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"Anderson, W"
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Turbulent channel flow over heterogeneous roughness at oblique angles
Large-eddy simulation has been used to model turbulent channel flow over a range of surfaces featuring a prominent spatial heterogeneity; the flow streamwise direction is aligned relative to the heterogeneity at a range of angles, defined herein with
$\\unicode[STIX]{x1D703}$
. Prior work has established that a sharp roughness heterogeneity orthogonal to the flow streamwise direction (
$\\unicode[STIX]{x1D703}=0$
) induces formation of an internal boundary layer, which originates at the heterogeneity and thickens in the downflow direction before being homogenized via ambient shear. In contrast, more-recent studies have shown that a sharp roughness heterogeneity parallel to the flow streamwise direction (
$\\unicode[STIX]{x1D703}=\\unicode[STIX]{x03C0}/2$
) induces streamwise-aligned, Reynolds-averaged secondary cells, where the spacing between adjacent surface heterogeneities regulates the spatial extent of secondary cells. No prior study has addressed intermediate (oblique) cases,
$0\\leqslant \\unicode[STIX]{x1D703}\\leqslant \\unicode[STIX]{x03C0}/2$
. Results presented herein show that the momentum penalty exhibits a nonlinear dependence upon obliquity, where internal boundary layer-like flow processes persist over a range of obliquity angles before abruptly vanishing for spanwise roughness heterogeneity (
$\\unicode[STIX]{x1D703}=\\unicode[STIX]{x03C0}/2$
). This result manifests itself within effective roughness lengths recovered
a posteriori
: the traditional approach to roughness modelling – predicated upon dependence with surface geometric arguments including height root-mean-square, skewness, frontal- and plan-area index, effective slope. and combinations thereof – is insufficient. A revised model incorporating dependence upon roughness frontal area index and flow-heterogeneity obliquity angle is able to accurately predict effective roughness length
a priori
.
Journal Article
Talking about making good choices
\"Guides readers through some decision-making processes and gives them the tools they need to choose their actions wisely. Also examined are influences that affect our choices, such as advertising campaigns, home environment, and peer pressure\"--P. [4] of cover.
Book
Buoyancy effects on large-scale motions in convective atmospheric boundary layers: implications for modulation of near-wall processes
A number of recent studies have demonstrated the existence of so-called large- and very-large-scale motions (LSM, VLSM) that occur in the logarithmic region of inertia-dominated wall-bounded turbulent flows. These regions exhibit significant streamwise coherence, and have been shown to modulate the amplitude and frequency of small-scale inner-layer fluctuations in smooth-wall turbulent boundary layers. In contrast, the extent to which analogous modulation occurs in inertia-dominated flows subjected to convective thermal stratification (low Richardson number) and Coriolis forcing (low Rossby number), has not been considered. And yet, these parameter values encompass a wide range of important environmental flows. In this article, we present evidence of amplitude modulation (AM) phenomena in the unstably stratified (i.e. convective) atmospheric boundary layer, and link changes in AM to changes in the topology of coherent structures with increasing instability. We perform a suite of large eddy simulations spanning weakly (
$-z_{i}/L=3.1$
) to highly convective (
$-z_{i}/L=1082$
) conditions (where
$-z_{i}/L$
is the bulk stability parameter formed from the boundary-layer depth
$z_{i}$
and the Obukhov length
$L$
) to investigate how AM is affected by buoyancy. Results demonstrate that as unstable stratification increases, the inclination angle of surface layer structures (as determined from the two-point correlation of streamwise velocity) increases from
$\\unicode[STIX]{x1D6FE}\\approx 15^{\\circ }$
for weakly convective conditions to nearly vertical for highly convective conditions. As
$-z_{i}/L$
increases, LSMs in the streamwise velocity field transition from long, linear updrafts (or horizontal convective rolls) to open cellular patterns, analogous to turbulent Rayleigh–Bénard convection. These changes in the instantaneous velocity field are accompanied by a shift in the outer peak in the streamwise and vertical velocity spectra to smaller dimensionless wavelengths until the energy is concentrated at a single peak. The decoupling procedure proposed by Mathis et al. (J. Fluid Mech., vol. 628, 2009a, pp. 311–337) is used to investigate the extent to which amplitude modulation of small-scale turbulence occurs due to large-scale streamwise and vertical velocity fluctuations. As the spatial attributes of flow structures change from streamwise to vertically dominated, modulation by the large-scale streamwise velocity decreases monotonically. However, the modulating influence of the large-scale vertical velocity remains significant across the stability range considered. We report, finally, that amplitude modulation correlations are insensitive to the computational mesh resolution for flows forced by shear, buoyancy and Coriolis accelerations.
