Investigation of the Effect of a Surface Protuberance on a High Speed Boundary Layer

Investigation of the Effect of a Surface Protuberance on a High Speed Boundary Layer

Ramaswamy, Deepak Prem (TU Delft Aerospace Engineering)

Schrijer, F.F.J. (mentor)
van Oudheusden, B.W. (graduation committee)
Avallone, F. (graduation committee)

Delft University of Technology

Aerospace Engineering

2017-12-18

One of the major challenges that engineers face when designing a high speed vehicle is aerodynamics heating. While ideally the external surfaces are expected to maintain a smooth profile, this is not the case in most vehicles, which might be laden with discontinuities like steps, gaps, and other protrusions. These could alter the local flow field and could have a drastic impact on the heating and hence should be properly studied. In this thesis, the effect of large protuberances, when placed in both a laminar and turbulent boundary layer is experimentally investigated. Cylindrical elements with height greater than or equal to the boundary layer height, placed on a flat plate were used as protuberances. Quantitative Infrared Thermography was used as the prime investigative tool, assisted by high speed Schlieren and oil flow visualisations. For the laminar interactions, a series of high and low heat flux regions were observed in the wake of the protuberance due to the presence of stream wise vortices. A local peak due to flow reattachment was found downstream of the cylinder along the centreline, whose length from the cylinder trailing edge scaled with the height of the cylinder and was nearly invariant with the diameter and the flow unit Reynolds number. The span wise distribution of Stanton number revealed that the location of heat flux peak formed due to the symmetry plane counter rotating vortices, scaled with the diameter of the cylinder with their peak heating increasing with increasing in diameter. This revealed the possible dependency of the strength and location of the symmetry plane vortex pair with the diameter of the protuberance. The flow transition induced by the elements considered here, moved upstream with increase in unit Reynolds number. Moving upstream, a presence of span wise vortex system was verified. A mean separation length was measured from the centreline heat transfer distribution and an empirical formulation was presented and shown to have good agreement with the current dataset along with recent literature. For the turbulent interaction, direct heat transfer measurements were acquired downstream of the protuberance for the first time. A centreline high heat transfer was observed, with heating of $1.5 - 2$ times the local turbulent case. A series of high and low heat flux regions were observed in the wake, dictating the possible presence of stream wise vortices. Similar to the laminar case, the location of reattachment heating was shown to scale with the height of the cylinder. Additionally, span wise heat transfer profiles show that the wake width increases with increase in height, while remaining fairly constant for a given condition, throughout the length of the measurement domain. Time resolved Schlieren images captured upstream of the protuberance showed a very unsteady separation shock and a relatively steady bow shock wake, together forming the lambda shock system. Probability density functions generated for the upstream separation length revealed that both the mean and the standard deviation of the separation length increases with increase in height of the cylinder.

Infrared Thermometry
protuberance
Transition
Boundary Layer
shock wave

http://resolver.tudelft.nl/uuid:f68ba044-79c6-441e-84de-12029d1650f6

Student theses

master thesis

© 2017 Deepak Prem Ramaswamy