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The multiple scattering of an ultrashort laser pulse by a turbid dispersive medium (namely a cloud of bubbles in water) is investigated by means of Monte Carlo simulations. The theory of Gouesbet and Gr\'ehan [Part. Part. Syst. Charact. 17 213-224 (2000)] is used to derive an energetic model of the scattering transient. The spreading and extinction of the pulse is decoupled from the transient of scattering to describe each phenomenon individually. The transient of scattering is modeled with the Lorenz-Mie Theory, thus also valid for a relative refractive index lower than one, contrary to the Debye series expansion which does not converge close to the critical angle. To this aim, the Scattering Impulse Response Function (SIRF) allows to detect the different modes of scattering transient in time and direction. The present approach is more generic and can simulate clouds of air bubbles in water. Two Monte-Carlo approaches are proposed. The first is a pure Monte Carlo approach where the delay due to the scattering is randomly drawn at each event, while the second is based on the transport of the whole scattering signal. They are both embedded in the Monte Carlo code Scatter3D [JOSA A 24, 2206-2219 (2007)]. Both models produce equivalent trends and are validated against published numerical results. They are applied to the multiple scattering of ultra short pulse by a cloud of bubble in water in the forward direction. The pulse spread due to the propagation in water is computed for a wide range of traveled distances and pulse durations, and the optimal pulse duration is given to minimize the pulse spread at a given distance. The main result is that the scattered photons exit the turbid medium earlier than the ballistic photons and produce a double peak related to the refraction in the bubble. This demonstrates the possibility to develop new diagnostics to characterize dynamic bubbly flows., Comment: 30 pages, 19 Figures

In order to better understand the hydroplaning phenomenon, local velocity measurements of water flow are performed inside the tire grooves of a real car rolling through a water puddle. Velocity fields are obtained by combining refraction Particle Image Velocimetry (r-PIV) illumination, seeding fluorescent particles and either the classical 2D2C or a 2D3C stereoscopic recording arrangements. The presence of some bubble columns inside the grooves is highlighted by separate visualisation using fluorescent contrast technique evidencing two phase flow characteristics. A simple predictive model is proposed supporting the r-PIV analysis. It provides useful information to adjust the focusing distance and to understand the effect of the bubble column presence on the recorded r-PIV images, especially for the seeding particles located in the upper part of the grooves, as fluorescent light is attenuated by the bubbles. Also, the predictions provided by the model are compatible with the measurements. The velocity fields inside the grooves are analysed using ensemble averaging performed over a set of independent snapshots, recorded with the same operating parameters. The variability of the longitudinal velocity distribution measured in a groove for several independent runs is explained by different mechanisms, like the random position of fluorescent seeding particles at various height of the groove, the hydrodynamic interactions between longitudinal and transverse grooves, and the random location of the transverse grooves from one run to an other. Three velocity components in cross-sections of the longitudinal grooves are obtained using the stereoscopic arrangement. They are compatible with the presence of some longitudinal vortices also assumed in the litterature., Comment: 41 pages, 25 figures. This is the version of the article before peer review or editing, as submitted by an author to Physics of Fluid Publishing Ltd is not responsible for any errors

We present an extensive experimental study focused on understanding the impact of cavitation in a high-pressure diesel nozzle on the macroscopic properties of fuel spray. Several high-speed videos of the liquid flow through a transparent, asymmetric cylindrical nozzle with a single orifice (phi = 0.35 mm) are recorded along with the videos of the resulting spray in the near-nozzle region, issued with an injection pressure of 300 bar at a frame-rate of 75 kHz. The high-repetition images of the internal flow are then used to estimate the onset of cavitation inside the transparent nozzle and the probability of development of cavitation in different regions of the nozzle with an average estimate of the amount of cavitation with time. On the other hand, recorded spray images are used to study spray penetration, cone-angles and velocity from the start of fuel injection. A novel approach is proposed for the measurement of perturbations that occur in form of big liquid structures along the spray boundary., Comment: in 27th European Conference on Liquid Atomization and Spray Systems, Sep 2016, Brighton, United Kingdom. 2016

The entire process of atomization of the fuel in an internal combustion engine plays a very important role in determining the overall efficiency of these engines. A good atomization process could help the fuel to mix with the air properly leading to its efficient combustion, thereby reducing the emitted pollutants as well. The recent trend followed by the engineers focused on designing fuel injectors for more efficient atomization is to increase the atomization pressure while decreasing the nozzle orifice diameter. A consequence of this is the development of cavitation (formation of vapor cavities or bubbles in the liquid) inside the injector close to the nozzle. The main reason behind this is the sudden changes in the pressure inside the injector and these cavities or bubbles are usually formed where the pressure is relatively low.This work mainly focuses on studying the formation of cavitation and its effect on the velocity of the spray in the near nozzle region using asymmetrical transparent nozzle equipped with a needle lift sensor with nozzle diameter of 0.35 mm at 300 bar of injection pressure. The experiment consists in recording of several image-pairs, which are separated by about 300 ns, capturing the dynamics of the spray, a few millimeters from the nozzle in the direction of the flow. These image-pairs are then used to compute the velocity from the displacement of the liquid structures and ligaments by correlating the first image with the second. About 200 of such velocity graphs are then averaged to obtain a velocity map and is compared with the similar average velocity maps obtained at different times from the start of the injection. The angular spread of the spray from each of these images is calculated as well. The images showing cavitation inside the injector are also recorded at these same instants of time so as to understand the effects of cavitation on the velocity and angular spread of the spray close to the nozzle., Comment: 13th International Conference on Liquid Atomization and Spray Systems, Aug 2015, Tainan, Taiwan. 2015, https://iclass2015.tw/