Publications (32)63.94 Total impact
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Article: Optimal Taylor-Couette flow: direct numerical simulations
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ABSTRACT: We numerically simulate turbulent Taylor-Couette flow for independently rotating in-ner and outer cylinders, focusing on the analogy with turbulent Rayleigh-Bénard flow. Reynolds numbers of Re i = 8 · 10 3 and Re o = ±4 · 10 3 of the inner and outer cylinders, respectively, are reached, corresponding to Taylor numbers Ta up to 10 8 . Effective scal-ing laws for the torque and other system responses are found. Recent experiments with the Twente turbulent Taylor-Couette (T 3 C) setup and with a similar facility in Mary-land at very high Reynolds numbers have revealed an optimum transport at a certain non-zero rotation rate ratio a = −ω o /ω i of about a opt = 0.33 − 0.35. For large enough T a in the numerically accessible range we also find such an optimum transport at non-zero counter-rotation. The position of this maximum is found to shift with the driving, reaching a maximum of a opt = 0.15 for T a = 2.5 · 10 7 . An explanation for this shift is elucidated, consistent with the experimental result that a opt becomes approximately independent of the driving strength for large enough Reynolds numbers. We furthermore numerically calculate the angular velocity profiles and visualize the different flow struc-tures for the various regimes. By writing the equations in a frame co-rotating with the outer cylinder a link is found between the local angular velocity profiles and the global transport quantities.Journal of Fluid Mechanics 12/2013; 719:14-46. · 2.46 Impact Factor -
Article: Optimal Taylor-Couette flow: Radius ratio dependence
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ABSTRACT: Taylor-Couette flow with independently rotating inner (i) and outer (o) cylinders is explored numerically and experimentally to determine the effects of the radius ratio {\eta} on the system response. Numerical simulations reach Reynolds numbers of up to Re_i=9.5 x 10^3 and Re_o=5x10^3, corresponding to Taylor numbers of up to Ta=10^8 for four different radius ratios {\eta}=r_i/r_o between 0.5 and 0.909. The experiments, performed in the Twente Turbulent Taylor-Couette (T^3C) setup, reach Reynolds numbers of up to Re_i=2x10^6$ and Re_o=1.5x10^6, corresponding to Ta=5x10^{12} for {\eta}=0.714-0.909. Effective scaling laws for the torque J^{\omega}(Ta) are found, which for sufficiently large driving Ta are independent of the radius ratio {\eta}. As previously reported for {\eta}=0.714, optimum transport at a non-zero Rossby number Ro=r_i|{\omega}_i-{\omega}_o|/[2(r_o-r_i){\omega}_o] is found in both experiments and numerics. Ro_opt is found to depend on the radius ratio and the driving of the system. At a driving of about Ta~10^{10}, Ro_opt saturates to an asymptotic {\eta}-dependent value. Theoretical predictions for the asymptotic value of Ro_{opt} are compared to the experimental results, and found to differ notably. Furthermore, the local angular velocity profiles from experiments and numerics are compared, and a link between a flat bulk profile and optimum transport for all radius ratios is reported.04/2013; -
Article: Spatial distribution of heat flux and fluctuations in turbulent Rayleigh-Benard convection
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ABSTRACT: We numerically investigate the radial dependence of the velocity and temperature fluctuations and of the time-averaged heat flux $\bar{j}(r)$ in a cylindrical Rayleigh-B\'enard cell with aspect ratio \Gamma=1 for Rayleigh numbers Ra between $2 \times 10^6$ and $2\times 10^{9}$ at a fixed Prandtl number Pr = 5.2. The numerical results reveal that the heat flux close to the sidewall is larger than in the center and that, just as the global heat transport, it has an effective power law dependence on the Rayleigh number, $\bar{j}(r)\propto Ra^{\gamma_j(r)}$. The scaling exponent $\gamma_j(r)$ decreases monotonically from 0.43 near the axis ($r \approx 0$) to 0.29 close to the side walls ($r \approx D/2$). The effective exponents near the axis and the side wall agree well with the measurements of Shang et al. (Phys.\ Rev.\ Lett.\ \textbf{100}, 244503, 2008) and the predictions of Grossmann and Lohse (Phys.\ Fluids \textbf{16}, 1070, 2004). Extrapolating our results to large Rayleigh number would imply a crossover at $Ra\approx 10^{15}$, where the heat flux near the axis would begin to dominate. In addition, we find that the local heat flux is more than twice as high at the location where warm or cold plumes go up or down, than in the plume depleted regions.Physical Review E 12/2012; 86:056315. · 2.26 Impact Factor -
Article: Spatial distribution of heat flux and fluctuations in turbulent Rayleigh-Bénard convection.
