Dan Liberzon

The MATERHORN: Unraveling the Intricacies of Mountain Weather

AbstractEmerging 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, multi-institutional, 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 platfor...

Effect of the initial spectrum on the spatial evolution of statistics of unidirectional nonlinear random waves

[1] Results of extensive experiments on propagation of unidirectional nonlinear random waves in a large wave tank are presented. The nonlinearity of the wavefield determined by the characteristic wave amplitude and the dominant wave length was retained constant in various series of experimental runs. In each experimental series, initial spectra of different shape and/or width were considered. Every series contained sufficient number of independent realizations to ensure reliable statistics. Evolution of various statistical parameters along the tank was investigated. It is demonstrated that the spectrum width plays an important role in the evolution of the random wavefield and strongly affects the variation of the wave spectrum as well as of parameters that characterize the deviation of the wavefield statistics from that corresponding to the Gaussian distribution. In particular, in a random wavefield that initially contains independent free harmonics within a narrow spectrum, extremely steep waves appear more often in the process of evolutions than predicted by a Rayleigh distribution, while for wider initial wave spectra the probability of those waves decreases sharply and is well below the Rayleigh values.

Upward-propagating capillary waves on the surface of short Taylor bubbles

Upward-propagating capillary waves are observed on the surface of short Taylor air bubbles rising in vertical pipes. The wave length distribution along the bubble surface is measured for bubbles rising in pipes of different diameters in stagnant and up-flowing water by digital image processing. It is shown that the waves are generated by bubble bottom oscillations, and their length is determined by wave-current interaction along the liquid film.

Lagrangian kinematics of steep waves up to the inception of a spilling breaker

Horizontal Lagrangian velocities and accelerations at the surface of steep water-waves are studied by Particle Tracking Velocimetry for gradually increasing crest heights up to the inception of a spilling breaker. Localized steep waves are excited using wavemaker-generated Peregrine breather-type wave trains. Actual crest and phase velocities are estimated from video recorded sequences of the instantaneous wave shape as well as from surface elevation measurements by wave gauges. Effects of nonlinearity and spectral width on phase velocity, as well as the relation between phase velocity and crest propagation speed are discussed. The inception of a spilling breaker is associated with the horizontal velocity of water particles at the crest attaining that of the crest, thus confirming the kinematic criterion for inception of breaking.

Statistical Analysis of the Spatial Evolution of the Stationary Wind Wave Field

Detailed investigation of wind-generated water waves in a 5-m-long wind wave flume facility is reported. Careful measurements were carried out at a large number of locations along the test section and at numerous airflow rates. The evolution of the wind wave field was investigated using appropriate dimensionless parameters. When possible, quantitative comparison with the results accumulated in field measurements and in larger laboratory facilities was performed. Particular attention was given to the evolution of wave frequency spectra along the tank, distinguishing between the frequency domain around the spectral peak and the highfrequency tail of the spectrum. Notable similarity between the parameters of the evolving wind wave field in the present facility and in field measurements was observed.

An Inexpensive Method for Measurements of Static Pressure Fluctuations

An application of a commercially available and inexpensive pressure probe and transducer, originally designed for pressure drop measurements in air conditioning conduits, is suggested for accurate and reliable measurements of static pressure fluctuations in airflow, with a particular application to wind interaction with water waves. It is demonstrated that this static pressure probe is a robust instrument that offers efficient dynamic pressure elimination while having low directional sensitivity and sufficiently high dynamic response. A series of measurements in a wind-wave flume was carried out to validate the characteristics of the sensor and of the pressure transducer.

