Imperial College London

Radu Cimpeanu

Faculty of Natural SciencesDepartment of Mathematics

Honorary Research Associate
 
 
 
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Contact

 

radu.cimpeanu11 CV

 
 
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Location

 

742Huxley BuildingSouth Kensington Campus

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Summary

 

Publications

Publication Type
Year
to

20 results found

Negus MJ, Moore MR, Oliver JM, Cimpeanu Ret al., 2021, Droplet impact onto a spring-supported plate: analysis and simulations, JOURNAL OF ENGINEERING MATHEMATICS, Vol: 128, ISSN: 0022-0833

Journal article

Dalwadi MP, Cimpeanu R, Ockendon H, Ockendon J, Mullin Tet al., 2021, Levitation of a cylinder by a thin viscous film, JOURNAL OF FLUID MECHANICS, Vol: 917, ISSN: 0022-1120

Journal article

Cimpeanu R, Gomes SN, Papageorgiou DT, 2021, Active control of liquid film flows: beyond reduced-order models, NONLINEAR DYNAMICS, Vol: 104, Pages: 267-287, ISSN: 0924-090X

Journal article

Galeano-Rios CA, Cimpeanu R, Bauman IA, MacEwen A, Milewski PA, Harris DMet al., 2021, Capillary-scale solid rebounds: experiments, modelling and simulations, JOURNAL OF FLUID MECHANICS, Vol: 912, ISSN: 0022-1120

Journal article

Ojiako CJ, Cimpeanu R, Bandulasena HCH, Smith R, Tseluiko Det al., 2020, Deformation and dewetting of liquid films under gas jets, JOURNAL OF FLUID MECHANICS, Vol: 905, ISSN: 0022-1120

Journal article

Wray AW, Cimpeanu R, 2020, Reduced-order modelling of thick inertial flows around rotating cylinders, JOURNAL OF FLUID MECHANICS, Vol: 898, ISSN: 0022-1120

Journal article

Kalogirou A, Cimpeanu R, Blyth MG, 2020, Asymptotic modelling and direct numerical simulations of multilayer pressure-driven flows, European Journal of Mechanics - B/Fluids, Vol: 80, Pages: 195-205, ISSN: 0997-7546

The nonlinear dynamics of two immiscible superposed viscous fluid layers in a channel is examined using asymptotic modelling and direct numerical simulations (DNS). The flow is driven by an imposed axial pressure gradient. Working on the assumption that one of the layers is thin, a weakly-nonlinear evolution equation for the interfacial shape is derived that couples the dynamics in the two layers via a nonlocal integral term whose kernel is determined by solving the linearised Navier–Stokes equations in the thicker fluid. The model equation incorporates salient physical effects including inertia, gravity, and surface tension, and allows for comparison with DNS at finite Reynolds numbers. Direct comparison of travelling-wave solutions obtained from the model equation and from DNS show good agreement for both stably and unstably stratified flows. Both the model and the DNS indicate regions in parameter space where unimodal, bimodal and trimodal waves co-exist. Nevertheless, the asymptotic model cannot capture the dynamics for a sufficiently strong unstable density stratification when interfacial break-up and eventual dripping occurs. In this case, complicated interfacial dynamics arise from the dominance of the gravitational force over the shear force due to the underlying flow, and this is investigated in detail using DNS.

Journal article

Tomlin R, Cimpeanu R, Papageorgiou D, 2020, Instability and dripping of electrified liquid films flowing down inverted substrates, Physical Review Fluids, Vol: 5, Pages: 013703-1-013703-34, ISSN: 2469-990X

We consider the gravity-driven flow of a perfect dielectric, viscous, thin liquid film, wetting a flatsubstrate inclined at a non-zero angle to the horizontal. The dynamics of the thin film is influencedby an electric field which is set up parallel to the substrate surface – this nonlocal physical mechanismhas a linearly stabilizing effect on the interfacial dynamics. Our particular interest is in fluid filmsthat are hanging from the underside of the substrate; these films may drip depending on physicalparameters, and we investigate whether a sufficiently strong electric field can suppress such nonlinearphenomena. For a non-electrified flow, it was observed by Brun et al. (Phys. Fluids 27, 084107, 2015)that the thresholds of linear absolute instability and dripping are reasonably close. In the presentstudy, we incorporate an electric field and analyse the absolute/convective instabilities of a hierarchyof reduced-order models to predict the dripping limit in parameter space. The spatial stability resultsfor the reduced-order models are verified by performing an impulse–response analysis with directnumerical simulations (DNS) of the Navier–Stokes equations coupled to the appropriate electricalequations. Guided by the results of the linear theory, we perform DNS on extended domains withinflow/outflow conditions (mimicking an experimental set-up) to investigate the dripping limit forboth non-electrified and electrified liquid films. For the latter, we find that the absolute instabilitythreshold provides an order-of-magnitude estimate for the electric field strength required to suppressdripping; the linear theory may thus be used to determine the feasibility of dripping suppressiongiven a set of geometrical, fluid and electrical parameters.

