Optimum Geometrical Conditions Research Articles

Decompressive craniectomy of post-traumatic brain injury: an in silico modelling approach for intracranial hypertension management

Traumatic brain injury (TBI) causes brain edema that induces increased intracranial pressure and decreased cerebral perfusion. Decompressive craniectomy has been recommended as a surgical procedure for the management of swollen brain and intracranial hypertension. Proper location and size of a decompressive craniectomy, however, remain controversial and no clinical guidelines are available. Mathematical and computational (in silico) models can predict the optimum geometric conditions and provide insights for the brain mechanical response following a decompressive craniectomy. In this work, we present a finite element model of post-traumatic brain injury and decompressive craniectomy that incorporates a biphasic, nonlinear biomechanical model of the brain. A homogenous pressure is applied in the brain to represent the intracranial pressure loading caused by the tissue swelling and the models calculate the deformations and stresses in the brain as well as the herniated volume of the brain tissue that exits the skull following craniectomy. Simulations for different craniectomy geometries (unilateral, bifrontal and bifrontal with midline bar) and sizes are employed to identify optimal clinical conditions of decompressive craniectomy. The reported results for the herniated volume of the brain tissue as a function of the intracranial pressure loading under a specific geometry and size of craniectomy are exceptionally relevant for decompressive craniectomy planning.

Heat removal enhancement in a channel with a single or an array of metallic foam obstacles

A single or an array of metallic porous structures of various geometries are introduced on the inside surface of a partially heated wall of a channel with air or water flow. Forced convective heat transfer rate from the heated wall to the fluid, as well as the average pressure drop along the channel are numerically studied. Effects of the aluminum porous block's structural parameters such as porosity (ϵ), Darcy number (Da) and Forchheimer coefficient (F) are taken into account. The optimum geometrical conditions of the system is extensively explored considering parameters such as blocks' height, width and spacing between them. Our results demonstrate great potential for metal foams to perform as heat sinks and pave the way for their further implementation in applications such as solar thermal collectors and electronic cooling.

Numerical investigation of the energy separation effect and flow mechanism inside convergent, straight, and divergent double‐sleeve RHVT

AbstractThis study provides analysis of a cooled Ranque–Hilsch vortex tube (RHVT) with various specifications. It shows how cooling influences energy conversion inside the RHVT and improves performance of the device in separation of hot gas from the cold stream within the fluid by presenting the temperature detachment (the temperature diminution of cold air (ΔTc = Ti − Tc), isentropic efficiency (ηis), and coefficient of performance (COP) of divergent, convergent, and straight VTs. Two key parameters including hot tube length and number of nozzles for cooling and insulated cases are investigated to find out how the performance of the VT is affected by different geometry configurations under cooling conditions. These influences were researched for straight, convergent, and divergent VT separators under different flow characteristics. The optimum geometrical conditions for the cooling cases were identified. Results are indicative of positive influence of cooling for energy separation inside a VT. The quantities of ΔTc, ηis, and COP for the cooled RHVT are greater than uncooled RHVT for various types of VTs. Cooling the VTs leads to an increase of 12.5% in ΔTc, 14.4% in ηis, and 15.1% in COP when the base case was an uncooled VT.

Defect Annihilation in the Directed Self-Assembly of Block Copolymers in Films with Increasing Thickness

We investigate directed self-assembly (DSA), with 3× density multiplication, of symmetric polystyrene-block-poly(methyl methacrylate) (L0 = 28 nm) with increasing film thicknesses and reveal a thickness limit above which the defect annihilation mechanism is different than in thinner films. Block copolymer films of increasing thickness underwent DSA on a macroscopic chemical pattern under optimum geometrical and chemical conditions. Scanning electron microscopy and grazing-incidence small-angle X-ray scattering (GISAXS) were used to characterize the long-range ordering and buried structures as a function of film thickness and thermal annealing time. Rotational GISAXS measurements revealed a critical thickness of ∼3L0, below which defect annihilation is cooperative across the film depth and above which it is faster at the free surface than in the film’s interior. In the latter case, defects can persist in the film’s interior despite perfect assembly and registration at the free surface of the film.

