Bulletin of the American Physical Society
69th Annual Meeting of the APS Division of Fluid Dynamics
Volume 61, Number 20
Sunday–Tuesday, November 20–22, 2016; Portland, Oregon
Session R28: Particle-laden Flows: Particle-Turbulence Interaction II |
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Chair: Laura Villafane, Stanford University Room: F149 |
Tuesday, November 22, 2016 1:30PM - 1:43PM |
R28.00001: Radiative heating of inertial particles in a turbulent square duct flow Andrew Banko, Laura Villafane, Christopher Elkins, John Eaton The coupled dynamics of small inertial particles, turbulence, and radiative heating is examined experimentally. A vertically downward airflow with Reynolds number of order 10,000 is laden with disperse Nickel particles which are smaller than all flow length scales. The particles have Stokes numbers of order 10 and the thermal time constant is similar to the aerodynamic time constant. This particle-air mixture is exposed to monochromatic near infrared radiation through one wall of the duct. While the gas and walls are nearly transparent to the incident radiation, the particles absorb energy and heat the gas with a spatial distribution dependent on the particle concentrations. The mass loading ratio of particles is varied in order to study the effect of increasing optical depth on the gas temperature rise. A fine wire thermocouple is used to measure the mean gas temperature variation along the full width of the duct, including the near wall region where particle concentrations mildly increase. Total energy absorption is inferred from measurements of transmitted light intensity. Comparisons are made to a 1-D model which assumes homogeneity of all flow quantities, low optical depth, and ignores preferential concentration. [Preview Abstract] |
Tuesday, November 22, 2016 1:43PM - 1:56PM |
R28.00002: Sensitivity of inertial particle response on turbulent duct flows to mass loading ratio and Reynolds number. Laura Villafane, Andrew Banko, Chris Elkins, John Eaton The momentum coupled dynamics of particles and turbulence are experimentally investigated in a vertical fully developed turbulent square duct flow of air laden with Nickel particles. Significant preferential concentration is present for the Stokes numbers investigated, which vary from 3 to 30 based on the Kolmogorov time scale. Higher order measures of preferential concentration, such as the sizes and shapes of clusters and voids, are analyzed for increasing mass loading ratios. The mass loadings chosen span the one-way and two-way coupled regimes, while the volume loading is kept low. The effect of Stokes number and mass loading is also evaluated for particle velocity statistics and compared to the unladen gas statistics. Planar laser scattering is used to record instantaneous particle images in the center of the duct. Preferential concentration statistics are computed from box counting and Voronoi tessellation algorithms. PIV and PTV techniques are used to calculate particle velocity statistics. The analysis is extended to the near wall region in the logarithmic layer for the case of low mass loading. These results are compared to those from the duct center to assess the effects of strong carrier phase inhomogeneity on the particle distributions. [Preview Abstract] |
Tuesday, November 22, 2016 1:56PM - 2:09PM |
R28.00003: Snowflakes as inertial particles in turbulence Filippo Coletti, Andras Nemes, Teja Dasari, Jiarong Hong, Michele Guala We report on the first direct measurements of trajectories and settling velocity of snow particles in the atmospheric surface layer. During a nocturnal snowfall we deploy an imaging system consisting of a searchlight and high speed cameras to illuminate and track thousands of snowflakes over a 7 m by 4 m vertical plane. We simultaneously characterize their shape and size using digital holography, while recording the air turbulence properties via sonic anemometry. We show that, in the meteorological conditions in object, the snowflake motion exhibits hallmark features identified by fundamental studies of particle-laden turbulence in both the Lagrangian and the Eulerian framework. The acceleration distribution displays stretched exponential tails, and by comparing with previous laboratory and computational studies we infer the Stokes number and aerodynamic response time of the snowflakes. The fall speed is found to be much greater than the expected value in still air, indicating that turbulence enhances settling according to the preferential sweeping mechanism. These observations demonstrate the major role of turbulence in determining the snow fall speed, and create the basis for leveraging results from particle-laden turbulence research towards improved snow precipitation models. [Preview Abstract] |
Tuesday, November 22, 2016 2:09PM - 2:22PM |
