  Using event-by-event viscous fluid dynamics to evolve fluctuating initial
density profiles from the Monte-Carlo Glauber model for U+U collisions, we
report a "knee"-like structure in the elliptic flow as a function of collision
centrality, located around the 0.5% most central collisions as measured by the
final charged multiplicity. This knee is due to the preferential selection of
tip-on-tip collision geometries by a high-multiplicity trigger. Such a knee
structure is not seen in the STAR data. This rules out the two-component
MC-Glauber model for initial energy and entropy production. Hence an enrichment
of tip-tip configurations by triggering solely on high-multiplicity in the U+U
collisions does not work. On the other hand, by using the Zero Degree
Calorimeters (ZDCs) coupled with event-shape engineering such a selection is
possible. We identify the selection purity of body-body and tip-tip events in
full-overlap U+U collisions. By additionally constraining the asymmetry of the
ZDC signals we can further increase the probability of selecting tip-tip events
in U+U collisions.
