Marine Implications of Molecular Hydrodynamics: Peter McArthur

August 30th, 2014 by JCB
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Despite common perceptions,  water is one of the most complex and poorly understood substances we know. It’s chemical structure and strange bonding characteristics give it peculiar mechanical qualities. In this short extract, ground-breaking research – originally intended to explain water’s biological importance – is applied to the maritime environment to explain some well-known phenomena.

Water’s structure and nature

In their article ‘Water: The Strangest Liquid’ (Cartlidge, 2010) two researchers, Anders Nilsson of Stanford University and Lars Pettersson of Stockholm University, explain that under ambient conditions ‘water as a (naturally occurring) liquid can form two types of structure, one tetrahedral and the other disordered’ (Fig 1)

They explain that both forms co-exist under ambient conditions and that when molecularly stressed, that is, undergoing only a slight increase or decrease in energy profile, one form can quickly convert to the other. These theories are supported by the work of Professor Martin Chaplin of Southbank University, London, whose physical and chemical domain of water diagram (Fig 2) underpins much of what mariners instinctively understand about water, though for present purposes we focus on the domain where thermal effects are less significant and the kinetic properties become highly relevant. Professor Chaplin noted that the two water-forms have a distinct nature, yet can simultaneously co-exist under normal conditions and have a propensity to readily convert from one form to the other – so that water can exhibit some rather ‘peculiar’ characteristics:

• It can exist in the mostly tetrahedral state – that is, ice
• Water may exhibit the viscous, non-Newtonian, qualities of a gel or behave in a highly mobile Newtonian fashion
• In its tetrahedral form, water behaves like a gel and is highly resistant to transient motion. In its disordered form, it becomes highly mobile and much less resistant to motion
• The energy values required for water-form conversion to take place are fairly modest
• The energy source to bring about conversion may be natural or artificial, thermal or mechanical

Researching various ship handling issues, the author realised that the molecular peculiarities of water held important implications. In order to make sense of them it is necessary to accept that water is neither stable nor uniform in nature. One property of water does remain consistent – it must comply with the second law of  thermodynamics so that, for a structural or chemical change to take place, an energy transfer must occur.

 

What happens during that energy transfer, when water-form conversion takes place, is of particular interest to the mariner as it offers a few solutions and, by its understanding, raises a number of interesting propositions.

 

Some maritime implications

The ship entering a lock   Experienced ship-handlers note that a high-blockage factor vessel entering a lock, reduces speed gradually – consistent with an apparent pressure build-up ahead of the vessel as the water has difficulty escaping. Eventually, without additional power being applied, the ship almost stops. As more engine power is applied, the vessel begins to move ahead, but very slowly. Then suddenly, and without warning, it surges forward. Contemporary teaching explains that once the water starts to move around the ship, the momentum of the water in motion will cause the whole body of water to be forced astern (around the ship) creating what is described as a slight low pressure ahead of the ship – and the vessel moves into it. Whilst this may explain the observed movement of the water, it does not adequately explain why water, initially so resistant to the ship’s advance, suddenly becomes highly mobile and starts moving – when it previously did not.

The theory describing conversion between the different water forms indicates that once sufficient energy is input and the water energy profile is raised, the tetrahedral form breaks down and conversion to the disordered state occurs very rapidly, with the water becoming significantly more mobile and correspondingly less gelatinous – so reducing resistance to the vessel’s advance. Thereafter, the additional power required to move the ship against the, more mobile, disordered water-form is suddenly no longer needed.

 

The only tell-tale sign indicating that water-form conversion has taken place is the sudden ‘forward surge’. In practice, usually accompanied by a realisation that the vessel needs to be slowed down – very quickly.

Molecular dynamic of breaking sea waves

Stating the obvious, sea-waves possess energy and can transfer that energy when they strike a vessel or some other structure.

Research shows that when a wave begins to break down, the crest can increase velocity by as much as 50% of the wave’s speed and accelerate away from the main structure of the wave – becoming confused and turbulent as it does so.

Consider, using water-form conversion theory, what is happening within the wave structure as its energy profile changes (Fig 3). What should become clear is that it is the dynamic which causes waves to break down.

