Browsing by Author "Lohse, Detlef"
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Item A multiple-resolution strategy for Direct Numerical Simulation of scalar turbulence(Journal of Computational Physics, 2015-02) Ostilla-Mónico, Rodolfo; Yang, Yantao; Van Der Poel, Erwin P.; Lohse, Detlef; Verzicco, RobertoIn this paper a numerical procedure to simulate low diffusivity scalar turbulence is presented. The method consists of using a grid for the advected scalar with a higher spatial resolution than that of the momentum. The latter usually requires a less refined mesh and integrating both fields on a single grid tailored to the most demanding variable produces an unnecessary computational overhead. A multiple resolution approach is used also in the time integration in order to maintain the stability of the scalars on the finer grid. The method is the more advantageous the less diffusive the scalar is with respect to momentum, therefore it is particularly well suited for large Prandtl or Schmidt number flows. However, even in the case of equal diffusivities the present procedure gives CPU time and memory occupation savings, due to the increased gradients and more intermittent behaviour of the scalars when compared to momentum.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 Asymmetry-induced particle drift in a rotating flow(The Physics of Fluids, 7/11/2005) Bluemink, Johanna J.; van Nierop, Ernst A.; Luther, S.; Deen, N. G.; Magnaudet, Jacques; Prosperetti, Andrea; Lohse, DetlefWe report on an intriguing phenomenon taking place in a liquid rotating around a fixed horizontal axis. Under suitable conditions, bubbles and particles are observed to drift along the axis of rotation maintaining a constant distance from it and a constant angle of elevation above the horizontal. Absence of fore-aft symmetry of the bubble or particle shape is a prerequisite for this phenomenon. For bubbles, this requires a volume sufficiently large for surface-tension effects to be small and large deformations to be possible. Particle image velocimetry and flow visualization suggest that the wake does not play a role. The dependence on bubble radius, particle shape, liquid viscosity, and speed of rotation is investigated.Item Boundary layer dynamics at the transition between the classical and the ultimate regime of Taylor-Couette flow(Physics of Fluids, 2014) Ostilla-Mónico, Rodolfo; Van Der Poel, Erwin P.; Verzicco, Roberto; Grossmann, Siegfried; Lohse, DetlefDirect numerical simulations of turbulent Taylor-Couette flow are performed up to inner cylinder Reynolds numbers of Rei = 105 for a radius ratio of ? = ri/ro = 0.714 between the inner and outer cylinders. With increasing Rei, the flow undergoes transitions between three different regimes: (i) a flow dominated by large coherent structures, (ii) an intermediate transitional regime, and (iii) a flow with developed turbulence. In the first regime the large-scale rolls completely drive the meridional flow, while in the second one the coherent structures recover only on average. The presence of a mean flow allows for the coexistence of laminar and turbulent boundary layer dynamics. In the third regime, the mean flow effects fade away and the flow becomes dominated by plumes. The effect of the local driving on the azimuthal and angular velocity profiles is quantified, in particular, we show when and where those profiles developItem Direct numerical simulation of Taylor–Couette flow with grooved walls: torque scaling and flow structure(Journal of Fluid Mechanics, 2016-03) Zhu, Xiaojue; Ostilla-Mónico, Rodolfo; Verzicco, Roberto; Lohse, DetlefWe present direct numerical simulations of Taylor–Couette flow with grooved walls at a fixed radius ratio n=ri/r0=0.714 with inner cylinder Reynolds number up to rei=3.76x10^4 , corresponding to Taylor number up to Ta=2.15x10^9 . The grooves are axisymmetric V-shaped obstacles attached to the wall with a tip angle of 90°. Results are compared to the smooth wall case in order to investigate the effects of grooves on Taylor–Couette flow. We focus on the effective scaling laws for the torque, flow structures, and boundary layers. It is found that, when the groove height is smaller than the boundary layer thickness, the torque is the same as that of the smooth wall cases. With increasing Ta , the boundary layer thickness becomes smaller than the groove height. Plumes are ejected from the tips of the grooves and secondary circulations between the latter are formed. This is associated with a sharp increase of the torque, and thus the effective scaling law for the torque versus Ta becomes much steeper. Further increasing Ta does not result in an additional slope increase. Instead, the effective scaling law saturates to the ‘ultimate’ regime effective exponents seen for smooth walls. It is found that even though after saturation the slope is the same as for the smooth wall case, the absolute value of torque is increased, and more so with the larger size of the grooves.