Thanasis Klonidis | Democritus University of Thrace (original) (raw)

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Papers by Thanasis Klonidis

International Conference of Computational Methods in Sciences and Engineering 2004 (ICCMSE 2004), 2019

Dam holding back a reservoir of water causes extensive destruction under sudden total collapse. T... more Dam holding back a reservoir of water causes extensive destruction under sudden total collapse. The Navier-Stokes equations were numerically solved for the dam-break phenomenon in lab scale. A set of numerical experiments was performed using the general scientific purposes computational fluid dynamics solver Ansys-Fluent. The position of the free-surface was determined applying the volume of fluid multiphase model. Calculated free-surface elevations and bottom static pressures were compared with available measurements for a converging-diverging flume. The calculations shed light into the flood wave front movement within the flume. The dam-break analysis was enhanced via water depth, static pressure and flow velocities presented in contours and vectors. Aerial nearly top view and a top view clearly depict the front wave tip. Furthermore, the volume of fluid method water volume fraction contour predictions graphics enabled the static pressure experimental data to be directly compared and to derive appropriate conclusions. Satisfactory qualitative and quantitative agreement was observed between current volume of fluid method results and acquired measurements by monitoring the wave front movement for nearly 10.0 s after the sudden and total dam-break collapse. Computational fluid dynamics analysis for the free-surface solution, achieved via volume of fluid method, proved to be a useful tool for dam-break problems.

This paper refers to the development of a moving grid implicit finite-volume numerical scheme cap... more This paper refers to the development of a moving grid implicit finite-volume numerical scheme capable of simulating 3D, steady, free-surface flows in irregular geometry channels with shockwave presence. The Navier-Stokes equations are used together with pseudo-compressibility technique to allow direct calculation of the pressure from the continuity equation. The position of the free-surface is determined by applying a moving boundary condition through the inclusion of the two-dimensional depth-averaged mass continuity equation. To improve the stability and accuracy of the model a new technique is introduced based on the use of two nested iteration steps. All of the mentioned equations are transformed into non-orthogonal body-fitted coordinate system to enable accurate representation of irregular geometries. The resulting numerical model is used to simulate super-critical free-surface flows including discontinuous flow featured with shock waves. The comparisons are satisfactory.

The proposed research work presents the development and application of an implicit finite-volume ... more The proposed research work presents the development and application of an implicit finite-volume numerical scheme capable of simulating 3D, steady, free-surface flows in irregular geometry channels with shockwave presence. The Navier-Stokes equations are used together with the utilization of the pseudocompressibility technique to calculate the pressure from the continuity equation. The position of the free surface is determined by applying a moving boundary condition through the inclusion of the two-dimensional depthaveraged mass continuity equation. All of the mentioned equations are transformed into non-orthogonal bodyfitted coordinate system and approximated using a second order implicit scheme resulting from the linearization of the governing equations. To deal with significant numerical instabilities, resulting from dispersion errors appearing during the abrupt change of the free-surface position, an alternative numerical technique is presented leading to the utilization of two nested iteration steps. The resulting numerical model is used to simulate super-critical free-surface flows including discontinuous flow featured with shock waves. The comparisons are remarkably satisfactory.

A non-hydrostatic, moving grid finite-volume implicit numerical scheme is applied to three-dimens... more A non-hydrostatic, moving grid finite-volume implicit numerical scheme is applied to three-dimensional steady free-surface flows over various crested bedform configurations featuring weirs and spillways. The Navier-Stokes equations are modified by exploiting the pseudo-compressibility technique to couple pressures with velocity components. The position of the free-surface is determined by applying a moving boundary condition through the inclusion of the two-dimensional depth-averaged mass continuity equation. To improve the stability and accuracy of the model a new technique is introduced based on the use of two nested iteration steps. All of the mentioned equations are transformed into non-orthogonal body-fitted coordinate system to enable accurate representation of irregular geometries. Calculated water surface elevations and bottom pressures are compared with available measurements of steady flows over curved and sharp crested weirs. The versatility of the model in properly capturing the non-hydrostatic nature of pressure distribution is emphasized.

A three-dimensional CFD numerical model has been utilized to simulate the 3D free-surface flow un... more A three-dimensional CFD numerical model has been utilized to simulate the 3D free-surface flow under supercritical flow conditions in a horizontal expansion channel. The program, ANSYS Fluent, solves the Navier-Stokes equations on a structured hexahedral grid using PISO method and the flow is treated as laminar and steady. The numerical simulation has been based on the Volume of Fluid method (VOF) approach. Available experimental measurements of the free-surface in an expansion channel, under various supercritical flow regimes, have been used to validate the proposed numerical methodology. In all test cases the 3D numerical model gives reasonable comparisons with measurements for the water depth. The presented methodology needs further improvement and testing it in abrupt open channel flow flumes.

