Daniil Sergeev

Atmospheric boundary layer over steep surface waves

Turbulent air-sea interactions coupled with the surface wave dynamics remain a challenging problem. The needs to include this kind of interaction into the coupled environmental, weather and climate models motivate the development of a simplified approximation of the complex and strongly nonlinear interaction processes. This study proposes a quasi-linear model of wind-wave coupling. It formulates the approach and derives the model equations. The model is verified through a set of laboratory (direct measurements of an airflow by the particle image velocimetry (PIV) technique) and numerical (a direct numerical simulation (DNS) technique) experiments. The experiments support the central model assumption that the flow velocity field averaged over an ensemble of turbulent fluctuations is smooth and does not demonstrate flow separation from the crests of the waves. The proposed quasi-linear model correctly recovers the measured characteristics of the turbulent boundary layer over the waved water surface.

Air-sea Interaction under Hurricane Wind Conditions

where U10 -the wind velocity at a standard meteorological height 〉10=10 m. which relate this coefficient to U10 are obtained either by generalizing empirical data (Garratt, 1977; Large & Pond, 1981, Taylor & Yelland, 2002; Fairall et al., 2003) or by numerical models (see, for example, Janssen, 1989; Janssen, 1991, Makin et.al, 1994; Hara & Belcher, 2004). Numerous field measurements give increasing dependencies of CD on wind speed, which relates to increasing of wave heights with the wind. The aerodynamic drag coefficient of the sea surface is a critical parameter in the theory of tropical hurricanes (Emanuel, 1995). To illustrate it we consider here the ideas of theory of energy balance in a tropical cyclone suggested by (Emanuel, 1986; Emanuel, 1995, Emanuel, 2003). According to this theory the mature tropical cyclone may be idealized as a steady, axisymmetric flow whose energy cycle is very similar to that of an ideal Carnot engine,

Adjusting of Wind Input Source Term in WAVEWATCH III Model for the Middle-Sized Water Body on the Basis of the Field Experiment

Adjusting of wind input source term in numerical model WAVEWATCH III for the middle-sized water body is reported. For this purpose, the field experiment on Gorky Reservoir is carried out. Surface waves are measured along with the parameters of the airflow. The measurement of wind speed in close proximity to the water surface is performed. On the basis of the experimental results, the parameterization of the drag coefficient depending on the 10 m wind speed is proposed. This parameterization is used in WAVEWATCH III for the adjusting of the wind input source term within WAM 3 and Tolman and Chalikov parameterizations. The simulation of the surface wind waves within tuned to the conditions of the middle-sized water body WAVEWATCH III is performed using three built-in parameterizations (WAM 3, Tolman and Chalikov, and WAM 4) and adjusted wind input source term parameterizations. Verification of the applicability of the model to the middle-sized reservoir is performed by comparing the simulated data with the results of the field experiment. It is shown that the use of the proposed parameterization improves the agreement in the significant wave height from the field experiment and from the numerical simulation.

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.

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).

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 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.

Laboratory modelling of air-sea interaction under severe wind conditions

Wind-wave interaction at extreme wind speed is of special interest now in connection with the problem of explanation of the sea surface drag saturation at the wind speed exceeding 30 m/s. The main objective of this work is investigation of factors determining momentum exchange under high wind speeds (up to U10>;40 m s-1) basing on the laboratory experiment in a well-controlled environment. Air flow velocity measurements were complemented by measurements of the wind wave spectra. The tendency to saturation of the surface drag was observed for wind speeds exceeding 25 m s-1, accompanied by the saturation of wind wave slopes. The effect of surface drag saturation can be explained quantitatively within the quasi-linear model of the air boundary layer above the waved water surface, when the contribution of the short-wave part of the wind wave spectrum to aerodynamic resistance of the water surface is taken into account.

Applying of PIV/PTV Methods for Physical Modeling of the Turbulent Buoyant Jets in a Stratified Fluid

Modern optical methods of flow visualization: Particle Tracking Velocimetry (PTV) and Particle Image Velocimetry (PIV) (see (Adrian, 1981)) are widely used in experimental investigations of air and liquid flows for scientific and industrial purposes. They are common in exploration of such processes as heat and mass transfer in power plants, flows in aircraft and shipbuilding, and medico-biological applications. Nowadays this technique is also employed in the laboratory modeling of geophysical flows (Sergeev  Troitskaya, 2011) including air-sea interaction (Reul et al., 1999; Veron et al., 2007); flows in water column and their interaction with bottom topography (Umeyama, 2008; Zhang et al., 2007); vortex flows (Beckers et al., 2002; Heist et al., 2003).