Physics problem: cumulative shell colliding with an armoured wall

Introduction and problem statement

This problem, as many others appearing here, took me a lot of time to wrap my head around. I almost lost faith, even tried to ask ChatGPT to solve it for me few times, but ChatGPT's solution was wrong or completely obscure for me.

Here is the problem statement (translated by ChatGPT from the Savchenko et al 1981 book).

Problem statement: In 1941, the Germans invented a cumulative anti-tank shell. The shell has a fuse on its front part, which, upon impact, causes detonation and ignites the entire charge. The shell penetrates the armor. In 1944, such German shells came into the hands of both the Soviets and their allies. Extensive experiments began. Various additional effects and paradoxes were discovered. The researchers started to investigate what exactly was penetrating the armor. At first, they thought it was a jet of hot gas that burns through the armor. However, it turned out that a metal jet is piercing through the armor traveling in a very puzzling way: in front of the plate at a speed of $v_{0} = {\rm 8\;km/s}$, inside the plate at ${\rm 4\;km/s}$, and behind the armor plate again at ${\rm 8\;km/s}$.

Explain this phenomenon and determine the speed of the metal wall of the conical cavity covering the charge, given that the cone’s vertex angle is 30°.

I have to admit that the translation is pretty accurate.

• You might prefer to read the notebook version of the post here.
• Latex sources for the plots are available here.

Figure 1: Cross-section of cumulative shell heading towards the armoured wall.

Solution

The problem consists of two parts. The first one asks to explain the phenomenon and the second one asks to determine the speed of the collapsing conical wall that is separating the fuse and the explosive charge of the shell, this collapsing wall is forming the metal jet piercing through the armor. I'll proceed in the order I solved it initially, starting with the part 2.

Part 2

Let's determine the speed of the conical wall collapsing into the jet flowing onto the armor at a speed of $v_{0} = {\rm 8\;km/s}$ as shown in Fig. 2. We focus here only on the upper part of the conical wall cross-section ($AB$, see Fig. 1 as well).

The breaking moment for me here was to realize that the velocity of the conical wall is orthogonal to its surface ($\vec{v}$ in Fig. 2). Since it is collapsing under the pressure force of the gas/liquid created to the left of the cone surface after the explosion of the charge.

Figure 2: Cross-section of the upper part of the collapsing conical wall.

Let's consider a reference frame in which the cone material flows along the cone surface at speed $\vec{v}_1$. This reference frame is moving to the right at a horizontal speed $\vec{u}$, so that the following relation is satisfied:

\begin{equation} \vec{v} - \vec{u} = \vec{v}_1 \end{equation}

In this reference frame the horizontal part of the jet is moving at speed $\left|\vec{v}_1\right| = v_1$ (here and below we ommit the arrow to denote scalar variables). This is relatively easy to prove using energy conservation. In summary we have the following system of equations:

\begin{align} \begin{cases} \vec{v} - \vec{u} = \vec{v}_1\\ v_0 - u = v_1\\ \end{cases} \end{align}

From the triangle formed by the speed vectors in Fig. 2 we get: \begin{align} \begin{cases} v_1 = v / \tan\left(\frac{\alpha}{2}\right)\\ u = v / \sin\left(\frac{\alpha}{2}\right) \end{cases} \end{align}

Now using the last two equations and the relation $v_0 - u = v_1$ we get an equation with respect to $v$:

\begin{align*} v_0 - u = v_1 & \Rightarrow v_0 - v / \sin\left(\frac{\alpha}{2}\right) = v / \tan\left(\frac{\alpha}{2}\right) \\ &\\ & \Rightarrow \boxed{v = \frac{v_0 \sin\left(\alpha/2\right)}{1 + \cos\left(\alpha/2\right)}}\\ &\\ & \Rightarrow v\approx 1.05\:{\rm km/s} \end{align*}

Part 1

The first part of the problem is to explain the phenomenon, mainly why the speed decreases by a factor of two in the armor. It is more or less clear that the jet will slow down due to the friction with the armor and then will accelerate back behind the armoured wall to the initial speed as it is being pushed forward. The factor of two for the velocity can be explained by the size of the hole pierced by the jet in the armoured wall (as shown in Fig. 3) which is two times larger than the diameter of the jet.

