Democritus University of Thrace

542390225756f78888142d54f3d17e01_LThe last three decades, the Laboratory of Hydraulics and Hydraulic Structures in the Civil Engineering Department of the Democritus University of Thrace, conducts high quality research in theoretical, numerical and experimental Fluid Mechanics and Hydraulics. Various experimental facilities exist for educational and research purposes (flumes, turn table etc. as well as various physical models) equipped with classical (flow visualization, velocity/pressure/salinity/temperature probes etc.) as well as modern (PIV, PTV and DAQ systems) measurement equipment.

The main research interests include:

  • Multiphase Flows (gravity currents, turbidity currents, sediments transport/deposition/erosion, jets, plumes etc.)
  • Free Surface Flows (open channels, dam break, hydraulics jumps etc.)
  • Aerodynamics (air flow over airfoils, terrains, building and other structures etc.)
  • Heat and Mass Transfer (species transport, natural and forced convection, pollutant transport and dispersant etc.)
  • Large Scale Flows (ocean and estuary hydrodynamics etc.)
  • Biomedical Engineering Application (blood flow, LDL concentration, Stent analysis etc.)

During the last ten years, “SimTec Software & Softwares Ltd” has been a valuable and close collaborator of the Laboratory of Hydraulics and Hydraulic Structures CFD research team, in terms of software provision, academic and technical support for the following CFD commercial packages

  • Fluent, Gambit, FlowLab
  • ANSYS CFD (Design Modeller, Meshing, Fluent, CFX, CFD-Post) 12.0 & 13.0 and ANSYS 14.0 (Workbench, Design Modeler, Meshing, Fluent, CFX, CFD-Post, Mechanical, Transient Mechanical, Multiphysics, Coupling etc.)

1. Multiphase Flow Application – Hydrodynamic and Depositional Characteristics of Turbidity Currents

During floods, the density of river water usually increases due to the increase in the concentration of the suspended sediment that the river carries, causing the river to plunge underneath the free surface of a receiving water basin and form a turbidity current that continues to flow along the bottom. The study and understanding of such complex and rare phenomena is of great importance, as they constitute one of the major mechanisms for suspended sediment transport from rivers into the ocean, lakes or reservoirs. The CFD Research Team (in the Laboratory of Hydraulics and Hydraulic Structures, in the Hydraulics Sector of the Department of Civil Engineering, at the Democritus University of Thrace), has successfully applied the CFD methods that are offered by ANSYS FLUENT for the conduction of fundamental and applied research regarding the hydrodynamic and depositional characteristics of these complex and highly important multiphase flows. In more detail, the examined flows are treated numerically as three-dimensional, turbulent, multiphase flows consisting of separate fluid (ambient water and suspended sediment carrier water) and solid phases (various classes of suspended sediment particles), through the application of the “Eulerian” multiphase model in conjunction with the RNG k-ε turbulence model for turbulence closure. Some indicative flow visualization results are depicted in the following images (Images 1 and 2).

Indicative References:

  • Georgoulas A., Tzanakis T., Angelidis P., Panagiotidis T. and Kotsovinos N. (2009), “Numerical Simulation of Suspended Sediment Transport And Dispersal from Evros River into the North Aegean Sea, by the Mechanism of Turbidity Currents”, Proceedings of the 11th International Conference on Environmental Science and Technology, Chania, Crete, Greece, 3-5 September 2009, Vol. A, pp. 343-350.
  • Georgoulas A., Angelidis P., Kotsovinos N. and Panagiotidis T. (2010a), “Numerical investigation of fresh water-suspended sediment mixtures discharging into saline ambient water”, Proceedings of the 6th International Symposium on Environmental Hydraulics, Athens, Greece, 23 -25 June 2010, Vol. 1, pp. 547-552.
  • Georgoulas A., (2010), “Study of density currents at rivers outflows, due to suspended sediment particles”, (Greek edition), Ph.D. Thesis, Democritus University of Thrace, Greece.
  • Georgoulas A., Angelidis P., Panagiotidis T. and Kotsovinos N. (2010b), “3D numerical modelling of turbidity currents”, Journal of Environmental Fluid Mechanics, Vol. 10, pp.603-635.
  • Georgoulas A., Kopasakis K., Angelidis P., and Kotsovinos N (2012a), “3D Multiphase Numerical Modelling for Turbidity Current Flows”, INTECH, Numerical Modelling, ISBN 979-953-307-542-5, (Accepted Book Chapter – In Press).
  • Anastasios N. Georgoulas, Kyriakos I. Kopasakis, Panagiotis B. Angelidis, Nikolaos E. Kotsovinos (2012b), “Numerical investigation of continuous, high density turbidity currents response, in the variation of fundamental flow controlling parameters”, Computers & Fluids, Available online 7 March 2012, ISSN 0045-7930, 10.1016/j.compfluid.2012.02.025.

