気相乱流中で微小液滴粒子が衝突する現象は，積乱rn雲などの鉛直方向に発達する対流雪中や様々な工業装rn置内の流れの中など，環境分野や機械工学分野においrnて数多く見られる．特に対流雲中においては，雲粒のrn衝突成長が乱流によって促進されることが指摘されてrnいる．したがって，このような雲粒の衝突頻度rnを正しく考慮することが，対流雲の発生・発達の正確rnな予測，さらには局地的集中豪雨の発生機構の解明やrn発生予測を行うために不可欠である．%We have developed a nonlinear inversion scheme for retrieving collision kernels of droplets from the time evolution of droplet size distributions in turbulent air flows. In order to obtain reference data to validate the scheme, we have performed three-dimensional direct numerical simulations (DNS) of colliding droplets in isotropic steady turbulent flows. In the DNS, air turbulence is calculated using a quasi-spectral method, and droplet motions are tracked by a Lagrange method. The collision kernels retrieved by the nonlinear inversion scheme are compared with those obtained by the DNS for flows with various Reynolds numbers. The collision kernels retrieved from our previous linear scheme are also compared to the reference data. The results show that both the linear and nonlinear schemes can retrieve the collision kernels with fair accuracy in low Reynolds number flows. In higher Reynolds number flows, however, retrieval errors of the linear scheme become larger but those of the nonlinear scheme stay small. Thus, the present nonlinear scheme is more robust than our earlier linear scheme.
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