Papers by Sergio Hernández Zapata
3D Numerical Simulation of Rayleigh-Bénard Convection in a Cylindrical Container
Environmental science and engineering, 2015
The heat transport by natural convection is a central mechanism in the explanation of many natura... more The heat transport by natural convection is a central mechanism in the explanation of many natural phenomena. Despite many existing work on the Rayleigh-Benard convection, often the phenomenon is studied by making a two-dimensional approach or using a rectangular container. In this work, we solve numerically the Navier-Stokes, continuity and energy equations in cylindrical coordinates. To this end a finite difference scheme is used for the time and spatial coordinates \(r\) and \(z\), whereas a Fourier spectral method is used for the angular coordinate. The advantage of this procedure is that it can be easily parallelized. The numerical results include the formation of concentric rolls and other patterns, which are compared with experimental results reported in the literature.
Bulletin of the American Physical Society, 2014
this work we present a numerical and experimental research of a vortex ring impinging a cone. Bot... more this work we present a numerical and experimental research of a vortex ring impinging a cone. Both the vortex and the conical wall have the same axis of symmetry. For this study we solve the Navier-Stokes and continuity equations in cylindrical coordinates using a finite difference scheme for r, z and time, whereas a Fourier spectral method is used for the angular variable. As initial conditions we assume that velocity is given by the Biot-Savart law for a vorticity distribution of constant magnitude inside a torus. With respect the experiments, measurements of velocity were made with a hot wire anemometer. To have a mapping in space we use a traverse system to place the hot wire probe in points of a grid. Additionally, the measurements of velocity are synchronized with the production of the vortex. Unlike the case of the impact with a flat wall, in this case the diameter of the vortex ring cannot grow. We study the shape of the vortex before the impact, the instabilities and the production of secondary vorticity during the impact. Finally, we made a comparison between experiment and the numerical simulations.
Bulletin of the American Physical Society, 2016
Submitted for the DFD16 Meeting of The American Physical Society A Numerical Simulation of the De... more Submitted for the DFD16 Meeting of The American Physical Society A Numerical Simulation of the Density Oscilator 1 SERGIO HER-NANDEZ ZAPATA, ERICK JAVIER LOPEZ SANCHEZ, GERARDO RUIZ CHAVARRIA, Facultad de Ciencias, UNAM-In this work we carry out a numerical simulation for the dynamics that originates when a fluid (salty water) is located on top of another less dense fluid (pure water) in the presence of gravity. This is an unstable situation that leads to the development of intercalating lines of descending salty water and ascending pure water. Another situation is studied where the fluids are in two containers joined by a small hole. In this case a time pattern of alternating flows develops leading to an oscillator. The study of the velocity field around the hole shows than in a certain interval of time it develops intercalating lines like in the former situation. An interesting result is the fact that when a given fluid is flowing in one direction a vorticity pattern develops in the other fluid. The Navier-Stokes, continuity and salt diffusion equations, are solved numerically in cylindrical coordinates, using a finite difference scheme in the axial and radial directions and a Fourier spectral method for the angular coordinate. On the other hand, the second order Adams-Bashfort method is used for the time evolution. The results are compared to a numerical simulation of a pedestrian oscillator we developed based on the Hebling and Molnar social force model.
Bulletin of the American Physical Society, 2016
this work we investigate the impact of an annular vortex on a conical surface when their symmetry... more this work we investigate the impact of an annular vortex on a conical surface when their symmetry axes are parallel but they do not coincide. For this purpose we solve the Navier-Stokes and continuity equations in cylindrical coordinates. We use a finite difference scheme for r and z coordinates whereas for the angular coordinate we use a Fourier spectral method. We study the development of asymmetrical modes when the vortex approaches the inner surface of the cone. The presence of the vortex ring induces the formation of a boundary layer which detaches and leads to the formation of a secondary vortex of opposite sign which moves away the cone. This secondary vortex also exhibits asymmetrical modes, which are attenuated as it moves. We present some results as the trajectories of the primary and the secondary vortices, their circulations as a function of time, the development of asymmetrical modes and the dependence of these properties on the Reynolds number and the distance between both symmetry axes. Finally we made a comparison of primary and secondary vortices with a free vortex.
Bulletin of the American Physical Society, 2015
a tidal induced flow between a channel and an open domain a pair of counterrotating vortices is p... more a tidal induced flow between a channel and an open domain a pair of counterrotating vortices is produced during each cycle. Such pair of vortices is known as a dipole. The Strouhal number (S) is the parameter determining if dipole escapes or is sucked during the stage of negative flowrate. Some years ago an analytical model has been proposed to determine the evolution of the vortices (Wells M.G. & Van Heijst G.J.F., Dynamics of atmospheres and oceans, 37(2003) 23-34). This model agrees with experimental and observational data when S is close to the critical value 0.13. However, no realistic predictions are given for small values of S. In this work we present a modification of this model to take into account some details not considered before. In particular the fact that not all vorticity created into the channel is incorporates into the dipole. This fact leads to have a lower translational velocity and also to the formation of a vorticity band behind the vortices. Our results have a better agreement with numerical simulations and experimental data. Finally we study the influence of the Reynolds number in the evolution of the vortices and the interaction between dipoles produced in subsequent cycles.
