4,483 research outputs found
Langmuir-Maxwell and Langmuir-Smoluchowski boundary conditions for thermal gas flow simulations in hypersonic aerodynamics
No full textThe simulation of nonequilibrium thermal gas flow is important for the aerothermodynamic design of re-entry and other high-altitude vehicles. In computational fluid dynamics, the accuracy of the solution to the Navier–Stokes–Fourier (N–S–F) equations depends on the accuracy of the surface boundary conditions. We propose new boundary conditions (called the Langmuir–Maxwell and the Langmuir–Smoluchowski conditions), for use with the N–S–F equations, which combine the Langmuir surface adsorption isotherm with the Maxwell/Smoluchowski slip/jump conditions in order to capture some of the physical processes involved in gas flow over a surface. These new conditions are validated for flat plate flow, circular cylinder in cross-flow, and the flow over a sharp wedge for Mach numbers ranging from 6 to 24, and for argon and nitrogen as the working gases. Our simulation results show that the new boundary conditions give better predictions for the surface pressures, compared with published experimental and DSMC data
Thermal rarefied gas flow investigations through micro/nano backward-facing step : Comparison of DSMC and CFD subject to hybrid slip and jump boundary conditions
No full textThis paper evaluates the suitability of a newly developed hybrid “Langmuir-Maxwell” and “Langmuir-Smoluchowski” slip/jump boundary conditions in the Navier-Stokes-Fourier equations for nano/micro backward-facing step geometry flow which experiences separation and reattachment. Additionally, this paper investigates the effect of different parameters such as step pressure ratio, inflow temperature and wall temperature on the separation zone in the nano/micro step geometry. We chose nitrogen as the working gas and use two DSMC solvers to assess the accuracy of the CFD solutions. DSMC results showed that the increase of the inlet temperatures extends the length of the separation zone and raises the mass flow rate. The change of pressure ratio does not affect the separation length while the increase of the step wall temperature decreases the length of this zone for both CFD and DSMC results. Compared to the DSMC results, the hybrid slip/jump boundary conditions predict better surface pressure, surface gas temperature and slip velocity in the separation zone than the standard Maxwell/Smoluchowski boundary conditions
Anisotropic scattering kernel : generalized and modified Maxwell boundary conditions
No full textThis paper presents a model of a scattering kernel of boundary conditions for the Boltzmann equation. The proposed scattering kernel is based on an anisotropic accommodation argument. Three parameters equal to the momentum accommodation coeffcients are shown as characterizing the influence of each direction. First the new scattering kernel is derived from a phenomenological criticism of the first form of the scattering kernel proposed by Maxwell; then the same result is established from an analytic approach based on the spectral nature of the linear integral operator associated to the scattering kernel problem. As a result, the model provides a correct form of scattering kernel to handle the influence of each direction in particle collisions with the wall. Finally independent accommodation of each internal mode is added to extend the model to the case of polyatomic gases
Will biological agents supplant systemic glucocorticoids as the first-line treatment for thyroid-associated ophthalmopathy?
No full textIn this article, the two authors present their opposing points of view concerning the likelihood that glucocorticoids will be replaced by newly developed biological agents in the treatment of active, moderate-to-severe thyroid-associated ophthalmopathy (TAO). TAO is a vexing, disfiguring and potentially blinding autoimmune manifestation of thyroid autoimmunity. One author expresses the opinion that steroids are nonspecific, frequently fail to improve the disease and can cause sometimes serious side effects. He suggests that glucocorticoids should be replaced as soon as possible by more specific and safer drugs, once they become available. The most promising of these are biological agents. The other author argues that glucocorticoids are proven effective and are unlikely to be replaced by biologicals. He reasons that while they may not uniformly result in optimal benefit, they have been proven effective in many reports. He remains open minded about alternative therapies such as biologicals but remains skeptical that they will replace steroids as the first-line therapy for active, moderate-to-severe TAO without head-to-head comparative clinical trials demonstrating superiority. Despite these very different points of view, both authors are optimistic about the availability of improved medical therapies for TAO, either as single agents or in combination. Further, both agree that better treatment options are needed to improve the care of our patients with active moderate-to-severe TAO
Discrete unified gas kinetic scheme for all Knudsen number flows. III. Binary gas mixtures of Maxwell molecules
No full textRecently a discrete unified gas kinetic scheme (DUGKS) in a finite-volume formulation based on the Boltzmann model equation has been developed for gas flows in all flow regimes. The original DUGKS is designed for flows of single-species gases. In this work, we extend the DUGKS to flows of binary gas mixtures of Maxwell molecules based on the Andries-Aoki-Perthame kinetic model [P. Andries, J. Stat. Phys. 106, 993 (2002)JSTPBS0022-471510.1023/A:1014033703134. A particular feature of the method is that the flux at each cell interface is evaluated based on the characteristic solution of the kinetic equation itself; thus the numerical dissipation is low in comparison with that using direct reconstruction. Furthermore, the implicit treatment of the collision term enables the time step to be free from the restriction of the relaxation time. Unlike the DUGKS for single-species flows, a nonlinear system must be solved to determine the interaction parameters appearing in the equilibrium distribution function, which can be obtained analytically for Maxwell molecules. Several tests are performed to validate the scheme, including the shock structure problem under different Mach numbers and molar concentrations, the channel flow driven by a small gradient of pressure, temperature, or concentration, the plane Couette flow, and the shear driven cavity flow under different mass ratios and molar concentrations. The results are compared with those from other reliable numerical methods. The results show that the proposed scheme is an effective and reliable method for binary gas mixtures in all flow regimes
The value of silence
No full textThis is an electronic version of the article published in Theatre Journal, 54(1):85-94, 2002 March. The published article is available at http://muse.jhu.edu/journals/theatre_journal/v054/54.1eng.pdfEng, David L.The Value of Silence.Theatre Journal, 54,(1):85-94, 2002.DOI: 10.1353/tj.2002.000
A 13-week, multicenter, randomized, double-blind study of lumiracoxib in hip osteoarthritis.
