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Ionisation Chambers
Published in W. P. M. Mayles, A. E. Nahum, J.-C. Rosenwald, Handbook of Radiotherapy Physics, 2021
The mass of air is equal to V × rair, where V is the cavity volume and rair the air density inside the chamber. The number of interactions and consequently, the charge produced are proportional to the mass of air, which in turn, is proportional to its density. As most ionisation chambers are vented to the ambient atmosphere, rair is a function of the atmospheric pressure, temperature and humidity.
Ionization Chamber Instrumentation
Published in Arash Darafsheh, Radiation Therapy Dosimetry: A Practical Handbook, 2021
Larry A. DeWerd, Blake R. Smith
Higher altitudes have a greater correction due to the lower pressure. It is common to have corrections of more than 10% in mountainous areas. When a calibrated device is used, the physicist must correct the reading using Equation 2.5 to that which would be obtained at standard temperature and pressure. However, for certain chambers with non–air-equivalent components, the standard correction breaks down for low-energy radiation [17–20]. This breakdown occurs when the ranges of the electrons entering the chamber cavity are short with respect to the dimensions of the cavity, and when the photon cross sections differ between the chamber wall and air. La Russa and Rogers [17] demonstrated such an effect for thimble-type chambers in kilovoltage x-rays using Monte Carlo calculations and later confirmed the results with an experimental investigation [20]. The investigators found a smaller deviation than observed for well-type ionization chambers, with a measured 2.5% effect at 0.7 atm for an unfiltered 60 kV x-ray beam with a graphite-walled thimble chamber. An effect of 13% was measured at an air density typical of Mexico City (0.76 atm) for a modified thimble chamber with an aluminum wall, using the same 60 kV x-ray beam. A chamber with an aluminum wall is not common and was used only to demonstrate the effect.
Acclimatization
Published in Andrew M. Luks, Philip N. Ainslie, Justin S. Lawley, Robert C. Roach, Tatum S. Simonson, Ward, Milledge and West's High Altitude Medicine and Physiology, 2021
Andrew M. Luks, Philip N. Ainslie, Justin S. Lawley, Robert C. Roach, Tatum S. Simonson
In addition to decreasing the PO2, hypobaria has other physiological effects, especially if imposed suddenly upon the subject. These relate to possible bubble formation in the blood and tissues as seen in decompression sickness experienced by divers with overly rapid ascent to the surface, as well as in aircraft crews subject to explosive decompression at great altitudes. This is not a problem for travelers or mountaineers taking some hours to days to reach a given altitude but may play a role in hypobaric chamber studies where rates of ascent may be fast enough to cause microbubble formation. There is also the physical effect of reduced air density in hypobaric hypoxia, which reduces the work of breathing (per liter) and may affect rates of diffusion of gas in the respiratory airways.
Numerical modeling of nanoparticle deposition in realistic monkey airway and human airway models: a comparative study
Published in Inhalation Toxicology, 2020
Nguyen Dang Khoa, Nguyen Lu Phuong, Kazuhide Ito
Inhaled airflow simulations were conducted in a physiologically steady-state, using Ansys-Fluent 19 (ANSYS, Inc., Canonsburg, PA) to solve the viscous, incompressible Navier–Stokes equations. The airflow conditions were specified at the trachea opening with the constant negative velocity which is perpendicular to the cross-section of the trachea opening. A zero-gauge pressure condition was set at the external cylinder domain in front of the nostril region to confirm the fully developed airflow entered the nostrils. The hydraulic diameter of the trachea opening was defined as the characteristic length scale of the model. The wall treatment was assumed no-slip condition for airflow simulation. A second-order upwind scheme was used for the convection term and the SIMPLE algorithm was employed. Airflow simulations were conducted with the inhalation of air at room temperature (20 °C), while the values of air density and viscosity were 1.225 kg/m3 and 1.789 × 10−5 kg/m-s, respectively. In the human airway models, the inhalation flow rates of 10 L/min and 20 L/min were used in accordance with the light and moderate activity conditions, respectively, as described by Phuong et al. (2018).
Influence of environmental factors on Olympic cross-country mountain bike performance
Published in Temperature, 2020
Franck Brocherie, Simon Fischer, Quentin De Larochelambert, Henri Meric, Florence Riera
Other environmental factors may also impact XCO performance. Here, LOESS polynomial regressions and stepwise generalized linear regression indicate that both altitude and absolute humidity have a negative effect on average speed. Humidity not only impacts the thermoregulation, but also the perception of ambient temperature [28] and/or the terrain (i.e. wet/sliding) which can have an impact on fatigue development during XCO races. This may explain the results observed in the present study. Similarly, altitude negatively affect XCO performance. This may be due to the fact that athletes with higher aerobic fitness are affected in a greater extent than lower level athletes [8,10]. Meanwhile, altitude reduces air density, which could have an influence on cycling aerodynamic drag, in particular in flat and downhill terrains [29]. Although caution is required regarding its putative effect [7], with a circuit design, this may favor XCO performance [11], in particular in lower levels groups such as U23 and elite female. Because XCO is an emergent cycling sport, more data are needed to better understand the relation of racing environment on XCO athletes’ performance.
Intraocular pressure changes in eyes with small incision lenticules and laser in situ keratomileusis
Published in Clinical and Experimental Optometry, 2019
Kuo‐jen Wang, Wai W Wang, Che‐liang Tsai, I‐jong Wang
The numerical tools used for simulating IOP were from OpenFoam (version 2.3.0) from OpenCFD Ltd. (OpenFOAM is an open source C++ computational continuum mechanics software and is a registered trademark of OpenCFD Ltd., Reading, UK; http://www.openfoam.com, and Scilab [version 5.5.1] from Scilab Enterprise, Rungis, France; http://www.scilab.org/). The OpenFoam program is a fluid dynamics simulator used to simulate the air‐puff pressure impinging on the corneal surface. The airflow parameters used for this modelling were incompressible airflow with an air density of 1.1855-kg/m3, an air viscosity of 15.68 × 10−6 m2/s, and a peak pressure profile of 30-mmHg with a duration of 28-ms. The geometric parameters of the air chamber were an internal diameter of the nozzle of 2.4-mm and a working distance from the tip of the air nozzle to the cornea of 11-mm. The airflow solver used was incompressible pisoFoam (PISO = pressure implicit split operator) of the Reynolds‐averaged stress turbulence model.1992 The temporal variation and spatial distribution profile of the flow pressure on the corneal surface was calculated. This pressure profile was then provided to shape the boundary conditions for simulating corneal deformation, which was also performed with a solver solid displacement foam in OpenFoam.