A. Kandaurov

Bag-breakup fragmentation as the dominant mechanism of sea-spray production in high winds

Showing the record strengths and growth-rates, recent hurricanes have highlighted needs for improving forecasts of tropical cyclone intensities most sensitive to models of the air-sea interaction. Especially challenging is the nature of sea-spray supposed to strongly affecting the momentum- and energy- air-sea fluxes at strong winds. Even the spray-generation mechanisms in extreme winds remained undetermined. Basing on high-speed video here we identify it as the bag-breakup mode of fragmentation of liquid in gaseous flows known in a different context. This regime is characterized by inflating and consequent bursting of the short-lived objects, bags, comprising sail-like water films surrounded by massive liquid rims then fragmented to giant droplets with sizes exceeding 500 micrometers. From first principles of statistical physics we develop statistical description of these phenomena and show that at extreme winds the bag-breakup is the dominant spray-production mechanism. These findings provide a new basis for understanding and modeling of the air-sea exchange processes at extreme winds.

Laboratory study of cross-polarized radar return under gale-force wind conditions

This paper presents data of laboratory experiments on a high-speed wind-wave flume of the Institute of Applied Physics (Nizhny Novgorod, Russian Federation), which are devoted to the investigation of the X-band co-polarized and de-polarized radar return in a wide range of high wind speeds (from 8 to 40 m s–1). Microwave measurements were accompanied by the measurements of airflow and wave field parameters. Experiments showed that alternatively to the co-polarized return, the dependency of the de-polarized return on the wind speed does not saturate, although the growth rate decreases at wind speed exceeding 30 m s–1. Comparison of the experimental data with the composite-surface Bragg scattering model for the measured parameters of the wind and waves showed that the model is in agreement with measurements of microwave co-polarized return, but fails to describe the de-polarized radar return. The obtained dependency of de-polarized radar return was compared with the empirical geophysical model function based on collocated airborne and satellite data.

Surface Manifestations of Internal Waves Induced by a Subsurface Buoyant Jet (Experiment and Theory)

In this contribution we summarized the main results of the work on internal waves generated by vertical turbulent plumes in stratified fluids, including the mechanisms of internal wave generation, the structure of internal waves, and their surface manifestations Particular attention is focused on the major series of experiments performed in the Large Thermally Stratified Tank of the Institute of Applied Physics of the Russian Academy of Sciences, Nizhny Novgorod, Russia. The majority of results are applicable for the monitoring of the coastal zone of the oceans. The other potential implications include buoyant plumes generated by subglacial discharge in Greenland fjords.

The “Bag Breakup” Spume Droplet Generation Mechanism at High Winds. Part I: Spray Generation Function

AbstractThis paper describes the results of an experimental and theoretical investigation into the mechanisms by which spume droplets are generated by high winds. The experiments were performed in ...

X-band radar cross-section at GALE force winds: Towards cross-polarization GMF for retrieval of hurricane wind speed and surface stress

Based on laboratory experiments, the logarithmic dependence of X-band cross-polarized NRCS on wind friction velocity was found for high wind conditions. Using this dependency, surface drag parameterizations at hurricane wind and available collocated field data on radar backscattering and wind speed, the geophysical model function (GMF) as NRCS via 10-m wind speed is derived. The GMP is applicable for wind speed retrieval at stormy and hurricane conditions.

The particular use of PIV methods for the modelling of heat and hydrophysical processes in the nuclear power plants

In this paper we describe PIV-system specially designed for the study of the hydrophysical processes in large-scale benchmark setup of promising fast reactor. The system allows the PIV-measurements for the conditions of complicated configuration of the reactor benchmark, reflections and distortions section of the laser sheet, blackout, in the closed volume. The use of filtering techniques and method of masks images enabled us to reduce the number of incorrect measurement of flow velocity vectors by an order. The method of conversion of image coordinates and velocity field in the reference model of the reactor using a virtual 3D simulation targets, without loss of accuracy in comparison with a method of using physical objects in filming area was released. The results of measurements of velocity fields in various modes, both stationary (workers), as well as in non-stationary (emergency).

Field Investigation and Numerical Simulation of Wind-Wave Interaction at the Middle-Sized Inland Reservoirs

An attempt is made to apply the modern methods of surface wave simulation developed for oceanic conditions to the modeling of waves in medium-size inland reservoirs (10–100 km). The results of field measurements of wind speed and waves are described, and on their basis the parameterization \( C_{D} \left( {U_{10} } \right) \) is proposed. WAVEWATCH III spectral wave model was adapted to the conditions of a medium-size inland reservoir. The simulated data are compared with the field data. The use of the new parameterization \( C_{D} \left( {U_{10} } \right) \) allowed reducing the values of the wind wave growth rate that improved consistency in data from the field experiment and numerical modeling concerning the height of significant waves. Further steps towards improving the quality of prediction of the adapted WAVEWATCH III model are discussed.

Cross-polarization geophysical model function for marine wind speed retrieval: Laboratory modeling and examples of hurricane wind speed retrieval from satellite imagery

Laboratory experiments directed to investigation of dependencies of the X-band normalized co-polarized and de-polarized radar cross-section on wind speed (U10) and incident angle (θ) are presented. Parameters of air-flow velocity and surface wind waves were measured simultaneously. It was shown that both co-polarized and depolarized radar return depend on incidence angle; the depolarized return is less sensitive. Analysis of the Doppler spectra of the radar backscatter enabled us to conclude that the radar return is formed by resonant scatters moving with the velocity exceeding in 20% the phase velocity of the energy containing surface waves. Basing on the measurements the X-band and C-band geophysical model functions (GMF) were derived for U10 =10-40 m/s and θ=30°-60°. Examples of hurricane wind speed retrieval with the use of the new GMF are discussed.

Laboratory modelling of cross-polarized radar return at severe wind conditions

Laboratory experiments directed to investigation of co-polarized and de-polarized X-band microwave radar return from the water surface at strong and hurricane wind were carried out in the high-speed wind-wave flume. Microwave measurements were accompanied by the measurements of air-flow and wave field parameters. Experiments showed that alternatively to the co-polarized return, the dependency of the de-polarized return on the wind speed do not saturate, although the growth rate decreases at wind speed exceeding 30 m/s. Comparison of the of the experimental data with the composite-surface Bragg scattering model for the measured parameters of the wind and waves showed, that the model is in agreement with measurements of microwave co-polarized return, but fails to describe the de-polarized radar return. The obtained dependency of de-polarized radar return was compared with the empirical geophysical model function based on collocated airborne and satellite data.

Laboratory investigation of short wind wave breaking modulation in the long surface wave field

Both the modulation of short wind wave breaking in the long surface wave field and the occurrence of breaking in the presence of a long wave were investigated in wind-wave tank. Short wind waves were generated by the air flow at various speeds, the long surface wave at various frequency and amplitude were generated by wave maker. Data of X- and Ka-band scatterometers, string wave recorder placed in the probing area, and web camera making video records of the probing area were synchronously recorded in the experiment. Also precision measurements using laser-optics system based on the high-speed video recording, laser illumination, and special algorithms of video processing allowed us to obtain a large ensemble of the wave profiles. It is shown that at relatively weak wind long surface wave causes short wind wave breaking, and at high wind speeds – breaking intensification. Wind wave breaking is mainly observed on the front slope of the long wave, the breaking observation area extends with increasing wind speed.