Surface water tension – strongest when the molecular structure is largely tetrahedral – normally binds the surface together, giving a glassy appearance. Energy input from wind, along with gravitational potential energy, results in conversion at the wave crest, so the peak becomes more mobile, causing it to accelerate and break-away from the wave’s main body. When unsupported, gravity takes over and the crest tumbles.This wave-breaking dynamic also explains why molecules at the water /air interface might gain energy locally – allowing small ‘packets’ to accelerate, causing streaks that we might see as foam carried aloft as spray.

The ‘Great Wave of Translation’ mystery!

In August 1834, John Scott Russell, undertaking research into more efficient ship-hull forms, used two horses to tow a barge along a narrow stretch of the Edinburgh-Glasgow canal. During one of the experiments the rope snapped and barge quickly came to a halt – but not the wave created by the boat (Fig 4).

He noted that the wave “… in a state of violent agitation … rolled forward with great velocity … continued its course along the channel apparently without change of form or diminution or speed (and) gradually diminished after a chase of one or two miles …”

 

Scott Russell, an engineer, recognised that he was seeing something unusual, for which he could not provide an adequate explanation. The wave, subject to various frictions and resistance, did not die down as might be expected. He concluded that something inexplicable was happening to keep the wave energised and in motion – it seemed to have its own internal power source that allowed it to keep moving.

Analysing Scott Russell’s description of his ‘Great Wave of Translation’ and applying what we now know of water’s molecular tendencies, a number of factors become apparent, permitting us to discover what was happening with the wave. Key to that understanding is the knowledge that once water-form conversion (to the disordered state) has taken place, the re-conversion process is relatively lengthy. We note:

1) The water ahead of the boat was trapped so that energy being transferred into the water did not dissipate.

2) With time, the localised energy profile increased and conversion from the tetrahedral to the disordered state took place.

3) Scott Russell’s description of the water becoming “… confused … and (in) a state of violent agitation” characteristically portrays conversion to the disordered state. Thus, the wave had acquired both internal energy and a disordered, more mobile, nature – necessary for its progress to continue unaided (Fig 5).

4) Lacking energy input and influenced by both resistance and the re-conversion process, the excess of internal energy powering the wave’s independent motion eventually depleted and the wave lost form.

Conclusions

This short article offers only a few examples of where water-form conversion has proved significant. New theories describing the multiple states that water can acquire have already helped in explaining particular ship-handling problems. This knowledge has led to a deeper understanding of the effect that water-state conversion has on the sea and has enabled us to address the ‘Great Wave of Translation’ mystery. Knowledge acquired during the research process has led to further discoveries, including:

• The molecular action underpinning water flow in pipelines, eductors and pumping systems
• Ship propulsion dynamics and the persistent nature of a ship’s wake
• The reason behind inconsistent water resistance readings when testing hull forms.
• Why a tsunami exhibits greater dynamism and destructive capacity than current
• theory suggests
• The molecular mechanisms behind ice flow and glacial dynamics

Ongoing research raises other prospects, for example:

• The energy transfers that take place during water-form conversion have implications for our understanding of  weather systems
• Ship-to-ship interaction and hydrodynamic calculations are likely to be influenced by the initial water state and the tendency of water to change form and nature
• Ship model test results may have to be tempered by an understanding that they may not bring about water-form conversion in the same way that a full-size ship might
• The idea of ‘exciting’ water ahead of a moving ship to initiate water-form conversion – so making the water more mobile and less resistant to vessel motion offers the prospect of reduced fuel consumption

Our understanding of water’s basic nature is changing and much of what we take for granted will, of necessity, require re-examination. Whilst this may present some short-term issues, a better understanding of water’s inherent nature and molecular dynamic also raises the possibility of significant benefits.

Acknowledgements 

The author gratefully acknowledges the input of Ron Branscombe for his insightful contribution on the ‘wave of translation mystery’.

Peter McArthur, Master Mariner, MBA, LLB, is a Manchester Ship Canal Pilot and a Chartered Marine Technologist (British Engineering Council) and a Fellow of the IMarEST.