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

Abstract

Constant-temperature hot-film anemometry measurements in a two-layer system of asilicone fluid layer over a magnetic fluid layer has been investigated. Complicating factorspotentially affecting the measurements were very slow velocities, the unusually highthermal conductivity of the magnetic liquid, the vicinity of solid boundaries, the presenceof the interface, and the possibility of magneto-convection in addition to naturalconvection. All these factors required a calibration procedure combining both experimentalcalibration and a set of numerical models of the system.The findings obtained during the calibration were that neither natural convection normagneto-convection were adversely affecting the measurements, probably due to theeffects of the solid boundaries and the interface. It was furthermore observedthat 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 preventedmeasurements close to the boundary. Spatial variation of the fluid velocities affected thecalibration of the sensor noticeably. While a simple calibration, either by movingthe probe or by assuming a uniform velocity, can give a qualitatively reasonablevelocity profile, it will quantitatively deviate significantly from the true velocities.