Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session A25: Particle-Laden Turbulence I |
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Chair: Lance Collins, Cornell University Room: 2005 |
Sunday, November 23, 2014 8:00AM - 8:13AM |
A25.00001: Bringing Clouds into Our Lab! - The Influence of Turbulence on Early Stage Rain Droplets Altug Yavuz, Rudie Kunnen, GertJan van Heijst, Herman Clercx We are investigating a droplet-laden flow in an air-filled turbulence chamber forced by speaker-driven synthetic jets in many axes. The speakers are running in a random manner; yet they allow us to control and define the statistics of the turbulence. The influence of the turbulence on the behaviour of particles, both individually and collectively, is not well known. Therefore we study the motion of the droplets with tunable size in a turbulent flow, mimicking the early stages of raindrop formation. 3D Particle Tracking Velocimetry (PTV) is chosen as the experimental method to track the droplets and carry out the statistics. Thereby it is possible to study the spatial distribution of the droplets in turbulence using the so-called Radial Distribution Function (RDF), which quantifies the clustering of the droplets under turbulence conditions. Additionally, this technique allows us to measure velocity statistics of the droplets and the influence of the turbulence on droplet trajectories, both individually and collectively. In this contribution, for different turbulence conditions, we will present velocity statistics of the droplets and quantify their clustering using the RDF. [Preview Abstract] |
Sunday, November 23, 2014 8:13AM - 8:26AM |
A25.00002: On free-stream correction methods for particle-laden flows Jeremy Horwitz, Ali Mani We examine the numerical implementation of point-particle drag laws for two-way coupled particle-laden flows. The Stokes drag formula is assumed to be valid for particles smaller than the smallest fluid scales and particle Reynolds numbers less than unity. Numerical implementations however can result in large errors in the computed drag force when the mesh size is comparable to the particle size. We present a quantification of this error and show that its source is rooted in estimation of the ``free-stream'' velocity. While the Stokes drag formula requires this ``free-stream'' velocity to be measured away from the particle, current numerical methods use sampling of fluid velocity at the location of the particle. We propose simple extrapolation procedures that estimate the true free-stream velocity in such systems. Investigations on test problems show that the proposed procedures successfully eliminate the steady-state drag error. [Preview Abstract] |
Sunday, November 23, 2014 8:26AM - 8:39AM |
A25.00003: On the mechanism for the clustering of inertial particles in the inertial range of isotropic turbulence Lance Collins, Andrew Bragg, Peter Ireland In this talk, we consider the physical mechanism for the clustering of inertial particles in the inertial range of turbulence. By comparisons with DNS data we demonstrate that the mechanism in the theory of Zaichik \emph{et.al.} (Phys. Fluids. 19:113308, 2007) quantitatively describes the clustering of particles in the inertial range. We then analyze the theory for isotropic turbulence in the limit $Re_\lambda\to\infty$. For arbitrary $St$ (Stokes number), there exists a separation in the inertial range beyond which $St_r\ll1$, where $St_r$ is the Stokes number based on the eddy turnover timescale at separation $r$. The inertial-range clustering in this limit can be understood to be due to the preferential sampling of the coarse-grained velocity gradient tensor at that scale. At smaller separations, there may be transitions to $St_r\sim 1$, where a path history symmetry breaking effect dominates the clustering mechanism, and in some cases $St_r\gg 1$, which implies ballistic behavior and a flat RDF. The scaling for each of these regimes is derived and compared to DNS, where applicable. Finally, we compare the results with the ``sweep-stick'' mechanism by Coleman and Vassilicos (Phys. Fluids 21:113301, 2009) and discuss the similarities and differences between the two theories. [Preview Abstract] |
Sunday, November 23, 2014 8:39AM - 8:52AM |
A25.00004: On the relationship between non-local clustering mechanisms and preferential accumulation Andrew Bragg, Lance Collins In a recent paper (New J. Phys. \textbf{16}:055013, 2014) we explained the physical mechanism for the clustering of inertial particles in turbulence contained within the theory of Zaichik \emph{et al.} (Phys. Fluids. \textbf{19}:113308, 2007). We showed that for particles with Stokes numbers in the limit $St\ll 1$, particles accumulate outside of vortices due to the ``centrifuge mechanism'' proposed by Maxey. However, for $St\geq O(1)$, the centrifuge mechanism gives way to a non-local path history symmetry breaking mechanism. Despite the change in the clustering mechanism, the instantaneous particle positions continue to correlate with high-strain, low-vorticity regions of the turbulence. In this talk we show how the non-local mechanism is influenced by, but not dependent upon, the preferential sampling of the fluid velocity gradient tensor along the particles path histories in such a way as to generate a bias for clustering in regions with strong straining motions. Finally, we show how the non-local mechanism still generates clustering, but without preferential accumulation, in the limit where the timescales of the fluid velocity gradient tensor measured along the inertial particle trajectories vanishes (i.e., a white noise velocity field). [Preview Abstract] |
