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Broadband coherent Rayleigh-Brillouin scattering (CRBS) was used to measure translational gas temperatures for nitrogen at the ambient pressure of 0.8 atm using a purpose-built Fabry-Perot etalon spectrometer. Temperatures derived from the CRBS spectral analysis were compared with experimentally-measured temperatures, and were found to be, on average, within 2% of the experimentally-measured value. Axial flow velocities from a double jet at a pressure ratio of 0.38 were also measured by looking at the Doppler shift of the CRBS line shape. With recent developments in chirped laser technology and the capacity of CRBS to simultaneously provide thermodynamic and bulk flow information, the CRBS line shape acquisition and analysis technique presented here may allow for future time-resolved, characterization of aerospace flows. [ABSTRACT FROM AUTHOR]

Coherent Rayleigh scattering was, for the first time, simulated using the Direct Simulation Monte-Carlo technique with the motivation of ultimately using experimental line shapes to directly measurement the velocity distribution of the gas. Carbon dioxide, at 300 K and low pressure, was simulated at various laser energies in the collisionless regime and at various pressures for a simulated laser intensity of 4×1015 W/m2. Simulated results compare favorably to published experimental results and existing kinetic line shape models, appropriately capturing line shape narrowing with increased optical potential well depth. Additionally, the expected line shape from a non-equilibrium gas with a bi-Gaussian velocity distribution was investigated. As with an equilibrium gas, the simulated line shape follows the velocity distribution and exhibits line narrowing with increasing laser intensity. [ABSTRACT FROM AUTHOR]

Theories for laser-atom interactions have been under development since the advent of laser technology. The theories have yet to be adequately integrated into kinetic flow solvers. Realizing this integration would greatly enhance the scaling of laser-species interactions beyond the realm of ultra-cold atomic physics. A representative numerical investigation was conducted using a custom collisionless gas particle trajectory code, demonstrating this goal in the present study. The investigation covered neutral atomic beam steering and collimation using near-resonant laser fields. In addition to the numerical investigation, a validating experiment was conducted. The experimental results showed good agreement with the numerical simulations when experimental parameters, such as finite laser line width, were taken into account. These simulations showed trends and some limitations associated with the use of a continuous-wave Gaussian laser fields for the steering and collimation of a geometrically skimmed cesium atomic beam using the photon scattering force and the near-resonant induced dipole gradient force. These simulations indicate possible integration of the resonant laser-atom interaction with other rarefied and collisional solvers for similar species such as alkali metals. [ABSTRACT FROM AUTHOR]

The DSMC method was used to investigate the interaction between a test gas and ten successive, pulsed, optical lattices. The one-dimensional lattices were formed by two 100 ps, 393 mJ, 532 nm, simulated laser pulses and assumed to be far from resonance. The maximum centerline temperature was simulated as a function of the intervening time between pulses. An optimum intervening time was found and related to the mean collision time and available energy modes. Subsequent pulses can act to cool the gas for intervening times near zero. For long intervening times, thermal diffusion carries energy away from the centerline, reducing the maximum temperature. At one atmosphere the optimal intervening time was found to be 0.7, 1.0 and 0.25 ns for argon, nitrogen, and methane respectively. The highest temperature simulated for nitrogen was 2480 K after 50 pulses with an optimal intervening time. The combination of small intervening time and high intensity requires the use of an optical cavity for successive laser pulses as commercially available high repetition rate lasers have neither the repetition rate nor the energy per pulse. [ABSTRACT FROM AUTHOR]

An experimental and numerical effort was undertaken to assess the effects of a cold gas (To=300K) nozzle plume impinging on a simulated spacecraft surface. The nozzle flow impingement is investigated experimentally using a nano-Newton resolution force balance and numerically using the Direct Simulation Monte Carlo (DSMC) numerical technique. The Reynolds number range investigated in this study is from 0.5 to approximately 900 using helium and nitrogen propellants. The thrust produced by the nozzle was first assessed on a force balance to provide a baseline case. Subsequently, an aluminum plate was attached to the same force balance at various angles from 0° (parallel to the plume flow) to 10°. For low Reynolds number helium flow, a 16.5% decrease in thrust was measured for the plate at 0° relative to the free plume expansion case. For low Reynolds number nitrogen flow, the difference was found to be 12%. The thrust degradation was found to decrease at higher Reynolds numbers and larger plate angles. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

With a focus on optical lattice gas heating, direct simulation Monte Carlo was used to investigate the interaction between molecular nitrogen, argon and methane, initially at 300 K and 0.8 atm, with pulsed, chirped optical lattices. Created from two 700 mJ, 532 nm, flattop laser pulses, the optical lattice parameters simulated are based on published optical lattice-based experiments, to ensure that pulse energies and durations do not exceed published optical breakdown (ionization) thresholds. Resultant translational gas temperatures, as well as induced bulk velocities, were used quantify energy and momentum deposition. To maximize available gas temperature changes achieved using the technique, laser pulses were linearly chirped to produce lattice velocities able to more effectively facilitate energy deposition throughout the pulse duration. From the initial conditions, the maximum, end pulse axial translational temperature obtained in nitrogen was approximately 755 K, at a lattice velocity of 400 m/s linearly chirped at 25 Gm/s² over the 40 ns pulse duration. To date, this stands as the single largest, numerically-predicted temperature change from optical lattice gas heating under the numerical integration of real world energy and laser-based limitations. [ABSTRACT FROM AUTHOR]

With a focus on optical lattice gas heating, direct simulation Monte Carlo was used to investigate the interaction between molecular nitrogen, initially at 300 K and 0.8 atm, and ten successive, pulsed optical lattices in the presence of two multipass cavity concepts. Created from two 5 ns, 130 mJ, 532 nm laser pulses, the optical lattice parameters simulated were chosen based on published optical lattice-based experiments. Similarly, the cavity concepts used were simulated with optical energy losses comparable to that observed experimentally. Translational gas temperatures were sampled on every cavity round trip and found to facilitate several hundred Kelvin increases in some cases. Importantly, these simulation results suggest that while optical energy losses significantly limit attainable gas temperatures in the context of multipass optical cavities, net momentum deposition into the gas profoundly impacts the energy deposition potential and gas temperature changes achievable through optical lattice gas heating. [ABSTRACT FROM AUTHOR]

Coherent Rayleigh-Brillouin scattering (CRBS) line shapes have been experimentally and numerically investigated for the first time in the high laser intensity regime (>1×1016 W/m2) for molecular nitrogen at 300 K. All narrowband pumps were used to generate deep, monochromatic potential wells in the experiment allowing for direct comparison with a DSMC numerical approach. Both results show a line shape narrowing compared to previous low intensity CRBS line shape models due to energy and momentum deposition from the laser field to the gas. The DSMC results indicate a temperature increase along the center of the laser pulse of 51 K and an induced bulk motion of 100 m/s. [ABSTRACT FROM AUTHOR]

Flow through circular orifices with thickness-to-diameter ratios varying from 0.015 to 1.2 is studied experimentally and numerically with kinetic and continuum approaches. Helium and nitrogen gases are used in the range of Reynolds numbers from 0.02 to over 700. Good agreement between experimental and numerical results is observed for mass flow and thrust corrected for the experimental facility background pressure. For thick-to-thin orifice ratios of mass flow and thrust vs pressure, a minimum is established. The thick orifice propulsion efficiency is much higher than that of a thin orifice. The effects of edge roundness and surface specularity on a thick orifice specific impulse were found to be relatively small. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]