<p>Boundary-layer flow over bodies such as aircraft wings or turbine blades is characterized by a pressure gradient due to the curved surface of the body. The boundary layer may experience modal and non-modal instability, and the type of dominant instability depends on whether the body is swept with respect to the oncoming flow or not. The growth of these disturbances causes transition of the boundary-layer flow to turbulence. Provided that they are convective in nature, the instabilities will only arise and persist if the boundary layer is continuously exposed to a perturbation environment. This may for example consist of turbulent fluctuations or sound waves in the free stream or of non-uniformities on the surface of the body. In engineering, it is of relevance to understand how susceptive to such perturbations the boundary layer is, and this issue is subject of <em>receptivity analysis</em>.</p><p> </p><p>In this thesis, receptivity of simplified prototypes for flow past a wing is studied. In particular, the three-dimensional swept-plate boundary layer and the boundary layer forming on a flat plate with elliptic leading edge are considered. The response of the boundary layer to vortical free-stream disturbances and surface roughness is analyzed, receptivity mechanisms are identified and their efficiency is quantified.</p> / 76218 VR Receptivity

Boundary-layer flow over bodies such as aircraft wings or turbine blades is characterized by a pressure gradient due to the curved surface of the body. The boundary layer may experience modal and non-modal instability, and the type of dominant instability depends on whether the body is swept with respect to the oncoming flow or not. The growth of these disturbances causes transition of the boundary-layer flow to turbulence. Provided that they are convective in nature, the instabilities will only arise and persist if the boundary layer is continuously exposed to a perturbation environment. This may for example consist of turbulent fluctuations or sound waves in the free stream or of non-uniformities on the surface of the body. In engineering, it is of relevance to understand how susceptive to such perturbations the boundary layer is, and this issue is subject of receptivity analysis.   In this thesis, receptivity of simplified prototypes for flow past a wing is studied. In particular, the three-dimensional swept-plate boundary layer and the boundary layer forming on a flat plate with elliptic leading edge are considered. The response of the boundary layer to vortical free-stream disturbances and surface roughness is analyzed, receptivity mechanisms are identified and their efficiency is quantified. / QC 20101022 / 76218 VR Receptivity

Computations and experiments were run to study heat transfer and overall effectiveness for a simulated turbine blade leading edge. Computational predictions were run for a film cooled leading edge model using a conjugate numerical method to predict the normalized “metal” temperatures for the model. This computational study was done in conjunction with a parallel effort to experimentally determine normalized metal temperatures, i.e. overall effectiveness, using a specially designed high conductivity model. Predictions of overall effectiveness were higher than experimentally measured values in the stagnation region, but lower along the downstream section of the leading edge. Reasons for the differences between computational predictions and experimental measurements were examined. Also of interest was the validity of Taw as the driving temperature for heat transfer into the blade, and this was examined via computations. Overall, this assumption gave reasonable results except near the stagnation line. Experiments were also conducted on a leading edge with no film cooling to gain a better understanding of the additional cooling provided by film cooling. Heat flux was also measured and external and internal heat transfer coefficients were determined. The results showed roughly constant overall effectiveness on the external surface. / text

The humpback whale is very maneuverable despite its enormous size and rigid body. This agility has been attributed to the use of its pectoral flippers, along the leading edge of which protuberances are present. The leading edge protuberances are considered by some biologists to be a form of passive flow control and/or drag reduction. Force and moment measurements along with qualitative and quantitative flow visualizations were carried out in water tunnel experiments on full-span and finite-span hydrofoil models with several different planforms and protuberance geometries. A NACA 634-021 cross-sectional airfoil profile was used for the baseline foil in all tests. Four planform geometries chosen included: a full-span set of foils which spanned the breadth of the water tunnel, a finite-span rectangular planform, a finite-span swept hydrofoil, and a scale flipper model that resembled the morphology of the humpback whale flipper. A variety of sinusoidal protuberance geometries which included three amplitudes equal to 2.5%, 5%, and 12% and wavelengths of 25% and 50% of the local chord were examined in combination with the different planform geometries. Testing included force and moment measurements and Particle Image Velocimetry (PIV) to examine the load characteristics and flow field surrounding the modified foils. Load measurements show that modified foils are capable of generating higher lift than the baseline at high angles of attack while at low angle of attack the baseline generally produces a lift coefficient equal to or greater than the modified cases. With the exception of the modified flipper model, the drag coefficients of the modified hydrofoils are either equal to or greater than their baseline counterparts. The increased drag reduces the lift-to-drag ratio. Flow visualizations show that vortical structures emanating from the shoulders of the protuberances are responsible for increased lift and drag at high angles. Cavitation tests show that modified foils cavitate in pockets behind the troughs of protuberances whereas the baseline foils produce cavitation along the entire foil span. Also, the cavitation numbers on modified hydrofoils were consistently higher than their baseline counterparts. This work shows the effect of leading edge protuberances on the aforementioned performance characteristics.

