Dynamics of local density perturbation in stably stratified fluid: Results of numerical experiments

Linear and nonlinear numerical models of dynamics of local density perturbation in a stably stratified medium are constructed. The influence of viscosity on the process of generation and propagation of internal waves generated by the local density perturbation in a pycnocline is evaluated. The problem on the dynamics of local density perturbation in the presence of wave background is considered.     

On numerical modeling of the dynamics of turbulent wake behind a towed body in linearly stratified medium

Two numerical models of the dynamics of a turbulent wake behind a towed body in a linearly stratified medium are compared, namely, the model based on direct numerical integration of Navier-Stokes equations in the Oberbeck-Boussinesq approximation and the mathematical model with applying a semiempirical turbulence model of the second order. The calculation results of the two models are similar to the known experimental data and are in good agreement.     

Water tank studies of plume rise and diffusion in a stably stratified layer under calm conditions

Summary Water tank experiments were conducted to investigate plume rise and diffusion of gases discharged from stacks under calm conditions with field wind velocity less than 0.4 m/s with stable thermal stratification. Similarity rules of scaled model experiments were obtained from nondimensional analysis of the fundamental equation. The densimetric Froude number of a plume and the buoyancy ratio of the plume and the atmosphere were varied during the water tank experiments while plume rise height, plume thickness, etc. were measured. It was found that the experimental results could be expressed in terms of these two dimensionless parameters. Papper presented at the GNFAO/EURASAP Meeting, Turin, September 1989. To speed up publication, proofs were not sent to the authors and were supervised by the Scientific Committee.     

A numerical study of a turbulent mixing layer and its generated noise

A direct numerical simulation of a turbulent mixing layer with the Reynolds number 500 and the convective Mach number 0.6 is performed and the results obtained are used to study the turbulent flow field and its generated noise.In the present simulation,the numerical techniques of absorbing buffer zones,artificial convection velocity and spatial filtering are used to achieve nonreflecting boundary conditions.The self-similarity is used to validate the present numerical simulations.The large-scale coherent structures are plotted together with the acoustic waves,which demonstrates the directivity of acoustic waves.The Lighthill’s source and space-time correlations are further investigated.The main contributions to mixing noise are identified in terms of large-scale coherent structures,Lighthill’s source and space-time correlations.     

Reaction driven convection around a stably stratified chemical front

A vertical stratification of a light and hot fluid over a heavy and cold one is expected to be stable with regard to buoyancy-driven convection. Here we show that chemical reactions can trigger convection around chemical fronts even in cases where concentration and heat both contribute to a stable density stratification. The balance between intrinsic thermal and solutal density gradients initiated by a spatially localized reaction zone and double diffusive mechanisms are at the origin of a new convective instability, the mechanism of which is explained by a displaced particle argument. Linear stability analysis of a reaction-diffusion-convection model confirmed by nonlinear simulations delimits the instability region in the parameter space spanned by the thermal and solutal Rayleigh numbers. Experimental systems in which to test our theoretical predictions are proposed.     

Large eddy simulation of stably stratified turbulence

Stable stratification turbulence, as a common phenomenon in atmospheric and oceanic flows, is an important mechanism for numerical prediction of such flows. In this paper the large eddy simulation is utilized for investigating stable stratification turbulence numerically. The paper is expected to provide correct statistical results in agreement with those measured in the atmosphere or ocean. The fully developed turbulence is obtained in the stable stratification fluid by large eddy simulation with different...     

On the penetration of the magnetic field into a superconductor

It is known that in superconductors the exponential decay of the magnetic field is an approximation, which breaks down if the dimension of a Cooper pair ξ _f is of the order or smaller than the London penetration depth δ. The appearance of a nonlocal relation between current and field yields deviations from the exponential decay especially a sign reversal of the field at a certain distance. This sign reversal is connected with a change: of the surface energy in superconductors and of the structure of fluxoids together with their interaction. In this paper we present results on the decay of magnetic field which is calculated from the exact BCS-integral-kernel for weak fields. As a result, the nonlocal effects in the framework of BCS-theory can be described in good approximation by the ratio of the London penetration depth δ(T, l) and the dimension of Cooper pairs ξ _f (T, l). The evaluations show, that one has still sign reversal, i.e. large nonlocal effects, in Type II superconductors with a κ(T _c )?,1.6. It should be mentioned that the limit κ?1.6 coincides roughly with the experimentally observed region of attraction of fluxoids. In addition results on the penetration depths are summarized.     

On the penetration depth of a strong field into superconductors

The field dependence of the magnetic penetration depth over the entire range of stability and metastability of the Meissner state was determined within the framework of the Ginzburg-Landau theory. A simple interpolation formula is suggested.     