Journal Article
An atlas of Middle Eastern affairs
\"The Middle East is a major focus of world interest. This revised and updated atlas provides accessible, concisely written entries on the most important current issues in the Middle East, combining maps with their geopolitical background. Providing a clear context for analysis of key concerns, it includes background topics, the position of the Middle East in the world and profiles of the constituent countries. \" -- from publisher.
Book
Non-periodic phase-space trajectories of roughness-driven secondary flows in high- boundary layers and channels
Turbulent flows respond to bounding walls with a predominant spanwise heterogeneity – that is, a heterogeneity parallel to the prevailing transport direction – with formation of Reynolds-averaged turbulent secondary flows. Prior experimental and numerical work has determined that these secondary rolls occur in a variety of arrangements, contingent only upon the existence of a spanwise heterogeneity (i.e. from complex, multiscale roughness with a predominant spanwise heterogeneity, to canonical step changes, to different roughness elements). These secondary rolls are known to be a manifestation of Prandtl’s secondary flow of the second kind: driven and sustained by the existence of spatial heterogeneities in the Reynolds (turbulent) stresses, all of which vanish in the absence of spanwise heterogeneity. Herein, we show results from a suite of large-eddy simulations and complementary experimental measurements of flow over spanwise-heterogeneous surfaces. Although the resultant secondary cell location is clearly correlated with the surface characteristics, which ultimately dictates the Reynolds-averaged flow patterns, we show the potential for instantaneous sign reversals in the rotational sense of the secondary cells. This is accomplished with probability density functions and conditional sampling. In order to address this further, a base flow representing the streamwise rolls is introduced. The base flow intensity – based on a leading-order Galerkin projection – is allowed to vary in time through the introduction of time-dependent parameters. Upon substitution of the base flow into the streamwise momentum and streamwise vorticity transport equations, and via use of a vortex forcing model, we are able to assess the phase-space evolution (orbit) of the resulting system of ordinary differential equations. The system resembles the Lorenz system, but the forcing conditions differ intrinsically. Nevertheless, the system reveals that chaotic, non-periodic trajectories are possible for sufficient inertial conditions. Poincaré projection is used to assess the conditions needed for chaos, and to estimate the fractal dimension of the attractor. Its simplicity notwithstanding, the propensity for chaotic, non-periodic trajectories in the base flow model suggests similar dynamics is responsible for the large-scale reversals observed in the numerical and experimental datasets.
Journal Article
A nanoliter-scale nucleic acid processor with parallel architecture
The purification of nucleic acids from microbial and mammalian cells is a crucial step in many biological and medical applications
1
. We have developed microfluidic chips for automated nucleic acid purification from small numbers of bacterial or mammalian cells. All processes, such as cell isolation, cell lysis, DNA or mRNA purification, and recovery, were carried out on a single microfluidic chip in nanoliter volumes without any pre- or postsample treatment. Measurable amounts of mRNA were extracted in an automated fashion from as little as a single mammalian cell and recovered from the chip. These microfluidic chips are capable of processing different samples in parallel, thereby illustrating how highly parallel microfluidic architectures can be constructed to perform integrated batch-processing functionalities for biological and medical applications.
Journal Article
Investigating the Toxicity, Uptake, Nanoparticle Formation and Genetic Response of Plants to Gold
We have studied the physiological and genetic responses of Arabidopsis thaliana L. (Arabidopsis) to gold. The root lengths of Arabidopsis seedlings grown on nutrient agar plates containing 100 mg/L gold were reduced by 75%. Oxidized gold was subsequently found in roots and shoots of these plants, but gold nanoparticles (reduced gold) were only observed in the root tissues. We used a microarray-based study to monitor the expression of candidate genes involved in metal uptake and transport in Arabidopsis upon gold exposure. There was up-regulation of genes involved in plant stress response such as glutathione transferases, cytochromes P450, glucosyl transferases and peroxidases. In parallel, our data show the significant down-regulation of a discreet number of genes encoding proteins involved in the transport of copper, cadmium, iron and nickel ions, along with aquaporins, which bind to gold. We used Medicago sativa L. (alfalfa) to study nanoparticle uptake from hydroponic culture using ionic gold as a non-nanoparticle control and concluded that nanoparticles between 5 and 100 nm in diameter are not directly accumulated by plants. Gold nanoparticles were only observed in plants exposed to ionic gold in solution. Together, we believe our results imply that gold is taken up by the plant predominantly as an ionic form, and that plants respond to gold exposure by up-regulating genes for plant stress and down-regulating specific metal transporters to reduce gold uptake.
Journal Article