Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session D25: Turbulence: General I |
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Chair: Eberhard Bodenschatz, Max Planck Institute for Dynamics and Self-Organization Room: 2005 |
Sunday, November 23, 2014 2:15PM - 2:28PM |
D25.00001: The role of wall confinement on the decay rate of an initially isotropic turbulent field David R. Dowling, Pooya Movahed, Eric Johnsen The problem of freely decaying isotropic turbulence has been the subject of intensive research during the past few decades due to its importance for modeling purposes. While isotropy and periodic boundary conditions assumptions simplify the analysis, large-scale anisotropy (e.g., caused by rotation, shear, acceleration or walls) is in practice present in most turbulent flows and affects flow dynamics across different scales, as well as the kinetic energy decay. We investigate the role of wall confinement and viscous dissipation on the decay rate of an initially isotropic field for confining volumes of different aspect ratios. We first generate an isotropic velocity field in a cube with periodic boundary conditions. Next, using this field, we change the boundary conditions to no-slip walls on all sides. These walls restrict the initial field to a confined geometry and also provide an additional viscous dissipation mechanism. The problem is considered for confining volumes of different aspect ratios by adjusting the initial field. The change in confining volume introduces an additional length scale to the problem. Direct numerical simulation of the proposed set-up is used to verify the scaling arguments for the decay rate of kinetic energy. [Preview Abstract] |
Sunday, November 23, 2014 2:28PM - 2:41PM |
D25.00002: Properties of Streamline Segments in Turbulent Channel Flows with Wavy Walls Fabian Hennig, Jonas Boschung, Norbert Peters We investigate the turbulent velocity field by means of instantaneous streamlines. The streamlines are partitioned into segments and decompose the velocity field in a non-arbitrary way. We conducted direct numerical simulations of a channel flow with one wavy and one plane wall. The results have been validated against DNS and experimental data from literature. Based on the DNS we investigate the properties of streamlines and streamline segments in detail. [Preview Abstract] |
Sunday, November 23, 2014 2:41PM - 2:54PM |
D25.00003: The kinematics of the reduced velocity gradient tensor in a fully developed turbulent free shear flow Oliver Buxton, Andrew Wynn, Paige Rabey The reduced velocity gradient tensor (VGT) is defined as a $2 \times 2$ block, from a single interrogation plane, of the full VGT, $\partial u_i/\partial x_j$. Direct numerical simulation data from the fully developed turbulent region of a nominally two-dimensional mixing layer is used in order to examine the extent to which information on the full VGT can be derived from the reduced VGT. It is shown that the reduced VGT is able to reveal significantly more information about regions of the flow in which strain-rate is dominant over rotation. It is thus possible to use the assumptions of homogeneity and local isotropy to place bounds on the first two statistical moments of the eigenvalues of the reduced strain-rate tensor (symmetric part of the reduced VGT), which in turn relate to the turbulent strain-rates. These bounds are shown to be dependent upon the kurtosis of $\partial u_1/\partial x_1$ and another variable defined from the constituents of the reduced VGT. The kurtosis is observed to be minimised on the centreline of the mixing layer and thus tighter bounds are are possible at the centre of the mixing layer than the periphery. Nevertheless, these bounds are observed to hold for the entirety of the mixing layer, despite some significant departures from local isotropy. [Preview Abstract] |
Sunday, November 23, 2014 2:54PM - 3:07PM |
