I R Praveen Krishna | Indian Institute of Space Science and Technology, Trivandrum, India (original) (raw)

Papers by I R Praveen Krishna

One of the primary concern in piezoresistive micro accelerometers is their cross axis sensitivity... more One of the primary concern in piezoresistive micro accelerometers is their cross axis sensitivity. The most direct approach to solve this problem is by means of electronic bridge configurations, which relies on the difference in stress profiles along different directions. In this paper, a nonplanar dual flexure geometry has been proposed as a scheme for cross axis sensitivity reduction, by the mechanical design itself. The device exhibits a very low cross axis sensitivity of <0.01% in Y direction and <0.05% in X direction while maintaining high prime axis sensitivity of 133μV/Vg. The device also shows very low non linearity (<1%) over a range of 100g acceleration and a resonant frequency of 2.2 kHz.

International Journal of Computational Methods, Mar 4, 2020

In this study, element-free Galerkin method (EFGM), a meshless method, is proposed for wrinkling ... more In this study, element-free Galerkin method (EFGM), a meshless method, is proposed for wrinkling analysis of pre-stressed rectangular membranes. The mathematical model for studying wrinkling of pre-stressed membranes is derived by considering the bending stiffness, though it is negligible. Moving least-square approximation for deflection is constructed by considering three degrees of freedom per node. Essential boundary conditions are imposed using scaled transformation matrix method. Initially, compression-induced buckling of a homogeneous thin plate without pre-stress is solved to validate the method and then a pre-stressed homogeneous membrane is analyzed for both compression-induced and shear-induced wrinkling. Capabilities of the proposed method for membrane analysis are compared with that of the finite element method (FEM). Comparative study on wrinkling analysis using EFGM and different FEM element types in a commercial FEM package shows that in lower modes both methods show satisfying consistency in eigenvalues with respect to the total of number of nodes, while at higher modes EFGM shows better consistency than FEM. Further, the study is extended to wrinkling of nonhomogeneous membranes subjected to linearly-varying in-plane load. The results obtained from EFGM analysis is compared and found to be matching well with those available in the literature.

Applied Acoustics, Oct 1, 2021

Nonlinear Dynamics, Mar 1, 2023

International Journal of Non-linear Mechanics, Jun 1, 2022

IOP conference series, Sep 1, 2020

The current paper presents a meshless formulation for the analysis of geometrically nonlinear one... more The current paper presents a meshless formulation for the analysis of geometrically nonlinear one-dimensional Euler-Bernoulli beams. Element-free Galerkin method (EFGM) is used as the meshless numerical tool and Newton-Raphson method as the iterative scheme. Numerical examples of beams with four different boundary conditions are solved, and the results are compared with that of the finite element method (FEM). EFGM results are found in good agreement with that of FEM. Also, it is found that EFGM eliminates the effect of membrane locking in thin beams.

Springer eBooks, Jul 19, 2021

Recent Advances in Materials, Mechanics and Management, 2019

Microsystem Technologies, 2017

Nonlinear Dynamics, Jul 1, 2020

The rotor–stator rubbing in rotating machinery generated as a consequence of rotor imbalance, sha... more The rotor–stator rubbing in rotating machinery generated as a consequence of rotor imbalance, shaft misalignment, and casing deformation is a potential threat to the machinery that seriously affects its performance. Timely prediction and correction of the rubbing are essential for the prolonged life of the machinery and its overall performance. A complete understanding of the system behavior during interaction is a great challenge for researchers working in the field of rotor dynamics. Rubbing phenomena involve complex contact nonlinearities and associated thermal effects, which makes the analysis very difficult. Research works in this field are started with the analysis of simple two degree-of-freedom models and now dealing with extensive three-dimensional finite element models. This paper provides a comprehensive review of different numerical models of rotor–stator rubbing with respect to their ability in simulating the actual response characteristics. A detailed description of contact modeling is also presented with the advantages and disadvantages of each model. Different methods for solving the numerical models are briefly explained. In addition, a commentary on different emerging techniques of rub identification is also reported. Finally, some informed recommendations on future directions are made by stating what lacks in the current research activities.

