Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session R2: Convection and Buoyancy-Driven Flows VII: Gravitational Effects and Flows Past Moving Bodies |
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Chair: Henry Burridge, University of Cambridge Room: 324 |
Tuesday, November 26, 2013 1:05PM - 1:18PM |
R2.00001: Effect of single silica gel particle adsorption on the transport processes in a humid air stream Apratim Sanyal, Saptarshi Basu, Pramod Kumar The effect of adsorption due to a single silica gel particle on a convective field consisting of humid air has been investigated numerically. The adsorption is incorporated as a sink term in the transport equation for species (water vapor) and has been modeled using Linear Driving Force model, while the heat released due to adsorption is taken as source term in the energy equation and proportional to the amount of water vapor adsorbed. The heat released creates a coupling between the species and the temperature field as the adsorption characteristics are directly influenced by particle temperature. The extent of species and temperature boundary layer show the diffusion of the adsorption effects into the free stream. Surface adsorption is found to decrease with Reynolds no. The particle surface temperature increases from forward stagnation point till downstream. This work provides a model for understanding the adsorption kinetics in convective stream for other adsorbate-adsorbent pair. Further more complex scenarios can be modeled such as presence of multiple adsorbent particles, the interaction of species and temperature boundary layers setup due to individual particles and their influence on the overall adsorption characteristics. [Preview Abstract] |
Tuesday, November 26, 2013 1:18PM - 1:31PM |
R2.00002: The effect of noncondensables on the thermocapillary-buoyancy convection in volatile fluids Tongran Qin, Laura Anfinson, Roman Grigoriev Convection in a liquid layer with free surface subject to a horizontal temperature gradient is one of the classic problems in fluid mechanics and heat transfer. In thicker layers the flow is driven by a combination of buoyancy and thermocapillarity in the liquid layer. Unlike buoyancy, thermocapillarity depends rather sensitively on the boundary conditions at the liquid-vapor interface. In particular, for volatile fluids, convective patterns are found to vary significantly with the composition of the gas phase. The interfacial temperature which defines thermocapillary stresses is controlled by the variation of the vapor concentration along the interface. At ambient conditions when the gas phase is dominated by air, the latter is found to be controlled by diffusion of vapor in (essentially static) air. At the opposite extreme where vapor dominates, we find that it is the diffusion of air in (quickly moving) vapor that controls the concentration and temperature distribution, leading to significant differences in the flow patterns found in these two limits. These findings show that the results of the studies conducted under atmospheric conditions are not applicable in the (near) absence of noncondensables (air) and resolve the disagreement between previous numerical and experimental results in the vapor-dominated limit. [Preview Abstract] |
Tuesday, November 26, 2013 1:31PM - 1:44PM |
R2.00003: Gravity driven current during sessile drop coalescence on a surface Ying Zhang, Samuel Oberdick, Stephen Garoff, Shelley Anna We study the mixing behavior of two sessile drops following coalescence on a flat surface. The surface is composed of silicone elastomer on which the drops exhibit contact angles of about 90 degree. The two drops are of equal volume at coalescence, but different densities and viscosities. Using laser induced fluorescence, we obtain both a top view of the contact line motion and a side view of the cross-sectional flow. During the coalescence stage, the initial healing of the meniscus bridge and damping of capillary waves occur on time scales comparable to the inertio-capillary time. However, the interface between the dyed and undyed components remains sharp, with diffusive mixing occurring at much longer timescales. At intermediate time scales the motion is controlled by a gravity current, which leads to the eventual stratification into two separate horizontal layers within the composite drop. Using lubrication analysis, we characterize the gravity current as a function of the drop sizes, and the density and viscosity differences between the two merging fluids. The numerical solution of the lubrication analysis captures the observed scaling of the time dependent interface movement as a function of fluid and geometric parameters. [Preview Abstract] |