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ABSTRACT: We numerically investigate the radial dependence of the velocity and temperature fluctuations and of the time-averaged heat flux j[over ¯](r) in a cylindrical Rayleigh-Bénard cell with aspect ratio Γ=1 for Rayleigh numbers Ra between 2×10^{6} and 2×10^{9} at a fixed Prandtl number Pr=5.2. The numerical results reveal that the heat flux close to the sidewall is larger than in the center and that, just as the global heat transport, it has an effective power law dependence on the Rayleigh number, j[over ¯](r)∝Ra^{γ_{j}(r)}. The scaling exponent γ_{j}(r) decreases monotonically from 0.43 near the axis (r≈0) to 0.29 close to the sidewalls (r≈D/2). The effective exponents near the axis and the sidewall agree well with the measurements of Shang et al. [Phys. Rev. Lett. 100, 244503 (2008)] and the predictions of Grossmann and Lohse [Phys. Fluids 16, 1070 (2004)]. Extrapolating our results to large Rayleigh number would imply a crossover at Ra≈10^{15}, where the heat flux near the axis would begin to dominate. In addition, we find that the local heat flux is more than twice as high at the location where warm or cold plumes go up or down than in plume depleted regions.Physical Review E 11/2012; 86(5-2):056315. · 2.26 Impact Factor -
Article: Optimal Taylor-Couette flow: direct numerical simulations
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ABSTRACT: We numerically simulate turbulent Taylor-Couette flow for independently rotating inner and outer cylinders, focusing on the analogy with turbulent Rayleigh-B\'enard flow. Reynolds numbers of $Re_i=8\cdot10^3$ and $Re_o=\pm4\cdot10^3$ of the inner and outer cylinders, respectively, are reached, corresponding to Taylor numbers Ta up to $10^8$. Effective scaling laws for the torque and other system responses are found. Recent experiments with the Twente turbulent Taylor-Couette ($T^3C$) setup and with a similar facility in Maryland at very high Reynolds numbers have revealed an optimum transport at a certain non-zero rotation rate ratio $a = -\omega_o / \omega_i$ of about $a_{opt}=0.33-0.35$. For large enough $Ta$ in the numerically accessible range we also find such an optimum transport at non-zero counter-rotation. The position of this maximum is found to shift with the driving, reaching a maximum of $a_{opt}=0.15$ for $Ta=2.5\cdot10^7$. An explanation for this shift is elucidated, consistent with the experimental result that $a_{opt}$ becomes approximately independent of the driving strength for large enough Reynolds numbers. We furthermore numerically calculate the angular velocity profiles and visualize the different flow structures for the various regimes. By writing the equations in a frame co-rotating with the outer cylinder a link is found between the local angular velocity profiles and the global transport quantities.07/2012; -
Article: Logarithmic temperature profiles in turbulent Rayleigh-Bénard convection
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ABSTRACT: We report results for the temperature profiles of turbulent Rayleigh-Bénard convection (RBC) in the interior of a cylindrical sample of aspect ratio Γ ≡ D/L = 0.50 (D and L are the diameter and height respectively). Results from experiment over the Rayleigh number range 4×10 12 < ∼ Ra < ∼ 10 15 for a Prandtl number Pr ≃ 0.8 and from direct numerical simulation (DNS) at Ra = 2 × 10 12 for Pr = 0.7 are presented. We find that the temperature varies as A * ln(z/L) + B where z is the distance from the bottom or top plate. This is the case in the classical as well as in the ultimate state of RBC. From DNS we find that A in the classical state decreases in the radial direction as the distance from the side wall increases and becomes small near the sample center.Physical Review Letters 05/2012; 109:114501. · 7.37 Impact Factor -
