Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: nudephil on 07/08/2020 16:56:11

Title: Does water fall faster through holes at the bottom of a trough than the side?
Post by: nudephil on 07/08/2020 16:56:11
Here's a scenario and a question sent in by Alan:

Picture a concrete trough 40 m L x 80 cm W x 50 cm H, which catches rainwater falling onto the roof of a six-storey building in Johannesburg.

The trough has 30 drain holes approximately 70 cm in diameter drilled into its side where it meets the bottom, spaced evenly along its length. Rainwater drains through these side holes and is routed through drainpipes to the outside of the building.

In addition to the side holes, the trough has 4 other open holes each of 100 mm diameter drilled through its concrete base, 13.8 m in from each end of the trough with a distance of 3 m between holes 1 & 2, 6.4 m between 2 & 3, and 3 m between 3 & 4. This allows water to fall through them at a height of 6 storeys to an open terrace below, and because the water hits the same areas each time, it damages by force the surface of the open terrace.

My question is this: will gravity pull the rainwater through the four open holes in the base of the trough at a faster and more forceful rate than through the side drain holes in the trough?


Can anyone answer?
Title: Re: Does water fall faster through holes at the bottom of a trough than the side?
Post by: alancalverd on 07/08/2020 20:06:44
It's a magic trough, only 50 cm high but with 70 cm holes in the side.

Of course it works because as any fule kno, gravity works upwards in  the southern hemisphere.
Title: Re: Does water fall faster through holes at the bottom of a trough than the side?
Post by: Bored chemist on 08/08/2020 22:36:33
It's a magic trough, only 50 cm high but with 70 cm holes in the side.

Of course it works because as any fule kno, gravity works upwards in  the southern hemisphere.
Ignore Alan C.
He's in a bad mood, possibly  because of this.
https://www.thenakedscientists.com/forum/index.php?topic=80136.0

I guess the holes are 70mm
But it won't matter much.
Water falling 6 storeys will mess up what it lands on. However, if it runs down a pipe  there will be friction against the sides and that will slow it down.
Title: Re: Does water fall faster through holes at the bottom of a trough than the side?
Post by: Petrochemicals on 09/08/2020 09:30:24
Here's a scenario and a question sent in by Alan:

Picture a concrete trough 40m L x 80cm W x 50cm H, which catches rainwater falling onto the roof of a six-storey building in Johannesburg.

The trough has 30 drain holes approximately 70cm in diameter drilled into its side where it meets the bottom, spaced evenly along its length.Rainwater drains through these side holes and is routed through drainpipes to the outside of the building.

In addition to the side holes, the trough has 4 other open holes each of 100mm diameter drilled through its concrete base, 13.8cm in from each end of the trough with a distance of 3cm between holes 1 & 2, 6.4cm between 2 & 3, and 3cm between 3 & 4. This allows water to fall through them at a height of 6 storeys to an open terrace below, and because the water hits the same areas each time, it damages by force the surface of the open terrace.

My question is this: will gravity pull the rainwater through the four open holes in the base of the trough at a faster and more forceful rate than through the side drain holes in the trough?


Can anyone answer?
Not if it is open to the same atmospheric conditions of pressure, the water will be under the same infuence. The water from the sides will fall at the rate that is governed by the ammount of water above the holes height, same for the bottom only the bottom will have more water above it.
Title: Re: Does water fall faster through holes at the bottom of a trough than the side?
Post by: Bored chemist on 09/08/2020 10:02:18
Not if it is open to the same atmospheric conditions of pressure,

So, it's the same...
the bottom will have more water above it.
but not the same.

Thanks for the clarification.
Title: Re: Does water fall faster through holes at the bottom of a trough than the side?
Post by: alancalverd on 09/08/2020 13:26:43
Assuming that the holes are actually 70 and 100 mm diameter, and given that the "side" holes are very near the bottom of the trough, the static pressure if the trough were full, would be near enough the same at both exit locations and not more than 50 cm water  so the flow would be reasonably laminar and thus proportional to the area of the holes. Interestingly this works out very close to a ratio of 2:1 per hole between "bottom" and "side", but there being only 4 bottom holes compared with 30 side holes, the total flow ratio would be 8:30.

However as we are dealing with rainwater, the trough will not start off full, so although the bottom holes will always work at maximum flow rate, the side holes will only begin to function when the rate of arrival of rainwater exceeds the rate of loss through the bottom holes.   

It is odd to design a distribution system (pipes in the side holes) that only works in a torrential downpour. My guess is that the bottom holes were intended to be fitted with vertical pipes say 40 cm high to act as overflow diverters. A steady stream of monsoon water 10 cm wide and falling 6 storeys sounds pretty lethal, so I would have fitted a fan tray below each overflow hole to break the flow into harmless droplets. Architects, engineers and builders are all fallible, but it is unusual to have all three contractors take their eye of the ball - which is why we also employ surveyors.
Title: Re: Does water fall faster through holes at the bottom of a trough than the side?
Post by: nudephil on 11/08/2020 10:51:23
Alan's messaged me to correct some of the measurements. The measurements referring to the four holes in the base - i.e. the distance in from each end and the distance between each hole - should read m rather than cm.

I've changed the original post accordingly.