Keld Rømer Rasmussen

Saltating particles in a turbulent boundary layer: experiment and theory

The work presented here focuses on the analysis of a turbulent boundary layer saturated with saltating particles. Experiments were carried out in a wind tunnel 15 m long and 0.6 m wide at the University of Aarhus in Denmark with sand grains 242 μm in size for wind speeds ranging from the threshold speed to twice its value. The saltating particles were analysed using particle image velocimetry (PIV) and particle-tracking velocimetry (PTV), and vertical profiles of particle concentration and velocity were extracted. The particle concentration was found to decrease exponentially with the height above the bed, and the characteristic decay height was independent of the wind speed. In contrast with the logarithmic profile of the wind speed, the grain velocity was found to vary linearly with the height. In addition, the measurements indicated that the grain velocity profile depended only slightly on the wind speed. These results are shown to be closely related to the features of the splash function that characterizes the impact of the saltating particles on a sandbed. A numerical simulation is developed that explicitly incorporates low-velocity moments of the splash function in a calculation of the boundary conditions that apply at the bed. The overall features of the experimental measurements are reproduced by simulation.

C6—C10-Dicarboxylic Aciduria: Investigations of a Patient with Riboflavin Responsive Multiple Acyl-CoA Dehydrogenation Defects

Summary: The abnormal metabolites-adipic, suberic, and sebacic acids-were detected in large amounts in the urine of a boy during a Reyes syndrome-like crisis. Substantial amounts of 5-OH-caproic acid, caproylglycine, glutaric acid, and 3-OH-butyric acid and moderately elevated amounts of ethylmalonic acid, methylsuccinic acid, 3-OH-isovaleric acid, and isovalerylglycine were also found. These metabolites were consistently present in urine samples collected in the boys habitual condition after the attack. 1-[14C]-Palmitic acid was oxidized at a normal rate, whereas U-[14C]-Palmitic acid was oxidized at a reduced rate in cultured skin fibroblasts from the patient, thus indicating a defect at the level of medium- and/or short-chain fatty acid oxidation. Riboflavin medication (100 mg three times a day) significantly reduced the excreted amounts of pathologic metabolites, suggesting a flavineadeninedinucleotide-related acyl-CoA dehydrogenation defect as the cause of the disease.Carnitine in plasma was low in the patient (6 μmole/liter, controls 26–74 μmole/liter), suggesting carnitine deficiency as a secondary effect of the acyl-CoA dehydrogenation deficiency.Speculation: The present patient, who presented with a Reyes syndrome-like attack, suffers from impaired dehydrogenation of acyl-CoA resulting in accumulation of acyl-CoA in the cells. Attacks with similar symptoms are seen in other acyl-CoA dehydrogenation deficiencies, such as glutaric aciduria types I and II, other types of C6—C10-dicarboxylic acidurias and isovaleric acidemia. Reduced flow through the acyl-CoA dehydrogenation steps may therefore be an ethiologic factor in Reyes syndrome. Several of the accumulated acyl-CoAs are toxic and may be responsible for some of the symptoms. The low carnitine level in plasma and the elevated esterified carnitine excretion in the present patient indicate that acyl-CoA accumulation may cause a functional carnitine deficiency by sequestration of carnitine as acyl-carnitines. As the inborn defect, systemic carnitine deficiency may exhibit symptoms like those of Reyes syndrome, it may be speculated whether functional carnitine deficiency in patients with accumulated acyl-CoA is another causal factor in the development of the symptoms during attacks.

Applications of spaceborne radar laboratory data to the study of aeolian processes

Aerodynamic roughness (z0) is an important parameter in studies of sand and dust transport, as well as atmospheric circulation models. Aerodynamic roughness is a function of the size and spacing of surface roughness elements and is typically determined at point locations in the field from wind velocity profiles. Because field measurements require complex logistics, z0 values have been obtained for very few localities. If radar can be used to map z0, estimates can be obtained for large areas. In addition, because aerodynamic roughness can change in response to surface processes (e.g., flooding of alluvial surfaces), radar remote sensing could obtain new measurements on short timescales. Both z0 and the radar backscatter coefficient σ0 are dependent on topographic roughness at the submeter scale, and correlation between these two parameters was developed based on radar data obtained from aircraft (AIRSAR). The Spaceborne Radar Laboratory (SRL) afforded the opportunity to test the correlation for data obtained from orbit. SRL data for sites in Death Valley, California; Lunar Lake, Nevada; and Gobabeb, Namibia, were correlated with wind data and compared with previous radar z0 relations. Correlations between σ0 and z0 for L band (λ=24 cm) HV (H, vertically and V, vertically polarized modes) L band HH, and C band (λ=5.6 cm) HV compare favorably with previous studies. Based on these results, maps of z0 values were derived from SRL data for each site, demonstrating the potential to map z0 for large vegetation-free areas from orbit using radar systems.