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 Drop Fragmentation at Impact onto a Bath of an Immiscible Liquid(Physical Review Letters, 6/28/2013) Lhuissier, Henri; Sun, Chao; Prosperetti, Andrea; Lohse, DetlefThe impact of a drop onto a deep bath of an immiscible liquid is studied with emphasis on the drop fragmentation into a collection of noncoalescing daughter drops. At impact the drop flattens and spreads at the surface of the crater it transiently opens in the bath and reaches a maximum deformation, which gets larger with increasing impact velocity, before surface tension drives its recession. This recession can promote the fragmentation by two different mechanisms: At moderate impact velocity, the drop recession converges to the axis of symmetry to form a jet which then fragments by a Plateau-Rayleigh mechanism. At higher velocity the edge of the receding drop destabilizes and shapes into radial ligaments which subsequently fragment. For this latter mechanism the number N?We3 and the size distribution of the daughter drops p(d)?d?4 as a function of the impact Weber number We are explained on the basis of the observed spreading of the drop. The universality of this model for the fragmentation of receding liquid sheets might be relevant for other configurations.Item Drop Impact on Superheated Surfaces(Physical Review Letters, 1/20/2012) Tran, Tuan; Staat, Hendrik J.J.; Prosperetti, Andrea; Sun, Chao; Lohse, DetlefAt the impact of a liquid droplet on a smooth surface heated above the liquid’s boiling point, the droplet either immediately boils when it contacts the surface (“contact boiling”), or without any surface contact forms a Leidenfrost vapor layer towards the hot surface and bounces back (“gentle film boiling”), or both forms the Leidenfrost layer and ejects tiny droplets upward (“spraying film boiling”). We experimentally determine conditions under which impact behaviors in each regime can be realized. We show that the dimensionless maximum spreading ? of impacting droplets on the heated surfaces in both gentle and spraying film boiling regimes shows a universal scaling with the Weber number We (??We 2/5), which is much steeper than for the impact on nonheated (hydrophilic or hydrophobic) surfaces (??We 1/4). We also interferometrically measure the vapor thickness under the droplet.Item Dynamic Leidenfrost effect: relevant time and length scales(Physical Review Letters, 2/10/2016) Shirota, Minori; van Limbeek, Michiel A.J.; Sun, Chao; Prosperetti, Andrea; Lohse, DetlefWhen a liquid droplet impacts a hot solid surface, enough vapor may be generated under it to prevent its contact with the solid. The minimum solid temperature for this so-called Leidenfrost effect to occur is termed the Leidenfrost temperature, or the dynamic Leidenfrost temperature when the droplet velocity is non-negligible. We observe the wetting or drying and the levitation dynamics of the droplet impacting on an (isothermal) smooth sapphire surface using high-speed total internal reflection imaging, which enables us to observe the droplet base up to about 100 nm above the substrate surface. By this method we are able to reveal the processes responsible for the transitional regime between the fully wetting and the fully levitated droplet as the solid temperature increases, thus shedding light on the characteristic time and length scales setting the dynamic Leidenfrost temperature for droplet impact on an isothermal substrate.Item Dynamics of Formation of a Vapor Nanobubble Around a Heated Nanoparticle(The Journal of Physical Chemistry, 8/17/2018) Maheshwari, Shantanu; van der Hoef, Martin; Prosperetti, Andrea; Lohse, DetlefWe study the formation of a nanobubble around a heated nanoparticle in a bulk liquid by using molecular dynamics simulations. The nanoparticle is kept at a temperature above the critical temperature of the surrounding liquid, leading to the formation of a vapor nanobubble attached to it. First, we study the role of both the temperature of the bulk liquid far away from the nanoparticle surface and the temperature of the nanoparticle itself on the formation of a stable vapor nanobubble. We determine the exact conditions under which it can be formed and compare this with the conditions that follow from a macroscopic heat balance argument. Next, we demonstrate the role of dissolved gas on the conditions required for nucleation of a nanobubble and on its growth dynamics. We find that beyond a certain threshold concentration, the dissolved gas dramatically facilitates vapor bubble nucleation due to the formation of gaseous weak spots in the surrounding liquid.Item Effect of roll number on the statistics of Taylor-Couette flow(Physical Review Fluids, 2016-09) Ostilla-Mónico, Rodolfo; Lohse, Detlef; Verzicco, RobertoA series of direct numerical simulations in large computational domains has been performed in order to probe the spatial feature robustness of the Taylor rolls in turbulent Taylor-Couette flow. The latter is the flow between two coaxial independently rotating cylinders of radius ri and ro, respectively. Large axial aspect ratios ?