Couette flow in channels with wavy walls

Summary. Three-dimensional Couette flows enclosed by a plane and by a wavy wall are addressed; the wave amplitude is proportional to the mean clearance of the channel multiplied by a small dimensionless parameter e. A perturbation expansion in terms of the powers of e of the full steady Navier-Stokes equations yields a cascade of boundary value problems which are solved at each step in closed form. The supremum value of e for which the expansion converges, is determined as a function of the Reynolds number Re: The analytical-numerical algorithm is applied to compute the velocity in the channel to O(e 4 ). Even in the first order approximation O(e), new results are obtained which complement the triple deck theory and its modifications. In particular, the incipient separation-detachment is discussed using the Prandtl-Schlichting criterion of starting eddies. The value e e for which eddies start in the channel, is analytically deduced as a function of Re as well as analytical formulas for the coordinates of the separation points. These analytical formulas show that e e in 3D channels is always less than e e in 2D channels. For non-smooth channels, a criterion of infinitesimally small e e is deduced. The critical value of e up to which bifurcation of the solutions can occur is estimated

Efficient optical-to-terahertz conversion in large-area InGaAs photo-Dember emitters with increased indium content

Optical-to-terahertz (THz) conversion of 800 nm femtosecond laser pulses in large-area bias-free InGaAs emitters based on photo-Dember (PD) and lateral photo-Dember (LPD) effects is experimentally investigated. We use metamorphic buffers to grow submicro-meter thick InxGa1-xAs layers with indium mole fractions x=0.37, 0.53, and 0.70 on a GaAs substrate. A strong enhancement of THz output energy with an increase of indium content is observed. On the surface of the sample providing the strongest emission (x=0.7), we have fabricated a 1.5 cm2 area of asymmetri-cally shaped metallic grating for LPD emission. This LPD emitter allows achieving high conversion efficiency of 0.24 . 10-3 and a broad generation band-width of up to 6 THz. We also demon-strate that there is no significant differ-rence in the conversion efficiency when operating at 1 and 200 kHz repetition rates. Our results show that large area LPD emitters give a convenient, competitive way to generate intense high-repetition-rate THz pulses

Northern Eurasia Future Initiative (NEFI): facing the challenges and pathways of global change in the 21st century

During the past several decades, the Earth system has changed significantly, especially across Northern Eurasia. Changes in the socio-economic conditions of the larger countries in the region have also resulted in a variety of regional environmental changes that can have global consequences. The Northern Eurasia Future Initiative (NEFI) has been designed as an essential continuation of the Northern Eurasia Earth Science Partnership Initiative (NEESPI), which was launched in 2004. NEESPI sought to elucidate all aspects of ongoing environmental change, to inform societies and, thus, to better prepare societies for future developments. A key principle of NEFI is that these developments must now be secured through science-based strategies co-designed with regional decision makers to lead their societies to prosperity in the face of environmental and institutional challenges. NEESPI scientific research, data, and models have created a solid knowledge base to support the NEFI program. This paper presents the NEFI research vision consensus based on that knowledge. It provides the reader with samples of recent accomplishments in regional studies and formulates new NEFI science questions. To address these questions, nine research foci are identified and their selections are briefly justified. These foci include: warming of the Arctic; changing frequency, pattern, and intensity of extreme and inclement environmental conditions; retreat of the cryosphere; changes in terrestrial water cycles; changes in the biosphere; pressures on land-use; changes in infrastructure; societal actions in response to environmental change; and quantification of Northern Eurasia's role in the global Earth system. Powerful feedbacks between the Earth and human systems in Northern Eurasia (e.g., mega-fires, droughts, depletion of the cryosphere essential for water supply, retreat of sea ice) result from past and current human activities (e.g., large scale water withdrawals, land use and governance change) and potentially restrict or provide new opportunities for future human activities. Therefore, we propose that Integrated Assessment Models are needed as the final stage of global change assessment. The overarching goal of this NEFI modeling effort will enable evaluation of economic decisions in response to changing environmental conditions and justification of mitigation and adaptation efforts

Macroscopic diffusion on rough surfaces

International audienceWe consider diffusion on rough and spatially periodic surfaces. The macroscopic diffusion tensor D is determined by averaging the local fluxes over the unit cell. D is proved to be the unit tensor for macroscopically isotropic surfaces. For general surfaces, an asymptotic analysis is applied, when the ratio of the oscillation amplitude to the size of the unit cell is a small parameter ɛ. The microscopic field is determined up to O(ɛ6) in analytical form and an algorithm is derived to calculate higher order terms. We also deduce general analytical formulas for D up to O(ɛ6) and derive an algorithm to compute D as a series in ɛ2

Nonlinear correction to Darcy's law for channels with wavy walls

International audienceFor low Reynolds numbers , the flow of a viscous fluid through a channel is described by the well-known Darcy's law which corresponds to a linear relation between the pressure gradient and the average velocity . When the channel is not straight and when the Reynolds number is not negligible, additional terms appear in this relation. Some previous authors investigated the first three coefficients in the expansion of in the powers of and they showed that the coefficient of vanishes for moderate . Other authors demonstrated that this coefficient can be non-zero. This question is addressed and solved. It is demonstrated that both cases occur; Forchheimer's law has a cubic correction for small and a quadratic one for large . Two analytical-numerical algorithms are constructed to prove this property. These algorithms are applied to the Navier-Stokes equations in three-dimensional channels enclosed by two wavy walls whose amplitude is proportional to , where 2b is the mean clearance of the channels and is a small dimensionless parameter. The first algorithm is applied for small by representing the velocity and the pressure in terms of a double Taylor series in and . The accuracy and following Pad, approximations yield analytical approximate formulae for Forchheimer's law. The first algorithm is applied to symmetric channels on the theoretical level (all terms on and are taken into account) to show that is an odd function of . This observation yields, in particular, a cubic correction to Darcy's law. Numerical examples for non-symmetrical channels yield the same cubic correction. The second algorithm is based on the analytical-numerical solution to the Navier-Stokes equations for arbitrary up to . This algorithm yields, in particular, a quadratic correction to Darcy's law for higher R

ACTIVE ANTENNA FOR GPS/GLONASS/EGNOS SATELLITE SYSTEMS SIGNALS RECEIVING

The article describes the result of development and laboratory testing of the active antenna for receiving signals of satellite systems GPS/GLONASS/EGNOS with powered from the RF cable. The antenna element is designed on a ceramic substrate with high permittivity and reduced dimensions