Saltation and wind-flow interaction in a variable slope wind tunnel

Abstract In a 15 m long horizontal wind tunnel, we studied the internal boundary layer over fixed arrays with roughness (z0) similar to the roughness of saltation of a sand bed. Utilizing turbulence spires the logarithmic overlap-layer is at least 75 mm thick some 5 m downwind of the entry. This permits a precise determination of u∗ from wind profile data. Over a sand bed in a variable slope wind tunnel with a 6 m long working section we studied how slope, grain size, and friction speed influence bed roughness. Owen (1964) proposed that z 0 = C · u ∗ 2 /2 g where C is a constant, but we found that C depends in a complex manner on grain size and friction speed. C increases from very low values near the threshold to an apparently limiting value at large friction speeds. For an almost uniform 125 μm sand, the limiting value of C is 0.06. The limiting value decreases, however, with particle size: for a uniform 544 μm sand, C only approaches 0.03 for friction speeds as high as 1 m/s. With the measured values of friction speed corrected for the effects of slope, our results appear to show that the limiting values of C do not depend on the slope angle, even though the slope affects the individual shapes of particle trajectory. C is perhaps independent of slope angle because the roughness height, and, therefore, C, is more directly related to the amount of momentum extracted from the air by the particles (i.e. the shear stress) rather than to the manner of extraction (particle trajectory shapes).

Barchan dune mobility in Mauritania related to dune and interdune sand fluxes

[1] We present a 1 year study of 50 dunes in a small field of barchan dunes in Mauritania. We documented the morphological evolution of the dunes and their migration rates and measured at 10 min intervals the interdune sand transport, the wind strength, and its direction for the same interval of time. The dune heights H 0 range between 2 and 5 m, and their celerity c is found to be well approximated by the standard migration law: c = Q 0 /H 0 , with Q 0 ≈ 50 m 3 /m yr. From both the interdune sand flux and the migration rate of the dunes we were able to estimate the spatially averaged sand flux at the dune crest as well as the bulk sand flux associated with the mass of sand transported by the dune. We found that the sand flux at the crest was about 3 times greater than the interdune mass transport rate, whereas the bulk sand flux was surprisingly of the same order as the interdune flux. Moreover, we analyzed carefully the interdune sand transport data, which can be well described by the Sorensen law. The cumulative mass of sand transported during moderate wind events within 1 year was much greater than that transported during strong wind events.

Assessment of aerodynamic roughness via airborne radar observations

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.

Roughness element effect on local and universal saltation transport

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.

Simulation of the Martian dust aerosol at low wind speeds

[1] Performing realistic simulations is crucial for developing, testing, and subsequently analyzing results of experiments sent to the surface of Mars. A wind tunnel has been constructed, in which the atmospheric conditions of pressure and wind speed are controlled to match those observed by the Pathfinder mission to Mars. Injection into the wind tunnel of an analogue dust from Salten Skov in Denmark allows simulation of the Martian aerosol. Here experiments can be tested in preparation for a planned mission to the planet (Mars Exploration Rovers to be launched in 2003). Observations of adhesion and cohesion effects have been made in the wind tunnel, which are relevant to particle transport and of significance for validating the performance of specific experiments on Mars. Preliminary studies have been made, at Mars atmospheric pressure, of dust capture on magnet arrays similar to those flown on the Mars Pathfinder mission.

Potential Transport of Windblown Sand: Influence of Surface Roughness and Assessment with Radar Data

The transport of windblown sand is controlled by many factors, including wind regime and sediment supply. Surface roughness at the sub-meter scale is also important because it influences both the threshold conditions for particle entrainment and the flux of sand once it is set into motion. In general, increases in surface roughness result in higher threshold speeds for particle movement and decreases in sand fluxes. Aerodynamic roughness (z 0) is the aeolian parameter related to surface roughness and is defined as the height above some mean level at which average wind speed is zero. Values of z 0 are derived from wind measurements through the boundary layer, but few z 0 values have been obtained over natural surfaces because of the expense and limitations of making such measurements. Rather, remote sensing using radar systems has the potential for addressing this problem. In this investigation, we derived z 0 values for a wide variety of surfaces in the southwestern United States and obtained radar data for these sites in P-band (wavelength = 68 cm), L-band (wavelength = 24 cm) and C-band (wavelength = 5.6 cm). We show that there are good correlations among z 0, the RMS height of the surface, and the radar backscatter coefficient, σ0, with the best correlation for L-band HV polarized radar data. This study shows the potential for mapping large regions with radar in order to derive aerodynamic roughness values, which in turn can be used in predictive models of sand transport.

Atmospheric ammonia exchange on a heathland in Denmark

Abstract Passive flux samplers were used to determine the ammonia exchange on an inland heath in Denmark over the last 2 years. The samplers measured the horizontal ammonia flux directly. Data were sampled continuously for periods of 1–4 weeks. The micro-meteorological gradient method was used with passive flux samplers and cup anemometers at different heights above the vegetation in order to calculate the vertical fluxes of ammonia. First a fixed sampler system was used with tubes mounted in four orthogonal horizontal directions. This system has been successfully applied to measure the emission from fertilized crops. Adapting this type of sampler to measure the deposition to heathlands did not prove to be straightforward. The precision of the calculated ammonia concentrations was too poor to give an acceptable accuracy for the concentration gradient. The problems were (a) driving rain which entered the tubes, (b) too few measuring points in each concentration profile, and (c) too long sampling periods to allow for the low-concentration levels above the heath area and the detection limits. Therefore, a passive flux sampler mounted on a wind vane and fitted with a rain shelter was developed. Results from the first 5 weekly periods are very promising, yielding accurate concentration gradients. The advantages of the passive flux samplers on the wind vane are (a) the minimum measuring period can be approximately halved compared to the fixed samplers, (b) some of the directional correction terms used with the fixed passive flux samplers are dispensed with, and (c) the field and laboratory work is minimized.