The effect of flow anisotropy and three dimensional separation on the turbulent structure and radiated turbulence ingestion noise of a rotor in two experimental configurations is studied. The first consists of a non-axisymmetric boundary layer wake ingested by a rotor mounted at the aft of a body of revolution inclined at 5 degree angle of attack. In the second configuration a transient disturbance is generated by an upstream wing body junction pitching from zero to 20 degree angle of attack . This disturbance is convected downstream and ingested into a rotor immersed in a wall boundary layer. In both cases flow velocimetry at the rotor inflow is done and the far field sound is measured. The flow velocimetry in the wake of the inclined body of revolution shows evidence of three dimensional separation and vortex rollup between the lee and body sides. A boundary layer embedded shear layer develops as the turbulent kinetic energy is pulled off the wall by the flow separation and is visible in the port side velocimetry. The turbulent structure of this shear layer and the boundary layer on the lee of the body is visualized using compact eddy structure representation and the modes on the port side are shown to be stretched versions of similar modes seen in an equilibrium, zero pressure gradient boundary layer. The effect these structures had on the radiated sound served to both increase blade to blade correlation and the overall broadband levels of the sound. Measurements of the sound using an acoustic array showed directivity effects that resulted from the location of the embedded shear layer and rollup vortices. Although the vortices likely have some effect on the spectra, most of the noise is dominated by the turbulence ingestion of the embedded shear layer.
For the second experimental configuration the transient motion was documented through repeated measurements of the flow field and sound, and an ensemble average of the measurements taken. Overall the flow was unsteady, particularly in the outer region of the boundary layer. The sound radiated was shown to be tonal during the first half of the interaction, where the flow is dominated by a deterministic mean flow change, and attributed to a form of periodic unsteady loading. During the latter half of the disturbance the broadband and overall sound levels increased significantly and are associated with the interaction of the rotor with flow separation over the wing body junction when it reached a critical, 16 degree angle of attack. / Doctor of Philosophy / The interaction of rotors and propellers with turbulence is commonly encountered when vehicles transit fluid mediums. In vehicles with aft mounted propellers, such as pusher type aircraft or underwater vehicles, turbulent boundary layers developed over the vehicle surface are ingested by the propeller. The size or scale of the average turbulent eddy greatly affects the type of sound generated by the interaction. For eddies that are small enough to only interact with one blade, the blade angle of attack varies randomly as it rotates through the turbulence and this radiates broadband sound. However, if the blade encounters eddies that are large or long enough to interact with multiple blades then the angle of attack, and thus the fluctuating lift force, begins to be correlated for each passage of the blade. This is known as blade to blade correlation and produces energy and sound concentration around the frequencies that correspond to the blade passage. This phenomenon is fairly well understood and many attempts have been made to model and predict the sound spectra from a rotor encountering turbulence in this manner. However these models often assume isotropic and homogenous turbulence when making predictions. This assumption works well in many applications, however, often the turbulence the rotor encounters is anisotropic with significant flow inhomogeneities. Thus, experimental investigations into the mechanisms and sources of sound in inhomogenous and anisotropic flows is necessary in an attempt to inform further flow and acoustic models.
In this dissertation the inflow and acoustic response of a rotor ingesting significantly complex and anisotropic flows is characterized. It focuses on two commonly encountered flow arrangements; a rotor mounted at the stern of a body of revolution at an angle of attack, and a rotor ingesting a turbulent wall boundary layer with transient disturbances introduced by an upstream wing body junction. In both cases the flow is three dimensional and the rotor encounters significant circumferential turbulence variation during its rotation through the resultant turbulent flow field. For the flow about the body of revolution the flow and noise appear to be driven by the rotor interaction with an embedded shear layer that results from three dimensional separation between the lee and windward sides of the body. For the transient disturbance interaction the rotor noise response shows two separate noise sources. During the first half of the disturbance the blade response is tonal and associated with a deterministic blade angle of attack change as the rotor interacts with the transient. In the latter half of the disturbance the rotor broadband noise is significantly increased due to flow separation over the wing body junction.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/120635 |
| Date | 27 June 2024 |
| Creators | Banks, Jarrod Thomas |
| Contributors | Aerospace and Ocean Engineering, Devenport, William J., Woolsey, Craig A., Alexander, William Nathan, Anderson, Jason M., Lowe, K. Todd |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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