Gregory T. Smedley

The wall shear stress produced by the normal impingement of a jet on a flat surface

A method for the theoretical determination of the wall shear stress under impinging jets of various congurations is presented. Axisymmetric and two-dimensional incompressible jets of a wide range of Reynolds numbers and jet heights are considered. Theoretical predictions from this approach are compared with available wall shear stress measurements. These data are critically evaluated based on the method of measurement and its applicability to the boundary layer under consideration. It was found that impingement-region wall shear stress measurements using the electrochemical method in submerged impinging liquid jets provide the greatest accuracy of any indirect method. A unique wall shear stress measurement technique, based on observing the removal of monosized spheres from well-characterized surfaces, was used to conrm the impinging jet analysis presented for gas jets. The technique was also used to determine an empirical relation describing the rise in wall shear stress due to compressibility eects in impinging high-velocity jets.

EFFECT OF PARTICLE SIZE AND MATERIAL PROPERTIES ON AERODYNAMIC RESUSPENSION FROM SURFACES

An experimental study of the aerodynamic entrainment of monodisperse spheres from glass substrates is presented. The spheres were made of ammonium fluorescein and polystyrene standards (M_W=3700, 18,700, and 114,200 amu). The use of monosized particles with carefully controlled properties leads to a narrow distribution of sphere-surface adhesion forces, facilitating the determination of the particle entrainment threshold. The spheres were exposed to well-characterized shear stresses in two different flows, (i) laminar channel flow; and (ii) a normally impinging gas jet. Threshold shear stresses were found to be more sensitive to particle size than predicted by the existing resuspension theories, which are based on equilibrium adhesion models. Furthermore, resuspension was also found to be sensitive to the duration of the applied shear stress. This sensitivity depends upon the particle size and material properties. A kinetic model of particle detachment is presented to account for these observed trends.

Entrainment of fine particles from surfaces by gas jets impinging at oblique incidence

Abstract This paper describes an experimental study of the removal of fine (12 μm) polystyrene particles from a glass substrate, using a gas jet that impinges obliquely onto a particle-laden surface. In order to avoid transient affects associated with jet start-up, the sample was slowly translated under a steady jet. The translating gas jet produces a long, clean path that provides very good statistics for exploring the effect of jet parameters. This study focuses on the dependence of the spatial distribution of removal on the jet pressure ratio and impingement angle. The jet is translated over the sample both longitudinally and transversely to determine both the width and the length of the particle removal footprint. The width of the removal footprint increases and the length decreases as the impingement angle is increased. Previous researchers have reported seemingly contradictory results regarding the dependence of removal efficiency on impingement angle; this paper seeks to resolve these differences. For the steady jet, the threshold jet pressure ratio required for 50% particle removal increases with decreasing impingement angle. In addition, studies of the entrainment of well-characterized particles from well-characterized substrates provide insight into the surface shear stress imposed by the oblique jet.

Method for characterization of adhesion properties of trace explosives in fingerprints and fingerprint simulations.

The near inevitable transfer of explosive particulate matter through fingerprints makes it possible to detect concealed explosives through surface sampling. Repeatable and well-characterized fingerprint simulation facilitates quantitative comparison between particulate sampling methods for subsequent detection of trace explosive residues. This study employs a simple, but reproducible sampling system to determine the accuracy of a fingerprint simulation. The sampling system uses a gas jet to entrain particles from a substrate and the resulting airborne particles are then aspirated onto a Teflon filter. A calibrated Barringer IonScan 400 ion mobility spectrometer was used to determine the mass of explosive material collected on the filter. The IonScan 400 was calibrated with known masses of 2,4,6-trinitrotoluene (TNT). The resulting calibration curve is in good agreement with that obtained by Garofolo et al. (1994) for an earlier model of the instrument. The collection efficiency of the sampling system was measured for three particle sizes (8.0. 10.0, and 13.0 microm) using spherical polystyrene particles laced with known quantities of TNT. Collection efficiency ranged from less than 1% for the larger particles to 5% for the smaller particles. Particle entrainment from the surface was monitored with dark field imaging of the remaining particles. The sampling system was then applied to two C4 test samples--a fingerprint transfer and a dry Teflon transfer. Over 100 ng of RDX was collected from the dry transfer sample, while less than 1 ng was collected from the fingerprint transfer. Possible explanations for this large difference are presented based on the system calibration.

The inviscid impingement of a jet with arbitrary velocity profile

Accurate determination of wall shear stress and heat and mass transfer rates under an impinging jet requires careful analysis of the boundary layer at the impingement surface due to the large pressure gradients near the stagnation point. Modeling the inviscid flow just outside the boundary layer provides the boundary conditions necessary for such an analysis. Previous inviscid models have considered only a small subset of possible jet velocity profiles and with limited spatial resolution. In the present work, analytical solutions to the stream-vorticity equation for two-dimensional and axisymmetric impingement flow with arbitrary velocity profile are found in terms of a surface integral involving the vorticity function, allowing an iterative determination of the stream function throughout the impingement region. Surface pressure distributions and streamline plots are calculated for various impinging jet configurations, including plane, round, and annular jet nozzles. The calculations show excellent agreement with previous experimental and numerical results, while requiring relatively short computation times. Flow predictions are also made for impinging jet configurations for which no previous data or calculations exist.

A study of laminar flow of polar liquids through circular microtubes

Recently, the validity of using classical flow theory to describe the laminar flow of polar liquids and electrolytic solutions through microtubes has been questioned for tube diameters as large as 500 μm [Brutin and Tadrist, Phys. Fluids 15, 653 (2003)]. This potential increase in flow resistance, which has been attributed to electrokinetic effects and enhanced surface roughness effects, is critical to the understanding of certain biofluid systems. We seek to characterize this phenomenon for a variety of capillary/liquid systems. Using a numerical solution to the Poisson–Boltzmann equation, we have calculated the electroviscous effect for the systems under consideration. We have also measured the pressure drop as a function of flow rate across well-characterized stainless steel and polyimide microtubes ranging in diameter from 120 μm to 440 μm. Deionized water, tap water, a saline solution, and a variety of glycerol/water mixtures were used. The calculations and measurements suggest that any deviation fro...

Preliminary drop-tower experiments on liquid-interface geometry in partially filled containers at zero gravity

Plexiglass containers with rounded trapezoidal cross sections were designed and built to test the validity of Concus and Finns existence theorem (1974, 1983) for a bounded free liquid surface at zero gravity. Experiments were carried out at the NASA Lewis two-second drop tower. Dyed ethanol-water solutions and three immiscible liquid pairs, with one liquid dyed, were tested. High-speed movies were used to record the liquid motion. Liquid rose to the top of the smaller end of the containers when the contact angle was small enough, in agreement with the theory. Liquid interface motion demonstrated a strong dependence on physical properties, including surface roughness and contamination.

Some transparent immiscible liquid pairs

This report identifies 121 transparent immiscible liquid pairs that have properties compatible with optical instrumentation based on laser-induced fluorescence. Physical data such as specific gravity, index of refraction, viscosity, flash point, and toxicity were found in the literature. Compatibility with Plexiglas (PMMA), contact angles of the internal meniscus on glass and PMMA, meniscus formation times, and clearing times were measured. A useful noninvasive technique for determining interfacial tensions is explained and used. Some dependence of interfacial tension on molecular structure is found.