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## Multivariable calculus

### Course: Multivariable calculus>Unit 2

Lesson 11: Divergence and curl (articles)

# Curl warmup, fluid rotation in two dimensions

Curl measures the rotation in a fluid flowing along a vector field.  Formally, curl only applies to three dimensions, but here we cover the concept in two dimensions to warmup.

## Background

• start bold text, i, end bold text, with, hat, on top represents the unit vector in the x-direction.
• start bold text, j, end bold text, with, hat, on top represents the unit vector in the y-direction.

## What we're building to

• Curl measures the "rotation" in a vector field.
• In two dimensions, if a vector field is given by a function start bold text, v, end bold text, with, vector, on top, left parenthesis, start color #0c7f99, x, end color #0c7f99, comma, start color #bc2612, y, end color #bc2612, right parenthesis, equals, start color #0c7f99, v, start subscript, 1, end subscript, end color #0c7f99, left parenthesis, x, comma, y, right parenthesis, start bold text, i, end bold text, with, hat, on top, plus, start color #bc2612, v, start subscript, 2, end subscript, end color #bc2612, left parenthesis, x, comma, y, right parenthesis, start bold text, j, end bold text, with, hat, on top, this rotation is given by the formula
start text, 2, d, negative, c, u, r, l, end text, start bold text, v, end bold text, with, vector, on top, equals, start fraction, \partial, start color #bc2612, v, start subscript, 2, end subscript, end color #bc2612, divided by, \partial, start color #0c7f99, x, end color #0c7f99, end fraction, minus, start fraction, \partial, start color #0c7f99, v, start subscript, 1, end subscript, end color #0c7f99, divided by, \partial, start color #bc2612, y, end color #bc2612, end fraction

## Rotation in fluid flow

Have yourself a nice swirly vector field:
This particular vector field is defined with the following function:
\begin{aligned} \quad \vec{\textbf{v}}(x, y) &= \left[ \begin{array}{c} y^3 - 9y \\ x^3 - 9x \end{array} \right] \\\\ &= (y^3 - 9y) \hat{\textbf{i}} + (x^3 - 9x)\hat{\textbf{j}} \end{aligned}
Now I want you to imagine that this vector field describes a fluid flow, perhaps in a chaotic part of a river. The following video shows a simulation of what this might look like. A sample of fluid particles, shown as blue dots, will flow along the vector field. This means that at any given moment, each dot moves along the arrow it is closest to. Focus in particular on what happens in the four circled regions.
Amidst all the chaos, you might notice that the fluid is rotating within the circled regions. In the left and right circles, the rotation is counterclockwise, and in the top and bottom circles, the rotation is clockwise.
• Key Question: If we are given a function start bold text, v, end bold text, with, vector, on top, left parenthesis, x, comma, y, right parenthesis that defines a vector field, along with some specific point in space, left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis, how much does a fluid flowing along the vector field rotate at the point left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis?
The vector calculus operation curl answer this question by turning this idea of fluid rotation into a formula. It is an operator which takes in a function defining a vector field and spits out a function that describes the fluid rotation given by that vector field at each point.
Technically, the curl operation only applies to three dimensions. You can see what that means and how it is computed in the next article, but in this article, we warm up by describing fluid rotation in two dimensions with a formula.

## Capturing two-dimensional rotation with a formula

One of the simplest examples of a vector field which describes a rotating fluid is
\begin{aligned} \quad \vec{\textbf{v}}(x, y) = \left[ \begin{array}{c} -y \\ x \end{array} \right] = -y \hat{\textbf{i}} + x\hat{\textbf{j}}. \end{aligned}
Here's what it looks like.
Animated, all the fluid particles just go in circles.
In some sense, this is the most perfect example of counterclockwise rotation, and you can understand the general formula for rotation in a two-dimensional vector field just by understanding why the function start bold text, v, end bold text, with, vector, on top, left parenthesis, x, comma, y, right parenthesis, equals, minus, y, start bold text, i, end bold text, with, hat, on top, plus, x, start bold text, j, end bold text, with, hat, on top gives counterclockwise rotation.