2. Free Surface Flow Applications – Dam Break Flow

A Dam Break Flow (DBF) constitutes an important practical problem in hydraulic engineering and its prediction is now a required element in the design of a dam and its surrounding environment. Flood waves generated by dam failures always lead to a great amount of property damage and loss of human lives. The CFD Research Team (in the Laboratory of Hydraulics and Hydraulic Structures, in the Hydraulics Sector of the Department of Civil Engineering, at the Democritus University of Thrace), has successfully applied the CFD methods that are offered by ANSYS FLUENT for the conduction of fundamental research regarding the hydrodynamic and depositional characteristics of these complex and highly important multiphase flows. In more detail, the examined flows are treated numerically as stratified/free surface flows of two immiscible fluids (air and water) that are separated by a clearly-defined interface, through the application of the “Volume Of Fluid – VOF” multiphase model in conjunction with various turbulence models for turbulence closure. Some indicative flow visualization results are depicted in the following images (Images 1-2).

References:

  • Georgoulas A., Pandremmenou A., Hrissanthou V. (2012). 3D Dam Dreak Numerical Modelling. Submitted for publication to the International Conference of Protection and Restoration of the Environment XI, Thessaloniki, Greece.
  • SPHERIC (2012). Validation Tests – Test 2 – 3D schematic dam break and evolution of the free surface, http://wiki.manchester.ac.uk/spheric/index.php/Test2 (accessed June 10, 2011).
    Soulis J.V., Chrisochoidis D.A., Fytanidis D.K., (2012), “Computational Analysis of Flood Waves”, 2nd Common Conference of Hellenic Hydrotechnical Association (HHA) and Greek Committee for Water Resources Management (GCWRM), (accepted abstract – paper in progress) (in Greek).
  • Bellos, C.V., Soulis, J.V, Sakkas, J.G., (1992).”Experimental investigation of two dimensional dam-break induced flows”. Journal of Hydraulic Research., Vol 29, 5:1-17.

3. Self Compacting Concrete (SCC) rheology and its fluidity test simulation

Self-Compacting Concrete (or Self-Consolidating Concrete, SCC) is a high fluidity type of concrete mixture which is able to flow freely under its own weight and gravity effect. Contrarily to conventional concrete, SCC allows the filling of forms of complex structural elements and confined areas without the need of internal or external vibration, guaranteeing at the same time superior concrete surface quality. Its evolutionary rheological characteristics come as a result of its different mixture design that has increased the need for further understanding of its rheology. During the last decade, Computational Fluid Dynamics (CFD) has proved to be a useful equipment for SCC rheological behavior and characteristics’estimation, providing further insight on this research area. Members of the CFD Research Team (in the Laboratory of Hydraulics and Hydraulic Structures in the Hydraulics Sector of the Department of Civil Engineering, at the Democritus University of Thrace), have successfully applied CFD techniques for SCC rheological behavior simulation. Using Continuous Mechanics Approach, CFD techniques have been applied in order to simulate SCC laboratory tests such as the widely used “L-box” and “V-funnel” apparatus tests. Free surface was captured using the Volume of Fluid (VOF) multiphase model. Governing equations were solved for transient, incompressible, isothermal, non-Newtonian and laminar flow using ANSYS FLUENT. The SCC was considered to be a single phase, non-Newtonian material obeying to a Bingham Plastic model, considering aggregate effects (blocking, segregation etc.) as neglected. The SCC properties (density, non-Newtonian viscosity parameters etc.) were estimated using laboratory tests through computer equipment as well as the “LCPC Box” apparatus. Instead of the classic Bingham model for the viscosity, a User Defined Function (UDF) was developed using ANSI C programming language for the Papanastasiou modification of Bingham model (Papanastasiou, 1987). The Papanastasiou modification is an exponential model used in order to avoid the discontinuity inherent of other viscoplastic models and to set up a continuous equation for both yielded (τ>το) and unyielded areas (τ<το) of fluid. Two and three-dimensional simulations were examined in order to verify the 2D and 3D effect. The results were compared with corresponding experimental data, showing the continuum approach model adequacy. Generally, CFD numerical results seem to follow experimental observations. Finally, the need for a model modification (possibly with an additional “momentum sink” term in the momentum equation) appeared in order to take in consideration the aggregate blocking between steel bars that takes place in some coarse aggregates’ mixtures.

Indicative References:

  • Georgiadis A.S., Fytanidis D.K., Anagnostopoulos N.S., (2010), “Simulating self-compacting concrete fluidity tests using computational fluid dynamics techniques”, 4th International Conference from Scientific Computing to Computational Engineering, Athens.

