Bulletin of the American Physical Society
71st Annual Meeting of the APS Division of Fluid Dynamics
Volume 63, Number 13
Sunday–Tuesday, November 18–20, 2018; Atlanta, Georgia
Session Q39: Stratified and Buoyancy Driven Flow 
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Chair: Guido Boffetta, University di Torino Room: Georgia World Congress Center Ballroom 3/4 
Tuesday, November 20, 2018 12:50PM  1:03PM 
Q39.00001: Investigation of effects equation of state and differential diffusion on fully developed stratified turbulent channel flow Maitreya Koneru Venkataswamy, Reetesh Ranjan, Suresh Menon Density stratification in turbulent flows observed in flow systems such as sea, lakes, and oceans, usually depends upon two scalars, namely, temperature and salinity. The density field is expressed in terms of these scalars through an equation of state (EoS). In addition, the molecular diffusion of temperature and salinity differs by an order of magnitude leading to differential diffusion phenomenon. In this study, the effects of linear/nonlinear EoS and differential diffusion on the instantaneous flow features and turbulent statistics in a fully developed stably stratified turbulent channel flow are examined using direct numerical simulations. The flow in the channel is simulated at a fixed friction Reynolds number of 180 and increasing level of stratification obtained by varying the friction Richardson number. The role of a mild differential diffusion is characterized by specifying two values of Schmidt number, namely, 1 and 4 for the salinity field and specifying Prandtl number to 1 for the temperature field. The results from the cases employing linear/nonlinear EoS and differential diffusion are compared with the corresponding reference simulations without considering salinity and EoS, where the transport equation for density is explicitly solved. 
(Author Not Attending)

Q39.00002: The evolution of flow structures and energy exchange in forced rotating stratified turbulence Tianyi Li, Minping Wan, Shiyi Chen We investigate the longtime evolution of the rotating stratified turbulence, concerning the flow structures and the exchange between kinetic and potential energy e_{KtoP}. Numerical simulations of forced threedimensional homogeneous rotating stratified turbulence are performed at a high hyperviscous Reynolds number and low Rossby number for a sequence of Froude numbers. In the presence of stratification, with two box scale structures with opposite signs of vertical vorticity formed at later times, there are short and small vortices spreading over the flow field. Compared with anticyclonic vortices, cyclonic vortices are lack in number, easier to grow up or merge, and form the boxscale structure earlier. Moreover, the intense area of e_{KtoP} is associated with the cyclonic structures. We also find a relation between vertical vorticity and the distribution of the density during the evolution of turbulence, which is serviceable for modeling density by velocity. 
Tuesday, November 20, 2018 1:16PM  1:29PM 
Q39.00003: NonOberbeckBoussinesq effects and prediction of the bulk temperature in turbulent RayleighBenard convection Eberhard Bodenschatz, Stephan Weiss, Marcel Wedi, Xuan Zhang, Olga Shishkina We report results of a complementary experimental and numerical study of turbulent nonOberbeckBoussinesq RayleighBenard convection (NOB RBC) in pressurized sulfur hexafluoride (SF6). The measurements are conducted in a cylindrical cell of a height 2.2m and diameter 1.1m in the Uboot of the Max Planck Turbulence Facility in Göttingen and direct numerical simulations (DNS) are performed with the Goldfish code, for a wide range of Rayleigh numbers and different NOB conditions. The numerical and experimental results are compared with predictions for the bulk and the reference temperatures of the top and bottom boundary layers made in Weiss, He, Ahlers, Bodenschatz, Shishkina, J. Fluid Mech., 2018. 
