{ "cells": [ { "cell_type": "markdown", "metadata": {}, "source": [ "# Control Volume Analysis\n", "\n", "## Learning outcomes\n", "\n", "* Understand the system representation of matter from a Lagrangian perspective\n", "* Learn about the conservation laws for mass, momentum and energy\n", "* Understand the concept of a control volume and how the conservation laws apply\n", "* Get comfortable with the idea of a surface normal vector.\n", "* Remind ourselves about the dot product with some Python\n", "\n", "# System Representation of Matter\n", "\n", "A system is a collection of matter that may move, flow and interact with its surroundings. You can imagine a fluid flow moving through a conduit as shown below and we somehow manage to tag a large number of Lagrangian particles in a cuboid of space and then we track those tagged particles as they move through the duct. **These tagged Lagrangian particles are a system** and they may move, flow and interact with their surroundings. As the duct's cross sectional area expands the shape of the system stretches in the *spanwise* direction and contracts in the *streamwise* direction since there are only so many particles in our system.\n", "\n", "At various moments in time we can mark the volume and boundary occupied by the system as shown in red. Note that the flow shown below is a simplified inviscid frictionless flow.\n", "\n", "" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "Lets look at that again in 2D. As the duct expands the fluid expands to fill the spanwise width and so the streamwise length of the system shrinks. It appears that the volume of the system is kept constant.\n", "\n", "" ] }, { "cell_type": "markdown", "metadata": {}, "source": [ "# Conservation Laws\n", "\n", "This observation about the volume of our system illustrated above suggests that the volume is conserved. Lets think about that a little more. We defined our system as a collection of matter in the form of particles. We cannot add or remove matter from that system — there is only so much stuff and we've tagged all of it in our system. More precisely we can state that the **time rate of change of the mass** of the system does not change. This is called the **Conservation of Mass**. How it relates to volume will become apparent.\n", "\n", "## Conservation of Mass\n", "\n", "The total mass of a system $M_{sys}$ can be defined as the sum of the mass of all the particles that make up the system:\n", "\n", "\\begin{equation*}\n", "M_{sys} = \\sum_\\text{mass sys}{\\delta m} = \\sum_\\text{volume sys}{\\rho \\delta \\rlap{V}-} \n", "\\end{equation*}\n", "\n", "Which is equivalent to the sum of the density times the volume of each particle. \n", "As the mass of each particle approaches zero, which is reasonable when dealing with small particles:\n", "\n", "\\begin{equation*}\n", "\\lim_{\\delta m \\rightarrow 0} \\Rightarrow M_{sys} = \n", "\\underbrace{\\int{\\delta m}}_\\text{mass sys} = \n", "\\underbrace{\\int{\\rho \\delta \\rlap{V}-}}_\\text{volume sys} \n", "\\end{equation*}\n", "\n", "So since by definition our system is a finite number of particles the mass of the system remains constant but also if the density also remains constant the volume is also conserved.\n", "The conservation of mass can be written:\n", "\n", "\\begin{equation*}\n", "\\frac{d M_{sys}}{dt} = 0\n", "\\end{equation*}\n", "\n", "We can also define conservation laws for momentum and energy. \n", "\n", "\n", "## Conservation of Momentum\n", "\n", "Momentum $\\vec{P}$ is simply the mass times the velocity of a body. Momentum is important because it allows us to understand how a mass of fluid responds to changes in velocity due to the forces acting on it. Each of the particles in our system have a constant infinitesimal mass and some measurable velocity as the system evolves. As above for conservation of mass, the momentum $\\vec{P} = M\\vec{V}$ can be written:\n", "\n", "\n", "\\begin{equation*}\n", "\\vec{P}_{sys} = M_{sys} \\vec{V} = \\underbrace{\\int{\\vec{V}\\delta m}}_\\text{mass sys} =\n", "\\underbrace{\\int{\\vec{V} \\rho \\delta \\rlap{V}-}}_\\text{volume sys} \n", "\\end{equation*}\n", "\n", "Newton's second law for a *linear system* states that the **time rate of change** of the *linear momentum* of the system equals the sum of the external forces acting on the system.\n", "\n", "\\begin{equation*}\n", "\\frac{d \\vec{P}_{sys}}{dt} = \\sum{\\vec{F}}\n", "\\end{equation*}\n", "\n", "## Conservation of Energy\n", "\n", "Like mass and momentum, our particles in our system have energy, $E$. They may have potential energy or kinetic energy depending on their trajectory. They may have internal energy due to addition or removal of heat.\n", "\n", "\\begin{equation*}\n", "E_{sys} = \\underbrace{\\int{e \\delta m}}_\\text{mass sys} =\n", "\\underbrace{\\int{e \\rho \\delta \\rlap{V}-}}_\\text{volume sys} \n", "\\end{equation*}\n", "\n", "The time rate of change of energy of the system equals the rate at which heat is added to the system across the system boundary plus the rate at which work is done on the system by body or surface forces.\n", "\n", "\\begin{equation*}\n", "\\frac{d E_{sys}}{dt} = \\dot{Q} - \\dot{W}\n", "\\end{equation*}\n", "\n", "where $\\dot{Q}$ is the rate of heat addition and $\\dot{W}$ the rate at which work is done on the system. You may recognise this as the **First Law of Thermodynamics**.\n", "\n", "The following table summarises the three conserved quantities that we have considered. \n", "\n", "| Property | Total amount ($N$)| Definition | Amount per unit mass ($n$) |\n", "|-----------------|-------------------|-------------------------------|-----------------------------|\n", "| Mass | $M$ | density $\\times$ volume |1 |\n", "| Linear momentum | $\\vec{P}$ | mass $\\times$ linear velocity |$\\vec{V}$ |\n", "| Energy | $E$ | mass $\\times$ specific energy |$e = u + \\frac{V^2}{2} + gz$ |\n", "\n", "Note the total amount of any conserved property in a system is denoted $N$ and the amount per unit mass is given the symbol $n$." ] }, { "cell_type": "markdown", "metadata": {}, "source": [ " \n", "# The Control Volume\n", "\n", "Not only is it extremely tedious to track every Lagrangian particle in a system it is practically impossible, tracking a system boundary precisely is also extremely difficult. We need a means to analysis fluid flow that doesn't place such an onerous requirement on us. Fortunately such a method exists and it is the *control volume*. Consider again the duct from the start of the notebook. The control volume is a fixed region of the flow geometry and instead of tracking fluid particles we record the rate of change of our conserved quantities across its boundary — the control surface.\n", "\n", "\n", "