Date of Award
Doctor of Philosophy
Trailing edge (TE) noise due to the interaction between a turbulent boundary layer (TBL) and an airfoil trailing edge is a major source of airfoil self-noise. This broadband noise source generates sound from a few 100 Hz well into the KHz range (~ 15,000 Hz). Wind turbine blades and other subsonic airfoils generate significant TE noise. Improvements such as serrations added to the trailing edge have shown to decrease the far-field noise generated without compromising the aerodynamic performance. A number of aeroacoustic TE noise theories have long been used as the basis for noise-reduction mechanisms. It is well known that they over-predict noise generation and reduction. Serrated trailing edges with different geometries are also known to decrease noise in certain frequency ranges and increase noise in others. In light of these discrepancies, recent work has been focused on understanding the flow mechanisms that cause noise and, by extension, the mechanisms that reduce noise.
In the present research, a NACA 0012 airfoil at a chord Reynold's number of ~850,000 and Mach number of 0.1 was chosen as a baseline configuration to study broadband noise generating flow mechanisms near the trailing edge. The results from the airfoil with a modified blunt trailing edge and 5° angle of attack were used as the baseline. Hot-wire anemometry with two traversing hot-wires were used as the main sensor for unsteady velocity measurements. Spanwise and streamwise correlations were obtained at different depths in the fully turbulent BL near the trailing edge of the baseline configuration to characterize flow structures. From these data, two regions having weak absolute and convective disturbances in the streamwise direction were isolated: the inner log region and the outer region respectively.
Correlation and BL profile data near trailing edges of varied thicknesses were compared and it was found that a decrease in thickness and wake deficit near the trailing edge corresponded to an increase in the convective speed of flow structures in the inner region. Sharpness transitions an unsteady wake to an unsteady mixing layer. It has been experimentally proven by other studies that an increase in TE noise occurs with an increase in its thickness. This follows that the interaction of weak absolute disturbances in the inner region with the trailing edge could be a major mechanism of noise generation.
Various serrated TE modifiers whose dimensions λ and 2h (λ is the width between two tips or roots and 2h is the length of a serration from root to tip) were scaled to the TBL thickness at the trailing edge of the baseline configuration were used as flow modifiers. The serrations formed a three dimensional flow structure with different convection speeds along the span. The effect of increasing the length of serrations was to transition the inner region of the turbulent boundary layer near the serration from weak absolute to convective disturbances and to decrease the spanwise correlation of flow structures near the tip. Results suggest that for a given serration width there exists a maximum length that corresponds to the greatest decrease in TE noise. This maximum length has been independently verified by other studies. It was also found that for a given large serration length there exists a minimum width that corresponds to an optimum convective speed of flow structures in the inner region near the serration. Evidence suggests that this width organizes the three dimensional flow structure most desirably to obtain the highest spanwise decay rate of flow-structures near the tip resulting in the maximum reduction of TE noise. Thus, the complex interaction of flow structures near the serration sheds light into the noise reduction mechanism of serrated trailing edges.
Clemons, Hephzibah, "Modification of flow structures associated with broadband trailing edge noise" (2016). Graduate Theses and Dissertations. 15116.