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Heated driver-tube air will be needed when the existing Mach-4 test section of the Purdue Quiet-flow Ludwieg tube is replaced with a Mach-6 test section. Thus, a system was developed to heat the air in the driver tube. The air was successfully heated without dramatic adverse effects on the quiet-flow performance. Measurements of the mean and fluctuating pressure were made with a Kulite pressure transducer about 10 cm downstream of the onset of uniform flow. Temperature measurements were made with thermocouples and a cold wire. In agreement with previous results, the cold-driver quiet Reynolds number was about 400,000. The quiet-length Reynolds number decreased with increasing driver-tube temperature, dropping to 380,000 at 90C and 340,000 at 140C. Thus, a nozzle wall that is colder than the gas moves transition forward on the nozzle wall. This trend agrees with Harvey's 1975 observation that a heated nozzle wall delays transition. Problems with nozzle cleanliness were encountered during the heating; there were also problems with maintaining a uniform driver-tube temperature due to heat transfer to the nozzle and to the unheated upstream section of the driver tube.

Health care is becoming increasingly complex. The pre-hospital setting is no exception especially when considering the unpredictable environment. To address complex clinical problems and improve quality of care for patients, researchers need to use innovative methods to create the necessary depth and breadth of knowledge. Quantitative approaches such as randomised controlled trials and observational (e.g. cross-sectional, case control, cohort) methods, along with qualitative approaches including interviews, focus groups, and ethnography have traditionally been used independently to gain understanding of clinical problems and how to address these. Both approaches however have drawbacks: quantitative methods focus on objective, numerical data and provide limited understanding of context, whereas qualitative methods explore more subjective aspects and provide perspective, but can be harder to demonstrate rigour. We argue that mixed methods research, where quantitative and qualitative methods are integrated, is an ideal solution to comprehensively understand complex clinical problems in the pre-hospital setting. The aim of this article is to discuss mixed methods in the field of pre-hospital research, highlight its strengths and limitations and provide examples. This article is tailored to clinicians and early career researchers and covers the basic aspects of mixed methods research. We conclude that mixed methods is a useful research design to help develop our understanding of complex clinical problems in the pre-hospital setting.