A. Tsinober

Experimental investigation of the field of velocity gradients in turbulent flows

We present results of experiments on a turbulent grid flow and a few results on measurements in the outer region of a boundary layer over a smooth plate. The air flow measurements included three velocity components and their nine gradients. This was done by a twelve-wire hot-wire probe (3 arrays × 4 wires), produced for this purpose using specially made equipment (micromanipulators and some other auxiliary special equipment), calibration unit and calibration procedure. The probe had no common prongs and the calibration procedure was based on constructing a calibration function for each combination of three wires in each array (total 12) as a three-dimensional Chebishev polynomial of fourth order. A variety of checks were made in order to estimate the reliability of the results. Among the results the most prominent are the experimental confirmation of the strong tendency for alignment between vorticity and the intermediate eigenvector of the rate-of-strain tensor, the positiveness of the total enstrophy-generating term ω i ω j s ij ( s ij = ½(∂ u i /∂ x j +∂ u j /∂ x i ), ω i = e ijk ∂ u j /∂ x k ) even for rather short records and the tendency for alignment in the strict sense between vorticity and the vortex stretching vector W i = ω j s ij . An emphasis is put on the necessity to measure invariant quantities, i.e. independent of the choice of the system of reference (e.g. s ij s ij and ω i ω j s ij ) as the most appropriate to describe physical processes. From the methodological point of view the main result is that the multi-hot-wire technique can be successfully used for measurements of all the nine velocity derivatives in turbulent flows, at least at moderate Reynolds numbers.

Lagrangian measurement of vorticity dynamics in turbulent flow

The full set of velocity derivatives,

Velocity derivatives in the atmospheric surface layer at Reλ=104

\partial u_{i}/\partial x_{j}

Turbulent transport of momentum and heat in magnetohydrodynamic rectangular duct flow with strong sidewall jets

, is measured experimentally in a Lagrangian way in quasi-homogeneous isotropic turbulence. This is achieved by applying the three-dimensional particle tracking velocimetry (3D-PTV) technique to an electromagnetically forced flow with

Velocity and temperature derivatives in high-Reynolds-number turbulent flows in the atmospheric surface layer. Part 1. Facilities, methods and some general results

\hbox{\it Re}_{\lambda}\,{\thickapprox}\,50

On the helical nature of three-dimensional coherent structures in turbulent flows

. Checks based on precise kinematic relations show that the technique presented measures the velocity derivatives with good accuracy. In a study on vorticity, characteristic properties of turbulent flows known from direct numerical simulations are reproduced. These are the positive skewness of the intermediate eigenvalue of the rate of strain tensor,

On the role of helical structures in three-dimensional turbulent flow

s_{ij}

Small-scale aspects of flows in proximity of the turbulent/nonturbulent interface

,

Random Taylor hypothesis and the behavior of local and convective accelerations in isotropic turbulence

\langle \Lambda_{2}\rangle \,{>}\,0

A study of properties of vortex stretching and enstrophy generation in numerical and laboratory turbulence

, the predominance of vortex stretching over vortex compression,