Figure 3: Metallic jet formed from the collapsed conic wall after piercing through the armoured wall, after the stationary flow regime is established.

So now we need to explain why the hole would be approximately twice the size of the metall jet piercing through the armor. For this we need to consider the moments before the hole is pierced completely. At this point there is a part of the jet that flows backwards reflected from the armor as shown in Fig.4 below, which expands the hole. The symmetry and mass conservation would hint us towards the factor of two for the hole diameter.

There is however a question, why would the backflow velocity be directed exactly in the opposite direction with respect to the incident jet. Indeed, this is not obvious.

Figure 4: Metallic jet formed from the collapsed conic wall prior to piercing through the armoured wall.

To find the angle at which the backward flow is reflected we can consider two colliding jets of liquid: smaller with a radius $r$ and a larger one with a radius $R$, moving towards each other at equal and parallel speeds (we can always switch to the reference frame where the metallic jet and the armoured wall move at equal speeds towards each other). As a result of the collision of the jets, a conical backflow in the direction of the smaller jet is formed. If we denote the angle of the cone as $2\beta$, then using momentum, mass and energy conseration it is relatively easy to show that the angle of the cone can be computed using dimensions of the colliding jets as follows: \begin{equation} \cos\beta = \frac{R^2-r^2}{R^2 + r^2} \end{equation}

Now, for our problem if we take the radius of the metallic jet to be $r$, it is much smaller than the effective radius of the armoured wall:

\begin{align} r \ll R & \Rightarrow \cos\beta = \frac{R^2-r^2}{R^2 + r^2} \approx 1 \\ & \Rightarrow \beta \approx 0 \end{align}

which means that indeed in our case the backflow is directed antiparallel to the incident jet velocity.

i3 (EndeavouOS edition): Multiple keyboard layouts

Here I will show you how to add a ukrainian keyboard layout to an existing layout (in this case us), so you can type using different languages.

• First add the following to your ~/.config/i3/config file:

  # add layouts
  exec setxkbmap -layout us,ua
  # set a switching shortcut
  exec setxkbmap -option 'grp:alt_shift_toggle'
  

• Now we want to show the current layout in the system tray (this might be distribution dependent, I guess). EndeavourOS provides some ready scripts for which I needed to install dependencies and uncomment some lines in the ~/.config/i3/i3blocks.config.
  • Install dependencies

    yay xkblayout-state
    

  • Uncomment the following block in ~/.config/i3/i3blocks.config:

    [keyboard-layout]
    command=~/.config/i3/scripts/keyboard-layout
    interval=2
    

Here is the screenshot of the way my env looks right now.

tmux config file

This is mostly a reminder for me whenever I set up a new system to remember where I should go to download my favourite tmux customizations: github link.

Physics problem: Horizontal cylindric water pump

First, let me give you the problem statement so you can decide for yourself if the post is worth your time.

Problem statement

A pump consists of a cylinder with a piston. The cylinder is positioned horizontally. The piston's active pushing area is $A$. There is a hole of area $a$, in the middle of the piston. The piston is pushing water to the left and the water is flowing out to the right through the hole. The force $F$ is applied to the piston to make it move to the left with a constant velocity. Assuming that the flow is laminar and stationary ($\frac{\partial X}{\partial t} = 0$), determine the flow velocity of the liquid through the hole.

[Source: Savchenko et al 1981 physics problems book]

Latex source for the above plot is here: link. In this one I used layers to draw the pump on top of water.

Solution

One thing that we need to realize is that we are asked to compute the outflow speed of the water relative to the piston.

Another one, which is important to get the same answer as in the source physics problem book, is to realize that the total cross-section area of the cylinder is $A+a$.

Now that these two points are clarified we can switch to the reference system moving to the left with the piston. In this reference system the water flows to the right and through the hole. Let's denote the speed of water to the left from the piston and to the right from the piston as $u$ and $v$ respectively. Therefore the speed $v$ is the actual outflow speed we are asked to compute here. Let's denote the pressure to the left and to the right from the piston as $P_1$ and $P_2$ respectively.

Let's write down the following three constraints imposed onto the pump system in the form of equations.