Article: A Non-Adiabatic Flamelet Progress–Variable Approach for LES of Turbulent Premixed Flames
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ABSTRACT: A progress variable/flame surface density/probability density function method has been employed for a Large Eddy Simulation of a CH4/Air turbulent premixed bluff body flame. In particular, both mean and variance of the progress variable are transported and subgrid spatially filtered gradient contributes to model the flame surface density (that introduces the effect of the subgrid flame reaction zone) and to presume a probability density function (that introduces the effect of subgrid fluctuations on chemistry). Chemistry is preliminarly tabulated in terms of laminar premixed flames and enthalpy is included as a new coordinate in their tabulation to take into account heat losses in the flowfield. Then, the PDF is used to build a turbulent flamelet library. The filtered mass, momentum, enthalpy and scalar equations mentioned above are integrated by an explicit scheme using finite differences, 2nd–order accurate in space and third order in time, over a cylindrical non-uniform grid using a staggered mesh. The bluff-body geometry is modelled by using the Immersed Boundary Method. The numerical predictions are compared with the available experimental data. KeywordsPremixed turbulent flame–Large Eddy Simulation–Progress variable–Flame surface density–Probability density function–Immersed Boundary MethodFlow Turbulence and Combustion 04/2012; 86(3):667-688. · 1.11 Impact Factor -
Article: Logarithmic temperature profiles in turbulent Rayleigh-B\'enard convection
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ABSTRACT: We report results for the temperature profiles of turbulent Rayleigh-B\'enard convection (RBC) in the interior of a cylindrical sample of aspect ratio $\Gamma \equiv D/L = 0.50$ ($D$ and $L$ are the diameter and height respectively). Results from experiment over the Rayleigh number range $4\times 10^{12} \alt Ra \alt 10^{15}$ for a Prandtl number $\Pra \simeq 0.8$ and from direct numerical simulation (DNS) at $Ra = 2 \times 10^{12}$ for $\Pra = 0.7$ are presented. We find that the temperature varies as $A*ln(z/L) + B$ where $z$ is the distance from the bottom or top plate. This is the case in the classical as well as in the ultimate state of RBC. From DNS we find that $A$ in the classical state decreases in the radial direction as the distance from the side wall increases and becomes small near the sample center.04/2012; -
Article: Combined Immersed Boundary/Large-Eddy-Simulations of Incompressible Three Dimensional Complex Flows
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ABSTRACT: In this paper we show how the Immersed Boundary (IB) method can be used with the Large-Eddy-Simulation (LES) to compute moderately high Reynolds number flows in complex geometric configurations. The resulting combination gives an easy-to-use, inexpensive and accurate technique which can be an important step towards the application of computational fluid dynamics (CFD) to industrially relevant problems. This paper aims at describing the main features of the method, some of the important drawbacks and possible solutions. Several representative examples are discussed in order to show the flexibility and the range of the applicability of this technique.Flow Turbulence and Combustion 04/2012; 77(1):3-26. · 1.11 Impact Factor -
Article: Thermal boundary layer profiles in turbulent Rayleigh-Bénard convection in a cylindrical sample.
