Application of Pressure-Sensitive Paints to Unsteady and High-Speed Flows

The Pressure-Sensitive Paint (PSP) technique allows the global pressure mapping of surfaces under aerodynamic conditions. The present study involves the application of TrisBathophenanthroline Ruthenium Perchlorate based PSP, developed in-house, to two different cases; a) the flow through a sonic nozzle, and b) the examination of the effect of dimples on glancing shock wave turbulent boundary layer interactions at transonic speeds. Introduction The process of flow visualisation is of tremendous value in the field of fluid dynamics research. Each technique has certain advantages and disadvantages. In some cases the researcher has economical and spatial limitation whereas in other cases the limitation might be due to the technique itself, for example achieving only qualitative data and a complementary technique is required to achieve better insight into the flow physics. The current standard for pressure measurement is an array of pressure taps. Pipes connect these holes to pressure transducers, which transform the mechanical force of the pressure to a digital or analog reading. Depending on the size and complexity of the model, the process of creating these models is timeconsuming and expensive. Thus the cost (both in terms of time and money) is the first drawback of using a pressure tap system. The second drawback of pressure taps is that they must be limited in number. Drilling small taps into models changes the local pressure distribution at that point, compared to the surface without a hole. Consequently, each tap drilled affects the total pressure distribution, ultimately leading to unrealistic results if too many taps are used. The limitation is in direct conflict with the need for a continuous image of the pressure distribution. Pressure-sensitive paint (PSP) has become a useful tool to augment conventional pressure taps in measuring the surface pressure distribution of aerodynamic components [8, 2, 7]. PSP offers the advantage of non-intrusive global mapping of the surface pressure. The PSP consists of a dispersion of luminescent probe molecules in an oxygen permeable binder layer. An excitation light source of wavelength λe and intensity Ie is used to promote molecules to an excited energy state [4, 10]. An increase in pressure causes a corresponding increase in the partial pressure of oxygen and an increase in the oxygen concentration within the binder layer. This results in a larger level of oxygen quenching and lower luminescence intensity [11]. Because the amount of oxygen in the test gas can be related to static pressures, one can obtain pressure signals from the change in the luminescent intensity of PSP. The relationship between the oxygen concentration and the pressure may be approximated by a second order polynomial [4] , as shown in equation (1). Ire f I = A(T )+B(T ) (