=7–8 [with ?=L/(ro?ri), and L the axial length of the domain] and a simulation with ?=14were used in order to allow the system to select the most unstable wave number and to possibly develop multiple states. The radius ratio was taken as ?=ri/ro= 0.909, the inner cylinder Reynolds number was fixed to Rei=3.4×10^4, and the outer cylinder was kept stationary, resulting in a frictional Reynolds number of Re??500, except for the ?=14simulation where Rei=1.5×10^4 and Re??240. The large-scale rolls were found to remain axially pinned for all simulations. Depending on the initial conditions, stable solutions with different number of rolls nrand roll wavelength ?z were found for ?=7.The effect of ?z and nron the statistics was quantified. The torque and mean flow statistics were found to be independent of both ?z and nr, while the velocity fluctuations and energy spectra showed some box-size dependence. Finally, the axial velocity spectra were found to have a very sharp dropoff for wavelengths larger than ?z, while for the small wavelengths they collapse.Item Effect of vapor bubbles on velocity fluctuations and dissipation rates in bubbly Rayleigh-B ?enard convection(Physical Review E, 9/19/2011) Lakkaraju, Rajaram; Schmidt, Laura E.; Oresta, Paolo; Toschi, Federico; Verzicco, Roberto; Lohse, Detlef; Prosperetti, AndreaNumerical results for kinetic and thermal energy dissipation rates in bubbly Rayleigh-Bénard convection are reported. Bubbles have a twofold effect on the flow: on the one hand, they absorb or release heat to the surrounding liquid phase, thus tending to decrease the temperature differences responsible for the convective motion; but on the other hand, the absorbed heat causes the bubbles to grow, thus increasing their buoyancy and enhancing turbulence (or, more properly, pseudoturbulence) by generating velocity fluctuations. This enhancement depends on the ratio of the sensible heat to the latent heat of the phase change, given by the Jakob number, which determines the dynamics of the bubble growth.Item Effect of velocity boundary conditions on the heat transfer and flow topology in two-dimensional Rayleigh-Bénard convection(Physical Review E, 2014-07) Van Der Poel, Erwin P.; Ostilla-Mónico, Rodolfo; Verzicco, Roberto; Lohse, DetlefThe effect of various velocity boundary condition is studied in two-dimensional Rayleigh-Bénard convection. Combinations of no-slip, stress-free, and periodic boundary conditions are used on both the sidewalls and the horizontal plates. For the studied Rayleigh numbers Ra between 10^8 and 10^11 the heat transport is lower for ?=0.33 than for ?=1in case of no-slip sidewalls. This is, surprisingly, the opposite for stress-free sidewalls, where the heat transport increases for a lower aspect ratio. In wider cells the aspect-ratio dependence is observed to disappear for Ra?10^10. Two distinct flow types with very different dynamics can be seen, mostly dependent on the plate velocity boundary condition, namely roll-like flow and zonal flow, which have a substantial effect on the dynamics and heat transport in the system. The predominantly horizontal zonal flow suppresses heat flux and is observed for stress-free and asymmetric plates. Low aspect-ratio periodic sidewall simulations with a no-slip boundary condition on the plates also exhibit zonal flow. In all the other cases, the flow is roll like. In two-dimensional Rayleigh-Bénard convection, the velocity boundary conditions thus have large implications on both roll-like and zonal flow that have to be taken into consideration before the boundary conditions are imposed.Item Effects of the computational domain size on DNS of Taylor-Couette turbulence with stationary outer cylinder(Physics of Fluids, 2015) Ostilla-Mónico, Rodolfo; Verzicco, Roberto; Lohse, DetlefIn search for the cheapest but still reliable numerical simulation, a systematic study on the effect of the computational domain (“box”) size on direct numerical simulations of Taylor-Couette flow was performed. Four boxes with varying azimuthal and axial extents were used. The radius ratio between the inner cylinder and the outer cylinder was fixed to ? = ri/ro = 0.909. The outer cylinder was kept stationary, while the inner rotated at a Reynolds number Rei = 105. Profiles of mean and fluctuation velocities are compared, as well as autocorrelations