D25.00004: Slow dynamics at $Re=10^8$ in turbulent Helium flows Javier Burguete, Philippe Roche, Bernard Rousset The presence of slow dynamics is a recurrent feature of many turbulent flows. This behaviour can be created by instabilities of the mean flow or by other mechanisms [1,2]. In this work we analyze the behavior of a highly turbulent flow (maximum Reynolds number $Re=10^8$, with a Reynolds based on the Taylor microscale $Re_{\lambda}=2000$). The experimental cell consists on a closed cavity filled with liquid Helium (330 liters) close to the lambda point (between 1.8 and 2.5 K) where two inhomogeneous and strongly turbulent flows collide in a thin region. The cylindrical cavity has a diameter of 78cm and two impellers rotate in opposite directions with rotation frequencies up to 2Hz. The distance between the propellers is 70cm. Different experimental runs have been performed, both in the normal and superfluid phases. We have performed velocity measurements using home-made Pitot tubes. Here we would like to present preliminary results on this configuration. The analysis of the data series reveals that below the injection frequencies there are different dynamical regimes with time scales two orders of magnitude below the injection scale. \\[4pt] [1] A. de la Torre, J. Burguete, Phys Rev Lett 99 (2007) 054101.\\[0pt] [2] M. Lopez-Caballero, J. Burguete, Phys Rev Lett 110 (2013) 124501. [Preview Abstract] |
Sunday, November 23, 2014 3:07PM - 3:20PM |
D25.00005: Decay Power Law in, High Intensity, Isotropic Turbulent Flow Timothy Koster, Alejandro Puga, John LaRue In the study reported here, isotropy is determined using the measure proposed by George (1992), where isotropy corresponds to those downstream positions where the product of the Taylor Reynolds number and the skewness of the velocity derivative is a constant. Straight forward approach can be used which is based on the observation of Batchelor (1953), that the square of the Talor micorscale is linearly related to downstream distance relative to the virtual origin. The fact that the decay of downstream velocity variance is described by a power law is shown to imply power law behavior for various other parameters such as the dissipation, the integral length scale, the Taylor microscale, the Kolmogorov microscale and the Taylor Reynolds number and that there is an algebraic relationship between the various power law exponents. Results are presented for mean velocities of 6 and 8 m/s for the downstream decay of the parameters listed in the preceding. The corresponding values of the Taylor Reynolds number at the start of the isotropic region are 290 and 400, and the variance decay exponent and virtual origin are found to be respectively -1.707 and -1.298 and -27.95 and -5.757. The exponents in the decay law for the other parameters are found to be within $\pm$ 3\% of the expected values. [Preview Abstract] |
Sunday, November 23, 2014 3:20PM - 3:33PM |
D25.00006: Direct and inverse energy cascades in a forced rotating turbulence experiment Antoine Campagne, Basile Gallet, Fr\'ed\'eric Moisy, Pierre-Philippe Cortet Turbulence in a rotating frame provides a remarkable system where 2D and 3D properties may coexist, with a possible tuning between direct and inverse cascades. We present here experimental evidence for a double cascade of kinetic energy in a statistically stationary rotating turbulence experiment. Turbulence is generated by a set of vertical flaps which continuously injects velocity fluctuations towards the center of a rotating water tank. The energy transfers are evaluated from two-point third-order three-component velocity structure functions, which we measure using stereoscopic PIV in the rotating frame. Without global rotation, the energy is transferred from large to small scales, as in classical 3D turbulence. For nonzero rotation rates, the horizontal kinetic energy presents a double cascade: a direct cascade at small horizontal scales and an inverse cascade at large horizontal scales. By contrast, the vertical kinetic energy is always transferred from large to small horizontal scales, a behavior reminiscent of the dynamics of a passive scalar in 2D turbulence. At the largest rotation rate, the flow is nearly 2D and a pure inverse energy cascade is found for the horizontal energy. [Preview Abstract] |
Sunday, November 23, 2014 3:33PM - 3:46PM |