Journal article

Moore MR, Cimpeanu R, Ockendon H, Ockendon JR, Oliver JMet al., 2020, Boundary layers in Helmholtz flows, Journal of Fluid Mechanics, Vol: 882, ISSN: 0022-1120

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Journal article

Ojiako CJ, Cimpeanu R, Bandulasena H, Smith R, Tseluiko Det al., 2019, Deformation of a Liquid Film by an Impinging Gas Jet: Modelling and Experiments, THE 6th NTERNATIONAL CONFERENCE ON FLUID FLOW, HEAT AND MASS TRANSFER, Publisher: Avestia Publishing

Conference paper

Cimpeanu R, Moore MR, 2018, Early-time jet formation in liquid-liquid impact problems: theory and simulations, Journal of Fluid Mechanics, Vol: 856, Pages: 764-796, ISSN: 0022-1120

We perform a thorough qualitative and quantitative comparison of theoretical predictions and direct numerical simulations for the two-dimensional, vertical impact of two droplets of the same fluid. In particular, we show that the theoretical predictions for the location and velocity of the jet root are excellent in the early stages of the impact, while the predicted jet velocity and thickness profiles are also in good agreement with the computations before the jet begins to bend. By neglecting the role of the surrounding gas both before and after impact, we are able to use Wagner theory to describe the early-time structure of the impact. We derive the model for general droplet velocities and radii, which encompasses a wide range of impact scenarios from the symmetric impact of identical drops to liquid drops impacting a deep pool. The leading-order solution is sufficient to predict the curve along which the root of the high-speed jet travels. After moving into a frame fixed in this curve, we are able to derive the zero-gravity shallow-water equations governing the leading-order thickness and velocity of the jet. Our numerical simulations are performed in the open-source software Gerris, which allows for the level of local grid refinement necessary for a problem with such a wide variety of length scales. The numerical simulations incorporate more of the physics of the problem, in particular the surrounding gas, the fluid viscosities, gravity and surface tension. We compare the computed and predicted solutions for a range of droplet radii and velocities, finding excellent agreement in the early stage. In light of these successful comparisons, we discuss the tangible benefits of using Wagner theory to confidently track properties such as the jet-root location, jet thickness and jet velocity in future studies of splash jet/ejecta evolution.

Journal article

Cimpeanu R, Papageorgiou DT, 2018, Three-dimensional high speed drop impact onto solid surfaces at arbitrary angles, International Journal of Multiphase Flow, Vol: 107, Pages: 192-207, ISSN: 0301-9322

The rich structures arising from the impingement dynamics of water drops onto solid substrates at high velocities are investigated numerically. Current methodologies in the aircraft industry estimating water collection on aircraft surfaces are based on particle trajectory calculations and empirical extensions thereof in order to approximate the complex fluid-structure interactions. We perform direct numerical simulations (DNS) using the volume-of-fluid method in three dimensions, for a collection of drop sizes and impingement angles. The high speed background air flow is coupled with the motion of the liquid in the framework of oblique stagnation-point flow. Qualitative and quantitative features are studied in both pre- and post-impact stages. One-to-one comparisons are made with experimental data available from the investigations of Sor and García-Magariño (2015), while the main body of results is created using parameters relevant to flight conditions with droplet sizes in the ranges from tens to several hundreds of microns, as presented by Papadakis et al. (2004). Drop deformation, collision, coalescence and microdrop ejection and dynamics, all typically neglected or empirically modelled, are accurately accounted for. In particular, we identify new morphological features in regimes below the splashing threshold in the modelled conditions. We then expand on the variation in the number and distribution of ejected microdrops as a function of the impacting drop size beyond this threshold. The presented drop impact model addresses key questions at a fundamental level, however the conclusions of the study extend towards the advancement of understanding of water dynamics on aircraft surfaces, which has important implications in terms of compliance to aircraft safety regulations. The proposed methodology may also be utilised and extended in the context of related industrial applications involving high speed drop impact such as inkjet printing and combustion.

