C. Randall Truman

Wind-tunnel modelling of the influence of vegetation structure on saltation threshold

The influence of vegetation structure on saltation threshold was investigated using uniformly spaced arrays of non-erodible roughness elements on a bed of erodible sand in an wind tunnel. Structural variables tested using arrays of solid cylinders included element aspect ratio (height/diameter) and lateral cover (total frontal-silhouette area per unit ground area). In agreement with previous studies, increase in saltation threshold above the value for bare sand was strongly related to lateral cover. Increasing aspect ratio from 0.25 to 4 tended to enhance the increase in saltation threshold at a given lateral cover. An approximate fit to the results could be obtained using a relation proposed by Raupach et al. (Journal of Geophysical Research, 1993, 98D, 3023–3029). Porous elements were constructed as clusters of narrow, vertically oriented cylinders (model ‘stems’), forming porous model plant bodies that were cylindrical in overall shape. Low-porosity elements were found to be approximately 50 per cent more effective in increasing saltation threshold than either solid or high-porosity elements. The ratio of plant-body frontal-silhouette area (based on overall dimensions) to total stem frontal-silhouette area was found to be a useful measure of plant-body porosity for wind erosion studies. Some conventional measures of vegetation amount fail to account for changes in vegetation structural attributes that may strongly influence aeolian processes.

Effects of organized turbulence structures on the phase distortion in a coherent optical beam propagating through a turbulent shear flow

Effects of organized turbulence structures on the propagation of an optical beam in a turbulent shear flow have been analyzed. An instantaneous passive‐scalar field in a computed homogeneous turbulent shear flow is used to represent index‐of‐refraction fluctuations, and phase distortion induced in a coherent optical beam by turbulent fluctuations is calculated. The organized vortical structures (‘‘hairpin‐shaped’’ eddies) in the turbulent flow give rise to a scalar distribution with elongated regions of intense fluctuation, which have an inclination (about 30°) with respect to the mean flow, similar to that of the characteristic ‘‘hairpin’’ eddies. Two‐point correlations of vorticity and scalar fluctuations support a proposed physical model in which the regions of intense scalar fluctuation are produced primarily by hairpin vortices. It is found that the spatial distribution of the phase distortion has a substantial variation with the direction of propagation. A highly localized distribution of intense ph...

Micronized Drug Adhesion and Detachment from Surfaces: Effect of Loading Conditions

Drug loading and processing conditions in a dry powder inhaler system are critical for performance of the system since it is these forces that must be overcome to properly redisperse respirable particles. In this work, we have investigated the effects of different loading forces for adhering micronized drug onto surfaces on the drug adhesion, detachment, and aerosol performance under controlled conditions. Drug loading onto a standardized surface was performed under three different loading conditions using turbula-mixer to actively induce powder–surface interactions. It was seen the drug loading increased with increased processing time. The presence of external press-on force also increased drug loading. Drug detachment studies performed using centrifugation method indicated that adhesion forces were the lowest at lower mixing time and increased with increasing press-on forces. Drug aerosolization performance from the surface was assessed using a prototype DPI and confirmed that external forces during drug loading and coating processing played an important role in drug dispersion. It was seen that the respirability of drug particles correlated with mixing times as well as the press-on forces.

Observation of the Development of Secondary Features in a Richtmyer–Meshkov Instability Driven Flow

Richtmyer–Meshkov instability (RMI) has long been the subject of interest for analytical, numerical, and experimental studies. In comparing results of experiment with numerics, it is important to understand the limitations of experimental techniques inherent in the chosen method(s) of data acquisition. We discuss results of an experiment where a laminar, gravity-driven column of heavy gas is injected into surrounding light gas and accelerated by a planar shock. A popular and well-studied method of flow visualization (using glycol droplet tracers) does not produce a flow pattern that matches the numerical model of the same conditions, while revealing the primary feature of the flow developing after shock acceleration: the pair of counter-rotating vortex columns. However, visualization using fluorescent gaseous tracer confirms the presence of features suggested by the numerics; in particular, a central spike formed due to shock focusing in the heavy-gas column. Furthermore, the streamwise growth rate of the spike appears to exhibit the same scaling with Mach number as that of the counter-rotating vortex pair (CRVP).

