Mahesh J Rao | IIT Madras (original) (raw)
Papers by Mahesh J Rao
Linear damping models have been used in the past for solving floating body dynamics, especially f... more Linear damping models have been used in the past for solving floating body dynamics, especially for simple geometries such as spar. However, due to the addition of heave damping elements to spar such as heave plate, complex flow around these elements may change the relationship between damping and velocity of the body to nonlinear. The damping plays a major role in accurate determination of motion response of spars, especially the heave. Free decay tests have been carried out for spar with and without heave plate in calm water condition. The Computational Fluid Dynamics (CFD) simulation of heave decay is carried out using ANSYS FLUENT and validated by free decay test results using scale models. Mesh convergence study has been conducted to determine the optimum mesh size. The heave motion obtained from CFD are used to derive the damping terms by matching the heave motion obtained using equation of motion by changing the damping term with linear, quadratic and the combination of linear and quadratic. The heave motion obtained from linear damping model matches well with that obtained from measured motion and CFD simulation for spar without heave plate. However, the linear / quadratic damping models alone are not suitable for spar with heave plate. Hence a combination of linear and quadratic damping model is proposed for spar with heave plate. The heave motion computed using a combination of linear and quadratic damping model matches well with that obtained from experimental studies and CFD simulations thus indicating the complexity of flow around heave plate in comparison to the spar alone. Further, the vortices around the spar models obtained from CFD simulations are also presented and discussed with regard to the higher order damping. NOMENCLATURE A wp Area of water plane (m 2
Applied Ocean Research, 2021
Abstract Heave plates are widely used in floating offshore structures to reduce the heave motion ... more Abstract Heave plates are widely used in floating offshore structures to reduce the heave motion by increasing heave damping and heave added mass, and these parameters are sensitive to the amplitude and frequency of heave motion. It is important to obtain the hydrodynamic parameters, namely, damping and added mass of the floater not only at its natural frequency, which can be obtained from free decay tests but also at other frequencies because the floater response is of interest over a wide range of amplitudes and frequencies. Forced oscillation tests in calm water can aid the investigation of these parameters at various motion frequencies and amplitudes. Heave damping and added mass of classic spar with heave plate are investigated in this study using experiments and numerical simulations of forced heave oscillation of a 1:100 scale model in calm water for various frequency-amplitude combinations. The least square method was used to determine the added mass and damping using three damping models, namely, linear, quadratic, and linear-plus-quadratic, and their applicability assessed. The effect of amplitude and frequency of oscillation on the parameters are discussed for various heave plate configurations with the aid of flow visualization from numerical simulations. Added mass effect is examined using flow visualization of the fluid acceleration field. The scale effect on the parameters is also addressed.
Applied Ocean Research, 2021
Abstract Spar, tension leg platform, and semi-submersibles use heave plates to reduce heave respo... more Abstract Spar, tension leg platform, and semi-submersibles use heave plates to reduce heave response by increasing heave damping and added mass. Conventional practice is to use a linear damping ratio, which is typically obtained from free heave decay tests or Computational Fluid Dynamics (CFD) simulations. However, with the addition of heave plates, the system damping becomes nonlinear. Understanding such nonlinear damping behavior of spar with circular heave plates in various configurations could be useful in the design of heave compensation devices. Experimental and numerical investigation on heave damping and added mass of a scaled model of spar with a variety of heave plate configurations have been carried out using free heave decay for a range of initial heave displacements. Applicability of the linear and quadratic damping models have been assessed for all configurations. The effects of parameters such as heave plate diameter, location of heave plate above the keel, and spacing between two plates on damping and added mass have also been studied. Flow fields obtained from numerical simulations are presented, and their implication on damping discussed.
2015 IEEE Underwater Technology (UT), 2015
ABSTRACT Autonomous Underwater Vehicles (AUV) are slow-moving small unmanned robots capable of sw... more ABSTRACT Autonomous Underwater Vehicles (AUV) are slow-moving small unmanned robots capable of swimming independently below the water surface on pre-defined mission paths and are commonly used for oceanographic exploration, bathymetric surveys and military applications. With the use of appropriate sensors and equipment, AUVs can perform underwater object recognition and obstacle avoidance. Amogh is a miniature AUV developed at Centre For Innovation (CFI), IIT Madras for AUVSI RoboSub competition. The vehicle has a non-conventional dual hull heavy bottom hydrodynamic design equipped with six thrusters which allow for motion control in 4 degrees of freedom. This paper presents various aspects of the unique design of the vehicle. The performance of a simple PID controller for steady depth and heading control has been discussed. Simulations performed on a decoupled mathematical model of the vehicle are compared against experimental results.
