Measurement of the lowest millimetre-wave transition frequency of the CH radical

The CH radical offers a sensitive way to test the hypothesis that fundamental constants measured on earth may differ from those observed in other parts of the universe. The starting point for such a comparison is to have accurate laboratory frequencies. Here we measure the frequency of the lowest millimetre-wave transition of CH, near 535 GHz, with an accuracy of 0.6 kHz. This improves the uncertainty by roughly two orders of magnitude over previous determinations and opens the way for sensitive new tests of varying constants.Comment: 5 pages, 5 figure

Shock enhancement and control of hypersonic mixing and combustion

The possibility that shock enhanced mixing can substantially increase the rate of mixing between coflowing streams of hydrogen and air has been studied in experimental and computational investigations. Early numerical computations indicated that the steady interaction between a weak shock in air with a coflowing hydrogen jet can be well approximated by the two-dimensional time-dependent interaction between a weak shock and an initially circular region filled with hydrogen imbedded in air. An experimental investigation of the latter process has been carned out in the Caltech 17 Inch Shock Tube in experiments in which the laser induced fluorescence of byacetyl dye is used as a tracer for the motion of the helium gas after shock waves have passed across the helium cylinder. The flow field has also been studied using an Euler code computation of the flow field. Both investigations show that the shock impinging process causes the light gas cylinder to split into two parts. One of these mixes rapidly with air and the other forms a stably stratified vortex pair which mixes more slowly; about 60% of the light gas mixes rapidly with the ambient fluid. The geometry of the flow field and the mixing process and scaling parameters are discussed here. The success of this program encouraged the exploration of a low drag injection system in which the basic concept of shock generated streamwise vorticity could be incorporated in an injector for a Scramjet combustor at Mach numbers between 5 and 8. The results of a substantial computational program and a description of the wind tunnel model and preliminary experimental results obtained in the High Reynolds Number Mach 6 Tunnel at NASA Langley Research Center are given here

The praxis and paradoxes of community engagement as the third mission of universities. A case of a selected South African university

Community Engagement (CE) is universities’ third mission (TM), with teaching and learning, and research as the first and second. Although endorsed as the TM of universities and despite clear policy mandates, CE is largely neglected in several South African universities for several reasons. There is growing pressure for universities to partake in CE in order to align their various disciplinary diversity and resources to connect with the broader society and produce socially engaged and responsible graduates. The long-term benefits of CE are linked to knowledge creation, improved community practices and improved social justice. The article aimed to explore academic staff members’ understanding of CE, its execution in the institution and its challenges. Semi-structured interviews were used to collect data, and data were analysed using thematic analysis. The study concluded that there are conceptual contradictions in academics’ view of CE and there is a mismatch between what is defined as CE and the actual practice. The CE challenges include lack of institutional guidance, lack of understanding on measuring its effectiveness, neglect, the uncertainty of who should conduct it and lack of finances channelled towards it. To curb the challenges, a clear conceptual framework/policy should be generated that stipulates a clear definition of CE, how to conduct it and which staff should engage communities. Finally, considering that CE benefits students, researchers and communities, provision for its funding should be made by universities to curb financial constraints

Characterization of a cryogenic beam source for atoms and molecules

We present a combined experimental and theoretical study of beam formation from a cryogenic buffer gas cell. Atoms and molecules are loaded into the cell by laser ablation of a target, and are cooled and swept out of the cell by a flow of cold helium. We study the thermalization and flow dynamics inside the cell and measure how the speed, temperature, divergence and extraction efficiency of the beam are influenced by the helium flow. We use a finite element model to simulate the flow dynamics and use the predictions of this model to interpret our experimental results.Comment: 10 pages, 14 figure

A high quality, efficiently coupled microwave cavity for trapping cold molecules

We characterize a Fabry-Perot microwave cavity designed for trapping atoms and molecules at the antinode of a microwave field. The cavity is fed from a waveguide through a small coupling hole. Focussing on the compact resonant modes of the cavity, we measure how the electric field profile, the cavity quality factor, and the coupling efficiency, depend on the radius of the coupling hole. We measure how the quality factor depends on the temperature of the mirrors in the range from 77 to 293K. The presence of the coupling hole slightly changes the profile of the mode, leading to increased diffraction losses around the edges of the mirrors and a small reduction in quality factor. We find the hole size that maximizes the intra-cavity electric field. We develop an analytical theory of the aperture-coupled cavity that agrees well with our measurements, with small deviations due to enhanced diffraction losses. We find excellent agreement between our measurements and finite-difference time-domain simulations of the cavity.Comment: 16 pages, 8 figure

