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  1. #21
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    Default Re: A Lesson in Physical Chemistry

    Quote Originally Posted by bedouin View Post
    I hadn't thought of this before - but don't the lock gates at Chichester marina have a curved profile on the marina side so they open without displacing any water - hence won't have this problem?
    The outer gates at Shotley are curved, inner gates flat.

  2. #22
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    Default Re: A Lesson in Physical Chemistry

    Quote Originally Posted by bedouin View Post
    I hadn't thought of this before - but don't the lock gates at Chichester marina have a curved profile on the marina side so they open without displacing any water - hence won't have this problem?
    It's a widely used principle and presumably works well, though there will still be some added load if there is a gradient across the gate. I don't think that they had thought of this when Heybridge was built. Anyway, flat gates are prettier and more fun to operate.
    Far away is near at hand in images of elsewhere

  3. #23
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    Default Re: A Lesson in Physical Chemistry

    Quote Originally Posted by johnalison View Post
    It's a widely used principle and presumably works well, though there will still be some added load if there is a gradient across the gate. I don't think that they had thought of this when Heybridge was built. Anyway, flat gates are prettier and more fun to operate.
    I know there are places that use curved gates even where there isn't a salinity gradient - allows you to open the gates earlier to supplement the sluces.

  4. #24
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    Default Re: A Lesson in Physical Chemistry

    Quote Originally Posted by alandalus11 View Post
    The answer that you are looking for is here. Hope this helps resolve the sticky gates issue.

    σt = (ρ - 1000) kg m−3 when p = -g ρ z(TDS=[(A-B) * 1000]/mL)

    What he said
    "I thought my daffodils were yellow until I met Joe Kennedy."

  5. #25
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    Default Re: A Lesson in Physical Chemistry

    Quote Originally Posted by Major_Clanger View Post
    What he said
    Are you sure?
    Far away is near at hand in images of elsewhere

  6. #26
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    Default Re: A Lesson in Physical Chemistry

    Quote Originally Posted by johnalison View Post
    Are you sure?
    Actually I haven't a clue, but it looked impressive!
    "I thought my daffodils were yellow until I met Joe Kennedy."

  7. #27
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    Sep 2016
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    338

    Default Re: A Lesson in Physical Chemistry

    Interestingly the magnitude of this effect will depend on the vertical position of the sluice. The pressure acting on each side of the gate varies from 0 at the water surface, increasing linearly to a maximum at the base. The rate of increase depends on the water density, which as we know is greater for salt water than fresh. This pressure gradient can be thought of as a right angle triangle, with its apex at the water surface, and the slope of the hypotenuse depending on the density. The total force on each side of the gate is the area of the triangle for that side.

    The reason that sluice position is relevant is that the pressure must be equal at that position, otherwise water would flow through the sluice.

    If the sluice is very close to the water surface (unlikely in a real lock) then the level would be equal on both sides of the gate. Because salt water is denser its pressure gets greater faster as we go down the gate, so the triangle on that side is larger. This results in a force acting to push the gate towards the fresh water side (which would open the gates at Heybridge).

    If on the other hand the sluice is at the bottom of the gate (a more likely scenario) then the effect will be reversed. The pressure at the base must be equal, so the water level on the salty side will be lower. The triangles will have equal base length but the salt one, being shorter, will have a smaller area. Thus there is a force acting to push the gate towards the salt water, holding it closed. When the gate is opened slightly the levels will attempt to equalise, resulting in rapid flow into the lock.

    Of course at some sluice depth between the two extremes the forces will balance, though there will still be flow as the gates are opened. Fresh water will flow into the lock at the top, and salt water will flow out at the bottom.

    The resultant force depends on difference in density, height of water (squared) and width of the gate. For a 4m deep lock, gate width 3.75m this could give a force of 10 kN ~ 1 Tonne which I guess most of us would notice.

  8. #28
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    Default Re: A Lesson in Physical Chemistry

    That's a great explanation. It's good to know that there is at least one intelligent person in East Anglia.
    Far away is near at hand in images of elsewhere

  9. #29
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    Default Re: A Lesson in Physical Chemistry

    Quote Originally Posted by johnalison View Post
    That's a great explanation. It's good to know that there is at least one intelligent person in East Anglia.
    The p = - g ρ z of alandalus11s cryptic set of formulae at #18 did give the gist of it: the calculation of the pressure at depth z. The σt bit just refers to the way oceanographers subtract 1000 from the density ρ in kg/m3 to give easy-to-handle two-digit density numbers - but the Total Dissolved Solids (TDS) bit was confusingly irrelevant.

    But I suspect you may perhaps have surmised that - and of course he may not be in East Anglia.

  10. #30
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    Default Re: A Lesson in Physical Chemistry

    Quote Originally Posted by Hydrozoan View Post
    The p = - g ρ z of alandalus11’s cryptic set of formulae at #18 did give the gist of it: the calculation of the pressure at depth z. The σt bit just refers to the way oceanographers subtract 1000 from the density ρ in kg/m3 to give easy-to-handle two-digit density numbers - but the Total Dissolved Solids (TDS) bit was confusingly irrelevant.

    But I suspect you may perhaps have surmised that - and of course he may not be in East Anglia.
    Oh Gawd - that makes three of you.
    Far away is near at hand in images of elsewhere

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