Sathyanarayana Ayyalasomayajula

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.

Modeling inertial particle acceleration statistics in isotropic turbulence

Our objective is to explain recent Lagrangian acceleration measurements of inertial particles in decaying, nearly isotropic turbulence [Ayyalasomayajula et al., Phys. Rev. Lett. 97, 144507 (2006)]. These experiments showed that as particle inertial effects increased, the variance in the particle acceleration fluctuations was reduced, and the tails of the normalized particle acceleration probability density function (PDF) became systematically attenuated. We model this phenomenon using a base flow that consists of a two-dimensional array of evenly spaced vortices with signs and intensities that vary randomly in time. We simulate a large sample of inertial particles moving through the fluid without disturbing the flow (one-way coupling). Consistent with Bec et al. [J. Fluid Mech. 550, 349 (2006)], we find that our model exhibits preferential concentration or clustering of particles in regions located away from the vortex centers. That is, inertial particles selectively sample the flow field, oversampling re...

Nonlinear interactions in strained axisymmetric high-Reynolds-number turbulence

We present measurements, over a wide range of Reynolds numbers (40 ≤ R λ ≤ 470), of grid-generated turbulence subjected to axisymmetric strain, and of the subsequent evolution of the turbulence after the strain is released. The Reynolds number was varied by the use of both passive and active grids and the strain was produced by a 4:1 area-change axisymmetric contraction placed at various distances from the grid. The time scale ratio of the turbulence to that of the mean strain varied from approximately 10 to 30. The results show reasonable agreement with (linear) rapid distortion theory (RDT) for the velocity variances but, contrary to linear theory, the strained longitudinal, u 1 , spectrum peaked at significantly higher wavenumber than the transverse, u 2 , spectrum. The mismatch in peaks increased with increasing R λ and at the highest Reynolds number (R λ = 470) the peak of the strained u 1 -spectrum occurred at a wavenumber 200 times greater than that of the u 2 -spectrum. As the flow relaxed toward isotropy after the contraction, further evidence of the non-locality in the flow field became apparent, with a second peak in the u 2 -spectrum emerging at a similar wavenumber to the high-frequency peak in the u 1 -spectrum. The strain also caused the longitudinal derivative skewness to change sign but as the flow evolved after the contraction the derivative skewness returned to its typical value of -0.4. We also show that single-point turbulence models are inadequate to describe the relaxation of the turbulence towards an isotropic state in the postcontraction region.

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.