14 research outputs found
Experimental characterization of starting jet dynamics
The dynamics of a laminar starting jet are explored in a series of laboratory experiments and numerical simulations.
We identify new, objective methods for characterizing the leading vortex ring, enabling robust comparisons with
results from a numerical model. Observations of circulation in the initial vortex ring and for the total jet are reported
along with strain rate at the leading stagnation point. Growth and pairing of shear instabilities trailing the leading
vortex ring is observed. Development of these secondary vortices and their subsequent interactions with the leading
vortex has significant effects on the characteristics of the primary vortex ring. Strong fluctuations in strain rate at
the leading edge are associated with the pairing of the initial vortex ring with a trailing secondary ringSupport for this research was provided by the Spanish MEC and European Union under Projects # ENE2005-08580-C02-01 and DPI2005-08654-C04-01Publicad
Negatively buoyant starting jets
The initial development of negatively buoyant jets has been investigated experimentally and numerically, focusing on the role played by gravity in the evolution of the leading vortex ring. Under the experimental conditions considered in this work, the densimetric Froude number, Fr= ρjU²j/[(ρ₀ − ρj) gD] , which represents the ratio between the jet momentum and the buoyancy forces,
emerges as the most relevant parameter characterizing the dynamics of the flow. Two different flow
regimes have been observed depending on the Froude number: for sufficiently small Fr, the vortex
ring generated initially is pushed radially away by gravity forces before it has time to detach from
the shear layer originating at the orifice. On the other hand, when the Froude number is larger than
a critical value, Fr> Frc∼ 1, the vortex ring detaches from the injection orifice and propagates
downstream into the stagnant ambient followed by a trailing jet until it eventually reaches a
maximum penetration depth. In order to clarify the mechanisms leading to the transition between the
two regimes, and to gain physical understanding of the formation dynamics of negatively buoyant
starting jets, the total and the vortex circulation, as well as the trajectory of the vortex center, have
been measured and compared to the case of neutrally buoyant jets. Finally, based on the
experimental measurements and on the results of the numerical computations, a kinematic model
that successfully describes the evolution of both total circulation and vortex trajectory is
proposed.This work was supported by the Spanish Ministry of Education under Project Nos. DPI2008-06624-C03-02 and ENE2008-0615-C04. This work has been extracted from the
Ph.D. thesis of Marugán-CruzPublicad
Quantization of Midisuperspace Models
We give a comprehensive review of the quantization of midisuperspace models.
Though the main focus of the paper is on quantum aspects, we also provide an
introduction to several classical points related to the definition of these
models. We cover some important issues, in particular, the use of the principle
of symmetric criticality as a very useful tool to obtain the required
Hamiltonian formulations. Two main types of reductions are discussed: those
involving metrics with two Killing vector fields and spherically symmetric
models. We also review the more general models obtained by coupling matter
fields to these systems. Throughout the paper we give separate discussions for
standard quantizations using geometrodynamical variables and those relying on
loop quantum gravity inspired methods.Comment: To appear in Living Review in Relativit
Thermal Stresses Analysis of a Circular Tube in a Central Receiver
AbstractThe tubes of central receiver power plants are designed to work under very demanding conditions, with heat fluxes varying between 0.2 to 1 MW/m2, depending on both the weather conditions and the operating regime of the plant. Also, the heat flux is localized along the circumferential coordinate of the tube, on the outward facing side of the tube. This results in a non-uniform temperature distribution with strong gradients both in the heat transfer fluid and the pipe walls. Predicting this distribution is important since it can induce high thermal stresses on the pipe walls and it is a key factor for the fluid decomposition.In this work we have studied different configurations to analyze the effects of tube diameter and wall thickness in the temperature distributions in the fluid and solid, for a typical concentrated solar heat flux distribution. HITEC salt and 316L stainless steel have been used as the heat transfer fluid and the material of the tubes in the receiver, respectively. A numerical method has been used to calculate the temperature distribution of the fluid and the convection coefficient along the circumference. Finally the Von-Mises thermal stresses of the tube receiver have been calculated