Visualization renders direct and intuitive realizations of the thermal and flow processes, as illustrated by the example shown in Figures 3.1-3.3. Figure 3.1 shows a 1/48-scale model of an F-18 aircraft being tested in a water tunnel at NASA Dryden Flow Visualization Facility. To simulate the Reynolds number encountered by the actual aircraft, water with a much higher density than air is used. Water also allows for easy introduction of liquid dyes, the streak pattern of which is imaged in Figure 3.1. The aerodynamics of the air flow over the aircraft body can be easily seen at a high angle of attack of the aircraft. Figure 3.2 shows a sequence of high-speed digital images of flame propagating in a turbulent flow. Each row of images is taken at a fixed equivalence ratio (or fuel-air ratio), and the turbulence intensity is increased from left to right. Some of the changes in the flame topology can be observed where higher level of turbulence causes smaller scales of flame corrugation or so-called flame wrinkles. Figure 3.3 is a somewhat more advanced technique using laser-induced fluorescence (LIF) imaging. Either a fluorescent dye can be injected into the fluid or, as in Figure 3.3, nitric oxide (NO) and hydroxyl (OH) molecules that are formed in the flow are optically induced to emit a fluorescence signal. The local concentrations of these molecules along with temperature are imaged when a laser light tuned to the excitation wavelengths of these molecules are sent in as a light sheet. The temperature field is imaged from the Rayleigh scattering signal from the molecules. Details of the fluorescence and Rayleigh scattering processes for diagnostics are discussed in Chapter 5 and also briefly in this chapter.