ABSTRACT An understanding of extreme waves is important in the design and analysis of offshore st... more ABSTRACT An understanding of extreme waves is important in the design and analysis of offshore structures, such as oil and gas platforms. With the increase of interest in the shipping of LNG, the design of import and export terminals in coastal water of slowly varying intermediate depth requires accurate analysis of steep wave shoaling. In this thesis, data from laboratory experiments involving random wave simulations on very gentle slopes have been analysed in terms of a model of large wave events, and the results interpreted by observation of the shape and magnitude of the large wave events. The auto-correlation function of the free surface elevation time histories, called NewWave, has been calculated from the wave spectrum and shown to fit very well up to the point where waves start to break (when compared to the ‘linear’ surface elevation time history). It has been shown that NewWave is an appropriate model for the shape of the ‘linear’ part of large waves provided kd > 0.5. A Stokes-like expansion for NewWave analysis has been demonstrated to match the average shape of the largest waves, accounting for the dominant vertical asymmetry. Furthermore, an appropriate local wave period derived from NewWave has been inserted into a Miche-based limiting criterion, using the linear dispersion equation, to obtain estimates for the limiting wave height. Overall, the analysis confirms the Miche-type criterion applies to limiting wave height for waves passing over very mild bed slopes. A derivation of general Green-Naghdi (GN) theory, which incorporates non-linear terms in its formulation, is also presented. This approach satisfies the boundary conditions exactly and approximates the field equations. The derived 2-dimensional vertical GN Level 1 model, capable of simulating steep waves on varying water depth, is validated against solitary waves and their interactions, and solitary waves on varying water depth and gives good qualitative agreement against the KdV equation. The developed and validated numerical model is used to simulate focussed wave groups on both constant depth and gentle slope. In general, the behaviour of waves simulated by the numerical model is very similar to that observed in the experimental data. There is evidence of vertical asymmetry as the water depth is reduced, owing to the non-linearity. Although the main physics is still controlled by linear dispersion, the higher order harmonics become increasingly important for shoaling waves. The numerical results also show a slope-induced wave set-up that keeps on increasing in amplitude as the wave group travels on the gentle slope.
IOP Conference Series: Earth and Environmental Science, 2020
Flow around a marine riser in water at the drag crisis regime was investigated using numerical mo... more Flow around a marine riser in water at the drag crisis regime was investigated using numerical modelling. In this regime, the drag coefficient drops off at a certain Reynolds number due to a change from laminar to turbulent flow. The aim is to investigate the capability of turbulence model to predict drag coefficient through COMSOL Multiphysics, a computational fluid dynamic (CFD) transient solver and compared against existing numerical models and experiment by Maritime Research Institute Netherlands (MARIN). Numerically, drag and lift forces depend on the point of separation from the cylinder in which different turbulence modelling will result in varying separation point and will lead to different vortex formation and the drag force. Reynolds Average Navier-Stokes (RANS) was employed using the k-ε and Menter’s Shear Stress Transport (SST) turbulence model in two-dimensional CFD simulation. Six Reynolds numbers, similar to the test case, were considered. It can be concluded that the...
IOP Conference Series: Earth and Environmental Science, 2020
Flow around a marine riser in water at the drag crisis regime was investigated using numerical mo... more Flow around a marine riser in water at the drag crisis regime was investigated using numerical modelling. In this regime, the drag coefficient drops off at a certain Reynolds number due to a change from laminar to turbulent flow. The aim is to investigate the capability of turbulence model to predict drag coefficient through COMSOL Multiphysics, a computational fluid dynamic (CFD) transient solver and compared against existing numerical models and experiment by Maritime Research Institute Netherlands (MARIN). Numerically, drag and lift forces depend on the point of separation from the cylinder in which different turbulence modelling will result in varying separation point and will lead to different vortex formation and the drag force. Reynolds Average Navier-Stokes (RANS) was employed using the k-ε and Menter’s Shear Stress Transport (SST) turbulence model in two-dimensional CFD simulation. Six Reynolds numbers, similar to the test case, were considered. It can be concluded that the standard k-ε turbulence model, can only provide a good approximation at high turbulence regime, which is Reynolds number of 3.15 × 105 and higher. While, SST turbulence model can provide a good approximation at subcritical regime which before the sudden drop of drag force regimes.
Water scarcity is mainly the lacking of satisfactory level of water for fulfilling water utilizat... more Water scarcity is mainly the lacking of satisfactory level of water for fulfilling water utilization of any area that includes stress, shortage or deficits, and crisis of water. Water scarcity in Bangladesh is also increasing following the global trend. Specially, water scarcity has turned to be a bottleneck in agricultural development in many parts of the country. The objective of present study is to assess the spatial distribution of agricultural water scarcity in Bangladesh using entropy weighting method. Five factors selected based on availability of data namely, frequency of pre-monsoon droughts, surface water availability, surface water quality, groundwater level, and groundwater quality and literature review were used to assess the agricultural water scarcity in Bangladesh. For this purpose, data related to monthly rainfall, groundwater level, groundwater quality, and surface water availability were collected from different sources. Results revealed that the western and the southwestern parts of Bangladesh are more vulnerable to agricultural water scarcity. The water stress entropy index was found in the range of 0.29 to 0.74 for southwest Bangladesh, which indicates severe water stress in the region. Analysis of data revealed high salinity is the major cause agricultural water scarcity in southwest Bangladesh.
