Biomedical subjects

Richard M Lueptow

Publications and source records attributed to Richard M Lueptow.

15 recordsLinked to original sources

PubMed2006

Capturing patterns and symmetries in chaotic granular flow.

Segregation patterns formed by time-periodic flow of polydisperse granular material (varying in particle size) in quasi-two-dimensional (quasi-2D) tumblers capture the symmetries of Poincaré sections, stroboscopic maps of the underlying flow, derived from a continuum model which contains no information about particle properties. We study this phenomenon experimentally by varying the concentration of small particles in a bidisperse mixture in quasi-2D tumblers with square and pentagonal cross sections. By coupling experiments with an analysis of periodic points, we explain the connection between the segregation patterns and the dynamics of the underlying flow. Analysis of the eigenvectors and unstable manifolds of hyperbolic points shows that lobes of segregated small particles stretch from hyperbolic points toward corners of the tumbler, demonstrating the connection between regions of chaotic flow and the shape of the segregation patterns. Furthermore, unstable manifolds map the shape of lobes of segregated particles. The techniques developed here can also be applied to nonpolygonal tumblers such as elliptical tumblers, as well as to circular tumblers with time-periodic forcing.

Journal Article↗

PubMed2006

Creeping granular motion under variable gravity levels.

In a rotating tumbler that is more than one-half filled with a granular material, a core of material forms that should ideally rotate with the tumbler. However, the core rotates slightly faster than the tumbler (precession) and decreases in size (erosion). The precession and erosion of the core provide a measure of the creeping granular motion that occurs beneath a continuously flowing flat surface layer. Since the effect of gravity on the subsurface flow has not been explored, experiments were performed in a 63% to 83% full granular tumbler mounted in a large centrifuge that can provide very high g-levels. Two colors of 0.5 mm glass beads were filled side by side to mark a vertical line in the 45 mm radius quasi-two-dimensional tumbler. The rotation of the core with respect to the tumbler (precession) and the decrease in the size of the core (erosion) were monitored over 250 tumbler revolutions at accelerations between 1g and 12g. The flowing layer thickness is essentially independent of the g-level for identical Froude numbers, and the shear rate in the flowing layer increases with increasing g-level. The degree of core precession increases with the g-level, while the core erosion is essentially independent of the g-level. Based on a theory for core precession and erosion, the increased precession is likely a consequence of the higher shear rate. Core erosion, on the other hand, is related to the creep region decay constant, which is connected with slow diffusion in the bed and unaffected by gravity.

Journal Article↗

PubMed2006

End-wall effects in granular tumblers: From quasi-two-dimensional flow to three-dimensional flow.

The flow of granular material in rotating tumblers is confined to a thin flowing layer at the free surface in which the particle velocity is primarily streamwise, perpendicular to the axis of rotation, with minimal axial motion. Particle tracking velocimetry was used to measure the surface velocity for 1 and 2 mm glass particles and sand in cylindrical tumblers of various diameters, lengths, and rotation rates for a flat continuously flowing surface. End-wall friction slows the streamwise surface velocity adjacent to the walls, yet material just inward from the end wall flows faster than the material in the center of a long tumbler. An axial velocity occurs near the end walls both upstream and downstream of the midlength of the flowing layer. Increasing the tumbler diameter and corresponding flowing layer length causes the magnitude of the axial and streamwise velocity components, as well as the axial thickness of the end-wall region, to increase. An increase of end-wall friction slows particles directly adjacent to the end wall, further enhancing the axial flow near the end wall. Increasing the rotation rate, while still maintaining a flat continuously flowing surface, causes a corresponding increase in both the streamwise and axial velocities in the tumbler. The boundary flow effects are localized to end walls when the axial length of the tumbler (L) is greater than the diameter (D), so that the flow in the center region of the tumbler is independent of the end walls and hence similar to the conditions at the center of an infinitely long tumbler. Decreasing the axial length of the tumbler so that L/D<1 causes the axial velocity to decrease as the regions affected by the end walls merge. This also leads to a streamwise velocity in quasi-two-dimensional tumblers that can be more than twice that at the center of three-dimensional tumblers.

