Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session L22: Turbulence Mixing III |
Hide Abstracts |
Chair: Katepalli Sreenivasan, New York University Room: 30C |
Monday, November 19, 2012 3:35PM - 3:48PM |
L22.00001: Turbulent mixing of substances which are highly diffusive K.R. Sreenivasan, P.K. Yeung How a substance gets mixed by a fluid, even when the motion is turbulent, depends to some extent on whether its diffusivity is small or large. The magnitude of the diffusivity is usually expressed by the Schmidt number ($Sc$, ratio of fluid viscosity to the diffusivity of the substance). The case of passive scalars (which have no back-reaction on the flow) with large $Sc$ (weak diffusivity) has received considerable attention, especially for its special features such as the -1 power roll-off of the spectrum of the fluctuations. Similar studies for passive scalars at low Schmidt numbers (or large diffusivity) do not yet exist, though the classical theory (Batchelor, Howells \& Townsend, J. Fluid Mech, {\bf 5}, 134 (1959)) is now more than fifty years old. In this talk we report direct numerical simulations for decaying scalar fields with $Sc$ as low as 1/2048, at grid resolution up to $4096^3$, in stationary isotropic turbulence with microscale Reynolds number in the range 140-390. We examine the validity of theoretical assumptions that lead to a spectral slope of -17/3 in the so-called inertial-diffusive range. Despite limitations on the range of scales in the simulations, the data support the theory as the Schmidt number decreases and the Reynolds number increases. [Preview Abstract] |
Monday, November 19, 2012 3:48PM - 4:01PM |
L22.00002: Turbulent mixing of highly diffusive substances in the presence of a uniform mean gradient P.K. Yeung, K.P. Iyer, K.R. Sreenivasan In the preceding talk, we verified the basic physical content and general validity of the theory of Batchelor, Howells \& Townsend (J. Fluid Mech. {\bf 5}, 134 (1959)) in the case of passive scalar fields decaying in the absence of any production mechanism. In this study we consider results from direct numerical simulations on passive scalars of very strong diffusivity (low Schmidt number), in a presence of a uniform mean gradient, which leads to production of scalar fluctuations at the large scales. Our results show that the presence of the mean gradient alters the physics of mixing fundamentally. While the spectrum of scalar fluctuations still follows a -17/3 power law in the so-called diffusive-inertial range, the constant is found to be non-universal and dependent on the magnitude of the mean scalar gradient. Spectral transfer activity is greatly reduced compared with that for moderately and weakly diffusive scalars. The resulting weakening of spectral cascade at high wavenumber leads to many distinctive features, including the failure of dissipative anomaly, and a new balance of terms in the spectral transfer equation for the scalar variance, different from the case of no gradient. We also present an alternative explanation for the scaling behaviors actually observed. [Preview Abstract] |
Monday, November 19, 2012 4:01PM - 4:14PM |
L22.00003: Turbulent mixing in microfluidics with Reynolds number in the order of 1 Fang Yang, Wei Zhao, Guiren Wang One important issue in microfluidic devices is the relatively slow mixing due to laminar flow at low Reynolds number (Re). In many cases, fast mixing is highly demanded. In macroscale, where Re is relatively high, mixing can be enhanced by forcing flow to be turbulent. However, although there can be elastic turbulence in low Re, it is conventionally believed that the flow in mirofluidics, where typical Re is in the order of 1 or less, can only be laminar. In present work, we demonstrate that turbulent mixing can be realized in a microchannel, where the Reynolds number is in the order of 1 when the flow is forced electrokinetically. The turbulent mixing in microfluidics can cause ultrafast scalar mixing. Confocal microscopic laser induced fluorescence with high tempo-spatial resolution is used to study the turbulent mixing in the microchannel. We report the fast mixing process, concentration profile, irregular concentration time trace, segregation intensity and continuous power spectrum of concentration fluctuation indicating multiscale structures of small eddies. The results indicate that turbulent mixing can be realized as well in microfluidics with Re in the order of 1. The study could open a new perspective view on transport phenomena in microfluidics. [Preview Abstract] |
Monday, November 19, 2012 4:14PM - 4:27PM |