Tuesday, November 20, 2018 1:29PM  1:42PM 
Q39.00004: The accessibility of strongly stratified turbulence in towedsphere wakes Qi Zhou, Peter Diamessis The strongly stratified turbulence (SST) flow regime (Brethouwer et al. 2007) is characterized by small turbulent horizontal Froude number, i.e. Fr_{h }« 1, and large buoyancy Reynolds number, i.e. Re_{h}Fr_{h}^{2 }» 1, two quantities that could vary significantly with time in temporally evolving freeshear turbulent flows, such as a towedsphere wake. We investigate a largeeddy simulation dataset of stratified towedsphere wakes of varying Reynolds number, Re ≡ UD/ν = [5×10^{3}, 10^{5}, 4×10^{5}], and Froude number, Fr ≡ 2U/(ND) = [4,16,64], where U is tow speed, D is sphere diameter and N is buoyancy frequency. By examining relevant stratified turbulence parameters in appropriately defined phase diagrams, we show that these wakes can indeed access the SST regime if ReFr^{2/3 }> O(10^{3}). The length of time during which the wake turbulence would operate within this regime also depends strongly on Re and Fr. This analysis constitutes a first attempt to obtain a predictive capability of stratified wake turbulence in terms of Re, while extending the investigations of SST from spacefilling homogeneous turbulence to a canonical inhomogeneous freeshear flow. 
Tuesday, November 20, 2018 1:42PM  1:55PM 
Q39.00005: Spectral structure of small scales in turbulent TaylorCouette flows Julio M Barros, Christian Butcher, Rory T Cerbus, Tinihau Meuel, Pinaki Chakraborty We study the spectral structure of smallscale fluctuations in turbulent TaylorCouette flows. In contrast to other canonical wallbounded turbulent flows, e.g., pipe flows and boundarylayer flows, it is unclear if the spectral structure of fluctuations in TaylorCouette flows is in accord with any theoretical framework. Using a combination of experiments and direct numerical simulations, we study the turbulent energy spectra in TaylorCouette flows. Our results show that the spectral structure of small scales conforms to the predictions of Kolmogorov's phenomenological theory of turbulence. 
Tuesday, November 20, 2018 1:55PM  2:08PM 
Q39.00006: ABSTRACT WITHDRAWN

Tuesday, November 20, 2018 2:08PM  2:21PM 
Q39.00007: Directional energy flux in anisotropic turbulence Naoto Yokoyama, Masanori Takaoka In NavierStokes turbulence, energy is transferred among three wave numbers via a triad interaction. The energy flux, which should be equal to the total energy dissipation rate if the energy transfer is local in the wavenumber space, plays a key role in the energycascading process that makes the selfsimilar structures in turbulent flows. The energy flux for a wave number in isotropic turbulence systems is given by a scalar and is independent of the magnitude of the wavenumbers in the inertial subrange. On the other hand, the energy flux in anisotropic turbulence systems depends on the directions as well as the magnitudes of the wave numbers, and should be a vector. The energy flux vector in the anisotropic systems cannot be uniquely determined in a similar way used for the isotropic energy flux, which is obtained from the scalar energy transfer rate according to the continuity equation of energy. In this work, we propose a way to determine the energy flux vector in anisotropic turbulence by using the pseudoinverse matrix, which selects the minimalnorm vector, and will discuss the validity of the energy flux vector in rotating turbulence. 
Tuesday, November 20, 2018 2:21PM  2:34PM 
Q39.00008: Extracting internal gravity waves from simulations of stratified turbulence Andrea Maffioli, Alexandre Delache, Fabien Godeferd In stratified turbulence internal gravity waves and advective motions coexist. A central discussion in the field has been what is the importance of one or the other motion in the dynamics. The main difficulty in answering this question has been the fact that these motions interact within stratified turbulence, which means that it is not straightforward to decompose a turbulent field from an experiment or simulation into waves and material motions. With the advent of modern computing, it has recently become possible to perform Direct Numerical Simulations (DNS) of the problem and to transform a time series of the velocity field into frequency and wavenumber space in a 4D Fourier transform. Since waves should exist only in a specific part of this 4D space which is described by the dispersion relation, this process allows to quantify the amount of energy in the waves. We present results from DNS with isotropic forcing, obtained using such spatiotemporal analysis. It is found that in DNS with buoyancy Reynolds number Re_{b} ∼ 1, the waves dominate the overall energy in the flow. This result however is dependent on the forcing not exciting the boxfilling shear modes directly. In DNS with higher Re_{b} this picture changes significantly.