The present work is aimed at CFD simulation and experimental study on frequency and amplitude dep... more The present work is aimed at CFD simulation and experimental study on frequency and amplitude dependency of heave damping of Spar hulls by forced oscillation along heave direction in calm water condition. Conventionally, the heave damping is obtained by free decay test of a scale model which gives damping only at the heave natural frequency, but the damping at frequencies other than natural frequency cannot be determined by this method. So the forced heave oscillation of Froude scaled Spar models were conducted in calm water condition over a range of frequencies and amplitudes of oscillation and the forces were measured along the heave direction to calculate added mass and damping from the equation of motion. The experimental investigations were performed on a classic Spar model with and without heave plate in a laboratory wave flume. The CFD simulations were carried out in a numerical tank using commercial CFD software. The measured heave force is compared with that obtained from C...
5th International Conference on Offshore Renewable Energy CORE 2021, 2021
The modelling of effect of gyroscopic and aerodynamic loads due to operation of rotor of the offs... more The modelling of effect of gyroscopic and aerodynamic loads due to operation of rotor of the offshore wind turbine on the hydrodynamic response of floater is important for predicting its response accurately. An in-house gyro-aero solver is developed for solving gyroscopic and aerodynamic forces acting on a horizontal axis wind turbine (HAWT) and vertical axis wind turbine (VAWT). This gyro-aero solver is coupled with a commercial potential flow based hydrodynamic solver ANSYS AQWA to perform gyroaero-hydro coupled time domain numerical simulations in prototype scale as a part of comparative study between a classic spar supporting 5 MW HAWT and a classic spar supporting 5 MW VAWT, both subjected to same wind-wave conditions. Appropriate nonlinear viscous damping corrections for heave and pitch (and roll) modes were applied to the potential flow solver for the considered spar using the scaledup damping data obtained from previous investigations by authors. The gyro-aero-hydro coupling and the nonlinear viscous damping is enforced on the commercial potential flow solver using a user defined function. Time domain analysis were performed for regular waves at operating condition and the results are presented as response amplitude operators for all degrees of freedom. Wind-wave and gyro loads which constitute the total loads are split up and discussed. The gyroscopic effect of VAWT and subsequent pitchroll coupling is brought out.
Proceedings of the ASME 2016 35th International conference on Ocean, Offshore and Arctic Engineering, 2016
Linear damping models have been used in the past for solving floating body dynamics, especially f... more Linear damping models have been used in the past for solving floating body dynamics, especially for simple geometries such as spar. However, due to the addition of heave damping elements to spar such as heave plate, complex flow around these elements may change the relationship between damping and velocity of the body to nonlinear. The damping plays a major role in accurate determination of motion response of spars, especially the heave. Free decay tests have been carried out for spar with and without heave plate in calm water condition. The Computational Fluid Dynamics (CFD) simulation of heave decay is carried out using ANSYS FLUENT and validated by free decay test results using scale models. Mesh convergence study has been conducted to determine the optimum mesh size. The heave motion obtained from CFD are used to derive the damping terms by matching the heave motion obtained using equation of motion by changing the damping term with linear, quadratic and the combination of linear and quadratic. The heave motion obtained from linear damping model matches well with that obtained from measured motion and CFD simulation for spar without heave plate. However, the linear / quadratic damping models alone are not suitable for spar with heave plate. Hence a combination of linear and quadratic damping model is proposed for spar with heave plate. The heave motion computed using a combination of linear and quadratic damping model matches well with that obtained from experimental studies and CFD simulations thus indicating the complexity of flow around heave plate in comparison to the spar alone. Further, the vortices around the spar models obtained from CFD simulations are also presented and discussed with regard to the higher order damping.
