James D. Iversen

Bio: James D. Iversen is an academic researcher from Iowa State University. The author has contributed to research in topics: Wind tunnel & Saltation (geology). The author has an hindex of 28, co-authored 66 publications receiving 3495 citations.

Merging of Aircraft Trailing Vortices

Abstract: A merging distance criterion for equal-strength corotational vortices is derived from low-turbulence wind tunnel flow visualization data. The vortex separation distance is normalized by defining a vortex core diameter based on circulation defect and angular momentum defect. Merging may take place for larger separation distances than predicted from earlier two-dimensional inviscid calculations, which indicates that viscosity and possibly three-dimensional effects are important factors in the merging phenomenon. Hot-wire velocity distributions and rolling moment measurements show that attenuation of the vortex hazard is associated with vortex merging. Nomenclature bg = generator wingspan bf = follower wingspan CL = lift coefficient cf = rolling-moment coefficient c = wing chord d = vortex separation distance d0 = initial vortex separation distance dc = vortex core diameter (usually based on peak tangential speed) dM = vortex core diameter (based on angular momentum defect equal to that in a Rankine vortex) dr = vortex core diameter (based on circulation defect equal to that in a Rankine vortex) dv —vortex core diameter (based on outer boundary of.the rotational portion of the vortex)

Roughness element effect on local and universal saltation transport

Abstract: Experimental results are presented which illustrate the effects of permanent surface obstructions on saltation phenomena. It is shown that the topographic drift geometry and the dimensionless erosion rates of windward erosion associated with cylindrical obstacles are strong functions of the cylinder aspect ratio. For short cylinders, there is also significant erosion taking place in the far wake. These two erosional areas develop due to different sets of separation vortex systems. For multi-element roughness arrays, sparse array data are presented which illustrate the increase of threshold friction speed with element frontal area density and roughness element drag coefficient.

Assessment of aerodynamic roughness via airborne radar observations

Abstract: The objective of this research is to assess the relationship among measurements of roughness parameters derived from radar backscatter, the wind, and topography on various natural surfaces and to understand the underlying physical causes for the relationship. This relationship will form the basis for developing a predictive equation to derive aerodynamic roughness (z0) from radar backscatter characteristics. Preliminary studies support the existence of such a relationship at the L-band (24 cm wavelength) direct polarization (HH) radar band frequencies. To increase the confidence in the preliminary correlation and to extend the application of the technique to future studies involving regional aeolian dynamics, the preliminary study has been expanded by: 1) defining the empirical relationship between radar backscatter and aerodynamic roughness of bare rocks and soils, 2) investigating the sensitivity of the relationship to microwave parameters using calibrated multiple wavelength, polarization, and incidence angle aircraft radar data, and 3) applying the results to models to gain an understanding of the physical properties which produce the relationship. The approach combines the measurement, analysis, and interpretation of radar data with field investigations of aeolian processes and topographic roughness.

Drifting Snow Similitude

Abstract: The problem of modeling the drifting of snow in a boundary layer wind tunnel is reviewed. The important dimensionless parameters that govern the drifting phenomena are introduced. Theoretical means such as the equations for mass transport rate of saltating material and the particle trajectory equations of motion are used to combine these dimensionless parameters into five similitude parameters. These final five parameters are analyzed as to their relative importance in the wind tunnel simulation of drifting snow. The properties of snow and of possible modeling materials are examined, and the final results of a previous study involving simulation of sand movement are presented. Use of a variety of model particles and the variation of vertical distortion of the modeled topography can aid in application of the results to full-scale conditions. Quantitative scaling can be achieved by measurement of the rate of accumulation of drifting material.

Sources and distributions of dust aerosols simulated with the GOCART model

Abstract: The global distribution of dust aerosol is simulated with the Georgia Tech/Goddard Global Ozone Chemistry Aerosol Radiation and Transport (GOCART) model. In this model all topographic lows with bare ground surface are assumed to have accumulated sediments which are potential dust sources. The uplifting of dust particles is expressed as a function of surface wind speed and wetness. The GOCART model is driven by the assimilated meteorological fields from the Goddard Earth Observing System Data Assimilation System (GEOS DAS) which facilitates direct comparison with observations. The model includes seven size classes of mineral dust ranging from 0.1–6 μm radius. The total annual emission is estimated to be between 1604 and 1960 Tg yr−1 in a 5-year simulation. The model has been evaluated by comparing simulation results with ground-based measurements and satellite data. The evaluation has been performed by comparing surface concentrations, vertical distributions, deposition rates, optical thickness, and size distributions. The comparisons show that the model results generally agree with the observations without the necessity of invoking any contribution from anthropogenic disturbances to soils. However, the model overpredicts the transport of dust from the Asian sources to the North Pacific. This discrepancy is attributed to an overestimate of small particle emission from the Asian sources.

