Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session CA: Turbulent Mixing II |
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Chair: Nicholas Ouellette, Yale University Room: Long Beach Convention Center 101A |
Sunday, November 21, 2010 1:00PM - 1:13PM |
CA.00001: ABSTRACT WITHDRAWN |
Sunday, November 21, 2010 1:13PM - 1:26PM |
CA.00002: ABSTRACT WITHDRAWN |
Sunday, November 21, 2010 1:26PM - 1:39PM |
CA.00003: Simultaneous velocity-temperature measurements in the heated wake of a cylinder with application to the modeling of turbulent passive scalars Arpi Berajeklian, Laurent Mydlarski The principal objective of this work is to study the sensitivity of (i) the turbulent Prandtl number ($Pr_T$), and (ii) the mechanical-to-thermal time-scale ratio ($r$) to differences in the scalar field's injection method within the same (hydrodynamic) flow. Both are recurring quantities employed in turbulence models, determined from experiments, and generally assumed to be (flow-dependent) constants. To this end, mixed velocity-temperature measurements were made in the heated wake of a circular cylinder. The passive scalar under consideration was temperature and the wake was heated by one of two ways: heating the cylinder itself, or by use of a mandoline placed downstream of the cylinder. For each case, the distributions of the turbulent Prandtl number and the mechanical-to-thermal time-scale ratio were compared. The experimental results demonstrate that both $Pr_{T}$ and $r$ differ for the two scalar injection methods (in addition to varying across the wake). Hence, both $Pr_{T}$ and $r$ not only depend on the type of flow, but on the scalar field injection method as well - a result that is generally not take into account when turbulent flows are modeled. [Preview Abstract] |
Sunday, November 21, 2010 1:39PM - 1:52PM |
CA.00004: Experiments and simulations of passive scalars released from concentrated sources in turbulent channel flow Emmanuel Germaine, Luca Cortelezzi, Laurent Mydlarski Turbulent mixing of a passive scalar ($\theta$) is studied by means of experiments and numerical simulations in turbulent channel flow, with an emphasis on the scalar dissipation rate ($\varepsilon_{\theta}$). The scalar (temperature) is injected at small scales by a heated line source, aligned in the spanwise direction. The present experiments focus on the evolution of $\varepsilon_{\theta}$ downstream of the line source, for different wall-normal source locations. In particular, knowledge of the different components of $\varepsilon_{\theta}$ (i.e., $\alpha \langle (\partial \theta / \partial x)^2 \rangle$, $\alpha \langle (\partial \theta / \partial y)^2 \rangle$, and $\alpha \langle (\partial \theta / \partial z)^2 \rangle$, where $\alpha$ is the thermal diffusivity) enable the quantification of the small-scale passive scalar statistics, and their (presumed) return to isotropy from an initially anisotropic injection. Measurements of temperature derivatives were performed by means of cold-wire thermometry. A direct numerical simulation was also undertaken to provide complementary data, difficult to obtain experimentally. The velocity field was independently computed using the freely-available channel flow code of Dr. John Gibson (http://www.channelflow.org). The advection-diffusion equation was solved using a third-order scheme with the flux integral method (Leonard \emph{et al.}, Appl. Math. Modelling, 1995). [Preview Abstract] |
Sunday, November 21, 2010 1:52PM - 2:05PM |
CA.00005: Phase relaxation of a cloud water droplet ensemble undergoing turbulent mixing Bipin Kumar, Raymond A. Shaw, Joerg Schumacher The understanding of the entrainment and mixing of clear (subsaturated) with cloudy air at the boundary of a cloud is still far from being complete. Mixing is determined by the ratio of two time scales: the mixing time and the phase relaxation time, which can be combined as a Damk\"ohler number. The phase relaxation time is connected with the water phase change and thus changes in the cloud water droplet size distribution and their number density. The mixing time of the advecting turbulent flow is determined by the size and velocity of the turbulent eddies. Here, we will outline a direct numerical simulation model that couples the Eulerian description of the velocity and water vapor fields with a Lagrangian ensemble of cloud water droplets. The simulations resolve a small cubic fraction of the cloud and simulate a homogeneous isotropic turbulent flow. Turbulence properties at larger scales are taken from field measurements of the helicopter-based measurement platform ACTOS. Cloud water droplets can grow and shrink, as determined by the advected vapor concentration field that sets the local supersaturation at the droplet position. First results of our direct numerical simulations are presented. [Preview Abstract] |
Sunday, November 21, 2010 2:05PM - 2:18PM |
CA.00006: Direct Computation of Two- and Three-dimensional FTLE/LCS from Particle Tracking Velocimetry Data Samuel Raben, Rod La Foy, Shane Ross, Pavlos Vlachos Finite-Time Lyapunov Exponents (FTLEs) and Lagrangian Coherent Structures (LCSs) are becoming more commonly utilized for the interpretation of unsteady experimental flow fields. FTLEs provide information on regions of high attraction, repulsion, and shear in a flow field and can be used to investigate transport and mixing. Elevated values in FTLE fields, or ridges, can be evaluated in time and are what are referred to as LCS. In order to compute the FTLE field from velocity fields, typically artificial particles are seeded into the field and then numerically integrated to find positions in time from the given velocity information. This process can be very computationally expensive. When dealing with experimental data such as PIV or PTV it is possible to decrease the computational cost by simply tracking the flow tracers already present in the flow, avoiding the additional steps of inferring the velocity and artificial particle seeding. Through the use of Lagrangian particle tracking this work finds that it is more computationally efficient, as well as more accurate, to calculate FTLEs this way. This work considers both 2D as well as 3D flow fields for this analysis. [Preview Abstract] |