Applied Ocean Research, 2021
Heave plates are widely used in floating offshore structures to reduce the heave motion by increa... more Heave plates are widely used in floating offshore structures to reduce the heave motion by increasing heave damping and heave added mass, and these parameters are sensitive to the amplitude and frequency of heave motion. It is important to obtain the hydrodynamic parameters, namely, damping and added mass of the floater not only at its natural frequency, which can be obtained from free decay tests but also at other frequencies because the floater response is of interest over a wide range of amplitudes and frequencies. Forced oscillation tests in calm water can aid the investigation of these parameters at various motion frequencies and amplitudes. Heave damping and added mass of classic spar with heave plate are investigated in this study using experiments and numerical simulations of forced heave oscillation of a 1:100 scale model in calm water for various frequency-amplitude combinations. The least square method was used to determine the added mass and damping using three damping models, namely, linear, quadratic, and linear-plus-quadratic, and their applicability assessed. The effect of amplitude and frequency of oscillation on the parameters are discussed for various heave plate configurations with the aid of flow visualization from numerical simulations. Added mass effect is examined using flow visualization of the fluid acceleration field. The scale effect on the parameters is also addressed.
Proceedings of International Conference on Advances in Computational and Experimental Marine Hydrodynamics (ACEMH 2014), 2014
The present work is aimed at CFD simulation and experimental study on frequency and amplitude dep... more The present work is aimed at CFD simulation and experimental study on frequency and amplitude dependency of heave damping of Spar hulls by forced oscillation along heave direction in calm water condition. Conventionally, the heave damping is obtained by free decay test of a scale model which gives damping only at the heave natural frequency, but the damping at frequencies other than natural frequency cannot be determined by this method. So the forced heave oscillation of Froude scaled Spar models were conducted in calm water condition over a range of frequencies and amplitudes of oscillation and the forces were measured along the heave direction to calculate added mass and damping from the equation of motion. The experimental investigations were performed on a classic Spar model with and without heave plate in a laboratory wave flume. The CFD simulations were carried out in a numerical tank using commercial CFD software. The measured heave force is compared with that obtained from CFD simulation. Further, the added mass and damping coefficient have been calculated by solving equation of motion from both experiment and simulation using the forces and heave motion as input. The amplitude and frequency dependency of heave added mass and heave damping for spar with and without heave plate are presented and discussed.
Underwater Technology (UT) 2015 IEEE, 2015
Autonomous Underwater Vehicles (AUV) are slow-moving small unmanned robots capable of swimming in... more Autonomous Underwater Vehicles (AUV) are slow-moving small unmanned robots capable of swimming independently below the water surface on pre-defined mission paths and are commonly used for oceanographic exploration, bathymetric surveys and military applications. With the use of appropriate sensors and equipment, AUVs can perform underwater object recognition and obstacle avoidance. Amogh is a miniature AUV developed at Centre For Innovation (CFI), IIT Madras for AUVSI RoboSub competition. The vehicle has a non-conventional dual hull heavy bottom hydrodynamic design equipped with six thrusters which allow for motion control in 4 degrees of freedom. This paper presents various aspects of the unique design of the vehicle. The performance of a simple PID controller for steady depth and heading control has been discussed. Simulations performed on a decoupled mathematical model of the vehicle are compared against experimental results.
Applied Ocean Research, Dec 28, 2020
Spar, tension leg platform, and semi-submersibles use heave plates to reduce heave response by in... more Spar, tension leg platform, and semi-submersibles use heave plates to reduce heave response by increasing heave damping and added mass. Conventional practice is to use a linear damping ratio, which is typically obtained from free heave decay tests or Computational Fluid Dynamics (CFD) simulations. However, with the addition of heave plates, the system damping becomes nonlinear. Understanding such nonlinear damping behavior of spar with circular heave plates in various configurations could be useful in the design of heave compensation devices. Experimental and numerical investigation on heave damping and added mass of a scaled model of spar with a variety of heave plate configurations have been carried out using free heave decay for a range of initial heave displacements. Applicability of the linear and quadratic damping models have been assessed for all configurations. The effects of parameters such as heave plate diameter, location of heave plate above the keel, and spacing between two plates on damping and added mass have also been studied. Flow fields obtained from numerical simulations are presented, and their implication on damping discussed.
