This work is focused on the aerodynamics for a proprietary laminar flow airfoil for Unmanned Aerial Vehicle (UAV) applications. The two main focuses are (1) aerodynamic performance at Reynolds number on the order of 10,000, (2) the effect of a conventional hot-wire probe on laminar separation bubbles. For aerodynamic performance, pressure and wake velocity distributions were measured at Re = 40,000 and 60,000 for a range of angles of attack. The airfoil performed poorly for these Reynolds numbers due to laminar boundary layer separation. 2-D boundary layer trips significantly improved the lift-to-drag ratio. For probe effects, three Reynolds numbers were investigated (Re = 100,000, 150,000, and 200,000), with three angles of attack for each. Pressure and surface shear distributions were measured. Flow upstream of the probe tip was not affected. Transition was promoted downstream due to the additional disturbances in the separated shear layer.

Micro Air Vehicles (MAV’s) have small size and extreme maneuverability which makes them ideal for surveillance. Propulsion mechanisms include propellers, rotors, and flapping airfoils. Flapping motions, along with biologically-inspired wing profiles, are of interest due to their use of natural physics. Corrugated airfoil structures appears to have poor aerodynamic performance at higher Reynolds numbers, but serve well at Re<10,000. Understanding flow structures around corrugated profiles and comparing them to a standard airfoil will aid in understanding how these corrugated profiles perform well and have been adopted by some of nature’s most acrobatic flyers. Motivation for this investigation is to compare static flow visualizations of corrugated profiles to a standard National Advisory Committee for Aeronautics (NACA) airfoil from low to high angles of attack and further observe flow structure development of a pitching and plunging flat plate at a Re<10,000 and a Strouhal number relevant to natural fliers. The static visualization was conducted at Re=1,000 with a NACA 0012 airfoil and two corrugated models. The Pitch and Plunge Fixture (PPF) developed was constructed by simplifying flapping wings as a two degree of freedom motion in plunge (translation) and pitch (rotation). Results obtained from the PPF were compared with a numerical simulation.

Use of an inflatable/rigidizable wing is explored for Mars airplane designs. The BIG BLUE (Baseline Inflatable-wing Glider Balloon Launched Unmanned airplane Experiment) project was developed at the University of Kentucky, with an objective to demonstrate feasibility of this technology with a flight-test of an high-altitude glider with inflatable/rigidizable wings. The focus of this thesis research was to design and analyze the wing for this project. The wings are stowed in the fuselage, inflate during ascent, and rigidize with exposure to UV light. The design of wings was evaluated by using aerodynamic and finite element software and wind tunnel testing. The profile is chosen based upon aerodynamic results and consideration of manufacturability of the inflatable wing structures. Flow over prototypes of inflatable/rigidizable and ideal shaped wings were also examined in the wind tunnel. Flow visualization, lift and drag measurements, and wake survey testing methods were performed. Results from the wind tunnel testing are presented along with suggestions in improving the inflatable/rigidizable wings aerodynamic efficiency and use on a low Reynolds number platform. In addition, high altitude wing deployment tests and low altitude flight tests of the inflatable/rigidizable wing were conducted.