D25.00007: A tractable representation of the non-linear terms in spectral space applied to isotropic turbulence Lawrence Cheung, Tamer Zaki The principal challenge in the analytical treatment of the Navier-Stokes equations in spectral space is the complex nature of the nonlinear triad interactions. In Fourier basis, these interactions are expressed as an infinite convolution sum over all wavenumber pairs. A tractable representation is introduced in terms of a combination matrix which recasts the convolution in a bilinear form. With this new representation, a spectral energy equation is derived, and its bilinear form facilitates the choice of the appropriate canonical basis. The utility of the formulation is demonstrated by considering the problem of homogeneous, isotropic turbulence. By invoking well-established assumptions, for example the presence of an inertial range where the energy decay rate is independent of wavenumber, we derive Kolmogorov's -5/3 scaling analytically, without any dimensional arguments. The same analytical framework also accurately predicts the spectral characteristics of scalar fluctuations. [Preview Abstract] |
Sunday, November 23, 2014 3:46PM - 3:59PM |
D25.00008: Pressure Rate of Strain, Pressure Diffusion and Velocity Pressure Gradient Tensor Measurements in a Cavity Shear Layer Flow Xiaofeng Liu, Joseph Katz Pressure related turbulence statistics of a 2D open cavity shear layer flow was investigated experimentally in a water tunnel at a Reynolds number of 40,000. Time-resolved PIV sampled at 4500 fps and a field of view of 25x25 mm was used to simultaneously measure the instantaneous velocity, material acceleration and pressure distributions. The pressure was obtained by spatially integrating the measured material acceleration. Results based on 150,000 measurement samples enable direct estimates of components of the pressure-rate-of-strain, pressure diffusion and velocity-pressure-gradient tensors. The pressure and streamwise velocity correlation changes its sign from negative values far upstream from the downstream corner to positive values near the corner due to the strong adverse pressure gradient imposed by the corner. Moreover, once its sign changes, the pressure-velocity correlation preserves its positive value for the streamwise correlations, and negative value for the spanwise correlations, even after the shear layer propagates beyond the adverse pressure gradient region along both the vertical and horizontal corner walls. The pressure diffusion term is of the same order as the production rate. In the shear layer, the streamwise pressure-rate-of-strain term, R$_{\mathrm{11}}$, is mostly negative while the perpendicular term, R$_{\mathrm{22}}$, is positive but with a smaller magnitude, implying turbulent energy redistribution from streamwise to lateral directions. [Preview Abstract] |
Sunday, November 23, 2014 3:59PM - 4:12PM |
D25.00009: Redistribution of kinetic energy by pressure forces in turbulence Alain Pumir, Haitao Xu, Eberhard Bodenschatz, Gregory Falkovich, Guido Boffetta In statistically homogeneous turbulent flows, pressure forces provide the main mechanism to redistribute kinetic energy among fluid elements, without net contribution to the overall energy budget. This holds true in both two-dimensional (2D) and three-dimensional (3D) flows, which show fundamentally different physics. As I will demonstrate, pressure forces act on fluid elements very differently in these two cases. Numerical simulations demonstrate that in 3D pressure forces strongly accelerate the fastest fluid elements, an effect which is absent in 2D. In 3D turbulence, these findings suggest a mechanism for a possibly singular build-up of energy, and thus may shed new light on the smoothness problem of the solution of the Navier-Stokes equation in 3D. [Preview Abstract] |
Sunday, November 23, 2014 4:12PM - 4:25PM |
D25.00010: Atmospheric Scintillations: A Clue for Bird Orientation and Navigation Charles Petty, Andrew Bowden, Andre Benard The index-of-refraction of the troposphere is anisotropic at all scales even if the local turbulent velocity field is statistically homogeneous. This anisotropy is partly due to the coupling between the fluctuating velocity field with the Coriolis field and the Lorentz field. Thus, the redistribution of turbulent kinetic energy and the concomitant anisotropy in the index-of-refraction may provide a practical means for birds (and other animals and insects) to orient and navigate. Consequently, if birds migrate between two points on the Earth by following a great circle path, then local anisotropic scintillation phenomena may provide a means to determine the latitude, the longitude, and the bearing along an orthodromic migration path. Thus, scintillation phenomena may be an important fundamental component in the underlying mechanics that support bird orientation and navigation. [Preview Abstract] |
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