Nonlinear Dynamics, Mar 30, 2021

The main objective of the present paper is to determine the influence of rub parameters on the st... more The main objective of the present paper is to determine the influence of rub parameters on the stability of a two-spool rotor system undergoing rub-impact. The parameters such as rotor–stator contact stiffness, coefficient of friction and clearance are varied for understanding their effects on the system response and stability. Moreover, the analysis is performed for two modes of rotor operations, namely co-rotation and counter-rotation, and determines their impacts on rotor–stator rubbing. A time variational method is employed to predict the nonlinear response of the system with a perturbation function applied at the steady-state solution points to investigate their stability. Two types of bifurcations, namely limit point and Neimark–Sacker bifurcations, are observed in the response by monitoring the Floquet exponents of the perturbed system. As the coefficient of friction is increased, the early onset of NS bifurcation has happened and the system enters into the quasi-periodic regime early. However, when the contact stiffness and clearance values are increased, the onset of NS bifurcation is delayed. It is also observed that the response characteristics of the co- and counter-rotating systems are entirely different. The separation between forward and backward whirling frequencies is reduced for the counter-rotating system due to the cancellation of gyroscopic moments. In addition, for the same set of parameters, the counter-rotating system enters into the quasi-periodic regime quickly once the disk starts rubbing.

Mechanisms and machine science, Aug 19, 2018

This paper discusses a modified model reduction technique for the nonlinear rubbing analysis of a... more This paper discusses a modified model reduction technique for the nonlinear rubbing analysis of a rotor disk with its stator. The rotor system consisting of a rigid disk, shaft and bearings is modeled using finite elements, incorporating the effects of rotary inertia and gyroscopic moments of both shaft and disk. The stator is modeled as an added stiffness to the rotor system without considering the stator dynamics and dry friction effect at the contact. The nonlinearities are localized at the rub location which permits the use of model reduction techniques, making the finite element model more compact. Component Mode Synthesis with a Craig-Bampton type sub-structuring is an efficient technique for model reduction. But, this method has some limitations due to the presence of nonlinearities in the system. In this paper, a modified Component Mode Synthesis method with dynamic sub-structuring is developed for the reduction of complete finite element model into a smaller model containing nonlinear degrees of freedom (DOF) only. This method has an advantage over existing methods is that it can be used for systems with non-symmetric element matrices. The reduced model is solved using Harmonic Balance Method (HBM) coupled with a hypersphere based continuation algorithm.

Springer eBooks, Jul 19, 2021

Nonlinear Dynamics, 2020

The rotor–stator rubbing in rotating machinery generated as a consequence of rotor imbalance, sha... more The rotor–stator rubbing in rotating machinery generated as a consequence of rotor imbalance, shaft misalignment, and casing deformation is a potential threat to the machinery that seriously affects its performance. Timely prediction and correction of the rubbing are essential for the prolonged life of the machinery and its overall performance. A complete understanding of the system behavior during interaction is a great challenge for researchers working in the field of rotor dynamics. Rubbing phenomena involve complex contact nonlinearities and associated thermal effects, which makes the analysis very difficult. Research works in this field are started with the analysis of simple two degree-of-freedom models and now dealing with extensive three-dimensional finite element models. This paper provides a comprehensive review of different numerical models of rotor–stator rubbing with respect to their ability in simulating the actual response characteristics. A detailed description of contact modeling is also presented with the advantages and disadvantages of each model. Different methods for solving the numerical models are briefly explained. In addition, a commentary on different emerging techniques of rub identification is also reported. Finally, some informed recommendations on future directions are made by stating what lacks in the current research activities.

Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, Oct 17, 2017

The focus of the current study is on the dynamics of rubbing between the rotor and stator parts i... more The focus of the current study is on the dynamics of rubbing between the rotor and stator parts in a rotating machine. Rub is a malfunction associated with the physical contact of rotating and stationary parts, which are otherwise not in contact. Because of the nonlinear nature of the problem the simulation time is significant even for small size systems. The rubbing is localized in space, either at the seal locations or at the interface between the rotor blade and stator. Since the nonlinearity is localized, reduced models can be developed for efficient computation. The objective of the present study is to develop a computationally efficient methodology for analyzing the rotor stator rub, by applying model reduction techniques using component mode synthesis, solving the reduced problem using harmonic balance method and time variational method. A hypersphere-based continuation algorithm is used for tracing the unstable branches and a backward differentiation formula based predictor is used for the Newton–Raphson update. The numerical results are validated by performing experiments.

