Asai et al. used a shock tube with a partially evacuated driven section and a pressurised driver section. This sub-atmospheric starting pressure gives PSP a higher sensitivity, regardless of the substrate used [10]. Gongora-Orozco et al. used a shock tube with an atmospheric driven pressure to capture a shock moving over grooves and successfully showed the modified shape of the shock front. Shock tubes are an excellent method of delivering predictable step changes in pressure, allowing us to look at the response times of the PSP. The aim of this work is to build upon the foundation laid by Asai et al. and Gongora-Orozco et al. and globally measure the pressure of a complex transient shock interaction at a baseline of atmospheric pressure.

Shock wave diffraction around a large corner gives large positive and negative pressure changes and has been studied using a variety of other flow diagnostic techniques [11�C13], meaning that this flow is relatively well understood. This work aims to investigate shock wave diffraction at two Mach numbers and compare the results with numerical data.2.?Background2.1. Shock TubeShock tubes are an ideal method to test the response time of an experimental technique. A region of high pressure (region 4) is separated from a region of low pressure (region 1) by a diaphragm. The diaphragm in the University of Manchester square shock tube is made up of acetate, which is burst by mechanical means. After the rupture of the diaphragm, compression waves begin to travel from region 4 into region 1. These compression waves eventually coalesce into a discontinuous shock wave.

The pressure at each location in the wave structure shown in Figure 1 can be analysed using the one-dimensional theory presented by Anderson [14].Figure 1.Shock tube flow after the diaphragm is ruptured.2.2. Shock DiffractionThe problem of shock wave diffraction around sharp corners has been investigated by several researchers since initial considerations by Anacetrapib Howard and Matthews [15]. Many experimental (all previous experimental work used density-based diagnostics) and numerical simulations have been performed, showing different levels of flow features. The basic structure of a strong shock wave diffracting around a corner was given by Skews [16]. As a shock wave reaches a sharp corner with an angle greater than 75��, the flow is independent of geometry and only a function of incident shock Mach number.

The diffracting shock wave loses strength along its length as it rounds the corner. The flow behind the incident shock (termed the ��complex region�� by Skews [16]) consists of a shear layer created by the inability of the flow to navigate the sharp corner. This shear layer rolls up into a spiral vortex (see Figure 2).Figure 2.Basic flow structure behind a shock wave diffracting around a sharp corner. (a) 1 < Mie �� 1.