Vegetated mixing layer around a finite-size patch of submerged plants: 1. Theory and field experiments

2012 ◽  
Vol 48 (10) ◽  
Author(s):  
Tatiana A. Sukhodolova ◽  
Alexander N. Sukhodolov
Keyword(s):  
Field Experiments ◽  
Mixing Layer ◽  
Finite Size ◽  
Submerged Plants

The mixing layer and its coherence examined from the point of view of two-dimensional turbulence

1988 ◽  
Vol 192 ◽  
pp. 511-534 ◽  
Author(s):  
Marcel Lesieur ◽  
Chantal Staquet ◽  
Pascal Le Roy ◽  
Pierre Comte
Keyword(s):  
Energy Spectrum ◽  
Kinetic Energy ◽  
White Noise ◽  
Mixing Layer ◽  
Finite Size ◽  
Spanwise Direction ◽  
Computational Domain ◽  
Two Dimensional ◽  
Infinite Domain ◽  
Kinetic Energy Spectrum

A two-dimensional numerical large-eddy simulation of a temporal mixing layer submitted to a white-noise perturbation is performed. It is shown that the first pairing of vortices having the same sign is responsible for the formation of a continuous spatial longitudinal energy spectrum of slope between k−4 and k−3. After two successive pairings this spectral range extends to more than 1 decade. The vorticity thickness, averaged over several calculations differing by the initial white-noise realization, is shown to grow linearly, and eventually saturates. This saturation is associated with the finite size of the computational domain.We then examine the predictability of the mixing layer, considering the growth of decorrelation between pairs of flows differing slightly at the first roll-up. The inverse cascade of error through the kinetic energy spectrum is displayed. The error rate is shown to grow exponentially, and saturates together with the levelling-off of the vorticity thickness growth. Extrapolation of these results leads to the conclusion that, in an infinite domain, the two fields would become completely decorrelated. It turns out that the two-dimensional mixing layer is an example of flow that is unpredictable and possesses a broadband kinetic energy spectrum, though composed mainly of spatially coherent structures.It is finally shown how this two-dimensional predictability analysis can be associated with the growth of a particular spanwise perturbation developing on a Kelvin-Helmholtz billow: this is done in the framework of a one-mode spectral truncation in the spanwise direction. Within this analogy, the loss of two-dimensional predictability would correspond to a return to three-dimensionality and a loss of coherence. We indicate also how a new coherent structure could then be recreated, using an eddy-viscosity assumption and the linear instability of the mean inflexional shear.

Impact of shelter on crop microclimates: a synthesis of results from wind tunnel and field experiments

2002 ◽  
Vol 42 (6) ◽  
pp. 679 ◽  
Author(s):  
H. A. Cleugh ◽  
D. E. Hughes
Keyword(s):  
Wind Speed ◽  
Wind Tunnel ◽  
Field Experiments ◽  
Surface Wind ◽  
Mixing Layer ◽  
Flow Region ◽  
Near Surface ◽  
Wind Speeds ◽  
Temperature And Humidity ◽  
Wake Zone

