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Use this search facility to find out more about the profile of our HPC-Europa2 visitors, the type of work they have been doing, and their project achievements.
In the present work, a numerical study is conducted in order to investigate the unsteady aerodynamics of finite-span flapping rigid wings. The non-dimensional incompressible Navier-Stokes equations are solved in their velocity-pressure formulation using a second-order accurate in space and time finite-difference scheme. To tackle the problem of moving boundaries, the governing equations are solved on overlapping structured grids. The main goal is to investigate the wake topology and aerodynamic performance of low aspect-ratio flapping wings and their dependence on the flapping parameters (such as flapping frequency, flapping amplitude and Reynolds number). Specifically, the root-flapping motion characteristic of flying animals is studied. The numerical simulations are performed at a Reynolds number of Re = 1000 (a Reynolds number value often found in nature) and at different values of Strouhal number St.
In the present work, we have studied the root-flapping motion characteristic of flying animals, a subject virtually unexplored. It was found that, indeed, root-flapping motion produces wake structures similar to those of heaving or coupled heaving-and-pitching motions, but with the difference that the latter motions generate larger vortices and forces than root-flapping motion, presumably because the average velocity is higher across the span; aside from this, similar wake regimes occurs at similar St. In general, it was found that for values less than St < 0.25 we are in the drag production regime, for values approximately equal to St = 0.25 we produce little or no drag (or thrust), whereas for values higher that St > 0.25 we are in the thrust production regime.
The simulations also show that the wake of thrust producing, rigid finite-span wings is formed by vortex loops close to the wing surface, that slowly convert into vortex rings as they are convected downstream. It was also observed that the vortex rings are themselves inclined with respect to the free-stream; the angle of inclination of the vortex rings is found to be in the direction of their travel and in the streamwise direction for thrust producing configurations, whereas for drag producing configurations the angle of inclination is opposite to the direction of travel.
Additionally, the effect of aspec- ratio AR on the aerodynamic forces of finite-span wings was also assessed. It was observed that as we increase the wing AR, the aerodynamic forces also increase and this is chiefly attributed to the large area of high aspect ratio wings and to the decrease of three-dimensional effects in long wings. This observation leads us to think that the assumption of two-dimensionality has some validity for birds and insects, where the wings of many species tend to have relatively large aspect ratio.
All the qualitative and quantitative results found during this research support the hypothesis that: “flying and swimming animals cruise at a Strouhal number tuned for high power efficiency”
The interaction of both short- and long-chain alcohols with biological membranes forms an important area of exploration because of the role of alcohols in metabolism, membrane fusion, drug delivery, alcohol toxicity, alcohol tolerance, and general anesthesia. Of particular interest therefore is the understanding of the molecular mechanism of action of alcohols on the lipid portions of cell biomembranes.
The presence of alcohols is believed both to modify the structural properties of membrane and to drastically affect their dynamics. Motions in biological membranes span a wide range of length and time scales and a large variety of these is essential for the functionality of membranes. In particular the short wavelength collective dynamics in the plane of the bilayer -in the picoseconds to nanosecond time scale- play a fundamental role in the transport of solutes across the bilayer.
The main purpose of this project was to probe the collective dynamics of membranes under the influence of alcohols using state of the art MD simulations. In particular membranes containing short chain alcohol (ethanol) and long chain alcohol (octanol) have been investigated. Moreover, the results of this study have been compared to neutron scattering experiments that we have performed on the same systems.
The key quantity, from both the theoretical and experimental points of view, for the determination of collective dynamics associated with density fluctuations is the dynamical structure factor S(Q,ω), the space and time Fourier transform of the particle pair correlation function I(Q,t), a quantity that can be calculated from MD trajectories.
The first task of this project was to generate equilibrated configurations of bare lipid bilayers and bilayers containing alcohols (ethanol and octanol) at the concentrations studied experimentally.
