow” conditions in which the convective flow velocity was large compared with the diffusion velocities so that the relaxation could be spatially resolved. The vibrational level populations were determined by laser-induced fluorescence measurements. By using modeling calculations to fit the spatial variation of the apparent vibrational temperature, we obtained a vibrational relaxation rate for BaO(X 1Σ +), ν = 1 → ν = 0, by Ar, of 9 × 10 −13 cm 3/molecule s.

Effects of Different Flow Field Conditions on the High Power Transmission Properties of a Reentry Plasma

A computer program for calculating the high power microwave transmission coefficients of a chemically seeded reentry plasma has been described in two previous reports. It has been explained how the computer code can be used to optimize the amount of high power that can be transmitted from a reentry vehicle. Using a half implicit – half explicit time integration scheme, the electron and ion transport equations coupled with Maxwell's equations are numerically integrated to determine the nonlinear reflection and transmission coefficients of a reentry plasma. Graphs are presented of the electron density profile and plane wave reflection and transmission coefficients corresponding to the flow field conditions of a blunt-nosed reentry vehicle. The nonlinear transmission effects are studied as a function of vehicle altitude, antenna location on the vehicle, incident power level, pulse length, pulse repetition frequency, and convective flow velocity. Good agreement is obtained between theoretical results and actual rocket flight data.