Linear damping models have been used in the past for solving floating body dynamics, especially f... more Linear damping models have been used in the past for solving floating body dynamics, especially for simple geometries such as spar. However, due to the addition of heave damping elements to spar such as heave plate, complex flow around these elements may change the relationship between damping and velocity of the body to nonlinear. The damping plays a major role in accurate determination of motion response of spars, especially the heave. Free decay tests have been carried out for spar with and without heave plate in calm water condition. The Computational Fluid Dynamics (CFD) simulation of heave decay is carried out using ANSYS FLUENT and validated by free decay test results using scale models. Mesh convergence study has been conducted to determine the optimum mesh size. The heave motion obtained from CFD are used to derive the damping terms by matching the heave motion obtained using equation of motion by changing the damping term with linear, quadratic and the combination of linear and quadratic. The heave motion obtained from linear damping model matches well with that obtained from measured motion and CFD simulation for spar without heave plate. However, the linear / quadratic damping models alone are not suitable for spar with heave plate. Hence a combination of linear and quadratic damping model is proposed for spar with heave plate. The heave motion computed using a combination of linear and quadratic damping model matches well with that obtained from experimental studies and CFD simulations thus indicating the complexity of flow around heave plate in comparison to the spar alone. Further, the vortices around the spar models obtained from CFD simulations are also presented and discussed with regard to the higher order damping. NOMENCLATURE A wp Area of water plane (m 2
Applied Ocean Research, 2021
Abstract Heave plates are widely used in floating offshore structures to reduce the heave motion ... more Abstract Heave plates are widely used in floating offshore structures to reduce the heave motion by increasing heave damping and heave added mass, and these parameters are sensitive to the amplitude and frequency of heave motion. It is important to obtain the hydrodynamic parameters, namely, damping and added mass of the floater not only at its natural frequency, which can be obtained from free decay tests but also at other frequencies because the floater response is of interest over a wide range of amplitudes and frequencies. Forced oscillation tests in calm water can aid the investigation of these parameters at various motion frequencies and amplitudes. Heave damping and added mass of classic spar with heave plate are investigated in this study using experiments and numerical simulations of forced heave oscillation of a 1:100 scale model in calm water for various frequency-amplitude combinations. The least square method was used to determine the added mass and damping using three damping models, namely, linear, quadratic, and linear-plus-quadratic, and their applicability assessed. The effect of amplitude and frequency of oscillation on the parameters are discussed for various heave plate configurations with the aid of flow visualization from numerical simulations. Added mass effect is examined using flow visualization of the fluid acceleration field. The scale effect on the parameters is also addressed.
Applied Ocean Research, 2021
Abstract Spar, tension leg platform, and semi-submersibles use heave plates to reduce heave respo... more Abstract Spar, tension leg platform, and semi-submersibles use heave plates to reduce heave response by increasing heave damping and added mass. Conventional practice is to use a linear damping ratio, which is typically obtained from free heave decay tests or Computational Fluid Dynamics (CFD) simulations. However, with the addition of heave plates, the system damping becomes nonlinear. Understanding such nonlinear damping behavior of spar with circular heave plates in various configurations could be useful in the design of heave compensation devices. Experimental and numerical investigation on heave damping and added mass of a scaled model of spar with a variety of heave plate configurations have been carried out using free heave decay for a range of initial heave displacements. Applicability of the linear and quadratic damping models have been assessed for all configurations. The effects of parameters such as heave plate diameter, location of heave plate above the keel, and spacing between two plates on damping and added mass have also been studied. Flow fields obtained from numerical simulations are presented, and their implication on damping discussed.
2015 IEEE Underwater Technology (UT), 2015
ABSTRACT Autonomous Underwater Vehicles (AUV) are slow-moving small unmanned robots capable of sw... more ABSTRACT Autonomous Underwater Vehicles (AUV) are slow-moving small unmanned robots capable of swimming independently below the water surface on pre-defined mission paths and are commonly used for oceanographic exploration, bathymetric surveys and military applications. With the use of appropriate sensors and equipment, AUVs can perform underwater object recognition and obstacle avoidance. Amogh is a miniature AUV developed at Centre For Innovation (CFI), IIT Madras for AUVSI RoboSub competition. The vehicle has a non-conventional dual hull heavy bottom hydrodynamic design equipped with six thrusters which allow for motion control in 4 degrees of freedom. This paper presents various aspects of the unique design of the vehicle. The performance of a simple PID controller for steady depth and heading control has been discussed. Simulations performed on a decoupled mathematical model of the vehicle are compared against experimental results.