Effect of a variable magnetic field on the instability of a stratified rotating fluid layer in porous medium

The instability of a stratified rotating fluid layer through porous medium in the presence of an inhomogeneous magnetic field is investigated. For exponentially varying density and magnetic field variations, an eigenvalue solution has been obtained. The dispersion relation is obtained and discussed for both the stable and unstable stratifications separately. It is found, for non-porous medium, that for the stable mode of disturbance, the system is always stable, and for the unstable mode of disturbance, it is stabilized only under a certain condition for the Alfvèn velocity, rotation and the stratification parameter. In the latter case, both rotation and magnetic field are found to have a stabilizing effect on the growth rate. In the presence of porous medium, it is found, for real growth rate n, that the inhomogeneous magnetic field has always a stabilizing effect on the considered system. It is found also, for complex growth rate n, that the system is stable for the stable stratification case, while it is stable or unstable for the unstable case under a certain wavenumbers range depending on the Alfvèn velocity and the stratification parameter. The presence of the magnetic field is found to stabilize a certain wavenumbers band, whereas the system was unstable for all wavenumbers in the absence of the magnetic field. Also, the presence of porous medium is found to hide the stabilizing effect played by rotation on the considered system for non-porous medium, i.e., rotation does not have any significant effect on the stability criterion in this case.     

Direct numerical simulation of a supersonic turbulent boundary layer on a flat plate and its analysis

In the past three decades, considerable progress has been made in the investigation of incompressible turbulent boundary layer through experiments, DNS and theoretical works, including: (1) the statistics characteristic and structure of turbulence; (2) the co-herent structures in turbulent flows; (3) turbulence modeling and the large eddy simula-tion (LES). In contrast, the progress was very slow for the compressible, in particular, the super-sonic turbulent boundary layer. Recent works on d…     

On magnetic field penetration into a granulated superconductor

The frequency dependence of the magnetic response of YBa₂Cu₃O_7−x high-temperature supercon-ducting ceramics is studied. The experimental results are obtained for different values of the constant ((0–99)×10⁻⁴ T) and variable ((0.18–7.24)×10⁻⁴ T) components of the external magnetic field in the frequency range from 60 Hz to 1 MHz. A simple qualitative model of the distributed Josephson medium is offered to account for the results obtained.     

Direct numerical simulation of shock/turbulent boundary layer interaction in a supersonic compression ramp

A direct numerical simulation of the shock/turbulent boundary layer interaction flow in a supersonic 24-degree compression ramp is conducted with the free stream Mach number 2.9.The blow-and-suction disturbance in the upstream wall boundary is used to trigger the transition.Both the mean wall pressure and the velocity profiles agree with those of the experimental data,which validates the simulation.The turbulent kinetic energy budget in the separation region is analyzed.Results show that the turbulent production term increases fast in the separation region,while the turbulent dissipation term reaches its peak in the near-wall region.The turbulent transport term contributes to the balance of the turbulent conduction and turbulent dissipation.Based on the analysis of instantaneous pressure in the downstream region of the mean shock and that in the separation bubble,the authors suggest that the low frequency oscillation of the shock is not caused by the upstream turbulent disturbance,but rather the instability of separation bubble.     

Dependence of the wind profile power law exponent on the Pasquill stability classes and height interval in a stably stratified surface boundary layer.

Summary Using the logarithmic wind profile under stable stratification, a classification scheme was obtained for the Monin-Obukhov stability length and for the wind profile power law exponent in terms of the surface roughness length and the PasquillD-G stability classes, anywhere in the surface layer.     

Phenomenology of two-dimensional stably stratified turbulence under large-scale forcing

In this paper, we characterise the scaling of energy spectra, and the interscale transfer of energy and enstrophy, for strongly, moderately and weakly stably stratified two-dimensional (2D) turbulence, restricted in a vertical plane, under large-scale random forcing. In the strongly stratified case, a large-scale vertically sheared horizontal flow (VSHF) coexists with small scale turbulence. The VSHF consists of internal gravity waves and the turbulent flow has a kinetic energy (KE) spectrum that follows an approximate k^?3 scaling with zero KE flux and a robust positive enstrophy flux. The spectrum of the turbulent potential energy (PE) also approximately follows a k^?3 power-law and its flux is directed to small scales. For moderate stratification, there is no VSHF and the KE of the turbulent flow exhibits Bolgiano–Obukhov scaling that transitions from a shallow k^?11/5 form at large scales, to a steeper approximate k^?3 scaling at small scales. The entire range of scales shows a strong forward enstrophy flux, and interestingly, large (small) scales show an inverse (forward) KE flux. The PE flux in this regime is directed to small scales, and the PE spectrum is characterised by an approximate k^?1.64 scaling. Finally, for weak stratification, KE is transferred upscale and its spectrum closely follows a k^?2.5 scaling, while PE exhibits a forward transfer and its spectrum shows an approximate k^?1.6 power-law. For all stratification strengths, the total energy always flows from large to small scales and almost all the spectral indicies are well explained by accounting for the scale-dependent nature of the corresponding flux.