Journal of Fluid Mechanics, 2021
We describe and model experimental results on the dynamics of a ‘ludion’ – a neutrally buoyant bo... more We describe and model experimental results on the dynamics of a ‘ludion’ – a neutrally buoyant body – immersed in a layer of stably stratified salt water. By oscillating a piston inside a cylinder communicating with a narrow (in one of its horizontal dimensions) vessel containing the stably stratified layer of salt water, it is easy to periodically vary the hydrostatic pressure of the fluid. The ludion or Cartesian diver, initially positioned at its equilibrium height and free to move horizontally, can then oscillate vertically when forced by the pressure oscillations. Depending on the ratio of the forcing frequency to the Brunt–Väisälä frequency of the stratified fluid, the ludion can emit its own internal gravity waves that we measure by a classical particle image velocimetry technique. Our experimental results describe first the resonance of the vertical motions of the ludion when excited at different frequencies. A theoretical oscillator model is then derived taking into account...
Latin American Journal of Physics Education, 2010
El que dos fases de una sustancia coexistan en equilibrio implica que la presión y la temperatura... more El que dos fases de una sustancia coexistan en equilibrio implica que la presión y la temperatura no son independientes. La curva que representa esta relación se llama de saturación. Utilizar la información de esta, es de particular importancia en fenómenos de refrigeración, ya que al controlar la presión se puede controlar la temperatura a la cual ocurre la evaporación, además de tener una mejor comprensión de los procesos termodinámicos involucrados. En el presente trabajo, se muestran los resultados de un experimento en donde se mide la curva de saturación de alcohol etílico. El objetivo es obtener un buen método para estudiar propiedades de saturación de diferentes sustancias y construir un dispositivo didáctico para mostrar en clase cómo funciona el evaporador de un refrigerador. El trabajo se complementa con una serie de experimentos demostrativos que ilustran la importancia del comportamiento alrededor de la saturación para explicar diversos procesos físicos y atmosféricos.
Pressure Behavior and Evolution of the Bubbles Inside the Pipes of an Experimental Installation of Centrifugal Pump During the Occurrence of Cavitation
Environmental Science and Engineering, 2011
Pressure drop inside a hydraulic installation was measured by piezoelectric transducers when cavi... more Pressure drop inside a hydraulic installation was measured by piezoelectric transducers when cavitation begins. Bubbles appear in the vicinity of blades of the centrifugal pump and after they moves through the outlet pipe following, in some cases, a line. The three last figures (second row) show different cavitation regimes. On the other side bubbles reveal regions of vorticity concentration both near the blades and in the outlet pipe.
Study of a Surface of a Liquid Layer in Evaporation
Environmental Science and Engineering, 2013
ABSTRACT In a liquid layer of alcohol or another volatile substance a pattern of hydrodynamic cel... more ABSTRACT In a liquid layer of alcohol or another volatile substance a pattern of hydrodynamic cells are formed when relative humidity decreases under a certain threshold. We describe the thermodynamical conditions under which the cells appear. We also study the temperature as a function of the vertical coordinate in the layer and we find that the temperature gradient can not be used to explain the origin of the cells. The deformation of the free surface is determined by the Schlieren technique. For that a sheet of paper with a random pattern of dots is placed in the bottom of the container. Using PIV software the displacement of the dots in the pattern is determined. The statistical study of the displacement allows us to study the slope of the surface and eventually its shape. We calculate the correlation lenght and we find that it is of the order of the size of the cells.
Foundations of Physics, 2010
A completely Lorentz-invariant Bohmian model has been proposed recently for the case of a system ... more A completely Lorentz-invariant Bohmian model has been proposed recently for the case of a system of non-interacting spinless particles, obeying Klein-Gordon equations. It is based on a multi-temporal formalism and on the idea of treating the squared norm of the wave function as a space-time probability density. The particle's configurations evolve in space-time in terms of a parameter with dimensions of time. In this work this model is further analyzed and extended to the case of an interaction with an external electromagnetic field. The physical meaning of is explored. Two special situations are studied in depth: (1) the classical limit, where the Einsteinian Mechanics of Special Relativity is recovered and the parameter is shown to tend to the particle's proper time; and (2) the non-relativistic limit, where it is obtained a model very similar to the usual non-relativistic Bohmian Mechanics but with the time of the frame of reference replaced by as the dynamical temporal parameter.
… -American Journal of …, 2010
Resumen: La cavitación un fenómeno recurrente en las bombas centrífugas fundamentalmente. Es un p... more Resumen: La cavitación un fenómeno recurrente en las bombas centrífugas fundamentalmente. Es un proceso físico muy parecido a la ebullición, la diferencia consiste en que en esta, la formación de burbujas se produce cerca de las superficies sólidas y es ...
Lat. Am. J. Phys. Educ. …, 2010
Lat. Am. J. Phys. Educ. Vol. 4, Suppl. 1, Nov. 2010 ... La refrigeración en la enseñanza de la fí... more Lat. Am. J. Phys. Educ. Vol. 4, Suppl. 1, Nov. 2010 ... La refrigeración en la enseñanza de la física ... Jimena Castro Gutiérrez, Sergio Hernández Zapata, Carlos Álvarez Macias Facultad de Ciencias, Universidad Nacional Autónoma de México. Universidad 3000, Circuito ...
In this paper we intend to extend some ideas of a recently proposed Lorentz-invariant Bohmian mod... more In this paper we intend to extend some ideas of a recently proposed Lorentz-invariant Bohmian model, obeying Klein-Gordon equations, but considering particles with a spin different than zero. First we build a Bohmian model for a single particle, closely related to the Bohm-Dirac model, but using Nikolic's ideas on a space-time probability density and the introduction of a synchronization parameter
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Papers by Sergio Hernández Zapata