Second mode unstable disturbance measurement of hypersonic boundary layer on cone by wavelet transform

Experimental investigation of hypersonic boundary layer instability on a cone is performed at Mach number 6 in a hypersonic wind tunnel.Time series signals of instantaneous fluctuating surface-thermal-flux are measured by Pt-thin-film thermocouple temperature sensors mounted at 28 stations on the cone surface in the streamwise direction to investigate the development of the unstable disturbance.Wavelet transform is employed as a mathematical tool to obtain the multi-scale characteristics of fluctuating surfacethermal-flux both in the temporal and spectrum space.The conditional sampling algorithm using wavelet coefficient as an index is put forward to extract the unstable disturbanceThe generic waveform for the second mode unstable disturbance is obtained by a phase-averaging technique.The development of the unstable disturbance in the streamwise direction is assessed both in the temporal and spectrum space.Our study shows that the local unstable disturbance detection method based on wavelet transformation offers an alternative powerful tool in studying the hypersonic unstable mode of laminar-turbulent transition.It is demonstrated that,at hypersonic speeds,the dominant flow instability is the second mode,which governs the course of laminar-turbulent transition of sharp cone boundary layer.     

Coarsening dynamics at unstable crystal surfaces

In this paper we focus on crystal surfaces led out of equilibrium by a growth or erosion process. As a consequence of that, the surface may undergo morphological instabilities and develop a distinct structure: undulations, mounds or pyramids, bunches of steps, ripples. The typical size of the emergent pattern may be fixed or it may increase over time through a coarsening process that in turn may last forever or it may be interrupted at some relevant length scale. We study dynamics in three different cases, stressing the main physical ingredients and the main features of coarsening: a kinetic instability, an energetic instability, and an athermal instability.     

“Tepid” Geysers above salt cavernsGeysers tièdes au-dessus des cavités salines

The formation of a brine geyser erupting from the wellhead of a large underground salt cavern is described. In most cases, the brine outflow from an opened cavern is slow; it results from the cavern creep closure and the thermal expansion of the cavern brine. These two processes are smooth; however, the brine outflow often is bumpy, as it is modulated by atmospheric pressure variations that generate an elastic increase (or decrease) of both cavern and brine volumes. In addition, when the flow is fast enough, the brine thermodynamic behavior in the wellbore is adiabatic. The cold brine expelled from the cavern wellhead is substituted with warm brine entering the borehole bottom, resulting in a lighter brine column. The brine outflow increases. In some cases, the flow becomes so fast that inertia terms must be taken into account. A geyser forms, coming to an end when the pressure in the cavern has dropped sufficiently. A better picture is obtained when head losses are considered. A closed-form solution can be reached. This proves that two cases must be distinguished, depending on whether the cold brine initially contained in the wellbore is expelled fully or not. It can also be shown that geyser formation is a rare event, as it requires both that the wellbore be narrow and that the cavern be very compressible. This study stemmed from an actual example in which a geyser was observed. However, scarce information is available, making any definite interpretation difficult.     

Investigation of interaction between reflected shocks and growing perturbation on an interface

This paper reports the result of investigation into Richtmyer–Meshkov instability (RMI) resulting from multiple interactions of shock waves with the interface between two media of different densities. The instability growth rates were measured after the interactions of the mixing zone with the refracted shock and the first and the second shocks reflected from the endwall. It was shown that for the contribution of separate shock–interface interactions to the instability growth rate, the condition of additivity is not realized. The values of the factor , accounting for the decrease in the RMI growth rate due to the thickening of the mixing zone, have been determined for a continuous interface and for a turbulent mixing zone. Received 27 January 1998 / Accepted 10 June 1998     

Strategies with Functional Materials in Tackling Instability Challenges of Non-aqueous Lithium-Oxygen Batteries

The instabilities of the battery including cathode corrosion/passivation,shuttling effect of the redox mediators,Li anode corrosion,and electrolyte decomposition are major barriers toward the practical implementation of lithium-oxygen(Li-O2)batteries.Functional materials offer great potential in high performance Li-O2 batteries owing to their functional tailorability of chemical modification for alleviating side reactions and improving catalysis activity,well-defined properties for discharge products storage,and fast mass and electron transfer paths.In this review,instability problems of non-aqueous Li-O2 batteries and recent studies related to the functional materials in tackling the instability issues from rational cathode construction,inhibition of redox mediators(RMs)shuttling,anode protection and novel electrolyte design are illustrated.Future research directions to overcome the critical issues are also proposed for this promising battery technology.The instability issues and the related strategies with functional materials based on the comprehensive consideration of all battery components proposed in this review provide the systematic,deep understanding and rational design of functional materials for Li-O2 batteries,which is beneficial to achieving the practical Li-O2 batteries.     

Magnetohydrodynamics instability of interfacial waves between two immiscible incompressible cylindrical fluids

The problem of nonlinear instability of interfacial waves between two immiscible conducting cylindrical fluids of a weak Oldroyd 3-constant kind is studied. The system is assumed to be influenced by an axial magnetic field, where the effect of surface tension is taken into account. The analysis, based on the method of multiple scale in both space and time, includes the linear as well as the nonlinear effects. This scheme leads to imposing of two levels of the solvability conditions, which are used to construct like-nonlinear Schr6dinger equations （1-NLS） with complex coefficients. These equations generally describe the competition between nonlinearity and dispersion. The stability criteria are theoret- ically discussed and thereby stability diagrams are obtained for different sets of physical parameters. Proceeding to the nonlinear step of the problem, the results show the appearance of dual role of some physical parameters. Moreover, these effects depend on the wave kind, short or long, except for the ordinary viscosity parameter. The effect of the field on the system stability depends on the existence of viscosity and differs in the linear case of the problem from the nonlinear one. There is an obvious difference between the effect of the three Oldroyd constants on the system stability. New instability regions in the parameter space, which appear due to nonlinear effects, are shown.