An implicit numerical scheme has been developed and subsequently applied to calculate steady, two... more An implicit numerical scheme has been developed and subsequently applied to calculate steady, two-dimensional depth averaged, free-surface flow problems. The implicit form of the scheme gives fast convergence. The scheme is second order accurate and unconditionally stable. The free-surface flow equations are transformed into a non-orthogonal, boundary-fitted coordinate system so as to simulate with accuracy irregular geometries. The model is used to analyze a wide variety of hydraulic engineering problems including subcritical flow in a converging-diverging flume, supercritical flow at a channel expansion with various Froude numbers, and mixed sub-and supercritical flow in a converging channel. The computed results are compared with measurements as well as with other numerical solutions and satisfactory agreement is achieved.

An implicit, fully coupled, two-dimensional, free-surface flow numerical model has been developed... more An implicit, fully coupled, two-dimensional, free-surface flow numerical model has been developed to calculate bed changes in alluvial channels. Vertically averaged free-surface flow equations in conjunction with sediment transport equation are transformed into a nonorthogonal, boundary fitted coordinate system and then are solved numerically. The model is used to analyze problems of local scour around bridge abutments. Measured data are compared with computational results and satisfactory agreement is achieved.

The present study presents a second order accurate implicit numerical scheme for the calculation ... more The present study presents a second order accurate implicit numerical scheme for the calculation of unsteady, two-dimensional depth averaged, free-surface flow problems. The introduction of a nonorthogonal, boundary-fitted coordinate system allows the accurate simulation of irregular geometries. The model is used to analyze dam-break flow in a converging-diverging flume. The computed results are compared with measurements and satisfactory agreement is achieved.

The present study proposes a finite-volume implicit numerical scheme for the simulation of two-an... more The present study proposes a finite-volume implicit numerical scheme for the simulation of two-and three-dimensional free-surface flow problems. The implicit form of the scheme guarantees fast convergence allowing the use of large time steps. The introduction of a non-orthogonal boundary-fitted coordinate system (local coordinates) makes it possible for the model to handle various types of boundary conditions with accuracy. In the case of two-dimensional flow problems, the conservative form of the equations of fluid dynamics is used while the Navier-Stokes equations in combination with the innovative technique of pseudocompressibility are used to describe mathematically the three-dimensional free-surface flow problems. The resulting flow equations are transformed into the local coordinate system and then are solved numerically. The three dimensional scheme is used to analyze the free-surface flow over a double-arc spillway which was mounted in a laboratory flume. Bottom pressures and water levels were measured at various points along the centerline. Successful comparisons between measurements and computed results ensure the credibility of the proposed scheme.

International Conference of Computational Methods in Sciences and Engineering 2004 (ICCMSE 2004), 2019

Dam holding back a reservoir of water causes extensive destruction under sudden total collapse. T... more Dam holding back a reservoir of water causes extensive destruction under sudden total collapse. The Navier-Stokes equations were numerically solved for the dam-break phenomenon in lab scale. A set of numerical experiments was performed using the general scientific purposes computational fluid dynamics solver Ansys-Fluent. The position of the free-surface was determined applying the volume of fluid multiphase model. Calculated free-surface elevations and bottom static pressures were compared with available measurements for a converging-diverging flume. The calculations shed light into the flood wave front movement within the flume. The dam-break analysis was enhanced via water depth, static pressure and flow velocities presented in contours and vectors. Aerial nearly top view and a top view clearly depict the front wave tip. Furthermore, the volume of fluid method water volume fraction contour predictions graphics enabled the static pressure experimental data to be directly compared and to derive appropriate conclusions. Satisfactory qualitative and quantitative agreement was observed between current volume of fluid method results and acquired measurements by monitoring the wave front movement for nearly 10.0 s after the sudden and total dam-break collapse. Computational fluid dynamics analysis for the free-surface solution, achieved via volume of fluid method, proved to be a useful tool for dam-break problems.

This paper refers to the development of a moving grid implicit finite-volume numerical scheme cap... more This paper refers to the development of a moving grid implicit finite-volume numerical scheme capable of simulating 3D, steady, free-surface flows in irregular geometry channels with shockwave presence. The Navier-Stokes equations are used together with pseudo-compressibility technique to allow direct calculation of the pressure from the continuity equation. The position of the free-surface is determined by applying a moving boundary condition through the inclusion of the two-dimensional depth-averaged mass continuity equation. To improve the stability and accuracy of the model a new technique is introduced based on the use of two nested iteration steps. All of the mentioned equations are transformed into non-orthogonal body-fitted coordinate system to enable accurate representation of irregular geometries. The resulting numerical model is used to simulate super-critical free-surface flows including discontinuous flow featured with shock waves. The comparisons are satisfactory.

