Armann Gylfason

Intermittency, pressure and acceleration statistics from hot-wire measurements in wind-tunnel turbulence

From hot-wire anemometer measurements in active-grid wind-tunnel turbulence we have determined the Reynolds number dependence of the velocity derivative moments, the mean-squared pressure gradient, χ, and the normalized acceleration variance, a0, over the Reynolds number range 100 Rλ 900. The values of χ and a0 were obtained from the fourth-order velocity structure functions. The derivative moments show power-law dependence on Reynolds number and the exponent is the same with or without shear. In particular, we find the derivative kurtosis, K∂u/∂x ∼ R 0.39 λ ,a nd there is no evidence of the transition that has been observed in this quantity in some recent work. We find that at high Reynolds numbers, χ and a0 tend to values similar to those obtained by direct particle tracking measurements and by direct numerical simulation. However, at lower Reynolds number our estimates of χ and a0 appear to be affected by the evaluation technique which imposes strict requirements on local homogeneity and isotropy.

On higher order passive scalar structure functions in grid turbulence

The scalar structure function scaling exponent ζn is experimentally determined for n⩽10 in decaying, grid-generated wind-tunnel turbulence with a constant mean temperature gradient. The Reynolds number is varied over the range 150⩽Rλ⩽700 by using static and active grids. The results show that up to n=10 the scaling exponent does not saturate although saturation is not precluded at higher orders. There appears to be no dependence of ζn on Reynolds number and the values of ζn are the same for the transverse (along the gradient) and the longitudinal (streamwise) structure functions. A compilation of previous work shows large variation in ζn, with a few results indicating saturation and most not. Reasons for the scatter are attributed to convergence problems at high orders, effects of flow or computational domain size causing clipping of large rare fluctuations, and differences in initial and boundary conditions.

Signatures of non-universal large scales in conditional structure functions from various turbulent flows

We present a systematic comparison of conditional structure functions in nine turbulent flows. The flows studied include forced isotropic turbulence simulated on a periodic domain, passive grid wind tunnel turbulence in air and in pressurized SF6, active grid wind tunnel turbulence (in both synchronous and random driving modes), the flow between counter-rotating

Lagrangian Measurements of Fluid and Inertial Particles in Decaying Grid Generated Turbulence

We present preliminary measurements of the Lagrangian acceleration probability density function (pdf) of fluid particles in decaying grid-generated turbulence and show that they are in very good agreement with direct numerical simulations determined at the same Reynolds number. We contrast these pdf’s with inertial particle acceleration pdf’s done in the same apparatus.

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 (

Heat-flux scaling in turbulent Rayleigh-Bénard convection with an imposed longitudinal wind.

\mathit{Re}_{{\it\tau}}

Effects of axisymmetric strain on a passive scalar field: modelling and experiment

) and Rayleigh number (

Numerical Optimization and Experimental Validation of a Low Speed Wind Tunnel Contraction

\mathit{Ra}

Lagrangian Measurements of Inertial Particle Accelerations in Grid Generated Wind Tunnel Turbulence

). 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 clustering of particles in high-Reynolds-number turbulence.

We present a numerical study of Rayleigh-Bénard convection disturbed by a longitudinal wind. Our results show that under the action of the wind, the vertical heat flux through the cell initially decreases, due to the mechanism of plume sweeping, and then increases again when turbulent forced convection dominates over the buoyancy. As a result, the Nusselt number is a nonmonotonic function of the shear Reynolds number. We provide simple models that capture with good accuracy all the dynamical regimes observed. We expect that our findings can lead the way to a more fundamental understanding of the complex interplay between mean wind and plume ejection in the Rayleigh-Bénard phenomenology.