Turbulence in the atmospheric boundary layer (ABL) is usually measured using sonic anemometers (sonics), but coarse spatial (
${\sim}10$
cm) and temporal (
${\sim}32$
Hz) resolutions of sonics ...preclude direct measurement of fine-scale parameters such as the turbulent kinetic energy (TKE) dissipation rate
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. Instead,
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is estimated using techniques based on Kolmogorov theory. Fine-scale measurements of ABL turbulence down to Kolmogorov scale were made with a sonic and hot-film anemometer dyad (a ‘combo’ probe) during the field campaigns of the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) programme. The hot-film probe was located on a gimbal within the sonic probe volume, and was automated to rotate in the horizontal plane to align with the mean flow measured by sonic. This procedure not only helped satisfy the requirement of hot-film alignment with the mean flow, but also allowed in situ calibration of hot-film probes. This paper analyses a period of nocturnal flow that was similar to a stratified parallel shear flow. The combo-probe measurements showed an interesting phenomenon – the occurrence of strong bursts, characterized by short-term increase of velocity fluctuations and simultaneous increase of TKE dissipation rate by orders of magnitude. These bursts were indicative of unusual turbulence activity at finer (
${\sim}0.1$
–0.4 m) scales that are not captured by sonics since the smallest scales resolved by the latter are greater than 0.6 m. With bursting present, the spectra exhibited bumps at scales intermediate to inertial and dissipation subranges, resembling a bottleneck phenomenon. Its manifestation, although unequivocally related to bursts, may not convincingly fit into the framework of previous bottleneck-effect theories that allude to either viscous effects or buoyancy effects modifying the local energy cascade via non-local effects. The origins of burst are yet to be identified. Stratified ABL with bursts exhibits non-Kolmogorov behaviour, and hence should be modelled with caution.
The Nile littoral cell, one of the world's longest, runs 650 km along the southeastern Mediterranean, from Abu Quir Bay near Alexandria, Egypt, to Haifa Bay on the northern Israeli coast.
Haifa Bay ...constitutes the northernmost final depositional sink of Nile-derived quartz sand, transported from the Nile delta by longshore currents generated by approaching breaking waves. The northward net sand transport along the Mediterranean coast of Israel results from larger waves approaching from west–south–west and south–west compared to their counterparts from west–north–west and north–west.
This study utilizes an extensive new database gathered from sediment drill cores, marine geophysical maps and field observations to measure the volume of sand deposited in Haifa Bay and the adjacent Zevulun Plain during the Holocene. It then compares this volume to recent data, including measurements of sand accumulation along Haifa Port's main breakwater (constructed in the southern entrance of the bay) as well as longshore sand transport estimates along the northern Carmel coast.
Research findings estimate the annual average quantity of sand transported to Haifa Bay throughout the period at 80,000–90,000 m
3. The findings further conclude that this amount has not changed appreciably over the past 75 years. Evaluating calculated values over the long term, it is suggested that the characteristics of longshore sand transport along the coast of Israel have not changed significantly during the past 7900–8500 years. It is obvious that this conjecture should be treated with reservations.
Effect of the capillary meniscus on the instability of large Prandtl number Czochralski melt flow is studied experimentally. The measurements are conducted in two experimental facilities by two ...independent non-intrusive optical techniques. The quantitative results are presented as dependencies of the critical Grashof number (critical temperature difference) on the meniscus height for different Prandtl numbers, radii and aspect ratios. The results show that with increase of the meniscus height the critical temperature difference noticeably grows and sometimes doubles. Recently reported parametric relations for the critical Grashof number and oscillations frequency are extended to include parameters of the meniscus.
•Experiments on capillary meniscus effect on instability of Czochralski flow.•Measurements by two independent and fully non-intrusive techniques.•Simple empirical relations for critical Grashof number and critical frequency.
THE MATERHORN Fernando, H. J. S.; Pardyjak, E. R.; Di Sabatino, S. ...