No full textDeterministic numerical solutions of the Boltzmann equation using the fast spectral method
No full textThe Boltzmann equation describes the dynamics of rarefied gas flows, but the multidimensional nature of its collision operator poses a real challenge for its numerical solution. In this paper, the fast spectral method [36], originally developed by Mouhot and Pareschi for the numerical approximation of the collision operator, is extended to deal with other collision kernels, such as those corresponding to the soft, Lennard–Jones, and rigid attracting potentials. The accuracy of the fast spectral method is checked by comparing our numerical solutions of the space-homogeneous Boltzmann equation with the exact Bobylev–Krook–Wu solutions for a gas of Maxwell molecules. It is found that the accuracy is improved by replacing the trapezoidal rule with Gauss–Legendre quadrature in the calculation of the kernel mode, and the conservation of momentum and energy are ensured by the Lagrangian multiplier method without loss of spectral accuracy. The relax-to-equilibrium processes of different collision kernels with the same value of shear viscosity are then compared; the numerical results indicate that different forms of the collision kernels can be used as long as the shear viscosity (not only the value, but also its temperature dependence) is recovered. An iteration scheme is employed to obtain stationary solutions of the space-inhomogeneous Boltzmann equation, where the numerical errors decay exponentially. Four classical benchmarking problems are investigated: the normal shock wave, and the planar Fourier/Couette/force-driven Poiseuille flows. For normal shock waves, our numerical results are compared with a finite difference solution of the Boltzmann equation for hard sphere molecules, experimental data, and molecular dynamics simulation of argon using the realistic Lennard–Jones potential. For planar Fourier/Couette/force-driven Poiseuille flows, our results are compared with the direct simulation Monte Carlo method. Excellent agreements are observed in all test cases, demonstrating the merit of the fast spectral method as a computationally efficient method for rarefied gas dynamics
Food consumption frequency and perceived stress and depressive symptoms among students in three European countries
No full textMikolajczyk RT, El Ansari W, Maxwell AE. Food consumption frequency and perceived stress and depressive symptoms among students in three European countries. Nutrition Journal. 2009;8(1):31.Background: Certain foods might be more frequently eaten under stress or when higher levels of depressive symptoms are experienced. We examined whether poor nutritional habits are associated with stress and depressive symptoms and whether the relationships differ by country and gender in a sample from three European countries collected as part of a Cross National Student Health Survey. Methods: A cross-sectional survey was conducted among first-year students in Germany (N = 696), Poland (N = 489) and Bulgaria (N = 654). Self-administered questionnaires included a 12-item food frequency questionnaire, Cohen's Perceived Stress Scale, and a modified Beck Depression Index. Linear regression analyses were conducted for two outcomes, perceived stress and depressive symptoms. Results: Food consumption frequencies differed by country and gender, as did depressive symptoms and perceived stress. For male students, none of the food consumption groups were associated with perceived stress or depressive symptoms. In females, perceived stress was associated with more frequent consumption of sweets/fast foods and less frequent consumption of fruits/vegetables. Additionally, depressive symptoms were associated with less frequent consumption of fruits/vegetables and meat. Conclusion: Our data show consistent associations between unhealthy food consumption and depressive symptoms and perceived stress among female students from three European countries, but not among male students. This suggests that efforts to reduce depressive symptoms and stress among female students may also lead to the consumption of healthier foods and/or vice-versa
Metabolic profiling and population screening of analgesic usage in nuclear magnetic resonance spectroscopy-based large-scale epidemiologic studies
No full textThe application of a 1H nuclear magnetic resonance (NMR) spectroscopy-based screening method for determining the use of two widely available analgesics (acetaminophen and ibuprofen) in epidemiologic studies has been investigated. We used samples and data from the cross-sectional INTERMAP Study involving participants from Japan (n = 1145), China (n = 839), U.K. (n = 501), and the U.S. (n = 2195). An orthogonal projection to latent structures discriminant analysis (OPLS-DA) algorithm with an incorporated Monte Carlo resampling function was applied to the NMR data set to determine which spectra contained analgesic metabolites. OPLS-DA preprocessing parameters (normalization, bin width, scaling, and input parameters) were assessed systematically to identify an optimal acetaminophen prediction model. Subsets of INTERMAP spectra were examined to verify and validate the presence/absence of acetaminophen/ibuprofen based on known chemical shift and coupling patterns. The optimized and validated acetaminophen model correctly predicted 98.2%, and the ibuprofen model correctly predicted 99.0% of the urine specimens containing these drug metabolites. The acetaminophen and ibuprofen models were subsequently used to predict the presence/absence of these drug metabolites for the remaining INTERMAP specimens. The acetaminophen model identified 415 out of 8436 spectra as containing acetaminophen metabolite signals while the ibuprofen model identified 245 out of 8604 spectra as containing ibuprofen metabolite signals from the global data set after excluding samples used to construct the prediction models. The NMR-based metabolic screening strategy provides a new objective approach for evaluation of self-reported medication data and is extendable to other aspects of population xenometabolome profiling
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