Tuesday, November 26, 2013 1:44PM - 1:57PM |
R2.00004: Long-Lasting Effect of Initial Configuration in Gravitational Spreading of Material Fronts Nadim Zgheib, Thomas Bonometti, S. Balachandar We present results from experiments and simulations pertaining to finite release gravity currents with a non-axisymmetric cross-section. First, we demonstrate that, contrary to expectation, the effects of the initial shape strongly influence the current's evolution well into the self-similar phases. Then we identify the physical mechanisms responsible for this dependence and propose a simple model capable of well capturing the dynamics of such releases. Finally, we show that this dependence on initial configuration is robust for various types of gravity currents (homogeneous and inhomogeneous) over a wide range of parameters such as Reynolds number, density ratio, wall friction and aspect ratio. We additionally inspect the deposition patterns pertaining to the abovementioned inhomogeneous currents. [Preview Abstract] |
Tuesday, November 26, 2013 1:57PM - 2:10PM |
R2.00005: Fountain behaviour from the frequency of fountain-top fluctuation and rise height Henry Burridge, Gary Hunt Focusing on the dynamics of established turbulent axisymmetric fountains in uniform quiescent environments, we present measurements for the frequency of fluctuations at the top of aqueous-saline fountains. Our results span source Froude numbers, $0.3 < Fr_0 < 40$, and clearly indicate four discrete bands of $Fr_0$ within which the dimensionless frequency of fluctuation is constant - the value of the constant changing between each band [Burridge \& Hunt (2013), {J. Fluid Mech.} 728, 91-119]. Our independent measurements of rise height [Burridge \& Hunt (2012),{J. Fluid Mech.} 691, 392-416] show that established fountains can be classified into four bands - within each, the mean rise height exhibits a unique dependence on $Fr_0$. Comparison of the $Fr_0$ bands show close agreement. This suggests that the dominant physics controlling the fountain's behaviour remains unchanged within each band and that monitoring the frequency of fluctuations at the fountain top provides a robust and independent means of classification. Within each class we identify unique time scales for the fluctuations and decompose these time scales into the relevant rise height and velocity scales, thereby demonstrating that the dominant length, time and velocity scales all change at the same $Fr_0$ boundaries. [Preview Abstract] |
Tuesday, November 26, 2013 2:10PM - 2:23PM |
R2.00006: Dynamic Separation on a Pitching and Surging Airfoil as a Model for Flow over Vertical Axis Wind Turbine Blades Reeve Dunne, Beverley McKeon The flow over a pitching and surging NACA 0018 airfoil at a chord Reynolds number of 100,000 is investigated using 2D time resolved particle image velocimetry. Sinusoidal pitch experiments between \(\pm 30^\circ\) at a reduced frequency \(\mathrm{k}=\frac{\Omega \mathrm{c}}{2 \mathrm{U}_\infty}=.12\) closely simulate the unsteady angle of attack experienced by the blade of a representative commercial vertical axis wind turbine (VAWT). The unsteady separation process is analyzed in detail with a focus on development of vorticity at the leading edge. Reduced order modeling techniques are used to deconstruct the flow and identify the evolution of dominant flow structures over the pitching cycle. Surging at the same reduced frequency and \(\frac{\mathrm{U}_{\mathrm{max}}-\mathrm{U}_{\mathrm{min}}}{\mathrm{U}_{\mathrm{mean}}}=.80\) is added to investigate the effect of the Reynolds number variation associated with the rotation of a VAWT blade in a non-rotating, laboratory frame. [Preview Abstract] |
Tuesday, November 26, 2013 2:23PM - 2:36PM |
R2.00007: Coriolis effect on dynamic stall in a vertical axis wind turbine Hsieh-Chen Tsai, Tim Colonius The immersed boundary method is used to simulate the flow around a two-dimensional rotating NACA 0018 airfoil at moderate (sub-scale) Reynolds number in order to investigate separated flow occurring on a vertical-axis wind turbine (VAWT). The influence of dynamic stall on the forces is characterized as a function of tip-speed ratio. The influence of the Coriolis effect is also investigated by comparing the rotating airfoil to one undergoing a surging and pitching motion that produces an equivalent speed and angle-of-attack variation over the cycle. While the Coriolis force produces only small differences in the averaged forces, it plays an important role during dynamic stall. Due to the fact that the Coriolis force deflects the fluid and propagates the vortices differently, the wake-capturing phenomenon of the trailing edge vortex is observed in the flow around the rotating airfoil during a certain range of azimuthal angle. This wake-capturing of the trailing edge vortex leads to a large decrease in lift. However, because of the phase difference between each wake-capturing, there are only small differences in the average forces. The simulations are also compared to results from companion water-tunnel experiments at Caltech. [Preview Abstract] |
