Incompressible Miscible Rayleigh-Taylor Instability Experiments on the University of Arizona Linear Induction Motor Drop Tower Using Planar Laser Induced Fluorescence

Abstract

Incompressible miscible Rayleigh-Taylor instability (RTI) experiments are described that utilize the University of Arizona’s vertical Linear Induction Motor (LIM) drop tower. RTI is a buoyancy driven instability occurring at the interface between two fluids of differing densities, represented by the Atwood number, where a destabilizing acceleration causes initial perturbations along the interface to develop into characteristic spike and bubble formations. The incompressible fluid pair used in the research is a miscible combination of isopropyl alcohol solution and calcium nitrate salt solution. Experiments were conducted at Atwood numbers of 0.150 and 0.216. The experiments conducted at an Atwood number of 0.216 are used as a replication of previous research to allow comparisons to the experiments conducted at Atwood values of 0.150. The fluid solutions used in experiments conducted at the Atwood value of 0.150 have been initially mismatched in refractive index as part of an examination of possible techniques for improving imaging issues commonly experienced by miscible experiments. An acrylic tank attached to an experimental aluminum reaction plate is mounted to the LIM drop tower and filled with the two fluids in an initially stratified configuration. The reaction plate, along with the fluid tank and diagnostic equipment, is vertically raised to the top of the LIM drop tower and subsequently accelerated downward at an effective constant acceleration of approximately 11.7 g, developing the fluid instability. Acceleration is accomplished by the interaction between the LIM drop tower’s parallel set of linear induction motors and the aluminum reaction plate that is vertically constrained within the rails of the drop tower. Deceleration is accomplished with a set of permanent magnetic brakes located at the base of the LIM drop tower. Acceleration is recorded using a single axis accelerometer mounted to the experimental test sled. Imaging equipment is rigidly attached to the test sled and provides visualization from a static viewpoint of the experimental tank. Both unforced and forced experiments are conducted, where an electrically driven actuator is used to provide vertical parametric forcing of the experimental test sled. Experiments are imaged using planar laser induced fluorescence (PLIF) to visualize the instability interface. A swept 445nm laser light beam that illuminates fluorescein dye mixed into the calcium nitrate solution allows planar visualization of the instability interface and experimental images are captured using a monochrome high-speed shock-rated digital camera that documents the experimental process. Resulting images expose a single plane of the developing RTI. Postprocessing follows in which a pixel level concentration profile is obtained. The concentration profile is used to provide measurements of the mixing process. From the concentration profile, a mixing region width is extracted and subsequent measurements of the instability growth constant, α, are obtained. Two methods for calculating α are used. Calculated α values for experiments at an Atwood number of 0.150 are in the range of 0.076–0.118 and the α values for Atwood numbers of 0.216 are in the range of 0.021-0.063, depending on measurement method. Comparisons of the measured α values to α values from previous experiments are made, noting that the reported α values are higher than what might be typically expected for similar experiments. A spectral analysis to examine the prevalent wavelengths during the experiment is completed and finds that forced experiments featured earlier development of dominant wavelengths when compared to unforced experiments. An analysis of the effects of refractive index variation on image sharpness using gradient-based focus measures is performed, but considered inconclusive.