Lattice Boltzmann Simulation of Natural Convection in an Annulus between a Hexagonal Cylinder and a Square Enclosure

Laminar natural convection in a water filled square enclosure containing at its center a horizontal hexagonal cylinder is studied by the lattice Boltzmann method. The hexagonal cylinder is heated while the walls of the cavity are maintained at the same cold temperature. Two orientations are treated, corresponding to two opposite sides of the hexagonal cross-section which are horizontal (case I) or vertical (case II). For each case, the results are presented in terms of streamlines, isotherms, local and average convective heat transfers as a function of the dimensionless size of the hexagonal cylinder cross-section (0.1≤B≤0.4), and the Rayleigh number (103≤Ra≤106)

Analysis of Turbulent Natural Convection by an Elliptic Relaxation Model in Tall Vertical Cavities with Linear Temperatures on Sidewalls

Turbulent natural convection of air is studied, by the elliptic-relaxation model v^2-f, in a tall vertical cavity whose hot and cold walls are maintained at linear temperatures of slopes γ_1 and γ_2, respectively. The average temperatures of the active walls are located at mid-height of the cavity. Four situations are analyzed, corresponding to γ_1=γ_2=γ (case I), γ_1=-γ_2=γ (case II), γ_1=0 and γ_2=γ (case III), γ_1=γ and γ_2=0 (case IV). These boundary conditions may be more representative or used to control heat transfer for certain systems. The effects of the slope (-1≤γ≤1), the aspect ratio of the cavity (10≤A≤80) and the average Rayleigh number (5×〖10〗^4≤〖Ra〗_m≤〖10〗^6 ) on the streamlines, isotherms, contours of the turbulent kinetic energy, heatlines, local and average Nusselt numbers are investigated. It is shown that the local and average heat transfers of cases III and IV can be deducted from those of cases I and II. The obtained dynamic and thermal fields as well as local and average heat transfers of the studied cases are quite different of those of the classical case corresponding to γ=0. A simplified procedure for calculating the average Nusselt number is also developed for each case

Room Temperature Coherent and Voltage Tunable Terahertz Emission from Nanometer-Sized Field Effect Transistors

We report on reflective electro-optic sampling measurements of TeraHertz emission from nanometer-gate-length InGaAs-based high electron mobility transistors. The room temperature coherent gate-voltage tunable emission is demonstrated. We establish that the physical mechanism of the coherent TeraHertz emission is related to the plasma waves driven by simultaneous current and optical excitation. A significant shift of the plasma frequency and the narrowing of the emission with increasing channel's current are observed and explained as due to the increase of the carriers density and drift velocity.Comment: 3 figure

Terahertz Emissions from Optically Excited Plasma Oscillations in HEMTs

International audienc

Terahertz Emissions from Optically Excited Plasma Oscillations in HEMTs

International audienc