Mechanical Engineering - Theses
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ItemRoughened Ultra High-Lift Blades with Grooves for Drag Reduction( 2023-03)Transitional boundary layers on low-pressure turbines (LPT) are prone to separation on the suction surface of the blade under strong local adverse pressure gradients. Intermittent free-stream turbulence, periodic wakes shed by the upstream blades, and surface roughness due to in-service degradation of the blades are shown to suppress the separation. Although this generally leads to a profile loss reduction, some of the benefits are offset by a loss increase associated with an increased turbulent wetted area. In this work, we explore a strategy where the losses in both the transitional and turbulent boundary layers can be reduced. In particular, we employ surface roughness in the transitional regime to reduce the separation bubble-related losses and riblets in the turbulent regime to further reduce the losses due to the turbulent wetted area. The efficacy of this ‘rough-riblet blade surface’ is studied using high fidelity scale resolving simulations on the configuration of a flat surface subjected to a streamwise varying pressure gradient. In the first phase of this investigation, the effects of riblet shapes, riblet tip curvature and the positioning of the riblets at different regimes of the flow are investigated under both zero-pressure gradient and a streamwise varying pressure gradient. Of all these considerations, it turns out that the most important aspect is the way in which riblets are positioned with respect to the flat surface level. This work shows that in order to achieve both a reduction in the skin-friction drag and boundary layer losses riblets have to flush mounted with reference to the flat suction surface. In the second phase of this investigation, a series of high-fidelity eddy resolving simulations were further carried out to discern the sensitivity of riblet dimensions to the profile loss. The performance of different riblet shapes and riblet dimensions are analysed by examining various first and second order statistical quantities in detail. In particular, the streamwise evolution of skin-friction coefficient, boundary layer integral parameters and shape factor are compared and contrasted among riblets of different dimensions. The instantaneous flow features and second order statistics such as the Reynolds stress, turbulent kinetic energy and its production are analyzed for different test cases to determine the impact of riblets on these quantities. When compared to the roughness alone configuration, the scalloped shape riblets with s^+ = 17 and h^+ = 22 reduced the net skin-friction drag by 7.3% and the trailing edge momentum thickness by 14.5% thereby demonstrating the efficacy of riblets in reducing the mixing losses under adverse pressure gradients. Through an analysis of flow blockage introduced by the application of riblets, the deleterious effects of increasing the riblet height along with the necessity of optimizing the riblet ramp are highlighted.
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ItemReduced-order modeling for membrane wings at low Reynolds numbers( 2019)The present work develops reduced-order models for the fluid-structure interaction of two-dimensional membrane wings at low Reynolds numbers. The fluid-structure interaction models are obtained from the coupling between a linear unsteady aerodynamic model and a structural membrane model. The linear aerodynamic model is derived from high-fidelity simulations at low Reynolds numbers using a system identification procedure. It is able to model the unsteady aerodynamic loads generated by a deflection of the wing, decomposed into its rigid and flexible degrees of freedom. The rigid degrees of freedom are represented by pitching and plunging, while the flexible degrees of freedom are decomposed into Fourier modes. The aerodynamic model is presented in state-space form, hence it is compatible with modern feedback control frameworks. Feedback control techniques are employed to investigate the performance of the individual degrees of freedom of the wing in tracking a reference lift and rejecting external disturbances. A one-dimensional nonlinear equation is adopted to model the structural dynamics of the membrane wing. From the nonlinear model, two lower-order approximations are derived by means of a truncated Taylor expansion: a quasi-linear model and a linear model. From the coupling with the linear aerodynamic model, a nonlinear, a quasi-linear and a linear fluid-structure interaction model are obtained. These models are adopted to investigate the static aeroelastic response and the stability of the membrane wing at different Reynolds numbers. Using Harmonic Balance methods, the forced aeroelastic response to pitching and the autonomous response of linearly unstable cases are also studied. Finally, an adjoint-based optimization procedure is employed to optimize the aerodynamic response of the membrane to an external disturbance in the form of a convective vortex. The results are compared against high-fidelity Direct Numerical Simulation to validate the predictive capabilities of the models and to discuss their range of applicability.