4. Wind flow Simulation over complex terrains

Wind turbines usage is a common way for environmental friendly energy production. Wind flow properties such as velocity and turbulence characteristics’ distribution as well as flow separation regions are important factors for wind farms’ location. The CFD Research Team (in the Laboratory of Hydraulics and Hydraulic Structures in the Hydraulics Sector of the Department of Civil Engineering, at the Democritus University of Thrace), has successfully applied the CFD techniques for air flow simulation over complex terrain in laboratory and field scale applications. Using Reynolds Averaged Navier-Stokes (RANs) equations as well as various turbulence models provided by ANSYS FLUENT, simulations have been carried out in order to evaluate their results, showing that they are in good agreement with the experimental ones as well as with field measurements. Simulations’ results are shown in Images 1-2.

Indicative References:

  • Dounias E., (2011), “Turbulence models over complex Terrains: a case of Cherso municipality”, MSc thesis, Civil-Engineering Department, Democritus University of Thrace, Supervision: Prof. Soulis J.V. (in Greek).
  • Georgiou S., (2011), “Numerical Simulation of wind flow in a canyon”, MSc thesis, Civil-Engineering Department, Democritus University of Thrace, Supervision: Prof. Soulis J.V. (in Greek).
  • shihara T., Hibi K., Oikawa S., (1999), “A wind tunnel study of turbulent flow over a three-dimensional steep hill”, J. Wind Eng. Ind. Aerodyn., vol. 83, pp. 95-107.

5. Biomedical Engineering Application – Blood flow and Low Density Lipoprotein transport

Although genetic factors seem to be important, basic mechanisms related to arterial wall cell malfunction, which leads to atherosclerosis formation, depend on the biomechanical blood flow properties. There are several studies concerning the relation of several haemodynamic parameters to atherosclerotic prone regions in the arterial system. These studies examine several arterial wall-related flow properties such as local low/high Wall Shear Stress(WSS) distribution as well as other transient flow properties namely; the time-Averaged Wall Shear Stress (AWSS), the time-Averaged Wall Shear Stress Vector (AWSSV), the Oscillatory Shear Index (OSI) and Relative Resident Time (RRT). Furthermore, Low Density Lipoprotein concentration has been examined in order to elucidate atherosclerotic regions.The CFD Research Team (in the Laboratory of Hydraulics and Hydraulic Structures, in the Hydraulics Sector of the Department of Civil Engineering, at the Democritus University of Thrace) has successfully applied computational fluid dynamics techniques in numerous biomedical engineering related projects. Using Continuous Mechanics Approach (CMA), the flow governing and the LDL mass transport equations are solved for incompressible, oscillatory, non-Newtonian, isothermal and laminar flow using ANSYS FLUENT. Several User Defined Functions (UDFs) were developed, using ANSI C programming language, in order to calculate and compute parameters such as the blood viscosity, the AWSS, the AWSSV, the OSI, the RRT as well as the wall boundary condition in order to simulate its semi-permeable behavior (LDL simulation). The OSI distribution of a patient specific arterial system is shown in Image 1. LDL concentration, AWSS, AWSSV OSI, RRT and LDL concentration distribution in a patient specific right coronary artery (RCAs) are shown in Image 2. Finally, the stents strut effect in haemodynamic flow properties is examined. Image 3 shows velocity vectors (a) and stream lines (b) in blood flow and WSS simulation through a stented artery.Following these analysis, it is evident that high OSI, RRT and LDL values tend to collocate with low AWSS, AWSSV and WSS values. Furthermore, instantaneous WSS vector presents higher oscillatory behavior (high OSI) where AWSS and AWSSV are low.


Indicative References:

  • Fytanidis D.K, Soulis J.V., Giannoglou G.D., (2012), “Patient-Specific Oscillatory haemodynamics and LDL transport”, Biomechanics and Modeling in Mechanobiology, (under review).
  • Soulis J.V., Fytanidis D.K, Papaioannou V.C., Giannoglou G.D., (2011),“Oscillating LDL accumulation in normal human aortic arch – shear dependent endothelium”, Hippokratia, Volume 1, Issue 1, pp. 22-25.
  • Soulis J.V., Fytanidis D.K, Papaioannou V.C., Giannoglou G.D., (2010), “Wall shear stress on LDL accumulation in human RCAs”, Journal of Medical Engineering and Physics, Volume 32, Issue 8, pp. 867-877.
  • Soulis J.V., Lampri O.P., Fytanidis D.K., (2011), Giannoglou G.D., “Relative Residence Time and Oscillatory Shear Index of Non-Newtonian Flow Models in Aorta”, 10th International Workshop on Biomedical Engineering, Kos, Greece.
  • Soulis, J.V., Fytanidis, D.Κ., Seralidou, K.D., Karagkiozaki, V.C., Giannoglou, G.D., (2011), “Wall Shear Stress and low density lipoprotein concentration in stented arteries”, 3rd Micro and Nano Flows Conference, August, Thessaloniki, Greece.