1,281 research outputs found
Turbulent drag reduction by hydrophobic surfaces with shear-dependent slip length
No full textThe stabilisation of a parabolic equilibrium profile in a three-dimensional (3D) turbulent channel flow for an incompressible fluid is addressed with the objective of achieving drag reduction. The formulation of this problem stems from Balogh’s work [1] where Lyapunov stability analysis was used to devise prototype feedback laws and prove global stability of the solutions. This treatment only considers the controller as a mathematical artefact, but it can actually be linked to physical control strategies modelling hydrophobic surfaces and porous media. In the former, only linear slip velocity boundary conditions (BC) were considered [8]. However, experiments [2] have suggested that the slip length may be shear-dependent. Motivated by these, the effect on drag reduction of a shear-dependent slip length surface is examined in the present study using Direct Numerical Simulations (DNS) at Re τ0 = u τ0 δ/ν ≃ 180. δ is the channel half height, u τ0 the wall-shear velocity for regular no-slip walls channel and ν the kinematic viscosity. The theoretical analysis in [5], is extended to this new model. The proposed formulation shows that the skin-friction coefficient can be reduced by tuning the parameters in the shear-dependent slip length model. The results, which verified by DNS simulations, show that by taking a slip length value based on a constant slip model [8] and combining it within a shear-dependent model, up to 50% drag reduction can be obtained. The effect of control is further assessed by formulating the Fukagata identity [4] with general boundaries; the weighted Reynolds shear-stress for each quadrant shows an enhanced reduction in the sweep/ejection events compared to the constant slip model
Hydrodynamical turbulence by fractal fourier decimation
No full textWe present a systematic numerical investigation of high-resolution 3D isotropic and homogeneous turbulence resolved on a decimated set of Fourier modes. Fractal decimation acts to decrease the effective dimensionality of the flow by allowing triadic interactions only in a set of Fourier modes N(k) proportional to k^DF for large k. While keeping the symmetries of the original 3D Navier-Stokes equations unchanged, a dramatic change in small-scale statistics is detected at decreasing the fractal dimension DF . Already at fractal dimension DF = 2.8, a global self-similar behaviour is observed in the inertial range of scales, the consequence of such transition are the restoration of the scaling symmetry and vorticity distribution that becomes close to Gaussian. We relate the results to the different roles of local vs non-local interactions in the energy transfer range
Least Squares Fitting of Chacón-Gielis Curves by the Particle Swarm Method of Optimization
Get PDFRicardo Chacón generalized Johan Gielis's superformula by introducing elliptic functions in place of trigonometric functions. In this paper an attempt has been made to fit the Chacón-Gielis curves (modified by various functions) to simulated data by the least squares principle. Estimation has been done by the Particle Swarm (PS) methods of global optimization. The Repulsive Particle Swarm optimization algorithm has been used. It has been found that although the curve-fitting exercise may be satisfactory, a lack of uniqueness of Chacón-Gielis parameters to data (from which they are estimated) poses an insurmountable difficulty to interpretation of findings.Least squares multimodal nonlinear curve-fitting; Ricardo Chacón; Jacobian Elliptic functions; Weierstrass ; Gielis super-formula; supershapes; Particle Swarm method; Repulsive Particle Swarm method of Global optimization; nonlinear programming; multiple sub-optima; global; local optima; fit; empirical; estimation; cellular automata; fractals
Introduction to mineralogy and petrology / S.K. Haldar.
