# Reducing pressure difference

1. Feb 15, 2010

### appelboom

hi

I have a plate moving back and forth in a closed space. The shape of the plate is a ring, it is moving around an axis up and down.

The pressure difference between the two sides is too big when the plate is moving, and hence the force on the plate. Now I want to make some holes in the plate in order to reduce the pressure difference between the 2 sides.
Do i only have to calculate with the reduced surface (plate minus holes) or do i have to take other things into the calculations. I think the dimensions of the holes are important too, one big hole will be different as several small holes. Any suggestion how to simulate this before drilling holes?

thanks!

2. Feb 15, 2010

### DaveC426913

Viscosity?
If viscosity is low it shouldn't be an issue. Note that it is the area of the holes that you are interested in, and area is proportional to the square of the hole diameter (but you know this).

That being said, several small holes will distribute the forces better than one (or few) large holes.

3. Feb 15, 2010

### appelboom

don't think that viscosity will play a roll, it is air.
But i will look at it closer

4. Feb 15, 2010

### DaveC426913

Air??

Then don't worry abvout the shape, size or number of holes. The only thing that will matter is the total area of the holes (and ensuring that they're symmetrically placed).

5. Feb 16, 2010

### appelboom

I have done some calculations and the air flow would be turbulent through the hole(s)

The situation is:
plate moving at 40 ms over a distance of 9 mm.The difference in volume by the movement is 42,5 cm³. Without holes the difference in pressure would be appr 3 bar.
I'm trying to look at the pressure drop if i use holes with a total surface of 500 mm²
(the total surface of the plate is appr 4500 mm²)

Is the turbulance playing a role?

6. Feb 16, 2010

### Naty1

Can you clarify post #5....

First you say there is turbulance, then ask if it has a role?? What do you conclude from your own efforts??

Turbulance may be playing a role in actual measurements; we have no idea what you included in your calculations. All this is dependent on the precision you seek.

7. Feb 16, 2010

### appelboom

I just calculated the pressure difference without the holes. It is around 3 bars, easy to calculate.
I am just wondering how to calculate the pressure difference between the 2 sides when i put holes into the plate. I consider 1 big hole to make the calculations easier.
When i calculate the reynolds number for the air moving through the hole i see it is a turbulent flow through the hole when the plate is moving.

by the way; thanks for the help!