Now showing 1 - 10 of 89
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    Spatiotemporal dynamics of a vortex induced vibration system in the presence of stochastic inflow fluctuations
    (01-08-2022)
    Aswathy, M. S.
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    In this study, influence of stochastic parametric inflow on the spatiotemporal dynamics of a vortex induced vibration system has been examined. The study has been carried out using an incompressible Navier–Stokes based flow solver at a Reynolds number of 100, and it highlights the role of stochastic fluctuations/noise in triggering complex spatiotemporal dynamics in the laminar vortex shedding regime. Wavelets and space–time plots were used to analyse the spatiotemporal data. Noise has been seen to advance the instability regime and alter the frequency characteristics of the response by exciting additional system frequencies, which in turn affects the phase dynamics as well. The study has also revealed that the time scale of the input is an important factor in deciding the dynamics. A short time-scaled noise can trigger aperiodic dynamics, whereas, a long time-scaled input invokes switch states or intermittent states. Noise affects the separation behaviour of the near-field vortices by delaying or advancing it, depending on the regime of operation. For the long time-scaled case, the flow-field becomes a combination of strong or weak von Kárman streets and transitionally disordered wakes. With the short time-scaled noise, the flow-field is predominantly disordered and characterised by phenomena such as vortex pairing and deflected wakes.
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    Estimation of modal characteristics of a reduced-order fluid–structure interaction system
    (01-01-2019)
    Lal, Hridya P.
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    Dheelibun Remigius, W.
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    Understanding of modal characteristics of fluid–structure interaction (FSI) systems plays a vital role in avoiding large oscillations and instabilities. Further, with parametric uncertainties, the onset of the instabilities may change and hence it is important to know the statistics of the modal characteristics of the coupled system to avoid system failures. The primary objective of the study is to estimate the statistics of modal characteristics of the stochastic FSI system which demands intensive computation resources especially for large ordered FSI systems. The present study also focusses on developing and implementing a stochastic reduced-order model to resolve pressure-induced oscillations of a disc-like structure as a generic fluid–structure interaction system. The stochastic reduced-order model uses a modal reduction approach together with spectral projection-based polynomial chaos expansion (PCE) to truncate both the system degrees of freedom (dofs) and the random modes. Interfacing algorithms have also been developed that enable finite element (FE) modelling of the FSI system using commercial software and in-house developed codes.
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    Interaction of a flexible splitter plate with vortex shedding past a rigid circular cylinder
    (01-01-2019)
    Bose, Chandan
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    Krishna Kumar, S.
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    This study deals with energy harvesting from vortex-induced vibration (VIV) of a flexible cantilever plate placed in the wake of a rigid cylinder. Conventionally, the plate is designed to resonate with the vortex shedding frequency behind the cylinder at a given flow velocity. The experimental results presented here show that the frequency of the plate oscillations undergoes an abrupt increase from a low value to a higher value at a critical flow velocity. Numerical simulations carried out in the present study suggest that this jump is due to the transition between two modes of wake oscillations in the system, one with a low Strouhal number range accompanied by impeded shedding and the other being the natural vortex shedding with a higher Strouhal number range. It is also found that the mode of vortex shedding depends on the flow velocity as well as the gap between the cylinder and the plate. There exists a critical gap for every flow velocity below which vortex shedding is inhibited resulting in reduced plate vibrations. The results indicate that the position of the plate in the wake affects the shedding frequency significantly and thus the gap between the plate and cylinder needs to be considered as a key parameter in design of such flow-induced energy harvesters.
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    Comparison of Stochastic Responses of Circular Cylinder Undergoing Vortex-Induced Vibrations with One and Two Degrees of Freedom
    (01-01-2021)
    Aswathy, M. S.
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    Vortex-induced vibration of a circular cylinder is a major research topic due to the immense applications they have in daily and industrial scenarios. Large numbers of studies have been conducted in this area in numerical and experimental domains with focus on understanding the response types, understanding the range of lock-in, the flow behavior, etc. However, most of the studies till date have been done in a deterministic environment; on the pretext that all factors about the incoming flow and input parameters are exactly known. In real-time flows, there can be a significant amount of uncertainties associated with various system parameters, which are traditionally not taken into consideration for the system. For example, randomness associated with the incoming flow might have significant effect on the associated dynamics. In this study, we do a stochastic modeling on a circular cylinder exhibiting free vibrations with one and two degrees of freedom. For this, we use Duffing Van der Pol combined system and impose fluctuations at every time step in the input flow by modeling them through a uniform distribution. The transverse oscillations of each of the cases under the presence of noise are individually studied. It is seen that noise brings in new dynamical states to the cylinder response compared to the deterministic cases. It is observed that there is a considerable difference between the responses of the single degree of freedom and two-degree of freedom cylinder. These qualitative differences are investigated in detail in the current study.
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    Effect of noise on vortex-induced vibrations of circular cylinders
    (01-01-2019)
    Aswathy, M. S.
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    In this study, we examine the effect of uniformly distributed noise superimposed on a steady, incompressible mean flow, past a circular cylinder undergoing transverse oscillations. We use a Duffing–Van der pol combined system for the present investigation in order to model the associated nonlinear behavior. We observe that as we vary the mean velocity as the bifurcation parameter, noise brings in major qualitative and quantitative changes on the structural response of the system compared to the deterministic cases. Noise induces new dynamical states in this vortex-induced vibration (VIV) system. The noise, depending on its coefficient of intensity, has a role in advancing the lock-in regime. The time histories of the response prior and post lock-in show different dynamics compared to the deterministic cases. Also, depending on the intensity of the noise coefficient, the probability density functions of the response amplitude also undergo qualitative and quantitative changes. We investigate the existence of P (phenomenological)-bifurcations in the VIV system by analyzing the joint probability density functions and quantify the response using Shannon entropy.
