Optical studies of leading-edge separation in high-enthalpy, low-density hypersonic flows

Nitric oxide (NO) rotational temperatures have been measured around atick mark shaped body in a hypersonic flow with total enthalpy of 11:3 MJ=kgand freestream density of 2:1 g=m3, and around a 30° wedge in a hypersonicflow with total enthalpy of 3:1 MJ=kg and freestream density of 6:4 g=m3. Thetemperature was measured with multi-line planar laser induced fluorescence(PLIF) thermometry, and the hypersonic flow generated by a free-piston shocktunnel. The temperature measurements at the high-enthalpy condition are thehighest total enthalpy NO-PLIF thermometry measurements made to date in ahypersonic flow. Additionally, this is the first experimental investigation of theflow around a tick mark shaped body.The tick mark body flow was visualised using time-resolved near-resonantlyenhanced shearing interferometry (RESI), with lithium seeded into the flow as arefractivity enhancing species. Several improvements to the RESI visualisationmethod were also investigated. The time-resolved visualisations of the tick markshaped body were analysed to provide quantitative measurements of the flowstructure. Additionally, the flow structures in lithium seeded and unseeded flows,visualised with both the RESI and NO-PLIF techniques, were compared. Thiscomparison showed that lithium seeding was not completely non-intrusive, as hadbeen previously supposed.The wedge thermometry experiments at the low-enthalpy condition wereconducted to validate the NO-PLIF system performance. The freestream temperaturemeasured here showed good agreement with prediction, and with previousNO-PLIF thermometry experiments conducted at this flow condition. Near thefluorescence discontinuity produced by the oblique shock wave, a fluorescencebleeding effect was found, which caused the freestream temperature to increaseand post-shock temperature to decrease near the shock wave. Analysis of previousNO-PLIF thermometry studies in free-piston shock tunnels shows this effect waspresent in previous experiments, but had not been previously identified. At thehigh-enthalpy condition, the freestream temperature was significantly lower thanthe predicted value, yet Boltzmann plots for various regions of the tick mark modelflow showed that a single rotational temperature is appropriate for this condition.This indicated that there is an as yet unidentified systematic error that becomessignificant for NO-PLIF measurements in high-enthalpy flows