The purpose of this paper is to synthesise data from the literature, and acquired during an extensive set of wind tunnel and field experiments, to quantify the effect of porous windbreaks on airflow, microclimates and evaporation fluxes. The paper considers flow oriented both normal (i.e. at right angles) and oblique to the windbreak, in addition to the confounding effects of topography. The wind tunnel results confirm the validity of the turbulent mixing layer as a model for characterising the airflow around a windbreak and for predicting the locations of the quiet and wake zones. This mixing layer is initiated at the top of the windbreak and grows with distance downwind until it intersects the vegetation or surface, marking the downwind extent of the quiet zone where the maximum shelter occurs. The 3 factors that determine the growth of this mixing layer are the windbreak porosity, windbreak height and the nature of the terrain upwind. For wind that is flowing normal to a porous windbreak in the field, the latter 2 have the primary influence on the size of the sheltered zone, while windbreak porosity is the main factor determining the amount of shelter. Analyses of the effect of porosity revealed that the amount of wind shelter increases as windbreak porosity is reduced, but the downwind extent of the sheltered zone does not vary with windbreak porosity. Thus, the suggestion from older studies that low-porosity (i.e. dense) windbreaks lead to a reduced sheltered area is not supported by the wind tunnel measurements. In the absence of shading effects, temperature and/or humidity are increased in the quiet zone, mirroring the pattern and magnitude of wind shelter. Thus, the increase in temperature and humidity is greatest where the minimum wind speed occurs, and the magnitude of the increase is smaller for more porous windbreaks. The humidity and air (but not surface) temperatures are decreased very slightly in the wake zone, although these small changes were not significant in a field situation. Microclimate changes, therefore, occur over a much smaller distance downwind than wind shelter, and are negligible for the very porous windbreak. For example, at 20 windbreak heights downwind, the wind speed may still be 80% of its upwind value, while the air and surface temperature and humidity have returned to their upwind values after 12–15 windbreak heights. Furthermore, these changes in temperature and humidity vary with the type of land cover, surface moisture status and the temperature and humidity of the 'regional' air. Over the course of a growing season, these changes can be masked by soil and climate variability. The turbulent scalar fluxes, i.e. evaporation and heat fluxes, also differ from the pattern of near-surface wind speeds. While significantly reduced in the quiet zone, they show a very large peak at the start of the wake zone — the location where the mixing layer intersects the surface. Thus, caution is required when extrapolating from the spatial pattern of shelter to microclimates and turbulent fluxes. Wind flowing at angles other than normal to the windbreak has 2 effects on the pattern of wind shelter. First, for the medium and low porosity windbreaks used in the wind tunnel, the amount of wind shelter is increased slightly in the bleed flow region near the windbreak, i.e. there is an apparent reduction in windbreak porosity as the wind direction becomes more oblique to the windbreak. Second, the profile of near surface wind speeds is similar to that for flow oriented normal to the windbreak, providing that the changes in distance from the windbreak are accounted for using simple geometry. The field data agree with these results, but show an even greater influence of the windbreak structure on the pattern of wind shelter in the bleed flow region, extending from the windbreak to at least 3 windbreak heights downwind, precluding any generalisations about the flow in this region.

Electron Microscopy of the Shape of Small Metal Particles

1977 ◽  
Vol 35 ◽  
pp. 300-301
Author(s):  
M. Jose Yacaman
Keyword(s):  
Electron Microscopy ◽  
Metal Particle ◽  
Field Experiments ◽  
Size Range ◽  
Dark Field ◽  
Metal Particles ◽  
Bragg Condition ◽  
Crystal Particle ◽  
Small Metal Particle ◽  
Small Metal Particles

In the Study of small metal particles the shape is a very Important parameter. Using electron microscopy Ino and Owaga(l) have studied the shape of twinned particles of gold. In that work electron diffraction and contrast (dark field) experiments were used to produce models of a crystal particle. In this work we report a method which can give direct information about the shape of an small metal particle in the amstrong- size range with high resolution. The diffraction pattern of a sample containing small metal particles contains in general several systematic and non- systematic reflections and a two-beam condition can not be used in practice. However a N-beam condition produces a reduced extinction distance. On the other hand if a beam is out of the bragg condition the effective extinction distance is even more reduced.

The partial realization theory of finite size two-dimensional arrays

1981 ◽  
Vol 64 (10) ◽  
pp. 1-8
Author(s):  
Tsuyoshi Matsuo ◽  
Yasumichi Hasegawa ◽  
Yoshikuni Okada
Keyword(s):  
Finite Size ◽  
Two Dimensional ◽  
Realization Theory

Successful field experiments: Getting in, getting the data, getting published

2010 ◽  
Author(s):  
Susan M. Kochanowski ◽  
Charles F. Seifert ◽  
Gary A. Yukl ◽  
Dov Eden ◽  
Gary P. Latham
Keyword(s):  
Field Experiments

Ignition of a reactive mixing layer

1999 ◽  
Vol 96 (6) ◽  
pp. 1016-1021 ◽  
Author(s):  
C. Nicoli ◽  
P. Haldenwang ◽  
B. Denet
Keyword(s):  
Mixing Layer ◽  
Reactive Mixing
Sign in / Sign up

Export Citation Format

Share Document