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    A model order reduction technique for systems with nonlinear frequency dependent damping
    (01-01-2020)
    Jith, Jithin
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    Frequency domain solution of systems with frequency dependent damping is a computationally expensive endeavour especially when dealing with large order three-dimensional systems. A moment-matching based reduced order model is proposed in this work which is capable of handling nonlinear frequency dependent damping in second-order systems. In the proposed approach, local linear systems with frequency independent matrices are derived from the original system, and using the principles of the Rational Krylov approach, orthogonal basis vectors are computed from these local systems through the second-order Arnoldi procedure. The system is then projected on to the basis set to obtain a numerically efficient reduced order model, accurate in the entire frequency domain of interest. The proposed approach is also shown to be more accurate than the popular modal projection based multi-model approach of the same order. The proposed tool is applied to the problem of determining the frequency response of an idealised centrifugal compressor impeller with non-viscous (frequency dependent) damping.
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    Transitional Flow Dynamics Past a Passively Flapping Airfoil in Gusty Flow
    (01-01-2021)
    Bose, Chandan
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    This paper investigates the transitional flow dynamics behind a passively flapping airfoil supported by nonlinear springs in the presence of gusty inflow. The fluid-structure interaction (FSI) framework is composed of an incompressible Navier-Stokes solver weakly coupled with a two degree-of-freedom (dof) nonlinear structural model. The fluid-elastic system shows a rich bifurcation behavior in terms of successive Hopf bifurcations in uniform flow condition as the mean wind speed is increased. Presence of gusty fluctuations in the inflow makes the dynamics more complex through transitional states that we refer to as ‘intermittency’ between different dynamical states. A regular intermittent state between quasi-periodic dynamics and low amplitude aperiodic response has been observed when the FSI system is subjected to a time harmonic gust in terms of sinusoidal fluctuation. A parametric study has been carried out for various amplitudes and frequencies of the sinusoidal fluctuation to demarcate the transitional regimes. Thereafter, the system is subjected to random gusts modeled as Ornstein-Uhlenbeck process and ‘on-off’ and ‘burst’ type intermittent dynamics have been observed for long time-scale and short time-scale input fluctuations respectively. The intermittent states have been characterized through time series analyses tools and the corresponding flow-field dynamics has been investigated in detail.
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    Analysis of a nonlinear aeroelastic system with parametric uncertainties using Polynomial Chaos Expansion
    (07-09-2011)
    Desai, Ajit
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    Aeroelastic stability remains an important concern for the design of modern structures such as wind turbine rotors, more so with the use of increasingly flexible blades. A nonlinear aeroelastic system has been considered in the present study with parametric uncertainties. Uncertainties can occur due to any inherent randomness in the system or modeling limitations, and so forth. Uncertainties can play a significant role in the aeroelastic stability predictions in a nonlinear system. The analysis has been put in a stochastic framework, and the propagation of system uncertainties has been quantified in the aeroelastic response. A spectral uncertainty quantification tool called Polynomial Chaos Expansion has been used. A projection-based nonintrusive Polynomial Chaos approach is shown to be much faster than its classical Galerkin method based counterpart. Traditional Monte Carlo Simulation is used as a reference solution. Effect of system randomness on the bifurcation behavior and the flutter boundary has been presented. Stochastic bifurcation results and bifurcation of probability density functions are also discussed.
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    Dynamical Stability Analysis of a Fluid Structure Interaction System Using a High Fidelity Navier-stokes Solver
    (01-01-2016)
    Bose, Chandan
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    Badrinath, Sandeep
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    The present paper investigates the flow induced dynamics of a non-linear fluid-structure interaction (FSI) system comprising of a symmetrical NACA 0012 airfoil supported by non-linear springs. Two methods are used in calculating the aerodynamic loads: a linear analytical approach and a full Navier-Stokes (N-S) solution. The analytical approach is based on the assumption of potential flow theory and a rigid wake. Wind velocity as a bifurcation parameter shows that the structural response undergoes a supercritical Hopf bifurcation. However, the analytical loads predict unrealistic bifurcation onset at low values of solid to fluid added mass ratio (μ) relevant to the application of flapping wing micro air vehicles (MAVs), showing the extremely large amplitude of oscillations. These observations render the use of an inviscid approach meaningless at such parametric regimes. Hence, we propose to use a N-S solver to emphasize the limit of applicability of the linear aerodynamic theory. Moreover, the inclusion of the viscous effects can potentially result in interesting dynamical behavior that has not been captured by the analytical approach. A bifurcation and stability analysis has been carried out for different parametric variations of μ in the fluid structure interaction system.
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    Bifurcation and response analysis of a nonlinear flexible rotating disc immersed in bounded compressible fluid
    (31-03-2017)
    Remigius, W. Dheelibun
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    Use of heavy gases in centrifugal compressors for enhanced oil extraction have made the impellers susceptible to failures through acousto-elastic instabilities. This study focusses on understanding the dynamical behavior of such systems by considering the effects of the bounded fluid housed in a casing on a rotating disc. First, a mathematical model is developed that incorporates the interaction between the rotating impeller - modelled as a flexible disc - and the bounded compressible fluid medium in which it is immersed. The nonlinear effects arising due to large deformations of the disc have been included in the formulation so as to capture the post flutter behavior. A bifurcation analysis is carried out with the disc rotational speed as the bifurcation parameter to investigate the dynamical behavior of the coupled system and estimate the stability boundaries. Parametric studies reveal that the relative strengths of the various dissipation mechanisms in the coupled system play a significant role that affect the bifurcation route and the post flutter behavior in the acousto-elastic system.