Law of the wall in an unstably stratified turbulent channel flow

We perform direct numerical simulations of an unstably stratified turbulent channel flow to address the effects of buoyancy on the boundary layer dynamics and mean field quantities. We systematically span a range of parameters in the space of friction Reynolds number ($Re_{\tau}$) and Rayleigh number ($Ra$). Our focus is on deviations from the logarithmic law of the wall due to buoyant motion. The effects of convection in the relevant ranges are discussed providing measurements of mean profiles of velocity, temperature and Reynolds stresses as well as of the friction coefficient. A phenomenological model is proposed and shown to capture the observed deviations of the velocity profile in the log-law region from the non-convective case

Inertial particle acceleration in strained turbulence

The dynamics of inertial particles in turbulence is modelled and investigated by means of direct numerical simulation of an axisymmetrically expanding homogeneous turbulent strained flow. This flow can mimic the dynamics of particles close to stagnation points. The influence of mean straining flow is explored by varying the dimensionless strain rate parameter $Sk_0/\epsilon_0$ from 0.2 to 20. We report results relative to the acceleration variances and probability density functions for both passive and inertial particles. A high mean strain is found to have a significant effect on the acceleration variance both directly, through an increase in wave number magnitude, and indirectly, through the coupling of the fluctuating velocity and the mean flow field. The influence of the strain on normalized particle acceleration pdfs is more subtle. For the case of passive particle we can approximate the acceleration variance with the aid of rapid distortion theory and obtain good agreement with simulation data. For the case of inertial particles we can write a formal expressions for the accelerations. The magnitude changes in the inertial particle acceleration variance and the effect on the probability density function are then discussed in a wider context for comparable flows, where the effects of the mean flow geometry and of the anisotropy at the small scales are present

Fluidelastic modeling of a weathercock stabilization in a uniform flow

International audienceThe relaxation dynamics of a weathercock free-to-rotate, in the presence of a uniform flow, as it aligns with the flow direction, is investigated experimentally in a wind-tunnel. The dynamics is observed to conveniently follow a damped harmonic oscillator behavior. At first order, the frequency is set by the aerodynamic coefficients. We show that a quasi static approach fails to precisely describe the relaxation dynamics and that non-stationary corrections are required to model the dynamics. A first strategy is to introduce added mass, added stiffness and added damping to the quasi-static approximation, following what is usually done in the context of vortex-induced vibrations. A second strategy is to introduce empirical corrections, whose scaling is obtained from the analysis of the experimental data. Finally, these two strategies are compared and we discuss the physical interpretations of the non-stationary corrections

Áhrif innkomu Costco á dagvöru- og eldsneytismarkaðinn á Íslandi

Í ritgerð þessari er fjallað um hver áhrifin urðu á íslenskan dagvöru- og eldsneytismarkað með tilkomu Costco á markaðinn. Tekin voru viðtöl við lykilstjórnendur fyrirtækja á dagvöru- og eldsneytismarkaði með því markmiði að fá betri innsýn inn í breytingar eða stefnu sem fyrirtækin fóru í til að svara tilvonandi samkeppni frá Costco. Einnig var lögð fram netkönnun til að sjá huglægni almennings í garð Costco og hvar þeirra viðskiptavinahópur lægi. Notast var því við blandaða aðferðafræði þar sem eigindleg rannsókn og megindleg rannsókn voru notaðar við greiningu. Auk þeirra rannsókna var notast við SVÓT greiningu og greiningu Porters til að greina markaðinn betur. Niðurstöður leiddu í ljós tímabundin áhrif á mörkuðum og hefur markaður náð nýju jafnvægi. Helstu breytingar fólust í samrunum fyrirtækja Haga og Olíuverzlunar Íslands, og N1 og Festis. Auk þess luku smærri fyrirtæki á dagvörumarkaðnum rekstri vegna samkeppninnar sem skapaðist vegna tilkomu Costco. Fyrirtæki sem rætt var við fór helst í breytingar við innflutning auk lítillegar verðbreytingar. Costco tók strax mikla markaðshlutdeild til sín en hún fór þó lækkandi eftir opnunaráhrif sem urðu við opnun Costco í Kauptúni

Inertial clustering of particles in high-reynolds-number turbulence

We report experimental evidence of spatial clustering of dense particles in homogenous, isotropic turbulence at high Reynolds numbers. The dissipation-scale clustering becomes stronger as the Stokes number increases and is found to exhibit similarity with respect to the droplet Stokes number over a range of experimental conditions (particle diameter and turbulent energy dissipation rate). These findings are in qualitative agreement with recent theoretical and computational studies of inertial particle clustering in turbulence. Because of the large Reynolds numbers a broad scaling range of particle clustering due to turbulent mixing is present, and the inertial clustering can clearly be distinguished from that due to mixing of fluid particles. © 2008 The American Physical Society