Hot-film measurements in a two-layer system of a magnetic liquid and a silicone oil

Constant-temperature hot-film anemometry measurements in a two-layer system of a silicone fluid layer over a magnetic fluid layer has been investigated. Complicating factors potentially affecting the measurements were very slow velocities, the unusually high thermal conductivity of the magnetic liquid, the vicinity of solid boundaries, the presence of the interface, and the possibility of magneto-convection in addition to natural convection. All these factors required a calibration procedure combining both experimental calibration and a set of numerical models of the system. The findings obtained during the calibration were that neither natural convection nor magneto-convection were adversely affecting the measurements, probably due to the effects of the solid boundaries and the interface. It was furthermore observed that the fluid interface resulted in an effect analogous to the near-wall effect, but deformations of the interface due to the sensor or its mounting prevented measurements close to the boundary. Spatial variation of the fluid velocities affected the calibration of the sensor noticeably. While a simple calibration, either by moving the probe or by assuming a uniform velocity, can give a qualitatively reasonable velocity profile, it will quantitatively deviate significantly from the true velocities.