More About Blind Holes – Some Surprises!

I’ve always known that cleaning inside blind holes was a difficult task.  In a previous blog, I demonstrated using video just how difficult it is to get liquids and/or air in and out of a blind hole.  My plan was to approach the problem logically (largely based on intuition), and offer readers some solutions to cleaning inside blind holes.  My basic thought process was along the following lines – –

The challenge in creating an exchange of liquid or gas within a blind hole is that whatever is going in and whatever is coming out have to share the same opening but moving in opposite directions.  The following illustration shows how a stream of liquid directed parallel to the orientation of the blind hole results in pressure that is (for practical purposes) evenly distributed across its opening.

Illustration showing the effect of a wide stream on penetration of a blind hole.
A stream of liquid directed parallel to the orientation of a blind hole results in more or less positive pressure distributed across its opening. There is no mechanism to encourage an exchange other than at the very mouth of the blind hole.

Neither increasing pressure nor changing the angle of attack of the liquid stream will have any effect on the above scenario.  The pressure across the opening will be positive.

The next logical though (for me) would be to devise a means to provide a pressure differential across the opening of the blind hole.  Perhaps this could be accomplished using a very fine or flat spray as shown in the following illustration?

Illustration showing the effect of a narrow, directed stream on the cleaning of a blind hole.
By narrowing and/or directing a stream of liquid it may be possible to cause the turbulent area to move deeper into the blind hole to create an exchange of air or soil.

In fact, a benefit of a narrow, off center stream can be demonstrated but the effect is not as pronounced as one might expect.  The depth of penetration decreases rapidly as the cross section of the blind hole is reduced and, as in the scenario of a wider stream as discussed above, increasing pressure and changing the angle of of incidence has surprisingly little beneficial effect.  It appears that the confinement of the cross section of the blind hole provides a dampening effect that is very difficult (or impossible) to overcome.  In the next blog, we’ll look at some video demonstrating the above and other effects of water jets on blind holes.  Warning, you may be surprised at what you see!

–  FJF  –

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