Tuesday, November 20, 2018 2:34PM  2:47PM 
Q39.00009: On the Orientation of Vortical Structures in Homogeneous Turbulent Shear Flows with Stable Stratification or System Rotation Frank G Jacobitz The orientation of vortical structures in homogeneous turbulent shear flows with stable stratification or system rotation is studied using results from direct numerical simulations. Inclined structures are observed in the plane of shear and the threedimensional twopoint autocorrelation coefficient of vorticity magnitude is computed to quantify their orientation. Isosurfaces of the autocorrelation coefficient closely resemble an ellipsoid with its major axis oriented into the direction of the structures. A leastsquares fit of an ellipsoid to the isosurfaces is performed and the major axis is determined. From the major axis, the inclination angle of the structures is computed. The inclination angle is observed to have a similar dependence on the Richardson number Ri, in the case of stratified shear flows, and on the rotation to shear rate ratio f/S, in the case of rotating shear flows, as the growth rate of the turbulent kinetic energy. Therefore, the structure orientation in homogeneous turbulent shear flows appears to be directly related to the dynamics of the flows. 
Tuesday, November 20, 2018 2:47PM  3:00PM 
Q39.00010: Threedimensional numerical simulation of MHD turbulent shear flows Long Chen, Mingjiu Ni Flow of an electrically conducting fluid in a cylindric container is analysed with direct numerical simulations (DNS). The flow geometry and the value of the injected current density(I) or magnetic fields(B) are corresponding to the experiment of Messadek(2002) . In this work, the cases with injected radius r_{e}=0.054m and 1T≤B ≤3T, 3A≤ I ≤20A have been selected to investigate, in which parameter space turbulence regime is wellestablished, whereas Hartmann layers remain laminar. Via a visualization way, we find that the flow experiences a complex evolution and the tendency of large structures variation with the increasing of I and B. The mean velocity profiles exhibit that the thickness of free shear layer is increased by the turbulence. For the present parameter space, the angular momentum of the numerics agrees well with that of the experiments and theoretical solutions. DNS shows that the energy spectra exhibits a power law close to k^{5/3} when the Joule dissipation is weak and close to k^{3} when it is significant, which appears consistent with the theory and the experimental results of Messadek(2002). 
Tuesday, November 20, 2018 3:00PM  3:13PM 
Q39.00011: Study of convection in rotating flow in a new configuration with bidirectional temperature gradients Ayan Kumar Banerjee, Amitabh Bhattacharya, Sridhar Balasubramanian Laboratory experiments were conducted in a novel configuration, comprising of nonhomogeneously heated rotating cylindrical annulus, to study dynamics of rotating convection. Temperature time series measurements and flow visualization were done over a range of Taylor number, Ta (spanning 6.5 X 10^{8 } 2.7 X 10^{9}) and Rayleigh number, Ra (spanning 2.2 X 10^{8} 6.2 X 10^{8}). Localized temperature time series data acquired by thermocouples, two point correlations of temperature data, Nusselt number characteristics, PIV imaging combined with 2D axisymmetric ANSYS Fluent simulation were used to understand the physics of baroclinic instabilities, convective structures and their interaction. Overall, it is speculated that heat transport in this new configuration is governed by the coexistence and interplay of convective plumes (above the heating zone that aid in the vertical transport of heat) and baroclinic waves/eddies (which aid in the horizontal transport of cold and hot fluids between the annuli). The two point temperature correlations show that these waves break into eddies as the value of Ta increases, which closely simulates the dynamics of geophysical motions. Scaling for the proposed system is derived and validated with thermal measurements. 
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