Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session GS: Turbulent Mixing |
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Chair: Julio Soria, Monash University Room: Hilton Chicago Stevens 4 |
Monday, November 21, 2005 10:34AM - 10:47AM |
GS.00001: Flow turbulence characterization in a large scale bubble plume facility Carlos Garcia, Marcelo Garcia The objective of the study consists of characterizing the flow turbulence in a large scale bubble plume facility based on a set of water velocity measurements conducted in a large tank (diameter = 15m) at a wastewater treatment plant. The tank was filled to a depth of 7m above the diffuser and six different air discharges were analyzed. Six acoustic Doppler velocimeter probes were placed on a down rod and arranged vertically at different distances above the diffuser. Data were collected during a period of 20 minutes at each of the more than ten radial locations. A hydrodynamic analysis of the generated bubble plume was performed first in order to characterize the frequency of the swinging motion of the plume. This information was used to remove this low frequency oscillation component by filtering the recorded velocity signals. Turbulence parameters such as turbulent kinetic energy, turbulence time and length scales, and dissipation rate of turbulent kinetic energy were then computed based on the processed signals. The results help to understand the bubble phenomenon and provide basis for the validation of numerical models of bubble plume systems used in the design of combined-sewer-overflow reservoirs. These reservoirs are being built by the Metropolitan Water Reclamation District of Greater Chicago as part of the Chicago land Tunnel and Reservoir Plan. [Preview Abstract] |
Monday, November 21, 2005 10:47AM - 11:00AM |
GS.00002: Experimental investigations of buoyancy effects in turbulent jet mixing L.K. Su, D.B. Helmer The effect of buoyancy on global turbulent jet scaling is well known, but its effect on mixing is less well understood. Here, planar laser imaging methods provide measurements of scalar and velocity fields (non-simultaneously) in turbulent, buoyant helium-air jets. Jet Reynolds numbers range up to 3000, and the measurements focus on the region of the flow where the global scaling is momentum-dominated, to isolate the local effects of buoyancy. Results indicate changes in scalar similarity profiles, for example, in comparison with neutrally buoyant flows. Other points of interest include the effects of buoyancy on flow organization, intermittency, entrainment, and scale interactions. [Preview Abstract] |
Monday, November 21, 2005 11:00AM - 11:13AM |
GS.00003: Bounds on stratified mixing with a mixing coefficient constraint Wenbo Tang, C.P. Caulfield, Rich Kerswell We derive non-trivial upper bounds for the long-time averaged vertical buoyancy flux ${\cal B}^* \!:=\! \langle \rho u_3 \rangle g/\rho_{0} $ for stably stratified Couette flow, with reference density $\rho_0$, kinematic viscosity $\nu$, thermal diffusivity $\kappa$, plate separation $d$, driven by constant relative velocity $\Delta U$, maintained at a statically stable density difference $\Delta \rho$. We numerically solve the variational problem using the ``background method'', and require that the mean flow is streamwise independent and statistically steady. We impose a coupling constraint such that a fixed fraction $\Gamma_c$ of the energy input into the system leads to enhanced irreversible mixing. We calculate the bound up to asymptotically large Reynolds numbers for a range of choices of $\Gamma_c$ and bulk Richardson numbers $J$. For any $Re$, the calculated upper bound increases with $J$, until a maximum possible value $J_{\max}(Re,\Gamma_c)$ at which the new constraint cannot be imposed, and the density field and velocity field become decoupled. The value of the bound at $J_ {\max}$ is a non-monotonic function of $\Gamma_c$, with $\Gamma_c=1/2$ leading to the largest possible values as $Re \! \to\! \infty$, consistently with the findings in Caulfield, Tang \& Plasting (2004) where this coupling constraint was not imposed. In fact, at any particular $Re$, the previous solution may be associated with a specific value of $\Gamma_c$. Imposing the coupling constraint with that $\Gamma_c$, as $J \! \to\! J_{\max}$, the new bound approaches from below the previous bound exactly. [Preview Abstract] |
Monday, November 21, 2005 11:13AM - 11:26AM |
