  As a generalization of the mass-flux based classical stream-tube, the concept
of momentum and energy transport tubes is discussed as a flow visualization
tool. These transport tubes have the property, respectively, that no fluxes of
momentum or energy exist over their respective tube mantles. As an example
application using data from large-eddy simulation, such tubes are visualized
for the mean-flow structure of turbulent flow in large wind farms, in fully
developed wind-turbine-array boundary layers. The three-dimensional
organization of energy transport tubes changes considerably when turbine
spacings are varied, enabling the visualization of the path taken by the
kinetic energy flux that is ultimately available at any given turbine within
the array.