The present work is aimed at CFD simulation and experimental study on frequency and amplitude dep... more The present work is aimed at CFD simulation and experimental study on frequency and amplitude dependency of heave damping of Spar hulls by forced oscillation along heave direction in calm water condition. Conventionally, the heave damping is obtained by free decay test of a scale model which gives damping only at the heave natural frequency, but the damping at frequencies other than natural frequency cannot be determined by this method. So the forced heave oscillation of Froude scaled Spar models were conducted in calm water condition over a range of frequencies and amplitudes of oscillation and the forces were measured along the heave direction to calculate added mass and damping from the equation of motion. The experimental investigations were performed on a classic Spar model with and without heave plate in a laboratory wave flume. The CFD simulations were carried out in a numerical tank using commercial CFD software. The measured heave force is compared with that obtained from C...
5th International Conference on Offshore Renewable Energy CORE 2021, 2021
The modelling of effect of gyroscopic and aerodynamic loads due to operation of rotor of the offs... more The modelling of effect of gyroscopic and aerodynamic loads due to operation of rotor of the offshore wind turbine on the hydrodynamic response of floater is important for predicting its response accurately. An in-house gyro-aero solver is developed for solving gyroscopic and aerodynamic forces acting on a horizontal axis wind turbine (HAWT) and vertical axis wind turbine (VAWT). This gyro-aero solver is coupled with a commercial potential flow based hydrodynamic solver ANSYS AQWA to perform gyroaero-hydro coupled time domain numerical simulations in prototype scale as a part of comparative study between a classic spar supporting 5 MW HAWT and a classic spar supporting 5 MW VAWT, both subjected to same wind-wave conditions. Appropriate nonlinear viscous damping corrections for heave and pitch (and roll) modes were applied to the potential flow solver for the considered spar using the scaledup damping data obtained from previous investigations by authors. The gyro-aero-hydro coupling and the nonlinear viscous damping is enforced on the commercial potential flow solver using a user defined function. Time domain analysis were performed for regular waves at operating condition and the results are presented as response amplitude operators for all degrees of freedom. Wind-wave and gyro loads which constitute the total loads are split up and discussed. The gyroscopic effect of VAWT and subsequent pitchroll coupling is brought out.
Proceedings of the ASME 2016 35th International conference on Ocean, Offshore and Arctic Engineering, 2016
Linear damping models have been used in the past for solving floating body dynamics, especially f... more Linear damping models have been used in the past for solving floating body dynamics, especially for simple geometries such as spar. However, due to the addition of heave damping elements to spar such as heave plate, complex flow around these elements may change the relationship between damping and velocity of the body to nonlinear. The damping plays a major role in accurate determination of motion response of spars, especially the heave. Free decay tests have been carried out for spar with and without heave plate in calm water condition. The Computational Fluid Dynamics (CFD) simulation of heave decay is carried out using ANSYS FLUENT and validated by free decay test results using scale models. Mesh convergence study has been conducted to determine the optimum mesh size. The heave motion obtained from CFD are used to derive the damping terms by matching the heave motion obtained using equation of motion by changing the damping term with linear, quadratic and the combination of linear and quadratic. The heave motion obtained from linear damping model matches well with that obtained from measured motion and CFD simulation for spar without heave plate. However, the linear / quadratic damping models alone are not suitable for spar with heave plate. Hence a combination of linear and quadratic damping model is proposed for spar with heave plate. The heave motion computed using a combination of linear and quadratic damping model matches well with that obtained from experimental studies and CFD simulations thus indicating the complexity of flow around heave plate in comparison to the spar alone. Further, the vortices around the spar models obtained from CFD simulations are also presented and discussed with regard to the higher order damping.