This thesis presents an investigation into the phenomena of scale effects on cavitation inception and noise of marine propellers. The overall aim is to extend the understanding of these phenomena and improve predicting methods. The investigations, which are largely experimental in nature, are restricted to the tip vortex and sheet types of cavitation. Chapter 1 includes a state-of-the-art review of the scale effect studies based on published papers to form the basis for the main objectives and structure of thesis. The objectives require systematic tests in a cavitation tunnel to explore the viscous scale effects contributing to the phenomena, particularly for the effect of the free-stream turbulence, and to include this effect in extrapolation procedures. Chapter 2 is concernedw ith the background flow measurementsin the cavitation tunnel under the effect of systematically varying levels of the free-stream turbulence generated by using wire meshes. This background information is obtained using a Laser Doppler Anemometry; measurements made with the latter provide a systematic basis on which the analyses of the cavitation inception and noise experiments can be performed. In Chapter 3, a set of cavitation inception tests is described with a NACA66 rectangular foil whose cross-section represents a typical blade section of a marine propeller. The inception measurements for systematically varying levels of the free-stream turbulence and that of the leading edge roughness are presented for different angles of attack and the results are discussed. Chapter 4 includes another set of cavitation inception experiments with a 5-bladed of model propeller of the Meridian Series. The measurements are taken for varying levels of the free-stream turbulence, blade roughness and dissolved gas contents. The results are analysed and discussed with a specific emphasis on the similarities between the effects of the free-stream turbulence and blade roughness. Chapter 5 presents a set of systematic noise measurements, with the same test propeller under the similar effects of the free-stream turbulence, blade roughness and dissolved gas content, using a single external hydrophone. The analyseso f these measurements,in terms of the tunnel background noise and net propeller noise, are presented and discussed for two operating conditions representing a typical non-cavitating and cavitating noise spectrum. In Chapter 6, a semi-empirical tool is developed to predict the inception of cavitation including the effect of the free-stream turbulence based on Lighthill's Leading Edge Correction factor (Lighthill, 1951). This tool is correlated with the inception tests results of the model propeller and its potential to be used as an extrapolator for the full-scale prediction is discussed. An attempt is made to establish a correspondence between the level of the free-stream turbulence and that of the blade roughness and its impact on the current test procedures is discussed. This chapter also includes an analysis of the similarity criteria to incorporate the effect of the free-stream turbulence in the inception of cavitation using the Dimensional Analysis procedure. In Chapter 7, a general review of the study together with the main conclusions from the thesis are presented and some recommendations for future work are made.

Vertical axis wind turbine, developed as one of the main methods to utilize the wind energy, has a promising future; however, the major issue to limit its performance is the uneven loading on the blade during operation. Flow control mechanisms have been employed in the aerodynamic field to improve the performance of airfoils. In this study, two types of leading-edge structures, including flexible leading-edge and leading-edge roughness, are experimentally investigated to analyze their effects on altering the aerodynamic characteristics of NACA 0018 airfoil under steady flow condition and dynamic pitching condition. Current experimental results indicate that 1) during the steady flow condition, both of leading-edge structures contribute to the delay of the static stall; 2) for the dynamic pitching process, the leading-edge structures either delayed the dynamic stall angle or increased the area of the coefficient of pressure loop as a function of angle of attack.

This thesis investigates vortex dynamics of a plunging airfoil by studying the vorticity transport mechanisms of two-dimensional direct numerical simulations. The simulations were used to study a simplified flat airfoil in a freestream that was subject to pure plunging motion. Quantitative and qualitative analyses were used the validate the two-dimensional simulations and gain insight into the effects of eliminating three-dimensional physics in a nominally two-dimensional flow. Additionally, a parametric study was conducted to analyze the effects of Reynolds and Strouhal numbers on the transport of vorticity.

