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We examine the problem of performing simultaneous and coplanar Particle ImageVelocimetry (PIV) and Laser-Induced Fluorescence (LIF) measurements in astratified fluid initially at rest. Our focus is on enabling detailed velocityand density measurements in long internal waves and gravity currents, throughrelatively small modifications of typical existing PIV systems comprisingpulsed lasers, using dye concentration as a proxy for fluid density. Severalissues have limited such measurements. These include: (1) variations in thelaser intensity and beam structure between laser pulses; (2) PIV particlesconcentrating preferentially at their neutral buoyancy depth, thereby yieldingnonuniform dye illumination; and (3) the need to maintain a constant index ofrefraction across large stratified fluid volumes. Here we focus on anexperimental setup comprising a stratified layer overlaying a deep homogeneousregion. We produce long internal waves using a lock-release setup, in order toinvestigate the structure of waves comprising recirculating fluid regions(known as ``trapped cores''), which are of current interest in oceanographicapplications. We maintain a short optical path and use velocity informationfrom PIV data to minimize index-of-refraction issues. We exploit the fact thatthe system is initially at rest to devise a mapping that links apparent andactual dye concentration, thus sidestepping nonuniform illumination issues dueto particle clustering. Finally, we devise a procedure to correct for laserpower variations along each ray in the sheet.

Simultaneous PIV and LIF measurements in stratified flows using pulsed lasers