1. The second Newton's law applied to the piston yields:

   $$ F = \left( P_1 - P_2 \right) \cdot A $$

2. The energy conservation law applied to a unit volume of the water (or else known as Bernoulli's law) yields:

   $$ P_1 + \rho \frac{u^2}{2} = P_2 + \rho \frac{v^2}{2} $$

3. The continuity equation yields:

   $$ u \cdot A = v \cdot a $$

Above, we have three equations and three unknowns to be found are $u$, $v$, and $P_1 - P_2$. By solving the above system of equations we obtain the following expression for the outflow speed $v$:

$$ \boxed{v = \sqrt{\frac{2FA}{\rho (A^2 - a^2)}}} $$

Making suspend work on an arch-flavoured linux

I have a custom installation of linux (EndeavourOS if you are curious) that I use with the i3 window manager. But whenever I was doing

systemctl suspend

it was going to suspend but then turned back on, without any error message... Recently, I've guessed that the reason for this was the missing swap file. If you find yourself in a similar situation there is a nice howto on the creation and activation of a swapfile on Linux: https://btrfs.readthedocs.io/en/latest/Swapfile.html

Physics problem: Air-filled balloon floating in a rotating cylindrical tank partially filled with water

I want to discuss this moderate complexity question about a balloon, filled with air, floating in a rotating tank, which is partially filled with water, as shown in the plot. This is a nice closure for 2023 as it happens to be the last problem on Archimedes' force in the edition of the Savchenko et al problem book that I have. I like this one as its key hint to the solution is in the illustration to the problem.

Problem statement In a cylindrical container, partially filled with water, an air-filled balloon is floating attached to the side wall with a rope as shown in the figure. The container is rotating and the rope is deviated from the side wall towards the center by an angle $\alpha$. The length of the rope is $l$ and the radius of the container is $R$. The radius of the balloon is $r$. Given the information above, determine the angular speed $\omega$ of rotation of the cylinder.

Latex code for the illustration can be found here.

Solution

Before I start describing the solution, I would like to draw your attention to the fact that the balloon is deviated towards the axis of rotation. What would be the reason for this? We need to have a force pulling it towards the center to provide the acceleration towards the rotation axis. It is clear that the rope, attaching the balloon to the wall, pulls the balloon away from the center. Therefore, in principle, I would expect the balloon to be pressed to the side wall of the tank just above the point where the rope is attached to the wall, due to buoyancy of air. And that would probably be true if we had a flat water surface. But here, judging from the illustration, the water surface is not level, but curved, actually its shape is parabolic. This creates a pressure gradient force pushing the air balloon towards the rotation axis.

We can easily compute the pressure gradient at the distance $x$ due to the curvature of the water surface, let's denote the horizontal profile of the water surface as $h(x)$, where $x$ is the distance from the rotation axis on the water surface.

Then

\begin{equation} \frac{\partial P(x)}{\partial x} = \rho_0 g \frac{\partial h(x)}{\partial x} \end{equation}

where $\rho_0$ is the water density, and $g$ is the acceleration due to gravity.

Now, to determine $\frac{\partial h(x)}{\partial x}$ we consider a water parcel at the surface at the distance $x$ from the rotation axis. There are two forces acting on it resulting in the acceleration directed towards the center of rotation: the first one is the reaction of the rest of the water, perpendicular to the water surface as there is no tangent acceleration, and the second one is the force of gravity. It is easy to show that the $\tan$ of the angle between the force of gravity ($dm \vec{g}$) and the reaction force $d\vec{N}$ is:

\begin{equation} \tan\sigma(x) = \frac{\partial h(x)}{\partial x} = \frac{\omega ^ 2 x}{g} \end{equation}

We can use the above to calculate the horizontal pressure gradient as a function of $x$ as follows

\begin{align*} \frac{\partial P(x)}{\partial x} = \rho_0 \cdot \omega ^ 2 x \Rightarrow \\ P(x) = P_0 + \frac{\rho_0 \cdot \omega ^ 2 x ^ 2}{2} \end{align*} where $P_0$ is the pressure at the center of the tank.

