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 D21: Turbulence: Mixing |
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Chair: Stavros Tavoularis, University of Ottawa Room: 209 |
Sunday, November 22, 2015 2:10PM - 2:23PM |
D21.00001: Turbulence structure and scalar diffusion in uniformly sheared flow distorted by a grid Stavros Tavoularis, Jovan Nedic Uniformly sheared flow, generated in a wind-tunnel by a shear generator, was let to develop a self-similar, strongly anisotropic turbulence structure and then it was disturbed by grids having square meshes with spacings larger than, comparable to and smaller than the spacing of the shear generator; a "fractal" grid was also used. The multi-scale, non-equilibrium turbulence structure downstream of each grid was documented and differences from the structures of the undisturbed shear flow and grid turbulence were identified. In addition, heat was injected passively from a line source located downstream of the grid and the growth of the heated plume under different conditions was examined. [Preview Abstract] |
Sunday, November 22, 2015 2:23PM - 2:36PM |
D21.00002: Turbulent diffusion from a heated line source in non-equilibrium grid turbulence Jovan Nedic, Stavros Tavoularis We have investigated turbulent diffusion of heat injected passively from a line source in equilibrium and non-equilibrium grid-generated turbulence, which are, respectively, flows in which the value of the non-dimensional rate of kinetic energy dissipation is constant or changes with streamwise distance from the grid. We used three grids with uniform square meshes and one fractal square grid (FSG), all of the same solidity, to generate non-equilibrium and equilibrium turbulence in a wind-tunnel. The regular grids have mesh sizes that are comparable to the first (RG160), second (RG80) and fourth (RG18) iterations of the fractal grid. The heated line source was inserted on the centre-plane of the grids at either of two downstream locations or an upstream one and it spanned the entire width of the wind-tunnel. We found that RG160 produced the greatest heat diffusion, followed by FSG, RG80 and RG18, in this order. The apparent turbulent diffusivity produced by the four grids also decreased in the same order. These findings conform with Taylor’s theory of diffusion by continuous movements. Moreover, the present study demonstrates that the fractal space-scale unfolding (SSU) mechanism does not apply to grids with the same solidity but different effective mesh sizes. [Preview Abstract] |
Sunday, November 22, 2015 2:36PM - 2:49PM |
D21.00003: ABSTRACT WITHDRAWN |
Sunday, November 22, 2015 2:49PM - 3:02PM |
D21.00004: Analysis of Cliff-Ramp Structures in Homogeneous Scalar Turbulence by the Method of Line Segments Michael Gauding, Jens Henrik Goebbert, Norbert Peters, Christian Hasse The local structure of a turbulent scalar field in homogeneous isotropic turbulence is analyzed by direct numerical simulations (DNS). A novel signal decomposition approach is introduced where the signal of the scalar along a straight line is partitioned into segments based on the local extremal points of the scalar field. These segments are then parameterized by the distance between adjacent extremal points and a segment-based gradient. Joint statistics of the length and the segment-based gradient provide novel understanding about the local structure of the turbulent field and particularly about cliff-ramp-like structures. Ramp-like structures are unveiled by the asymmetry of joint distribution functions. Cliff-like structures are further analyzed by conditional statistics and it is shown from DNS that the width of cliffs scales with the Kolmogorov length scale. [Preview Abstract] |
Sunday, November 22, 2015 3:02PM - 3:15PM |
D21.00005: A Simple Parameterization of Mixing of Passive Scalars in Turbulent Flows Ajithshanthar Nithianantham, Karan Venayagamoorthy A practical model for quantifying the turbulent diascalar diffusivity is proposed as $K_s=1.1\gamma'L_T k^{1/2}$ , where $L_T$ is defined as the Thorpe length scale, $k$ is the turbulent kinetic energy and $\gamma'$ is one-half of the mechanical to scalar time scale ratio, which was shown by previous researchers to be approximately $0.7$. The novelty of the proposed model lies in the use of $L_T$, which is a widely used length scale in stably stratified flows (almost exclusively used in oceanography), for quantifying turbulent mixing in unstratified flows. $L_T$ can be readily obtained in the field using a Conductivity, Temperature and Depth (CTD) profiler. The turbulent kinetic energy is mostly contained in the large scales of the flow field and hence can be measured in the field or modeled in numerical simulations. Comparisons using DNS data show remarkably good agreement between the predicted and exact diffusivities. [Preview Abstract] |
Sunday, November 22, 2015 3:15PM - 3:28PM |
D21.00006: Space-scale unfolding mechanism in canonical multi-scale flows Pawel Baj, Paul J.K. Bruce, Oliver R.H. Buxton Some recent studies on fractal generated turbulence revealed a highly increased transverse turbulent scalar flux downstream of fractal grids compared to regular grids. The complexity of these flows makes it impossible to track the origins of this phenomenon, often referred to as the space-scale unfolding mechanism (SSU). Thus research on flows past canonical examples of single and multi-scale obstacles, which are arrays of bars of the same and different thicknesses, was undertaken in order to investigate the SSU's roots. The velocity field and the scalar concentration field were measured simultaneously downstream of the obstacles by means of particle image velocimetry and laser induced fluorescence techniques. It is observed that the concentration field behind the multi-scale obstacle undergoes intense quasi-periodic transverse scalar bursts, which are believed to be the manifestation of the SSU, whereas such events are either weak or absent in the single scale configuration. Investigation of the velocity field reveals a phase locking between wakes of different scale objects in terms of the phase-conditioned transverse integral length scale. Both phenomena are observed to be triggered at the downstream position corresponding to the wakes' intersection point. [Preview Abstract] |