Bulletin of the American Meteorological Society,
11/2015, Letnik:
96, Številka:
11
Journal Article
Recenzirano
Odprti dostop
Emerging application areas such as air pollution in megacities, wind energy, urban security, and operation of unmanned aerial vehicles have intensified scientific and societal interest in mountain ...meteorology. To address scientific needs and help improve the prediction of mountain weather, the U.S. Department of Defense has funded a research effort—the Mountain Terrain Atmospheric Modeling and Observations (MATERHORN) Program—that draws the expertise of a multidisciplinary, multiinstitutional, and multinational group of researchers. The program has four principal thrusts, encompassing modeling, experimental, technology, and parameterization components, directed at diagnosing model deficiencies and critical knowledge gaps, conducting experimental studies, and developing tools for model improvements. The access to the Granite Mountain Atmospheric Sciences Testbed of the U.S. Army Dugway Proving Ground, as well as to a suite of conventional and novel high-end airborne and surface measurement platforms, has provided an unprecedented opportunity to investigate phenomena of time scales from a few seconds to a few days, covering spatial extents of tens of kilometers down to millimeters. This article provides an overview of the MATERHORN and a glimpse at its initial findings. Orographic forcing creates a multitude of time-dependent submesoscale phenomena that contribute to the variability of mountain weather at mesoscale. The nexus of predictions by mesoscale model ensembles and observations are described, identifying opportunities for further improvements in mountain weather forecasting.
Celotno besedilo
Dostopno za:
BFBNIB, DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
A parametric experimental study of the cold plume instability that appears in the large-Prandtl-number Czochralski melt flows is reported. The critical temperature difference (the critical Grashof ...number) and the frequency of appearing oscillations were measured for varying Prandtl numbers, aspect ratios of the melt, and crystal/crucible radii ratio. The measurements were carried out by two independent and fully non-intrusive experimental techniques. The results are reported as dimensional and dimensionless parametric dependences, and then are joined into relatively simple empirical relations showing how the critical Grashof number and the frequency of emerging oscillations depend on other parameters.
•Study of cold plume instability in large-Prandtl-number Czochralski melt flows.•Measurements by two independent and fully non-intrusive experimental techniques.•Measured critical parameters are reported in form of semi-empirical relations.
The performance of an in situ calibration technique, implementing neural network (NN) algorithms for co-located multi-wire hot-film and sonic anemometers, is studied. The NN-based calibration ...technique, proposed by Kit et al (2010 J. Atmos. Ocean. Technol. 27 23-41), allows performing direct measurement of fine scales in turbulent air flow, and offers a robust tool for measurements of micro-scale properties in atmospheric flows. Accuracy of the suggested calibration technique is examined in view of isotropic and anisotropic flow fields of various turbulence intensity (TI). Anisotropic velocity datasets of various TIs were generated using a 'virtual probe' simulating hot-film anemometer response to the sensed flow, while a kinematic model of homogeneous isotropic turbulent flow was implemented to generate isotropic flow datasets. NN calibration performance is examined by quantitative comparison between the original and reconstructed velocity components, using a specially constructed norm and by visual comparison of original and reconstructed time series. The examined NN calibration technique performance is shown to be of reasonable accuracy for all TI velocity fields examined, while a notable drop in accuracy is detected with the increase in TI. The reconstruction of isotropic velocity fields, using the NN algorithm for calibration, is apparently of slightly lower accuracy than in the case of anisotropic flows. The results are discussed in view of possible implementation of the suggested technique in direct field measurements of atmospheric turbulence fine scales.
