Analytical propeller theory

[Guest]

I am a phd student at Queen's University Belfast and my research topic is, "Mixing of a stratified flow due to the passage of a vessel." I shall explain it briefly:
It involves research on the River Lagan that flows through Belfast. The river has a barrage across the mouth to maintain a constant head of water upstream. It is tidal and the saltwater that flows in over the barrage forms what is known as a stratified flow where the denser saline water subducts underneath the lighter fresh water which flows slowly over the saline water. This stratification leads to all sorts of problems, with the main one being that the oxygen transfer to the high oxygen demanding sediments on the bed is limited across the stratification layer which is very pronounced.
Attempts have been made to use a curtain of bubbles to mix up the stratification at certain points with the hope of adding vital oxygen into the oxygen depleted areas. This however has not been successful and other methods are being researched. My topic is looking at the relative mixing of the stratification achieved by the propeller on a simple taxi boat that operates on a charter basis on the river. It is roughly 30ft in length. As well as carrying out laboratory and field tests, I aim to gain a better understnading of the theory of propellers.
I have looked in many books and websites to try and find the best ways of analysing analytically the hydrodynamics of the propeller action. The problem is that all texts deal with the design of a propeller in terms of the speed of travel required by the vessel, the resistance to the vessel and the efficiency of the propeller. What I want to do is be able to calculate the downstream velocities of water flow at certain points based on the relevant paramenters of the propeller used and the speed of rotation of the propeller. I have heard mention of a thing called efflux velocity. What is it, how can I calculate its magnitude and its location away from the propeller? Can anyone help me with this or direct me to somewhere were I could find out more? What are the relevant characteristics that I will need to know about the propellers I am using to be able to perform analytical anlysis of them?
Also, the tests I have done so far have used a mounted stationary propeller in a hydraulics flume. If the propeller was to be mounted on a rig that moves through the tank , would the travel of the propeller induce a greater inflow of water into the propeller and would this be recipricated by an increase in the downstream velocities of the propeller wash?
If anyone can help me with these quesries I would be very grateful.
Thank you
Paul Wilson-

[SailDesign]

Paul,
most of what you can do theoretically will be totally useless in the Real World (tm) as the boat will be subject to wave action, and that will alter the flow considerably, by introducing a random vertical component.
Your best bet for guaging flow downstream of the prop would be to mount a sensor boom aft of the boat to measure real-time flows in real conditions, and then average the results to allow for waves. You may have to run tests in various conditions 9i.e. at different wave heights) to get across-the-board readings.
Enjoy..
Steve

[Tim B]

Paul,

The question you're trying to solve looks a lot like a rotating propeller in a CFD package. See www.fluent.com for info on Fluent CFD. Failing that, there should be enough information around to put together a routine to calculate velocity vecors aft of a prop.

Good Luck,

Tim B.

[Paul Wilson]

waves are negliable since it is in a menadering river with a width of 30m at most so the fetch distance is not substantial enough to form waves. Also it is mainly the horizontal velocities that I am primarily concerned with and what we have found is that they are substantially large enough to be effected by small ripples on the river surface. In the lab there will be no waves.
we need to calculate the expected flows downstream of the propeller to be able to use Laser Doppler velocimetry, in the lab. conditions.
Thank you anyway, it is something we must consider to be able to answer all questions put to us.
Paul Wilson

[trouty]

interesting study Paul,

Wish I could help you with effluxion calculations - you will find some in civil engineering formulae for bridges and culverts across watercourses - they tend to use the effluxion rate in calculating pipe diameter and numbers for culverts.... don't ask me how - I just wasted half a lifetime working in the civil engineering department of a large railway company is all and that little tidbit springs to mind....(I once made the mistake of askin an engineer what he was labouring over, with his maps and calculator...and effluxion rates was the rather lengthy answer).

As for hypoxic conditions in stratified water - in terms of effecting fish's ability to survive - sometimes it's actually elevated CO2 levels in the water that kills the fish before actual oxygen depletion does.

Bubbling pure oxygen alone thru oxygen depleted water isn't always sufficient to solve the issues unless any built up dissolved CO2 is removed as well.

Bubbled air tends to remove excess dissolved CO2 from water more effectively, than oxygen. Bubbling oxygen thru certainly increases the waters fish varrying capacity, but on it's own is insufficient.

I picked this up from a US company who make oxygen infusers for recirculating baitwells in boats....usually a combination of both bubbled air AND bubbled pure oxygen achieves the best results and I'm inclined to suggest this approach might work best for your river situation.

Curiously enough the Swan River running thru the Capital City of Perth Western Australia has recently been sufffering mass fishkills thru algal blooms A/c excess nutrient/stratification oxygen depletion problems and The 'Swan River Trust' (A state govt body) have been running bubbled air trials for a couple years, with a view to solving the stratification O2 depletion issues, and I'm sure if you contacted them you'd likely get some good realtime data. (Use google search Swan River Trust Western Australia, should turn up their website and a contact email address).

Back to the prop tho...I'm not aware of any 3D prop wash modelling programs..as you say - more often than not designers are more interested in less prop slip and more forward propulsion, than what happens to the water aft of the vessel.

Be keen to hear what you come up with in the end.

Cheers!

[yipster]

making it even more complicated but interesting also is the endflow optimizer (http://pbcf.motech.co.jp/)
( with thanks to Peter T for mentioning it in the dual prop shaft thread )

yipster

[Paul Wilson]

Tim
I tried the fluent website but it didnt turn up anything. I guessed that there might have been some work done on props using cfd and I know that there is at least one person in my department doing just that but I was wondering if anyone had comleted any work on it yet.
Trouty
Thanks very much, very helpful. As far as I am aware, it is simply air that is pumped into the river and not solely oxygen. Even still the results have shown only localised mixing and improved O2 levels. This summer was very warm and dry here and the hydrogen sulphide smells along the river were strong. The algal bloom had also crept down through the estuary and the river looked in a bad state. I will look around about for the stuff for culvert design ad see if there is any correlations.
My first degree was actually in Environmental and Civil Engineering so this topic is right down my street.
Thanks Yall

[tspeer]

I would look at the literature for aircraft wake turbulence and helicopter vortex wakes. I would expect the persistent wake effects to be due to rollup of the shed vortices from the prop.

Many panel codes have the abiltity to calculate wake rollup for arbitrary shaped wakes. Helicopter codes are specifically designed to deal with rotor wakes, so they may be the most directly applicable. Not sure how you'd model the two different fluids with a potential flow code, however.

If you're trying to accelerate mixing with the taxi and it's acceptable to incur some additional drag while doing so, you might be able to use a hydrofoil, either vertical or horizontal, to shed a vortex wake that would help move the fluids around. A lifting horizontal hydrofoil will generate a pair of vortices that will descend under their mutual interference. The boat might actually benefit from the lift and the descending wake could be more effective than a disturbance that is limited to the surface.

[Guest]

nice project