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Title: Stability of the flow over a rough, rotating disk
Author: Harris, Joseph H.
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2013
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This thesis is concerned with discovering the effect of a distributed roughness on the boundary-layer stability of a rotating disk. The investigation uses both a local, linear stability analysis and machined aluminium disks rotating in water in conjunction with a hot-film anemometer system. The stability analysis applies a sinusoidal function to the surface of the disk which mimics anisotropic roughness similar to a grooved record. The new surface is used with the governing equations in order to calculate the new mean flow profiles for the now grooved surface at a variety of roughnesses. These new flow profiles are then used in the stability analysis. The results show that the roughness has the effect of increasing the stability of the cross-flow instability mechanism by decreasing the velocity of the radial wall jet. Conversely, increasing roughness levels cause the growth of the streamlinecurvature instability mechanism, something which is probably caused by a thickening of the boundary-layer seen in the velocity profiles. These two outcomes result in a predicted switch of the dominant instability mechanism on the disk. The experimental arrangement confirms the results of the mean velocity profiles, and appears to show the appearance of the enlarged streamline-curvature instability at higher roughness levels. This instability appears as a small burst of frequencies at low Reynolds numbers centred on the numerically predicted neutral curve lobe. This burst dies down as it moves downstream, but appears to increase the amount of energy in the flow which hastens the onset of the cross-flow instability earlier than predicted. Before the emergence of this other mode at lower roughness levels, the roughness appears to delay the onset of the spiral vortices by pushing back the location of the initial cross-flow instability. The experimental results also see a decrease in the number of spiral vortices seen around the circumference of the disk as roughness is increased. This result is thought to be due to the decrease in the growth rate of the cross-flow instability rather than any switch in the neutral curve positions.
Supervisor: Not available Sponsor: Engineering and Physical Sciences Research Council (EPSRC)
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: QA Mathematics ; TJ Mechanical engineering and machinery