R28.00004: Marine particle aggregate breakup in turbulent flows Matthew Rau, Steven Ackleson, Geoffrey Smith The dynamics of marine particle aggregate formation and breakup due to turbulence is studied experimentally. Aggregates of clay particles, initially in a quiescent aggregation tank, are subjected to fully developed turbulent pipe flow at Reynolds numbers of up to 25,000. This flow arrangement simulates the exposure of marine aggregates in coastal waters to a sudden turbulent event. Particle size distributions are measured by in-situ sampling of the small-angle forward volume scattering function and the volume concentration of the suspended particulate matter is quantified through light attenuation measurements. Results are compared to measurements conducted under laminar and turbulent flow conditions. At low shear rates, larger sized particles indicate that aggregation initially governs the particle dynamics. Breakup is observed when large aggregates are exposed to the highest levels of shear in the experiment. Models describing the aggregation and breakup rates of marine particles due to turbulence are evaluated with the population balance equation and results from the simulation and experiment are compared. Additional model development will more accurately describe aggregation dynamics for remote sensing applications in turbulent marine environments. [Preview Abstract] |
Tuesday, November 22, 2016 2:22PM - 2:35PM |
R28.00005: Turbulence Modulation and Particle Segregation in a Turbulent Channel Flow Kee Onn Fong, Mostafa Toloui, Omid Amili, Jiarong Hong, Filippo Coletti Particle-laden flows are ubiquitous in biological, environmental, and engineering flows, but our understanding of the mechanism by which particles modulate turbulence is incomplete. Simulations involve a wide range of scales, and shall be corroborated by measurements that reconstruct the motion of both the continuous and dispersed phases. We present experimental observations on the interaction between inertial particles and turbulent flow through a vertical channel in two-way coupled regime. The working fluid is air laden with size-selected glass particles, which we investigate by planar particle image velocimetry and digital inline holography. Unlike most previous experiments, we focus on a regime in which particle segregation and turbulence modulation are both strong. PIV shows that turbulence modulation is especially pronounced near the wall, where particles accumulate by turbophoresis. The segregation, however, is much weaker than what suggested by one-way coupled simulations. Results from digital holography confirm the trends in particle concentration and velocities, and additionally provide information on the three-dimensional clustering. The findings are compared to previous investigations and discussed in the context of modeling strategies. [Preview Abstract] |
Tuesday, November 22, 2016 2:35PM - 2:48PM |
R28.00006: Effects of solid inertial particles on the velocity and temperature statistics of wall bounded turbulent flow Bamdad Lessani, Hadi Nakhaei The effect of solid inertial particles on the velocity and temperature statistics of a non-isothermal turbulent channel flow is studied using direct numerical simulation.A two-way coupled Eulerian-Lagrangian approach is adopted.Three different particle Stokes numbers of St= 25, 60, 200, at a constant particle mass loading of $\phi_m = 0.57$, are considered. The variations of different budget terms for the turbulent kinetic energy equation and fluctuating temperature variance equation in the presence of particles are reported. It is shown that the near wall dissipation and viscous transport terms are larger for St = 25 particles compared to the ones of higher inertia particles (St = 60, 200). The same behavior is observed for the dissipation and viscous transport terms of the fluctuating temperature variance equation. The fluid turbulent heat flux is also reduced by the presence of particles, but as a result of fluid-particle heat exchange, the total heat transfer rate stays always higher for particle-laden flow even for the largest particles considered. The total Nusselt number is split into a turbulence contribution and a particle contribution, and the effects of particles inertia on fluid turbulent heat flux and fluid-particle heat transfer are examined. [Preview Abstract] |
Tuesday, November 22, 2016 2:48PM - 3:01PM |
