Boundary layer transition observations on a body of revolution with surface heating and cooling in water

Boundary layer flow visualization in water with surface heat transfer was carried out on a body of revolution which had the predicted possibility of laminar separation under isothermal conditions. Flow visualization was by in-line holographic technique. Boundary layer stabilization, including elimination of laminar separation, was observed to take place on surface heating. Conversely, boundary layer destabilization was observed on surface cooling. These findings are consistent with the theoretical predictions of Wazzan et al. in The stability and transition of heated and cooled incompressible laminar boundary layers, in Proceedings of the Fourth International Heat Transfer Conference, Vol. 2, FCI 4. Elsevier, Amsterdam (1970).

Laser Raman spectra of semicarbazide hydrobromide

Semicarbazide hydrobromide which is isomorphous with SEM.HCl, was expected to belong to a new family of ferroelectrics. Dielectric, thermal and other studies on these crystals have yielded results which show many peculiarities but not confirmed ferroelectricity in the low temperature phase. As such a Laser Raman spectrosocopic study of oriented singe crystals of SEM.HBr was made at 298°K and at 253°K. The results have been correlated with structural features and compared with SEM.HCl.

Theoretical study of a 16-µm CO2 downstream-mixing gasdynamic laser: A two-dimensional approach

A 16-µm CO2-N2 downstream-mixing gasdynamic laser, where a cold CO2 stream is mixed with a vibrationally excited N2 stream at the exit of the nozzle, is studied theoretically. The flow field is analyzed using a two-dimensional, unsteady, laminar and viscous flow model including appropriate finite-rate vibrational kinetic equations. The analysis showed that local small-signal gain up to 21.75 m−1 can be obtained for a N2 reservoir temperature of 2000 K and a velocity ratio of 1:1 between the CO2 and N2 mixing streams. Applied Physics Letters is copyrighted by The American Institute of Physics.

Laser Raman spectroscopic study of ferroelastic LiCsSO4

Lithium caesium sulphate has been reported to undergo a phase transition from the room temperature orthorhombic phase with space groupP cmn to a final phase with space groupP 22/n. Though a sharp anomaly in its physical properties has been found at 202.0;K, it was found that there was a need for careful investigations in the vicinity of 240 and 210.0;K. Since the changes in the crystal structure involve primarily a rotation of the SO4 tetrahedron about thec-axis and as this may be reflected both in the intensity and polarisation of the internal as well as external phonon modes, the laser Raman spectra of oriented single crystals of LiCsSO4 at different temperatures were investigated. For correlation and definite identification of the spectral features, its infrared absorption spectrum was also studied. An analysis of the intensities and polarizations of the internal modes of the sulphate ions reveals the change in symmetry of the crystal. The integrated intensity and peak height of thev 1 line, plotted against temperature show anomalous peaks in the region of the phase transition. Differential scanning calorimetric study gives the enthalpy change ΔH across the phase transition to be 0.213 kJ/mol.

Simple period timing device for laser Doppler signals

A period timing device suitable for processing laser Doppler anemometer signals has been described here. The important features of this instrument are: it is inexpensive, simple to operate, and easy to fabricate. When the concentration of scattering particles is low the Doppler signal is in the form of a burst and the Doppler frequency is measured by timing the zero crossings of the signal. But the presence of noise calls for the use of validation criterion, and a 5–8 cycles comparison has been used in this instrument. Validation criterion requires the differential count between the 5 and 8 cycles to be multiplied by predetermined numbers that prescribe the accuracy of measurement. By choosing these numbers to be binary numbers, much simplification in circuit design has been accomplished since this permits the use of shift registers for multiplication. Validation accuracies of 1.6%, 3.2%, 6.3%, and 12.5% are possible with this device. The design presented here is for a 16-bit processor and uses TTL components. By substituting Schottky barrier TTLs the clock frequency can be increased from about 10 to 30 MHz resulting in an extension in the range of the instrument. Review of Scientific Instruments is copyrighted by The American Institute of Physics.

Study of synchronously mode-locked and internally frequency-doubledcw dye laser

The analysis of the characteristics of a synchronously mode-locked and internally frequency-doubled dye laser is presented. Dependence of dye laser pulse characteristics on the cavity length mismatch of the pump laser and dye laser is studied. Variation of the minimum pulsewidth with intracavity bandwidth and the harmonic conversion efficiency is presented in the form of graphs.

Growth and physical properties of a new chiral open-framework crystal for NLO applications: Cesium hydrogen l-malate monohydrate

Cesium hydrogen l-malate monohydrate, CsH(C4H4O5)·H2O, is a new chiral open-framework semi-organic crystalline material with a second-harmonic generation efficiency one order of magnitude greater than KDP. Single crystals of this new material have been grown by the conventional slow cooling technique from aqueous solution. Grown crystals display both platy and prismatic morphologies depending on the imposed supersaturation. Hardness values measured using Vickers hardness indenter show considerable anisotropy. The resistivity behavior at room temperature and above, places the crystal between an ionic conductor and a dielectric. The single-crystal SHG efficiency estimated through Maker fringes experiment gives deff which is 4.24 times that of KDP. Single and multiple shot experiments performed on the grown crystals for the fundamental and second harmonic of pulsed Nd:YAG laser (1064 and 532 nm) show that it exhibits a high laser damage threshold which is a favorable property for nonlinear optical applications.

