questions about flow

six.line · 04/17/2006, 09:42 AM

I haven't had fluid dynamics, so if you're answer contains fluid dynamics principles, remember they'll be lost on me.

Target turnover rate, 10xtotal, right?

Is it possible to have flow without that turnover rate? Say for example you have a powerhead with a teeny output hose, say a diameter of a centimeter or a half an inch. To move a certain amount of water, it's going to move a smaller water column a lot faster, especially compared with a powerhead with a larger output hose diameter, say twice or even three times that.

So what do we want? A ton of water moving slowly, or a little water moving quickly?

And what if your live rock gets in the way?

TekCat · 04/17/2006, 09:58 AM

Turnover rate is generally referred to volume of water (which is typically powerhead's rating), not the speed of the stream.

The important factors in waterflow flow are in order (IMO):
1. coverage
2. how turbulent it is
3. flow rate

If you create stronger stream of water, (like you said) using smaller diameter output hose; what you make is a stong localized laminar flow. This is highly undesirable. Because this way you will decrease coverage (see dead spots) of the stream and make it laminar (corals don't like that). Interesting thing is that your powerhead will be moving the same amount of water independent of type of output you rig. So, save yourself some time and spend it better by tuning flow so it is turbulent and doesn't leave any corner of your tank untouched.

hth

six.line · 04/17/2006, 10:17 AM

That does help, thanks. It also introduces a new question.

Is it better to use multiple lower rated powerheads, or one or two high rated power heads?

bertoni · 04/17/2006, 12:16 PM

I suspect that over a fairly wide range, it doesn't much matter. At the extreme of one powerhead for a tank, you might not be able to distribute the flow all that well, leaving dead spots. At some number of powerheads, that'll cease to be an issue. I think a lot depends on the rock structure, if present.

04/17/2006, 09:42 AM	#1
six.line Registered Member Join Date: Dec 2005 Location: Logan, UT Posts: 775	questions about flow I haven't had fluid dynamics, so if you're answer contains fluid dynamics principles, remember they'll be lost on me. Target turnover rate, 10xtotal, right? Is it possible to have flow without that turnover rate? Say for example you have a powerhead with a teeny output hose, say a diameter of a centimeter or a half an inch. To move a certain amount of water, it's going to move a smaller water column a lot faster, especially compared with a powerhead with a larger output hose diameter, say twice or even three times that. So what do we want? A ton of water moving slowly, or a little water moving quickly? And what if your live rock gets in the way? __________________ This post approved by Mrs. Sixline. assign <= six.line(#FFFFFF && string("nerdy")) Current Tank Info: 30 gallon with 10 gallon sump

04/17/2006, 10:17 AM	#3
six.line Registered Member Join Date: Dec 2005 Location: Logan, UT Posts: 775	That does help, thanks. It also introduces a new question. Is it better to use multiple lower rated powerheads, or one or two high rated power heads? __________________ This post approved by Mrs. Sixline. assign <= six.line(#FFFFFF && string("nerdy")) Current Tank Info: 30 gallon with 10 gallon sump

04/17/2006, 12:16 PM	#4
bertoni RC Mod Join Date: Mar 2002 Location: Mountain View, CA, USA Posts: 88,616	I suspect that over a fairly wide range, it doesn't much matter. At the extreme of one powerhead for a tank, you might not be able to distribute the flow all that well, leaving dead spots. At some number of powerheads, that'll cease to be an issue. I think a lot depends on the rock structure, if present. __________________ Jonathan Bertoni

04/17/2006, 09:58 AM	#2
TekCat Registered Member Join Date: Sep 2005 Location: MN Posts: 3,130	Turnover rate is generally referred to volume of water (which is typically powerhead's rating), not the speed of the stream. The important factors in waterflow flow are in order (IMO): 1. coverage 2. how turbulent it is 3. flow rate If you create stronger stream of water, (like you said) using smaller diameter output hose; what you make is a stong localized laminar flow. This is highly undesirable. Because this way you will decrease coverage (see dead spots) of the stream and make it laminar (corals don't like that). Interesting thing is that your powerhead will be moving the same amount of water independent of type of output you rig. So, save yourself some time and spend it better by tuning flow so it is turbulent and doesn't leave any corner of your tank untouched. hth