Date of Award
MS in Aerospace Engineering
This thesis involves the experimental testing of a bluff body with a blunt base to evaluate the effectiveness of end-plate tabs in reducing drag. The bluff body is fitted with interchangeable end plates; one plate is flush with the rest of the exterior and the other plate has small tabs protruding perpendicularly into the flow. The body is tested in the Cal Poly 3ft x 4ft low speed wind tunnel. Testing is conducted in three phases.
The first phase was the hot-wire measurement of streamwise velocity of the near wake behind the bluff body. An IFA300 thermal anemometry system with a hot-wire probe placed behind the model measures the wake velocity fluctuations. The power spectral density on the model without tabs shows large spikes at Strouhal numbers of 0.266, 0.300, and 0.287 at corresponding Re = 41,400, 82,800, 124,200 where vortex shedding occurs. The model with tabs shows no such peaks in power and therefore has attenuated vortex generation in the wake flow at that location.
The second phase of testing was pressure testing the model through the use of pressure ports on the exterior of the bluff body. A Scanivalve pressure transducer measured multiple ports almost simultaneously through tubing that was connected to the model internally and routed through the model’s strut mount and outside of the wind tunnel. This pressure testing shows that the model with tabs is able to achieve up to 36% increase in Cp at Reh = 41,400 on the base region of the bluff body and no negative pressure spikes that occur as a result of vortex shedding.
The last phase of testing is the measurement of total drag on the model through a sting balance mount. This testing shows that the drag on the model is reduced by 14% at Re = 41,400. However it also shows that as velocity increased, the drag reduction is reduced and ultimately negated at Re = 124,200 with no drag loss at all.
The addition of tabs as a passive flow control device did eliminate vortex shedding and alter the base pressure of the bluff body. This particular model however showed no reduction in total drag on the model at high Reynolds numbers higher than 124,000. Further study is necessary to isolate the exact geometry and flow velocities that should be able to produce more favorable drag results for a bluff body with this type of passive flow control device.