Vibrational branching ratios and hyperfine structure of 11^{11}BH and its suitability for laser cooling

The simple structure of the BH molecule makes it an excellent candidate for direct laser cooling. We measure the branching ratios for the decay of the ${\rm A}^{1}\Pi (v'=0)$ state to vibrational levels of the ground state, ${\rm X}^{1}\Sigma^{+}$, and find that they are exceedingly favourable for laser cooling. We verify that the branching ratio for the spin-forbidden transition to the intermediate ${\rm a}^{3}\Pi$ state is inconsequentially small. We measure the frequency of the lowest rotational transition of the X state, and the hyperfine structure in the relevant levels of both the X and A states, and determine the nuclear electric quadrupole and magnetic dipole coupling constants. Our results show that, with a relatively simple laser cooling scheme, a Zeeman slower and magneto-optical trap can be used to cool, slow and trap BH molecules.Comment: 7 pages, 5 figures. Updated analysis of A state hyperfine structure and other minor revision

Measurements of mixed convective heat transfer to low temperature helium in a horizontal channel

A horizontal 2.85 m long, 19 mm i.d. stainless steel heated circular channel was employed to measure coefficients of heat transfer to low temperature helium flow. Experimental parameters range from 6.5 to 15 K, from 0.12 to 0.3 MPa at heat fluxes up to 1000 W/m square and Reynolds numbers from 9,000 to 20,000. A significantly nonuniform distribution of heat transfer coefficients over the tube periphery is observed. Difference between temperatures on the upper and lower surfaces of the stainless steel channel wall was found to reach 9 K. It was noted that the highest temperature on the wall outer surface is displaced from its uppermost point. Measurements of local flow temperatures revealed vortical structure of the flow. The displacement of the point with the highest temperature is attributable to the effect of vortices. The relationships for calculating local and averaged coefficients of heat transfer are proposed

Some heat transfer and hydrodynamic problems associated with superconducting cables (SPTL)

To study some effects of thermogravitation on (CIIK-SPTL) systems, a heated tube experiment was set up at Krzhizhanovsky Power Engineering Institute Moscow, U.S.S.R. Heat transfer data were taken with fluid helium flowing through a 2.85 m, 19 mm diameter uniformly heated horizontal tube. Temperatures were measured on the top and bottom of the tube at six axial locations with three other circumferential measurements made at (X/L) =57. Typical temperature profiles show significant variations both axially and circumferentially. The data are grouped using reduced Nusselt number (NuR) and the bulk expansion parameter for each axial location. The average data for 0.26 less than or equal to X/L less than or equal to 0.76 follow a power law relation with the average expansion parameter. System instabilities are noted and discussed. Future work including heat transfer in coaxial cylinders is discussed

Preliminary experimental results for a cryogenic brush seal configuration

Preliminary fluid nitrogen flow data are reported for a five-brush, ceramic-coated-rub-runner brush seal system, where the brushes and the rub runner were placed at each end of a centrally pressurized multifunction tester ('back-to-back' set of brushes) and tested at rotor speeds of 0, 10, 18, and 22.5 krpm. After testing, both the brushes and the ceramic-coated rub runner appeared pristine. The coating withstood both the thermomechanical and dynamic loadings with minor wear track scarring. The bristle tips showed some indication of material shearing (smearing) wear. The Ergun porous flow equation was applied to the brush seal data. The Ergun relation, which required heuristic information to characterize the coefficients, fit the gaseous data but was in poor agreement with the fluid results. The brush seal exit conditions were two phase. Two-phase, choked-flow design charts were applied but required one data point at each rotor speed to define the (C(sub f)A x Constant) flow and area coefficients. Reasonable agreement between prediction and data was found, as expected, but such methods are not to be construed as two-phase-flow brush seal analyses

Experimental study of ceramic coated tip seals for turbojet engines

Ceramic gas-path seals were fabricated and successfully operated over 1000 cycles from flight idle to maximum power in a small turboshaft engine. The seals were fabricated by plasma spraying zirconia over a NiCoCrAlX bond boat on the Haynes 25 substrate. Coolant-side substrate temperatures and related engine parameters were recorded. Post-test inspection revealed mudflat surface cracking with penetration to the ceramic bond-coat interface