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ABSTRACT: We numerically investigate the structures of the near-plate temperature profiles close to the bottom and top plates of turbulent Rayleigh-Bénard flow in a cylindrical sample at Rayleigh numbers Ra = 10(8) to Ra = 2 × 10(12) and Prandtl numbers Pr = 6.4 and Pr = 0.7 with the dynamical frame method [Zhou and Xia, Phys. Rev. Lett. 104, 104301 (2010)], thus extending previous results for quasi-two-dimensional systems to three-dimensional systems. The dynamical frame method shows that the measured temperature profiles in the spatially and temporally local frame are much closer to the temperature profile of a laminar, zero-pressure gradient boundary layer (BL) according to Pohlhausen than in the fixed reference frame. The deviation between the measured profiles in the dynamical reference frame and the laminar profiles increases with decreasing Pr, where the thermal BL is more exposed to the bulk fluctuations due to the thinner kinetic BL, and increasing Ra, where more plumes are passing the measurement location.Physical Review E 02/2012; 85(2 Pt 2):027301. · 2.26 Impact Factor -
Article: Thermal boundary layer profiles in turbulent Rayleigh-Bénard convection in a cylindrical sample
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ABSTRACT: We numerically investigate the structures of the near-plate temperature profiles close to the bottom and top plates of turbulent Rayleigh-Bénard flow in a cylindrical sample at Rayleigh numbers Ra=108 to Ra=2×1012 and Prandtl numbers Pr=6.4 and Pr=0.7 with the dynamical frame method [ Zhou and Xia Phys. Rev. Lett. 104 104301 (2010)], thus extending previous results for quasi-two-dimensional systems to three-dimensional systems. The dynamical frame method shows that the measured temperature profiles in the spatially and temporally local frame are much closer to the temperature profile of a laminar, zero-pressure gradient boundary layer (BL) according to Pohlhausen than in the fixed reference frame. The deviation between the measured profiles in the dynamical reference frame and the laminar profiles increases with decreasing Pr, where the thermal BL is more exposed to the bulk fluctuations due to the thinner kinetic BL, and increasing Ra, where more plumes are passing the measurement location.Physical Review E 02/2012; 85:027301. · 2.26 Impact Factor -
Article: Prandtl and Rayleigh number dependence of heat transport in high Rayleigh number thermal convection
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ABSTRACT: Results from direct numerical simulation for three-dimensional Rayleigh–Bénard convection in samples of aspect ratio and up to Rayleigh number are presented. The broad range of Prandtl numbers is considered. In contrast to some experiments, we do not see any increase in with increasing , neither due to an increasing , nor due to constant heat flux boundary conditions at the bottom plate instead of constant temperature boundary conditions. Even at these very high , both the thermal and kinetic boundary layer thicknesses obey Prandtl–Blasius scaling.Journal of Fluid Mechanics 10/2011; 688:31-43. · 2.46 Impact Factor -
Article: Effect of vapor bubbles on velocity fluctuations and dissipation rates in bubbly Rayleigh-Bénard convection.
