P. L. García-Ybarra

Bénard–Marangoni convection with a deformable interface and poorly conducting boundaries

Dispersion relations and threshold values for the onset of buoyancy–thermocapillary instability are given for the case of prescribed heat flux on the boundaries of a liquid layer heated from below when the upper boundary is a deformable surface open to the ambient air. The nonlinear evolution equations of this free surface under various circumstances are also provided (without and with buoyancy, for microgravity or standard ground conditions).

Unsteady Effects in Droplet Vaporization Lifetimes at Subcritical and Supercritical Conditions

Abstract The heating and vaporization of a pure cold fluid package in a hot environment of the same fluid has been analyzed. The model applies to subcritical as well as supercritical fluid conditions and relics on the assumption of constant pressure and quasisteady conditions in the gas phase (in a reference system receding with the cold front). An asymptotic analysis is performed using the ratio of the hot fluid density to the density of the cold fluid package as the smallness parameter. Then, a transcendental equation is obtained which provides the evolution of the cold package radius. For longer times when isothermal conditions are achieved in the cold region, the d2 law is obtained. Some deviations from this law. due to the unsteadiness of the heating process in the cold region, are evaluated and discussed.

Catastrophic Yaw Prevention for Cruciform Tailed Missiles

The amplified resonance called catastrophic yaw can be associated to large amplitude equilibrium points of slightly asymmetric spinning cruciform missiles. A representative set of the steady-state equations of motion permits the analysis of the equilibrium points that define the long-term behavior of the missile in order to avoid this flight condition. In this paper the inclusion of a high order aerodynamic model to accurately represent the high angle characteristics of the missile is discussed and the expression of the fin cant deflection that assures the low amplitude of the single equilibrium point is proposed.

Multiple Roll Angle Fitting of Static Aerodynamic Coefficients of Cruciform Missiles

The aerodynamic characterization of unguided missiles at high angles of attack has been of primary interest for researchers, since nonlinear effects may lead to unexpected steady-state solutions. The usual approach is to add cubic terms to the linear aerodynamic coefficients. However, experiments show the strong roll-orientation dependence of the static normal force and pitch moment, which cannot be addressed with the mere inclusion of cubic terms. This paper presents a fitting procedure based on the Maple-Synge expansion to analytically represent high order roll orientation-dependent aerodynamics of cruciform finned missiles. The study shows that, even at moderate angles of attack of about 15o, expansions to the seventh power may be insufficient to accurately describe the high degree of nonlinearity in the static coefficients.

Simulation of Flame Fronts by Sources of Fluid Volume

Gaseous combustion waves are self propagating exothermic chemical reactions. In the low Mach number limit, these waves are the usual premixed flames where a gas mixture, in an initially unstable molecular configuration at a given temperature, reaches its thermodynamical equilibrium at the flame temperature. Heat release by combustion reaction diffuses towards the fresh gas whose volume increases by thermal expansion. Each heated fluid element expands, acting like a piston that pushes the surrounding fluid. A propagating planar flame front is rendered unstable by such a phenomenon because when it becomes slightly corrugated, crests and valleys push themselves away each other. The linear stability analysis of the planar propagation was performed, independent and almost simultaneously, by Darrieus1 and Landau2 who calculated the potential flow induced in the fresh gas by an infinitesimally distorted flame front. By relating the gas thermal expansion effects to appropriate hydrodynamic jump conditions through the flame, they found that the burned gas is in rotational motion and that each Fourier component of the front distortion grows exponentially with a positive growing rate σDL that increases linearly with the corrugation wave number k.

Unsteady potential flows computation by cellular automata: The premixed flame instability

Abstract A cellular automaton leading to self-propagating fronts in a discrete fluid has been implemented. In spite of the stabilizing diffusive-convective mechanisms, planar propagation of these fronts becomes unstable due to a feed-back mechanism with the flow generated by the front itself. The stability properties have been analyzed in the continuum limit and compared with the discrete simulations as well as with the stability properties of premixed flame fronts.

Discrete Potential Flow Simulation of a Premixed Flame Front

The propagation of a premixed flame front in a gaseous medium provides an example of non-Laplacian growth, contrary to cases as viscous fingering, non-equilibrium solidification, electrochemical deposition, etc., where interfacial pattern formation is controlled by Laplace equation.1 In the flame front problem Laplace equation for the velocity potential holds only in the upstream fresh gas zone (when the flow is assumed potential at infinity) where the temperature is constant and low enough to keep the chemical reaction frozen. In fact, through the flame front, the temperature rises exponentially, up to the combustion temperature, due to the diffusion of heat released in a narrower reaction zone.2 Then, inside the thermal flame thickness (typically of 0.1 mm width) gas thermal expansion occurs and the continuity equation, written in terms on a velocity potential, φ, takes the form of a Poisson equation with a non-homogeneous term equal to the relative rate of gas volume increase n n

EXPERIMENTAL EVIDENCE OF SELF-EXCITED RELAXATION OSCILLATIONS LEADING TO HOMOCLINIC BEHAVIOR IN SPREADING FLAMES

Delta phi = - frac{{dln rho }}{{dt}}