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Register a new userSpatial decay of the vorticity field of time-periodic viscous flow past a body
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We study the asymptotic spatial behavior of the vorticity field, $omega(x,t)$, associated to a time-periodic Navier-Stokes flow past a body, $mathscr B$, in the class of weak solutions satisfying a Serrin-like condition. We show that, outside the wake region, $mathcal R$, $omega$ decays pointwise at an exponential rate, uniformly in time. Moreover, denoting by $bar{omega}$ its time-average over a period and by $omega_P:=omega-bar{omega}$ its purely periodic component, we prove that inside $mathcal R$, $bar{omega}$ has the same algebraic decay as that known for the associated steady-state problem, whereas $omega_P$ decays even faster, uniformly in time. This implies, in particular, that sufficiently far from $mathscr B$, $omega(x,t)$ behaves like the vorticity field of the corresponding steady-state problem.
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We study the deformation and transport of elastic fibers in a viscous Hele-Shaw flow with curved streamlines. The variations of the global velocity and orientation of the fiber follow closely those of the local flow velocity. The ratios of the curvatures of the fibers by the corresponding curvatures of the streamlines reflect a balance between elastic and viscous forces: this ratio is shown experimentally to be determined by a dimensionless {it Sperm number} $Sp$ combining the characteristic parameters of the flow (transverse velocity gradient, viscosity, fiber diameter/cell gap ratio) and those of the fiber (diameter, effective length, Youngs modulus). For short fibers, the effective length is that of the fiber; for long ones, it is equal to the transverse characteristic length of the flow. For $S_p lesssim 250$, the ratio of the curvatures increases linearly with $Sp$; For $S_p gtrsim 250$, the fiber reaches the same curvature as the streamlines.
In this paper, we address the space-time decay properties for strong solutions to the incompressible viscous resistive Hall-MHD equations. We obtained the same space-time decay rates as those of the heat equation. Based on the temporal decay results in cite{cs}, we find that one can obtain weighted estimates of the magnetic field $B$ by direct weighted energy estimate, and then by regarding the magnetic convection term as a forcing term in the velocity equations, we can obtain the weighted estimates for the vorticity, which yields the corresponding estimates for the velocity field. The higher order derivative estimates will be obtained by using a parabolic interpolation inequality proved in cite{k01}. It should be emphasized that the the magnetic field has stronger decay properties than the velocity field in the sense that there is no restriction on the exponent of the weight. The same arguments also yield the sharp space-time decay rates for strong solutions to the usual MHD equations.
This paper is concerned with a simplified epidemic model for West Nile virus in a heterogeneous time-periodic environment. By means of the model, we will explore the impact of spatial heterogeneity of environment and temporal periodicity on the persistence and eradication of West Nile virus. The free boundary is employed to represent the moving front of the infected region. The basic reproduction number $R_0^D$ and the spatial-temporal risk index $R_0^F(t)$, which depend on spatial heterogeneity, temporal periodicity and spatial diffusion, are defined by considering the associated linearized eigenvalue problem. Sufficient conditions for the spreading and vanishing of West Nile virus are presented for the spatial dynamics of the virus.
Consider the resolvent problem associated with the linearized viscous flow around a rotating body. Within a setting of classical Sobolev spaces, this problem is not well posed on the whole imaginary axis. Therefore, a framework of homogeneous Sobolev spaces is introduced where existence of a unique solution can be guaranteed for every purely imaginary resolvent parameter. For this purpose, the problem is reduced to an auxiliary problem, which is studied by means of Fourier analytic tools in a group setting. In the end, uniform resolvent estimates can be derived, which lead to the existence of solutions to the associated time-periodic linear problem.
A hydrodynamic flow of electrons driven by an oscillating electric field is investigated. It is found that a double-peak profile of the electric current can appear. Such a profile originates from the interplay of viscous and inertial properties of the electron fluid as well as the boundary conditions. The nontrivial profile of the current results in a characteristic stray magnetic field where peaks could also occur in one of the field components. Analytical results are supported by numerical calculations in samples of different geometries such as straight channel, nozzle, and cavity and are found to be qualitatively insensitive to a specific form of the oscillating electric field. In addition, it is shown that nozzle and cavity provide an efficient means to locally enhance or reduce the fluid velocity.
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