Streamwise-traveling waves of spanwise wall velocity for turbulent drag reduction

Waves of spanwise velocity imposed at the walls of a plane turbulent channel flow are studied by Direct Numerical Simulations. We consider sinusoidal waves of spanwise velocity which vary in time and are modulated in space along the streamwise direction. The phase speed may be null, positive or negative, so that the waves may be either stationary or traveling forward or backward in the direction of the mean flow. Such a forcing includes as particular cases two known techniques for reducing friction drag: the oscillating wall technique (a traveling wave with infinite phase speed) and the recently proposed steady distribution of spanwise velocity (a wave with zero phase speed). The traveling waves alter the friction drag significantly. Waves which slowly travel forward produce a large reduction of drag, that can relaminarize the flow at low values of the Reynolds number. Faster waves yield a totally different outcome, i.e. drag increase. Even faster waves produce a drag reduction effect again. Backward-traveling waves instead lead to drag reduction at any speed. The traveling waves, when they reduce drag, operate in similar fashion to the oscillating wall, with an improved energetic efficiency. Drag increase is observed when the waves travel at a speed comparable with that of the convecting near-wall turbulence structures. A diagram illustrating the different flow behaviors is presented

Pattern-based Pressure Drop Of Air–water Flow Across A 90° Sharp Mitre Elbow

Air–water flow in a 90° sharp elbow (mitre bend) is studied in a new purpose-built experimental facility at the University of Sheffield. For the first time, the two-phase flow is investigated in a mitre bend for water-based Reynolds numbers Rew = 5600–12800 and water-to-air mass flow rate ratios mw/ma=10–3800. Four different flow patterns are observed in the upstream pipe (plug, slug, slug-annular and annular) by using a high-speed high-resolution camera. The results show that the perturbation length upstream and downstream of the elbow and the pressure drop are significantly affected by the flow patterns. Two new values of the Lockhart–Martinelli parameter C are found for the pressure drop across the elbow

Condensation heat transfer coefficient for rectangular multiport microchannels at high ambient temperature

We experimentally compute the local heat transfer coefficient of blend refrigerant R-410A condensing inside horizontal rectangular multiport aluminium microchannels with hydraulic diameters equal to 0.52 mm and 1.26mm. The refrigerant flows at near-critical pressure and the cooling air flows at high temperatures proper of hot climates. The experiments are conducted in a bespoke experimental facility and micro-foil sensors are used to measure the local condensation heat flux. The heat transfer coefficient is found to increase with the mass flow rate per unit area and the vapour quality and to decrease with the ambient temperature. Correlations available in the literature do not predict our experimental data satisfactorily because of our extreme operating conditions of high pressure and high cooling air temperature. A novel correlation is therefore obtained to successfully compute the Nusselt number for the condensing annular flow regime in our high pressure and high temperature conditions

Water-air flow in straight pipes and across 90 degrees sharp-angled mitre elbows

Water-air flow in horizontal and vertical straight pipes and through 90∘ sharp-angled mitre elbows is investigated by visualizing the flow patterns by means of a high-precision camera and by measuring the pressure drop. The flow is studied in pipes with three diameters for about six hundred conditions of water-air flows, characterized by superficial velocities in the ranges of =0.3-1m/s for water and =0.15-34m/s for air. The portion of the pipe upstream of the elbow is always positioned horizontally, while the portion of the pipe downstream of the sharp bend is oriented horizontally or vertically with the flow moving upward. Plug, slug, slug-annular, and annular flows are visualized in horizontal straight pipes, while slug, churn, and annular regimes are recorded in vertical straight pipes downstream of the sharp bend. These patterns are well predicted by the Mandhane map (J. Mandhane, G. Gregory, K. Aziz, A flow pattern map for gas-liquid flow in horizontal pipes, Int. J. Multiph. Flow 1 (4) (1974) 537-553) for horizontally oriented straight pipes and by the Hewitt-Roberts map (G. Hewitt, D. Roberts, Studies of two-phase flow patterns by simultaneous 292 X-ray and flash photography, Tech. Rep., Atomic Energy Research Establishment, Harwell (England) (1969)) for vertically oriented straight pipes. The changes of the flow patterns as the fluids pass through the mitre elbows are discussed. A multiple membrane flow structure is observed in the vertical upward flow at much higher Reynolds numbers based on the water superficial velocity than in the vertical downward case previously reported in the literature. A dimensional analysis is employed to obtain the non-dimensional parameters that describe the flows through the straight pipes and across the elbows. It is proved that a rigorous way to present the flow regimes and the pressure-drop coefficients of the water-air flow for a given geometry is the space of the Reynolds numbers based on the superficial velocities of air and water for fixed Froude number because the flow is incompressible and isothermal. By expressing the maps in scaled form, the prediction of the flow patterns along the straight portions of the pipe improves. The flow patterns through the elbows are expressed in terms of rescaled Mandhane maps, which, for the first time, simultaneously represent the flow patterns both upstream and downstream of the elbows. New pattern-based empirical correlations are obtained for the scaled pressure drops for the water-air flows through the horizontal and vertical pipes and through the elbows. The correlations are based on the flow patterns and do not rely on widely used prediction models, thereby predicting the pressure drop more accurately than the models presently available in the literature