The later stages of separated shear layer transition within separation bubbles developing over a NACA0018 airfoil operating at a chord Reynolds number of 105 and at angles of attack of 0, 5, 8, and 10 degrees were investigated experimentally in a wind tunnel. Several experimental tools, including a rake of six boundary-layer hot-wire anemometers, were used to perform measurements over the model. Novel high-speed flow visualization performed with a smoke-wire placed within the separated shear layer showed that roll-up vortices are shed within separation bubbles forming on the suction side of the airfoil. The structures were found to convect downstream and eventually break down during laminar-to-turbulent transition. Top view visualizations revealed that, at angles of attack of 0, 5, and 8 degrees, roll-up vortices form coherently across the span and undergo significant spanwise deformations prior to breaking down. At angles of attack of 5 and 8 degrees, rows of streamwise-oriented structures were observed to form during vortex breakdown. Statistics regarding the formation and development of shear layer roll-up vortices were extracted from high-speed flow visualization sequences and compared to the results of boundary layer measurements. It was found that, on the average, roll-up vortices form following the initial exponential growth of unstable disturbances within the separated shear layer and initiate the later stages of transition. The onset of these nonlinear stages was found to occur when the amplitude of velocity disturbances reached approximately 10% of the free-stream velocity. The rate of vortex shedding was found to fall within the frequency band of the unstable disturbances and lie near the central frequency of this band. The formation of vortices has been linked to the generation of harmonics of these unstable disturbances in velocity signals acquired ahead of mean transition. Once shed, vortices were found to drift at speeds between 33% and 44% of the edge velocity. Vortex merging at an angle of attack of 5?? was investigated. It was found that the majority of roll-up vortices proceed to merge with either one or two other vortices. Vortex merging between two and three vortices was found to occur periodically in a process similar to vortex merging in plane mixing layers undergoing subharmonic forcing of the most amplified disturbance. The flapping motion of the separated shear layer was investigated by performing a cross-correlation analysis on the high-speed flow visualization sequences to extract vertical displacement signals of the smoke within the shear layer. The frequency of flapping was found to correspond to the unstable disturbance band. At an angle of attack of 5??, it was found that the separated shear layer has a low-frequency component of flapping that matches a strong peak in velocity and surface pressure spectra that lies outside the unstable disturbance frequency band. The spanwise development of disturbances was assessed in the aft portion of the separation bubbles by performing a cross-correlation analysis on signals acquired simultaneously across the span with the rake of hot-wires. The spanwise correlations between signals was found to be well-correlated ahead of shear layer roll-up, after which disturbances became rapidly uncorrelated ahead of mean reattachment. These results were found to be linked to the coherent roll-up and subsequent breakdown of roll-up vortices.

The accuracy of existing methods for estimating the drag based on experimental flow field measurements were assessed for two-dimensional bodies. The effects of control volume boundary placement and inherent simplifying assumptions were also investigated. Wind tunnel experiments were performed on a circular cylinder operating at a Reynolds number of 8,000 and 20,000, and on a NACA 0018 airfoil operating at a chord Reynolds number of 100,000 for three angles of attack (α), specifically, 5◦, 10◦, and 15◦. The circular cylinder experiments fall within the the shear layer transition flow regime. Airfoil investigations span both types of flow development common to low Reynolds number airfoil operation. For α = 5◦ and 10◦, a separation bubble forms on the upper surface of the airfoil, while, for α = 15◦, the flow separates without reattaching, resulting in a stalled airfoil. Wake velocity and pressure measurements were performed at several downstream locations to investigate the impact of control volume boundary placement. Wake profiles were measured between 3 and 40 diameters downstream from the circular cylinder axis and between 1 and 4.5 chord lengths from the trailing edge of the airfoil. In addition to wake profiles, the outer flow velocity variation was quantified to investigate the appropriate location to measure freestream flow characteristics in a test section with streamwise-varying outer flow conditions. The results show that drag estimates are strongly dependent on the streamwise position of the measured wake profile for all methods investigated. Drag estimates improved, and streamwise variation decreased, with increasing streamwise position of the flow measurements. For the pressure based method examined, wake measurements should be taken at least 10 times the projected model height downstream of the model. In the case of the circular cylinder, this is equivalent to 10 diameters and, for the airfoil investigated, it is approximately 1 chord length from the trailing edge. For the methods relying on velocity measurements, acceptable estimates of drag were possible when based on measurements taken at least 30 projected heights downstream, i.e., 30 diameters for the circular cylinder and 3 chord lengths for the airfoil model investigated. The findings highlight the importance of providing a detailed description of the methodology and experimental implementation for drag estimates based on flow field measurements. Finally the study offers guidelines for implementing momentum integral based drag calculations in future investigations.

