INSTABILITIES AND TRANSITION IN THREE-DIMENSIONAL FLOWS WITH ROTATION EXCITATION OF INERTIAL MODES IN A CLOSED GRID TURBULENCE EXPERIMENT UNDER ROTATION

Translating a grid in a closed volume of fluid is a convenient way to generate an approximately homogeneous and isotropic turbulence when a compact system is needed, e.g. when experiments are performed in a rotating frame [1, 2, 3, 4]. Such flow excited by a grid translation is, in general, composed of a superposition of (i) a reproducible flow (determined through ensemble averaging) and (ii) a non-reproducible turbulent flow. In the presence of background rotation, both of these two flow components may excite inertial waves [5], which propagate because of the restoring nature of the Coriolis force. The excited inertial waves are respectively (i) reproducible —and therefore detectable in the ensemble average— and (ii) non-reproducible —detectable in the individual realizations only — [6]. In a closed container, inertial waves may appear in the form of standing inertial modes, which are the eigenmodes of the container geometry [7, 8]. Excitation of inertial modes in grid-generated turbulence has been first observed by Dalziel [1] and are also visible in the experiments of Morize and Moisy [9] and Moisy et al. [10]. They have been characterized by Bewley et al. [3], who found good agreement between the measured frequencies and the numerical results of Maas [8]. We investigate here the structure of these inertial modes and we explore to what extent they may be reduced [6]. A square water tank is mounted on a rotating platform, and velocity fields in a vertical plane are measured using a corotating Particle Image Velocimetry system. Two grid configurations have been used: a “simple ” grid, of mesh size 40 mm, and a “modified ” grid, on the top of which a set of inner sidewalls is attached. We demonstrate that, in the latter configuration