Matching the refractive index [in] density stratified flows

The use of optical methods such as Laser Doppler Velocimetry (LDV) and laser induced fluorescence techniques (LIF) in experimental fluid mechanics is becoming very common. The greatest advantage of such methods is that measurements are made without disturbing the flow. A major impediment to using optical methods to study density stratified flows is the variation of the refractive index within the flow field. McDougall (1979a) has proposed a method for the reduction of refractive index variations while maintaining a density difference. The method relies on the fact that various solutes in, say, water can contribute to the density and to the refractive index of the solution in different proportions. A new pair of solutes (ethanol and NaCl) were found to be suitable for use in LDV and LIF studies of stratified flows. In some circumstances the new pair is more useful than that described by McDougall (1979a).