1,107 research outputs found
In therapy with avatars
No full textCombating phobias and psychotic disorders using virtual technology: that is what the work of Dr. Willem-Paul Brinkman of the Faculty of Electrical Engineering, Mathematics and Computer Science involves. Of course one does not have any of these disorders oneself – or at least that’s what our reporter also thought
Structure and dynamics of turbulent flows over highly permeable walls
No full textHighly porous materials are found in various industrial applications and environmental flows. In previous studies it was found that a turbulent flow along a highly porous wall experiences a higher skin friction as compared to a solid wall with similar surface roughness when the so-called permeability Reynolds number (Re_K) is larger than O(1). The main objective of the present study was to gain understanding of the characteristic structures and auto-generation mechanisms of turbulence for Re_K >> 1. To this purpose the Volume-Averaged Navier-Stokes (VANS) equations were solved in a Direct Numerical Simulation (DNS) of a turbulent flow through a plane channel with an upper solid wall and a lower porous wall at Re_K = 5.91. The DNS results are in good agreement with available Particle Image Velocimetry (PIV) data for the same flow geometry. A linear stochastic estimation technique was used to capture the structure associated with the characteristic ejection event that contributes most to the Reynolds shear stress near the porous wall. This structure is similar to a horseshoe vortex. Contrary to the conventional hairpin vortex found near solid walls, this horseshoe vortex has a significantly higher inclination angle with the wall and its legs are much shorter. The latter is consistent with the observed absence of low and high-speed streaks near highly permeable walls. Next, the auto-generation mechanisms of the horseshoe vortex were studied in another DNS in which the horseshoe vortex was released in the Reynolds-averaged flow field obtained from the former DNS. Two distinct auto-generation mechanisms were observed: (1) the generation of new structures at the upstream end of the horseshoe vortex, which evolve rapidly into a turbulent spot with an arrowhead shape, and (2) the interaction of the horseshoe vortex with spanwise oriented Kelvin-Helmholtz vortex rollers originating from the inflexion point in the mean velocity profile near the porous wall
Direct numerical simulations of drag reduction in turbulent channel flow over bio-inspired herringbone riblet-texture
No full textThe use of drag reducing surface textures is a promising passive method to reduce fuel consumption. Probably most wellknown is the utilisation of shark-skin inspired ridges or riblets parallel to the mean flow. They can reduce drag up to 10%. Recently another bio-inspired texture based on bird flight feather riblets has been proposed. It differs from the standard riblets in two ways. First, the riblets are arranged in a converging/diverging or herringbone pattern. Second, the riblet height or groove depth changes gradually. Drag reductions as high as 20% have been claimed [2]. The objective of the present work is to study the drag reducing properties and mechanisms of this texture. To that purpose Direct Numerical Simulations (DNSs) of turbulent plane channel flow have been performed. Structured roughness has been applied to both walls and several geometric parameters have been varied. Marginal drag reductions on the order of 2.5% and significant drag increases well beyond 100% were found. The latter is attributed to a strong secondary flow that mixes momentum through the whole channel. In future optimization studies we might look for conditions at which secondary motions affect the near-wall cycle of turbulence only
Frye, Hon. Senator Wm. P. of Maine
No full textTitle from unverified information on negative sleeve.Annotation from negative, scratched into emulsion: this, Sen. W.P. Fry, Maine, 2922 [crossed out], 129266, 1603, 931 [crossed out] W.P. Fry; penciled on emulsion: Maine.Forms part of Brady-Handy Photograph Collection (Library of Congress)
Mechanics of dense suspensions in turbulent channel flows
No full textDense suspensions are usually investigated in the laminar limit where inertial effects are insignificant. When the flow rate is high enough, i.e. at high Reynolds number, the flow may become turbulent and the interaction between solid and liquid phases modifies the turbulence we know in single-phase fluids. In the present work, we study turbulent channel flows laden with finite-size particles at high volume fraction by means of Direct Numerical Simulations. A direct-forcing Immersed Boundary Method has been adopted to couple liquid and solid phases. We will show that the turbulence is attenuated in dense cases, even though the overall drag is increased because of the particle contribution to the total stress
Flow regimes of inertial suspensions of finite size particles
No full textInertial regimes in a channel flow of suspension of finite-size neutrally buoyant particles are studied for a wide range of Reynolds numbers: , and particle volume fractions: . The flow is classified in three different regimes according to the phase-averaged stress budget across the channel \cite{Lashgari2014}. The laminar viscous regime at low and where the viscous stress is the dominating term in the budget, the turbulent regime at high and relatively low where the momentum is mainly transferred by the action of the Reynolds stress and the inertial shear-thickening regime where the particle stress contributes the most to the significant enhancement of the wall shear stress. Particle distribution and dispersion properties provide additional evidence for the existence of the three different regimes
External and Internal interfacial turbulent shear layers
No full textSimulation, PIV data, and local models show characteristics and conditional statistics of turbulence either side and within interfacial layers [I] depending on the mean profile and the presence of resistive/porous walls. Key words; turbulence, interface structure, conditional statistics, numerical model
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