arXiv subjects

Yuri M. Shtemler

Publications and source records attributed to Yuri M. Shtemler.

14 recordsLinked to original sources

arXiv2024

Momentum transfer and foam production via breaking waves in hurricane conditions

Generated under hurricane conditions, a slip layer composed of foam, bubble emulsion, and spray determines the behavior of the surface drag with wind speed. This study enables us to estimate foam's contribution to this behavior. A logarithmic parametrization of surface drag is introduced, wherein the effective roughness length of the underlying surface is decomposed into three fractional roughness lengths. These correspond to the foam-free area (as determined by laboratory data, which includes the effects of spray and bubble emulsion) and ocean areas covered by whitecaps and streaks, each weighted by their respective coverage coefficients. A key concept of this approach is the use of well-established experimental bubble-size spectra produced by breaking surface waves to obtain the effective roughness length. This method provides a fair correlation of the logarithmic parametrization of surface drag against wind speed with a wide class of respective experimental data. Additionally, this approach estimates the hurricane's potential intensity, demonstrating reasonable agreement with experimental findings.

physics.ao-ph↗

arXiv2017

Correlation between Foam-Bubble Size and Drag Coefficient in Hurricane Conditions

Recently proposed model of foam impact on the air sea drag coefficient Cd has been employed for the estimation of the efficient foam-bubble radius Rb variation with wind speed U10 in hurricane conditions. The model relates Cd (U10) with the efficient roughness length Zeff (U10) represented as a sum of aerodynamic roughness lengths of the foam free and foam covered sea surfaces Zw (U10 ), and Zf (U10) weighted with the foam coverage coefficient. This relation is treated for known phenomenological distributions Cd (U10), Zw (U10) at strong wind speeds as an inverse problem for the efficient roughness parameter of foam-covered sea surface Zf (U10).

physics.ao-ph↗

arXiv2014

Drag coefficient for the air-sea exchange: foam impact in hurricane conditions

A physical model is proposed for the estimation of the foam impact on the variation of the effective drag coefficient, C_d, with reference to the wind speed U10 in stormy and hurricane conditions. In the present model C_d is approximated by partitioning the sea surface into foam-covered and foam-free areas. Based on the available optical and radiometric measurements of the fractional foam coverage and the characteristic roughness of the sea-surface in the saturation limit of the foam coverage, the model yields the resulting dependence of C_d vs U10. This dependence is in fair agreement with that evaluated from field measurements of the vertical variation of the mean wind speed.

physics.ao-ph↗

arXiv2014

Resonant instability of the nonlinearly-saturated magnetorotational mode in thin Keplerian discs

The magneto-rotational decay instability (MRDI) of thin Keplerian discs threaded by poloidal magnetic fields is introduced and studied. The linear magnetohydrodynamic problem decouples into eigenvalue problems for in-plane slow- and fast- Alfv'een-Coriolis (AC), and vertical magnetosonic (MS) eigenmodes. The magnetorotational instability (MRI) is composed of a discrete number of unstable slow AC eigenmodes that is determined for each radius by the local beta. In the vicinity of the first beta threshold a parent MRI eigenmode together with a stable AC eigenmode (either slow or fast) and a stable MS eigenmode form a resonant triad. The three-wave MRDI relies on the nonlinear saturation of the parent MRI mode and the exponential growth of two daughter linearly stable waves, slow-AC and MS modes with an effective growth rate that is comparable to that of the parent MRI. If, however, the role of the AC daughter wave is played by a stable fast mode, all three modes remain bounded.

physics.plasm-ph↗

arXiv2011

Regimes of the non-exponential temporal growth in thin Keplerian discs under toroidally-dominated magnetic fields

