  In three dimensional turbulence there is on average a cascade of kinetic
energy from the largest to the smallest scales of the flow. While the dominant
idea is that the cascade occurs through the physical process of vortex
stretching, evidence for this is debated. In the framework of the
Karman-Howarth equation for the two point turbulent kinetic energy, we derive a
new result for the average flux of kinetic energy between two points in the
flow that reveals the role of vortex stretching. However, the result shows that
vortex stretching is in fact not the main contributor to the average energy
cascade; the main contributor is the self-amplification of the strain-rate
field. We emphasize the need to correctly distinguish and not conflate the
roles of vortex stretching and strain-self amplification in order to correctly
understand the physics of the cascade, and also resolve a paradox regarding the
differing role of vortex stretching on the mechanisms of the energy cascade and
energy dissipation rate. Direct numerical simulations are used to confirm the
results, as well as provide further results and insights on vortex stretching
and strain-self amplification at different scales in the flow. Interestingly,
the results imply that while vortex stretching plays a sub-leading role in the
average cascade, it may play a leading order role during large fluctuations of
the energy cascade about its average behavior.
