FLOW VISUALIZATION TECHNIQUE 

Flow visualization is concerned with a study of fluid flow near the surface of objects or test specimens. The technique is commonly used in wind tunnel testing and has many forms, ranging from smoke flow visualization, particle tracker method, oil flow visualization, Special Clay Mixture Method, Fluorescent Dye Technique, visualizing tufts and streamers stuck to a surface.

Surface flow visualization is a well-established technique used to help in understanding flow fields, particularly complex, three-dimensional flows. Pressure sensitive paints are majorly used to determine the surface pressure of the test object and it is one of the main surface flow visualization techniques used.

Pressure Sensitive Paints are a just a coating, such as paints which behave as a fluorescent material under a particular wavelength with respect to different intensities depending on the external air pressure which is applied to its surface. PSP techniques have been applied widely to the study of high-speed flows. PSP is a non-contact technique. PSP is the first global optical technique that is able to give information on flow structures that cannot be easily obtained using conventional pressure sensors and provide non-contact, quantitative surface pressure visualization for complex aerodynamic flows.

BASICS AND WORKING OF PRESSURE SENSITIVE PAINTS

Pressure sensitive paint is a luminescent dye dispersed in an oxygen permeable binder. The dye undergoes excitation when they absorb light. This usually happens in the UV portion of the electromagnetic spectrum. After this, it returns to its ground state by emitting light, in the red portion of the spectrum. Also, we can make the dye to return to its ground state without emitting light, by interaction with an oxygen molecule. This process is known as oxygen quenching, which involves the non-radiative deactivation of an excited photo-active molecule(luminophore). 

Non-radiative processes include internal conversion to a different electronic state and then the release of heat or external conversion via contact with an external molecule, in this case, oxygen. Thus, the increase in pressure of the oxygen above the PSP causes an increase in the oxygen concentration within the binder, leading to a decrease in the intensity of the emitted radiation. As the pressure on the surface varies, the intensity of light emitted varies measured by the detector. This gives us a measure of variation of pressure on the surface of the test specimen.