Performance analysis of the airfoil-slat arrangements for hydro and wind turbine applications

Standard airfoils historically used for wind and hydrokinetic turbines had maximum lift coefficients of around 1.3 at stall angles of attack, which is about 12°. At these conditions, the minimum flow velocities to generate electric power were about 7 m/s and 2 m/s for the wind turbine and the hydrokinetic turbine cases, respectively. In this study, NACA4412-NACA6411 slat–airfoil arrangement was chosen for these two cases in order to investigate the potential performance improvements. Aerodynamic performances of these cases were both numerically and experimentally investigated. The 2D and 3D numerical analysis software were used and the optimum geometric and flow conditions leading to the maximum power coefficient or the maximum lift to drag ratio were obtained. The maximum lift to drag ratio of 24.16 was obtained at the optimum geometric and flow parameters. The maximum power coefficient of 0.506 and the maximum torque were determined at the tip speed ratios of 5.5 and 4.0 respectively. The experimental work conducted in a towing tank gave the power coefficient to be 0.47 which is about %7 lower than the numerical results obtained. Hence, there is reasonable agreement between numerical end experimental values. It may be concluded that slat-hydrofoil or airfoil arrangements may be applied in the design of wind and hydrokinetic turbines for electrical power generation in lower wind velocities (3–4 m/s) and current velocities (about 1 m/s).

Theoretical modeling of the divergence of a flat-topped beam from a two-stage beam shaper into a conical intensity profile after propagation in free space

In this article, a two-stage beam shaping device based on a plastic fiber-bundle prism duct coupled waveguide is theoretically and experimentally described. ZEMAX software is used to simulate and investigate the divergence phenomenon on the beam shape outputted and the radiance profiles in both position and angle space. The effect of prism's geometrical structure on the beam divergence is also investigated and the optimum geometric conditions are reported. According to the theoretical results, due to an asymmetrical divergence effect the beam's cross section is increased with distance by a variable aspect ratio. The results also show that propagation in free space transformed a square beam with a flat-top intensity distribution into a rectangular beam with a conical intensity distribution. For experimental study, an imaging technique is applied to investigate the beam's output images and intensity profiles. A source light is used to illuminate the optical beam shaping elements of the reported design. Digital photographs of the beam's output for different axial distances are taken by a camera and the image data is converted into a response curve for comparison with the simulated image profiles. The obtained experimental results are strongly in agreement with the theoretical ones.

Aeration Cost of Propeller-Aspirator Pump Aerator at Optimum Geometric and Dynamic Conditions

Aeration Cost of Propeller-Aspirator Pump Aerator at Optimum Geometric and Dynamic Conditions

Narrow-bandwidth tunable picosecond pulses in the visible produced by noncollinear optical parametric amplification with a chirped blue pump

Narrow-bandwidth (approximately 27 cm(-1)) tunable picosecond pulses from 480 nm-780 nm were generated from the output of a 1 kHz femtosecond titanium:sapphire laser system using a type I noncollinear optical parametric amplifier (NOPA) with chirped second-harmonic generation (SHG) pumping. Unlike a femtosecond NOPA, this system utilizes a broadband pump beam, the chirped 400 nm SHG of the Ti:sapphire fundamental, to amplify a monochromatic signal beam (spectrally-filtered output of a type II collinear OPA). Optimum geometric conditions for simultaneous phase- and group-velocity matching were calculated in the visible spectrum. This design is an efficient and simple method for generating tunable visible picosecond pulses that are synchronized to the femtosecond pulses.

Separation Characteristics of Visco-Elastic Fluid in a Cavity

In order to investigate the optimum geometric conditions for a heat transfer augmentation technique using visco-elastic fluid, flow visualization experiments have been performed in a cavity between rectangular ribs. The rib height, the rib length and the cavity size were changed in several steps while the mean velocity at narrow cross-sections was kept constant. From the results, it is expected that the case having small rib height, small rib length and large cavity size has an advantage for heat transfer augmentation. It is also found that the recirculation size is a simple function of contraction ratio and Weissenberg number.

Investigation of surface radius variation in design of a lens duct delivery system

In this article a ray tracing method is used to study the variation of the entrance and exit surface shape in the different lens ducts systems. Designs with a variety of plane and spherical first and second surfaces are studied. The optimum geometrical conditions for each design in terms of the aperture, length, and the radius of curvature are reported. A comparison is made between the results obtained for ducts with the spherical and plane surfaces and the more efficient design is described. The effect of second-order aspheric shape factor on the device performance is also investigated. This study shows that each design has some advantages and disadvantages, and considering specific application, the result of this study can lead to a choice of the optimum design in terms of the required lens duct performance.