L22.00004: Spinodal turbulence enhances heat transfer in micro devices Stefano Faris\'e, Pietro Poesio, Gian Paolo Beretta We experimentally prove the possibility of using spinodal mixtures to increase heat transfer in micro devices as a consequence of an evenly distributed micro agitation, which increases the effective diffusivity. Despite the $Re$-number is as low as 5, turbulence-like mixing can be achieved by mass transfer effects. A mixture of acetone-hexadecane is quenched in a micro heat exchanger to induce spinodal decomposition. The heat transfer rate is enhanced by self-induced convective motion (spinodal turbulence) because the drops of one phase move against each others under the influence of non-equilibrium capillary forces, Korteweg stresses,which are sustained by the free energy liberated during phase separation. The heat transfer is increased up to the 200\% and the effect become larger as the bulk $Re$ decreses, while no dramatic increase in the pressure drop is observed. We built two different experimental set-ups: in the first we measure the heat transfer with a feedback method and in the second we measure the pressure drop and we visualize the induced convection. High-speed camera visualization,pressure drop and temperature measurements allow a complete characterization of the phenomenon, with a special attention to the quantification of the heat transfer coefficent enhancement. [Preview Abstract] |
Monday, November 19, 2012 4:27PM - 4:40PM |
L22.00005: Spectral transfer and scale locality characteristics in turbulent mixing over a wide range of Schmidt numbers Dhawal Buaria, P.K. Yeung, J.A. Domaradzki A classical picture of turbulent mixing is that advective transport by the velocity field causes blobs of scalar fluctuations to be broken down into smaller and smaller scales, where the fluctuations are ultimately dissipated by molecular diffusivity. In Fourier space this corresponds to a spectral cascade, which is generally understood to be local in nature (i.e., occurring among scales similar in size). However, recent numerical simulations show that at very low values of the Schmidt number ($Sc$) the spectral cascade is strongly suppressed. To understand this observation we examine the detailed spectral transfer characteristics of scalar fields with $Sc$ ranging from 1/2048 to 64 in isotropic turbulence with Taylor-scale Reynolds number 140. We also compute so-called scale locality functions which measure contributions from ``resolved'' and ``subgrid'' scales to the transfer flux across a specified cutoff scale. Our results suggest that the classical cascade scenario holds well at $Sc\ge 1$. However, in the limit $Sc\ll 1$ transfer is dominated by nonlocal triadic interactions involving low wavenumber scalar modes and high wavenumber scalar modes, modulated by a high wavenumber velocity mode, corresponding to advection by the small-scale velocity field in physical space. [Preview Abstract] |
Monday, November 19, 2012 4:40PM - 4:53PM |
L22.00006: Extreme responses of a coupled scalar-particle system during turbulent mixing Joerg Schumacher, Bipin Kumar, Raymond Shaw Extreme responses of a droplet ensemble during an entrainment and mixing process as present at the edge of a cloud are investigated by means of three-dimensional direct numerical simulations. We combine therefore the Eulerian description of the turbulent velocity and vapor content fields with a Lagrangian ensemble of cloud water droplets which are advected in the flow and can shrink and grow in correspondence with the supersaturation at their position. We find that the Damk\"ohler number $Da$, a dimensionless parameter which relates the fluid time scale to the typical evaporation time scale, can capture all aspects of the initial mixing process. The mixing process is characterized by the limits of strongly homogeneous ($Da\ll 1$) and strongly inhomogeneous ($Da\gg 1$). We explore these two extreme regimes and study the response of the droplet size distribution to the corresponding parameter settings through enhancement and reduction the response constant $K$ in the droplet growth equation. Thus, $Da$ is varied while Reynolds and Schmidt numbers are held fixed, and initial microphysical properties are held constant. In the homogeneous limit minimal broadening of the size distribution is observed as the new steady state is reach, whereas in the inhomogeneous limit the size distr [Preview Abstract] |
Monday, November 19, 2012 4:53PM - 5:06PM |
L22.00007: Lyapunov exponents of inertial particles in isotropic turbulence Li Guo, Guodong Jin, Dong Li, Guowei He The Lyapunov exponents of inertial particles in isotropic turbulence are calculated using direct numerical simulation (DNS), filtered DNS and large-eddy simulation (LES). Here, filter operators are taken as Eulerian space filter and Lagrangian time filter. The Lyapunov exponents obtained are qualitatively consistent but their magnitudes are different: the Lyapunov exponents from DNS are largest and the ones from LES are smallest while the ones from filter DNS are in between. The comparisons imply that the filters could reduce both stretching and compression in turbulent flows. Furthermore, the Lagrangian time filter allows the filtered trajectories to share the similar statistics of true particle trajectories in turbulent flows. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700