# Doing an EE in fluid mechanics, physics HL, I need advice on some things :)

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Hi, so my essay has for title "An investigation of shear thickening material in military purposes." I am almost at the finish line, but there are still some things that are trying to kill me slowly. So in my essay I have to study the viscosity of shear thickening fluid called cornstarch, as a way to model liquid body armour used in the military.

The first "step" of my ee is to first establish the shear thickening property of my liquid. TO do this i dropped a metallic sphere into a cylinder filled with the liquid and measure velocity of fall and velocity through the liquid. This allowed me to use an equation to find the viscosity and I also used this to find the shear stress using the velocity gradient rule that says, shear stress = viscosity * du/dy. (sorry if this looks messy but it's a very specific ee). The ISSUE that I am having is to be able to find literature data to support my own data. And that has proven difficult because while I have the literature data, my ee data does not match it at all. So I am stuck with data that just isn't working the way i want. And this is beyond trying to tweak numbers to get what i want, because the calculation process is legitimately insane.

So let's say i have a relationship that says: shear stress = viscosity*shear rate, and that I plotted the viscosity and shear stress that I found, but when I do so, I almost get a constant value for the shear stress, which means that there are points superimposed over each other. This also means that I cannot produce a error bar for it because it would literally take over the whole graph and give me horrible presentation marks. While the shear stress is not even what i am interested in, when I find the shear rate by doing shear stress/viscosity (according to equation above), I get a graph of Viscosity in function of shear rate that is exponentially negative, which is the exact opposite of literature values, which show the viscosity in function of shear rate as positively exponential :/

So what do i do people? I worked countless hours on this and now I'm getting a crappy end result lol I need some outside help rn

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Shear stress is a sliding force acting on a surface. An example is if you have jelly and apply a force on a surface, you distort the geometry. In the shear stress = shear rate * viscosity, you would literally have to distort geometry of fluid. I don't know how to this in a lab, but if you vary the stress, you can directly measure the shear rate, which is simply surface velocity divide by height of fluid, and so you don't really need viscosity. The hard part is how to set it up and know the stress each time.

Just dropping a ball only finds viscosity, but there is no stress.

Edited by kw0573
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16 hours ago, kw0573 said:

Shear stress is a sliding force acting on a surface. An example is if you have jelly and apply a force on a surface, you distort the geometry. In the shear stress = shear rate * viscosity, you would literally have to distort geometry of fluid. I don't know how to this in a lab, but if you vary the stress, you can directly measure the shear rate, which is simply surface velocity divide by height of fluid, and so you don't really need viscosity. The hard part is how to set it up and know the stress each time.

Just dropping a ball only finds viscosity, but there is no stress.

But if I want ti find both the shear stress and viscosity at once, then I pretty much can't use the sphere dropping experiment? Cause that's what i did to find the viscosity already. But in terms of lab equipment I have no idea how I could set up a working experiment for find the shear stress (of that same identical fluid. Because what I did was that I dropped the sphere into the cylinder filled with liquid at different heights, to measure the change in viscosity as the height increase (increase in velocity).

At first I also looked at normal stress to replace shear stress. But that doesn't work either because normal stress is just the impact force of the sphere onto the surface of the liquid.

So I am very stuck for this

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You have to have shear stress to use an equation involving shear stress. I understand that you have already done your experiments and if you do not have time to design new experiments, you can consider just submit the viscosity-measuring aspects.

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