sloshing water: effect on energy/effort

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sloshing water: effect on energy/effort

Postby wildwanderer » Mon 19 Aug, 2019 9:04 pm

This is a bit of a side diversion to the below soft water bottle topic. Possibly belongs in techno babble. :mrgreen:

I commonly use 2 x 1 L PET bottles. I couldn’t find two cheap 1L ones last time i visited the super market, so grabbed 2x 1.5L.
Typically, I fill with around 500ml each (and occasionally 1L each if it’s a big day with no resup points)

On my osprey pack I have these in the side pockets on a angle. With 500ml of water and 1.5L bottles I very much noticed the slosh effect.. the water bouncing and water weight shifting back and forwards as I moved.

And I wonder if this sloshing is causing me to use up more energy... either through my muscles needing to stabilise the load more or because the water sloshing backwards is energy heading in the reverse of my walking direction..

Thoughts?

(its not something I’m overly concerned about... more scientifically curious)
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Re: sloshing water: effect on energy/effort

Postby crollsurf » Mon 19 Aug, 2019 10:20 pm

Scientifically speaking I do like to keep my pack balanced so swap around my bottles because I favour my left when reaching for water, but other than that...

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Re: sloshing water: effect on energy/effort

Postby slparker » Tue 20 Aug, 2019 4:33 pm

I am not a physicist but there is no more energy required to carry a 1kg/1l sloshing bottle up an incline, or when walking, than any other 1kg weight.

that said, the postural muscles will need to do more work to accommodate the shifting load as the weight moves across the lateral plane.

So no more energy is required to bear the static weight but there is more physiological cost to accommodate balancing the shifting weight.

I don't know how big a deal it is but if you're worried carrying the load centrally (i.e. in the vertical plane) will virtually negate this effect. hard to reach a water bottle carried in your pack though...
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Re: sloshing water: effect on energy/effort

Postby skibug » Mon 26 Aug, 2019 12:40 pm

"...And I wonder if this sloshing is causing me to use up more energy... either through my muscles needing to stabilise the load more or because the water sloshing backwards is energy heading in the reverse of my walking direction.."

1. The sloshing water will be constantly changing the position of the Centre Of Mass of the (combined) pack and body, and so will require larger correctional forces applied to the ground (by the feet) to maintain balance and consistent velocity across the ground. This effect would be relatively minor (ie undetectable) for 1- 3 kg of water, more substantial for larger masses or smaller people (ie children).
2. The extra kinetic energy cost of the sloshing water will depend on several factors including bottle shape and orientation of the bottle in the three planes, gait style (particularly positive and negative accelerations due to heel strike and pre-toe-off ground reaction forces), etc. Typical gait entails a continuous change in horizontal velocity (of the C.O.M.) due to the (negative) heel-strike g.r.f. followed by the (positive) propulsive g.r.f. The sloshing water will create a "lag" effect on C.O.M. forward velocity, and so again may change the size of g.r.forces and thus muscle energy production; but similarly this will be relatively minor for typical water quantities. Whether that energy cost adds up to something substantial over many hours you'd have to come up with some numbers and model it.

Having said the above, I find if you have a "loose" item (like a water bottle) strapped to a pack so that it swings during walking, it is incredibly annoying - though that is probably the fear that it will come loose, or cause some damage, rather than any increased energy cost.

If sloshing water bothers you, use a soft bottle (or wine bladder).

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Re: sloshing water: effect on energy/effort

Postby Gippsmick » Wed 05 Feb, 2020 3:17 pm

Not sure about the science of water sloshing and extra energy but if want to stop that slosh, buy one of those plastic exfoliating body scrub ball thingos for a dollar, cut it up and put a piece inside your bottles. Works a treat for trail running.
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Re: sloshing water: effect on energy/effort

Postby flingebunt » Sun 09 Feb, 2020 7:47 pm

I love a good physics discussion, and went for the experiment.

Put about 10 kgs of water into half-full water bottles into my backpack and walked around at home. This is what I observed
  • Your body pretty much compensates for the change in centre of gravity as you walk. You do this without thinking.
  • With a normal pace you don't really feel that much with the shifting weight
  • If I twist my body around I can feel the weight of the water sloshing around, but it is not that pronounced. This is where all the weight in my pack is sloshing water.
  • It sort of has a cushioning effect because the pack doesn't just stop, rather the water arrives at different times, as well as bounces back.

My conclusion is that if you are carrying 1 litre of water spread across 2 water bottles, you would barely notice the effect. You already have the pack rising and falling as well as moving left and right, forwards and backwards and it is not perfectly aligned to your centre of gravity. Put on a heavy pack and start walking it feels strange, after a 1 few ks it can feel natural.

The impact of the extra motion of 1 kg of water moving at a different pace to the rest of the packs movement is barely noticeable. In most cases it would be less than this as you would only have 1 half empty container that you are carrying at any time.

But am now thinking, that if you put arranged the water to a certain weight in different ways in your backpack, it might even be more efficient than carrying the same solid weight.
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Re: sloshing water: effect on energy/effort

Postby slparker » Sun 09 Feb, 2020 8:37 pm

I don't think that it could use less energy, conservation of momentum and all that, the energy required to move 1l of water would be the same. I suppose it is possible to be more biomechanically efficient if the timing of the water movement coincided with movement of the wearer's COG (i.e. water sloshes up when the COG moves up in the swing phase of walking) but I don't see how it would be possible to harmonise this movement except on a treadmill - not practical for variable terrain.