Liquid recirculation in microfluidic channels by the interplay of capillary and centrifugal forces

We demonstrate a technique to recirculate liquids in a microfluidic device, maintaining a thin fluid layer such that typical diffusion times for analytes to reach the device surface are < 1 min. Fluids can be recirculated at least 1000 times across the same surface region, with no change other than slight evaporation, by alternating the predominance of centrifugal and capillary forces. Mounted on a rotational platform, the device consists of two hydrophilic layers separated by a thin pressure-sensitive adhesive (PSA) layer that defines the microfluidic structure. We demonstrate rapid, effective fluid mixing with this device

Comparative study of risk factors among children and adolescents with an anthropometric diagnosis of overweight or obesity

OBJETIVO: O excesso de peso é um grande problema de saúde pública em todo o mundo, atingindo crianças e adolescentes. O presente estudo teve por objetivo verificar se a condição de portador de sobrepeso se associa à presença de fatores de risco à saúde. MÉTODOS: Foram estudados 84 sujeitos entre seis e 17 anos, constituindo-se um grupo de 34 pacientes portadores de sobrepeso e outro de 50 obesos, atendidos em ambulatório de Nutrologia Pediátrica da Universidade de Ribeirão Preto (Unaerp), durante o período de outubro de 2004 a outubro de 2005. Foram obtidas as variáveis: pressão arterial, circunferência abdominal, glicemia de jejum, glicemia de segunda hora, colesterol total, HDL-c, LDL-c e triglicérides. Comparam-se os dois grupos por meio de duas abordagens estatísticas: comparação entre as distribuições dos valores absolutos (teste de Mann-Whitney) e entre as frequências de valores considerados alterados (teste exato de Fisher). RESULTADOS: Quanto à primeira abordagem, verificou-se diferença estatística apenas para o HDL-c, com maiores valores para o subgrupo de portadores de sobrepeso de seis a dez anos. Quanto à segunda comparação, verificou-se diferença estatística para a circunferência abdominal (segundo os critérios de Freedman), HDL-c e glicemia de jejum, com maiores frequências de valores alterados entre os obesos. CONCLUSÕES: Os portadores de sobrepeso apresentaram alterações indicativas de prejuízos à sua saúde, semelhantemente aos obesos, sendo necessário também incluí-los nos programas de prevenção e tratamento.OBJECTIVE: Excess weight is a serious public health problem all over the world, affecting children and adolescents. The objective of the present study was to determine whether the condition of being overweight is associated to changes in health parameters. METHODS: 84 subjects aged 6-17 years old were studied (34 overweight and 50 obese patients), all of them assisted at the Pediatric Nutrology outpatient clinic of the University of Ribeirão Preto (Unaerp) from October/2004 to October/2005. Arterial pressure, abdominal circumference, fast glycemia, second hour glycemia, total cholesterol and its fractions (HDL-c and LDL-c) and triglycerides were determined. The two groups were compared by two statistical approaches: comparison of absolute values by Mann-Whitney test, and between frequencies of altered values by Fisher exact test. RESULTS: There was a statistical difference only for HDL-c, with higher concentrations in the overweight group when the subgroup of patients with 6-10 years old was considered. Regarding the second approach, there were statistical differences in the abdominal circumference by Freedman criteria, HDL-c and fast glycemia, with higher frequencies of altered values among obese subjects. CONCLUSIONS: Overweight patients presented alterations harmful to their health, similarly to obese individuals. It is necessary to enroll overweight children and adolescents in programs dedicated to the prevention and treatment of nutritional problems

