On turbulence measurement in rotating magnetic field-driven flow

Abstract

Small-scale turbulence measurements in liquid metals are generally conducted using potential difference probes (PDPs). However, the use of PDPs in alternating magnetic field-driven flows is limited as the driving magnetic field (DMF) induces a noise signal, which cannot be separated from the turbulence signal using a filtering algorithm that does not take into consideration the physics of both the noise and the flow. Therefore, we developed a post-measurement method for the rejection of DMF-induced signals. The method was demonstrated for a case study of rotating magnetic field (RMF)-driven flow in which the interaction between the PDP and DMF was investigated. The probe was successfully aligned so that the DMF-induced signal could be linearly separated from the turbulence signal. Consequently, our method enables the properties of both the cyclostationary induced signal and turbulence to be utilized to reconstruct DMF-induced signals. The induced signal is reconstructed using a moving ensemble (or ergodic) averaging algorithm that removes the turbulence contribution to the original signal. The algorithm was tested by applying it to velocity measurements that were conducted inside liquid Ga–In–Sn alloy subjected to RMF driven flow, and it was found that the developed method enabled the successful rejection of the DMF-induced signal. The rejection of the induced signal revealed an unknown spectral distribution of the turbulent kinetic energy for RMF-driven flow at a high magnetic Taylor number. This distribution could not have been identified without the rejection of the induced signal.