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ItemDirect numerical simulations of flow past a rotating sphere and droplet( 2011)The transport of a spherical particle or droplet has been subjected to intensive research for centuries as the hydrodynamic forces acting on the spherical particle or droplet are known to affect its flight path. Typical engineering applications of the transport of a particle or droplet can be found in areas such as internal engine combustion, inkjet printing, drug and chemical delivery where the particle or droplet's trajectory is usually predicted using the standard drag coefficient correlation for a stationary sphere. The objective of this study is focused on the flow past a solid rotating sphere at small to moderate Reynolds numbers at different rotation axis angles. The deformation of the droplet on the total hydrodynamic forces is also investigated. At moderate Reynolds numbers, $\Rey = 100$, $250$ and $300$, a parametric study on the effect of rotation axis angles was performed. The goal was to identify the change in behaviour for the flow past a rotating sphere over a range of rotation axis angles, $\alpha = 0$, $\pi/6$, $\pi/3$ and $\pi/2$. The sphere was rotated at dimensionless rotation rates, $\varOmega^* = 0.05$, $0.20$, $0.50$ and $1.00$. For $\Rey = 100$, the flow is steady and the effect of rotation axis angles on both near wake flow fields and forces are insignificant at $\varOmega^* = 0.05$. The effect of rotation axis angles becomes more pronounced with increasing $\varOmega^*$. For $\Rey = 250$ and $300$, the dynamic behaviours of both wake structures and forces are highly correlated to the rotation axis angle, $\alpha$, and rotation rate, $\varOmega^*$. The flow was classified into five different regimes for all parameters considered at $\Rey = 250$ and $300$, the hydrodynamic forces acting on the sphere are closely related to the corresponding flow regime. The changes to the time-averaged flow fields as a result of increasing Reynolds numbers are less pronounced. The flow past a rotating sphere was also numerically simulated at a higher Reynolds number, $\Rey = 500$ and $1,\!000$ for streamwise and transverse rotation only. The non-dimensional rotation rates, $\varOmega^*$, were considered over the range of $0.00$ and $1.20$. For streamwise rotation at $\Rey = 500$, a dimensionless parameter was defined to differentiate the transition of the flow structures from rotating vortex shedding to spiral structures. For $\Rey = 1,\!000$, a reverse rotation is observed due to small-scale eddies release mechanism. The phase diagram $\left( C_{Ly}, C_{Lx} \right)$ no longer forms a closed curve for the reverse rotation flow regime. For transverse rotation, a newly observed flow regime is calculated for $\Rey = 500$ and $\varOmega^* = 1.00$; and $\Rey = 1,\!000$ and $\varOmega^* \geq 0.80$. At this flow regime, stable foci are formed in the near wake increasing the hydrodynamic forces oscillation amplitude. The deformation and dynamic behaviours of a droplet rotating in streamwise and transverse directions, released into a free stream were studied at initial Reynolds number, $\Rey_i = 40$, for different initial Weber numbers, $We_i$, viscosity ratios, density ratios and dimensionless rotation rates $\left( \varOmega^* \leq 1.00 \right)$. The upper limit of $\varOmega^*$ is chosen to be unity to avoid droplet breakup. For large $We_i$, the droplet shape pancakes along the free stream as a result of streamwise rotation. Hence, its frontal area increases and leads to an increase in the total drag coefficients. But a decrease in $We_i$ shows a negative total drag coefficient. For a transversely rotating sphere, the deformation is divided into along the free stream direction and along the rotating axis. The different deformation leads to two distinctively different droplet dynamic behaviours.
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ItemNumerical study of internal wall-bounded turbulent flows( 2011)Direct numerical simulation (DNS) of turbulent pipe flow has been performed at Reynolds numbers ranging from Reτ ≈ 170 to 2000. A literature review highlights a need for higher Reynolds number pipe flow DNS data. There have been many numerical studies for internal geometry (pipe and channel) wall-bounded turbulent flows. Many of the numerical data for both pipe and channel flows, which are now readily accessible are at lower Reynolds numbers. At higher Reynolds numbers, there is a lack of pipe flow DNS data as compared to channel flow DNS data. As the highest Reynolds numbers in numerical simulations are starting to overlap the lower region of experiments, validation of both experimental and numerical results is now possible. Moreover, numerical simulations are extremely useful in complementing experimental results in the near-wall region where accurate experimental data are often difficult to obtain. However, available DNS data of internal wall-bounded turbulent flows are performed with different grid resolutions and computational domain sizes, making it difficult to directly compare between them. An undertaking of this thesis involves a systematic study (using constant grid resolutions) of the domain length effect on the convergence of turbulence statistics. Investigations carried out using numerical data from fully developed pipe flow simulations indicate a recommended computational length of 8π pipe radius or half channel height for turbulence statistics to converge. It is hoped that this will serve as a benchmark computational domain length for future numerical simulations performed. A study is also carried out to better understand the similarities and differences of the flow physics between turbulent channel and pipe flows. This is performed using the newly obtained pipe flow DNS data and channel flow DNS data of del ´ Alamo et al. (2004) at a comparable Reynolds number of Reτ ≈ 1000. Different turbulence statistics investigated including mean flow, turbulence intensities, correlations and energy spectra. Comparison of both wall-bounded channel and pipe flows shows little discrepancies in the near-wall region but differences are observed in the outer-region. Although there is abundant literature for both experimental and numerical wall bounded turbulent flows, further analysis reveals discrepancies in the open literature. One of the primary contributing factors that plagues reported results are spatial resolution issues. In this thesis, the numerical data is used to investigate the effects of insufficient spatial resolution in wall-bounded turbulence by averaging the streamwise velocity component in the spanwise direction. A correction scheme is proposed to correct experimental results suffering from insufficient spatial resolution. The correction scheme is applied to attenuated experimental results such as streamwise turbulence intensity and one-dimensional energy spectra and is shown to be effective. The method of using DNS data to analysis and correct experimental results can be extended to other experimental techniques such as particle image velocimetry and laser doppler velocimetry.