Journal Article↗

PubMed2006

Surface roughness effects in granular matter: influence on angle of repose and the absence of segregation.

We investigate the effect of nanoscale variations in the surface roughness of individual particles on macroscale granular flow characteristics. Experiments were conducted in circular rotating tumblers with smooth and rough 2 and 3 mm steel particles. The smooth beads had a rms surface roughness of approximately 30 to 60 nm; rough beads had a surface roughness of approximately 240 to 350 nm. The dynamic angle of repose for rough particles increased by 10 degrees to 25 degrees over that of smooth particles over a wide range of rotation speeds. Even though surface roughness affects the angle of repose, we were unable to detect any segregation of bidisperse mixtures of rough and smooth particles in the radial direction in two-dimensional (2D) tumblers. Furthermore, no axial banding segregation occurred in 3D tumblers, both cylindrical and spherical. For mixtures of smooth and rough particles, the angle of repose increased monotonically with increasing concentration of rough particles. Particle dynamics simulations verified that the dependence of the angle of repose on the concentration of rough particles can be directly related to the coefficient of friction of the particles. Simulations over a broad range of friction parameters failed to induce segregation solely from differences in the angle of repose. These results indicate that nanoscale surface roughness can affect the flowability and angle of repose of granular matter without driving demixing of the bulk granular material.

Journal Article↗

PubMed2005

Removal of organic contaminants by RO and NF membranes.

Rejection characteristics of organic and inorganic compounds were examined for six reverse osmosis (RO) membranes and two nanofiltration (NF) membranes that are commercially available. A batch stirred-cell was employed to determine the membrane flux and the solute rejection for solutions at various concentrations and different pH conditions. The results show that for ionic solutes the degree of separation is influenced mainly by electrostatic exclusion, while for organic solutes the removal depends mainly upon the solute radius and molecular structure. In order to provide a better understanding of rejection mechanisms for the RO and NF membranes, the ratio of solute radius (r(i,s)) to effective membrane pore radius (r(p)) was employed to compare rejections. An empirical relation for the dependence of the rejection of organic compounds on the ratio r(i,s)/r(p) is presented. The rejection for organic compounds is over 75% when r(i,s)/r(p) is greater than 0.8. In addition, the rejection of organic compounds is examined using the extended Nernst-Planck equation coupled with a steric hindrance model. The transport of organic solutes is controlled mainly by diffusion for the compounds that have a high r(i,s)/r(p) ratio, while convection is dominant for compounds that have a small r(i,s)/r(p) ratio.

2-Propanol↗

PubMed2005

Synthesizing primary molecular relaxation processes in excitable gases using a two-frequency reconstructive algorithm.

Identifying molecular relaxation processes in excitable gases remains challenging. An algorithm that reconstructs the primary relaxation processes is presented. Based on measurements of acoustic attenuation and sound speed at two frequencies, it synthesizes the entire frequency dependence of the complex effective specific heat of the gas, which is the macroscopic "footprint" of relaxation effects. The algorithm is based on the fact that for a simple relaxation process, such as occurs in many polyatomic gases at temperatures around 300 K, the effective specific heat traces a semicircle in the complex plane as a function of frequency. Knowing the high-frequency or instantaneous value of the specific heat provides the capability to not only sense the presence, but also infer the nature and, for mixtures of unlike-symmetry molecules, the concentration of foreign molecules leaking in a host gas.

Journal Article↗

PubMed2005

Combined size and density segregation and mixing in noncircular tumblers.

Flowing granular materials segregate due to differences in particle size (driven by percolation) and density (driven by buoyancy). For noncircular tumblers the additional interaction between the segregation mechanisms and chaotic advection complicates the physics. Experiments are performed using a bi-disperse mixture of equal volumes of different sizes of steel and glass beads in a quasi-two-dimensional square tumbler. Mixing is observed instead of segregation when the denser beads are larger than the lighter beads so that the ratio of particle sizes is greater than the ratio of particle densities. This can be expressed in terms of the particle diameters and masses as d(heavy)/d(light) >(m(heavy)/m(light))(1/4) . Segregation patterns vary from a semicircular core when the fill level is below 50% to more complicated patterns including lobes and streaks for fill levels above 50%. Temporal evolution of segregated patterns is quantified in terms of a "segregation index" (based on the area of the segregated pattern) to capture both the rate and extent of segregation at different particle properties. The circular and noncircular tumblers have no significant difference in the segregation index, even though the segregation patterns differ significantly.