I have carried a jerry of water when in the army, I recall, the sloshing from side to side added to the aggravation so minimising slosh is probably optimal for postural muscle endurance.
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Re: sloshing water: effect on energy/effort

Postby flingebunt » Mon 10 Feb, 2020 12:33 am

Well imagine a tall office tower in a earthquake. When the earth moves the building goes from side to side. So they install springs that cushion the movement and push it back into position.

Now imagine a backpack when you are walking. The pack will move forward/backward, side to side and up and down.

As you walk the pack moves from one side to the other. A rigid load moves from left to right all at the same time. But the water in a bottle moves slightly slower, so that the swing of the pack is reduced. If there are different bottles with different frequencies (because of the size of the bottle, amount of water and orientation) the water will be arriving at its end point and bouncing back at different times, meaning the force of the pack's swing is reduced. The same applies with forward movement.

When you step forward and the pack's weight does not all arrive at the same time. The impact as you land on each foot is less than with a rigid load.

This water movement may feel strange and unpredictable when you start walking, but the body rapidly learns and adapts.

Of course with the right frequency and all bottles of water containing the same amount of water and with the same orientation, a resonance effect could come into play, causing problems. For the earthquake metaphor, there was an Earth quake that destroyed all buildings of a certain size. Not every tall building, but every building above a certain number of stories and below a certain number of stories. This happened because of this resonance.

Carrying a jerry can is very different because it is much more awkward than having something on your back. There is a narrow optimal way to carry something heavy in your hand, and the sloshing has a big impact on this, especially as it moves in a single wave. Your backpack moves with your back, and the movement of the weight of the pack moves your body, rather than moves around on your back.

Look, do the experiment. Put some partly empty water bottles in your backpack. 10 kgs would be enough and see how it feels. I only did an at home test.
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Re: sloshing water: effect on energy/effort

Postby slparker » Mon 10 Feb, 2020 9:37 am

Not sure that this is right:

As you walk the pack moves from one side to the other. A rigid load moves from left to right all at the same time. But the water in a bottle moves slightly slower, so that the swing of the pack is reduced


The swing of the pack is the same - the torso and the pack affixed to it describes the same lateral arc away from the midline. What you might be describing is a phenomenon where there is latency in the water moving away from the midline to the extremity of the arc - because it is a fluid. Assuming that the torso swings in the same arc from left to right the water may tend to 'peak' in the midline but this would only be a portion of the fluid and it doesn't mean that the walker uses any less energy to walk up and down hills but it might mean that the walker uses less energy to move the torso from left to right. remember that in level walking oscillation of the torso away from the midline is minimal so any energy saved by postural muscles is likely to be similarly minimal. There is movement in two other planes but the principle is the same even if the movement of fluid id highly variable.

However, when the walker negotiates obstacles and leans the torso over to one side (i.e. crossing a log, scrambling on rocks) that water will tend to spill over to whichever side that the walker is leaning towards, increasing the energy required to get that water back to the midline (I forget the physical principle but the mass at the end of an arc requires more energy to return to the midline than if the weight were closer). This is really noticeable when you are carrying a lot of water in one container. This property of the fluid might well might negate any advantage that was described above.

I am not sure that there is any effective energy return in the system - what you seem to be describing in your discussion on springs is a kind of sloshing backwards of water helping to bring the torso back to the midline and increasing efficiency of the postural muscles. This is plausible but in reality would require harmonics - the net direction of flow of the water in the vessel corresponding exactly with the movement of the torso. This system still requires enrgy to move the water from side to side unless you are proposing a perpetual motion machine....
In reality what i have noticed is that, when carrying a large water vessel (the 25l jerry on my back), the flow of the water might be in the opposite direction of my torso - so that you are 'fighting' the weight of the water that is moving independently of the torso. Runners who carry water really notice this effect and try and reduce sloshing as much as possible by placing baffles in their water containers.

I am not sure that an experiment would be of much benefit because, as you state, perceiving any energy gain or loss using your perception is pretty flawed. You could do a controlled experiment in a physiology lab to measure energy consumption but you would need a number of repeats, a number of controls etc to do it properly.

Fluid dynamics is really, really complicated maths and I don't pretend to have a grasp on it but I think that carrying bottles in the midline and minimising slosh is the most efficient method based upon my limited understanding. I try to carry water bottles on my shoulder straps with my spare 2L in my pack at my midline.
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Re: sloshing water: effect on energy/effort

Postby Baka Dasai » Mon 10 Feb, 2020 1:54 pm

flingebunt wrote:Well imagine a tall office tower in a earthquake. When the earth moves the building goes from side to side. So they install springs that cushion the movement and push it back into position.


In Japan, tall buildings are often built with swimming pools (or aquariums, or other large bodies of water) about halfway up the building to minimise the effects of earthquakes.

But I imagine this effect would only work for walking if your walking rhythm was very consistent, as slparker says above.
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Re: sloshing water: effect on energy/effort

Postby Warin » Mon 10 Feb, 2020 2:44 pm

flingebunt wrote:Well imagine a tall office tower in a earthquake. When the earth moves the building goes from side to side. So they install springs that cushion the movement and push it back into position.


Just where are these springs 'that push it back into position'?

Ones I am aware of are in the base to support the structure- so as the earth moves the building movement is reduced through inertia. Nothing there about 'pushing it back into position'.

To 'push it back into position' there would need to be something stationary .. not something you find in an earthquake.
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Re: sloshing water: effect on energy/effort

Postby flingebunt » Mon 10 Feb, 2020 11:20 pm

Ones I am aware of are in the base to support the structure- so as the earth moves the building movement is reduced through inertia. Nothing there about 'pushing it back into position'.


Well a spring works by both providing resistance, but also as an active force. When you push a spring, it has a cushioning effect, but at the same time, when you stop pushing against the spring, the tension built up in the spring will be released, pushing back.
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