## The role of Computational Science and Engineering

Many of the problems that must be tackled in order to advance technology and science increasingly require synergetic approaches across disciplines. Computational Science refers to interdisciplinary research aiming at the solution of complex scientific and engineering problems under the unifying theme of computation. The explosive growth of computer power of the last few decades, and the advancement of computational methods, have enabled the application of computational approaches to an ever-increasing set of problems. Thus, a Computational Science core in a modern Engineering School offers the opportunity to bring together, in collaborative work, scientists and engineers from various disciplines, and complements experimental activities.

UCY-CompSci is active in research projects that span several fields, such as aerodynamics, turbulent fluid and plasma flows, biological fluid flows, environmental flows, controlled thermonuclear fusion and molecular dynamics. The common thread that joints these activities together is the application of computational methods to solve physical problems that involve fluids. This operational model has been chosen because it creates opportunities for synergies and transfer of know-how from one sub-area of application to another. For example, consistent and conservative numerical techniques that have developed for the accurate computation of the Lorentz forces in Magnetohydrodynamics have been used successfully at the Laboratory to improve the accuracy of simulations of saltwater intrusion in porous media. UCY-CompSci offers a stimulating research environment where scientists with different backgrounds, who nevertheless speak the common language of computational mechanics, can bring together their collective knowledge in search of innovative solutions to difficult problems.

The broad research areas where UCY-CompSci is active are described below. Synergies and overlaps occur between these areas as explined above. Details about specific projects can be found under the "Research Projects" menu item.

## Heat Transfer

With the ever-increasing significance of efficient energy production methods and renewable energy utilization, the subject of heat transfer is in the heart of many interesting applications. UCY-CompSci is active in the study of solar thermal systems such as solar air/water heaters, concentrated thermal/photovoltaic systems, thermal storage and basic research in buoyancy driven flows inside heated cavities.

Our capabilities in the demanding field of heat transfer extend to high-fidelity Direct Numerical Simulations (DNS) and Large Eddy Simulations (LES) of turbulent flows inside three dimensional configurations. Natural, mixed and forced convection flows incorporating conjugate and radiative heat transfer can be simulated with a wide range of computational tools, both commercial and in-house. Recently, the potential to efficiently simulate variable property flows that result from large temperature differences in the heated medium, was added.

## Turbulence fundamentals: theory, simulation and modeling

Turbulence theory, modeling and simulation is one the core areas of activity for UCY-CompSci. Turbulence remains one of the great challenges of modern science and engineering. While fluid turbulence was first described at end of the 19th Century, and had been the focus of many great minds during the 20th century, it remains today a great challenge for science and engineering. We are still learning new aspects about the fundamental mechanisms at work in turbulent flows, and despite great progress, the need to compute turbulent flows with accuracy and efficiency is still a key challenge holding back progress in many engineering applications.

UCY-CompSci is involved in activities that span the whole range, from fundamental studies probing the physics of turbulent flows using theory and Direct Numerical Simulations (DNS), to the development of engineering turbulence models, such as the Structure-Based Models, to the use of RANS models and Large Eddy Simulations (LES) for the simulation of turbulent flows in complex configurations related to engineering applications.

A key idea that what introduced along with Structure-Based Turbulence models, is the distinction between the componentality and the dimensionality of the turbulence. It might seem obvious now, but researchers were often missing the difference between the number of active fluctuation components in turbulence (the componentality) and the number of axes of dependence (dimensionality) of the turbulence. Thus, they would often talk about 2D turbulence when they really meant 2D-2C turbulence. In general 2D turbulence, can 2D-1C (jetal eddies), 2D-2C (vortical eddies), or 2D-3C (correlated jetal-vortical motions). The one-point turbulence tensors provide the mathematical framework, both for understanding the role of componentality and dimensionality in turbulent flows and for building turbulence models that are consistent with these concepts.

__Selected Publications__

*Journal of Turbulence*, 636046, (2012)

## Biomedical flows

Biomedical flows was one of the focus areas of the TOK-DEV project and an area where UCY-CompSci has developed significant know-how. We have developed the capability to carry out high-fidelity simulations of aerosol deposition in models of the human respiratory system. We have shown that turbulence generated in the upper airways persists at least down to the first few generations of the bronchi and at least for a part of the inhalation cycle. Hence, Large Eddy Simulations (LES) are ideally suited for probing the fundamental aspects of these flows and for gaining a better understanding of the factors (such anatomy, chronic disease etc.) that affect aerosol deposition. On the other hand, for routine patient evaluation of customized treatment strategies LES is still too expensive. We are thus also engaged at developing the capability to carry RANS based simulations using the Algebraic Structure-Based Model (ASBM).