ABSTRACT An understanding of extreme waves is important in the design and analysis of offshore st... more ABSTRACT An understanding of extreme waves is important in the design and analysis of offshore structures, such as oil and gas platforms. With the increase of interest in the shipping of LNG, the design of import and export terminals in coastal water of slowly varying intermediate depth requires accurate analysis of steep wave shoaling. In this thesis, data from laboratory experiments involving random wave simulations on very gentle slopes have been analysed in terms of a model of large wave events, and the results interpreted by observation of the shape and magnitude of the large wave events. The auto-correlation function of the free surface elevation time histories, called NewWave, has been calculated from the wave spectrum and shown to fit very well up to the point where waves start to break (when compared to the ‘linear’ surface elevation time history). It has been shown that NewWave is an appropriate model for the shape of the ‘linear’ part of large waves provided kd > 0.5. A Stokes-like expansion for NewWave analysis has been demonstrated to match the average shape of the largest waves, accounting for the dominant vertical asymmetry. Furthermore, an appropriate local wave period derived from NewWave has been inserted into a Miche-based limiting criterion, using the linear dispersion equation, to obtain estimates for the limiting wave height. Overall, the analysis confirms the Miche-type criterion applies to limiting wave height for waves passing over very mild bed slopes. A derivation of general Green-Naghdi (GN) theory, which incorporates non-linear terms in its formulation, is also presented. This approach satisfies the boundary conditions exactly and approximates the field equations. The derived 2-dimensional vertical GN Level 1 model, capable of simulating steep waves on varying water depth, is validated against solitary waves and their interactions, and solitary waves on varying water depth and gives good qualitative agreement against the KdV equation. The developed and validated numerical model is used to simulate focussed wave groups on both constant depth and gentle slope. In general, the behaviour of waves simulated by the numerical model is very similar to that observed in the experimental data. There is evidence of vertical asymmetry as the water depth is reduced, owing to the non-linearity. Although the main physics is still controlled by linear dispersion, the higher order harmonics become increasingly important for shoaling waves. The numerical results also show a slope-induced wave set-up that keeps on increasing in amplitude as the wave group travels on the gentle slope.
IOP Conference Series: Earth and Environmental Science, 2020
Flow around a marine riser in water at the drag crisis regime was investigated using numerical mo... more Flow around a marine riser in water at the drag crisis regime was investigated using numerical modelling. In this regime, the drag coefficient drops off at a certain Reynolds number due to a change from laminar to turbulent flow. The aim is to investigate the capability of turbulence model to predict drag coefficient through COMSOL Multiphysics, a computational fluid dynamic (CFD) transient solver and compared against existing numerical models and experiment by Maritime Research Institute Netherlands (MARIN). Numerically, drag and lift forces depend on the point of separation from the cylinder in which different turbulence modelling will result in varying separation point and will lead to different vortex formation and the drag force. Reynolds Average Navier-Stokes (RANS) was employed using the k-ε and Menter’s Shear Stress Transport (SST) turbulence model in two-dimensional CFD simulation. Six Reynolds numbers, similar to the test case, were considered. It can be concluded that the...
IOP Conference Series: Earth and Environmental Science, 2020
Flow around a marine riser in water at the drag crisis regime was investigated using numerical mo... more Flow around a marine riser in water at the drag crisis regime was investigated using numerical modelling. In this regime, the drag coefficient drops off at a certain Reynolds number due to a change from laminar to turbulent flow. The aim is to investigate the capability of turbulence model to predict drag coefficient through COMSOL Multiphysics, a computational fluid dynamic (CFD) transient solver and compared against existing numerical models and experiment by Maritime Research Institute Netherlands (MARIN). Numerically, drag and lift forces depend on the point of separation from the cylinder in which different turbulence modelling will result in varying separation point and will lead to different vortex formation and the drag force. Reynolds Average Navier-Stokes (RANS) was employed using the k-ε and Menter’s Shear Stress Transport (SST) turbulence model in two-dimensional CFD simulation. Six Reynolds numbers, similar to the test case, were considered. It can be concluded that the standard k-ε turbulence model, can only provide a good approximation at high turbulence regime, which is Reynolds number of 3.15 × 105 and higher. While, SST turbulence model can provide a good approximation at subcritical regime which before the sudden drop of drag force regimes.
Water scarcity is mainly the lacking of satisfactory level of water for fulfilling water utilizat... more Water scarcity is mainly the lacking of satisfactory level of water for fulfilling water utilization of any area that includes stress, shortage or deficits, and crisis of water. Water scarcity in Bangladesh is also increasing following the global trend. Specially, water scarcity has turned to be a bottleneck in agricultural development in many parts of the country. The objective of present study is to assess the spatial distribution of agricultural water scarcity in Bangladesh using entropy weighting method. Five factors selected based on availability of data namely, frequency of pre-monsoon droughts, surface water availability, surface water quality, groundwater level, and groundwater quality and literature review were used to assess the agricultural water scarcity in Bangladesh. For this purpose, data related to monthly rainfall, groundwater level, groundwater quality, and surface water availability were collected from different sources. Results revealed that the western and the southwestern parts of Bangladesh are more vulnerable to agricultural water scarcity. The water stress entropy index was found in the range of 0.29 to 0.74 for southwest Bangladesh, which indicates severe water stress in the region. Analysis of data revealed high salinity is the major cause agricultural water scarcity in southwest Bangladesh.
Uploads
Papers by Mohd Ridza