Impact of Irregular Anisotropic Surface Roughness on the Near-wall Region of Turbulent Channel Flow

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Reynolds and dispersive shear stress contributions above highly skewed roughness

The roughness functions induced by irregular peak- and/or pit-dominated surfaces in a fully-developed turbulent channel flow are studied by direct numerical simulation. A surface generation algorithm is used to synthesise an irregular Gaussian heightmap with periodic boundaries. The Gaussian heightmap is decomposed into “pits-only" and “peaks-only" components which produces two additional surfaces with similar statistical properties, with the exception of skewness, which is equal and opposite (S=±1.6). While the peaks-only surface yields a roughness function comparable to that of the Gaussian surface, the pits-only surface exhibits a far weaker roughness effect. Analysis of results is aided by deriving an equation for the roughness function which quantitatively identifies the mechanisms of momentum loss and/or gain. The statistical contributions of “form-induced" and stochastic fluid motions to the roughness function are examined in further detail using quadrant analyses. Above the Gaussian and peaks-only surfaces, the contributions of dispersive and Reynolds shear stresses show a compensating effect, whereas, above the pits-only surface, an additive effect is observed. Overall, the results emphasise the sensitivity of the near-wall ow with respect to higher-order topographical parameters, which can, in turn, induce significant differences in the roughness function above a peak- and/or pit-dominated surface

Reynolds number dependence of Reynolds and dispersive stresses in turbulent channel flow past irregular near-Gaussian roughness

Direct numerical simulations of fully-developed turbulent channel flow with irregular rough walls have been performed at four friction Reynolds numbers, namely, 180, 240, 360 and 540, yielding data in both the transitionally- and fully-rough regime. The same roughness topography, which was synthesised with an irregular, isotropic and near-Gaussian height distribution, is used in each simulation. Particular attention is directed towards the wall-normal variation of flow statistics in the near-roughness region and the fluid-occupied region beneath the crests, i.e. within the roughness canopy itself. The goal of this study is twofold. (i) Provide a detailed account of first- and second-order double-averaged velocity statistics (including profiles of mean velocity, dispersive stresses, Reynolds stresses, shear stress gradients and an analysis of the mean force balance) with the overall aim of understanding the relative importance of “form-induced” and “turbulence-induced” quantities as a function of the friction Reynolds number. (ii) Investigate the possibility of predicting the levels of streamwise dispersive stress using a phenomenological closure model. Such an approach has been applied successfully in the context of idealised vegetation canopies (Moltchanov & Shavit, 2013, Water Resour. Res., vol. 49, pp. 8222-8233) and is extended here, for the first time, to an irregular rough surface. Overall, the results reveal that strong levels of dispersive stress occur beneath the roughness crests and, for the highest friction Reynolds number considered in this study, show that the magnitude (and gradient) of these “form-induced” stresses exceed their Reynolds stress counterparts. In addition, this study emphasises that the dominant source of spatial heterogeneity within the irregular roughness canopy are “wake-occupied” regions and that a suitable parameterisation of the wake-occupied area is required to obtain an accurate prediction of streamwise dispersive stress

Peran Kerjasama antara Interpol Indonesia dengan Malaysia Ditinjau dari Perspektif Hukum Internasional

The trans-national criminal that passes the national border only can be caught by authorized law officer and one of them is Interpol. ICPO-Interpol as organization of Police in the world has a National Central Bureau (NCB) in each state as member. The existence of NCB or its representative in each state will enable the police performance in seek, and arrest the International fugitive. The method of this research is a descriptive analytic method and applies the normative law approach refer to the law norm, especially the International law norm and described into the general section to the specific section based on the primary law, secondary law material and tertiary law material. The data was collected by library study. In seek and arrest the transnational criminal needs a mutual cooperation between polices in any nations. The mutual cooperation between Indonesian Interpol and Malaysia in eradicate the transnational crime enforced in three agreements. First, the extradition contract between Republic of Indonesia and Government of Malaysia that validated in Act No. 9 of 1974. Second, is a mutual agreement in criminal code or that known as Mutual Legal Assistance in Criminal Matters (MLA). Third, it is a Memorandum of Understanding between the Government of Republic of Indonesia and Malaysia about the narcotic. Although a mutual cooperation between Interpol of Indonesia and Malaysia had enforced in an agreement, but the agreement is not yet effective, because there are many weakness on the agreement. Therefore, an effective cooperation between both of nations is an informal cooperation through diplomatic relations

Wall-bounded Turbulence Over Highly Positively and Negatively Skewed Roughness

While most engineering rough surfaces have only moderately skewed height distributions, examples of irregular rough surfaces with very high positive or negative skewness are not unusual. In the current study, the fluid dynamic properties of a set of seven irregular rough surfaces with skewness varying from -2.3 to +2.3 are investigated using direct numerical simulations of turbulent channel flow. We observe that the roughness function saturates both for very high positive and very high negative skewness, showing that extrapolation of existing empirical correlations for moderate skewness will overpredict the roughness effect for extremely skewed cases

Performance of a C4F8O Gas Radiator Ring Imaging Cherenkov Detector Using Multi-anode Photomultiplier Tubes

We report on test results of a novel ring imaging Cherenkov (RICH) detection system consisting of a 3 meter long gaseous C4F8O radiator, a focusing mirror, and a photon detector array based on Hamamatsu multi-anode photomultiplier tubes. This system was developed to identify charged particles in the momentum range from 3-70 GeV/c for the BTeV experiment.Comment: 28 pages, 23 figures, submitted to Nuclear Instruments and Method