Experimental Observation of Inclined Waves in a Continuous Double Gradient of Salt and Sediment

Linear stability analysis of stratified fluids wherein the density depends on two scalar fields has predicted the existence of inclined waves arising from a phase shift between the peak amplitudes of two scalars when one of the scalars has a settling velocity [ A. Alsinan, E. Meiburg, P. Garaud, JFM (217) 816 243-267]. Here are reported what to our knowledge are the first experimental observations of candidate inclined waves. The settling scalar was 1200 grit (approximately 3 micron) silicon carbide and the diffusing scalar was sodium chloride salt. A continuously-stratified double gradient was established in a nominally 30 cm x 20 cm x 7.5 cm cell using a continuously stirred, gravity-flowed, double-bucket technique. Typical parameters were 0 – 2.5% by mass particles and 0 – 2% by mass salt with the salt denser at the bottom of the cell and the particles denser at the top. Imaging consisted of backlighting and a 25 MPixel Nikon Z6 mirrorless camera and Nikkor 24-70 lens. Images were typically acquired once per minute over a period of 30 minutes or more and later combined to form video. Results are compared to predictions of linear stability theory.