At small length scales, low velocities, and high viscosity, the effects of inertia on motion through fluid become insignificant and viscous forces dominate. Microswimmer propulsion, of necessity, is achieved through different means than that achieved by macroscopic organisms. We describe in detail the hydrodynamics of microswimmers consisting of colloidal particles and their interactions. In particular we focus on two-bead swimmers and the effects of asymmetry on collective motion, calculating analytical formulae for time-averaged pair interactions and verifying them with microscopic time-resolved numerical simulation, finding good agreement. We then examine the long-term effects of a swimmer's passing on a passive tracer particle, finding that the force-free nature of these microswimmers leads to loop-shaped tracer trajectories. Even in the presence of Brownian motion, the loop-shaped structures of these trajectories can be recovered by averaging over a large enough sample size. Finally, we explore the phenomenon of synchronisation between microswimmers through hydrodynamic interactions, using the method of constraint forces on a force-based swimmer. We find that the hydrodynamic interactions between swimmers can alter the relative phase between them such that phase-locking can occur over the long term, altering their collective motion.

No description available.

No description available.

<p>Fluids conveyed in deformable conduits are often encountered in  microfluidic applications, which makes fluid--structure interactions (FSIs) an unavoidable phenomenon. In particular, experiments reported the existence of FSI instabilities in compliant microchannels at low Reynolds numbers, Re, well below the established values for rigid conduits. This observation has significant implications for new strategies for mixing at the microscale, which might harness FSI instabilities in the absence of  turbulence. In this thesis, we conduct research on the modeling and stability of microscale FSIs. Understanding the steady response, the dynamics and the stability of these FSIs are the three major objectives. This thesis begins with the analysis of the steady-state scalings and the linear stability of a previously derived mathematical model, through which we emphasize the power of reduced modeling in making the FSI problems tractable. Next, we turn to a more realistic problem regarding FSIs in a common configuration of low-Re flows through long, shallow rectangular three-dimensional microchannels. Through a scaling analysis, which takes advantage of the geometric separation of scales, we find that the flow can be simplified under the lubrication approximation, while the wall deforms like a variable-stiffness Winkler foundation at the leading order. Coupling these dominant effects, we obtain a new fitting-parameter-free flow rate--pressure drop relation for a thick-walled microchannel, which rationalizes previous experiments. Then, we derive a one-dimensional (1D) steady model, at both vanishing and finite Re, by coupling the reduced flow and deformation models. To satisfy the displacement constraints along the channel edges, weak tension is introduced to regularize the underlying Winkler-foundation-like mechanism. This model is then made dynamic by introducing flow unsteadiness and the elastic wall's inertia. We conduct a global stability analysis of this system by perturbing the non-flat steady state with infinitesimal perturbations. We identify the existence of globally unstable modes, typically in the weakly inertial flow regime, whose features are consistent with experimental observations. The unstable eigenmodes oscillate at frequencies close to the natural frequency of the wall, suggesting that the instabilities are resonance phenomena. We also capture the transient energy amplification of perturbations through a linear non-normality analysis of the proposed reduced 1D FSI model.</p>

A numerical method for modeling a low Reynolds number turbine blade, the L1M, is presented along with the pitfalls encountered. A laminar solution was confirmed to not accurately predict the flow features known in low Reynolds number turbine blade flow. Three fully turbulent models were then used to try to predict the separation and reattachment of the flow. These models were also found to be insufficient for transitioning flows. A domain was created to manually trip the laminar flow to turbulent flow using a predictive turbulence transition model. The trip in the domain introduced an instability in the flow field that appears to be dependent on the discretization order, turbulence model, and transition location. The method was repeated using the Pack B blade and the same obstacles were apparent. The numerical method developed was then used in an optimization technique developed to design a wind tunnel simulating periodic flow conditions using only 2 blades. The method was first used to predict a c_p distribution for the aft loaded L1A research blade provided by the U.S. Air Force. The method was then extended to a larger domain emulating the 2 blade, 2D wind tunnel. The end-wall geometry of the tunnel was then changed using previously defined control points to alter the distribution of c_p along the suction surface of the interior blades. The tunnel c_p's were compared to the computationally acquired periodic solution. The processed was repeated until an acceptable threshold was reached. The optimization was performed using the commercially available software iSIGHT by Engineous Solutions. The optimization algorithms used were the gradient based Successive Approximation Method, the Hooke Jeeves, and Simulated Annealing.