Browsing by Author "Spandan, Vamsi"
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Item A parallel interaction potential approach coupled with the immersed boundary method for fully resolved simulations of deformable interfaces and membranes(Journal of Computational Physics, 2016-12) Spandan, Vamsi; Meschini, Valentina; Ostilla-Mónico, Rodolfo; Lohse, Detlef; Querzolo, Giorgio; De Tulio, Marco D.; Verzicco, RobertoIn this paper we show and discuss how the deformation dynamics of closed liquid–liquid interfaces (for example drops and bubbles) can be replicated with use of a phenomenological interaction potential model. This new approach to simulate liquid–liquid interfaces is based on the fundamental principle of minimum potential energy where the total potential energy depends on the extent of deformation of a spring network distributed on the surface of the immersed drop or bubble. Simulating liquid–liquid interfaces using this model require computing ad-hoc elastic constants which is done through a reverse-engineered approach. The results from our simulations agree very well with previous studies on the deformation of drops in standard flow configurations such as a deforming drop in a shear flow or cross flow. The interaction potential model is highly versatile, computationally efficient and can be easily incorporated into generic single phase fluid solvers to also simulate complex fluid–structure interaction problems. This is shown by simulating flow in the left ventricle of the heart with mechanical and natural mitral valves where the imposed flow, motion of ventricle and valves dynamically govern the behaviour of each other. Results from these simulations are compared with ad-hoc in-house experimental measurements. Finally, we present a simple and easy to implement parallelisation scheme, as high performance computing is unavoidable when studying large scale problems involving several thousands of simultaneously deforming bodies in highly turbulent flows.Item AFiD-GPU: A versatile Navier–Stokes solver for wall-bounded turbulent flows on GPU clusters(Computer Physics Communications, 2017-05) Zhu, Xiaojue; Phillips, Everett; Spandan, Vamsi; Donners, John; Ruetsch, Gregory; Romero, Joshua; Ostilla-Mónico, Rodolfo; Yang, Yantao; Lohse, Detlef; Verzicco, Roberto; Fatica, Massimiliano; Stevens, Richard J. A. M.The AFiD code, an open source solver for the incompressible Navier–Stokes equations (http://www.afid.eu), has been ported to GPU clusters to tackle large-scale wall-bounded turbulent flow simulations. The GPU porting has been carried out in CUDA Fortran with the extensive use of kernel loop directives (CUF kernels) in order to have a source code as close as possible to the original CPU version; just a few routines have been manually rewritten. A new transpose scheme has been devised to improve the scaling of the Poisson solver, which is the main bottleneck of incompressible solvers. For large meshes the GPU version of the code shows good strong scaling characteristics, and the wall-clock time per step for the GPU version is an order of magnitude smaller than for the CPU version of the code. Due to the increased performance and efficient use of memory, the GPU version of AFiD can perform simulations in parameter ranges that are unprecedented in thermally-driven wall-bounded turbulence. To verify the accuracy of the code, turbulent Rayleigh–Bénard convection and plane Couette flow are simulated and the results are in excellent agreement with the experimental and computational data that have been published in literature.Item Drag reduction in numerical two-phase Taylor–Couette turbulence using an Euler–Lagrange approach(Journal of Fluid Mechanics, 2016-05) Spandan, Vamsi; Ostilla-Mónico, Rodolfo; Verzicco, Roberto; Lohse, DetlefTwo-phase turbulent Taylor–Couette (TC) flow is simulated using an Euler–Lagrange approach to study the effects of a secondary phase dispersed into a turbulent carrier phase (here bubbles dispersed into water). The dynamics of the carrier phase is computed using direct numerical simulations (DNS) in an Eulerian framework, while the bubbles are tracked in a Lagrangian manner by modelling the effective drag, lift, added mass and buoyancy force acting on them. Two-way coupling is implemented between the dispersed phase and the carrier phase which allows for momentum exchange among both phases and to study the effect of the dispersed