UCY-CompSci is also active in hemodynamics, for example we are using LES to study the flow factors that contribute to the rapture of aneurysms in the carotid. Aneurysm rapture remains notoriously difficult to predict, an indication that we still do not understand all the mechanism that determine the evolution of aneurysms to the point of rapture. Using real carotid geometries and velocity data we have shown that even for nominal Reynolds numbers that would correspond to laminar flow, in the vicinity of the aneurysm the flow becomes unstable and turbulent-like at least for part of the heartbeat cycle. This inherent flow unsteadiness is thought to contribute to a modified distribution of shear stresses on the walls of the aneurysm and could prove to play role in raptures.

__Selected Publications__

*Proceedings of the 8th Euromech Fluid Mechanics Conference*, Bad Reichenshall, Germany (2010)

*6th World Congress on Biomechanics*, Singapore (2010)

*Proceedings of the 11th World Congress on Medical Physics and Biomedical Engineering*, Munich, Germany (2009)

*7th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements*, Limassol, Cyprus (2008)

*6th GRACM International Congress on Computational Mechanics*, Thessaloniki, Greece (2008)

## Environmental flows

In synergy with fundamental studies of two-phase flow and the project on aerosol inhalation and deposition in the human lugs, UCY-CompSci is engaged in Large-Eddy Simulations (LES) of aerosol dispersion in atmospheric flows. Cases of turbulent dispersion around isolated obstacles under various flow conditions, from point, line or volume sources. Ucy-CompSci has developed expertise in both Eulerian and Lagrangian approaches for the description of the dispersed phase. In the latter case, the trajectories of a large population (O(10^6)) of particles are computed using advanced particle-tracking algorithms. The dispersion of these particles is either considered as "passive" (one-way coupling) or "active" where the particle motion is affecting the carrier phase (two-way coupling).

In a somewhat different direction, which nevertheless forms again synergy with the oscillating turbulent flow in aneurysms, UCY-CompSci is looking at the physics of oscillating turbulent flows over modulated surfaces such as river dunes or ripples. These flows are challenging both from the physical-modeling and the computational point of view. Due to the accelerating/decelerating nature of the boundary layer, interesting flow features and transition phenomena appear during each cycle. In collaboration with the University of Maryland (U.S.) and the University of Patras (Greece), UCY-CompSci focuses on the effects of ripple geometry and current strength intensity on the formation of coherent structures and secondary motions over the bed.
In a third area of activity, we are using Direct Numerical Simulations, Large Eddy Simulations and RANS (ASBM) to simulate the flow over complex terrain, such as the modified 'Witch of Agnesi" hill. These benchmark cases contain challenging flow phenomena for the validation of present and future modelling CFD approaches.

__Selected Publications__

D. G. E. Grigoriadis, A. Dimas, E. Balaras (2012), **Large-eddy simulation of wave turbulent boundary layer over rippled bed**, *Journal of Coastal Engineering,* **60**, pp. 174-189, doi:10.1016/j.coastaleng.2011.10.003.

D. G. E. Grigoriadis, A. Dimas, E. Balaras, **LES of oscillating turbulent boundary layer over rippled bed**, ETMM-9, *9th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements*, 6-8 June 2012, Thessaloniki, Greece.

D.G.E. Grigoriadis, B. J. Geurts, **Reduced turbophoresis in two-way coupled particle-ladden flows**, *ERCOFTAC Workshop on Direct and Large-Eddy Simulation DLES-8*, 7-9 Jul. 2010, Eindhoven, The Nederlands.

D. G. E. Grigoriadis, S. C. Kassinos (2009), **Lagrangian particle dispersion in turbulent flow over a wall mounted obstacle**, *International Journal of Heat and Fluid Flow*, vol. **30**, Issue 2, June 2009, pp.462-470.

