Tuesday, May 12, 2015

Waterjetting 33a - Waterjet structure and its effect

When a waterjet first comes out of an orifice the flow (providing the upstream conditions are properly aligned) will form a cylindrical stream, with the jet pressure across that stream relatively constant. Within about an inch, depending on the flow conditions, ripples start to appear on that smooth cylinder (Rayleigh waves) and these grow and gradually disrupt the jet as it travels further from the nozzle.

Looking at the jet under ordinary light, this makes the jet appear to grow larger, and potentially more powerful as it moves away from the nozzle. However, when the jet is back-lit, or when a pressure profile is taken of the jet at different distances from the nozzle, a different picture emerges.

Figure 1. Pressure profiles across a 6,000 psi jet at 6-inch intervals from the nozzle

What this shows is that the initial even pressure distribution across the stream gradually transforms into a curve very similar to that described as “Gaussian” in mathematical literature. These pressure profiles are generally carried out only at lower jet pressures, because of the way that we have to protect the pressure transducer from the direct jet impact.

Figure 2. Instrumentation for measuring pressure profile.

At higher jet pressures the erosion from the jet on the protective steel cap very rapidly wears the entry hole into the pressure transducer channel, and makes the readings less reliable as the channel shape begins to change. For this reason we have relied on either physical damage to the target, or photographs of the jet, to see what the jet structure had transitioned into, with backlit photographs giving the better set of information.

This damage, interestingly, comes more from droplet impact caused by the breakup of the outside of the jet, which can be easier shown through a front-lit photo.

Figure 3. Microsecond exposure of an ultra-high pressure jet, the orifice is at the 10-inch mark on the rule.

At the nozzle the jet emerges as a smooth cylinder, but small ripples develop on the edge of the jet as it flows. There has been a considerable amount of study of this, and the development of the waves relates to the surface tension in the liquid, and the relative densities of the two fluids (in the case the water in the jet, and the air into which it is injected). At very low pressures (such as water from a tap) surface tension effects dominate and the jet stream is pulled into droplets over a relatively short distance.

Figure 4. Breakup of a low pressure jet into droplets. (Taken from an MIT lecture on fluid jets)

Incidentally, for my male readers, this is why you should stand within 6-inches of the wall of a urinal. (Read to the bottom of the post).

As the jet velocity increases small surface waves develop on the outside of the jet. They look similar to these stationary capillary waves, except that they grow in magnitude as they move away from the nozzle, and were first discussed by Rayleigh, for whom they are named.

Figure 5. Wave generation on the surface of a jet (the grid has a 1-mm spacing for scale). (From an MIT lecture on fluid jets ).

At the jet velocity grows higher – as can be seen by looking closely at the jet in Figure 3, these waves grow large enough to be pulled from the surface of the jet, and, being slowed by the surrounding air, appear to be pulled backwards as the jet flows.

The wave deceleration and break up into a fine mist reduces the size of the central core of the jet, and also the drag reduces the velocity of the outer layers of the jet, giving the pressure profile that is shown in Figure 1.

When cutting with a plain jet (i.e. without polymers or abrasive) a slight deceleration over the edge of the jet is helpful in eroding and removing the material in the target. When a jet impacts close to the nozzle, and the pressure across the target is relatively uniform (as it is 15-cm or 6-inches from the nozzle in Figure 1) then there is no pressure difference between the water that penetrates into the cracks on the target within that central zone. Because of this, while the material may compress, there isn’t enough difference in the forces on the material to cause it to be removed, and the central stub of material will therefore remain in place.

However, in the zone where the pressure differential has developed, on the sides of the jet, there will be a large difference in the pressures in the fluid within the cracks on the target as one moves away from the jet center. This will cause significant material removal. I wrote about this in an earlier post and used the same illustration of what we called a butterfly – which is the erosion pattern that a 10,000 psi jet makes on an aluminum target, where the pressure profile of the jet is still relatively even, close to the nozzle.

Figure 6. Erosion pattern around the point of impact of a waterjet on an aluminum target. The erosion occurs on the outside of the impacting jet, while the central core, being under an even jet pressure, is not removed.

Having digressed a little from the initial topic to explain what happens with the water on the outside of the jet as it is peeled away from the central core, I will return to the topic in the next post, because it is the tapering in of the cut, as the central higher-pressure cutting jet moves away from the nozzle, that is the subject of this short sequence, and so we will return to that topic, next time.


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