The linear stability of thin vertically-isothermal density-stratified Keplerian discs in toroidally-dominated magnetic fields is treated by asymptotic expansions in the small aspect ratio of the discs. The discs are found to be spectrally stable. The great variety of possible initial conditions leads to three regimes of non-exponential growth of perturbations, which are classified according to different relative levels of the in-plane and axial perturbed velocities. The first two regimes of instability are characterized by the decoupling of the magneto-sonic (MS) and inertia-Coriolis (IC) modes, as well as by algebraic temporal growth of the perturbations, which are driven by either MS or IC modes (hereafter MS - and IC -regimes of instability, respectively). The third, mixed IC-MS regime of non-exponential, non-algebraic growth is due only to nonaxisymmetric perturbations. The latter regime is characterized by high radial and azimuthal wavenumbers, and growth time of the order of tens of rotating periods. The mixed IC-MS regime most likely exhibits the maximal growth as compared with the IC- and MS- regimes. In the first two regimes of instability the compressible MS mode plays a principal role either as the driver of the growth or the driven growing mode, while the mixed IC-MS regime is described by the Bousinesq approximation for incompressible fluid. The latter is obtained as a natural limit of the expansion scheme. The presence of magnetic field in the mixed IC- MS regime may drastically increase the growth rates of the perturbations as compared with the pure hydrodynamic system.

physics.plasm-ph↗

arXiv2011

Spectral and algebraic instabilities in thin Keplerian discs under poloidal and toroidal magnetic fields

Linear instability of two equilibrium configurations with either poloidal (I) or toroidal (II) dominant magnetic field components are studied in thin vertically-isothermal Keplerian discs. Solutions of the stability problem are found explicitly by asymptotic expansions in the small aspect ratio of the disc. In both equilibrium configurations the perturbations are decoupled into in-plane and vertical modes. For equilibria of type I those two modes are the Alfvén-Coriolis and sound waves, while for equilibria of type II they are the inertia-Coriolis and magnetosonic waves. Exact expressions for the growth rates as well as the number of unstable modes for type I equilibria are derived. Those are the discrete counterpart of the continuous infinite homogeneous cylinder magnetorotational (MRI) spectrum. It is further shown that the axisymmetric MRI is completely suppressed by dominant toroidal magnetic fields (i.e. equilibria of type II). This renders the system prone to either non-axisymmetric MRI or non-modal algebraic growth mechanisms. The algebraic growth mechanism investigated in the present study occurs exclusively due to the rotation shear, generates the inertia-Coriolis driven magnetosonic modes due to non-resonant or resonant coupling that induces, respectively, linear or quadratic temporal growth of the perturbations.

astro-ph.GA↗

arXiv2009

Wind-wave stabilization by a foam layer between the atmosphere and the ocean

The study is motivated by recent findings of the decrease in the momentum transfer from strong winds to sea. The Kelvin-Helmholtz instability (KHI) of a three-fluid system of air, foam and water is examined within the range of intermediately short surface waves. The foam layer thickness necessary for effective separation of the atmosphere and the ocean is estimated. Due to high density contrasts in the three-fluid system, even a relatively thin foam layer between the atmosphere and the ocean can provide a significant stabilization of the water surface by the wavelength shift of the instability towards smaller scales. It is conjectured that such stabilization qualitatively explains the observed reduction of roughness and drag.

physics.ao-ph↗

arXiv2009

Amplification of whistler waves in a thin Keplerian disc embedded in an axisymmetric magnetic field

The linear stability of thin Keplerian discs of weakly ionized polytropic plasma embedded in a mixed (toroidal-poloidal) axisymmetric magnetic field is studied. The effects of the central body on the equilibrium state of the discs are modeled by an axial total current and a dipole magnetic field. Studying the amplification of high-frequency waves shows that Keplerian discs can be destabilized due to combined effects of equilibrium density stratification and poloidal magnetic field. By considering a boundary value problem in the limit of strong Hall effect, it is shown that whistler waves which are free of direct influence of compressibility, rotation and gravity, are amplified along their trajectory while other modes of wave propagation do not change their amplitude.

physics.plasm-ph↗

arXiv2009

Hall equilibrium of thin Keplerian disks embedded in mixed poloidal and toroidal magnetic fields