phase on the carrier phase dynamics. The radius ratio of the TC setup is fixed to ? = 0.833, and a maximum inner cylinder Reynolds number of Rei = 8000 is reached. We vary the Froude number (Fr), which is the ratio of the centripetal to the gravitational acceleration of the dispersed phase and study its effect on the net torque required to drive the TC system. For the two-phase TC system, we observe drag reduction, i.e. the torque required to drive the inner cylinder is lower compared with that of the single-phase system. The net drag reduction decreases with increasing Reynolds number Rei, which is consistent with previous experimental findings (Murai et al., J. Phys.: Conf. Ser., vol. 14, 2005, pp. 143–156; Phys. Fluids, vol. 20(3), 2008, 034101). The drag reduction is strongly related to the Froude number: for fixed Reynolds number we observe higher drag reduction when Fr < 1 than for with Fr > 1. This buoyancy effect is more prominent in low Rei systems and decreases with increasing Reynolds number Rei. We trace the drag reduction back to the weakening of the angular momentum carrying Taylor rolls by the rising bubbles. We also investigate how the motion of the dispersed phase depends on Rei and Fr, by studying the individual trajectories and mean dispersion of bubbles in the radial and axial directions. Indeed, the less buoyant bubbles (large Fr) tend to get trapped by the Taylor rolls, while the more buoyant bubbles (small Fr) rise through and weaken themItem Identifying coherent structures and vortex clusters in Taylor-Couette turbulence(Journal of Physics: Conference Series, 2016) Spandan, Vamsi; Ostilla-Mónico, Rodolfo; Lohse, Detlef; Verzicco, RobertoThe nature of the underlying structures in Taylor-Couette (TC) flow, the flow between two co-axial and independently rotating cylinders is investigated by two methods. First, the quadrant analysis technique for identifying structures with intense radial-azimuthal stresses (also referred to as 'Q's) of Lozano-Durán et al., (J. Fluid Mech. 694, 100-130) is used to identify the main structures responsible for the transport of angular velocity. Second, the vortex clusters are identified based on the analysis by del Álamo et al., (J. Fluid. Mech., 561, 329-358). In order to test these criteria, two different radius ratios ? = ri/ro are considered, where ri and ro are the radii of inner and outer cylinder, respectively: (i) ? = 0.5 and (ii) ? = 0.909, which correspond to high and low curvature geometries, respectively and have different underlying structures. The Taylor rolls, i.e. the large-scale coherent structures, are effectively captured as 'Q's for the low curvature setup and it is observed that curvature plays a dominant role in influencing the size and volumes of these 'Q's. On the other hand, the vortex clusters are smaller in size when compared to the 'Q' structures. These vortex clusters are found to be taller in the case of ? = 0.909, while the distribution of the lengths of these clusters is almost homogenous for both radius ratios.Item Life stages of wall-bounded decay of Taylor-Couette turbulence(Physical Review Fluids, 2017-11) Ostilla-Mónico, Rodolfo; Zhu, Xiaojue; Spandan, Vamsi; Verzicco, Roberto; Lohse, DetlefThe decay of Taylor-Couette turbulence, i.e., the flow between two coaxial and independently rotating cylinders, is numerically studied by instantaneously stopping the forcing from an initially statistically stationary flow field at a Reynolds number of Re =3.5×10^4.The effect of wall friction is analyzed by comparing three separate cases, in which the cylinders are either suddenly made no-slip or stress-free. Different life stages are observed during the decay. In the first stage, the decay is dominated by large-scale rolls. Counterintuitively, when these rolls fade away, if the flow inertia is small a redistribution of energy occurs and the energy of the azimuthal velocity behaves nonmonotonically, first decreasing by almost two orders of magnitude and then increasing during the redistribution. The second stage is dominated by non-normal transient growth of perturbations in the axial (spanwise) direction. Once this mechanism is exhausted, the flow enters the final life stage, viscous decay, which is dominated by wall friction. We show that this stage can be modeled by a one-dimensional heat equation, and that self-similar velocity profiles collapse onto the theoretical solution.