Nonlinear Dynamics, Jul 9, 2011

In this paper, the dynamic behavior of a cantilever beam impacting two flexible stops as well as ... more In this paper, the dynamic behavior of a cantilever beam impacting two flexible stops as well as rigid stops is studied both experimentally and numerically. The effect of contact stiffness, clearance, and contacting materials is studied in detail. For the numerical study of the system, a finite element model is created and the resulting differential equations are solved using a Time Variational Method (TVM). To achieve higher computational efficiency, the Newton–Krylov method is used along with TVM. Experimental results validate the contact model proposed for predicting the first mode system dynamics. A new nonlinear force estimation function has been proposed based on measured accelerations, which enables the understanding of the impact dynamics.

IET Science, Measurement & Technology, 2012

Journal of Nondestructive Evaluation, 2019

Oil whirl and whip phenomena are fundamental to rotor systems supported by journal bearings. Publ... more Oil whirl and whip phenomena are fundamental to rotor systems supported by journal bearings. Published studies opt for a reduced formulation of the Reynolds equation of lubrication in order to aid the computations. In the current study, the complete Reynolds equation is solved using Pseudo Spectral Methods (PSM) and results compared with reduced solutions. The possibility of certain trends being missed in reduced model simulations is also brought out using a simple example. Rotor shaft and journal bearing systems are numerically modelled and coupled simulations have been carried out for test rotors inspired from literature. A semi analytical derivative estimation method is demonstrated to be superior to conventional finite difference methods in terms of processor load. This will be a useful addition for iterative solvers applied on rotors with more complicated geometry. Time transient analysis is carried out for two test rotors in order to bring out the oil whirl and whip phenomena,...

One of the primary concern in piezoresistive micro accelerometers is their cross axis sensitivity... more One of the primary concern in piezoresistive micro accelerometers is their cross axis sensitivity. The most direct approach to solve this problem is by means of electronic bridge configurations, which relies on the difference in stress profiles along different directions. In this paper, a nonplanar dual flexure geometry has been proposed as a scheme for cross axis sensitivity reduction, by the mechanical design itself. The device exhibits a very low cross axis sensitivity of <0.01% in Y direction and <0.05% in X direction while maintaining high prime axis sensitivity of 133μV/Vg. The device also shows very low non linearity (<1%) over a range of 100g acceleration and a resonant frequency of 2.2 kHz.

International Journal of Computational Methods, Mar 4, 2020

In this study, element-free Galerkin method (EFGM), a meshless method, is proposed for wrinkling ... more In this study, element-free Galerkin method (EFGM), a meshless method, is proposed for wrinkling analysis of pre-stressed rectangular membranes. The mathematical model for studying wrinkling of pre-stressed membranes is derived by considering the bending stiffness, though it is negligible. Moving least-square approximation for deflection is constructed by considering three degrees of freedom per node. Essential boundary conditions are imposed using scaled transformation matrix method. Initially, compression-induced buckling of a homogeneous thin plate without pre-stress is solved to validate the method and then a pre-stressed homogeneous membrane is analyzed for both compression-induced and shear-induced wrinkling. Capabilities of the proposed method for membrane analysis are compared with that of the finite element method (FEM). Comparative study on wrinkling analysis using EFGM and different FEM element types in a commercial FEM package shows that in lower modes both methods show satisfying consistency in eigenvalues with respect to the total of number of nodes, while at higher modes EFGM shows better consistency than FEM. Further, the study is extended to wrinkling of nonhomogeneous membranes subjected to linearly-varying in-plane load. The results obtained from EFGM analysis is compared and found to be matching well with those available in the literature.