Evolution of unidirectional nonlinear wave groups with wide spectra is studied experimentally and numerically. As an example of such an evolution, focusing of an initially wide wave train that is ...modulated both in amplitude and in frequency, to a single steep wave at a prescribed location along the laboratory wave tank is investigated. When numerous frequency harmonics arrive at the focusing location in phase, a very wave steep single emerges. The experimental study was carried out in two wave flumes that differ in size by an order of magnitude: a 330 m long Large Wave Channel in Hanover, and in 18 m long Tel-Aviv University wave tank. The spatial version of the Zakharov equation was applied in the numerical simulations. Detailed quantitative comparison is carried out between the experimental results and the numerical simulations. Spectra of the 2nd order bound waves are calculated using the theoretical model adopted. It is demonstrated that with the contribution of bound waves accounted for, a very good agreement between experiments and simulations is achieved.
A record of a tsunami event riding on the usual tide signal was recorded by a floating‐type tidal gauge installed in the port of Yafo, Israel. The tsunami was triggered by an earthquake in the Aegean ...Sea on 9 July 1956. This paper presents a retrieval of tsunami waves from the record. At the first stage of the study an attempt had been undertaken to reproduce the 1956 tsunami assuming a coseismic nature of its generation source. Although these simulations resulted in tsunami waves with their amplitude close to that obtained from the record measured at Yafo, they did not contain significant spectral energy components with periods of ∼15 min as appear in the spectra of 1956 tide‐gauge records. When landslide movement, triggered by the main shock and/or by the largest aftershock, is suggested as a source of these tsunami waves, the spectra of the resulted marigram obtained in the proximity to Yafo contain harmonics with frequencies very close to those measured. This corroborates the landslide nature of the tsunamigenic source responsible for generation of higher‐frequency (relative to the tidal waves) energy components. The peak periods determined via spectral analysis of the recent tide‐gauge records (1 year and longer) in the absence of tsunami events vary from 50 to 60 min. Similar periods have been revealed in a special numerical study dealing with longwave propagation toward the coast of Israel, thus confirming that their origin is related to continental shelf resonance. These resonance periods differ significantly from those found for the 1956 tsunami.
Abstract
In a recent paper by Kit et al., a novel algorithm for the calibration of hot-film probes using a collocated sonic anemometer combined with the neural network approach is described. An ...important step in the algorithm is the generation of a calibration dataset by an appropriate low-pass filtering of the voltage and velocity time series obtained from hot-film probes and a sonic anemometer, correspondingly. Kit et al. report that a polynomial least squares fit that was used to approximate the relations of these voltage–velocity data from the dataset failed while a neural network approach worked satisfactorily. The same polynomial fit worked successfully with a calibration dataset obtained using a standard calibration unit that enables one to generate calibration data at evenly distributed yaw angles, varying in a wide range (−30°, 30°). In the current study, an attempt is made to uncover the reason for the failure of the polynomial fit algorithm with a sonic anemometer–based calibration dataset (SBS-PF). The probability densities of the velocity angles for the calibration dataset, as well as for a full velocity dataset obtained using the neural network approach, are computed. Also developed are theoretical expressions for the same angular density probability distributions based on the following assumptions: (i) an axisymmetric turbulent velocity field, (ii) Gaussian density probability distribution for velocity components, and (iii) weak correlations between the velocity components (i.e., the probability density distribution of the entire velocity vector is a product of probabilities of its components). The agreement between measured and theoretical angular probability distributions is good. The results herein indicate that the angular density probability of the low-pass-filtered calibration dataset is twice as narrow as that of the full velocity time series. This result can explain the failure of the polynomial fit to reconstruct the full velocity time series satisfactorily as resulting from the intrinsic property of this algorithm to ascribe a large weight to the highly concentrated points and a light weight to the thinly concentrated points while performing fitting.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
A new experimental facility for experimental validation of numerical codes calculating Czochralski melt flow is described. The setup is built to make all the boundary conditions reproducible in a ...mathematical model. Measurements are focused on the steady-oscillatory flow transition, which is defined by appearance of temperature oscillations measured independently by thermocouples and interferometer. Location of thermocouples is defined according to numerical predictions, which ensures measurements at locations where oscillations amplitude is large. Current state of comparison of experimental and numerical results is discussed.
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