A digital micromirrors device is used to reproduce the speckle-like interferometric images that would produce rough particles. Time-dependent index inhomogeneities induced by a flame are added between the particle and the imaging system. The size measurements deduced from 2D-Fourier analysis of the interferometric patterns show a less than 10% error when the programmed object is fixed, and a less than 20% error when a scintillation of the object is programmed.

In this study, entrainment ratios are evaluated for jets with different physical properties i.e. viscosity. An academic test rig has been designed in order to insulate the jet from undefined boundary conditions. Two configurations are studied. The first one is a nitrogen jet issuing into a coflow of nitrogen. This allows us to evaluate the entrainment ratio in this baseline case. The viscosity ratio between the surrounding and the jet is unity. The second case is a propane jet issuing into a nitrogen coflow. Here, nitrogen is 3.5 more viscous than propane. Both PLIF and PIV measurements are performed in order to obtain simultaneous maps of scalar and velocity components. Therefore, entrainment ratios as well as the phenomenological processes for fluids issuing into a more viscous environment can be evaluated. These typical flow configurations exhibit in the very near field i) enhanced mixing due to detrainment process, ii) more important turbulent fluctuations. It is worth noting that, whereas the viscosity itself acts at the level of smallest scales, flows with viscosity variations at a large scale (such as jets issuing in different environment) are characterized by effects of viscosity variations at any scale, including the largest. A simple visualization of the scalar dispersion allows us to observe a significant disparity between variable viscosity flow and constant viscosity flow behaviors, leading us to state that the viscosity affects the topology and the dynamic of the whole flow at all scales.

Velocity measurements in the vicinity of an obstacle remain very complicated even when optical diagnostics based on displacement of micrometric tracers are considered. In the present paper, digital in-line holography with a divergent beam is proposed to measure the three-dimensional (3D) velocity vector fields in a turbulent boundary layer and, in particular, on the near wall region of a wind tunnel. The seeding droplets (1-5 mu m) transported by a turbulent airflow are illuminated by a couple of laser pulses coming from a fiber coupled laser diode. These double exposure holograms are then recorded through a transparent glass reticle specially designed for this application with an accurate surface positioning combined with a particularly attractive in situ calibration method of the investigation volume (less than 10 mm(3)). The method used for processing holograms recorded in such a configuration is detailed. Our original calibration procedure and the assessment of its accuracy are presented. Our holographic probe has been tested in a wind tunnel for a large range of different velocities. Then 3D velocity vector fields extracted from more than 13000 holograms are analyzed. Statistical results show the capability of our approach to access in a turbulent boundary layer. In particular, it leads to relevant measurements for fluid mechanics such as velocity fluctuation and the shear stress in the very close vicinity of a wall. (c) 2012 Optical Society of America

Digital in-line holography (DIH) with a divergent beam is used to measure size and concentration of cavitation bubbles (6-100 μm) in hydrodynamic facilities. A sampling probe is directly inserted in the cavitation tunnel, and the holograms of the bubbles are recorded through a transparent test section specially designed for DIH measurements. The recording beam coming from a fiber-coupled laser diode illuminates the sample volume, and holograms are recorded by a CMOS camera. From each hologram, the sampling volume can be reconstructed slice by slice by applying a wavelet-based reconstruction method. Because of the geometry of the recording beam, a magnification ratio must be introduced for recovering the 3D location and size of each bubble. The method used for processing holograms recorded in such a configuration is presented. Then, statistical results obtained from 5000 holograms recorded under different pressures in the cavitation tunnel are compared and discussed.

Digital holography is a well established technique for the study of dynamic volume of particle fields. In this work, an original digital in-line holography system is developed and used to determine 3D displacements of particles tracer in an air-flow. This system allows us to investigate a sample volume of few mm 3 and reconstruct a moving particle with velocities ranging up to 1m.s -1 . The sample volume is reconstructed by computing the wavelet transformation (WT) for different value of the scale parameter. This parameter is related to the axial distance between a reconstruction plane and the CCD sensor. A fiber coupled laser diode is used as the recording light sources. The curvature radius of the reference wave emerging from this fiber forms geometrically magnified diffraction patterns of particles and thus acts as an optical magnification. We show that this enables to enhance the spatial resolution of the system. Here, a transverse magnification ratio of 4 allows us to reconstruct images of droplets with a diameter smaller than 5μm. Note that this configuration needs an accurate knowledge of curvature radius of the recording wave. By using an original calibration method, the droplets can be localized from the reconstructed holograms with an accuracy of 1μm for lateral position and 20μm in depth. The potential of this system is shown in the case of a laminar air flow seeded with oil micro-droplets.