Optically addressable single-use microfluidic valves by laser printer lithography

We report the design, fabrication, and characterization of practical optofluidic valves fabricated using laser printer lithography. Valves are opened by directing optical energy from a solid-state laser, with similar power characterisitcs to those used in CD/DVD drives, to a spot of printed toner where localized heating melts an orifice in the polymer layer in as little as 500 ms, connecting previously isolated fluidic components or compartments. Valve functionality, response time, and laser input energy dependence of orifice size are reported for cyclo-olefin copolymer (COC) and polyethylene terephthalate (PET) films. Implementation of these optofluidic valves is demonstrated on pressure-driven and centrifugal microfluidic platforms. In addition, these “one-shot” valves comprise a continuous polymer film that hermetically isolates on-chip fluid volumes within fluidic devices using low-vapor-permeability materials; we confirmed this for a period of one month. The fabrication and integration of optofluidic valves is compatible with a range of polymer microfabrication technologies and should facilitate the development of fully integrated, reconfigurable, and automated lab-on-a-chip systems, particularly when reagents must be stored on chip for extended periods, e.g. for medical diagnostic devices, lab-on-a-chip synthetic systems, or hazardous biochemical analysis platforms

Scaling of boundary-layer disturbances exposed to free-stream turbulence

We present theoretical results related to the experimental findings of Matsubara & Alfredsson (J. Fluid Mech., vol. 430, 2001, pp. 149–168) on the scaling of the energy spectra of the Klebanoff modes, i.e. streamwise-elongated vortical disturbances generated by free-stream turbulence in a flat-plate transitional boundary layer. The scaling is explained by a model that describes the streamwise evolution of the streamwise and spanwise energy spectra. The theoretical framework is based on the quasi-steady asymptotic solution of the boundary-region equations, on an axial-symmetric model of the free-stream spectrum, and on the spectral response of the boundary layer to the external perturbations

Self-Consistent Data Analysis of the Proton Structure Function g1 and Extraction of its Moments

The reanalysis of all available world data on the longitudinal asymmetry A|| is presented. The proton structure function g1 was extracted within a unique framework of data inputs and assumptions. These data allowed for a reliable evaluation of moments of the structure function g1 in the Q2 range from 0.2 up to 30 GeV2. The Q2 evolution of the moments was studied in QCD by means of Operator Product Expansion (OPE).Comment: Proceeding of 3rd International Symposium on the Gerasimov-Drell-Hearn Sum Rule and its extensions, Old Dominion University, Norfolk, Virginia June 2-5, 200

Growth of boundary-layer streaks due to free-stream turbulence

The growth of laminar boundary-layer streaks caused by free-stream turbulence encountering a flat plate in zero-pressure-gradient conditions is investigated experimentally in a wind tunnel and numerically by solving the unsteady boundary-region equations. A comparative discussion amongst the most relevant theoretical frameworks, such as the Goldstein theory, the Taylor-Stewartson theory, the optimalgrowth theory and the Orr-Sommerfeld theory, is first presented and parallels and complimentary aspects of the theories are pointed out to justify the use of the Goldstein theory in our study. The statistical properties of the positive and negative fluctuations of the laminar streaks are discussed, showing how the total time average of the boundary-layer fluctuations masks the true character of the disturbance flow and revealing that the maximum values and the root-mean-square of positive and negative fluctuations grow downstream at the same rate. The downstream growth rate of the low-frequency disturbances and the decay rate of the high-frequency disturbances are also computed for the first time. The numerical solutions of the unsteady boundary-region equations are compared successfully with the streak profiles measured in the wind tunnel and with direct numerical simulation results available in the literatur