Sunday, November 23, 2014 8:52AM - 9:05AM |
A25.00005: Dynamics of light particles in turbulence Varghese Mathai, Vivek Prakash, Jon Brons, Chao Sun, Detlef Lohse Particle-laden turbulent flows occur widely in nature and industrial applications. The accelerations experienced by these particles can be extreme and intermittent, and are a measure of the forces acting on them. Most of the previous studies have focused on neutrally buoyant and heavy particles in turbulence. In this work, we experimentally study the Lagrangian dynamics of finite-size light particles in a nearly homogeneous and isotropic turbulent channel flow. We explore a range of size ratios and density differences to arrive at the transitional regime when the wake effects start to dominate particle dynamics. Our results suggest that light particle dynamics in turbulence is a strongly two-way coupled problem even for very small density differences with the continuous phase. [Preview Abstract] |
Sunday, November 23, 2014 9:05AM - 9:18AM |
A25.00006: Secondary flow and particle transport in a square duct Hoora Abdehkakha, Gianluca Iaccarino Particle transport and deposition play a significant role in various industrial applications. Previous studies have shown that high magnitudes of the vorticity near the corners of a duct can cause higher accumulation of the particles close to the wall. The objective of this study is to investigate the effects of the secondary flows in the transport and deposition of particles in a turbulent square duct flow. In order to enhance our understanding of particle deposition, we performed three-dimensional direct numerical simulation of a square duct in low Mach number turbulent flow using a Lagrangian model for prediction of particle transport and deposition. To have a more comprehensive understanding of the effects of turbulent flow on particle deposition, simulations with different Reynolds numbers and particle Stokes numbers are performed. [Preview Abstract] |
Sunday, November 23, 2014 9:18AM - 9:31AM |
A25.00007: Turbulent pair dispersion in the presence of gravity Kelken Chang, Benedict Malec, Raymond Shaw We present numerical evidence of the alteration in the turbulent pair dispersion of heavy particles with two different Stokes numbers (bidisperse), whose effect on the dispersion is further compounded when a uniform gravitational acceleration is present. Lagrangian particle trajectories for fluid tracers, and bidisperse inertial particles with and without gravity were calculated from a direct numerical simulation of homogeneous, isotropic turbulence. Particle pair dispersion shows a short-time, ballistic (Batchelor) regime and a transition to super-ballistic dispersion that is suggestive of the emergence of Richardson scaling. The commonly used equation of motion for inertial, sedimenting particles and Kolmogorov scaling arguments are shown to capture the essential features of the pair dispersion at very short time and length scales. Between the ballistic and super-ballistic regions, the dispersions of both tracers and monodisperse inertial particles display a sub-ballistic behavior that is strongly suppressed in the bidisperse case. We attribute the suppression of the dispersion to a reduction in the correlation between velocity and acceleration increments, whose behavior we attempt to capture using a stagnation point model. [Preview Abstract] |
Sunday, November 23, 2014 9:31AM - 9:44AM |
A25.00008: Collision statistics of inertial particles in two-dimensional homogeneous isotropic turbulence with an inverse cascade Ryo Onishi, J.C. Vassilicos This study investigates the collision statistics of inertial particles in inverse-cascading 2D homogeneous isotropic turbulence by means of a direct numerical simulation (DNS). A collision kernel model for particles with small Stokes number (St) in 2D flows is proposed based on the model of Saffman {\&} Turner (1956) (ST56 model). The DNS results agree with this 2D version of the ST56 model for St \textless 0.1. It is then confirmed that our DNS results satisfy the 2D version of the spherical formulation of the collision kernel. The fact that the flatness factor stays around 3 in our 2D flow confirms that the present 2D turbulent flow is nearly intermittency-free. Collision statistics for St $=$ 0.1, 0.4 and 0.6, i.e. for St \textless 1, are obtained from the present 2D DNS and compared with those obtained from the three-dimensional (3D) DNS of Onishi et al. (2013). We have observed that the 3D radial distribution function at contact (g(R), the so-called clustering effect) decreases for St $=$ 0.4 and 0.6 with increasing Reynolds number, while the 2D g(R) does not show a significant dependence on Reynolds number. This observation supports the view that the Reynolds-number dependence of g(R) observed in three dimensions is due to internal intermittency of the 3D turbulence. We have further investigated the local St, which is a function of the local flow strain rates, and proposed a plausible mechanism that can explain the Reynolds-number dependence of g(R). [Preview Abstract] |
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