The leading-edge vortex (LEV) is a structure found in unsteady aerodynamics that can alter the forces induced on wings and other rotating structures. This thesis presents an experimental study on LEV development on low aspect-ratio wing rolling in a uniform flow at high angles of attack. The flow structure dynamics of rotating wings in the presence of a free stream are not well understood due to the limited studies under these conditions. In this study, a broad parameter space with varying advance ratio and wing radius of gyration are analyzed using dye-visualizations. In most cases, either a conical LEV structure developed on the inboard part of the wing and persisted to a significant roll angle, as well as the arch structure. Plenoptic PIV was used to validate observations in flow visualizations as well as identify finer structures. A binary classification criterion was defined based on the formation and persistence of the inboard conical LEV structure. This criterion identified the LEV as either conical ,non-conical or transitional. Previous studies inspired the proposal of a ”rotation parameter” ,ΠRot, that was a based on a non-dimensional velocity gradient. A value of ΠRot = 0.17 was found to separate conical and non-conical LEV parameter, suggesting the fundamental importance of this parameter to LEV dynamics. Furthermore, the forces were analyzed to understand the impact of the flow structure on the forces. The conical LEVs had a transient peak followed by irregular udulations while the non-conical LEVs produced high frequency oscillations. In both cases, the force could be understood based on the time-evolution of the LEVs. Passive bleeding was considered within this study to perturb the flow. Four passive bleed configurations were experimented with at different hole locations and sizes. It was found that a hole applied near the wing root with a large diameter perturbed the flow and transformed the structure from conical to non-conical classifications. This provides a platform to further understand the flow mechanisms that govern LEV formation and evolution by drastically changing flow structures and maintaining the same geometric and kinematic parameters. Additional studies were done analyzing the changes on the forces on the wing. The lift on the passive bleeding did not seem to be affected however, the thrust was decreased to nearly 0.

High turbine inlet temperature becomes necessary for increasing thermal efficiency of modern gas turbines. To prevent failure of turbine components, advance cooling technologies have been applied to different portions of turbine blades. The detailed film cooling effectiveness distributions along a rotor blade has been studied under combined effects of upstream trailing edge unsteady wake with coolant ejection by the pressure sensitive paint (PSP). The experiment is conducted in a low speed wind tunnel with a five blade linear cascade and exit Reynolds number is 370,000. The density ratios for both blade and trailing edge coolant ejection range from 1.5 to 2.0. Blade blowing ratios are 0.5 and 1.0 on suction surface and 1.0 and 2.0 on pressure surface. Trailing edge jet blowing ratio and Strouhal number are 1.0 and 0.12, respectively. Results show the unsteady wake reduces overall effectiveness. However, the unsteady wake with trailing edge coolant ejection enhances overall effectiveness. Results also show that the overall effectiveness increases by using heavier coolant for ejection and blade film cooling. Leading edge film cooling has been investigated using PSP. There are two test models: seven and three-row of film holes for simulating vane and blade, respectively. Four film holes’ configurations are used for both models: radial angle cylindrical holes, compound angle cylindrical holes, radial angle shaped holes, and compound angle shaped holes. Density ratios are 1.0 to 2.0 while blowing ratios are 0.5 to 1.5. Experiments were conducted in a low speed wind tunnel with Reynolds number 100,900. The turbulence intensity near test model is about 7%. The results show the shaped holes have overall higher effectiveness than cylindrical holes for both designs. As increasing density ratio, density effect on shaped holes becomes evident. Radial angle holes perform better than compound angle holes as increasing blowing and density ratios. Increasing density ratio generally increases overall effectiveness for all configurations and blowing ratios. One exception occurs for compound angle and radial angle shaped hole of three-row design at lower blowing ratio. Effectiveness along stagnation row reduces as increasing density ratio due to coolant jet with insufficient momentum caused by heavier density coolant, shaped hole, and stagnation row.

A novel numerical method solves the VIScous Vorticity Equation (VISVE) in 3D in order to model the Leading Edge Vortex (LEV) of propellers is proposed and implemented in this dissertation. The spatial concentration of the vorticity is exploited in the method, which is designed to be spatially compact and numerically efficient, in the meantime, capable of modeling complicated vorticity/solid boundary interaction in 2D and 3D. The numerical model can work as a viscous correction on top of the traditional Boundary Element Method (BEM) results. The proposed method is first applied in the case of a 2D hydrofoil at high angle of attack. The results are correlated with those from Navier-Stokes (N-S) simulation. The method is then used to model the LEV and tip vortex of a 3D swept wing. The results of the 3D simulation show great similarity to those from N-S. In the end, the method is applied in the case of propellers at low advance ratios. All the essential flow characteristics (LEV and tip vortex) are predicted. The objective of this dissertation is not developing a mathematically equivalent numerical method to the full-blown Reynolds-Averaged Navier-Stokes (RANS) solver, but inventing an accurate and computationally efficient tool to model the effects of the LEV on the propeller performance for engineering's purpose. / text