Applied Acoustics, Oct 1, 2021

Nonlinear Dynamics, Mar 1, 2023

International Journal of Non-linear Mechanics, Jun 1, 2022

IOP conference series, Sep 1, 2020

The current paper presents a meshless formulation for the analysis of geometrically nonlinear one... more The current paper presents a meshless formulation for the analysis of geometrically nonlinear one-dimensional Euler-Bernoulli beams. Element-free Galerkin method (EFGM) is used as the meshless numerical tool and Newton-Raphson method as the iterative scheme. Numerical examples of beams with four different boundary conditions are solved, and the results are compared with that of the finite element method (FEM). EFGM results are found in good agreement with that of FEM. Also, it is found that EFGM eliminates the effect of membrane locking in thin beams.

Springer eBooks, Jul 19, 2021

Recent Advances in Materials, Mechanics and Management, 2019

Microsystem Technologies, 2017

Nonlinear Dynamics, Jul 1, 2020

The rotor–stator rubbing in rotating machinery generated as a consequence of rotor imbalance, sha... more The rotor–stator rubbing in rotating machinery generated as a consequence of rotor imbalance, shaft misalignment, and casing deformation is a potential threat to the machinery that seriously affects its performance. Timely prediction and correction of the rubbing are essential for the prolonged life of the machinery and its overall performance. A complete understanding of the system behavior during interaction is a great challenge for researchers working in the field of rotor dynamics. Rubbing phenomena involve complex contact nonlinearities and associated thermal effects, which makes the analysis very difficult. Research works in this field are started with the analysis of simple two degree-of-freedom models and now dealing with extensive three-dimensional finite element models. This paper provides a comprehensive review of different numerical models of rotor–stator rubbing with respect to their ability in simulating the actual response characteristics. A detailed description of contact modeling is also presented with the advantages and disadvantages of each model. Different methods for solving the numerical models are briefly explained. In addition, a commentary on different emerging techniques of rub identification is also reported. Finally, some informed recommendations on future directions are made by stating what lacks in the current research activities.

Nonlinear Dynamics, Mar 30, 2021

The main objective of the present paper is to determine the influence of rub parameters on the st... more The main objective of the present paper is to determine the influence of rub parameters on the stability of a two-spool rotor system undergoing rub-impact. The parameters such as rotor–stator contact stiffness, coefficient of friction and clearance are varied for understanding their effects on the system response and stability. Moreover, the analysis is performed for two modes of rotor operations, namely co-rotation and counter-rotation, and determines their impacts on rotor–stator rubbing. A time variational method is employed to predict the nonlinear response of the system with a perturbation function applied at the steady-state solution points to investigate their stability. Two types of bifurcations, namely limit point and Neimark–Sacker bifurcations, are observed in the response by monitoring the Floquet exponents of the perturbed system. As the coefficient of friction is increased, the early onset of NS bifurcation has happened and the system enters into the quasi-periodic regime early. However, when the contact stiffness and clearance values are increased, the onset of NS bifurcation is delayed. It is also observed that the response characteristics of the co- and counter-rotating systems are entirely different. The separation between forward and backward whirling frequencies is reduced for the counter-rotating system due to the cancellation of gyroscopic moments. In addition, for the same set of parameters, the counter-rotating system enters into the quasi-periodic regime quickly once the disk starts rubbing.

Mechanisms and machine science, Aug 19, 2018

This paper discusses a modified model reduction technique for the nonlinear rubbing analysis of a... more This paper discusses a modified model reduction technique for the nonlinear rubbing analysis of a rotor disk with its stator. The rotor system consisting of a rigid disk, shaft and bearings is modeled using finite elements, incorporating the effects of rotary inertia and gyroscopic moments of both shaft and disk. The stator is modeled as an added stiffness to the rotor system without considering the stator dynamics and dry friction effect at the contact. The nonlinearities are localized at the rub location which permits the use of model reduction techniques, making the finite element model more compact. Component Mode Synthesis with a Craig-Bampton type sub-structuring is an efficient technique for model reduction. But, this method has some limitations due to the presence of nonlinearities in the system. In this paper, a modified Component Mode Synthesis method with dynamic sub-structuring is developed for the reduction of complete finite element model into a smaller model containing nonlinear degrees of freedom (DOF) only. This method has an advantage over existing methods is that it can be used for systems with non-symmetric element matrices. The reduced model is solved using Harmonic Balance Method (HBM) coupled with a hypersphere based continuation algorithm.

