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1. Stochastic theory and direct numerical simulations of the relative motion of high-inertia particle pairs in isotropic turbulence

   https://doi.org/10.1017/jfm.2016.859  

   Dhariwal, Rohit; Rani, Sarma L.; Koch, Donald L. (February 2017, Journal of Fluid Mechanics)

   The relative velocities and positions of monodisperse high-inertia particle pairs in isotropic turbulence are studied using direct numerical simulations (DNS), as well as Langevin simulations (LS) based on a probability density function (PDF) kinetic model for pair relative motion. In a prior study (Rani et al. , J. Fluid Mech. , vol. 756, 2014, pp. 870–902), the authors developed a stochastic theory that involved deriving closures in the limit of high Stokes number for the diffusivity tensor in the PDF equation for monodisperse particle pairs. The diffusivity contained the time integral of the Eulerian two-time correlation of fluid relative velocities seen by pairs that are nearly stationary. The two-time correlation was analytically resolved through the approximation that the temporal change in the fluid relative velocities seen by a pair occurs principally due to the advection of smaller eddies past the pair by large-scale eddies. Accordingly, two diffusivity expressions were obtained based on whether the pair centre of mass remained fixed during flow time scales, or moved in response to integral-scale eddies. In the current study, a quantitative analysis of the (Rani et al. 2014) stochastic theory is performed through a comparison of the pair statistics obtained using LS with those from DNS. LS consist of evolving the Langevin equations for pair separation and relative velocity, which is statistically equivalent to solving the classical Fokker–Planck form of the pair PDF equation. Langevin simulations of particle-pair dispersion were performed using three closure forms of the diffusivity – i.e. the one containing the time integral of the Eulerian two-time correlation of the seen fluid relative velocities and the two analytical diffusivity expressions. In the first closure form, the two-time correlation was computed using DNS of forced isotropic turbulence laden with stationary particles. The two analytical closure forms have the advantage that they can be evaluated using a model for the turbulence energy spectrum that closely matched the DNS spectrum. The three diffusivities are analysed to quantify the effects of the approximations made in deriving them. Pair relative-motion statistics obtained from the three sets of Langevin simulations are compared with the results from the DNS of (moving) particle-laden forced isotropic turbulence for $$St_{\unicode[STIX]{x1D702}}=10,20,40,80$$ and $$Re_{\unicode[STIX]{x1D706}}=76,131$$ . Here, $$St_{\unicode[STIX]{x1D702}}$$ is the particle Stokes number based on the Kolmogorov time scale and $$Re_{\unicode[STIX]{x1D706}}$$  is the Taylor micro-scale Reynolds number. Statistics such as the radial distribution function (RDF), the variance and kurtosis of particle-pair relative velocities and the particle collision kernel were computed using both Langevin and DNS runs, and compared. The RDFs from the stochastic runs were in good agreement with those from the DNS. Also computed were the PDFs $$\unicode[STIX]{x1D6FA}(U|r)$$ and $$\unicode[STIX]{x1D6FA}(U_{r}|r)$$ of relative velocity $$U$$ and of the radial component of relative velocity $$U_{r}$$ respectively, both PDFs conditioned on separation $$r$$ . The first closure form, involving the Eulerian two-time correlation of fluid relative velocities, showed the best agreement with the DNS results for the PDFs. 

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2. Almost Mathieu operators with completely resonant phases

   https://doi.org/10.1017/etds.2018.133  

   LIU, WENCAI (July 2020, Ergodic Theory and Dynamical Systems)

   Let $$\unicode[STIX]{x1D6FC}\in \mathbb{R}\backslash \mathbb{Q}$$ and $$\unicode[STIX]{x1D6FD}(\unicode[STIX]{x1D6FC})=\limsup _{n\rightarrow \infty }(\ln q_{n+1})/q_{n}<\infty$$ , where $$p_{n}/q_{n}$$ is the continued fraction approximation to $$\unicode[STIX]{x1D6FC}$$ . Let $$(H_{\unicode[STIX]{x1D706},\unicode[STIX]{x1D6FC},\unicode[STIX]{x1D703}}u)(n)=u(n+1)+u(n-1)+2\unicode[STIX]{x1D706}\cos 2\unicode[STIX]{x1D70B}(\unicode[STIX]{x1D703}+n\unicode[STIX]{x1D6FC})u(n)$$ be the almost Mathieu operator on $$\ell ^{2}(\mathbb{Z})$$ , where $$\unicode[STIX]{x1D706},\unicode[STIX]{x1D703}\in \mathbb{R}$$ . Avila and Jitomirskaya [The ten Martini problem. Ann. of Math. (2), 170 (1) (2009), 303–342] conjectured that, for $$2\unicode[STIX]{x1D703}\in \unicode[STIX]{x1D6FC}\mathbb{Z}+\mathbb{Z}$$ , $$H_{\unicode[STIX]{x1D706},\unicode[STIX]{x1D6FC},\unicode[STIX]{x1D703}}$$ satisfies Anderson localization if $$|\unicode[STIX]{x1D706}|>e^{2\unicode[STIX]{x1D6FD}(\unicode[STIX]{x1D6FC})}$$ . In this paper, we develop a method to treat simultaneous frequency and phase resonances and obtain that, for $$2\unicode[STIX]{x1D703}\in \unicode[STIX]{x1D6FC}\mathbb{Z}+\mathbb{Z}$$ , $$H_{\unicode[STIX]{x1D706},\unicode[STIX]{x1D6FC},\unicode[STIX]{x1D703}}$$ satisfies Anderson localization if $$|\unicode[STIX]{x1D706}|>e^{3\unicode[STIX]{x1D6FD}(\unicode[STIX]{x1D6FC})}$$ . 

