V. I. Kazakov

Surface manifestations of internal waves investigated by a subsurface buoyant Jet: 1. The mechanism of internal-wave generation

In a large test reservoir with artificial temperature stratification at the Institute of Applied Physics, Russian Academy of Sciences, we have performed a major laboratory simulation of the nonstationary dynamics of buoyant turbulent jets generated by wastewater flows from underwater collector diffusers. The interaction of buoyant jets with the pycnocline leads to an active generation of internal waves. An analysis of the dependence of wave amplitude on the control parameter proportional to the rate of liquid flow from the collector diffuser has indicated that this dependence is adequately described by a function that is characteristic for the presence in the Hopf bifurcation system, which occurs for a soft actuation mode of self-oscillations of the globally instable mode. To check the conditions for the actuation of the globally instable mode, we have performed an auxiliary experiment in a small reservoir with a salt stratification formulated similar to the experiment in the big reservoir. Using the particle image velocimetry (PIV) method, we have measured the velocity field in the buoyant jet and constructed the profiles of transverse velocity in several sections. When the jet approaches the pycnocline, a counterflow is generated at the edges. A stability analysis for the resulting profiles of flow velocities performed by the method of normal modes has revealed that, for the jet portions with counterflow, the condition of absolute instability by the Briggs criterion for axisymmetric jet oscillations is satisfied, which testifies to the fact that the globally instable mode is actuated. The estimates for oscillation frequencies of the globally instable mode are well consistent quantitatively with the measured spectrum of jet oscillations.

Surface manifestations of internal waves investigated by a subsurface buoyant jet: 3. Surface manifestations of internal waves

In a large test reservoir at the Institute of Applied Physics, Russian Academy of Sciences, a series of experiments were performed to investigate the surface manifestations of internal waves radiated by a subsurface buoyant jet. The field of currents on the water surface of the reservoir was studied through the distribution of temperature with shallow thermocline. Using Particle Tracking Velocimetry (PTV), the velocity field of surface currents was measured. A theoretical model was developed to calculate the rates of disturbances on the surface. A comparison with experimental data indicated that the calculated data of the surface rate value are overestimated. This discrepancy was explained by the presence of a film of surface-active substances (SASs) with experimentally obtained parameters. Using scale modeling coefficients, we estimated the parameters of internal waves radiated by the subsurface wastewater system and the values of their surface manifestations in field conditions. We estimated the hydrodynamic contrasts in the field of surface waves, which can be caused by these inhomogeneous currents on the surface. For a wind velocity of 5 m/s, the magnitude of the contrast in the field of short waves can reach up to 10–25%, which is detected with confidence by remote-sensing methods.

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 investigated by a subsurface buoyant jet: Part 2. Internal wave field

In a large test reservoir with artificial temperature stratification at the Institute of Applied Physics, Russian Academy of Sciences, a major simulation of internal wave actuation by buoyant turbulent jets generated by wastewater flows from underwater collector diffusers in conditions of temperature stratification with deep and shallow thermocline has been performed. Using a modification of the particle tracing velocimetry (PTV) method in the stratification mode with shallow thermocline, the velocities of currents generated by internal waves at the surface of the water area are measured. A theoretical model is developed describing the fields of internal waves in the presence of jet stream. Dispersion relations and structures of lower (first and second) modes of internal waves in the stratified basin for different rates of liquid outflow from the collector model are obtained. The experimentally measured field of isothermal shifts with respect to the system of characteristic modes of internal waves is decomposed. A mixed regime of internal wave actuation with the simultaneous existence of the first and second modes is observed. The characteristics of perturbations in the liquid column and on its surface are compared. This analysis allows us to prove that the velocity fields on the surface are indeed surface manifestations of internal waves.

Field studies of variation scales of gravitational capillary waves and near-water wind in the Black Sea shelf area

In this study we present the results of data processing from the 2004 Gelendzhik Bay expedition on the Akvanavt research vessel. The data were processed using the method of wavelet analysis. We found an emerging kilometer-scale variation in the surface heaving and near-water wave over the shelf area. This effect was shown to be possibly caused by the inhomogeneity of current in the depth-drop area.

Subsatellite experiments in the north-eastern part of the Black Sea

Space-time variability of current field characteristics, surface waves, and parameters of the near-surface atmospheric layer above the shelf zone of the Black Sea in the Gelendzhik-city region are studied. A joint analysis of the field measurement data obtained on August, 2007, September, 2008 and of the synchronous SAR images made by the Envisat satellite of the European Space Agency is carried out.

Polarized X-band Doppler radar scatterometer for investigation of microwave scattering of the wavy water surface in laboratory conditions

The paper describes an experimental model of continuous wave X-band Doppler radar scatterometer (sine frequency modulation) designed for physics investigation of radio waves scattering from sea surface in controlled conditions. The prototype is developed and fabricated at the IAP RAS. Its main feature is adaptation to the conditions of a laboratory modeling in the wind-wave flumes to investigate the dependence of the normalized radar cross-section (NRCS) on the wind speed. The design of the microwave and antenna systems allows measurement of scattered radiation power and its Doppler spectrum both at linear co- and cross-polarizations (in a sequential switching mode). This is important from the viewpoint of studying the waves at high wind speeds. The detailed description of the design and its specifications are presented. Also the problems of calibration and results of experimental operation on the high-speed wind-wave flume of IAP RAS 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.

On the structure of internal waves excited by buoyant plumes in stratified fluid

Laboratory scale models of sewage disposal from submerged diffusers of the wastewater outfalls was carried out in the Large Thermostratified Tank of IAP RAS. It is shown that intensive interaction of internal waves occurs due to the interaction of turbulent buoyant plumes and a region of temperature bound (thermocline) and a stream propagates under a thermocline. A theoretical model of the field of internal waves is constructed for experimentally obtained velocity and density fields. It is demonstrated that the bimodal regime of internal wave excitation takes place with the first mode localized in the thermocline region and the second one in the stream. The mode excitation coefficients can be described well in the framework of self-induced internalwave generation by buoyant plumes.