Kristy L. Hansen

Performance Variations of Leading-Edge Tubercles for Distinct Airfoil Profiles

An experimental investigation has been undertaken to determine the influence of sinusoidal leading-edge protrusions on the performance of two NACA airfoils with different aerodynamic characteristics. Force measurements on full-span airfoils with various combinations of tubercle amplitude and wavelength reveal that when compared to the unmodified equivalent, tubercles are more beneficial for the NACA 65-021 airfoil than the NACA 0021 airfoil. It was also found that for both airfoil profiles, reducing the tubercle amplitude leads to a higher maximum lift coefficient and larger stall angle. In the poststall regime, however, the performance with largeramplitude tubercles is more favorable. Reducing the wavelength leads to improvements in all aspects of lift performance, including maximum lift coefficient, stall angle, and poststall characteristics. Nevertheless, there is a certain point at which further reduction in wavelength has a negative impact on performance. The results also suggest that tubercles act in a manner similar to conventional vortex generators.

The formation mechanism and impact of streamwise vortices on NACA 0021 airfoil's performance with undulating leading edge modification

Wings with tubercles have been shown to display advantageous loading behavior at high attack angles compared to their unmodified counterparts. In an earlier study by the authors, it was shown that an undulating leading-edge configuration, including but not limited to a tubercled model, induces a cyclic variation in circulation along the span that gives rise to the formation of counter-rotating streamwise vortices. While the aerodynamic benefits of full-span tubercled wings have been associated with the presence of such vortices, their formation mechanism and influence on wing performance are still in question. In the present work, experimental and numerical tests were conducted to further investigate the effect of tubercles on the flow structure over full-span modified wings based on the NACA 0021 profile, in the transitional flow regime. It is found that a skew-induced mechanism accounts for the formation of streamwise vortices whose development is accompanied by flow separation in delta-shaped regions n...

The effect of undulating leading-edge modifications on NACA 0021 airfoil characteristics

In spite of its mammoth physical size, the humpback whales manoeuvrability in hunting has captured the attention of biologists as well as fluid mechanists. It has now been established that the protrusions on the leading-edges of the humpbacks pectoral flippers, known as tubercles, account for this species’ agility and manoeuvrability. In the present work, Prandtls nonlinear lifting-line theory was employed to propose a hypothesis that the favourable traits observed in the performance of tubercled lifting bodies are not exclusive to this form of leading-edge configuration. Accordingly, a novel alternative to tubercles was introduced and incorporated into the design of four airfoils that underwent wind tunnel force and pressure measurement tests in the transitional flow regime. In addition, a Computation Fluid Dynamics study was performed using the Shear Stress Transport transitional model in the context of unsteady Reynolds-Averaged Navier-Stokes at several attack angles. The results from the numerical investigation are in reasonable agreement with those of the experiments, and suggest the presence of features that are also observed in flows over tubercled foils, most notably a distinct pair of streamwise vortices for each wavelength of the tubercle-like feature.

Reduction of Flow Induced Tonal Noise through Leading Edge Tubercle Modifications

A sinusoidal modification to the leading edge of an airfoil (tubercles) has led to the elimination of tonal noise for a NACA 0021 airfoil at a Reynolds number, Re ~ 120,000. It has also been found that the overall broadband noise is reduced for a considerable range of frequencies surrounding the peak in tonal noise. Investigations have also revealed that changing the amplitude and spacing between the tubercles has an effect on noise reduction. The mechanism of noise reduction is believed to be strongly related to the formation of streamwise vortices which are generated by tubercles. These vortices most likely have an effect on the stability characteristics of the boundary layer, hence influencing the velocity fluctuations of the shear layer near the trailing edge. In addition, spanwise variations in separation location are thought to affect the vortex shedding process, which could influence the feedback mechanism.

Monitoring pressure profiles across an airfoil section with a fibre Bragg grating sensor array

Fluid flow over an airfoil section creates a pressure difference across the upper and lower surfaces, thus generating lift. Successful wing design is a combination of engineering design and experience in the field, with subtleties in design and manufacture having significant impact on the amount of lift produced. Current methods of airfoil optimization and validation typically involve computational fluid dynamics (CFD) and extensive wind tunnel testing with pressure sensors embedded into the airfoil to measure the pressure over the wing. Monitoring pressure along an airfoil in a wind tunnel is typically achieved using surface pressure taps that consist of hollow tubes running from the surface of the airfoil to individual pressure sensors external to the tunnel. These pressure taps are complex to configure and not ideal for in-flight testing. Fiber Bragg grating (FBG) pressure sensing arrays provide a highly viable option for both wind tunnel and inflight pressure measurement. We present a fiber optic sensor array that can detect positive and negative pressure suitable for validating CFD models of airfoil profile sections. The sensing array presented here consists of 6 independent sensing elements, each capable of a pressure resolution of less than 10 Pa over the range of 70 kPa to 120 kPa. The device has been tested with the sensor array attached to a 90mm chord length airfoil section subjected to low velocity flow. Results show that the arrays are capable of accurately detecting variations of the pressure profile along the airfoil as the angle of attack is varied from zero to the point at which stall occurs.

Infrasound and Low-Frequency Noise from Wind Turbines

Infrasound, low-frequency noise (ILFN) and amplitude modulation of the noise are known to disturb some residents living near wind farms. However, the mechanisms responsible for ILFN and amplitude modulation are not well understood. In an attempt to shed some light on these mechanisms, acoustic measurements were taken close to a wind farm, at residences located two or more kilometres from the nearest turbine in a wind farm and in an anechoic chamber using a scale-model, electrically-driven, wind turbine. The measured spectra reveal distinct peaks at the frequencies corresponding to the blade-pass frequency and its harmonics, and the characteristics of these peaks are remarkably similar for field and laboratory measurements, indicating that the zero mean flow simulation is a good representation of an actual wind turbine. Near field acoustic holography measurements on the scale-model turbine confirm that tonal components at the frequencies corresponding to the blade-pass frequency and its harmonics are generated as a result of blade-tower interaction, suggesting that it is likely to be an important mechanism of infrasound generation for industrial wind turbines. Inaccuracies in the assumed location of sources of noise generated by a wind turbine affect the accuracy of community noise predictions. This is because the source height affects the distance from the turbine beyond which sound rays arrive at the receiver having been reflected from the ground more than once, thus reducing the attenuation with distance from the turbine.

Examination of the predictions versus measurements of an acoustic interaction model for spinning modes, and concurrent low frequency amplitude modulation acoustic signatures from a 3MW wind turbine

A recently presented hypothesis and model relating to the generation of spinning modes from wind turbines, as a direct result of acoustic interaction involving the tower, results in a far field infrasound sound pressure level prediction, which is higher than that predicted by point source method. The model also predicts a significant attenuation of the fundamental blade passing frequency component relative to the second and higher harmonics. The model concurrently predicts a low frequency (~20 Hz), amplitude modulated harmonic series as a side effect of the acoustic interaction on a 1.6 MW 80 m diameter wind turbine. This study examines the model predictions of a 3.0 MW 90 m diameter wind turbine, and compares the predictions to measurements of the low frequency harmonic series and blade passing frequency harmonics at several different distances from the wind turbine.