The second task consisted on writing a Fortran parallel analysis code (MPI), that generated the intermediate scattering functions I(Q,t) from the MD trajectories.
The last task was devoted to the proper analyses of the data: it consisted in generating the necessary intermediate scattering curves (I(Q,t) and their Fourier transforms, S(Q,ω) for comparison with experiments.
Equilibrated configurations of bare lipid bilayers and bilayers containing alcohols (ethanol and octanol), at the concentrations studied experimentally have been generated.
Initial configurations of membrane patches containing octanol were easy to construct since this long chains alcohol partition completely in the lipid membrane. The partitioning of ethanol was less obvious and necessitated equilibrating a system were the alcohol is both in the lipid and aqueous phase.
In a second phase, we have generated trajectories with appropriate length scale (typically 10-20 nanoseconds) to overlap with the energy resolutions used in experiments.
The following part of the project consisted on writing the parallel analysis code to calculate the intermediate scattering function I(Q,t) and the dynamical structure factor S(Q, ω). Moreover another quantity, the longitudinal current spectra, CL(Q,ω), that can be obtained as CL(Q,ω) = S(Q,ω) ω2/Q2, was taken into account to characterize the vibrational spectra since it highlights inelastic features. In addition information on the transverse dynamics have been reliably determined from the MD data by calculating the transverse current spectra, CT(Q,ω), a quantity that is not experimentally accessible.
The analysis of these functions necessitated computation of time correlation functions considering, as in experiments, an in-plane or out-of-plane powder average over Q the momentum transfers, for every Q value. Due to the fact that estimates of the considered correlation functions for long correlation times are often too computationally expensive, accordingly, the gain from parallelizing the code turned out to be a great added value to the project.
The results of this study concerning the lipid/ethanol system showed that ethanol modifies collective lipid-chain fluctuations . The impact of ethanol on membrane core dynamics was quantified by determining the collective short wavelength fluctuations on the 100-ps time scale from all atom molecular dynamics (MD) simulations and inelastic neutron scattering experiments. In addition to the acoustic mode that has been reported previously in phospholipid bilayers, we provided evidence for a new low-energy mode associated with the acyl chain dynamics, when ethanol is embedded within the lipid bilayers. This excitation may be related to the enhanced permeability observed in the presence of ethanol molecules. Finally, from experiment and simulations the collective hydrophobic membrane core dynamics show transverse properties, related to molecular motions perpendicular to the membrane.
As long as the lipid/octanol system is concerned, the data analysis and interpretation are still in progress.
 M. D. Kaye, K. Schmalzl, V. Conti Nibali, M. Tarek and M. C. Rheinstädter, “Ethanol enhances collective dynamics of lipid membranes”, Physical Review E 83, 050907(R) (2011)
Ultimately our aim is to gather knowledge of fluid slip in flows over structured surfaces with entrapped micro- or nanobubbles. We pursue further understanding on this topic with extensive computer simulations. The numerical method to be used in the simulations is the lattice-Boltzmann method. As the understanding on this specific topic is still very rudimentary, almost any results obtained from simulations are going to be new and help us to better understand the role of surface bubbles in microchannel flows. Especially, we expect to obtain results which help us to propose new surface structures that could be utilised in practical microfluidic devices.
Keijo Mattila visited Department of Applied Physics in the Technical University of Eindhoven for six weeks (from May 20 to June 30, 2009). Dr. Jens Harting hosted the visit. HPC-Europa2 programme gave us an opportunity to access the high-performance computing resources at SARA in Amsterdam. In addition, visit under HPC-Europa2 programme enabled intensive collaboration. During this visit, we implemented modifications needed in the previously developed simulation and analysis software. Major investment of time was made on the computational efficiency of the modified simulation software. With code optimization, the efficiency was increased by more than a factor of three. At the end of the visit, we began to run flow slippage simulations and to tentatively analyse obtained results. Due to the large number of individual simulations needed to gain new understanding, we will continue to run more simulations.