Boris Jacob

Drag reduction by microbubbles in a turbulent boundary layer

An experimental characterization of the turbulent boundary layer over a flat plate in the presence of small amounts of microbubbles is performed. The average diameter of the injected bubbles is comparable with the local Kolmogorov lengthscale, and the bulk void fraction C¯ is approximately 0.1%. The velocity field of the liquid phase, as well as the bubble characteristics, is acquired by optical techniques. Even at the small void fraction typical of this investigation, the interaction between microbubbles and turbulence leads to significant modifications of the underlying flow field. In accordance with previous investigations, the main global effect consists in a reduction of the magnitude of the viscous drag. This has been checked with preliminary tests conducted in a towing tank, as well as by inferring the wall stress from the boundary layer velocity profile measured in a laboratory facility at the same conditions. Here, the local wall stress is found to drop by approximately 25%. An analysis of the tu...

The residual anisotropy at small scales in high shear turbulence

It has always been believed that turbulence in fluids can achieve a universal state at small scales with fluctuations that, becoming statistically isotropic, are characterized by universal scaling laws. In fact, in different branches of physics it is common to find conditions such that statistical isotropy is never recovered and the anisotropy induced by large scale shear contaminates the entire range of scales up to velocity gradients. We address this issue here, of particular significance, for wall bounded flows. The systematic decomposition in spherical harmonics of the correlation functions of velocity fluctuations enables us to extract the different anisotropic contributions. They vanish at small scale at a relatively fast rate under weak shear. Under strong shear instead they keep a significant amplitude up to viscous scales, thus leaving a persistent signature on the gradients which can be detected even in the statistics of low order, e.g., in the energy dissipation tensor.

Anisotropic fluctuations in turbulent shear flows

An experimental analysis of small-scale anisotropic fluctuations has been performed in two different flows in order to assess possible universal features of shear dominated turbulence, such as the decay rate of anisotropy at small scales and the recovery of isotropy. The two data sets—a homogeneous shear flow and a turbulent boundary layer—are characterized by turbulent Reynolds numbers in the range Reλ=230−330. The measurements in the logarithmic region of the turbulent boundary layer extend from y+=90 up to y+=350. The explicit use of the Taylor hypothesis is avoided by resorting to two-point measurements, with separation in the spanwise direction, z. Streamwise (u) and wall-normal (v) velocity components are simultaneously measured in both positions by means of cross-wire probes. This procedure allows the construction of the mixed transverse structure functions, here addressed up to order 6. According to the SO(3) formalism, the observables we selected are free of any isotropic contribution, thus allow...

On the spatial distribution of small heavy particles in homogeneous shear turbulence

We report on a novel experiment aimed at investigating the effects induced by a large-scale velocity gradient on the turbulent transport of small heavy particles. To this purpose, a homogeneous shear flow at Reλ = 540 and shear parameter S* = 4.5 is set-up and laden with glass spheres whose size d is comparable with the Kolmogorov lengthscale η of the flow (d/η ≈ 1). The particle Stokes number is approximately 0.3. The analysis of the instantaneous particle fields by means of Voronoi diagrams confirms the occurrence of intense turbulent clustering at small scales, as observed in homogeneous isotropic flows. It also indicates that the anisotropy of the velocity fluctuations induces a preferential orientation of the particle clusters. In order to characterize the fine-scale features of the dispersed phase, spatial correlations of the particle field are employed in conjunction with statistical tools recently developed for anisotropic turbulence. The scale-by-scale analysis of the particle field clarifies tha...

Experimental assessment of a new form of scaling law for near-wall turbulence

Scaling laws and intermittency in the wall region of a turbulent flow are addressed by analyzing data obtained by single component hot wire anemometry in the boundary layer of a flat plate at Reθ=2200. The article aims in particular at the experimental validation of a new form of refined similarity recently proposed for the shear dominated range of turbulence, where the classical Kolmogorov–Oboukhov inertial range theory is inappropriate. An approach inspired by the extended self-similarity allows for the extraction of the different power laws for the longitudinal structure functions at several wall normal distances. A double scaling regime is found in the logarithmic region, confirming previous experimental results. Approaching the wall, the scaling range corresponding to the classical cascade-dominated range tends to disappear and, in the buffer layer, a single power law is found to describe the available range of scales. The double scaling is shown to be associated with two different forms of refined s...

Inertial particles in homogeneous shear turbulence

The characteristics of inertial particles distribution in a uniformly sheared turbulent flow are investigated, with the aim of quantifying the effects associated with the large-scale anisotropy induced by the mean velocity gradient. The focus of the analysis is on clustering aspects, and in particular on the dependence of the radial distribution function on both the directionality and the magnitude of the observation scale. We discuss experimental data measured in a homogeneous shear flow seeded with particles of size comparable with the Kolmogorov length scale and Stokes number St ≈ 0.3, and discuss their distribution properties in comparison with results provided by related one-way coupled direct numerical simulations which make use of the point-force approximation.