Journal Article↗

PubMed2005

Creeping motion in granular flow.

The core of a quasi-two-dimensional rotating cylinder filled more than half full with glass beads rotates slightly faster than the cylinder itself and decreases in radius over time. Core precession depends linearly on the number of tumbler revolutions while core erosion varies logarithmically. Both processes serve to quantify the slow granular motion in the "fixed" bed and depend on the filling fraction and the tumbler rotation rate. A simple model, based on experimental observations of an exponential decrease in velocity parallel to the free surface, captures the primary features of the core dynamics.

Journal Article↗

PubMed2005

Dynamics of granular band formation: Long-term behavior in slurries, parameter space, and tilted cylinders.

Band formation (axial segregation) and subsequent coarsening of bidisperse mixtures in long circular tumblers is well documented for the case where the cylinder is at a single fill level and the interstitial fluid is air. However, little information is available for a range of fill levels, nor is the effect of rotational speed on segregation clear. Moreover just a handful of studies have focused on slurry systems, where the interstitial fluid is a liquid. This is precisely the parameter space covered in this study. Experiments are conducted using a 2:1 mixture of 882 and 272 mum glass beads with water as the interstitial fluid. Several different phenomena are uncovered. Results indicate that bands are less likely to form at low rotational speeds and low fill levels. As the fill level and rotational speed increase, more bands form and they form more quickly. However, at fill levels near 50% and high rotational speeds the bands contain a mixture of particles rather than being relatively pure. Furthermore, the evolution of the core of small beads that forms deep in the bed depends on the fill level and the rotational speed. For certain fill levels and rotational speeds, the core remains prominent as bands form, while in other cases the core disappears entirely between bands. Finally, when the tumbler is tilted so that the fill level varies from 14% at one end to slightly more than half full at the other end, the bands and core that form locally qualitatively correspond with those that would form for the corresponding fill level in a horizontal cylinder.

Journal Article↗

PubMed2005

Reverse osmosis membrane rejection for ersatz space mission wastewaters.

Adequate rejection of a variety of inorganic and organic compounds is necessary if reverse osmosis (RO) and nanofiltration (NF) membranes are to be used for space mission wastewater reuse. Three ersatz space mission wastewaters defined by NASA having different pH (2.6-8.9), conductivities (3980-12,640 microS/cm), and amounts of organic compounds (50-2400 mg/L as carbon) were tested to determine the membrane flux and the solute rejection for five RO and two NF membranes that are commercially available. The results show that the rejection of ions depends upon the solution pH which influences electrostatic repulsion. However, the rejection of dissolved organic carbon (DOC) depends upon the composition of the wastewater. The DOC rejection (80-95%) was the highest for the wastewater containing dextran (molecular weight 15-20 k) compared with the other ersatz wastewaters having detergent and urea as the major carbon sources (31-83%). The wastewater having the greatest conductivity (12,640 microS/cm) and DOC (2400 mg/L) showed a greater flux decline (71-96%) than the other ersatz wastewaters (37-82%) having lower conductivities (3980-6980 microS/cm) and DOC (50-660 mg/L) for the RO and NF membranes. The ratio of solute radius (r(i,s)) to effective membrane pore radius (r(p)) was employed to compare ion rejection. For ionic compounds, the rejection is higher than 70% when the r(i,s)/r(p) ratio is greater than 0.5 for both the RO and NF membranes with all wastewaters.

Carbon↗

PubMed2005

Fine-tuning molecular acoustic models: sensitivity of the predicted attenuation to the Lennard-Jones parameters.

