Muhammad Saif Ullah Khalid

Analysis of Strouhal number based equivalence of pitching and plunging airfoils and wake deflection

Numerical simulations have been used to analyze the equivalence of pitching and plunging motions found in a flapping NACA0012 airfoil. Two-dimensional incompressible Navier–Stokes equations are solved at Reynolds number of 103 over a range of Strouhal numbers. A novel criterion based on the Strouhal number is proposed which provides equivalence of pitching and plunging motions using the length scale traversed by the trailing edge in each case. Aerodynamic coefficients are found to match well for both the kinematics in temporal as well as spectral domains. Detailed analysis provides contribution of different mechanisms, such as vortex shedding, added mass, interaction of leading and trailing edge vortices, in the overall aerodynamic forces produced by a pitching or plunging airfoil. Wake deflection is observed for a plunging airfoil at high Strouhal numbers resulting in a bias in the lift coefficient. Further investigation reveals the dominance of second harmonic of the fundamental frequency in the lift spectrum emphasizing the role of quadratic nonlinearity in the observed phenomenon.

Reduced-Order Modeling of Torque on a Vertical-Axis Wind Turbine at Varying Tip Speed Ratios

Vertical-axis wind turbine (VAWT) has received significantattention due to its application in urban environment. Torque produced by VAWT determines its efficiency and power o In this paper, we develop a reduced-order model of torque VAWT at different tip speed ratios (TSR). We n umerically simulate both two- and three-dimensional flows past a three-bladed Darrieus H-type VAWT and compute ov erall torque acting on the turbine. We then perform higher-order spectral analysis to identify dominant frequ encies and nonlinear couplings. We propose a reducedorder model of torque in the form of modified van der Pol equati on with additional quadratic term to allow for even harmonics in addition to odd harmonics present in the sy stem. Using, a perturbation approach of method of multiple scales, we solve the proposed model and compute t he coefficients at different TSR. The model not only predicts torque accurately in time domain but also in spectr al domain. These reduced-order models provide an accurate and computationally efficient means to predict ove rall performance and output of the turbine with varying free-stream conditions even in predictive setting.

Aerodynamics of a flying wing UAV with backward facing stepped wing profile

A flying wing possesses no definite fuselage and no horizontal tail surfaces resulting in lesser drag surfaces, a larger wing area with a better range and endurance as compared to a conventional unmanned aerial vehicle (UAV). The present work explores an efficient and stable flight of a flying wing UAV at a lower Reynolds number using backward facing step wing profile. Numerical simulations for flow over a wing with NACA0012 airfoil, having a backward facing step along the length of its chord, on its upper surface were performed. The depth of the step was also varied to find out an optimum depth of the step to improve its aerodynamic performance. This work involves two-dimensional and three-dimensional steady state and transient simulations followed by the introduction of the wing end-plates to control the wingtip vortices. Here, the wings were designed, based on their aspect- and taper-ratios. Structural analysis was carried out to investigate the natural modes and frequencies in the current wing configurations.

Nonlinear Characterization of Heart Rate Variability in Normal Sinus Rhythm, Atrial Fibrillation and Congestive Heart Failure

This paper highlights the application of methods and techniques from nonlinear analysis to illustrate their far superior capability in revealing complex cardiac dynamics under various physiological and pathological states. The purpose is to augment conventional (time and frequency based) heart rate variability analysis, and to extract significant prognostic and clinically relevant information for risk stratification and improved diagnosis. In this work, several nonlinear indices are estimated for RR intervals based time series data acquired for Healthy Sinus Rhythm (HSR) and Congestive Heart Failure (CHF), as the two groups represent different cases of Normal Sinus Rhythm (NSR). In addition to this, nonlinear algorithms are also applied to investigate the internal dynamics of Atrial Fibrillation (AFib). Application of nonlinear tools in normal and diseased cardiovascular states manifest their strong ability to support clinical decision support systems and highlights the internal complex properties of physiological time series data such as complexity, irregularity, determinism and recurrence trends in cardiovascular regulation mechanisms.Copyright

Nonlinear Characterization of Flow over Oscillating Elliptic Airfoils

Using immersed boundary methods, we perform numerical simulation for flow over a heaving elliptical airfoil at Re = 500 for a range of Strouhal numbers at reduced frequency 6.283. We quantify the response of this nonlinear system through time-histories of CL and CT , their Fourier spectra, phase maps, poincare sections, and higher-order spectral analysis. With increasing oscillation amplitude, this nonlinear system undergoes bifurcations achieving chaotic solutions finally. Aerodynamic response takes route to chaos through quasi-periodicity. We also relate these behavioral changes in the response of the system with the wake transitions. We also present cross-bispectrum between CL and CT , and autotripsectrum for CL to study cubic coupling among various frequency components. We find the strongest quadratic and cubic coupling between the excitation frequency components.

Proper orthogonal decomposition closure models for Burgers and Navier-Stokes Equations

Proper orthogonal decomposition based reduced-order models hold importance in fluid dynamics due to their utilization in flow control, design, and optimization. The reducedordered models have shown promising results for laminar flows, however, lack accuracy for complex and turbulent flows. In this study, we consider Burgers and Navier-Stokes equations and develop their reduced order models. Burgers equation is considered with homogenous boundary condition and Navier-Stokes equations are used for simulating the incompressible fluid flow past a circular cylinder. The literature shows that the conventional reduced-order models do not perform well in case of less number of modes and require closure model for the discarded modes. We investigate the effect of closure modeling on the accuracy of the reduced-order model for 1D Burgers equation and 2D Navier-Stokes equations. We consider three mode-dependent closure models, based on eddy viscosity model. This study compares the performance of all considered closure models for wide range of mathematical parameter νe and choose the best closure approach for the reducedorder model of Navier-Stokes equations. The numerical results show that the selected closure model greatly improves the accuracy of the reduced-order model for both Burgers and Navier-Stokes equations.