Flow Visualization

Many of the most exciting discoveries in the field of fluid mechanics have been possible due to careful flow visualization. Flow visualization allows us to gain an overall view of flow patterns. It helps in identifying the vortex structures and other secondary flow features. The introduction of tracer particles and the study of their movement provides considerable information of the flow physics. In liquids, colored dyes and gas bubbles are common tracers, whereas for gas flows, smoke, helium-lled `soap' bubbles or gas molecules made luminous by an ionizing electric spark have served as tracers.For the present investigation, flow visualization was carried out in the test cell using light generated from the pulsed Nd:YAG laser. Visualization was carried out in the nearwake region of the cylinder. The flow was seeded with small diameter oil droplets that were produced by the commercial particle generator discussed in Seeding Arrangement For PIV. There is an important difference between the nature of seeding for flow visualization compared to that of PIV measurements. In the latter, seeding is spatially homogeneous with a high particle density. The homogeneously seeded images appear featureless i.e. no flow structure is visible. Structures become visible only when the velocity field is evaluated. During flow visualization, seeding is done in an inhomogeneous manner. It clusters around the cylinder and the seeding density is reduced to highlight the flow structures. The CCD camera is once again synchronized with the ring of the laser, though image pairs are not required. The images of the CCD camera are each of 1280 1024 pixels. During experiments, images were acquired through a PC at a rate of 8 Hz. The light sheet and the camera were perpendicular to each other. The particle traces were further processed with an imaging software (Paint-shop available with MS-Windows) to improve clarity.

Figure 3.24: Schematic drawing of the flow visualization arrangement.