Session R21: General Fluids V

Spectral characteristics of atmospheric surface layer turbulence in Qatar

Turbulent characteristics of atmospheric boundary layer are of utmost importance in modeling the large-scale meteorological processes, diffusion of atmospheric contaminants, heat transfer and evaporation from the earth surface. Meteorological data are available for some areas of the globe but are sparse in tropical regions. There had been some recent studies in tropical weather in southwestern Asia but no study is carried out in Persian Gulf region. The present study reports the micrometeorological data collected from an atmospheric measurement station in the coastal region of Doha, Qatar, to characterize the nature of atmosphere surface layer (ASL) and ocean wave in this region. In the present work turbulence velocity spectra in this region is presented and compared with the available data from other locations. Also, empirical relationship for the normalized dissipation function in this region is suggested. Finally, variation of different length scales with the stability parameter $z$/$L$ is investigated and compare with the existing values in available literatures. This is the first ever study of ASL in this area, and is expected to be a foundation of further atmospheric research endeavors in Qatar.   

Spray Characterization of Gas-to-Liquid Synthetic Jet Fuels

Gas-to-Liquid (GTL) Synthetic Paraffinic Kerosene (SPK) fuel obtained from Fischer-Tropsch synthesis has grabbed the global attention due to its cleaner combustion characteristics. GTL fuels are expected to meet the vital qualities such as atomization, combustion and emission characteristics of conventional jet fuels. It is imperative to understand fuel atomization in order to gain insights on the combustion and emission aspects of an alternative fuel. In this work spray characteristics of GTL-SPK, which could be used as a drop-in fuel in aircraft gas turbine engines, is studied. This work outlines the spray experimental facility, the methodology used and the results obtained using two SPK's with different chemical compositions. The spray characteristics, such as droplet size and distribution, are presented at three differential pressures across a simplex nozzle and compared with that of the conventional Jet A-1 fuel. Experimental results clearly show that although the chemical composition is significantly different between SPK's, the spray characteristics are not very different. This could be attributed to the minimal difference in fluid properties between the SPK's. Also, the spray characteristics of SPK's show close resemblance to the spray characteristics of Jet A-1 fuel.   

Optical properties of nanofluids and its implication in nPIV measurements

Nanofluids have shown potential as heat transfer fluids in recent times due to their anomalous enhancement in heat transfer characteristics. Optical experimental methods are used to study near-wall flow characteristics in nanofluids to better understand this phenomenon. It is important to characterize the optical properties of the fluid under consideration as accuracy of these measurement techniques highly depends on these characteristics. For example, evanescent wave based nano-Particle Image Velocimetry (nPIV) technique, that measures near-wall velocity fields with an out of plane resolution of O(100nm), is an effective tool for such studies In the present study, optical properties of SiO$_{2}$--water nanofluids at various particle concentrations are investigated. Measurements of refractive indices and the optical transmittance of nanofluids, which are directly related to the out-of-plane resolution of nPIV measurements, are reported. The effects of the modification of these optical properties on the nPIV measurements of nanofluids in a micro channel are then discussed.   

Compressible cavitation with stochastic field method

Non-linear phenomena can often be well described using probability density functions (pdf) and pdf transport models. Traditionally the simulation of pdf transport requires Monte-Carlo codes based on Lagrange particles or prescribed pdf assumptions including binning techniques. Recently, in the field of combustion, a novel formulation called the stochastic field method solving pdf transport based on Euler fields has been proposed which eliminates the necessity to mix Euler and Lagrange techniques or prescribed pdf assumptions. In the present work, part of the PhD Design and analysis of a Passive Outflow Reducer relying on cavitation, a first application of the stochastic field method to multi-phase flow and in particular to cavitating flow is presented. The application considered is a nozzle subjected to high velocity flow so that sheet cavitation is observed near the nozzle surface in the divergent section. It is demonstrated that the stochastic field formulation captures the wide range of pdf shapes present at different locations. The method is compatible with finite-volume codes where all existing physical models available for Lagrange techniques, presumed pdf or binning methods can be easily extended to the stochastic field formulation.   

Surface manifestations of an underlaying turbulent flow

We present an experimental study of a turbulent flow in a quasi bidimensional configuration and with a free surface. Turbulence is excited in the volume of a liquid metal by using an electromagnetic forcing with a spatially tunable magnetic field. We will present our measurements of the velocity field at the surface, obtained by tracking particles, and of the surface deformation, obtained by a direct optical measurement. The turbulent flows under study show a strong correlation between the imposed forcing geometry and the mean velocity field. We also observed considerable deformation of the free surface and the preferential concentration of the particles used for visualization. Concequently, we will discusse possible physical scenarios at the origin of this concentration, which also depends on the forcing geometry.   

Dissolution without shrinking: a microfluidic study of multicomponent gas bubble dissolution

Spherical CO$_{2}$ bubbles generated in a flow-focusing microfluidic channel first shrink rapidly, and reach an equilibrium size. In the first -- rapid dissolution -- regime, the time needed for the bubbles to shrink in the channel is 30 ms regardless of surfactant concentrations in the liquid phase. After 30 ms following generation, all bubbles stop shrinking and reach an equilibrium radius, which varies with the surfactant concentration. We interpret the results by considering three major factors: interfacial tension, effects of other gases (O$_{2}$, N$_{2})$ that are already dissolved in the liquid phase, and the pressure drop along the channel. Our theoretical model, based on multicomponent 1D (radial) diffusion model, explains both shrinking and equilibrium regimes with different dissolution behavior of three gases and composition changes inside the bubble. We solve the model for a single gas bubble in an infinite liquid phase where the pressure changes with time, and compare with our experimental results.   

Generation and self-assembly of multiple droplets inside microchannels

When two immiscible microscopic droplets immersed in a host fluid phase are brought into contact three different equilibrium topologies can form. Depending on the relative values of the three possible interfacial tensions one observes either complete engulfing of one drop by the other one, partial engulfing in which all three possible interfaces are present or non-engulfing when the drops remain separated by the host phase. In the case of multiple drops these surface interactions lead to self-assembly of complex stable and metastable architectures corresponding to local minima of the total interfacial energy. We study those equilibrium configurations experimentally by using automated microfluidic devices in which droplets are generated and merged on-demand inside microchannels. Guided by theoretical considerations and numerical energy minimization we propose stability diagrams spanned by ratios of surface tensions and volume fractions. We also explain how linear sequences of droplet volumes generated inside narrow microfluidic channels relax, after entering a wide chamber, towards given two- or three-dimensional architectures. Our findings provide routes towards synthesis of polymeric particles with predesigned internal structure and may find use in generation of autonomous droplet networks with application as biosensors.