and velocity spectra. The smallest box is found to accurately reproduce the torque and mean azimuthal velocity profiles of larger boxes, while having smaller values of the fluctuations than the larger boxes. The axial extent of the box directly reflects on the Taylor-rolls and plays a crucial role on the correlations and spectra. The azimuthal extent is found to play a minor role in the simulations, as the boxes are large enough. For all boxes studied, the spectra do not reach a box independent maximum.Item Efficient sonochemistry through microbubbles generated with micromachined surfaces(Angewandte Chemie, 8/3/2012) Fernandez Rivas, David; Prosperetti, Andrea; Zijlstra, Aaldert G.; Lohse, Detlef; Gardeniers, Han J.G.E.It's the pits: Increased efficiency and controllability of sonochemical reactions was achieved with silicon surfaces on which pits were micromachined to entrap gas, which, upon ultrasonic excitation, emits a stream of microbubbles (see picture). The microbubbles are chemically active at ultrasonic amplitudes well below those necessary for sonochemical activity in conventional reactors.Item Entrapped air bubbles in piezo-driven inkjet printing: Their effect on the droplet velocity(The Physics of Fluids, 12/8/2006) de Jong, Jos; Jeurissen, Roger; Borel, Huub; van den Berg, Marc; Wijshoff, Herman; Reinten, Hans; Versluis, Michel; Prosperetti, Andrea; Lohse, DetlefAir bubbles entrapped in the ink channel are a major problem in piezo-driven inkjet printing. They grow by rectified diffusion and eventually counteract the pressure buildup at the nozzle, leading to a breakdown of the jetting process. Experimental results on the droplet velocity udrop as a function of the equilibrium radius R0 of the entrained bubble are presented. Surprisingly, udrop(R0) shows a pronounced maximum around R0=17?m before it sharply drops to zero around R0=19?m. A simple one-dimensional model is introduced to describe this counterintuitive behavior which turns out to be a resonance effect of the entrained bubble.Item Giant Bubble Pinch-Off(Physical Review Letters, 4/21/2006) Bergmann, Raymond; van der Meer, Devaraj; Stijnman, Mark; Sandtke, Marijn; Prosperetti, Andrea; Lohse, DetlefSelf-similarity has been the paradigmatic picture for the pinch-off of a drop. Here we will show through high-speed imaging and boundary integral simulations that the inverse problem, the pinch-off of an air bubble in water, is not self-similar in a strict sense: A disk is quickly pulled through a water surface, leading to a giant, cylindrical void which after collapse creates an upward and a downward jet. Only in the limiting case of large Froude numbers does the purely inertial scaling h(?log h)1/4??1/2 for the neck radius h [J.?M. Gordillo et al., Phys. Rev. Lett. 95, 194501 (2005)] become visible. For any finite Froude number the collapse is slower, and a second length scale, the curvature of the void, comes into play. Both length scales are found to exhibit power-law scaling in time, but with different exponents depending on the Froude number, signaling the nonuniversality of the bubble pinch-off.Item Growing bubbles in a slightly supersaturated liquid solution(Review of Scientific Instruments, 6/26/2013) Enríquez, Oscar R.; Hummelink, Christian; Bruggert, Gert-Wim; Lohse, Detlef; Prosperetti, Andrea; van der Meer, Devaraj; Sun, ChaoWe have designed and constructed an experimental system to study gas bubble growth in slightly supersaturated liquids. This is achieved by working with carbon dioxide dissolved in water, pressurized at a maximum of 1MPa and applying a small pressure drop from saturation conditions. Bubbles grow from hydrophobic cavities etched on silicon wafers, which allows us to control their number and position. Hence, the experiment can be used to investigate the interaction among bubbles growing in close proximity when the main mass transfer mechanism is diffusion and there is a limited availability of the dissolved species.Item Harmonic enhancement of single-bubble sonoluminescence(Physical Review E, 5/22/2003) Lu, Xiaozhen; Prosperetti, Andrea; Toegel, Ruediger; Lohse, DetlefIt is known from experiment that the light emission from a sonoluminescing bubble can be increased by using more than one driving frequency. In this paper, a systematic method to determine the optimal conditions of pressure amplitude and relative phase for this effect is described. As a specific application, a two-frequency system—26.5 kHz and 53 kHz—is considered. It is found that the maximum temperatures achievable can be appreciably increased with respect to single-frequency drive, still maintaining spherical stability, provided the dissolved inert gas concentration is kept extremely low in order to maintain diffusive stability.
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