So now we would have to find the resulting force, actually the horizontal component of the force, acting on the balloon due to the above pressure field in the water as:

\begin{align*} F_P = \int\limits_{B} P(\beta, \gamma) \cdot \cos \phi(\beta, \gamma) dA \end{align*}

Where $B$ denotes the surface of the balloon, $dA$ is an element of its surface area, $(\beta, \gamma)$ are the angles defining a position of an element $dA$ on the surface of the balloon, and $\phi$ is the angle between the horizontal axis and the normal to the surface of the balloon pointing outside of the balloon (just to have the force positive when it is pushing the balloon towards the center).

It is easy to see that:

$$ dA = r^2\cos\gamma d\gamma d\beta $$

Let's denote the position of the center of the air ballon as $x_0$. We can split the balloon into two equal parts: one closer to the rotation axis and the other one further away from the rotation axis, let's denote water pressure on these parts as $P_{-}(\beta, \gamma)$ and $P_{+}(\beta, \gamma)$.

To fully and independently cover these parts the angles should be $\gamma \in [-\pi/2, \pi/2]$ and $\beta \in [-\pi/2, \pi/2]$. Then we can use $\beta, \gamma$ pair to parameterize the pressure on those sides of the balloon as follows.

$$ P_{+}(\beta, \gamma) = P_0 + \frac{\rho_0 \omega ^ 2}{2}\cdot(x_0 + r \cos\gamma \cdot \cos\beta)^2 $$

And for the half closer to the rotation axis:

$$ P_{-}(\beta, \gamma) = P_0 + \frac{\rho_0 \omega ^ 2}{2}\cdot(x_0 - r \cos\gamma \cdot \cos\beta)^2 $$

Now we pair opposite points with the same $\beta, \gamma$ on both hemispheres to get the contribution to the pressure gradient across the balloon:

$$ \Delta P(\beta, \gamma) = P_{+} - P_{-} = 2 \rho_0\omega^2 r x_0 \cos\gamma \cdot \cos\beta $$

Then the resulting horizontal component of the force acting on the balloon from the two parts is calculated as follows:

\begin{align*} dF_{P} &= \Delta P(\beta, \gamma) dA \cdot \cos\gamma \cos\beta \\ &= 2\rho_0 \omega^2 r^3 x_0 \cos^3\gamma \cos^2\beta d\gamma d\beta \end{align*}

To get the net force we integrate the above expression:

\begin{align*} F_{P} & = 2\rho_0 \omega^2 r^3 x_0 \int\limits_{-\pi/2}^{\pi/2} \cos^3\gamma d\gamma \int\limits_{-\pi/2}^{\pi/2} \cos^2\beta d\beta \\ & = \frac{4}{3} \pi r ^3 \rho_0 \omega^2 x_0 \end{align*}

Now we are ready to write down the second Newton's law projected on the vertical and horizontal axes:

\begin{cases} F_{A} - mg - T \cos\alpha = 0 \\ F_{P} - T \sin\alpha = m \omega ^ 2 x_0 \end{cases}

where $F_{A}$ - is the Archimedes' force acting on the balloon, $mg$ - is the force of gravity pulling the balloon down, and $T$ - is the tension of the rope attaching the balloon to the side wall of the tank.

Let's denote the volume of the balloon as $V$ and the air density as $\rho$, then using the Newton's law expression projected on the vertical axis we can express the tension force as follows:

$$ T = Vg (\rho_0 - \rho) \frac{1}{\cos\alpha} $$

Then we plug the above expression for $T$ in to the equation for the horizontal force components and obtain the following eqaution with respect to the rotation frequency $\omega$:

\begin{align*} T\sin\alpha & = Vg (\rho_0 - \rho) \tan\alpha \\ & = F_{P} - m \omega ^ 2 x_0 = (\rho_0 - \rho) \omega ^ 2 x_0 V \end{align*}

Simplifying the above we get: \begin{align*} g\tan\alpha = \omega ^ 2 x_0 \Rightarrow \omega = \sqrt{\frac{g\tan\alpha}{x_0}} \end{align*}

Now we need to express the distance from the rotation axis to the center of the balloon ($x_0$) through the quantities given in the problem statement. From the geometry considerations it is easy to obtain the following: $$ x_0 = R - (l + r) \sin\alpha $$

Therefore, the final answer to the problem question is:

$$ \boxed{\omega = \sqrt{\frac{g\tan\alpha}{R - (l + r) \sin\alpha}}}. $$