Ratio Of Cavity Length Research Articles

Analysis of Spatiotemporal Variations and Flow Structures in a Periodically Driven Cavity

The time-dependent fluid flow in a square cavity was studied using model fluids of glycerol-water solution at different frequencies and amplitudes of motion of the top plate. The range of Reynolds numbers in our investigation varied from 5 to 3700. The experiments were carried out in a square cavity with a periodically driven lid, and planar velocity measurements were obtained using particle image velocimetry. The flow was driven by moving the top surface of the cavity in a simple harmonic motion. The aspect ratio, defined as the ratio of cavity length to the cavity height, is unity. The ratio of cavity spanwise width to the length of the cavity is 0.2. The temporal variation of velocity at fixed locations in the cavity exhibits a periodic variation. The basic frequency of the fluid motion at a point in the flow domain was observed to be the same as that of plate motion for low Reynolds number Re. However, existence of dominant secondary frequencies was observed along the central vertical plane. The velocity variation as a function of time at a fixed position and the velocity profiles along horizontal and vertical planes are also quantitatively described. These were compared to computational fluid dynamics (CFD) simulations based on the finite volume technique. Comprehensive details of the flow as a function of Reynolds number are analyzed. The evolution of secondary vortices at different plate positions as a function of Reynolds number is also presented. The planar velocity measurements acquired are indicative of the flow behavior in a periodically driven cavity with a narrow span width even at high Re. At very low Re, the flow throughout the periodically driven cavity qualitatively resembles the classical steady lid-driven cavity flow. At high Re, the entire cavity is occupied with multiple vortices. The qualitative features of the bulk flow observed are valid even for cavities with infinite span width.

Effects of lateral heat and carrier diffusion on thermal runaway of cw DH semiconductor lasers

Effects of lateral heat and carrier diffusion on the maximum output intensity, above which thermal runaway takes place, are determined for cw semiconductor lasers. The effects are depicted by an aspect ratio of cavity length to stripe width and a dimensionless quantity representing the efficiency of heat transfer. A concise criterion for no thermal runaway is given. The results show that the output intensity can be increased by an order of magnitude by reducing the stripe width sufficiently but that only about a twofold increase in the output power can be realized. However, a laser with a smaller stripe width can be operated at a higher temperature for the same output power. An optimization with respect to the aspect ratio and the dimensionless quantity should result in a three-to-fourfold increase even in the output power. >

An experiment of cavitating flow around a finite span hydrofoil

This paper describes the experimental results of a finite span hydrofoil of NACA0012 section with elliptic planform (aspect ratio of 3).Lift and drag coefficients were measured at two angles of attack (α=10 and 20 deg.), where the cavitation number was changed from non-cavitating condition to super-cavitating condition.The pressure distribution of the hydrofoil and the wake distribution were measured at some typical cases. With this experiment the following results are obtained : (1) As the cavity grows, the ratio of cavity length to chord length becomes larger near the tip.(2) Under the non-cavitating condition or the condition of tip vortex cavitation there is a velocity acceleration near the tip-vortex. This acceleration changes to deceleration after inception of sheet type cavitation.

Finite Difference Calculation of Cavitating Flow around a Finite Span Hydrofoil

Characteristics of cavitating flow around a finite span hydrofoil are studied by a finite difference computation using Bubble Two-phase Flow model. The flow with Reynolds number of 103 is calculated at the angle of attack of 10 deg. on a NACA 0012 hydrofoil with elliptic planform of aspect ratio of 3.The mechanisms of the cavitating flow, particularly the relationships between the vortex system around the hydrofoil and the cavitation, are investigated with the use of the high spatial resolution in the numerical calculation although the Reynolds number is low because of the computation time and memory capacity problem. The discussions on the calculated results give the following results : (1) Since the planform of the hydrofoil is elliptic, the ratio of cavity length to chord length becomes almost constant along the span when the cavity is small. But as the cavity grows, the tip region begins to generate larger cavity. The reason for this is that the tip vortex expands the separating flow region on the suction side of the hydrofoil near the tip.(2) The larger cavity weakens the tip vortex because of the reduction of the lift force and/or the diffusion of flow due to cavity. The diffusion due to cavity also increases the width of its wake.(3) The above calculated results agrees qualitatively with the existing experimental results. It can be said that even the present calculation with low Reynolds number has clarified many of the mechanisms of cavitating flow around a finite span hydrofoil.

Optimum coupling junction and cavity lengths for coupled-cavity semiconductor lasers

In this paper we investigate the dependence of the mode suppression in coupled-cavity semiconductor lasers on coupling junction parameters and cavity lengths. An effective mirror formalism is developed and combined with exact numerical analysis to explore the mode behavior across the entire relative cavity gain operating range of coupled-cavity resonators. A gap coupling model is also presented which permits one to deduce the optimum gap spacing for best mode suppression, as well as the dependence of this optimum gap spacing on relative cavity lengths. Finally, the exact numerical analysis is combined with spectral gain roll-off to explore the mode suppression of all modes of the system versus cavity lengths. Long-long and long-short geometries are shown to be desirable as well as some interesting new intermediate cavity length ratios. Calculations indicate that coupled-cavity 1.55-μm lasers with mode suppression in excess of 10 cm−1 to both near in and repeat modes are realizable.

Analysis and design of coupled-cavity lasers - Part I: Threshold gain analysis and design guidelines

The analysis and design of two-section and multisection coupled-cavity lasers are treated in two parts. In this first part, the focus is on two-section laser design and control using threshold gains. In the second part numerical analysis of the transient behavior is given. The present treatment begins by using the poles of a linear transfer function for the coupled-cavity laser to obtain mode wavelengths and wavelength dependent threshold gains. A general wave scattering matrix describes the intercavity coupling, so that simple waveguide discontinuities or complex arrays of discontinuities can be modeled. Numerical examples are given. Design relationships obtained primarily from the scattering analysis, together with numerical examples, show the dependence and interdependence of the laser mode wavelengths and threshold gain minima on various parameters such as cavity and coupler lengths, indexes of refraction, and gains. These relationships show optimum or preferred coupling gap lengths and cavity length ratios for various design criteria. For example, for optimum mode and spurious rejection stability a short ( \lsim40 \mu m) cavity can be coupled via a gap approximately an integer number of half wavelengths long to a second longer cavity ( \sim 100- 250 \mu m). Conversely, for maximum tunability (or sensitivity to input changes) together with good mode selectivity, two medium length cavities ( \sim 100-250 \mu m) differing in length by a small amount ( \lsim40 \mu m) should be chosen.

Cleaved-coupled-cavity lasers with large cavity length ratios for enhanced stability

The fabrication and operation of the first cleaved-coupled-cavity (C3) semiconductor lasers with large cavity length ratios are described. The internal cleaved facet is precisely positioned by photochemically etching a groove through most of the wafer. Single longitudinal mode operation is obtained over a temperature range of 21 °C and over a current range of threshold to greater than four times threshold. Sidemode suppression of 100:1 was measured when the laser was modulated at 350 MHz with an extinction ratio greater than 10:1. These results are experimentally and theoretically compared to approximately equal length C3 lasers.