Bulletin of the American Physical Society
68th Annual Meeting of the APS Division of Fluid Dynamics
Volume 60, Number 21
Sunday–Tuesday, November 22–24, 2015; Boston, Massachusetts
Session A4: Particle-Laden Flows: Turbulence Modulation |
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Chair: Krishnan Mahesh, University of Minnesota Room: 103 |
Sunday, November 22, 2015 8:00AM - 8:13AM |
A4.00001: Investigation of turbulence modulation in particle-laden flows using the lattice Boltzmann method. Cheng Peng, Nicholas Geneva, Haoda Min, Lian-Ping Wang Turbulent modulation by finite-size solid particles has been studied experimentally and numerically in the past several decades. Previous studies have revealed that resolving the interfaces between particle surfaces and fluid is crucial to properly include finite-size effects on local fluid turbulence. Finite-size particles also produce pseudo-turbulence that may not decay locally, leading to a stronger nonlinear dependence of the level of turbulence modulation on the particle volume fraction. In this study we apply the lattice Boltzmann method (LBM) to perform interface-resolved simulations of turbulent particle-laden flow, focusing on local turbulence dynamics at the scale of particle size. We will discuss the accuracy of this mesoscopic approach when compared to other macroscopic methods. We consider both fully developed homogeneous isotropic (HI) turbulent flows and turbulent channel flows laden with finite-size particles. The particle volume fraction is around 10\% and the particle-to-fluid density ratio is of the order of one. Conditional statistics as a function of distance from the moving particle surfaces are studied in detail, and are used to help interpret global turbulence modulation by particles. Grid convergence of these conditional statistics will be discussed. [Preview Abstract] |
Sunday, November 22, 2015 8:13AM - 8:26AM |
A4.00002: Modulation to the compressible homogenous turbulence by heavy point particles: Effect of particles' density Zhenhua Xia, Yipeng Shi, Shiyi Chen In this paper, two-way interactions between heavy point particles and forced compressible homogenous turbulence are simulated by using a localized artificial diffusivity scheme and an Eulerian-Lagrangian approach. The initial turbulent Mach number is around 1.0 and the Taylor Reynolds number is around 110. Seven different simulations of 10$^{\mathrm{6}}$ particles with different particle densities (or Stokes number) are considered. The statistics of the compressible turbulence, such as the turbulence Mach number, kinetic energy, dilatation, and the kinetic energy spectra, from different simulations are compared with each other, and with the one-way undisturbed case. Our results show that the turbulence is suppressed if the two-way coupling backward interactions are considered, and the effect is more obvious if the density of particles is higher. The kinetic energy spectrum at larger Stokes number (higher density) exhibits a reduction at low wave numbers and an augmentation at high wave numbers, which is similar to those obtained in incompressible cases. The probability density functions of dilatation, and normal upstream Mach number of shocklets also show that the modulation to the shocklet statistics is more apparent for particles with higher density. [Preview Abstract] |
Sunday, November 22, 2015 8:26AM - 8:39AM |
A4.00003: Particle-Resolved Direct-Numerical Simulation of Turbulent Particle-Laden Flows Using Unstructured Overset Meshes Wyatt Horne, Krishnan Mahesh Particle-laden turbulent flows involve a large range of length scales, ranging from the larger convective length scales down to length scales smaller than particle size. We develop a particle-resolved direct-numerical simulation (PR-DNS) method to enable the accurate study of the physics of particle-laden flow at particle length scales. Unstructured meshes are attached directly to particle surfaces and to the background flow field. The different meshes are allowed to arbitrarily overlap with each other to create a single cohesive solution. A dynamic connectivity procedure is used that cuts solid bodies out of each mesh and establishes interpolation pairs between overlapping meshes. The flow is incompressible, and the numerical method is based on that developed by Mahesh et al. [J. Comput. Phys. (2004) 197:215-240]. Several cases are presented illustrating the method's efficacy for studying particle-laden flow. Included are cases featuring freely moving particles within turbulent fluid flow. [Preview Abstract] |
Sunday, November 22, 2015 8:39AM - 8:52AM |
A4.00004: Effects of finite-size particles on the turbulent flows in a square duct Zhaosheng Yu, Zhaowu Lin, Xueming Shao, Lian-Ping Wang Fully resolved numerical simulations of the particle-laden turbulent flows in a square duct are performed with a direct-forcing fictitious domain method. The effects of the finite-size particles on the mean and root-mean-square (RMS) velocities are investigated at the friction Reynolds number of 150 (based on the friction velocity and half duct width) and the particle volume fractions ranging from 0.78\% to 7.07\%. For the neutrally buoyant case, our results show that the mean secondary flow is enhanced and its circulation center shifts closer to the center of the duct cross-section when the particles are added. The reason for the particle effect on the mean secondary flow is analyzed by examining the terms in the mean streamwise vorticity equation. The particles enhance the wall-tangential component of the RMS velocity (i.e. Reynolds normal stress) more than its wall-normal component in the near-wall region near the corners, resulting in the enhancement in the gradients of the normal stress difference, which we think is mainly responsible for the enhancement in the mean secondary flow. The particles accumulate preferentially in the near-corner region in the neutrally buoyant case. In addition, the effects of particle sedimentation are examined at different Shields numbers. [Preview Abstract] |