GS.00004: Turbulence generation behind orificed, perforated plate Arindam Singha, Rui Liu, David S.-K. Ting There were many investigations on grid generated turbulence, but almost all of them concentrate on the decaying region of turbulence, at a considerable distance from the grid. The investigation at the region just behind the grid, where the ``grid shadow'' is still visible, is limited. In this region, considerable inhomogeniety and anisotropy of turbulence flow field is expected. Past studies show that the identification of the point of onset of decaying turbulence is important in unification of the theory of turbulence. This present study aims to focus on the different aspects of identification of the starting position of the decaying turbulence region. In the present study, the turbulence characteristics in the region behind an orificed, perforated plate are explored. Orificed, perforated plate is used instead of conventional grid, as it is expected to produce more isotropic turbulence due to the absence of the effects of finite grid thickness. The experiments are carried out at freestream Reynolds number of 12700 in a closed circuit wind tunnel of 0.75$\times $0.75 m$^{2}$ working cross section. The measurement plane is chosen as a square plane of 0.25$\times $0.25 m$^{2}$ at the central portion of the test section at streamwise distances of 10$M$, 15$M$ and 20$M$ respectively from the perforated plate, where $M$ is the mesh characteristic dimension. The turbulence flow field is interpreted with the help of the theory of isotropic turbulence. [Preview Abstract] |
Monday, November 21, 2005 11:26AM - 11:39AM |
GS.00005: 2D multiple-scale flow generated by fractal electromagnetic forcing Lionel Rossi, John Christos Vassilicos, Yannis Hardalupas We generate a class of multi-scale quasi-steady laminar flows in the laboratory by controlling a quasi-two-dimensional shallow layer brine flow by multi-scale Lorentz body forcing. The flows' multi-scale topology is invariant over a broad range of Reynolds numbers, $Re_{2D}$ from 600 to 9900. Our multi-scale flows have a power-law energy spectrum $E(k)\sim k^{-p}$ with $p=2.5$ in agreement with the formula $p+D_{s}=3$ of Davila \& Vassilicos (PRL, 2003) where $D_s \approx 0.5$ is the fractal dimension of the set of stagnation points. The exponents $D_s$ and $p$ are controlled by the multi-scale electromagnetic forcing over the entire range of scales. The pair dispersion properties are also controlled by their multi-scale hyperbolic stagnation point topology which generates a sequence of exponential separation processes starting from the smaller scale hyperbolic points and ending with the larger ones. The average mean square separation $\overline{\Delta^{2}}$ has an approximate power law behaviour $\sim t^{\gamma}$ with ``Richardson exponent'' $\gamma \approx 2.45$ in the range of time scales controlled by the hyperbolic stagnation points. This exponent $\gamma = 3-D_{s}$ is itself controlled by the multi-scale quasi-steady hyperbolic stagnation point topology of the flow. [Preview Abstract] |
Monday, November 21, 2005 11:39AM - 11:52AM |
GS.00006: Turbulence transport in the presence of a macroscale gradient Daniela Tordella, Michele Iovieno In the absence of kinetic energy production, the intermediate asymptotics of the turbulent shear-free mixing layer is considered. By means of a theoretical analysis based on the use of the two-point correlation equations and their intermediate-similarity solutions, we examine the following two results obtained via numerical experiments on the interaction between different decaying homogeneous and isotropic turbulences: \begin{enumerate} \item mixing is highly intermittent for shear-free decaying homogeneous isotropic interacting flows with kinetic-energy ratios far from unity in contrast to a Gaussian asymptotic state, and \item the intermittency increases/decreases when the kinetic-energy gradients and integral-scale gradients are aligned/opposite. \end{enumerate} [Preview Abstract] |
Monday, November 21, 2005 11:52AM - 12:05PM |
GS.00007: Modeling and Simulation of Fluid Mixing for Laser Experiments and Supernova Yongmin Zhang, James Glimm, R. Paul Drake, Srabasti Dutta, John Grove, David Sharp Recently, laboratory astrophysics has been playing an important role in the study of astrophysical systems, especially in the case of supernova explosions through the creation of scaled reproductions of astrophysical systems in the laboratory. In collaboration with a team centered at U. Michigan and LLNL, we have conducted front tracking simulations for axisymmetrically perturbed spherical explosions relevant to supernovae as performed on NOVA laser experiments, with excellent agreement with experiments. We have extended the algorithm and its physical basis for preshock interface evolution due to radiation preheat. The preheat simulations motivate direct experimental measurements of preheat as part of any complete study of shock-driven instabilities by such experimental methods. Our second focus is to study turbulent combustion in a type Ia supernova (SN Ia) which is driven by Rayleigh-Taylor mixing. We have extended our front tracking to allow modeling of a reactive front in SN Ia. Our 2d axisymmetric simulations show a successful level of burning. Our front model contains no adjustable parameters so that variations of the explosion outcome can be linked directly to changes in the initial conditions. [Preview Abstract] |
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