D. G. E. Grigoriadis, A. Dimas, E. Balaras (2009),** Turbulence statistics and structures by large-eddy simulation of open-channel sub-critical flow over bed dune**, *Journal of Geophysical Research*, (E) Earth Science, 114, F02022, doi:10.1029/2008JF001014.

D.G.E. Grigoriadis, C. Langer, S.C. Kassinos, **Diagnostic properties of structure tensors in turbulent flows**, *ERCOFTAC Workshop on Direct and Large-Eddy Simulation DLES-7*, University of Trieste, Abdus Salam International Centre for Theoretical Physics, 8-10 Sep. 2008, Trieste, Italy.

D.G.E. Grigoriadis, S. Kassinos, **Eulerian-Lagrangian dispersion using LES**, ETMM-7, *7th International ERCOFTAC Symposium on Engineering Turbulence Modelling and Measurements*, 4-6 June 2008, Limassol, Cyprus.

## Aerodynamics

UCY-CompSci is engaged in the development of the Algebraic Structure-Based Model (ASBM) for application in aerodynamics. For this purpose we are carrying our Large Eddy Simulations (LES), Unsteady RANS (URANS) and RANS simulations of steady and unsteady turbulent flow over various arifoils.

__Selected Publications__

## Water related topics (in collaboration with NIREAS)

Being located in Cyprus, on the easternmost European outpost in the Mediterranean, where rainfall is often short and water scarcity a constant concern, UCY-CompSci is naturally contributing where possible to the pursue of engineering solutions for improving water quality and availability. Despite having one of the most elaborate networks of fresh water reservoirs in Europe, Cyprus is often faced with water shortfalls. A large percentage of the water captured in reservoirs is lost to evaporation, while the interruption of the natural flow of rivers to the costal areas by man-made reservoirs has brought issues of salt-water intrusion to the forefront as well.

UCY-CompSci is one of the key laboratories contributing to the activities of the NIREAS International Water Research Center. Currently UCY-CompSci is engaged in the two major activities. The first concerns the development of a complete hydrological model of the main fresh water reservoirs in Cyprus aiming at solving issues related to evaporative losses and water quality such as bacterial loads. The second concerns the development of an advanced computational tool for the simulation of variable density flows in porous media for use in monitoring and controlling salt-water intrusion in coastal areas. Both activities are carried out in close collboration with the Water Development Department (WDD) and the Meteorological Service.

Finally, on collaboration with the Environmental Engineering Laboratory GAIA, also one of the main laboratories contributing to the activities of NIREAS, UCY-CompSci is using Large Eddy Simulations (LES) and modeling in an effort to optimize the performance Solar Photocatalytic Reactors.

__Selected Publications__

*Journal of Membrane Science*

**337**, 81 (2009), Bad Reichenshall, Germany (2010)

*Lakes & Reservoirs: Research and Management*

**13**, 51, (DOI:10.1111/j.1440-1770.2007.00353.x) (2008)

**10**, 350 (2008)

*Industrial & Engineering Chemistry Research*

**47**, 1636 (DOI:10.1021/ie0713893) (2008)

*Industrial & Engineering Chemistry Research,*

**45**, 8134 (2006)

## Two-phase flows

Two-phase flows are ubiquitous in science and engineering. and this is also reflected that UCY-CompSci activities in this area overlap with many of the other research activities described here. UCY-CompSci has been steadily developing in-house tools for the accurate and efficient simulation of two-phase flows, and in particular of various coupling between the fluid flow and a particle phase.

On one hand, we are interested in the fundamental aspects of two-phase turbulent flow and we use Direct Numerical Simulations (DNS) of particle and passive scalar transport in canonical homogeneous turbulence, including complex flows such magnetohydrodynamic turbulence. Currently we involved in the development of structure-based models for this class of turbulent flows.

On the other hand, we have the capability to simulate two-phase flows in the context of engineering applications as reflected in the range of publications that cover this area.

__Selected Publications__

X. Xu, D. Rouson, S. C. Kassinos, and H. Radhakrishnan, **Dispersed-phase structure in sheared MHD turbulence**, *Journal of Turbulence*, 636046, in press (2011).

D. G. E. Grigoriadis, and S. C. Kassinos**, Lagrangian Particle Dispersion In Turbulent Flow Over A Wall Mounted Obstacle****, ***International Journal of Heat and Fluid Flow,*** 30, **462 (2009).