Two-dimensional simulation of a shock-accelerated gas cylinder

The Richtmyer-Meshkov Instability (RMI) arises when a density gradient in a fluid (gas) is subjected to an impulsive acceleration (e.g., due to a shock wave passage). The evolution of RMI is non-linear and hydrodynamically complex and hence is a very good test problem to validate numerical codes. In this paper, we present a two-dimensional numerical simulation of RMI-driven evolution of the flow produced by shock acceleration of a diffuse heavy gaseous cylinder embedded in lighter gas. The initial conditions employed in the simulation are a very close match to the initial conditions realised in a well-characterised experiment, facilitating a detailed quantitative comparison with experimental measurements, as well as with other simulations of the same experiment. Comparison of the late-time flow statistics between experiment and numerics elucidates the limitations inherently present in a two-dimensional simulation of a spatially three-dimensional flow, even if the large-scale flow structure is nominally two-dimensional.

PLIF Flow Visualization of a Supersonic Injection COIL Nozzle

This paper describes Planar Laser-Induced Fluorescence (PLIF) flow visualization of a supersonic nozzle with supersonic injection. The nozzle simulates Chemical Oxygen Iodine Laser (COIL) flow conditions with non-reacting, cold flows, where the injected flow was seeded with iodine. A laser sheet near 565nm excited the iodine, and the fluorescence was imaged with a gated, CCD camera. Spanwise and streamwise images were taken, where the relative concentration of the injected to primary flow, turbulent structures, and penetration distance of the injected flow were identified. These images qualitatively revealed a lack of mixing of the secondary (injected) and primary flows at the centerline of the nozzle, even far downstream of the throat. Quantitative data of the penetration of the secondary flow, with varying primary to secondary flow rate ratios, helped identify the shallow angle of the injectors as an inhibiter of secondary penetration even at relatively low primary flow rates. From the PLIF results, this nozzle is characterized as a poor mixer and would not be recommended as a nozzle that produces a well-mixed medium, as required with chemical lasers. This work precedes a project that will use PLIF results to design a well-mixed supersonic nozzle with supersonic injection. The results will be compared to and enable validation of computational fluid dynamics (CFD) predictions of the designed nozzle.

Tunable diode laser gain measurements on the HF(2-0) overtone transistions in a small-scale HF laser

A tunable diode laser was used to probe the overtone gain medium of a small-scale HF laser. Two-dimensional, spatially resolved small signal gain and temperature maps were generated for the P(3) ro-vibrational transition in the first HF overtone band.

Numerical Simulation of a Shock-Accelerated Multiphase Fluid Interface

A Richtmyer-Meshkov Instability (RMI) [1, 2] is generated when an interface between two different fluids is impulsively accelerated. The instability develops due to misalignment of the density and pressure interfaces. This misalignment results in the deposition of vorticity, causing the formation of an instability that grows nonlinearly with time and eventually may transition to fully turbulent flow. It has been recently shown that a similar class of instability can evolve in a multi-phase flow [3], where the density gradient is caused by a second, non-fluid phase.

Non-Intrusive mach number measurement in a supersonic hydrogen fluoride laser

An experimental technique has been developed to directly measure flow velocity and Mach number in a supersonic hydrogen fluoride laser. The technique uses a tunable diode laser source to probe the laser cavity at an angle to the flow creating a Doppler shifted lineshape. The amount of Doppler shift can be related to the flow velocity. The diode laser was traversed in the vertical direction to produce velocity and Mach number profiles.

Vortex deposition in shock-accelerated gas with particle/droplet seeding

We present an experimental and numerical study of post-shock evolution of gas initially seeded with small droplets or particles. In two-phase media with gas being the embedding phase occupying most of the volume, shock acceleration can lead to vortex formation. The physical mechanism responsible for the vorticity deposition in this case is different from that of Richtmyer-Meshkov instability that would emerge on a gas-gas density interface. After the shock passage, the particles or droplets lag behind the surrounding gas. Momentum exchange between the embedded phase and the embedding phase leads to non-uniform local equilibrium velocity distribution, and thus to shear and vortex formation. Here we investigate shock interaction with a cylindrical particle-seeded column (with and without reshock).