Measurements and analysis of hypersonic tripped boundary layer turbulence

Surface roughness elements have been used to overcome the large prediction uncertainties of hypersonic boundary layer transition. They provide the ability to force transition at short distances downstream from the tripping element to ensure turbulent boundary layers are present. This is useful on vehicle surfaces where a laminar boundary layer could produce separation bubbles heavily affecting the flow field, as well as in ground test facilities that do not have the space to allow for natural transition to occur. A large number of investigations have looked into the effects that the shape and size of a boundary layer trip has on the transition location. However, few have investigated the effect that trips have on downstream boundary layer turbulence. This investigation uses a flat plate instrumented with heat transfer and pressure sensors coupled with schlieren imaging and a cylindrically focused laser differential interferometry system to analyse the effects that boundary layer trips have on downstream boundary layer turbulence. A Mach 7.3 flow with an enthalpy of 1.5 MJ/kg was used to analyse various boundary layer trips with k/δ ratios of 1.9, 2.7 and 4.0. Analysis of the schlieren images showed an increase in boundary layer height of approximately 50% when a boundary layer trip was introduced into the flow. The disturbance spectra obtained from the cylindrically focused laser differential interferometry showed the shape of the spectrum to match well between the untripped and tripped cases. The magnitude of the fluctuations decreased when boundary layer trips were used, however the magnitude of the fluctuations did not change when the trip height was increased.