Larbi Labraga

Surface Roughness Effects on Turbulent Boundary Layer Structures

A turbulent boundary layer structure which develop over a κ-type rough wall displays several differences with those found on a smooth surface. The magnitude of the wake strength depends on the wall roughness. In the near-wall region, the contribution to the Reynolds shear stress fraction, corresponding to each event, strongly depends on the wall roughness. In the wall region, the diffusion factors are influenced by the wall roughness where the sweep events largely dominate the ejection events. This trend is reversed for the smooth-wall. Particle Image Velocimetry technique (PIV) is used to obtain the fluctuating flow field in the turbulent boundary layer in order to confirm this behavior. The energy budget analysis shows that the main difference between rough- and smooth-walls appears near the wall where the transport terms are larger for smooth-wall. Vertical and longitudinal turbulent flux of the shear stress on both smooth and rough surfaces is compared to those predicted by a turbulence model. The present results confirm that any turbulence model must take into account the effects of the surface roughness

Flow Bistability Downstream of Three-Dimensional Double Backward Facing Steps at Zero-Degree Sideslip

The flow downstream of a three-dimensional double backward facing step (3D DBWFS) is investigated for Reynolds number Re h ranging from 5 x 10 3 to 8 × 10 4 (based on the first step height h). The flow is studied both qualitatively by means of laser tomoscopy and oil-flow visualizations and quantitatively by means of particle image velocimetry (PIV) measurements. In particular, the results show a mean flow asymmetry. A sensitivity study around zero degree sideslip has shown that the flow is bistable for this geometry. This bistability has been observed in two different wind tunnels for very different upstream conditions. As a main consequence, the zero degree drift angle could be a relevant validation case of unstable flow computation. More tests are carried out to understand and control this particular flow feature.

Turbulence Structures Downstream of a Localized Injection in a Fully Developed Channel Flow

The effects of localized blowing through a porous strip on a turbulent channel were studied experimentally. The measurements were conducted downstream of the porous strip for three blowing rates: 3%, 5%, and 8% (of the velocity at the centerline of the channel)

VOLUMETRIC 3-COMPONENT VELOCIMETRY MEASUREMENTS OF THE FLOW FIELD ON THE REAR WINDOW OF A GENERIC CAR MODEL

Volumetric 3-component Velocimetry measurements are carried out in the flow field around the rear window of a generic car model, the so-called Ahmed body. This particular flow field is known to be highly unsteady, three dimensional and characterized by strong vortices. The volumetric velocity measurements from the present experiments provide the most comprehensive data for this flow field to date. The present study focuses on the wake flow modifications which result from using a simple flow control device, such as the one recently employed by Fourrie et al. [1]. The mean data clearly show the structure of this complex flow and confirm the drag reduction mechanism suggested by Fourrie et al. The results show that strengthening the separated flow leads to weakening the longitudinal vortices and vice versa. The present paper shows that the Volumetric 3-component Velocimetry technique is a powerful tool used for a better understanding of a threedimensional unsteady complex flow such that developing around a bluffbody.

Use of the POD and the Coherent Structure Detection Criteria to Study the Flow Dynamics Downstream a Confined Square Obstacle

Studying the concept of the vortex is an essential tool in the comprehension of fluid dynamics. Despite this, it is still very difficult to find a universal definition of a vortex. Several methods of detection and characterization of vortex structures has been developed and performed. Specifically tailored for PIV data, they present an important topic in modern experimental fluid mechanics.In fact, the performance of the method is related to its effectiveness in the velocity data analysis and to successfully detect and locate the vortices as well as calculate the characteristic vortex parameters.In the present study, we explore the efficiency of different vortex detection algorithms such as the vorticity ω, Γ2 and Q criteria by studying the vortices structures in the wake of a confined square obstacle.Proper orthogonal decomposition (POD) of the velocity fields was used to extract the energetic contribution of the different instabilities modes.These methods were firstly applied to an experimentally two-dimensional instantaneous velocity fields obtained by Particle Image Velocimetry (PIV) technique. Then, in a second step, we tested these criterions on a numerical velocity fields determined from Lattice Boltzmann simulations and compared to experimental results.Copyright

Wet Foam Flow in Horizontal Square Duct and Through Singularities

In this study, we characterize experimentally, the behavior of a wet foam flow along a square channel and through some singularities (an enlargement and a fence). The enlargement characteristics are a square section 21×21 mm2 at the upstream and a rectangular section of 21 × 42 mm2 at the downstream, for an aspect ratio of 0.5. As for the fence, it presents a height of 10 mm, a width of 5 mm and an inclination at its point of 45°. Experimental data were obtained in order to well understand the singularity effects over the foam flow and its texture in the immediate vicinity of this latter. The experimental investigation is focused into finding the wall shear stress and the liquid film which links the duct walls to the foam flow. The properties of the foam and the influence of this phenomenon might have an important role. We try to relate the wall shear stress obtained experimentally to determine the rheological properties of the foam. The use of different experimental devices and measurement techniques allow the comprehension of physical phenomena aspects that govern the foam structure. The PIV method gives the dynamics of the wet foam including the velocity fields near the walls. These ones are experimentally obtained from a series of instantaneous snapshots that will capture the bubbles movement. The innovative use of the polarographic method hands the measurements of the mass transfers, from which wall shear stress between the foam flow and the Plexiglas walls of the channel can be determined. The mass transfer measurement technique is based on a polarography method, by measuring the redox reaction controlled by the convection and diffusion phenomenon. The conductimetry technique, based on the measurement of the electric resistance in a liquid volume, allows the accurate perception of the liquid film thickness formed between the foam flow and the walls. The static pressure losses are directly obtained by the measurements taken from a series of pressures taps placed in the bottom wall of the channel. This parameter will allow the determination of the foam flow behavior. It is taken into consideration a one-dimensional flow, meaning that the established flow behaves as a piston or move as a bloc, with a foam velocity of 2.3 cm/s. All experiments lead to determine the general tendency of the foam flow structure along the square channel and the singularities, including its rheology behavior and drainage ability.Copyright