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Over 25 years of access to the velocity gradient tensor: what we have learned from experiments... Imprimir
James M. Wallace
Professor Emeritus, Dept. of Mechanical Engineering
Director, Burgers Program for Fluid Dynamics
Institute for Physical Science & Technology
University of Maryland
Over 25 years of access to the velocity gradient tensor: what we have learned from experiments and DNS about turbulence? 

Viernes 3 de octubre, 16:15hrs
Departamento de Física (DFI), F12
Av Blanco Encalada 2008.

A little over Twenty-five  years ago there was no experimental access to the velocity gradient tensor for turbulent flows. Without such access, knowledge of fundamental and defining properties of turbulence, such as vorticity, dissipation and strain rates and helicity, could not be studied in the laboratory.  Although a few direct simulations at very low Reynolds numbers had been performed, most of these did not focus on properties of the small scales of turbulence defined by the velocity gradient tensor.   In 1987 the results of the development and first successful use of a multi-sensor hot-wire probe for measurements of all the components of the velocity gradient tensor in a turbulent boundary layer were published by Balint, Vukoslavcevic and Wallace (Adv. in Turbulence: Proc. of 1st Euro. Turb. Conf.,Springer-Verlag, 456). That same year the first DNS of a turbulent channel flow was successfully carried out and reported by Kim, Moin and Moser (J. Fluid Mech. 177), including statistics of the vorticity field.  Also in that year a DNS of homogeneous shear flow by Rogers and Moin (J. Fluid Mech. 176) was published in which the authors examined the structure of the vorticity field. Additionally, Ashurst, Kerstein, Kerr and Gibson (Phys. Fluids 30) examined alignment of the vorticity and strainrate fields using this homogeneous shear flow data as well as the DNS of isotropic turbulence of Kerr (J. Fluid Mech. 153) who had initiated such studies.  Furthermore, Metcalfe, Orszag, Brachet, Menon and Riley (J. Fluid Mech. 184) also published in 1987 results from their direct simulation of a temporally developing planar mixing layer in which they examined coherent vortical states resulting from secondary instabilities.  Since then several experimentalists have used multi-sensor hot-wire probes of increasing complexity in turbulent boundary layers, wakes, jets, mixing layers and grid flows. Numerous computationalists have employed DNS in a wide variety of turbulent flows at ever increasing Reynolds numbers.  PIV and other optical methods have been rapidly developed and advanced during these two and a half decades which have provided other means of access to these fundamental properties of turbulence. This presentations reviews highlights of these remarkable developments and points out some of the most important things we have learned about turbulence as a result.

Seminarios Anteriores

Red de paredes tipo zig-zag en un cristal líquido nemático Imprimir
Ignacio Andrade
Departamento de Física
FCFM, U. de Chile

Viernes 26 de septiembre, 16:15hrs
Departamento de Física (DFI), F12
Av Blanco Encalada 2008.

Liquid crystals displays (LCD's) with tailoring electrodes exhibit complex spatiotemporal dynamics when a large voltage is applied. We report experimental observations of the appearance of a programmable zig-zag lattice using an in-plane switching cell filled with a nematic liquid crystal. Applying a small voltage to a wide range of frequencies, the system exhibits an Ising wall lattice. Increasing the voltage, this lattice presents a spatial instability generating an undulating wall lattice, and to higher voltages it becomes zig-zag type. Experimentally, we characterize the bifurcations and phase diagram of the wall lattice. Theoretically, we develop, from first principles, a descriptive model. This model has a good qualitative agreement with experimental observations. 
Formación de laberintos: De estructuras localizadas a patrones extendidos Imprimir
Ignacio Bordeu
Departamento de Física
FCFM, U. de Chile

Viernes 12 de septiembre, 16:15hrs
Departamento de Física, F12
Av Blanco Encalada 2008.

La naturaleza de los fenómenos macroscópicos involucra un constante intercambio de energía entre el sistema en estudio y su entorno. Estos sistemas, denominados sistemas disipativos, abarcan desde sistemas granulares y cristales líquidos a fluidos y sistemas biológicos. Éstos, exhiben comportamientos de auto organización, relajando muchas veces a equilibrios con un ordenamiento espacial como son los patrones. Además, pueden presentar la existencia de estructuras localizadas, es decir, estructuras que solo abarcan una porción del espacio disponible. En éste seminario se presentará el estudio analítico y numérico de la evolución temporal de una estructura localizada, la cual, vía inestabilidades consecutivas cae a un equilibrio estacionario dado por un patrón extendido tipo laberinto. 
Illuminating the hidden sector with photons Imprimir
Paola Arias
Departamento de Física

Jueves 4 de septiembre, 16:15hrs
Departamento de Física
Av Blanco Encalada 2008, 3er piso
Sala de seminarios


Physics beyond the Standard Model naturally gives rise to very light and weakly interacting particles, dubbed WISPs (Weakly Interacting Slim Particles). A prime example is the axion, invented to solve the strong CP problem and a natural candidate for cold dark matter. In this talk we will review two strongly motivated candidates for WISPs, observational hints for them, and the present status of searches with photon regeneration experiments.
Making a Cocktail: Neutrino Masses, Mixing Angles and Dark Matter Imprimir
Maximiliano Rivera
Departamento de Física
UTFSM, Campus Santiago

Viernes 29 de agosto, 16:15hrs
Departamento de Física - Sala F12
Av Blanco Encalada 2008

Several question in High Energy Physics are still opened, for example the nature of neutrino (Majorana or Dirac), the mechanism that gives small neutrino masses and the nature of Dark Matter (wimps, axions, etc..), among others. In this seminar an extension of the standard model that naturally generate small neutrino masses and provide a dark matter candidate is presented. The dark matter particle is part of a new scalar doublet field that plays a crucial role in radiatively generating neutrino masses. The symmetry that stabilises the dark matter also suppresses neutrino masses to appear first at three-loop level. Without the need of right-handed neutrinos or other very heavy new fields, this offers an attractive explanation of the hierarchy between the electroweak and neutrino mass scales. The model has distinct verifiable predictions for the neutrino masses, flavor mixing angles and dark matter signals.
Observación: Charla en Español 
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