Axisymmetric steady-state weakly ionized Hall-MHD Keplerian thin disks are investigated by using asymptotic expansions in the small disk aspect ratio ε. The model incorporates the azimuthal and poloidal components of the magnetic fields in the leading order in ε. The disk structure is described by an appropriate Grad-Shafranov equation for the poloidal flux function ψthat involves two arbitrary functions of ψfor the toroidal and poloidal currents. The flux function is symmetric about the midplane and satisfies certain boundary conditions at the near-horizontal disk edges. The boundary conditions model the combined effect of the primordial as well as the dipole-like magnetic fields. An analytical solution for the Hall equilibrium is achieved by further expanding the relevant equations in an additional small parameter δthat is inversely proportional to the Hall parameter. It is thus found that the Hall equilibrium disks fall into two types: Keplerian disks with (i) small (Rd ~δ^0) and (ii) large (Rd > δ^k, k > 0) radius of the disk. The numerical examples that are presented demonstrate the richness and great variety of magnetic and density configurations that may be achieved under the Hall-MHD equilibrium.

astro-ph↗

arXiv2008

An asymptotic model for the Kelvin-Helmholtz and Miles mechanisms of water wave generation by wind

The generalized Kelvin-Helmholtz (KH) and Miles mechanisms of the water wave generation by wind are investigated for two-layer piece-wise linear model of the wind profile. It is shown by asymptotic expansions in small air-to-water density ratio that two mechanisms of the instability operate in quite different scales. Miles' short waves are generated by weak winds, in particular, Miles' regime is responsible for initiation of the instability at the minimum wind speed, while the generalized KH regime dominates at strong winds and raises moderately-short waves.

physics.flu-dyn↗

arXiv2007

Shock-Induced Flows through Packed Beds: Transient Regimes

The early stage of the transient regimes in the shock-induced flows within solid-packed beds are investigated in the linear longwave and high-frequency approximation. The transient resistance law is refined as the Duhameltime integral that follows from the general concept of dynamic tortuosity and compressibility of the packed beds. A closed-form solution is expected to describe accurately the early stage of the transient regime flow and is in qualitative agreement with available experimental data.

physics.class-ph↗

arXiv2007

Wind instability of a foam layer sandwiched between the atmosphere and the ocean

Wind shortwave instability of a foam layer between the atmosphere and the ocean is examined in order to reach greater understanding of the recent findings of the decrease in momentum transfer from hurricane winds to sea waves. The three-fluid configuration with the high contrasts in densities of the air, foam and water provides for an effective mechanism to stabilize the water surface.

physics.ao-ph↗

arXiv2007

Heat transfer in sound propagation and attenuation through gas-liquid polyhedral foams

A cell method is developed, which takes into account the bubble geometry of polyhedral foams, and provides for the generalized Rayleigh-Plesset equation that contains the non-local in time term corresponding to heat relaxation. The Rayleigh-Plesset equation together with the equations of mass and momentum balances for an effective single-phase inviscid fluid yield a model for foam acoustics. The present calculations reconcile observed sound velocity and attenuation with those predicted using the assumption that thermal dissipation is the dominant damping mechanism in a range of foam expansions and sound excitation frequencies.

physics.gen-ph↗

arXiv2007

The Hall instability of thin weakly-ionized stratified Keplerian disks

The stratification-driven Hall instability in a weakly ionized polytropic plasma is investigated in the local approximation within an equilibrium Keplerian disk of a small aspect ratio. The leading order of the asymptotic expansions in the aspect ratio is applied to both equilibrium as well as the perturbation problems. The equilibrium disk with an embedded purely toroidal magnetic field is found to be stable to radial, and unstable to vertical short-wave perturbations. The marginal stability surface is found in the space of the local Hall and inverse plasma beta parameters, as well as the free parameter of the model which is related to the total current through the disk. To estimate the minimal values of the equilibrium magnetic field that leads to instability, the latter is constructed as a sum of a current free magnetic field and the simplest approximation for magnetic field created by a distributed electric current.

astro-ph↗