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3. Analysis of passive flexion in propelling a plunging plate using a torsion spring model

   https://doi.org/10.1017/jfm.2018.736  

   Arora, N.; Kang, C.-K.; Shyy, W.; Gupta, A. (December 2018, Journal of Fluid Mechanics)

   We mimic a flapping wing through a fluid–structure interaction (FSI) framework based upon a generalized lumped-torsional flexibility model. The developed fluid and structural solvers together determine the aerodynamic forces, wing deformation and self-propelled motion. A phenomenological solution to the linear single-spring structural dynamics equation is established to help offer insight and validate the computations under the limit of small deformation. The cruising velocity and power requirements are evaluated by varying the flapping Reynolds number ( $$20\leqslant Re_{f}\leqslant 100$$ ), stiffness (represented by frequency ratio, $$1\lesssim \unicode[STIX]{x1D714}^{\ast }\leqslant 10$$ ) and the ratio of aerodynamic to structural inertia forces (represented by a dimensionless parameter $$\unicode[STIX]{x1D713}$$ ( $$0.1\leqslant \unicode[STIX]{x1D713}\leqslant 3$$ )). For structural inertia dominated flows ( $$\unicode[STIX]{x1D713}\ll 1$$ ), pitching and plunging are shown to always remain in phase ( $$\unicode[STIX]{x1D719}\approx 0$$ ) with the maximum wing deformation occurring at the end of the stroke. When aerodynamics dominates ( $$\unicode[STIX]{x1D713}>1$$ ), a large phase difference is induced ( $$\unicode[STIX]{x1D719}\approx \unicode[STIX]{x03C0}/2$$ ) and the maximum deformation occurs at mid-stroke. Lattice Boltzmann simulations show that there is an optimal $$\unicode[STIX]{x1D714}^{\ast }$$ at which cruising velocity is maximized and the location of optimum shifts away from unit frequency ratio ( $$\unicode[STIX]{x1D714}^{\ast }=1$$ ) as $$\unicode[STIX]{x1D713}$$ increases. Furthermore, aerodynamics administered deformations exhibit better performance than those governed by structural inertia, quantified in terms of distance travelled per unit work input. Closer examination reveals that although maximum thrust transpires at unit frequency ratio, it is not transformed into the highest cruising velocity. Rather, the maximum velocity occurs at the condition when the relative tip displacement $${\approx}\,0.3$$ . 

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4. Interface-resolved simulations of particle suspensions in Newtonian, shear thinning and shear thickening carrier fluids

   https://doi.org/10.1017/jfm.2018.532  

   Alghalibi, Dhiya; Lashgari, Iman; Brandt, Luca; Hormozi, Sarah (October 2018, Journal of Fluid Mechanics)

   We present a numerical study of non-colloidal spherical and rigid particles suspended in Newtonian, shear thinning and shear thickening fluids employing an immersed boundary method. We consider a linear Couette configuration to explore a wide range of solid volume fractions ( $$0.1\leqslant \unicode[STIX]{x1D6F7}\leqslant 0.4$$ ) and particle Reynolds numbers ( $$0.1\leqslant Re_{p}\leqslant 10$$ ). We report the distribution of solid and fluid phase velocity and solid volume fraction and show that close to the boundaries inertial effects result in a significant slip velocity between the solid and fluid phase. The local solid volume fraction profiles indicate particle layering close to the walls, which increases with the nominal $$\unicode[STIX]{x1D6F7}$$ . This feature is associated with the confinement effects. We calculate the probability density function of local strain rates and compare the latter’s mean value with the values estimated from the homogenisation theory of Chateau et al. ( J. Rheol. , vol. 52, 2008, pp. 489–506), indicating a reasonable agreement in the Stokesian regime. Both the mean value and standard deviation of the local strain rates increase primarily with the solid volume fraction and secondarily with the $$Re_{p}$$ . The wide spectrum of the local shear rate and its dependency on $$\unicode[STIX]{x1D6F7}$$ and $$Re_{p}$$ point to the deficiencies of the mean value of the local shear rates in estimating the rheology of these non-colloidal complex suspensions. Finally, we show that in the presence of inertia, the effective viscosity of these non-colloidal suspensions deviates from that of Stokesian suspensions. We discuss how inertia affects the microstructure and provide a scaling argument to give a closure for the suspension shear stress for both Newtonian and power-law suspending fluids. The stress closure is valid for moderate particle Reynolds numbers, $$O(Re_{p})\sim 10$$ . 

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5. COUNTING CONJUGACY CLASSES IN

   https://doi.org/10.1017/S0004972718000047  

   HULL, MICHAEL; KAPOVICH, ILYA (June 2018, Bulletin of the Australian Mathematical Society)

   We show that if a finitely generated group $$G$$ has a nonelementary WPD action on a hyperbolic metric space $$X$$ , then the number of $$G$$ -conjugacy classes of $$X$$ -loxodromic elements of $$G$$ coming from a ball of radius $$R$$ in the Cayley graph of $$G$$ grows exponentially in $$R$$ . As an application we prove that for $$N\geq 3$$ the number of distinct $$\text{Out}(F_{N})$$ -conjugacy classes of fully irreducible elements $$\unicode[STIX]{x1D719}$$ from an $$R$$ -ball in the Cayley graph of $$\text{Out}(F_{N})$$ with $$\log \unicode[STIX]{x1D706}(\unicode[STIX]{x1D719})$$ of the order of $$R$$ grows exponentially in $$R$$ . 

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