The motion of a heavy tethered sphere and its wake were measured in a closed loop water channel using a time resolved, high-speed particle image velocimetry technique in a horizontal plane. Measurements were performed for nondimensional reduced velocities ranging from 2.8 to 31.1 that include three bifurcation regions. In order to analyze the vortex shedding characteristics, the directional swirling strength parameter was computed in addition to the vorticity as the former enables vortex identification. In the first bifurcation region, the sphere remained stationary and the wake was characterized by a train of hairpin vortices exhibiting symmetry in the vertical plane similar to visualization results obtained for stationary spheres. The second bifurcation region was characterized by large amplitude periodic oscillations transverse to the flow. Phase-averaged results for the swirling strength showed that although the shedding mechanism was identical for several reduced velocities, the phase at which vortices were shed increased with V R . Spatiotemporal swirling strength characteristics revealed counter-rotating vortex pairs in the far wake of the sphere. In addition to primary vortex pairs, secondary weaker vortical structures were also observed. In the third bifurcation region, nonstationary vortex shedding was characterized by high frequencies associated with shear layer instabilities causing pinch-off of small scale vortices. In addition, large scale undulations of the wake associated with the sphere motion were observed.

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The main purpose of this benchmark paper is to study and compare point and spatial neutronic approaches used to calculate ULOF and UTOP transients in sodium cooled fast reactors. A second objective is to compare deterministic and Monte Carlo calculations with two different calculation codes. The first one is based on a deterministic (discrete ordinate S N) approach, using tabulated self-shielded cross sections, where the core reactivity and the power shape distribution are evaluated at each time step of the transient calculation. The second model relies on the Transient Fission Matrix (TFM) approach, condensing the response of a Monte Carlo neutronic code in time dependent Green functions characterizing the local transport in the reactor. This second approach allows a fast estimation of the reactivity and of the flux redistribution in the system during the transient with a precision closed to that of the Monte Carlo code. Both models have been coupled to the thermalhydraulics and applied on an ASTRID representative assembly. This application case is supposed to be sensitive to power redistributions. A second comparison between spatial kinetics and point kinetics calculations has been led to study this point. Finally we obtain a good agreement between spatial and point kinetics on ULOF and UTOP calculations, while some discrepancies are observed between the TFM and the S N approaches on the power level stabilization, due to difference on the feedback estimation in both models.