Particle−Liquid Mass Transfer Coefficient in Two-/Three-Phase Stirred Tank Reactors

Knowledge of particle−liquid mass transfer coefficient (kSL) is important for a rational design of a solid−liquid/solid−liquid−gas stirred tank reactor (STR). Considerable efforts have been made by different investigators to study and correlate the behavior of kSL in STRs for a variety of geometric and operating conditions. This review attempts to put together and analyze the information available in the literature on kSL in STRs systematically. Different approaches used by researchers to correlate kSL are critically discussed. The effect of system configurations (type of the impeller, D/T, and C/T), operating parameters (impeller speed/power input, particle loading, and gas flow rate), and physical properties (dP, Dm, μ, and Δρ) on kSL have been discussed elaborately. Some new data for six-bladed impellers have been generated to determine the optimum geometric configuration and operating conditions for particle−liquid mass transfer in three-phase STRs. It is concluded that kSL in two-/three-phase STRs ca...

Calibration of ionization chambers in air kerma length

Pencil ionization chambers are employed for the direct determinationof the computed tomography dose index (CTDI), which is used for patientdosimetry in CT examinations. The chambers are calibrated in air kerma lengthin a reference x-ray field. The precision of calibration is influenced by severalfactors which are investigated in the present work. It is shown that the bestirradiation length is 50% of the chamber rated length. The reading must becorrected for a residual signal due to the irradiation of the sensitivechamber volume by scattered radiation. The origin of this radiation isidentified mainly at the edges of the diaphragm limiting the useful field. Theresidual signal is determined by a linear fit of measurements performed atminimum of three diaphragm apertures. The optimum geometric conditions are alsodiscussed.

Mixing mechanism of three‐tip kneading block in twin screw extruders

AbstractIn recent years, twin screw extruders have been applied to various kinds of polymer processing. It has been important to find their optimum geometrical configurations and operational processing conditions for the best performance of extrusions and products. Many engineers have been evolving numerical and the experimental methods to characterize the mixing performance for twin screw extruders. We have carried out three‐dimensional flow simulations of kneading blocks in intermeshing co‐rotating twin screw extruders by using the finite element method to quantify their ability in distributive and dispersive mixing. We discuss their performance in distributive mixing for three different type of kneading blocks in terms of the residence time distribution and the nearest distance between markers at various periods of time, by using the marker tracking method. Those numerical techniques and applications of mixing indices have enabled us to quantify and evaluate their abilities in distributive mixing of kneading blocks in twin screw extruders.

A computer simulation study on the detection efficiencies of parallel-plate electron multipliers

The detection efficiencies of parallel-plate electron multipliers with a parallel electric field (PPEM-parallel) and with a tilted electric field (PPEM-tilt) have been calculated by a Monte Carlo simulation method. The detection efficiencies vary with the geometrical conditions of the dynodes such as the dynode spacing, the dynode displacement, and the dynode length. Under the optimum geometrical conditions that give the maximum mean gains, the PPEM-parallels have a detection efficiency of 75% and the PPEM-tilts have a detection efficiency of 70% for an incident electron energy of 300 eV. These values do not vary with the applied voltage. Detection efficiency versus incident electron energy curves has been obtained for the optimum geometrical conditions. The forms of these curves are almost the same for the PPEM-parallel and the PPEM-tilt and do not change with the applied voltage.

A computer simulation study on electron multiplication of parallel-plate electron multipliers

Electron multiplication of parallel-plate electron multipliers with a parallel electric field (PPEM-parallel) and with a tilted electric field (PPEM-tilt) have been studied by a Monte Carlo simulation method. The optimum geometrical condition is represented with a ratio of the dynode length Ld to the dynode spacing Ds, Ld/Ds=0.040Vd, for the PPEM-parallel, and with a ratio of the dynode displacement Xd to the dynode spacing Ds, Xd/Ds=0.40, for the PPEM-tilt, where Vd is an applied voltage. For optimum geometrical conditions, the PPEM-parallel and the PPEM-tilt have almost equal characteristic properties of operation such as mean gain, pulse height distribution, and transit time distribution of secondary electrons. These are inherent results of the almost equivalent electron multiplication processes such as collision energy of secondaries and number of electron multiplication steps. A simplified analytical method is used to explain the computer simulation results.