Tuesday, November 26, 2013 2:36PM - 2:49PM |
R2.00008: Unsteady vortex dynamics for finite-aspect-ratio pitching wings Ryan Jantzen, Kunihiko Taira, Kenneth Granlund, Michael Ol We examine the vortex dynamics around low-aspect-ratio pitching wings and the corresponding unsteady aerodynamic loading using direct numerical simulations with the immersed boundary projection method. Finite-aspect-ratio wings of two and four with rectangular planforms are considered to pitch about the leading edge, from an angle of attack of $\alpha=0$ to $45^\circ$, over a range of reduced frequencies for a Reynolds number of 300. These parameters are chosen to uncover the influence of fast and slow wing motion on the formation of the wake vortices. We also study the relationship between the vortex dynamics and the unsteady forces exerted on the pitching wing emphasizing the influence from the leading-edge and tip vortices. Companion water tunnel experiments are performed with fluorescent dye visualization and direct force measurements at a Reynolds number of $20,000$, which highlight qualitative similarities between the flow fields despite the large difference in Reynolds numbers. The insights obtained from the numerical and experimental results are used as a foundation for the development of closed-form models to predict aerodynamic forces for wings undergoing large-amplitude maneuvers representative of flapping wing kinematics and wings in gusty operating conditions. [Preview Abstract] |
Tuesday, November 26, 2013 2:49PM - 3:02PM |
R2.00009: Force Coefficients on Surging Rigid and Flexible Wings Peter Mancini, Anya Jones, Kenneth Granlund, Michael Ol This study considers an aspect ratio 4 rigid flat plate and an aspect ratio 4.5 flexible wing, undergoing rectilinear motion in a water tunnel over several chord lengths at a Reynolds number of 20,000. Varying incidence angle, Reynolds number, and acceleration profile led to an extensive parameter study for both wings. Acceleration regions were linear with time and varied with distances of 0.25 to 6.0 chord-lengths. Measurements include lift and drag histories along with flow visualization of leading and trailing edge vortices throughout the entire motion by fluorescent dye injection illuminated by a laser sheet. A non-circulatory bump in lift coefficient at the end of the acceleration region was observed for each rigid wing case. The rigid wing also experienced a significant decrease in lift shortly after the wing reached its terminal velocity. This dip was followed by a second peak in lift around 6 chords traveled for every case, although the magnitudes differed among the acceleration profiles. Conversely, the flexible wing exhibited little to no non-circulatory peak at the end of acceleration and did not experience this dip and rise in lift. This study explores the influence of planform and chordwise flexibility on leading edge vortex formation, retention, and shedding. [Preview Abstract] |
Tuesday, November 26, 2013 3:02PM - 3:15PM |
R2.00010: Unsteady pitching flat plates Kenneth Granlund, Michael Ol, Luis Bernal Direct force measurements and qualitative flow visualization were used to compare flowfield evolution vs. lift and drag for a nominally 2D rigid flat plate executing smoothed linear pitch ramp maneuvers in a water tunnel. Non-dimensional pitch rate was varied from 0.01 to 0.5, incidence angle from 0 to 90 degrees, and pitch pivot point from the leading to the trailing edge. For low pitch rates, the main unsteady effect is delay of stall beyond the steady incidence angle. Shifting the time-base to account for different pivot points leads to collapse of both lift/drag history and flowfield history. For higher rates, a leading edge vortex forms; its history also depends on pitch pivot point, but linear shift in time-base is not successful in collapsing lift/drag history. Instead, a phenomenological algebraic relation, valid at the higher pitch-rates, accounts for lift and drag for different rates and pivot points, through at least 45 degree incidence angle. [Preview Abstract] |
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