Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session P18: Turbulence Theory: Dispersion and Stochastic Processes 
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Chair: Raul Bayoan Cal, Portland State University Room: 400 
Monday, November 25, 2019 5:16PM  5:29PM 
P18.00001: On the stochastic modeling of Lagrangian velocity and acceleration in turbulent flows Laurent Chevillard, Bianca Viggiano, Jan Friedrich, Romain Volk, Mickael Bourgoin, Raul Bayoan Cal We propose to answer the following question: can we build up an infinitely differentiable stochastic process, such that asymptotically, when the Reynolds number goes to infinity, it becomes irregular (in a Holder sense) and intermittent (in a way we will clarify)? This has importance while modeling velocity and acceleration of particles following their trajectories in a turbulent flow. We propose such a process as a solution of a stochastic differential equation, making it causal. We proceed with analytical and numerical solutions, and compare against experimental and numerical data. Come, it will be fun. [Preview Abstract] 

P18.00002: ABSTRACT WITHDRAWN 
Monday, November 25, 2019 5:42PM  5:55PM 
P18.00003: Linear Response Theory of Single Particle Diffusion in Turbulence Yukio Kaneda A theory is proposed for the statistics of single particle diffusion in stationary homogenous isotropic turbulence of incompressible fluid. The theory is based on a generalization of the idea of linear response theory that is known in the statistical mechanics for systems at or near the thermal equilibrium state. The theory gives $< \Delta {\bf v}(t)^2 >/(\epsilon t) = C_0+ B_1+ \cdots$, for the inertial time interval $t$ such that $ T \gg t \gg \tau$, where $\Delta {\bf v}(t)$ is the velocity increment of a particle during the time interval $t$, $\epsilon$ is the kineticenergy dissipation rate per unit mass, $T$ is the integral time scale, $\tau$ is the Kolmogorov microtimescale, $B_1=C_{1T}(t/T)+C_{1\tau}(\tau/t)$, and $C_0,C_{1T}, C_{1\tau}$ are nondimensional universal constants. $B_1$ represents the effect of small but finite $t/T$ and $\tau/t$. An examination of the theory by comparison with the data of direct numerical simulation (DNS) [B. L. Sawford and P. K. Yeung, Phys. Fluids 23, 091704 (2011)] suggests that the theory is in good agreement with the DNS. [Preview Abstract] 
Monday, November 25, 2019 5:55PM  6:08PM 
P18.00004: The origin of turbulence in thermal convection. Katepalli Sreenivasan, Joerg Schumacher, Ambrsih Pandey, Victor Yakhot If a fluid flow is driven by a weak Gaussian random force, the nonlinearity in the NavierStokes equations is negligibly small and the resulting velocity field obeys Gaussian statistics. Nonlinear effects become important as the driving becomes stronger and a transition occurs to turbulence with anomalous scaling of small scales. This process is reasonably well understood~for homogeneous and isotropic turbulence. In this paper, we discuss the Reynoldsnumber dependence of moments of the kinetic energy dissipation rate in the bulk of thermal convection in the RayleighBenard system. The data for different Reynolds numbers are obtained from direct numerical simulations using threedimensional spectral element method in a convection cell with square cross section and aspect ratio 25. The normalized moments of the kinetic energy dissipation rate show a nonmonotonic dependence for small Reynolds numbers before obeying the algebraic scaling for the turbulent state. This feature is explained via the transition akin to ``softtohard turbulence'' in convection where, depending on the Rayleigh number, turbulence is produced by either the instabilities of the bulk and the wall boundary layers. [Preview Abstract] 
Monday, November 25, 2019 6:08PM  6:21PM 
P18.00005: Fluctuationinduced forces in homogeneous isotropic turbulence Rodolfo Ostilla Monico, Daniel Putt, Vamsi Arza Spandan, Alpha Albert Lee Understanding force generation in nonequilibrium systems is a significant challenge in statistical physics. We uncover a surprising fluctuationinduced force between two plates immersed in homogeneous isotropic turbulence using Direct Numerical Simulation. The force is a nonmonotonic function of plate separation. The mechanism of force generation reveals an intriguing analogy with fluctuationinduced forces: energy in the fluid is localised in regions of high vorticity, or "worms", which have a characteristic length scale. The magnitude of the force depends on the packing of worms inside the plates, with the maximal force attained when the plate separation is comparable to the characteristic worm length. A key implication of our study is that the length scaledependent partition of energy in an active or nonequilibrium system determines force generation. [Preview Abstract] 
