Experimental Investigation Of Formation Research Articles

Experimental Investigations of Formation of Quasiperiodic Water States due to Electrophysical Processes in the Atmosphere

Experimental Investigations of Formation of Quasiperiodic Water States due to Electrophysical Processes in the Atmosphere

Experimental investigation of formation and decomposition of roaldite in ammonia atmosphere at 300–700 °C and associated nitrogen isotope fractionations

Roaldite (Fe4N) is one of the few nitride minerals found in meteorites. Their nitrogen (N) isotopic signatures carry important information for understanding the early N cycle in the proto-solar nebula. However, the lack of knowledge on the N isotopic effects from nitride formation to its survival from frictional heating during landing impedes the interpretation and application of N isotope compositions of nitride minerals in meteorites. Here, we carried out laboratory experiments under a recently proposed roaldite forming condition, i.e., NH3 (as starting N source) reacting with metallic Fe at medium temperatures. We observed Fe4N formation over a large range of temperatures from 300 °C to 700 °C. The formation of Fe4N was associated with equilibrium N isotope fractionations with αFe4N-NH3 values of 0.9907 (±0.0004) at 300 °C and 0.9936 (±0.0004) at 500 °C, respectively. In the experimental pressure conditions (initial PNH3 = 3.9–6.4 bar, PTotal < 8.3 bar), the formed Fe4N remained stable at 300 °C, but was unstable and quickly decomposed to Fe and N2 at 500 °C and 700 °C. The decomposition of Fe4N was associated with large kinetic isotope fractionations with αN2-Fe4N values of 0.9811 (±0.0009) at 500 °C and 0.9839 (±0.0011) at 700 °C, respectively. Our experimental results suggest that roaldite formed from NH3 can carry an isotopic signature very close to that of its source, but partial decomposition (if there is any) can easily shift its N isotope composition for several tens of per mil, and in extreme cases, to >300‰. Thus, great caution is needed when using N isotope composition of roaldite (and probably other nitride minerals as well) to trace source information.

Turbulence production in an APG-boundary-layer transition induced by randomized perturbations

This paper is devoted to an experimental investigation of formation and development of coherent vortical structures at late stages of a laminar-turbulent transition initiated by a harmonic, almost two-dimensional Tollmien-Schlichting (TS) wave perturbed by weak (initially) broadband disturbances. The initial base flow represented a self-similar boundary layer with an adverse pressure gradient (APG) with Hartree parameter βH = −0.115. Experiments were performed at controlled disturbance conditions with the help of the ‘deterministic noise’ method and a universal disturbance source of instability waves. The main measurements were carried out by means of a hot-wire anemometer in a broad spatial region of the flow starting with stages of quasi-sinusoidal small-amplitude instability wave and ending with final stages of transition characterized by formation of concentrated localized vortical structures. The excited perturbations were partly random (within 20 TS-wave fundamental periods) but periodical at very large time scales during which the flow passes the model several times. The detailed measurements and the experimental data processing gave us the possibility to obtain instantaneous velocity and vorticity fields in the (x, y, z, t)-space and to perform computer-aided ‘visualization’ of the instantaneous flow structure. Specific features of the turbulence production mechanism occurring at late stages of transition are studied and compared with previously reported data obtained at sinusoidal excitation. A qualitative similarity is found between essentially nonlinear stages of transition observed in the present (randomized) case and those studied previously in cases of transition initiated by a harmonic TS wave or by a TS wave packet. It is found that interaction of primary wave with a broadband ‘noise’ of 3D TS waves leads at late stages of transition to formation of Λ-vortices, intensive Λ -shaped high-shear (HS) layers, Ω -shaped vortices, ring-like vortices and associated ‘spikes’ in time-traces, and other phenomena. These structures are found to be disposed in a random order in time and space and have somewhat distorted shapes (in contrast to the periodic-excitation case studied earlier). However, their general properties are qualitatively similar. The obtained instantaneous pictures of velocity and vorticity fields seem to correspond qualitatively to those present in the near-wall region of the developed turbulent boundary layer.

The gaseous bubble formation in crystal growth from the liquid phase under microgravity

Some results of theoretical and experimental investigation of formation and behaviour of gaseous inclusions during crystal growth from the liquid phase on the example of GaP:N model system have been presented. The thermodynamic analysis of gas-liquid system carried out gives the general picture of the bubble formation process and the evolution of critical gas nuclei which are significantly influenced by the gravity. The consideration taken out has been applied to some experimental crystal growth situations. Some methods for avoiding the formation and incorporation of gas bubbles into growing solid have also suggested.