Springer eBooks, Jul 19, 2021

Nonlinear Dynamics, 2020

The rotor–stator rubbing in rotating machinery generated as a consequence of rotor imbalance, sha... more The rotor–stator rubbing in rotating machinery generated as a consequence of rotor imbalance, shaft misalignment, and casing deformation is a potential threat to the machinery that seriously affects its performance. Timely prediction and correction of the rubbing are essential for the prolonged life of the machinery and its overall performance. A complete understanding of the system behavior during interaction is a great challenge for researchers working in the field of rotor dynamics. Rubbing phenomena involve complex contact nonlinearities and associated thermal effects, which makes the analysis very difficult. Research works in this field are started with the analysis of simple two degree-of-freedom models and now dealing with extensive three-dimensional finite element models. This paper provides a comprehensive review of different numerical models of rotor–stator rubbing with respect to their ability in simulating the actual response characteristics. A detailed description of contact modeling is also presented with the advantages and disadvantages of each model. Different methods for solving the numerical models are briefly explained. In addition, a commentary on different emerging techniques of rub identification is also reported. Finally, some informed recommendations on future directions are made by stating what lacks in the current research activities.

Proceedings of the Institution of Mechanical Engineers, Part C: Journal of Mechanical Engineering Science, Oct 17, 2017

The focus of the current study is on the dynamics of rubbing between the rotor and stator parts i... more The focus of the current study is on the dynamics of rubbing between the rotor and stator parts in a rotating machine. Rub is a malfunction associated with the physical contact of rotating and stationary parts, which are otherwise not in contact. Because of the nonlinear nature of the problem the simulation time is significant even for small size systems. The rubbing is localized in space, either at the seal locations or at the interface between the rotor blade and stator. Since the nonlinearity is localized, reduced models can be developed for efficient computation. The objective of the present study is to develop a computationally efficient methodology for analyzing the rotor stator rub, by applying model reduction techniques using component mode synthesis, solving the reduced problem using harmonic balance method and time variational method. A hypersphere-based continuation algorithm is used for tracing the unstable branches and a backward differentiation formula based predictor is used for the Newton–Raphson update. The numerical results are validated by performing experiments.

Nonlinear Dynamics, Jul 9, 2011

In this paper, the dynamic behavior of a cantilever beam impacting two flexible stops as well as ... more In this paper, the dynamic behavior of a cantilever beam impacting two flexible stops as well as rigid stops is studied both experimentally and numerically. The effect of contact stiffness, clearance, and contacting materials is studied in detail. For the numerical study of the system, a finite element model is created and the resulting differential equations are solved using a Time Variational Method (TVM). To achieve higher computational efficiency, the Newton–Krylov method is used along with TVM. Experimental results validate the contact model proposed for predicting the first mode system dynamics. A new nonlinear force estimation function has been proposed based on measured accelerations, which enables the understanding of the impact dynamics.

IET Science, Measurement & Technology, 2012

Journal of Nondestructive Evaluation, 2019

Oil whirl and whip phenomena are fundamental to rotor systems supported by journal bearings. Publ... more Oil whirl and whip phenomena are fundamental to rotor systems supported by journal bearings. Published studies opt for a reduced formulation of the Reynolds equation of lubrication in order to aid the computations. In the current study, the complete Reynolds equation is solved using Pseudo Spectral Methods (PSM) and results compared with reduced solutions. The possibility of certain trends being missed in reduced model simulations is also brought out using a simple example. Rotor shaft and journal bearing systems are numerically modelled and coupled simulations have been carried out for test rotors inspired from literature. A semi analytical derivative estimation method is demonstrated to be superior to conventional finite difference methods in terms of processor load. This will be a useful addition for iterative solvers applied on rotors with more complicated geometry. Time transient analysis is carried out for two test rotors in order to bring out the oil whirl and whip phenomena,...