Modeling the atmospheric dust cycle: 1. Design of a soil-derived dust emission scheme

Abstract: A soil-derived dust emission scheme has been designed to provide an explicit representation of the desert dust sources for the atmospheric transport models dealing with the simulation of the desert dust cycle. Two major factors characterizing the erodible surface are considered: (1) the size distribution of the erodible loose particles of the soil which controls the erosion threshold and the emission strength and (2) the surface roughness which imposes the efficient wind friction velocity acting on the erodible surface. These two parameters are included in a formulation of the threshold wind friction velocity by adapting a size-dependent parameterization proposed by Iversen and White (1982) and by applying to the rough erodible surfaces a drag partition scheme derived from Arya (1975). This parameterization of the threshold friction velocity has been included in an horizontal flux equation proposed by White (1979). This allows to attribute a specific production rate to each soil size range for each type of surface. The dust flux F is then considered as a fraction of the total horizontal flux G, the value of the ratio F/G being imposed, at this time, by the soil clay content. In summary, the computed mass fluxes depend on the soil size distribution, the roughness lengths, and the wind friction velocity. The different steps of this scheme have been independently validated by comparison with relevant experimental data. Globally, the agreement is satisfying, so that the dust fluxes could be retrieved with less uncertainties than those observed in previous simulations of the desert dust cycle.

The physics of wind-blown sand and dust

Abstract: The transport of sand and dust by wind is a potent erosional force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols This article presents an extensive review of the physics of wind-blown sand and dust on Earth and Mars Specifically, we review the physics of aeolian saltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission, the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scale vortices We also discuss the physics of wind-blown sand and dune formation on Venus and Titan

Context Camera Investigation on board the Mars Reconnaissance Orbiter

Abstract: [1] The Context Camera (CTX) on the Mars Reconnaissance Orbiter (MRO) is a Facility Instrument (i.e., government-furnished equipment operated by a science team not responsible for design and fabrication) designed, built, and operated by Malin Space Science Systems and the MRO Mars Color Imager team (MARCI). CTX will (1) provide context images for data acquired by other MRO instruments, (2) observe features of interest to NASA's Mars Exploration Program (e.g., candidate landing sites), and (3) conduct a scientific investigation, led by the MARCI team, of geologic, geomorphic, and meteorological processes on Mars. CTX consists of a digital electronics assembly; a 350 mm f/3.25 Schmidt-type telescope of catadioptric optical design with a 5.7° field of view, providing a ∼30-km-wide swath from ∼290 km altitude; and a 5000-element CCD with a band pass of 500–700 nm and 7 μm pixels, giving ∼6 m/pixel spatial resolution from MRO's nearly circular, nearly polar mapping orbit. Raw data are transferred to the MRO spacecraft flight computer for processing (e.g., data compression) before transmission to Earth. The ground data system and operations are based on 9 years of Mars Global Surveyor Mars Orbiter Camera on-orbit experience. CTX has been allocated 12% of the total MRO data return, or about ≥3 terabits for the nominal mission. This data volume would cover ∼9% of Mars at 6 m/pixel, but overlapping images (for stereo, mosaics, and observation of changes and meteorological events) will reduce this area. CTX acquired its first (instrument checkout) images of Mars on 24 March 2006.

The physics of wind-blown sand and dust

Abstract: The transport of sand and dust by wind is a potent erosional force, creates sand dunes and ripples, and loads the atmosphere with suspended dust aerosols. This paper presents an extensive review of the physics of wind-blown sand and dust on Earth and Mars. Specifically, we review the physics of aeolian saltation, the formation and development of sand dunes and ripples, the physics of dust aerosol emission, the weather phenomena that trigger dust storms, and the lifting of dust by dust devils and other small-scale vortices. We also discuss the physics of wind-blown sand and dune formation on Venus and Titan.