The proposed research work presents the development and application of an implicit finite-volume ... more The proposed research work presents the development and application of an implicit finite-volume numerical scheme capable of simulating 3D, steady, free-surface flows in irregular geometry channels with shockwave presence. The Navier-Stokes equations are used together with the utilization of the pseudocompressibility technique to calculate the pressure from the continuity equation. The position of the free surface is determined by applying a moving boundary condition through the inclusion of the two-dimensional depthaveraged mass continuity equation. All of the mentioned equations are transformed into non-orthogonal bodyfitted coordinate system and approximated using a second order implicit scheme resulting from the linearization of the governing equations. To deal with significant numerical instabilities, resulting from dispersion errors appearing during the abrupt change of the free-surface position, an alternative numerical technique is presented leading to the utilization of two nested iteration steps. The resulting numerical model is used to simulate super-critical free-surface flows including discontinuous flow featured with shock waves. The comparisons are remarkably satisfactory.

A non-hydrostatic, moving grid finite-volume implicit numerical scheme is applied to three-dimens... more A non-hydrostatic, moving grid finite-volume implicit numerical scheme is applied to three-dimensional steady free-surface flows over various crested bedform configurations featuring weirs and spillways. The Navier-Stokes equations are modified by exploiting the pseudo-compressibility technique to couple pressures with velocity components. The position of the free-surface is determined by applying a moving boundary condition through the inclusion of the two-dimensional depth-averaged mass continuity equation. To improve the stability and accuracy of the model a new technique is introduced based on the use of two nested iteration steps. All of the mentioned equations are transformed into non-orthogonal body-fitted coordinate system to enable accurate representation of irregular geometries. Calculated water surface elevations and bottom pressures are compared with available measurements of steady flows over curved and sharp crested weirs. The versatility of the model in properly capturing the non-hydrostatic nature of pressure distribution is emphasized.

A three-dimensional CFD numerical model has been utilized to simulate the 3D free-surface flow un... more A three-dimensional CFD numerical model has been utilized to simulate the 3D free-surface flow under supercritical flow conditions in a horizontal expansion channel. The program, ANSYS Fluent, solves the Navier-Stokes equations on a structured hexahedral grid using PISO method and the flow is treated as laminar and steady. The numerical simulation has been based on the Volume of Fluid method (VOF) approach. Available experimental measurements of the free-surface in an expansion channel, under various supercritical flow regimes, have been used to validate the proposed numerical methodology. In all test cases the 3D numerical model gives reasonable comparisons with measurements for the water depth. The presented methodology needs further improvement and testing it in abrupt open channel flow flumes.

An implicit numerical scheme has been developed and subsequently applied to calculate steady, two... more An implicit numerical scheme has been developed and subsequently applied to calculate steady, two-dimensional depth averaged, free-surface flow problems. The implicit form of the scheme gives fast convergence. The scheme is second order accurate and unconditionally stable. The free-surface flow equations are transformed into a non-orthogonal, boundary-fitted coordinate system so as to simulate with accuracy irregular geometries. The model is used to analyze a wide variety of hydraulic engineering problems including subcritical flow in a converging-diverging flume, supercritical flow at a channel expansion with various Froude numbers, and mixed sub-and supercritical flow in a converging channel. The computed results are compared with measurements as well as with other numerical solutions and satisfactory agreement is achieved.

An implicit, fully coupled, two-dimensional, free-surface flow numerical model has been developed... more An implicit, fully coupled, two-dimensional, free-surface flow numerical model has been developed to calculate bed changes in alluvial channels. Vertically averaged free-surface flow equations in conjunction with sediment transport equation are transformed into a nonorthogonal, boundary fitted coordinate system and then are solved numerically. The model is used to analyze problems of local scour around bridge abutments. Measured data are compared with computational results and satisfactory agreement is achieved.

The present study presents a second order accurate implicit numerical scheme for the calculation ... more The present study presents a second order accurate implicit numerical scheme for the calculation of unsteady, two-dimensional depth averaged, free-surface flow problems. The introduction of a nonorthogonal, boundary-fitted coordinate system allows the accurate simulation of irregular geometries. The model is used to analyze dam-break flow in a converging-diverging flume. The computed results are compared with measurements and satisfactory agreement is achieved.

The present study proposes a finite-volume implicit numerical scheme for the simulation of two-an... more The present study proposes a finite-volume implicit numerical scheme for the simulation of two-and three-dimensional free-surface flow problems. The implicit form of the scheme guarantees fast convergence allowing the use of large time steps. The introduction of a non-orthogonal boundary-fitted coordinate system (local coordinates) makes it possible for the model to handle various types of boundary conditions with accuracy. In the case of two-dimensional flow problems, the conservative form of the equations of fluid dynamics is used while the Navier-Stokes equations in combination with the innovative technique of pseudocompressibility are used to describe mathematically the three-dimensional free-surface flow problems. The resulting flow equations are transformed into the local coordinate system and then are solved numerically. The three dimensional scheme is used to analyze the free-surface flow over a double-arc spillway which was mounted in a laboratory flume. Bottom pressures and water levels were measured at various points along the centerline. Successful comparisons between measurements and computed results ensure the credibility of the proposed scheme.