Hi Vince, That info must have taken some time to find, so thanks for the effort. Iv'e just looked at both of these Formuli and they are very complex, in their structure to say the least.

Perhaps Iv'e aimed my question too high, though in simplicity, as you say, most obstructions under water, will cause flow variations, which in turn cause current eddies.

It's what happens when these eddies collide, that is of more significance to the angler. Particularly how they bear out further downstream or out of flow.

Thanks again Vince, some very good information, Now I just want to find out more info.

Best Regards

Stuart

Stuart

Chaotic flow simulation makes those equations look like noddy goes to toyland and is way beyond me, although I know some tefal heads that do it for a living. Stolen from wikipedia - Werner Heisenberg was asked what he would ask God, given the opportunity. His reply was: "When I meet God, I am going to ask him two questions: Why relativity? And why turbulence? I really believe he will have an answer for the first."

I think that the intuitive answer you are looking for is that turbulent flow is very energetic and will stir up the bottom releasing food items. Also the compressed flow will channel the food coming in from upstream into a constricted flow.

The output flow from the colliding eddies on a given day are predictable given the rocks and water flow do not change.

The sum of chaotic flow of the 2 eddies will tend to laminar flow and this stirred up conveyer belt of food will line up in the same place and deposit the food items along a line with the depth only varying by the food objects mass. I suppose this is why the fish tend to feed along a line on the output of a weir. The line will change from day to day depending on water conditions.

regards

Vince

Hi Vince, I have to say, I printed and read both the Equation of State and Bernouli's Equation last night. Bernouli's Equation is more in line with measuring flow, which is what I'm trying to find out more about.

Trying to imagine how the sum works is sometimes complex, but given time and prompting I think I'll come to terms with it.

You have a very good understanding of flow and physics. What courses did you do and where? A big Thank You for the info you've sent in and your patience.

Best Regards

Stuart

Stuart

I trained as an aerosystems engineer and computational fluid dynamics was part of the course. I have not often used it in anger as i have tended more towards avionics and the later management (less hard sums).

I included the equation of state to give you a feel for the stillwater element of your question. However, I would suggest that instead of getting too wrapped around the maths, focus on the fact that since density is a constant for a low speed problem, the only variables are pressure and velocity.

It has been an interesting exercise for me as I had not thought too deeply about it. As I am on my first river season, there have been many other things to consider. I fished the Lynn earleir this year and took a fish in front of a boulder. As the fly reached the boulder, it seemed to pause and the fish took it. It seemed like slow motion but now I realise that it was this relationship described above acting in the real world. When I answered the weirpoint above, it dawned on me why the feed line in some pools extends so far behind the turbulent water, in some cases I have consistently taken fish 10-20 metres behind a weir.

regards

Vince

Hi Vince, With the grounding you've had, you are a step ahead of most, because you understand how feed and seam lines run. Most people spend years trying to learn, how River Dynamics effect a particular run or pool.

I stepped away from Rivers some years ago, because of life changes and now where we live. I dare say I may try running water again, not being too far from the Dee at Llangollen.

It's been good to read and try to get my head around the calculations, but very provoking.

If I hear of any difficult runs or eddies I'll PM you Ok.

Best Regards

Stuart

I'm not sure that that what you see tells the whole story - there is a 60 degree bend on our beat and the serious flow passes through about 1-1.5 metres of a 10 metre possible bed. I regularly see six-inchers jumping about a foot out of the water to catch some fly - youthful exuberance? maybe - but there isn't any escape left or right, if they want to get out of that flow then its either upstream or downstream over a couple of rocks. Either they are highly mobile or they must be resting under some undercut rock (river bed is basically rock) or something else I can't see - I frankly fail to see otherwise why the foodstuffs in that particular concentration should warrant competition (where they are 6-inchers there are also footers+) and the energy required to keep in position in that kind of flow (concentrated 2-4 (cubic metres per second) without there being some resting place in the deep.

Next year I'm going to try some dapping on that bend (I have found it impossible to place a fly convincingly on that part and somehow wets/nymphs don't do it for me) - anyone got a loop opti peak 3-weight they want to sell me come spring? Doesn't have to be pristine.

@vgb - Are you saying that the turbulence created by the weir doesn't "eject" food until 10-20 metres later?

DB

If there are sub-surface rocks or sharp features in shallow water then there will be resting places. When I mentioned the Lynn, the fly came to a near stop just in front of the rock as the fish took. Similar to below centre but the rock was mostly submenrged and water was flowing high either side of it.




If you consider the tube in the bernoulli picture and sliced it in half, it paints the picture. As the flow hits the pinch point, the static pressure increases due to the flow constriction and the velocity drops. The velocity drop will incrase with the size of the constriction. It seems to me that this gives the fish time to see the food and take it.

When I mentioned the weir, I had 2 particular pools in mind, this is one of them:



Both of these pool run into a straight stretch of river, with a bubble run running along the bank. While I have been fishing with my "L" plates on, I have started casting early, to check my set up and practice. I found that I was into decent fish in a line with the bubble run whilst being set back from the more turbulent water. I think that the food line is being concentrated by the constriction and ejected at a comparatively high velocity along the flowstream. It appears that the fish are lining up with the flowstream but picking it off as the velocity slows (less work for them) but while the food concentration is still high enough to make it worthwhile. After significant rain, I have picked up fish further away than during the dog days.

It is all conjecture on my part but it seems to fit what I am seeing.

regards

Vince

Not a direct answer to the question but if you haven't seen this you may find it useful.

http://www.midcurrent.com/articles/b...ing_water.aspx

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