A double-diffusive interface tank for dynamic-response studies

Abstract

A large tank capable of long-term maintenance of a sharp temperature-salinity interface has been developed and applied to measurements of the dynamical response of oceanographic sensors. A two-layer salt-stratified system is heated from below and cooled from above to provide two convectively mixed layers with a thin double-diffusive interface separating them. A temperature jump exceeding 10°C can be maintained over 1-2 cm (a vertical temperature gradient of order 10 3°C /m) for several weeks. A variable speed-lowering system allows testing of the dynamic response of conductivity and temperature sensors in full-size oceanographic instruments. An acoustic echo sounder and shadowgraph system provide nondisruptive monitoring of the interface and layer microstructure. Tests of several sensor systems show how data from the facility is used to determine sensor response times using several fitting techniques and the speed dependence of thermometer time constants is illustrated. The linearity of the conductivity-temperature relationship across the interface is proposed as a figure of merit for design of lag-correction filters to accurately match temperature and conductivity sensors for the computation of salinity. The effects of finite interface thickness, slow sensor sampling rates and the thermal mass of the conductivity cell are treated. Sensor response characterization is especially important for autonomous instruments where data processing and compression must be performed in-situ, but is also helpful in the development of new sensors and in assuring accurate salinity records from traditional wire-lowered and towed systems.

Laboratory observations of double-diffusive convection using high-frequency broadband acoustics

“…Only the lower interface showed significant disagreement, where (aside from the first profile) the PME estimate is consistently 2 to 3 times smaller than what was observed. This could be because the buoyancy flux through the bottom of the tank has been underestimated, although estimating the flux from the four observed migration rates that disagree leads to heat fluxes 2 to 3 times those measured previously (Q=800 to 1100 W as compared with 400 W from Schmitt et al (2005)). …”

“…The temperatures that either the heater or chiller were set to were noted throughout the experiments, but no direct measurements of these temperatures were made. Previous measurements of the heat supplied to the tank through the bottom boundary showed Q ≈ 400W (Schmitt et al, 2005). Inverting (5), this would give J ≈ 3 to 4 × 10 −7 m 2 /s 3 .…”

“…High-frequency broadband acoustic observations of doublediffusive interfaces were performed in a insulated cylindrical tank (470 cm deep and 91.4 cm in diameter) (Schmitt et al, 2005;Lavery and Ross, 2007). Single double-diffusive interfaces were formed by filling the bottom of the tank with salt water, then very slowly floating fresh water on top through a sponge.…”

“…Before calculating salinity, density and sound speed profiles (Fofofnoff and R.C. Millard, 1983), the conductivity data were filtered to match the response of the temperature sensors (following the method outlined in (Schmitt et al, 2005)). These profiles showed very little variability in the temperature and conductivity in the layers surrounding the interface, confirming that the CT sensor measurements are representative of these well-mixed layers.…”

Chapter 26. Summary and Recommendations

“…This effect can either be investigated experimentally, theoretically or numerically given sufficient resources. We have recently become aware of an experimental facility at Woods Hole Oceanographic Institution operated by the Schmitt Fluid Dynamics group in Quissett, MA [111]. The facility includes a tank with vertical dimensions of 10 m and a circular cross section with a diameter of 2 m. The tank has the added capability of being temperature or salt stratified, controlled by internal sensors to set the degree of stratification.…”