Numerical Simulation on the Opening Delay of a Discharge Reed Valve in Compressors

The opening delay of a discharge reed valve for compressors caused by the stiction force of the oil film between the reed and the valve seat is investigated. The coupling simulation algorithm including models of cavitation in the oil film is developed. In order to check the validity, the numerical simulation is performed under several conditions of a model experiment. The coupling model consists of the reed deformation, the gas flow, and the oil film flow with cavitation. The two-dimensional reed deformation is expressed by expanding natural modes that are obtained by the finite element method using shell elements. The gas flow is expressed by the thermodynamic model under the assumption of the internal reversible process. The two-dimensional oil film pressure distribution is obtained by solving numerically the Reynolds’ equation through the finite volume method. On the model of cavitation in the oil film, in addition to the case in which cavitation is not taken into account, two cavitation models are tested; the static cavitation model (Reynolds’ condition) in which the minimum oil pressure is set to absolute zero, and the dynamic cavitation model in which the oil film pressure can fall to the absolute negative (tensile in the liquid) due to the direct simulation of the bubble expansion. Comparisons on the opening delay time between simulation results and the experimental result show that the static cavitation model gives the earlier delay time than that in the experiment, and the dynamic cavitation model which simulates the absolute negative pressure in the oil film reproduces accurately the experimental delay time. The simulation through the dynamic cavitation model also reproduces well behaviors of cavitation and blow-by flow (oil film rupture) in the oil film captured by the high-speed camera in the experiment. The relation among the pressure, the thickness, and the cavitation area of the oil film that are obtained in the simulation shows the mechanism of the oil film rupture (valve opening), i.e., the cavitation bubbles promote the increase of the oil film thickness during the stiction and the increased oil film thickness causes the recovery of the oil film pressure, and the subsequent blow-by flow (oil film rupture)

A Streamwise Constant Model of Turbulence in Plane Couette Flow

Streamwise and quasi-streamwise elongated structures have been shown to play a significant role in turbulent shear flows. We model the mean behavior of fully turbulent plane Couette flow using a streamwise constant projection of the Navier Stokes equations. This results in a two-dimensional, three velocity component ($2D/3C$) model. We first use a steady state version of the model to demonstrate that its nonlinear coupling provides the mathematical mechanism that shapes the turbulent velocity profile. Simulations of the $2D/3C$ model under small amplitude Gaussian forcing of the cross-stream components are compared to DNS data. The results indicate that a streamwise constant projection of the Navier Stokes equations captures salient features of fully turbulent plane Couette flow at low Reynolds numbers. A system theoretic approach is used to demonstrate the presence of large input-output amplification through the forced $2D/3C$ model. It is this amplification coupled with the appropriate nonlinearity that enables the $2D/3C$ model to generate turbulent behaviour under the small amplitude forcing employed in this study.Comment: Journal of Fluid Mechanics 2010, in pres

Staggered grids discretization in three-dimensional Darcy convection

We consider three-dimensional convection of an incompressible fluid saturated in a parallelepiped with a porous medium. A mimetic finite-difference scheme for the Darcy convection problem in the primitive variables is developed. It consists of staggered nonuniform grids with five types of nodes, differencing and averaging operators on a two-nodes stencil. The nonlinear terms are approximated using special schemes. Two problems with different boundary conditions are considered to study scenarios of instability of the state of rest. Branching off of a continuous family of steady states was detected for the problem with zero heat fluxes on two opposite lateral planes.Comment: 20 pages, 9 figure

Relating statistics to dynamics in axisymmetric homogeneous turbulence

The structure and the dynamics of homogeneous turbulence are modified by the presence of body forces such that the Coriolis or the buoyancy forces, which may render a wide range of turbulence scales anisotropic. The corresponding statistical characterization of such effects is done in physical space using structure functions, as well as in spectral space with spectra of two-point correlations, providing two complementary viewpoints. In this framework, second-order and third-order structure functions are put in parallel with spectra of two-point second- and third-order velocity correlation functions, using passage relations. Such relations apply in the isotropic case, or for isotropically averaged statistics, which, however, do not reflect the actual more complex structure of anisotropic turbulence submitted to rotation or stratification. This complexity is demonstrated in this paper by orientation-dependent energy and energy transfer spectra produced in both cases by means of a two-point statistical model for axisymmetric turbulence. We show that, to date, the anisotropic formalism used in the spectral transfer statistics is especially well-suited to analyze the refined dynamics of anisotropic homogeneous turbulence, and that it can help in the analysis of isotropically computed third-order structure function statistics often used to characterize anisotropic contexts.Comment: Physica

Avaliação dos efeitos do sulfato de atropina em filhotes de cães

O artigo não apresenta resumo

PHOSPHATEMIC INDEX EVALUATES PHOSPHORUS LOAD

Objective: Dietary phosphorus (P) restriction is crucial to treat hyperphosphatemia and reduce cardiovascular disease risk and mortality in patients with chronic kidney disease (CKD) and the wider population. Various methods for dietary P restriction exist, but the bioavailability of P in food should also be considered when making appropriate food choices to maintain patients’ quality of life. Here, we propose the ‘‘Phosphatemic Index’’ (PI) as a novel tool for evaluating dietary P load based on P bioavailability; we also evaluated the effect of continuous intake of different PI foods in mixed meals on serum intact fibroblast growth factor 23 concentration. Design and Methods: A 2-stage crossover study was conducted: Study 1: 20 healthy participants consumed 10 different foods containing 200 mg of P, and the PI was calculated from the area under the curve of a time versus serum P concentration curve; Study 2: 10 healthy participants consumed 4 different test meals (low, medium, or high PI meals or a control) over a 5-day period. Results: Study 1 showed milk and dairy products had high PI values, pork and ham had medium PI values, and soy and tofu had low PI values. In Study 2, ingestion of high PI test meals showed higher fasting serum intact fibroblast growth factor 23 levels and lower serum 1,25-dihydroxyvitamin D levels compared with ingestion of low PI test meals. Conclusion: These findings suggest that the PI can usefully evaluate the dietary P load of various foods and may help to make appropriate food choices for dietary P restriction in CKD patients