Journal article

Anderson TG, Cimpeanu R, Papageorgiou DT, Petropoulos PGet al., 2017, Electric field stabilization of viscous liquid layers coating the underside of a surface, PHYSICAL REVIEW FLUIDS, Vol: 2, ISSN: 2469-990X

We investigate the electrostatic stabilization of a viscous thin film wetting the underside of a horizontal surface in the presence of an electric field applied parallel to the surface. The model includes the effect of bounding solid dielectric regions above and below the liquid-air system that are typically found in experiments. The competition between gravitational forces, surface tension, and the nonlocal effect of the applied electric field is captured analytically in the form of a nonlinear evolution equation. A semispectral solution strategy is employed to resolve the dynamics of the resulting partial differential equation. Furthermore, we conduct direct numerical simulations (DNS) of the Navier-Stokes equations using the volume-of-fluid methodology and assess the accuracy of the obtained solutions in the long-wave (thin-film) regime when varying the electric field strength from zero up to the point when complete stabilization occurs. We employ DNS to examine the limitations of the asymptotically derived behavior as the liquid layer thickness increases and find excellent agreement even beyond the regime of strict applicability of the asymptotic solution. Finally, the asymptotic and computational approaches are utilized to identify robust and efficient active control mechanisms allowing the manipulation of the fluid interface in light of engineering applications at small scales, such as mixing.

Journal article

Cimpeanu R, Devine MT, O'Brien C, 2017, A simulation model for the management and expansion of extended port terminal operations, Transportation Research Part E: Logistics and Transportation Review, Vol: 98, Pages: 105-131, ISSN: 1366-5545

This study introduces a discrete event simulation model for the analysis of bulk carrier unloading and material transport, storage and discharge at Europe’s largest alumina refinery, RUSAL Aughinish Alumina. With novel features such as the integration of additional unloading functionality, auxiliary infrastructure units, as well as efficient maintenance scheduling into the material processing chain, the model is used to predict and evaluate the performance gain in the port system in the context of long-term investment and planning scenarios. Promising strategic directions in terms of large scale performance indicators such as berth occupancy and costs have been identified.

Journal article

Kalogirou A, Cîmpeanu R, Keaveny EE, Papageorgiou DTet al., 2016, Capturing nonlinear dynamics of two-fluid Couette flows with asymptotic models, Journal of Fluid Mechanics, Vol: 806, Pages: R1-R13, ISSN: 1469-7645

The nonlinear stability of two-fluid Couette flows is studied using a novel evolution equation whose dynamics is validated by direct numerical simulation (DNS). The evolution equation incorporates inertial effects at arbitrary Reynolds numbers through a non-local term arising from the coupling between the two fluid regions, and is valid when one of the layers is thin. The equation predicts asymmetric solutions and exhibits bistability, features that are essential observations in the experiments of Barthelet et al. (J. Fluid Mech., vol. 303, 1995, pp. 23–53). Related low-inertia models have been used in qualitative predictions rather than the direct comparisons carried out here, and ad hoc modifications appear to be necessary in order to predict asymmetry and bistability. Comparisons between model solutions and DNS show excellent agreement at Reynolds numbers of O(103)O(103) found in the experiments. Direct comparisons are also made with the available experimental results of Barthelet et al. (J. Fluid Mech., vol. 303, 1995, pp. 23–53) when the thin layer occupies 1/51/5 of the channel height. Pointwise comparisons of the travelling wave shapes are carried out, and once again the agreement is very good.

