Scale-dependent breakdown of isotropic turbulence by off-axis rotation.

Abstract

An experimental investigation was conducted to explore the interplay of turbulence and noninertial effects introduced by off-axis rotation on isotropic-turbulence dynamics using tomographic particle image velocimetry. A cubic box, generating isotropic turbulence with the aid of eight mixers positioned at each corner, rotated with a turntable, was subjected to different rotational speeds. Particular focus was placed on the changes in distinct turbulence quantities, including the Kolmogorov, Taylor, and integral scales, as well as changes in the velocity spectra. An inspection within a central subregion featuring isotropic flow with nearly zero mean flow revealed distinct rotation-induced anisotropic effects. The analysis of the longitudinal and transverse autocorrelation functions indicated rotation-induced changes predominantly at larger scales. The compensated spectral counterparts demonstrated a scale-dependent response to rotation, where energy distribution exhibited minor changes at smaller scales and significant alterations at larger scales, affecting the extent of the inertial subrange. The inverse-turbulent Rossby number Ro_{ε}^{-1}=2Ω〈k〉/ε, where Ω represents the rotational speed, 〈k〉 is the turbulence kinetic energy, and ε denotes the energy-dissipation rate, demonstrated a monotonic relationship between rotational speed and various turbulence characteristics. As the Ro_{ε}^{-1} value increases, there is an increase in turbulence levels and the dissipation rate, while the Kolmogorov microscale, Taylor microscales, and integral scales decrease. Also, changes in the tangential direction were more significant than those in the vertical axis of rotation.