D. Rouson, S. C. Kassinos, I. Moulitsas, I. Sarris, and X. Xu**, "****Dispersed-phase structural anisotropy in homogeneous magnetohydrodynamic turbulence at low magnetic Reynolds number,"**** ***Phys. Fluids,* **20**, 025101 (2008) {DOI:10.1063/1.2832776}.

S. C. Kassinos, B. Kanepen, and D. Carati**, "Passive scalar transport in MHD turbulence sheared in a rotating frame," **Physics of Fluids** 19, **015105 (2007) {DOI:10.1063/1.2409732}.

D.G.E. Grigoriadis, B. J. Geurts**, Reduced turbophoresis in two-way coupled particle-ladden flows, ***ERCOFTAC Workshop on Direct and Large-Eddy Simulation DLES-8*, 7-9 Jul. 2010, Eindhoven, The Nederlands.

H. Radhakrishnan, and S. C. Kassinos, **Numerical modeling of turbulent airflow and particle deposition in a bifurcating airway model**, Proceedings of the 11th World Congress on Medical Physics and Biomedical Engineering, Munich September 2009, Germany.

## Nanofluidics, Molecular dynamics and ab initio methods

We are interested in the interaction of fluids with nano-structures, such Carbon Nanotubes (CNTs) and we use Molecular Dynamics and ab initio methods to probe fundamental aspects of this interaction.

__Selected Publications__

*Chemical Reviews,*

**108**, 5014 (DOI:10.1021/cr078140f) (2008)

*Chemical Physics Letters,*

**460**, 512 (2008)

*Molecular Simulation,*

**34**, 671 (2008)

*Chemical Engineering Science,*

**63**, 2793 (DOI:10.1016/j.ces.2008.03.004) (2008)

*Chemical Engineering Science,*

**63**, 2047 (DOI:10.1016/j.ces.2007.12.035) (2008)

## Activites in Controlled Themonuclear Fusion (FUTERE-CY Unit)

Since 2007, Cyprus is participating in the European Union's Fusion Energy Programme through the **FU**sion **T**ransnational **U**nit for **RE**search in **CY**prus (FUTURE-CY). FUTURE-CY is the national fusion research unit of Cyprus and participates in the fusion programme as a transnational unit of HellasFusion, the National Fusion Association of Greece.

UCY-CompSci is the main laboratory contributing to the activities of the FUTURE-CY and currently is involved in the following areas of activities:

► Direct and Large Eddy Simulations of Low Magnetic Reynolds Numbers MHD flow (blanket modules)

► Plasma-Wall Interaction: Molecular Dynamics (MD) simulations of the formation of He bubbles in Be and W.

► ITM Task Force activities: application of the Dispersed Phase Structure Dimensionality to EDGE turbulence

__Selected Publications__

*Phys. of Fluids*,

**23**, 047101, doi:10.1063/1.3570686 (2011)

*Journal of Nuclear Materials*, Volume

**396**, Issues 2-3, 307-308, doi:10.1016/j.jnucmat.2009.10.066 (2010)

*Fusion Science and Technology*(2011)

*Fusion Engineering and Design*, Volume

**86**, Issue 1, Pages 5-14 (2011)

*Computers & Fluids*, vol.

**39**, 345-358, doi:10.1016/j.compfluid.2009.09.012 (2010)

*Theoretical and Computational Fluid Dynamics,*

**23**(2009)

*Phys. of Fluids*,

**21**, 097102, doi:10.1063/1.3231833 (2009)

*Journal of Computational Physics,*

**228**, 903 (2009)

*Phys. Fluids,*

**20**, 025101 (DOI:10.1063/1.2832776) (2008)

*Phys. of Fluids,*

**19**, 015105 (DOI:10.1063/1.2409732) (2007)

*Journal of Turbulence,*

**7**, No. 26 (2006)

*J. Fluid Mech.*

**513**, 199 (2004)

**,**

**Thermal Conduction in Magnetized Turbulent Gas**,

*The Astrophysical Journal,*

**589**, L77-L80 (2003)

UCY-CompSci in collaboration with NIREAS International Water Research Center and the Water Development Department (WDD) is looking into ways to reduce water loss thourgh evaporation from major dams of Cyprus. | ||

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