T. Gotoh

National Institute for Fusion Science (NIFS-Repository)

We have developed a code to study the transfermechanism by turbulent ﬂow of particles with or withoutelectric charges such as cloud droplets, very ﬁne yellowsand from the the microscopic view point. Key ingredi-ents of the code are to develop the eﬃcient code whichis able to generate the turbulence ﬁeld with enough spa-tial resolution, to track the Lagrangian particles and todevelop a model of the particle dynamics such as evapo-ration and condensation, collision, electric static interac-tion which occur below the Kolmogorov scale. We haveconsidered in this study the turbulent transfer of thecloud droplets and interaction between them.The continuum part is assumed to be describedby the equation of continuity, the Navier-Stokes equa-tion with buoyancy and external force, and the equa-tions of temperature and moisture mixing ratio. Thewater droplets evaporate and condensate and thus inter-act with the turbulent ﬂow through the buoyancy due tothe latent heat. A dynamical model of the cloud dropletsis such that a small water droplet particle experiences thedrag force in proportion to the relative velocity and thegravity force. The second order interpolation and PICwere used in the interpolation of the velocity and theredistribution of the particle attributes to the ﬂuid gridpoints.In a cubic box with length 25cm the steady turbu-lence is generated and begins to decay by removing theexternal force. The small water droplets with radius of10µm are initially dispersed randomly within a thin hor-izontal layer and then allowed to be convected. Figure 1shows the distribution of the cloud droplets in which thecolor pallet from red to blue indicates their radius fromlarger to smaller radius. The cloud droplets are quicklydispersed and convected downward. The distribution ofparticle radius is shown in Fig.2. The peak of the distri-bution moves toward smaller radius and the distributionbecomes wider and asymmetric as time goes on. One in-teresting observation is that the turbulent kinetic energytends to slowly decay in time when compared to the caseof no buoyancy eﬀects (i.e. dry air turbulence) as seenin Fig.3. This is due to the heat absorption by the sur-rounding air due to the evaporation of the cloud dropletsand considered to be a driving force of turbulent ﬂow.Fig. 1: Cloud particle distribution at 0.6 [sec] after release.Red: droplets of larger radius, blue: droplets of smaller ra-dius.Fig. 2: Evolution of the probability density function ofdroplet radius.Fig. 3: Decay of the total kinetic energy. Red: Buoyancyon, Blue: Buoyancy oﬀ.386§15. Development of Direct Numerical Simulation Code for Particles Convected by TurbulenceGotoh, T. (Nagoya Inst. Tech.)

Development of Direct Numerical Simulation Code for Particles Convected by Turbulence

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