Two-dimensional asymmetric turbulent flow in ducts

An experimental and theoretical investigation is reported on the asymmetric, quasi-parallel flow of turbulent incompressible fluids. The experimental programme consisted of providing detailed measurements of mean and turbulent characteristics of the fully developed flow in a plane channel having one smooth wall, while the other was roughened by transverse square ribs. The dissimilar wall conditions imposed a strong asymmetry upon both mean and turbulent flow fields bringing into prominance several interating features that are concealed in the symmetric flow situations. The theoretical investigation concerned the provision of a procedure capably of accurate prediction of strongly asymmetric quasi-parallel flows. The research was concentrated upon the physical aspect of the problem, that is the establishment and testing of an approximate closed set of the transport equations, sufficient for the accurate description of the considered flows. Two physical models have been explored, both of which used the Spalding-Patankar numerical method for the solution of resulting equations. The first model, based upon the extension of Kolmogaa Prandtl eddy viscosity formula was tested in plane all-smooth and smooth-rough channels. It showed several deficiencies and was subsequently discarded. A second model was established that is described by a closed set of four partial differential equations for conservation of mean momentum, turbulent shear stress, turbulent kinetic energy and its clissipation. This model was extensively tested is several types of duct flows, wall boundary layers and quasi-parallel free flows. With a single set of empirical constants, the model yielded predictions of various flow properties which were in good agreement with experiments.Open acces

Statistical Significance Testing in Information Retrieval: An Empirical Analysis of Type I, Type II and Type III Errors

Statistical significance testing is widely accepted as a means to assess how well a difference in effectiveness reflects an actual difference between systems, as opposed to random noise because of the selection of topics. According to recent surveys on SIGIR, CIKM, ECIR and TOIS papers, the t-test is the most popular choice among IR researchers. However, previous work has suggested computer intensive tests like the bootstrap or the permutation test, based mainly on theoretical arguments. On empirical grounds, others have suggested non-parametric alternatives such as the Wilcoxon test. Indeed, the question of which tests we should use has accompanied IR and related fields for decades now. Previous theoretical studies on this matter were limited in that we know that test assumptions are not met in IR experiments, and empirical studies were limited in that we do not have the necessary control over the null hypotheses to compute actual Type I and Type II error rates under realistic conditions. Therefore, not only is it unclear which test to use, but also how much trust we should put in them. In contrast to past studies, in this paper we employ a recent simulation methodology from TREC data to go around these limitations. Our study comprises over 500 million p-values computed for a range of tests, systems, effectiveness measures, topic set sizes and effect sizes, and for both the 2-tail and 1-tail cases. Having such a large supply of IR evaluation data with full knowledge of the null hypotheses, we are finally in a position to evaluate how well statistical significance tests really behave with IR data, and make sound recommendations for practitioners.Comment: 10 pages, 6 figures, SIGIR 201

Boundary layer scaling in Rayleigh-Benard convection

Accepted versio

Wind and boundary layers in Rayleigh-Benard convection. Part 2: boundary layer character and scaling

The effect of the wind of Rayleigh-Benard convection on the boundary layers is studied by direct numerical simulation of an L/H=4 aspect-ratio domain with periodic side boundary conditions for Ra={10^5, 10^6, 10^7} and Pr=1. It is shown that the kinetic boundary layers on the top- and bottom plate have some features of both laminar and turbulent boundary layers. A continuous spectrum, as well as significant forcing due to Reynolds stresses indicates undoubtedly a turbulent character, whereas the classical integral boundary layer parameters -- the shape factor and friction factor (the latter is shown to be dominated by the pressure gradient) -- scale with Reynolds number more akin to laminar boundary layers. This apparent dual behavior is caused by the large influence of plumes impinging onto and detaching from the boundary layer. The plume-generated Reynolds stresses have a negligible effect on the friction factor at the Rayleigh numbers we consider, which indicates that they are passive with respect to momentum transfer in the wall-parallel direction. However, the effect of Reynolds stresses cannot be neglected for the thickness of the kinetic boundary layer. Using a conceptual wind model, we find that the friction factor C_f should scale proportional to the thermal boundary layer thickness as C_f ~ lambda_Theta, while the kinetic boundary layer thickness lambda_u scales inversely proportional to the thermal boundary layer thickness and wind Reynolds number lambda_u ~ lambda_Theta^{-1} Re^{-1}. The predicted trends for C_f and \lambda_u are in agreement with DNS results