Using Direct Numerical Simulations (DNS), we examine the effects of Taylor Reynolds number, R λ , and Froude number, F r, on the motion of settling, bidisperse inertial particles in isotropic turbulence. Particle accelerations play a key role in the relative motion of bidisperse particles, and we find that reducing F r leads to an enhancement of the accelerations, but a suppression of their intermittency. For Stokes numbers St > 1, the effect of R λ on the accelerations is enhanced by gravity, since settling causes the particle accelerations to be affected by a larger range of flow scales. The results for the Probability Density Function (PDF) of the particle relative velocities show that for bidisperse particles, decreasing F r leads to an enhancement of their relative velocities in both the vertical (parallel to gravity) and horizontal directions. Importantly, our results show that even when the particles are settling very fast, turbulence continues to play a key role in their vertical relative velocities, and increasingly so as R λ is increased. This occurs because although the settling velocity may be much larger than typical velocities of the turbulence, due to intermittency, there are significant regions of the flow where the turbulence contribution to the particle motion is of the same order as that from gravitational settling. Increasing R λ enhances the non-Gaussianity of the relative velocity PDFs, while reducing F r has the opposite effect, and for fast settling particles, the PDFs become approximately Gaussian. Finally, we observe that low-order statistics such as the Radial Distribution Function (RDF) and the particle collision kernel, are strongly affected by F r and St, and especially by the degree of bidispersity of the particles. Indeed, even when the difference in the value of St of the two particles is ≪ 1, the results can differ strongly from the monodisperse case, especially when F r ≪ 1. However, we also find that these low-order statistics are very weakly affected by R λ when St O(1), irrespective of the degree of bidispersity. Therefore, although the mechanisms controlling the collision rates of monodisperse and bidisperse particles are different, they share the property of a weak sensitivity to R λ when St O(1).

Using 3D Voronoï analysis, we explore the local dynamics of small, settling, inertial particles in isotropic turbulence using Direct Numerical Simulations (DNS). We independently vary the Taylor Reynolds number R λ ∈ [90, 398], Froude number F r ≡ a η /g ∈ [0.052, ∞] (where a η is the Kolmogorov acceleration, and g is the acceleration due to gravity), and Kolmogorov scale Stokes number St ≡ τ p /τ η ∈ [0, 3]. In agreement with previous results using global measures of particle clustering, such as the Radial Distribution Function (RDF), we find that for small Voronoï volumes (corresponding to the most clustered particles), the behavior is strongly dependent upon St and F r, but only weakly dependent upon R λ , unless St > 1. However, larger Voronoï volumes (void regions) exhibit a much stronger dependence on R λ , even when St ≤ 1, and we show that this, rather than the behavior at small volumes, is the cause of the sensitivity of the standard deviation of the Voronoï volumes that has been previously reported. We also show that the largest contribution to the particle settling velocities is associated with increasingly larger Voronoï volumes as the settling parameter Sv ≡ St/F r is increased.Our local analysis of the acceleration statistics of settling inertial particles shows that clustered particles experience a net acceleration in the direction of gravity, while particles in void regions experience the opposite. The particle acceleration variance, however, is a convex function of the Voronoï volumes, with or without gravity, which seems to indicate a non-trivial relationship between the Voronoï volumes and the sizes of the turbulent flow scales. Results for the variance of the fluid acceleration at the inertial particle positions are of the order of the square of the Kolmogorov acceleration and depend only weakly on Voronoï volumes. These results call into question the "sweep-stick" mechanism for particle clustering in turbulence which would lead one to expect that clustered particles reside in the special regions where the fluid acceleration is zero (or at least small).We then consider the properties of particles in clusters, which are regions of connected Voronoï cells whose volume is less than a certain threshold. The results show self-similarity of the clusters, and that the statistics of the cluster volumes depends only weakly on St, with a stronger dependance on F r and R λ . Finally, we compare the average settling velocities of all particles in the flow with those in clusters, and show that those in the clusters settle much faster, in agreement with previous work. However, we also find that this difference grows significantly with increasing R λ and exhibits a non-monotonic dependence on F r. The kinetic energy of the particles, however, are almost the

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Two-phase cross-flow takes place in a wide range of industrial equipment, including heat exchangers and measurement devices. The aim of this paper is to establish a numerical model and experimental methodology for comprehensive study and visualization of void fraction and wake region in gas-liquid cross-flow over immersed bodies with various cross-section geometries. Conservation of mass and momentum for both-phase free streams, along with constitutive relationships, were used for modeling turbulence. The input parameters for the numerical simulations were two-phase Reynolds number, free-stream void fraction, bubble size in the inlet, and cross-section geometry of prisms inserted in the two-phase flow path. Because the wake region and phase distribution around an immersed object are time-dependent, we report time average values of drag, lift, and pressure coefficients. The results show that drag and lift coefficients are strongly dependent on the two-phase Reynolds number; this dependency is a more moderate function of void fraction. The results are in good agreement with available empirical correlations and experimental work. Furthermore, experiments were conducted to visualize phase distribution and wake region in two-phase cross-flow. Comparison of the experimental and numerical results verifies the developed numerical model.