Sunday, November 22, 2015 8:52AM - 9:05AM |
A4.00005: ABSTRACT WITHDRAWN |
Sunday, November 22, 2015 9:05AM - 9:18AM |
A4.00006: Near-wall turbulence modification by small, heavy particles in a horizontal channel flow Junghoon Lee, Changhoon Lee Near-wall turbulence modification by particles in a horizontal channel flow is investigated via direct numerical simulation coupled with a point-force approximation for small, heavy particles with a diameter smaller than the Kolmogorov length scale of the flow. The Stokes numbers considered are 0.72, 0.81 and 5.3 in wall units and the Froude number is much smaller than 1, indicating that the influence of gravity on particle motion is strong. Particle-particle collisions are not taken into account to focus on the interactions of particles with turbulence. Water droplets in air turbulence are considered. When a particle touches the wall, it is removed, and a new particle is introduced at a random location, with a velocity identical to the fluid velocity at the new position, maintaining a constant particle mass loading. It is shown that the turbulence intensities are enhanced near the bottom wall and reduced in the outer flow region due to the presence of particles, consistent qualitatively with the previous experimental observation by Li et al. (2012). The root-mean-squared vorticity, turbulence production and viscous dissipation are modified in a similar manner to the turbulence intensities. The physical mechanisms responsible for this turbulence modification behavior are discussed by examining the modification of coherent turbulent structures. [Preview Abstract] |
Sunday, November 22, 2015 9:18AM - 9:31AM |
A4.00007: ABSTRACT WITHDRAWN |
Sunday, November 22, 2015 9:31AM - 9:44AM |
A4.00008: Numerical flow Simulation around a flat plate during heavy rainfall using Lagrangian Eulerian approach Gaurav Dhir, Sawan Suman Experimental evidence shows that aircrafts operating under heavy rainfall conditions face deterioration of lift and increase in drag. This scenario can be a critical design challenge especially for slow moving vehicles such as airships. Effective roughening of airfoil surface caused by an uneven water film, loss of flow momentum and the loss of vehicle momentum due to its collision with the raindrops are the primary reasons causing the drag to increase. Our work focuses primarily on the numerical quantification of boundary layer momentum loss caused due to raindrops. The collision of raindrops with a solid surface leads to formation of an ejecta fog of splashed back droplets with their sizes being of the order of micrometers and their acceleration leads to boundary layer momentum loss. We model the airflow within a flat plate boundary layer using a Lagrangian-Eulerian approach with the raindrops being considered as non-deformable, non-spinning and non-interacting droplets. We employ an inter-phase coupling term to account for the interaction between the boundary layer flow and the droplets. Our presentation will focus on several comparisons (velocity field, lift and drag at various angles of attack) with the results of the standard (rain-free) Prandtl boundary layer flow. [Preview Abstract] |
Sunday, November 22, 2015 9:44AM - 9:57AM |
A4.00009: Particle-induced influences on the spectral TKE budget in wall-bounded turbulence David Richter The phenomenon of turbulence modification in wall-bounded turbulence has been studied extensively in both numerical and experimental contexts, and results from a complex interaction between particle dynamics and turbulent motions. An important question which remains unresolved, however, is the scale over which small particles modify the turbulence, how this is a function of the particle inertia relative to flow timescales, and how this potentially upscale influence should be modeled in subgrid or Eulerian-based models. In this work, data from direct numerical simulations of turbulent planar Couette flow, two-way coupled with Lagrangian point particles, are used to compute spectral energy budgets at several Reynolds (friction Reynolds numbers up to 900) and Stokes (St = [1, 10, 100] based on the centerline Kolmogorov scale) numbers in order to understand the spectral extent of the particle influence on turbulence. It will be shown in this talk that particles modify the surrounding turbulence in two distinct but related ways: (1) through a direct energy source/sink, which is highly wavenumber and Stokes number dependent and (2) through a potentially severe reduction in TKE production across all spatial scales. [Preview Abstract] |
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