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ABSTRACT: Numerical 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.Physical Review E 09/2011; 84(3 Pt 2):036312. · 2.26 Impact Factor -
Article: Axially-homogeneous Rayleigh-Benard convection in a cylindrical cell
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ABSTRACT: Previous numerical studies have shown that the "ultimate regime of thermal convection" can be attained in a Rayleigh-Benard cell when the kinetic and thermal boundary layers are eliminated by replacing the walls with periodic boundary conditions (homogeneous Rayleigh-Benard convection). Then, the heat transfer scales like Nu ~ Ra^{1/2} and turbulence intensity as Re ~ Ra^{1/2}, where the Rayleigh number Ra indicates the strength of the driving force. However, experiments never operate in unbounded domains and it is important to understand how confinement might alter the approach to this ultimate regime. Here we consider homogeneous Rayleigh-Benard convection in a laterally confined geometry - a small aspect-ratio vertical cylindrical cell - and show evidence of the ultimate regime as Ra is increased: In spite of the confinement and the resulting kinetic boundary layers, we still find Nu ~ Re ~ Ra^{1/2}. The system supports exact solutions composed of modes of exponentially growing vertical velocity and temperature fields, with Ra as the critical parameter determining the properties of these modes. Counterintuitively, in the low Ra regime, or for very narrow cylinders, the numerical simulations are susceptible to these solutions which can dominate the dynamics and lead to very high and unsteady heat transfer. As Ra is increased, interaction between modes stabilizes the system, evidenced by the increasing homogeneity and reduced fluctuations in the r.m.s. velocity and temperature fields. We also test that physical results become independent of the periodicity length of the cylinder, a purely numerical parameter, as the aspect ratio is increased.04/2011; -
Article: Prandtl and Rayleigh number dependence of heat transport in high Rayleigh number thermal convection
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ABSTRACT: Results from direct numerical simulation for three-dimensional Rayleigh-B\'enard convection in samples of aspect ratio $\Gamma=0.23$ and $\Gamma=0.5$ up to Rayleigh number $Ra=2\times10^{12}$ are presented. The broad range of Prandtl numbers $0.5<Pr<10$ is considered. In contrast to some experiments, we do not see any increase in $Nu/Ra^{1/3}$, neither due to $Pr$ number effects, nor due to a constant heat flux boundary condition at the bottom plate instead of constant temperature boundary conditions. Even at these very high $Ra$, both the thermal and kinetic boundary layer thicknesses obey Prandtl-Blasius scaling.Journal of Fluid Mechanics 02/2011; 688:31-43. · 2.46 Impact Factor -
Article: Modification of turbulence in Rayleigh–Bénard convection by phase change
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ABSTRACT: Heavy or light particles introduced into a liquid trigger motion due to their buoyancy, with the potential to drive flow to a turbulent state. In the case of vapor bubbles present in a liquid near its boiling point, thermal coupling between the liquid and vapor can moderate this additional motion by reducing temperature gradients in the liquid. Whether the destabilizing mechanical feedback or stabilizing thermal feedback will dominate the system response depends on the number of bubbles present and the properties of the phase change. Here we study thermal convection with phase change in a cylindrical Rayleigh–Bénard cell to examine this competition. Using the Reynolds number of the flow as a signature of turbulence and the intensity of the flow, we show that in general the rising vapor bubbles destabilize the system and lead to higher velocities. The exception is a limited regime corresponding to phase change with a high latent heat of vaporization (corresponding to low Jakob number), where the vapor bubbles can eliminate the convective flow by smoothing temperature differences of the fluid.New Journal of Physics 02/2011; 13(2):025002. · 4.18 Impact Factor -
Article: Fluid velocity fluctuations in a collision of a sphere with a wall
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ABSTRACT: We report on the results of a combined experimental and numerical study on the fluid motion generated by the controlled approach and arrest of a solid sphere moving towards a solid wall at moderate Reynolds number. The experiments are performed in a small tank filled with water for a range of Reynolds numbers for which the flow remains axisymmetric. The fluid agitation of the fluid related to the kinetic energy is obtained as function of time in the experiment in a volume located around the impact point. The same quantities are obtained from the numerical simulations for the same volume of integration as in the experiments and also for the entire volume of the container. As shown in previous studies, this flow is characterized by a vortex ring, initially in the wake of the sphere, that spreads radially along the wall, generating secondary vorticity of opposite sign at the sphere surface and wall. It is also observed that before the impact, the kinetic energy increases sharply for a small period of time and then decreases gradually as the fluid motion dies out. The measure of the relative agitation of the collision is found to increase weakly with the Reynolds number Re. The close agreement between the numerics and experiments is indicative of the robustness of the results. These results may be useful in light of a potential modelling of particle-laden flows. Movies illustrating the spatio-temporal dynamics are provided with the online version of this paper. V C 2011 American Institute of Physics. [doi:Physics of Fluids 01/2011; 23(6):063301. · 1.93 Impact Factor -