3D-calibration of three- and four-sensor hot-film probes based on collocated sonic using neural networks

High resolution measurements of turbulence in the atmospheric boundary layer (ABL) are critical to the understanding of physical processes and parameterization of important quantities, such as the turbulent kinetic energy dissipation. Low spatio-temporal resolution of standard atmospheric instruments, sonic anemometers and LIDARs, limits their suitability for fine-scale measurements of ABL. The use of miniature hot-films is an alternative technique, although such probes require frequent calibration, which is logistically untenable in field setups. Accurate and truthful calibration is crucial for the multi-hot-films applications in atmospheric studies, because the ability to conduct calibration in situ ultimately determines the turbulence measurements quality. Kit et al (2010 J. Atmos. Ocean. Technol. 27 23–41) described a novel methodology for calibration of hot-film probes using a collocated sonic anemometer combined with a neural network (NN) approach. An important step in the algorithm is the generation of a calibration set for NN training by an appropriate low-pass filtering of the high resolution voltages, measured by the hot-film-sensors and low resolution velocities acquired by the sonic. In Kit et al (2010 J. Atmos. Ocean. Technol. 27 23–41), Kit and Grits (2011 J. Atmos. Ocean. Technol. 28 104–10) and Vitkin et al (2014 Meas. Sci. Technol. 25 75801), the authors reported on successful use of this approach for in situ calibration, but also on the methods limitations and restricted range of applicability. In their earlier work, a jet facility and a probe, comprised of two orthogonal x-hot-films, were used for calibration and for full dataset generation. In the current work, a comprehensive laboratory study of 3D-calibration of two multi-hot-film probes (triple- and four-sensor) using a grid flow was conducted. The probes were embedded in a collocated sonic, and their relative pitch and yaw orientation to the mean flow was changed by means of motorized traverses. The study demonstrated that NN-calibration is a powerful tool for calibration of multi-sensor 3D-hot film probes embedded in a collocated sonic, and can be employed in long-lasting field campaigns.

Pressure Distribution in Confined Jet Flow

A momentum jet injected into a confined container breaks up to “diffusive turbulence” after traveling a critical distance. It has been argued that an adverse pressure gradient developing within the container, acting against the jet momentum flux, is responsible for this break up (Risso and Fabre, 1997,“Diffusive Turbulence in a Confined Jet Experiment,” J. Fluid Mech., 337, pp. 233–261; Voropayev et al., 2011, “Evolution of a Confined Turbulent Jet in a Long Cylindrical Cavity: Homogeneous Fluids,” Phys. Fluids, 23, 115106). Experimental evidence for this adverse pressure gradient is presented in this paper, supplemented by a control-volume analysis to explain the results. The rise of pressure from the jet-injection level to a location beyond the jet break up xb is shown to be proportional to the jet momentum flux. The overall (integrated) sidewall friction on a control volume is negligible, compared to the increase of pressure, if the flow control volume extends beyond xb. For smaller lengths of the control volume, the side wall drag is not negligible compared to the pressure rise. The Reynolds number similarity was evident for jet Reynolds numbers above 6000. This work was motivated by its applications to degassing of crude oil stored in the U.S. Strategic Petroleum Reserves, which are slender salt caverns. To improve its quality, periodically oil is cycled through a degassing plant and injected back to the cavern as a jet, and the degassing time is critically dependent on jet dynamics.

Lagrangian Kinematic Criterion for the Breaking of Shoaling Waves

AbstractAn experimental study was conducted with the aim of validating the Lagrangian kinematic criterion (LKC) for the case of breaking of shoaling waves. Monochromatic wave trains were generated in a large wave flume and allowed to shoal and break naturally on an artificial inclination changeable shore, thus allowing inspection of a range of slopes. Instantaneous horizontal Lagrangian water surface velocity was measured by particle tracking velocimetry and compared to the instantaneous propagation speed of the crest on a verge of breaking, the latter calculated using time series produced by resistance-type wave gauges staged along the flume. The inception of a breaker was found to occur when the monotonically increasing horizontal water velocity on the crest, during the process of steepening, approached that of the slowing steep crest, thus confirming the LKC for shoaling conditions. In addition, an objective method of breaking detection was developed utilizing the phase–time method and wavelet analysis...