Applied Ocean Research, 2021
Heave plates are widely used in floating offshore structures to reduce the heave motion by increa... more Heave plates are widely used in floating offshore structures to reduce the heave motion by increasing heave damping and heave added mass, and these parameters are sensitive to the amplitude and frequency of heave motion. It is important to obtain the hydrodynamic parameters, namely, damping and added mass of the floater not only at its natural frequency, which can be obtained from free decay tests but also at other frequencies because the floater response is of interest over a wide range of amplitudes and frequencies. Forced oscillation tests in calm water can aid the investigation of these parameters at various motion frequencies and amplitudes. Heave damping and added mass of classic spar with heave plate are investigated in this study using experiments and numerical simulations of forced heave oscillation of a 1:100 scale model in calm water for various frequency-amplitude combinations. The least square method was used to determine the added mass and damping using three damping models, namely, linear, quadratic, and linear-plus-quadratic, and their applicability assessed. The effect of amplitude and frequency of oscillation on the parameters are discussed for various heave plate configurations with the aid of flow visualization from numerical simulations. Added mass effect is examined using flow visualization of the fluid acceleration field. The scale effect on the parameters is also addressed.
Proceedings of International Conference on Advances in Computational and Experimental Marine Hydrodynamics (ACEMH 2014), 2014
The present work is aimed at CFD simulation and experimental study on frequency and amplitude dep... more The present work is aimed at CFD simulation and experimental study on frequency and amplitude dependency of heave damping of Spar hulls by forced oscillation along heave direction in calm water condition. Conventionally, the heave damping is obtained by free decay test of a scale model which gives damping only at the heave natural frequency, but the damping at frequencies other than natural frequency cannot be determined by this method. So the forced heave oscillation of Froude scaled Spar models were conducted in calm water condition over a range of frequencies and amplitudes of oscillation and the forces were measured along the heave direction to calculate added mass and damping from the equation of motion. The experimental investigations were performed on a classic Spar model with and without heave plate in a laboratory wave flume. The CFD simulations were carried out in a numerical tank using commercial CFD software. The measured heave force is compared with that obtained from CFD simulation. Further, the added mass and damping coefficient have been calculated by solving equation of motion from both experiment and simulation using the forces and heave motion as input. The amplitude and frequency dependency of heave added mass and heave damping for spar with and without heave plate are presented and discussed.
Underwater Technology (UT) 2015 IEEE, 2015
Autonomous Underwater Vehicles (AUV) are slow-moving small unmanned robots capable of swimming in... more Autonomous Underwater Vehicles (AUV) are slow-moving small unmanned robots capable of swimming independently below the water surface on pre-defined mission paths and are commonly used for oceanographic exploration, bathymetric surveys and military applications. With the use of appropriate sensors and equipment, AUVs can perform underwater object recognition and obstacle avoidance. Amogh is a miniature AUV developed at Centre For Innovation (CFI), IIT Madras for AUVSI RoboSub competition. The vehicle has a non-conventional dual hull heavy bottom hydrodynamic design equipped with six thrusters which allow for motion control in 4 degrees of freedom. This paper presents various aspects of the unique design of the vehicle. The performance of a simple PID controller for steady depth and heading control has been discussed. Simulations performed on a decoupled mathematical model of the vehicle are compared against experimental results.
Applied Ocean Research, Dec 28, 2020
Spar, tension leg platform, and semi-submersibles use heave plates to reduce heave response by in... more Spar, tension leg platform, and semi-submersibles use heave plates to reduce heave response by increasing heave damping and added mass. Conventional practice is to use a linear damping ratio, which is typically obtained from free heave decay tests or Computational Fluid Dynamics (CFD) simulations. However, with the addition of heave plates, the system damping becomes nonlinear. Understanding such nonlinear damping behavior of spar with circular heave plates in various configurations could be useful in the design of heave compensation devices. Experimental and numerical investigation on heave damping and added mass of a scaled model of spar with a variety of heave plate configurations have been carried out using free heave decay for a range of initial heave displacements. Applicability of the linear and quadratic damping models have been assessed for all configurations. The effects of parameters such as heave plate diameter, location of heave plate above the keel, and spacing between two plates on damping and added mass have also been studied. Flow fields obtained from numerical simulations are presented, and their implication on damping discussed.