R28.00007: Momentum transfer and particle stress in polydisperse, particle-laden flow David Richter, Omar Garcia, Christopher Astephen Direct numerical simulations are performed in combination with two-way coupled Lagrangian point particles to study the effects of polydispersity on particle-induced modifications to momentum transfer in turbulent wall-bounded flow. Turbulent Couette flow is chosen as an idealized testbed for this purpose since total momentum flux is uniform in the wall-normal direction. Monodisperse simulations are first used to characterize momentum flux modification and particle stress as a function of particle Stokes number, and from this understanding bidisperse and continuously polydisperse mixtures of particle Stokes number are simulated. A simple model is then constructed to predict the total particle stress of these particle mixtures. While in the dilute limit particle stresses are nearly linearly additive, the entire mixture cannot simply be modeled by a single monodisperse particle with an effective Stokes number. [Preview Abstract] |
Tuesday, November 22, 2016 3:01PM - 3:14PM |
R28.00008: Cell structures caused by settling particles in turbulent Rayleigh-Bénard convection Changhoon Lee, Sangro Park Turbulent thermal convection is an important phenomenon frequently found in nature and industrial processes, often with laden particles. In the last several decades, the vast majority of studies have addressed single phase convective flow with focus on the scaling relation of flow parameters associated with heat transfer. Particle-laden Rayleigh-Bénard convection, however, has not been sufficiently studied. In this study, modulation of cell structures by settling particles in turbulent Rayleigh-Bénard convection in a doubly periodic square channel is investigated using direct numerical simulation with a point particle approach. Flow parameters are fixed at Rayleigh number=$10^6$, Prandtl number=0.7, the aspect ratio=6, and Froude number=0.19. We report from the simulations that settling heavy particles modulate irregular large-scale thermal plume structures into organized polygonal cell structures. Different shapes of flow structures are obtained for different particle diameters and mass loadings. We found that polygonal cell structures arise due to asymmetric feedback force exerted by particles onto hot and cold plumes. Increasing the number of particles augments the asymmetry and the polygonal cell structures become smaller, eventually going to the hexagonal structures. [Preview Abstract] |
Tuesday, November 22, 2016 3:14PM - 3:27PM |
R28.00009: The effect of collision, Stokes and Reynolds numbers on turbophoresis Mahdi Esmaily-Moghadam, Ali Mani Migration of inertial particles toward solid boundaries in turbulent flows is known as turbophoresis. In this study, we investigate the effect of various parameters on turbophoresis through direct numerical simulations of turbulent flow laden with Lagrangian point-particles. We consider a flow of air in a square duct at a bulk Reynolds number of 5,000 to 20,000 dispersed with nickel particles ranging in size from 4 to 16 micron in diameter. We examine the effect of the Stokes and Reynolds numbers on the near-wall particle concentration and its relationship to the turbophoretic velocity. Our results are consistent with the previously published results pertaining to the saturation of the turbophoretic velocity for Stokes numbers larger than ~10. Adopting a hard sphere collision model, we examine the role of collisions on the near wall concentration and demonstrate the sensitivity of the results to the restitution coefficient. Our findings show that while reducing the restitution coefficient leads to a higher degree of turbophoresis; collision can decrease the near wall concentration by orders of magnitude for a global particle volume fraction of $O(10^{-5})$. [Preview Abstract] |
Tuesday, November 22, 2016 3:27PM - 3:40PM |
R28.00010: Direct numerical simulation of powder electrification in a turbulent channel flow Holger Grosshans, Miltiadis Papalexandris Particle electrification is often encountered in process industries. Sometimes it has useful applications, such as the control of particle trajectories through an electric field. In other situations is has negative effects. For example, during pneumatic transport it can cause particle deposition or, even worse, spark discharges and subsequent fires and explosions. Despite its frequent occurrence, due to the complexity of the underlying physical mechanisms, there are still many open questions regarding particle electrification and inconsistent theoretical predictions have been reported. The objective of our work is to gain a better understanding and physical insight of this phenomenon. To this end, we performed Direct Numerical Simulations to analyze the turbulent flow of a carrier fluid with immersed particles in a channel. Moreover, the motion of the particles was computed in a Lagrangian framework and dynamic models accounting for the particle-wall and particle-particle charge exchange were implemented. In our talk, we discuss in detail the effect of the fluid turbulence to the build-up of the electrostatic charge of the particles. Furthermore, we elaborate on the influence of the particle Stokes number and gravitational forces to the process of powder charging. [Preview Abstract] |
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