Article: Radial boundary layer structure and Nusselt number in Rayleigh–Bénard convection
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ABSTRACT: Results from direct numerical simulation (DNS) for three-dimensional Rayleigh– Bénard convection in a cylindrical cell of aspect ratio 1/2 and Prandtl number P r = 0.7 are presented. They span five decades of Rayleigh number Ra from 2 × 10 6 to 2 × 10 11 . The results are in good agreement with the experimental data of Niemela et al. (Nature, vol. 404, 2000, p. 837). Previous DNS results from Amati et al. (Phys. Flu-ids, vol. 17, 2005, paper no. 121701) showed a heat transfer that was up to 30 % higher than the experimental values. The simulations presented in this paper are performed with a much higher resolution to properly resolve the plume dynamics. We find that in under-resolved simulations the hot (cold) plumes travel further from the bottom (top) plate than in the better-resolved ones, because of insufficient thermal dissipation mainly close to the sidewall (where the grid cells are largest), and therefore the Nusselt number in under-resolved simulations is overestimated. Furthermore, we compare the best resolved thermal boundary layer profile with the Prandtl–Blasius profile. We find that the boundary layer profile is closer to the Prandtl–Blasius profile at the cylinder axis than close to the sidewall, because of rising plumes close to the sidewall.Journal of Fluid Mechanics 01/2010; 643:495-507. · 2.46 Impact Factor -
Article: Nonlinear spin-up of a thermally stratified fluid in cylindrical geometries
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ABSTRACT: This is an entry for the Gallery of Fluid Motion of the 62nd Annual Meeting of the APS-DFD (fluid dynamics videos). This video shows the three-dimensional time-dependent incremental spin-up of a thermally stratified fluid in a cylinder and in an annulus. The rigid bottom/side wall(s) are non-slip, and the upper surface is stress-free. All the surfaces are thermally insulated. The working fluid is water characterized by the kinematic viscosity $\nu$ and thermal diffusivity $\kappa$. Initially, the fluid temperature varies linearly with height and is characterized by a constant buoyancy frequency $N$, which is proportional to the density gradient. The system undergoes an abrupt change in the rotation rate from its initial value $\Omega_i $, when the fluid is in a solid-body rotation state, to the final value $\Omega_f$. Our study reveals a feasibility for transition from an axisymmetric initial circulation to non-axisymmetric flow patterns at late spin-up times.09/2009; -
Article: Heat transfer mechanisms in bubbly Rayleigh-Bénard convection.
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ABSTRACT: The heat transfer mechanism in Rayleigh-Bénard convection in a liquid with a mean temperature close to its boiling point is studied through numerical simulations with pointlike vapor bubbles, which are allowed to grow or shrink through evaporation and condensation and which act back on the flow both thermally and mechanically. It is shown that the effect of the bubbles is strongly dependent on the ratio of the sensible heat to the latent heat as embodied in the Jakob number Ja. For very small Ja the bubbles stabilize the flow by absorbing heat in the warmer regions and releasing it in the colder regions. With an increase in Ja, the added buoyancy due to the bubble growth destabilizes the flow with respect to single-phase convection and considerably increases the Nusselt number.Physical Review E 08/2009; 80(2 Pt 2):026304. · 2.26 Impact Factor
Top co-authors
Top Journals
Institutions
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2012
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Politecnico di Bari
Bari, Apulia, Italy
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2009–2012
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Universiteit Twente
- • Group of Physics of Fluids
- • Group of Physics of Fluids (POF)
Enschede, Provincie Overijssel, Netherlands -
University of California, Santa Barbara
- Department of Physics
Santa Barbara, CA, USA
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2008–2012
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University of Rome Tor Vergata
- Dipartimento di Ingegneria Meccanica
Roma, Latium, Italy -
Arizona State University
Tempe, AZ, USA
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