Sunday, November 21, 2010 2:18PM - 2:31PM |
CA.00007: Flow around finite-size neutrally buoyant Lagrangian particles in fully developed turbulence Mathieu Gibert, Simon Klein, Antoine B\'erut, Eberhard Bodenschatz By using an innovating technique based on Lagrangian Particle Tracking (LPT), we have been able to follow the motion of finite-size neutrally buoyant particles together with the trajectories of tracer particles in the surrounding fluid. The particles we study have diameters of about 200 times the dissipative scale of the flow, and their density is almost that of the fluid. The experiments are conduced in a von Karman swirling water flow at Taylor microscale Reynolds numbers up to 500. By measuring the full motion of the big particles (translation and rotation), we are able to ``sit'' in their frame of reference and measure the flow properties around them. We will report experimental results on the flow properties and its correlations with the big particle trajectories in this Lagrangian frame. [Preview Abstract] |
Sunday, November 21, 2010 2:31PM - 2:44PM |
CA.00008: Measurements of 3D velocity and scalar field for a film-cooled airfoil trailing edge Michael Benson, Christopher Elkins, John Eaton Turbine blade tips commonly are cooled by venting air through slots upstream of the trailing edge. The effectiveness of this approach is governed by the rate of mixing of the coolant with the mainstream flow, which is strongly under-predicted by conventional RANS models. Experiments were conducted for a simple airfoil with a modern trailing edge cooling geometry. The full 3D coolant concentration distribution was measured using Magnetic Resonance Imaging (MRI). The scans measured the concentration distribution with a spatial resolution of 0.5 mm$^{3}$ and an uncertainty near 5{\%}. Magnetic Resonance Velocimetry (MRV) was used to provide 3D, mean velocity measurements in the identical flow. Blowing ratios of 1.0, 1.3, and 1.5 were examined at Reynolds numbers of 50,000 and 100,000 based on airfoil chord length. The coupled concentration and velocity measurements were used to develop a qualitative picture of the flow structures contributing to the rapid mixing. Surface concentration measurements provide film cooling effectiveness data, which were compared for validation purposes with traditional thermal measurements. The MRI-based technique for measuring film cooling effectiveness avoids the large uncertainties caused by conduction in the thermal tests. [Preview Abstract] |
Sunday, November 21, 2010 2:44PM - 2:57PM |
CA.00009: Coupled Velocity and Cooling Effectiveness Measurements of a Film Cooling Hole With Varied Blowing Rates and Ejection Angles Emin Issakhanian, Chris J. Elkins, John K. Eaton Film cooling is used to shield turbine blades from combustion gases which are at temperatures above the melting point of the blade's constituent alloy. Maximizing film cooling effectiveness allows higher combustion temperatures and decreases need for bypass air. The present experiment studies flow through a single film cooling hole jetting into a square channel. The momentum thickness Reynolds number of the main flow is 500. The diameter of the cooling flow is 10 times the momentum thickness at the hole exit. The cooling flow Reynolds number varies between 1250 and 5000. Magnetic Resonance Velocimetry (MRV) and Concentration (MRC) are used to measure mean velocity and coolant concentration of the 3-D field both inside the main channel and inside the cooling hole and feed plenum. By marking only the main flow with a passive scalar, the MRC data allow measurement of cooling flow concentration, which by analogy is related to the temperature of the fluid. The velocity data shows the development of a counter-rotating vortex pair downstream of the jet. These vortices transport cooling flow away from the channel floor resulting in a lifted kidney-shaped coolant cross-section and reduced effectiveness. The varying strength of this flow feature and of surface effectiveness due to different ejection angles and blowing ratios is studied. [Preview Abstract] |
Sunday, November 21, 2010 2:57PM - 3:10PM |
CA.00010: The highly excited confined mixing layer Wei Zhao, Guiren Wang In order to understand the mechanism of ultrafast mixing observed in confined mixing layer in a pipe, flow velocity and vorticity fields are quantitatively measured with PIV. Under strong forcing, the responses of the velocity and vorticity field are completely different, leading to a different mixing process. In free mixing layer, under strong forcing level, there will be saturation, and the initial mixing depends only on the convective scalar transport by velocity fluctuations of large scale spanwise vortex. However in confined mixing layer under strong forcing, it is found that not only the velocity fluctuations play important roles in mixing process, but also the mean velocity field. The local averaged vertical velocity V and spanwise velocity W are not negligible anymore and can be significantly affected by the forcing. These mean velocity constituents have two direct effects: One is to transport scalar outward from the axial line, which leads to a larger spreading rate. Another is to accelerate the breakdown of vortex structures by stretching. Hence, the mixing can be significantly enhanced. From the distributions of velocity, in cross-section, it can be found that both the corner vortex and strongly asymmetric influence of actuating coexist. When the forcing amplitude is increased to 11{\%}, the instant vortex structures near the pipe axis indicate a high consistency with the mean vortex structures. Hence, at high forcing level, we tend to believe that the corner vortex is the primary source of V constituent. [Preview Abstract] |
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