# Hydrostatic Pressure Curved Surface

1. Jan 26, 2016

### joshmccraney

Hi PF!

I have been reading and reading for a clean explanation for the hydrostatic force of a curved submerged 2-D (or 3-D if you're up to it) surface. I really don't care what the curve looks like: quadratic, circular, partway sinusoidal, etc. All articles I read involve the centroid, but could someone help me through a derivation where we do all the math ourself? If so you choose the geometry (or I can) and let's start into it!

Thanks either way!

Josh

2. Jan 26, 2016

### Staff: Mentor

The pressure increases with depth according to the hydrostatic equation, correct? The pressure acts on every surface in the direction normal to that surface, correct? So the pressure force on a differential area dA is $p\vec{n}dA$, where $\vec{n}$ is a unit normal to the surface. You need to add up (integrate) all these pressure forces vectorially.

Chet

3. Jan 27, 2016

### joshmccraney

Correct. So the pressure at any submerged point is $p = \rho g d$ where $p$ is pressure, $\rho$ is density, $g$ is acceleration of gravity, and $d$ is the vertical distance submerged?

So pressure is then $$\vec{p} = \iint_A \rho g d \cdot \vec{n} dA$$ (I used the $\cdot$ so my depth variable doesn't look like a differential, not to mistake it with a dot product).

4. Jan 27, 2016

### Staff: Mentor

$$\vec{F} = \int_A \rho g z \vec{n} dA$$
OK. Let's take a simple example. Let the area of interest be parallel to the y axis, and have a width w. Let the equation for the surface be x = x(z). Then $dA=w\sqrt{(dx)^2+(dz)^2}$. What is the equation for the unit normal to the contour?

5. Jan 27, 2016

### joshmccraney

Of course, not sure why I didn't put a force.

I think $\vec{n} = - \hat{j}$. I can't really picture the surface you describe though. Is it something like a cylinder in the $y$ direction with an arbitrary cross section that is a function of $x$ and $z$?

6. Jan 27, 2016

### Staff: Mentor

At every value of y, x is the same function of z. On the surface of interest, y runs from 0 to W.

The unit normal to the surface is not j. The unit tangent to the surface is $\vec{t}=\frac{(dx)\vec{i}+(dz)\vec{k}}{\sqrt{(dx)^2+(dz)^2}}$. The unit normal to the surface is then $$\vec{n}=\frac{(dz)\vec{i}-(dx)\vec{k}}{\sqrt{(dx)^2+(dz)^2}}$$

So, if we call p(z) is the pressure at depth z, what is the resultant force vector acting on the surface, according to our integral?

7. Jan 27, 2016

### joshmccraney

Ohhhhhh I think I see the shape you're describing now, and you're talking about the sides? Is the force then $$\vec{F} = \int_0^W \int_{z_0}^{z_1} \rho g z \left( \hat{i} - x'(z) \hat{k} \right) \, dz dw$$

8. Jan 27, 2016

### Staff: Mentor

Very nice. Now the w can be integrated immediately, and the x component of the force can be obtained as an exact differential. The z component should be partially handled using integration by parts. Check out the x component carefully. You should be able to see some kind of center of mass thing from this. BTW, if you are integrating from z0 to z1, you need to include the pressure a z0 in the pressure expression at depth z.

Chet

9. Jan 30, 2016

### joshmccraney

$$\vec{F} = \int_0^W \int_{z_0}^{z_1} \rho g z \left( \hat{i} - x'(z) \hat{k} \right) \, dz dw =\\ W g \rho \hat{i}\int_{z_0}^{z_1} z \, dz - W g \rho \hat{k}\int_{z_0}^{z_1}\rho z x'(z) \, dz =\\ W g \rho \hat{i}\int_{z_0}^{z_1} z \, dz - W g \rho \hat{k} \left( z_1 x(z_1)-z_0 x(z_0) - \int_{z_0}^{z_1}x \, dz\right)$$
where I believe the $z \, dz$ integral can be rewritten as a center of mass. You agree with this?

10. Jan 30, 2016

### Staff: Mentor

Yes. It's going to be $\Delta z$ times the pressure at the center of mass.