E. Phillip Muntz

Nonequilibrium gas flows. I - A detailed validation of Monte Carlo direct simulation for monatomic gases

One‐dimensional shock wave properties in helium and argon are predicted using Monte Carlo direct simulation. The collision model is based directly on the interatomic potential, taking angular scattering into account. The potential is assumed to be of the Maitland–Smith [n(r)−6] form. The detailed validity of the simulation is studied by comparing the predicted macroscopic and microscopic flow properties in shock waves to a wide range of available data.

Transient motion of a confined rarefied gas due to wall heating or cooling

The transient motion that arises in a confined rarefied gas as a container wall is rapidly heated or cooled is simulated numerically. The Knudsen number based on nominal gas density and characteristic container dimension is varied from nearcontinuum to highly rarefied conditions. Solutions are generated with the direct simulation Monte Carlo method. Comparisons are made with finite-difference solutions of the Navier-Stokes equations, the limiting free-molecular values, and (continuum) results based on a small perturbation analysis. The wall heating and cooling scenarios considered induce relatively large acoustic disturbances in the gas, with characteristic flow speeds on the order of 20 % of the local sound speed. Steadystate conditions are reached after on the order of 5 to 10 acoustic time units, here based on the initial speed of sound in the gas and the container dimension. As rarefaction increases, the initial gas response time is decreased. For the case of a rapid increase in wall temperature, transient rarefaction effects near the wall greatly alter gas response compared to the continuum predictions, even at relatively small nominal Knudsen number. For wall cooling, the continuum solution agrees well with direct simulation at that same Knudsen number. A local Knudsen number, based on the mean free path and the scale length of the temperature gradient, is found to be a more suitable indicator of transient rarefaction effects.

DETECTIVE QUANTUM EFFICIENCY (DQE) ANALYSIS OF ELECTROSTATIC IMAGING AND SCREEN-FILM IMAGING IN MAMMOGRAPHY

Sensitometric response and modulation transfer function (MTF) measurements are used to scale noise power spectral (NPS) measurements to the exposure axis. The resulting value of noise equivalent quanta (NEQ) can be compared with the actual number of exposure quanta required to form the image (q ) ; the ratio yields the system detective quantum efficiency (DQE). Two systems used for mammography are studied.

Comparison of Kinetic Models for Gas Damping of Moving Microbeams

Numerical investigations of the gas o w structure and the gas-damping force on moving and heated microbeams are carried out using the Navier-Stokes equations with rst-order velocity-slip and temperature-jump boundary conditions (the NSSJ method) and two kinetic numerical techniques: the particle-based direct simulation Monte Carlo (DSMC) method, and a deterministic discrete-ordinate solution of the ellipsoidal statistical (ES) kinetic model equation. The gas-damping coecien ts on a moving microbeam for quasi-static isothermal conditions are estimated by the three numerical methods for Kn = 0.1-1.0. The NSSJ simulations tend to overestimate the gas-damping coecien t for Knudsen numbers larger than 0.1, whereas the DSMC and ES kinetic approaches are in good agreement for the slip and transitional o w regimes. The o w structure and the Knudsen force are calculated using the ES kinetic model for a heated microbeam over a wide range of Knudsen numbers. The Knudsen force peaks in the transitional regime (Kn 2), and the numerically predicted variation of the force with Knudsen number is consistent with experimentally observed displacements of the heated microbeam.

Analysis of a Two Wrap Meso Scale Scroll Pump

The scroll pump is an interesting positive displacement pump. One scroll in the form of an Archimedes spiral moves with respect to another, similarly shaped stationary scroll, forming a peristaltic pumping action. The moving scroll traces an orbital path but is maintained at a constant angular orientation. Pockets of gas are forced along the fixed scroll from its periphery, eventually reaching the center where the gas is discharged. A model of a multi‐wrap scroll pump was created and applied to predict pumping performance. Meso‐scale scroll pumps have been proposed for use as roughing pumps in mobile, sampling mass spectrometer systems. The main objective of the present analysis is to obtain estimates of a scroll pump’s performance, taking into account the effect of manufacturing tolerances, in order to determine if the meso scale scroll pump will meet the necessarily small power and volume requirements associated with mobile, sampling mass spectrometer systems. The analysis involves developing the govern...

Multiparameter optimization of mammography: an update

Previously in this forum we have reported the application of multiparameter optimization techniques to the design of a minimum dose mammography system. The approach used a reference system to define the physical imaging performance required and the dose to which the dose for the optimized system should be compared. During the course of implementing the resulting design in hardware suitable for laboratory testing, the state of the art in mammographic imaging changed, so that the original reference system, which did not have a grid, was no longer appropriate. A reference system with a grid was selected in response to this change, and at the same time the optimization procedure was modified, to make it more general and to facilitate study of the optimized design under a variety of conditions. We report the changes in the procedure, and the results obtained using the revised procedure and the up- to-date reference system. Our results, which are supported by laboratory measurements, indicate that the optimized design can image small objects as well as the reference system using only about 30% of the dose required by the reference system. Hardware meeting the specification produced by the optimization procedure and suitable for clinical use is currently under evaluation in the Diagnostic Radiology Department at the Clinical Center, NH.