Measurement of sensitivities for the thermal recording of laser beams

A simple technique for determining the energy sensitivities for the thermographic recording of laser beams is described. The principle behind this technique is that, if a laser beam with a known spatial distribution such as a Gaussian profile is used for imaging, the radius of the thermal image formed depends uniquely on the intensity of the impinging beam. Thus by measuring the radii of the images produced for different incident beam intensities the minimum intensity necessary (that is, the threshold) for thermographic imaging is found. The diameter of the laser beam can also be found from this measurement. A simple analysis based on the temperature distribution in the laser heated material shows that there is an inverse square root dependence on pulse duration or period of exposure for the energy fluence of the laser beam required, both for the threshold and the subsequent increase in the size of the recording. It has also been shown that except for low intensity, long duration exposure on very low conductivity materials, heat losses are not very significant.

CO2-laser induced X-ray emission from high density gaseous targets

A theory for the emission of X-rays from a high density gaseous plasma interacting with CO2 laser is given. It predicts a sharp increase in the X-ray intensity for densities close to the critical.

Theory of CO2-laser induced x-ray emission from high density gaseous targets

Abstract is not available.

Anomalous plasma heating by an intense laser beam

Heating of laser produced plasmas by an instability is investigated. For intense laser beams anomalous absorption is found. A comparison is made with the experiment.

Cooling of a composite slab

A mixed boundary value problem associated with the diffusion equation, that involves the physical problem of cooling of an infinite parallel-sided composite slab, is solved completely by using the Wiener-Hopf technique. An analytical expression is derived for the sputtering temperature at the quench front being created by a cold fluid moving on the upper surface of the slab at a constant speed v. The dependence of the various configurational parameters of the problem under consideration, on the sputtering temperature, is rather complicated and representative tables of numerical values of this important physical quantity are prepared for certain typical values of these parameters. Asymptotic results in their most simplified forms are also obtained when (i) the ratio of the thicknesses of the two materials comprising the slab is very much smaller than unity, and (ii) the quench-front speed v is very large, keeping the other parameters fixed, in both the cases.

Transition from radiatively inefficient to cooling dominated phase in two temperature accretion disks around black holes

We investigate the transition of a radiatively inefficient phase of a viscous two temperature accreting flow to a cooling dominated phase and vice versa around black holes. Based on a global sub-Keplerian accretion disk model in steady state, including explicit cooling processes self-consistently, we show that general advective accretion flow passes through various phases during its infall towards a black hole. Bremsstrahlung, synchrotron and inverse Comptonization of soft photons are considered as possible cooling mechanisms. Hence the flow governs a much lower electron temperature similar to 10(8) - 10(9.5) K compared to the hot protons of temperature similar to 10(10.2) - 10(11.8) K in the range of the accretion rate in Eddington units 0.01 less than or simiar to (M) over dot less than or similar to 100. Therefore, the solutions may potentially explain the hard X-rays and the gamma-rays emitted from AGNs and X-ray binaries. We finally compare the solutions for two different regimes of viscosity and conclude that a weakly viscous flow is expected to be cooling dominated compared to its highly viscous counterpart which is radiatively inefficient. The flow is successfully able to reproduce the observed minosities of the under-fed AGNs and quasars (e.g. Sgr A*), ultra-luminous X-ray sources (e.g. SS433), as well as the highly luminous AGNs and ultra-luminous quasars (e.g. PKS 0743-67) at different combinations of the mass accretion rate and ratio of specific heats.

EIT as a tool to observe dual wavelength operation of a semiconductor laser

We propose a simplified technique for dual wavelength operation of an extended cavity semiconductor laser, and its characterization using electromagnetically induced transparency (EIT). In this laser cavity scheme light beam is made converging before it incidences on the cavity grating. The converging angle of the beam creates two longitudinal oscillating modes of resonating cavity. Frequency separation between the longitudinal modes are measured with the help of beat frequency generation in a photodiode and creating pair of EIT spectra in Rb vapor. The pair of EIT dips that are generated due to dual wavelength of this laser (that is used as control laser) can be used to estimate frequency difference between the generated wavelengths. Width of EIT spectra can be used to estimate line width of individual wavelength components.

Two-temperature accretion around rotating black holes: a description of the general advective flow paradigm in the presence of various cooling processes to explain low to high luminous sources

We investigate viscous two-temperature accretion disc flows around rotating black holes. We describe the global solution of accretion flows with a sub-Keplerian angular momentum profile, by solving the underlying conservation equations including explicit cooling processes self-consistently. Bremsstrahlung, synchrotron and inverse Comptonization of soft photons are considered as possible cooling mechanisms. We focus on the set of solutions for sub-Eddington, Eddington and super-Eddington mass accretion rates around Schwarzschild and Kerr black holes with a Kerr parameter of 0.998. It is found that the flow, during its infall from the Keplerian to sub-Kepleria transition region to the black hole event horizon, passes through various phases of advection: the general advective paradigm to the radiatively inefficient phase, and vice versa. Hence, the flow governs a much lower electron temperature similar to 10(8)-10(9.5) K, in the range of accretion rate in Eddington units 0.01 less than or similar to (M) over dot less than or similar to 100, compared to the hot protons of temperature similar to 10(10.2)-10(11.8) K. Therefore, the solution may potentially explain the hard X-rays and gamma-rays emitted from active galactic nuclei (AGNs) and X-ray binaries. We then compare the solutions for two different regimes of viscosity. We conclude that a weakly viscous flow is expected to be cooling dominated, particularly at the inner region of the disc, compared to its highly viscous counterpart, which is radiatively inefficient. With all the solutions in hand, we finally reproduce the observed luminosities of the underfed AGNs and quasars (e. g. Sgr A*) to ultraluminous X-ray sources (e. g. SS433), at different combinations of input parameters, such as the mass accretion rate and the ratio of specific heats. The set of solutions also predicts appropriately the luminosity observed in highly luminous AGNs and ultraluminous quasars (e. g. PKS 0743-67).