Sunday, November 22, 2015 3:28PM - 3:41PM |
D21.00007: Turbulent mixing by buoyancy-driven flows in long tubes Stuart Dalziel, Liyong Zou We explore the buoyancy-driven turbulent flow that develops due to a change of orientation for a long tube initially filled with a statically stable stratification. For simple orientation histories, the flow may be characterised by the low mixing of a gravity current, the modest mixing of Kelvin-Helmholtz instability, or the much greater mixing of Rayleigh-Taylor instability. However, precise details of the orientation history can prove to be important. We present experimental results from a range of orientation histories, exploring both the temporal development of the flow and the level of mixing achieved. [Preview Abstract] |
Sunday, November 22, 2015 3:41PM - 3:54PM |
D21.00008: Structural Composition and Turbulent Mixing Mechanisms of a Subsonic Boundary Layer Patrick Bechlars, Richard Sandberg Turbulent mixing is a key mechanism for redistributing energy in a wide range of flows. The effect of this mixing on the flow is similar to that of viscous diffusion and the process is therefore often described as turbulent diffusion. Turbulence models based on the Boussinesq approximation rely on the accuracy of the model's description of the mixing to capture the correct energy redistribution. In this presentation the basic mechanism is illustrated using a subsonic turbulent boundary layer (TBL) as a case study, and the direct influence of turbulence on the mean flow is quantified. Through a characteristic analysis the structures involved in the mixing mechanism are identified and further analyzed. The key structures for the mixing in a TBL are large clusters of smaller turbulent structures that are known as large scale motions (LSMs). While the smaller structures are located in the near-wall region they mainly align in the stream-wise direction and pack densely, which affects production and dissipation. Within the LSMs the single vortices reach towards the outer regions and develop an arbitrary alignment as soon as their distance to the wall is sufficiently large. The discussed mechanisms are not limited to TBLs and a comparison to a jet flow is provided in the talk. [Preview Abstract] |
Sunday, November 22, 2015 3:54PM - 4:07PM |
D21.00009: Wave propagation in inhomogeneous media as turbulent mixing in six-dimensional incompressible flow Jaan Kalda, Mihkel Kree Using the approximation of geometrical optics, light propagation in media with fluctuating coefficient of refraction can be described as Hamiltonian dynamics of wave vectors in 6-dimensional phase space where the spatial coordinates are complemented by the respective wave vector components. Hence, according to the Liouville's theorem, the dynamics of the wave front can be described as mixing in an incompressible 6D velocity field. As the wave energy is transferred along the ray trajectories, the energy density fluctuations follow the dilution of the wave front. We use the theory of turbulent mixing to show that the intensity-distribution of speckles (regions of high energy density) follows a power law, and to derive the scaling exponents. If the velocity field were isotropic, these exponents would be determined by the dimensionality of the flow. However, there is a strong anisotropy of the field due to the asymmetry between the spatial and wave vector coordinates. Also, the effective dimensionality of the flow is reduced by one due to the energy (wave frequency) conservation law: any ray trajectory is bound to a five-dimensional manifold within the 6D phase space. Implications of the anisotropy, and of the effective reduction of the dimensionality are studied numerically [Preview Abstract] |
Sunday, November 22, 2015 4:07PM - 4:20PM |
D21.00010: Probability density function of a puff dispersing from the wall of a turbulent channel Quoc Nguyen, Dimitrios Papavassiliou Study of dispersion of passive contaminants in turbulence has proved to be helpful in understanding fundamental heat and mass transfer phenomena. Many simulation and experimental works have been carried out to locate and track motions of scalar markers in a flow. One method is to combine Direct Numerical Simulation (DNS) and Lagrangian Scalar Tracking (LST) to record locations of markers. While this has proved to be useful, high computational cost remains a concern. In this study, we develop a model that could reproduce results obtained by DNS and LST for turbulent flow. Puffs of markers with different Schmidt numbers were released into a flow field at a frictional Reynolds number of 150. The point of release was at the channel wall, so that both diffusion and convection contribute to the puff dispersion pattern, defining different stages of dispersion. Based on outputs from DNS and LST, we seek the most suitable and feasible probability density function (PDF) that represents distribution of markers in the flow field. The PDF would play a significant role in predicting heat and mass transfer in wall turbulence, and would prove to be helpful where DNS and LST are not always available. [Preview Abstract] |
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