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Title: The impact of surface roughness on transonic inlet lip-flows at incidence
Author: Coles, Charlotte
ISNI:       0000 0004 8501 0143
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 2019
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At high angles of incidence, the flow-field around the lower inlet lip of a typical engine can accelerate to a supersonic value, creating a local supersonic region, terminated by a shock wave. This creates a shock wave-boundary-layer interaction, which could lead to detrimental consequences on the global flow-field. To meet future environmental regulations, new, larger, engines are being designed with a higher bypass ratio. However, to offset any additional drag, these engine inlets are also shorter and slimmer than previous designs. Shortening the inlet lip is likely to place the lower lip interaction closer to surface roughness features, such as the acoustic liner. This is made up of small, tightly packed holes over the inner surface. The acoustic liner is not the only source of roughness during take-off. Other examples include ice which can form around the inlet lip, or insects which impact and leave a residual roughness adhered to the inlet. The aim of this study is to better understand the impact of roughness on this scale on the lower lip transonic flow-field. The study uses an experimental set-up with a unique working section designed to assess the effects of shock-induced separation on a inlet lip at high angles of incidence. This rig was designed to match typical intake conditions during take-off conditions. The leading edge characteristics of the modelled lip geometry matched those of a typical engine inlet. A variety of methods such as Schlieren imaging and Laser Doppler Velocimetry were used to investigate the flow-field. In particular, boundary-layer properties were compared at a location called the equivalent fan-face position. In total, seven rough cases were compared during this study. A smooth, baseline, surface was also investigated as a reference case. Five of the rough cases had a continuous roughness across the whole model width, extending from approximately the stagnation point to the equivalent fan-face position. These cases had different roughness heights, ranging from 1x10−5 to 1.33x10−3, when normalised by the maximum lip thickness. The other two, discrete rough cases, had span-wise roughness either located at roughly the interaction position or at the equivalent fan-face position. With the addition of continuous roughness, compression and expansion waves were seen in the supersonic region around the model lip nose. The supersonic region was smaller than the smooth reference case, and as a result the shock location was further upstream. The shock wave-boundary-layer interaction was likely separated for all the continuous cases. Increasing the roughness height, resulted in a larger separation, reflected through a bigger λ-foot in the Schlieren images, and a thicker, less full, boundary-layer was measured at the equivalent fan-face position. The size of the separation appeared to correlate with the boundary-layer properties at the equivalent fan-face position. For the highest roughness levels, the increase in boundary-layer thickness plateaued to a value 40% larger than the baseline reference case. It was seen that even with relatively small levels of surface roughness, there was still a noticeable impact on the flow-field compared to the baseline condition. With roughness in specific locations, some appeared to have a more severe impact on the flow-field than others. The presence of roughness in the interaction region, for example, appears to lead to a larger separation at the base of the interaction, compared to other locations of roughness. Modelling an increase in the mass flow engine demand, all the rough surfaces show similar changes to their flow field. For example, they had a larger supersonic region, separation size and a thicker boundary-layer. For all the roughness cases examined, three key drivers of the boundary-layer properties at the equivalent fan-face position were determined. These were the separation size, the pressure gradient downstream of the interaction, and the roughness height itself.
Supervisor: Babinsky, Holger Sponsor: EPSRC ; Rolls Royce
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: fluid dynamics ; supersonic flow ; shock wave