No full textIncludes bibliographical references (p. 325-326) and index.xviii, 338 pages
Affordable homeownership policy: implications for housing markets
Get PDFAffordable homeownership is a policy that is often accorded a great deal of policy attention by governments of many countries. In this paper, we examine the market implications of setting a housing price to income ratio target for a market segment by the government. The policy requires active intervention by the government with regard to the targeted sector. We use a simple model of the housing market with a homeownership affordability target to derive the market implications of such targets. In the presence of uncertainty and resource constraints, the objective of homeownership affordability is achieved for the targeted group at the expense of greater volatility in residential construction activity. When the size of the targeted sector is significant in size, there are spillover price and crowding out effects on the non-targeted housing market segment. This results in political pressure on the government to expand homeownership affordability targets to increasing segments of the population. Housing price to income ratios tend to be fairly constant over time and across targeted groups, the housing supply is relatively price inelastic and the income elasticity of housing demand is less than one. The Singapore government intervenes extensively in the housing sector to ensure homeownership affordability, with a resulting homeownership rate of 91 percent for the resident population. The above hypotheses regarding the implications of setting housing price to income ratio targets are tested using the Singapore housing market. The experience and data for Singapore were found to support the above hypotheses.Affordable homeownership policy, market implications, Singapore
Affordable homeownership policy : implications for housing markets
Get PDFAffordable homeownership is a policy that is often accorded a great deal of policy attention by governments of many countries. In this paper, we examine the market implications of setting a housing price to income ratio target for a market segment by the government. The policy requires active intervention by the government with regard to the targeted sector. We use a simple model of the housing market with a homeownership affordability target to derive the market implications of such targets. In the presence of uncertainty and resource constraints, the objective of homeownership affordability is achieved for the targeted group at the expense of greater volatility in residential construction activity. When the size of the targeted sector is significant in size, there are spillover price and crowding out effects on the non-targeted housing market segment. This results in political pressure on the government to expand homeownership affordability targets to increasing segments of the population. Housing price to income ratios tend to be fairly constant over time and across targeted groups, the housing supply is relatively price inelastic and the income elasticity of housing demand is less than one. The Singapore government intervenes extensively in the housing sector to ensure homeownership affordability, with a resulting homeownership rate of 91 percent for the resident population. The above hypotheses regarding the implications of setting housing price to income ratio targets are tested using the Singapore housing market. The experience and data for Singapore were found to support the above hypotheses.Affordable homeownership policy, market implications, Singapore
Some Experiments on Fitting of Gielis Curves by Simulated Annealing and Particle Swarm Methods of Global Optimization
Get PDFIn this paper an attempt has been made to fit the Gielis curves (modified by various functions) to simulated data. The estimation has been done by two methods - the Classical Simulated Annealing (CSA) and the Particle Swarm (PS) methods - of global optimization. The Repulsive Particle Swarm (RPS) optimization algorithm has been used. It has been found that both methods are quite successful in fitting the modified Gielis curves to the data. However, the lack of uniqueness of Gielis parameters to data (from which they are estimated) is corroborated. From a technical viewpoint, this exercise may be considered as an application of CSA and RPS to extremely nonlinear least-squares curve-fitting to data that may exhibit a large number of local optima.Gielis curves; superformula; nonlinear curve-fitting; Least squares; multi-modal; local optima; global optimization; simulated annealing; particle swarm; parameters estimation
Symbols for Hijabs in The Proudest Blue The Story of Hijab and Family by Ibtihaj Muhammad and S.K. Ali
Get PDFThis study aimed at analyzing symbols that represent hijab as depicted in The Proudest Blue The Story of Hijab and Family written by Ibtihaj Muhammad and S.K. Ali and illustrated by Hatem Aly. This study was a qualitative study that analyze a text, so that the data were in the form of sentences and illustrations taken from the book. The data were collected by reading the book, identifying the data, and classifying the data. Then, the collected data were analyzed using the semiotic theory from Peirce. Based on the findings, there were five symbols representing hijab. The first two symbols are related to the authors’ choice of color. The author uses the pink to illustrate that hijabs are related to love and the blue color that is related to religious symbol of piety and chastity or sincerity. The blue color was the most dominant symbol in the book. In addition, the authors recommend that hijab is not a whisper, a laugh, and a tablecloth. These three symbols represent hijabs as something strong, not a joke to be laughed at and not only a thing to cover. All symbols used by the author are positive ones which challenge the negative stereotypes about hijab in which it is associated with radicalism and terrorism
CASESIAN : a knowledge-based system using statistical and experiential perspectives for improving the knowledge sharing in the medical prescription process
No full textAuthor name used in this manuscript: S.K. KwokAuthor name used in this manuscript: A.H.C. TsangAccepted ManuscriptPublishedGreen (AAM
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