Monday, November 25, 2019 6:21PM  6:34PM 
P18.00006: Largescale structure of a passive scalar field in homogeneous turbulence Katsunori Yoshimatsu, Yukio Kaneda We study the largescale structure of a passive scalar field without any scalar source in incompressible homogeneous turbulence. We assume that the initial scalar spectrum at time $t=0$ takes the form $C k^2+o(k^2)$ at the wavenumber $k \to 0$, where $C$ is independent of $k$. Theoretical analysis [Yoshimatsu and Kaneda, Phys. Rev. Fluids 3, 104601 (2019)] shows that the spectrum keeps the form at $k \to 0$ and $C$ is time independent for $t \ge 0$. On the basis of the independence andã€€an assumption of a certain selfsimilar evolution of the scalar field at the large scales including the scales comparable to the scalar integral length scales, it is shown that a certain measure of the anisotropy of the scalar field remains timeindependent at the large scales in a selfsimilar state, irrespective of the velocity field. In addition, we performed direct numerical simulation (DNS) of the passive scalar field in a periodic box. It is found that the DNS results are consistent with the theory. [Preview Abstract] 
Monday, November 25, 2019 6:34PM  6:47PM 
P18.00007: Threepoint statistics of passive scalars at high Schmidt numbers M. P. Clay, K. P. Iyer, D. Buaria, P. K. Yeung, K. R. Sreenivasan The turbulent mixing of passive scalars is a fundamental problem relevant to many natural and engineering flows. While traditionally analyzed via one or twopoint statistics, threepoint statistics have also been used to gain insight into the structure of the scalar field [Warhaft, {\it Annu. Rev. Fluid Mech} {\bf 32}, 203240 (2000)]. Experimental data are scarce, and for the important case of scalar fluctuations generated under the presence of a mean gradient in isotropic turbulence, measurements are limited to Schmidt numbers ($Sc$) near unity [Mydlarski and Warhaft, {\it Phys. Fluids} {\bf 10}, 28852894 (1998)]. Here we analyze threepoint statistics from direct numerical simulations of scalars under a uniform mean gradient in $R_\lambda\approx 140$ forced isotropic turbulence. By using grids with up to $8192^3$ points and passive scalars with $Sc$ up to 512, threepoint statistics are gathered in the emerging viscousconvective range to study the approach to local isotropy exhibited by high$Sc$ scalars. [Preview Abstract] 
Monday, November 25, 2019 6:47PM  7:00PM 
P18.00008: Triad interactions in a fourdimensional fluctuating velocity field Preben Buchhave, Clara Velte High intensity turbulence should be considered a stochastic function of four independent parameters, three spatial coordinates and time. The interaction between the different velocity structures is caused by the nonlinear term in NavierStokes equation and is conventionally analyzed by a threedimensional spatial Fourier transform in a homogeneous velocity field resulting in the socalled triad interactions caused by the second order convection term. We present some conclusions resulting from an analysis of a fourdimensional Fourier transform of the fluctuating velocity. Moreover, we consider a case corresponding to a realistic experiment, where the velocity is digitally sampled with a finite sample rate and the flow is limited to a finite spatial and temporal range. Our results explain features observed in experiments such as the time development of the power spectrum, deviations from the conventional Richardson cascade and perseverance of initial largescale spectral features. [Preview Abstract] 
Monday, November 25, 2019 7:00PM  7:13PM 
P18.00009: TripleCorrelations in Decaying Isotropic Turbulence Clayton Byers, Jonathan MacArt, Michael Mueller, Marcus Hultmark The selfsimilar scaling approach for decaying isotropic turbulence is utilized to extract constraints on the temporally dependent scaling parameters. The resulting similarity solution and constraints show that the temporal evolution of the length scale, which is shown to be the Taylor microscale, sets the exponent of decay. This exponent is found to retain a dependence on the initial conditions of the flow. Additionally, a new triplecorrelation scaling parameter, $u^3/Re_\lambda$, is found. The validity of this new scaling is checked in three different ways, each resulting in a collapse of the triple correlation data. The usefulness of this scale becomes apparent when compared to the results of Stewart \& Townsend (1951), which utilized the classic $u^3$ scaling and did not show collapse in their data. Three Direct Numerical Simulations at differing initial Reynolds numbers were performed to test the theoretical results. [Preview Abstract] 
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