In a previous paper [Y. Dain and R. M. Lueptow, J. Acoust. Soc. Am. 109, 1955 (2001)], a model of acoustic attenuation due to vibration-translation and vibration-vibration relaxation in multiple polyatomic gas mixtures was developed. In this paper, the model is improved by treating binary molecular collisions via fully pairwise vibrational transition probabilities. The sensitivity of the model to small variations in the Lennard-Jones parameters--collision diameter (sigma) and potential depth (epsilon)--is investigated for nitrogen-water-methane mixtures. For a N2(98.97%)-H2O(338 ppm)-CH4(1%) test mixture, the transition probabilities and acoustic absorption curves are much more sensitive to sigma than they are to epsilon. Additionally, when the 1% methane is replaced by nitrogen, the resulting mixture [N2(99.97%)-H2O(338 ppm)] becomes considerably more sensitive to changes of sigma(water). The current model minimizes the underprediction of the acoustic absorption peak magnitudes reported by S. G. Ejakov et al. [J. Acoust. Soc. Am. 113, 1871 (2003)].

Acoustics↗

PubMed2003

Theory for a gas composition sensor based on acoustic properties.

Sound travelling through a gas propagates at different speeds and its intensity attenuates to different degrees depending upon the composition of the gas. Theoretically, a real-time gaseous composition sensor could be based on measuring the sound speed and the acoustic attenuation. To this end, the speed of sound was modelled using standard relations, and the acoustic attenuation was modelled using the theory for vibrational relaxation of gas molecules. The concept for a gas composition sensor is demonstrated theoretically for nitrogen-methane-water and hydrogen-oxygen-water mixtures. For a three-component gas mixture, the measured sound speed and acoustic attenuation each define separate lines in the composition plane of two of the gases. The intersection of the two lines defines the gas composition. It should also be possible to use the concept for mixtures of more than three components, if the nature of the gas composition is known to some extent.

Acoustics↗

PubMed2003

Acoustic attenuation in gas mixtures with nitrogen: experimental data and calculations.

Attenuation in a gas results from a combination of classical attenuation, attenuation from diffusion, and attenuation due to molecular relaxation. In previous papers [J. Acoust. Soc. Am. 109, 1955 (2001); 110, 2974 (2001)] a model is described that predicts the attenuation from vibrational relaxation in gas mixtures. In order to validate this model, the attenuation was measured using a pulse technique with four transducer pairs, each with a different resonant frequency. The attenuation calculated using the model was compared to the measured values for a variety of gases including: air, oxygen, methane, hydrogen, and mixtures of oxygen/nitrogen, methane/nitrogen, carbon dioxide/nitrogen, and hydrogen/nitrogen. After the measured data is corrected for diffraction, the model matches the trends in the measured attenuation spectrum for this extensive set of gas mixtures.

Acoustics↗

PubMed2003

Diffraction and attenuation of a tone burst in mono-relaxing media.

The influence of intrinsic absorption in a relaxing medium and the resulting three-dimensional diffraction correction of the magnitude of the acoustic pressure averaged over the surface of a receiver is investigated for a tone burst. A rigorous formula for the damped acoustic pressure average at the receiver was obtained for arbitrary pulsed waves in a mono-relaxing medium. Depending on the pulse oscillation frequency, envelope duration, and relaxation frequency of the media, the plane wave burst envelope can be reduced, amplified, or otherwise deformed.

Journal Article↗

PubMed2002

Design parameters for rotating cylindrical filtration.

Rotating cylindrical filtration displays significantly reduced plugging of filter pores and build-up of a cake layer, but the number and range of parameters that can be adjusted complicates the design of these devices. Twelve individual parameters were investigated experimentally by measuring the build-up of particles on the rotating cylindrical filter after a fixed time of operation. The build-up of particles on the filter depends on the rotational speed, the radial filtrate flow, the particle size and the gap width. Other parameters, such as suspension concentration and total flow rate are less important. Of the four mechanisms present in rotating filters to reduce pore plugging and cake build-up, axial shear, rotational shear, centrifugal sedimentation and vortical motion, the evidence suggests rotational shear is the dominant mechanism, although the other mechanisms still play minor roles. The ratio of the shear force acting parallel to the filter surface on a particle to the Stokes drag acting normal to the filter surface on the particle due to the difference between particle motion and filtrate flow can be used as a non-dimensional parameter that predicts the degree of particle build-up on the filter surface for a wide variety of filtration conditions.

Biomechanical Phenomena↗