Marko Princevac

Observations of Flow and Turbulence in the Nocturnal Boundary Layer over a Slope

Measurements were conducted on an eastern slope of the Salt Lake Basin (SLB) as a part of the Vertical Transport and Mixing Experiment (VTMX) conducted in October 2000. Of interest was the nocturnal boundary layer on a slope (in particular, katabatic flows) in the absence of significant synoptic influence. Extensive measurements of mean flow, turbulence, temperature, and solar radiation were made, from which circulation patterns on the slope and the nature of stratified turbulence in katabatic winds were inferred. The results show that near the surface (,25‐50 m) the nocturnal flow is highly stratified and directed downslope, but at higher levels winds strongly vary in magnitude and direction with height and time, implying the domination of upper levels by air intrusions. These intrusions may peel off from different slopes surrounding the SLB, have different densities, and flow at their equilibrium density levels. The turbulence was generally weak and continuous, but sudden increases of turbulence levels were detected as the mean gradient Richardson number ( ) dropped to Rig about unity. With a short timescale fluctuated on the order of a few tens of seconds while modulating with Rig a longer (along-slope internal waves sloshing) timescale of about half an hour. The mixing efficiency (or the flux Richardson number) of the flow was found to be a strong function of , similar to that found in laboratory Rig experiments with inhomogeneous stratified shear flows. The eddy diffusivities of momentum and heat were evaluated, and they showed a systematic variation with when scaled with the shear length scale and the rms Rig vertical velocity of turbulence.

Unsteady Thermally Driven Flows on Gentle Slopes

Abstract The theoretical and laboratory studies on mean velocity and temperature fields of an unsteady atmospheric boundary layer on sloping surfaces reported here were motivated by recent field observations on thermally driven circulation in very wide valleys in the presence of negligible synoptic winds. The upslope (anabatic) flow on a long, shallow, heated (with a buoyancy flux Fbs) slope of inclination α located adjoining a level plane and the effects of cooling of the slope on this flow during the evening transition are studied for the case of a gentle slope for which the length of the sloping plane far exceeds the thickness h of the convective boundary layer. First, a theoretical analysis is presented for the mean upslope flow velocity UM, noting that the turbulence but not the mean flow structure therein is similar to that on a level terrain. The analysis, which is based on mean momentum and heat equations as well as closure involving level-terrain turbulence parameterizations, shows that UM is pro...

Reciprocating flow-based centrifugal microfluidics mixer

Proper mixing of reagents is of paramount importance for an efficient chemical reaction. While on a large scale there are many good solutions for quantitative mixing of reagents, as of today, efficient and inexpensive fluid mixing in the nanoliter and microliter volume range is still a challenge. Complete, i.e., quantitative mixing is of special importance in any small-scale analytical application because the scarcity of analytes and the low volume of the reagents demand efficient utilization of all available reaction components. In this paper we demonstrate the design and fabrication of a novel centrifugal force-based unit for fast mixing of fluids in the nanoliter to microliter volume range. The device consists of a number of chambers (including two loading chambers, one pressure chamber, and one mixing chamber) that are connected through a network of microchannels, and is made by bonding a slab of polydimethylsiloxane (PDMS) to a glass slide. The PDMS slab was cast using a SU-8 master mold fabricated by a two-level photolithography process. This microfluidic mixer exploits centrifugal force and pneumatic pressure to reciprocate the flow of fluid samples in order to minimize the amount of sample and the time of mixing. The process of mixing was monitored by utilizing the planar laser induced fluorescence (PLIF) technique. A time series of high resolution images of the mixing chamber were analyzed for the spatial distribution of light intensities as the two fluids (suspension of red fluorescent particles and water) mixed. Histograms of the fluorescent emissions within the mixing chamber during different stages of the mixing process were created to quantify the level of mixing of the mixing fluids. The results suggest that quantitative mixing was achieved in less than 3 min. This device can be employed as a stand alone mixing unit or may be integrated into a disk-based microfluidic system where, in addition to mixing, several other sample preparation steps may be included.

Virtual Towers Using Coherent Doppler Lidar during the Joint Urban 2003 Dispersion Experiment

Abstract During the Joint Urban 2003 (JU2003) atmospheric field experiment in Oklahoma City, Oklahoma, of July 2003, lidar teams from Arizona State University and the Army Research Laboratory collaborated to perform intersecting range–height indicator scans. Because a single lidar measures radial winds, that is, the dot product of the wind vector with a unit vector pointing along the lidar beam, the data from two lidars viewing from different directions can be combined to produce horizontal velocity vectors. Analysis programs were written to retrieve horizontal velocity vectors for a series of eight vertical profiles to the southwest (approximately upwind) of the downtown urban core. This technique has the following unique characteristics that make it well suited for urban meteorology studies: 1) continuous vertical profiles from far above the building heights to down into the street canyons can be measured and 2) the profiles can extend to very near the ground without a loss of accuracy (assuming clear l...

Turbulent Entrainment Into Natural Gravity-Driven Flows

Observations of entrainment into natural gravity-driven flows on sloping surfaces are described. It is shown that the laboratory-based entrainment law of Ellison & Turner (1959), which is often used for modelling of atmospheric and oceanic flows, underestimates the entrainment rates substantially, arguably due to the fact that the laboratory flows have been conducted at Reynolds numbers

Quasi-Steady Katabatic Winds on Slopes in Wide Valleys: Hydraulic Theory and Observations

(Re \lesssim 10^3)

Retrieval of Microscale Wind and Temperature Fields from Single- and Dual-Doppler Lidar Data

below what is required for mixing transition (

Morning breakup of cold pools in complex terrain

Re\sim10^3

Flow and Turbulence in an Urban Canyon

– 10^4 ) whereas natural flows occur at much higher Reynolds numbers

Fluid Dynamic Structures in a Fire Environment Observed in Laboratory-Scale Experiments

(Re\sim10^7)