Radioisotopic X-ray analysis of uranium ores by K shell fluorescence

Uranium ores from various locations in Canada were analyzed by X-ray spectroscopy. A 10 mCi 57Co point source was employed and mounted on a one dimensional goniometer. This instrument was designed with adjustable scatter angle, primary and secondary collimation settings. The optimum geometrical conditions for analysis of uranium using partially thick powdered ore samples were found. Assumptions can be drawn for innate geological specimens which lead to the direct calculation of the matrix and thickness factors. This is accomplished by simultaneously and experimentally measuring the Compton scattering and transmitted peak intensities. These factors are then used to modify the U K αi peak intensity so that no comparative standards are required to maintain accuracy. As such, the technique is a prime candidate for application to on-stream industrial analysis.

Off-shell effects in the (p, 2p) reaction using optimum geometrical conditions

Abstract Calculations of off-shell effects in the (p, 2p) reaction are presented using a fixed condition geometry and the Reid soft core potential for the proton-proton interaction. It is shown that ratios of cross sections in the chosen geometry clearly exhibit the off-shell behaviour of the p-p interaction, and that this method therefore offers a means of discriminating between phase equivalent nucleon-nucleon potentials.

Theoretical maximum of the power efficiency of a pulsed injection laser with a Fabry-Perot cavity

The maximum power efficiency of an injection laser operating in a Fabry-Perot mode is analyzed with a phenomenological theory which assumes that internal heating can be neglected. The efficiency is expressed in terms of two dimensionless geometric variables related to the length and the reflection coefficients of the cavity, and of a pair of dimensionless device parameters determined by the threshold constants, the product device resistance times area, and the output per unit width of junction. The power efficiency is maximized subject to independent variation of the two geometrical variables, and the optimum geometrical conditions are expressed as the only positive real root of a cubic equation. At low and at high efficiencies, the root has a simple approximate form. A graph of the root for all values of the device parameters is presented. The limiting efficiency is a monotonic function of a single parameter, the electrical quality factor, which is the ratio of the equivalent photon voltage of the radiation to the ohmic drop in the device at threshold. At low temperature where the electrical quality factor is large, the limiting efficiency is nearly independent of temperature, is proportional to the internal quantum efficiency, and can be close to unity. At higher temperatures where the factor becomes small, the limiting efficiency is proportional to the square of the internal quantum efficiency and drops toward zero. Measured values of the electrical quality factor of GaAs lasers show that the maximum power efficiency at 300°K is less than five per cent while at 80°K it exceeds fifty per cent. For device development, the best monitor of relative efficiency of a low-temperature laser is the differential external quantum efficiency, while for higher temperatures it is the threshold current density. The cavity length and reflection coefficients for optimum efficiency depend on the threshold parameters and the design power, but not in a critical way. A graph of the power efficiency against the electrical quality factor with cavity design as a parameter shows that a change from the optimum length by the factor one-half or two leaves the efficiency above 90 per cent of its optimum. The reflectivity has even less effect. The design tolerance for operation of a series-connected array of lasers is also discussed.

The laser interferometer: Application to plasma diagnostics

A laser interferometer employing a reference arm with a spherical mirror is discussed. This interferometer is extremely promising in many fields of scientific endeavor, for instance, in plasma physics, due to its simplicity and high sensitivity. Its sensitivity is greater than that of a similar interferometer with planar mirrors because the closely spaced normal modes with nonzero transverse indexes can easily be resolved in the former but not in the latter. Special attention is given to describing the principle of operation, discussing the optimum geometrical conditions for both the laser resonant structure and the reference arm, and discussing the sensitivity limitations. Some experimental measurements of electron densities in gaseous plasmas are also presented.

Optimum geometric conditions in the design and use of X-ray diffraction tubes and cameras

A theory is developed which enables optimum geometric conditions to be determined under various conditions. The validity of the simplifying assumptions is discussed. The theory shows (a) that stationary-anode X-ray tubes should be employed with focal dimensions three times those of the specimen; (b) that rotating-anode X-ray tubes should have focal dimensions five times those of the specimen. In each case, the distance between specimen and focus and the film distance are determined by the spacings which it is desired to record. Optimum conditions are also calculated for cases where the focal size, specimen distance, and/or film distance are restricted by experimental conditions. The dosage received by the specimen and the contrast produced at the film are also considered. The implications of this analysis are discussed, and, in particular, it is concluded that a need exists for X-ray tubes of variable focal size.