Journal article

Cimpeanu R, Papageorgiou DT, 2015, Electrostatically induced mixing in confined stratified multi-fluid systems, International Journal of Multiphase Flow, Vol: 75, Pages: 194-204, ISSN: 1879-3533

Electrostatic control mechanisms underpin a wide range of modern industrial processes, from lab-on-a-chip devices to microfluidic sensors for security applications. During the last decades, the striking impact of fluid interface manipulation in contexts such as polymer self-assembly, micromanufacturing and mixing in viscous media has established the field of electrically driven interfacial flows as invaluable. This work investigates electrostatically induced interfacial instabilities and subsequent generation of nonlinear coherent structures in immiscible, viscous, dielectric multi-layer stratified flows confined in channels with plane walls. The present study demonstrates theoretically that interfacial instabilities can be utilized to achieve efficient mixing in different immiscible fluid regions. This is accomplished by electrostatically driving stable flows far from their equilibrium states to attain time-oscillatory and highly nonlinear flows producing mixing. The nonlinear electrohydrodynamic instabilities play the role of imposed background velocity fields or moving device parts in more traditional mixing protocols. Initially, simple yet efficient on–off voltage protocols are investigated and subsequently symmetry-breaking voltage distributions are considered and shown to considerably enhance the achieved level of mixing. Both two- and three-dimensional flows, containing realistic fluid configurations (water and oils), are computed using direct numerical simulations based on the Navier–Stokes equations. Such numerical investigations facilitate the quantitative study of the flow into the fully nonlinear regime and constitute the basis of optimization methods in the context of microfluidic mixing applications in two- and three-dimensional geometries.

Journal article

Cimpeanu R, Martinsson A, Heil M, 2015, A parameter-free perfectly matched layer formulation for the finite-element-based solution of the Helmholtz equation, Journal of Computational Physics, Vol: 296, Pages: 329-347, ISSN: 1090-2716

This paper presents a parameter-free perfectly matched layer (PML) method for the finite-element-based solution of the Helmholtz equation. We employ one of Bermúdez et al.'s unbounded absorbing functions for the complex coordinate mapping underlying the PML. With this choice, the only free parameter that controls the accuracy of the numerical solution for a fixed numerical cost (characterised by the number of elements in the bulk and the PML regions) is the thickness of the perfectly matched layer, δPML. We show that, for the case of planar waves, the absorbing function performs best for PMLs whose thickness is much smaller than the wavelength. We then perform extensive numerical experiments to explore its performance for non-planar waves, considering domain shapes with smooth and polygonal boundaries, different solution types (smooth and singular), and a wide range of wavenumbers, k , to identify an optimal range for the normalised PML thickness, kδPML, such that, within this range, the error introduced by the presence of the PML is consistently small and insensitive to change. This implies that if the PML thickness is chosen from within this range no further PML optimisation is required, i.e. the method is parameter-free. We characterise the dependence of the error on the discretisation parameters and establish the conditions under which the convergence of the solution under mesh refinement is controlled exclusively by the discretisation of the bulk mesh.

Journal article

Cimpeanu R, Devine MT, Tocher D, Clune Let al., 2015, Development and analysis of a port terminal loader model at RUSAL Aughinish, SIMULATION MODELLING PRACTICE AND THEORY, Vol: 51, Pages: 14-30, ISSN: 1569-190X

Journal article

Cimpeanu R, Papageorgiou DT, 2014, On the generation of nonlinear travelling waves in confined geometries using electric fields, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol: 372, Pages: 1-14, ISSN: 1364-503X

We investigate electrostatically induced interfacial instabilities and subsequent generation of nonlinear coherent structures in immiscible, viscous, dielectric multi-layer stratified flows confined in small-scale channels. Vertical electric fields are imposed across the channel to produce interfacial instabilities that would normally be absent in such flows. In situations when the imposed vertical fields are constant, interfacial instabilities emerge due to the presence of electrostatic forces, and we follow the nonlinear dynamics via direct numerical simulations. We also propose and illustrate a novel pumping mechanism in microfluidic devices that does not use moving parts. This is achieved by first inducing interfacial instabilities using constant background electric fields to obtain fully nonlinear deformations. The second step involves the manipulation of the imposed voltage on the lower electrode (channel wall) to produce a spatio-temporally varying voltage there, in the form of a travelling wave with pre-determined properties. Such travelling wave dielectrophoresis methods are shown to generate intricate fluid–surface–structure interactions that can be of practical value since they produce net mass flux along the channel and thus are candidates for microfluidic pumps without moving parts. We show via extensive direct numerical simulations that this pumping phenomenon is a result of an externally induced nonlinear travelling wave that forms at the fluid–fluid interface and study the characteristics of the generated velocity field inside the channel.

Journal article

Cimpeanu R, Papageorgiou DT, Petropoulos PG, 2014, On the control and suppression of the Rayleigh-Taylor instability using electric fields, PHYSICS OF FLUIDS, Vol: 26, ISSN: 1070-6631

Journal article

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