Why does a Pressure Washer clean better than a Garden Hose?

[JoeyF]

TL;DR

If you think about a pressure washer and its ability to wash debris from a surface much more effectively than a garden hose, is this explained by it's increased fluid velocity, pressure, or both? Due to its increased velocity and pressure, can we assume that the forces acting on debris are higher?

In Bernoulli's equation, you have static, dynamic, and hydrostatic pressures equal to a constant. In one sense, I'm wondering if the dynamic part of the equation explains this since -> Pressure = (1/2*rho*velocity^2) and pressure can be expressed as a force / unit area, therefore, an increase in velocity translates to an increase in Force.

There is also the drag equation:

An increase in velocity would also be an increase in force. It's the same exponential relationship, so I'm wondering if it's synonymous.

Is it safe to say an increase in velocity results in an increase in force on an object in that streamline? Can somebody help answer/explain this? Thanks!

 

[Drakkith]

Summary:: If you think about a pressure washer and its ability to wash debris from a surface much more effectively than a garden hose, is this explained by it's increased fluid velocity, pressure, or both? Due to its increased velocity and pressure, can we assume that the forces acting on debris are higher?

Yes to everything. The velocity is higher, therefore the force and pressure on the incident surface are both higher.

Is it safe to say an increase in velocity results in an increase in force on an object in that streamline? Can somebody help answer/explain this?

Yes, that's correct. This is why it takes more power to move a ship faster through the water or a plane faster through the air.

 

[Baluncore]

Summary:: If you think about a pressure washer and its ability to wash debris from a surface much more effectively than a garden hose, is this explained by it's increased fluid velocity, pressure, or both? Due to its increased velocity and pressure, can we assume that the forces acting on debris are higher?

Is it safe to say an increase in velocity results in an increase in force on an object in that streamline? Can somebody help answer/explain this?

Cleaning concrete with water requires the water open and enter the gap between the dirt and the concrete. The dirt may be held in place by atmospheric pressure and some chemical bonds. Materials such as oil, bitumen or wax, may be quite difficult to separate from what was dry concrete. When the only solvent available is water, the pressure must break the bond between the dirt and the originally dry concrete.

What water velocity is needed for the ram-pressure, ½·ρ·v², to exceed atmospheric pressure?

 

[JoeyF]

Cleaning concrete with water requires the water open and enter the gap between the dirt and the concrete. The dirt may be held in place by atmospheric pressure and some chemical bonds. Materials such as oil, bitumen or wax, may be quite difficult to separate from what was dry concrete. When the only solvent available is water, the pressure must break the bond between the dirt and the originally dry concrete.

What water velocity is needed for the ram-pressure, ½·ρ·v², to exceed atmospheric pressure?

14.24 m/s

 

[Baluncore]

I think 14.142 m/s would qualify as “slower than a pressure washer”, and “faster than a garden hose”.

The geometry of the contact and the chemical bonds will make a difference, but so will disintegration of a particulate material, or the chemical solution of a cement or glue in the water.