The particles in the flow won't hit the nozzle much at all, since the average flow is tangential to the nozzle walls. Even if the flow is highly turbulent insid

The particles in the flow won't hit the nozzle much at all, since the average flow is tangential to the nozzle walls. Even if the flow is highly turbulent inside the nozzle, there is a boundary layer where the velocity becomes zero. However, when the flow impacts a solid object, the particles can't "keep up" with the fluid splashing away due to their inertia, so they hit the solid object at high speed. Particles in turbulent flow are characterised by (among others) the Stokes number [1], which says something about how well the particles follow the flow. This depends e.g. on the particle size, roughness, and the density difference between solid and fluid. I imagine they have tuned the Stokes number to an optimal tradeoff between cutting speed and nozzle/fluid line wear. [1] https://en.wikipedia.org/wiki/Stokes_number https://en.wikipedia.org/wiki/Stokes_number