The Airbus A340-600 (the long one) had problems with gravity transfer from the tail (trim) tank under certain circumstances. As I recall, the two issues were the sharp corners of the trim tank outlet causing cavitation and the fuel in the pipe between the wing and tail degassing when the internal pressure went from sea level (valves at each end shut) to cruise altitude (valves open).
A full-size rig was built at Filton which replicated the connecting pipe in transparent material. The rig could be adjusted for angle and internal 'altitude'.
The waves that went down the length of the pipe would have been great fun for an inch high surfer who didn't mind JetA1!