## The $\hat{\textbf{i}}$start bold text, i, end bold text, with, hat, on top-component

First, let's understand why the minus, y, start bold text, i, end bold text, with, hat, on top component suggests counterclockwise rotation. Imagine a small twig sitting in our fluid, oriented parallel to the y-axis. More specifically, let's say one end is at the origin left parenthesis, 0, comma, 0, right parenthesis, and the other is at the point left parenthesis, 0, comma, 2, right parenthesis. What does the minus, y, start bold text, i, end bold text, with, hat, on top component of the vector field imply for the fluid velocity at points on this twig?
This means the velocity at the top of the twig is minus, 2, start bold text, i, end bold text, with, hat, on top, a leftward vector, while the velocity at the bottom of the twig is 0.
For the twig, this means the important factor for counterclockwise rotation is that vectors point more to the left as we move up the vector field. Said with a few more symbols, the important point here is that the start bold text, i, end bold text, with, hat, on top-component of a vector attached to a point left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis decreases as y, start subscript, 0, end subscript increases.
Said with even more symbols,
start fraction, \partial, divided by, \partial, y, end fraction, left parenthesis, minus, y, right parenthesis, equals, minus, 1, is less than, 0
Let's generalize this idea a bit.
• Question: Consider a more general vector field.
start bold text, v, end bold text, with, vector, on top, left parenthesis, start color #0c7f99, x, end color #0c7f99, comma, start color #bc2612, y, end color #bc2612, right parenthesis, equals, start color #0c7f99, v, start subscript, 1, end subscript, end color #0c7f99, left parenthesis, x, comma, y, right parenthesis, start bold text, i, end bold text, with, hat, on top, plus, start color #bc2612, v, start subscript, 2, end subscript, end color #bc2612, left parenthesis, x, comma, y, right parenthesis, start bold text, j, end bold text, with, hat, on top
The components start color #0c7f99, v, start subscript, 1, end subscript, end color #0c7f99 and start color #bc2612, v, start subscript, 2, end subscript, end color #bc2612 are any scalar-valued functions. If you place a small twig at some point left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis, oriented parallel to the y-axis, how can you tell if the twig will rotate just by looking at start color #0c7f99, v, start subscript, 1, end subscript, end color #0c7f99, comma, start color #bc2612, v, start subscript, 2, end subscript, end color #bc2612 and left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis?
• Answer: Look at the rate of change of start color #0c7f99, v, start subscript, 1, end subscript, end color #0c7f99 as start color #bc2612, y, end color #bc2612 varies near the point of interest, left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis:
start fraction, \partial, start color #0c7f99, v, start subscript, 1, end subscript, end color #0c7f99, divided by, \partial, start color #bc2612, y, end color #bc2612, end fraction, left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis, left arrow, start color gray, start text, space, S, u, g, g, e, s, t, s, space, c, o, u, n, t, e, r, c, l, o, c, k, w, i, s, e, space, r, o, t, a, t, i, o, n, space, i, f, space, n, e, g, a, t, i, v, e, space, end text, end color gray
If this is negative, it indicates that vectors point more to the left as y, start subscript, 0, end subscript increases, so rotation would be counterclockwise. If it is positive, vectors point more to the right as y, start subscript, 0, end subscript increases, indicating a clockwise rotation.

## The $\hat{\textbf{j}}$start bold text, j, end bold text, with, hat, on top-component

Next, let's see why the x, start bold text, j, end bold text, with, hat, on top component of the original vector field suggests counterclockwise rotation as well. This time, imagine a twig which is parallel to the x-axis. Specifically, put one end of the twig at the origin left parenthesis, 0, comma, 0, right parenthesis, and put the other at the point left parenthesis, 2, comma, 0, right parenthesis.
The vector attached to the origin is 0, but the vector attached to the other end at left parenthesis, 2, comma, 0, right parenthesis is 2, start bold text, j, end bold text, with, hat, on top, an upward vector. Therefore, the fluid pushes the right end of the stick upwards, and the left end experiences no force, so there will be a counterclockwise rotation.
For this second twig, the vertical component of vectors increases as we move right, suggesting counterclockwise rotation. That is to say, the y component of a vector attached to a point left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis increases as x, start subscript, 0, end subscript increases.
In the case of a more general vector field function,
start bold text, v, end bold text, with, vector, on top, left parenthesis, start color #0c7f99, x, end color #0c7f99, comma, start color #bc2612, y, end color #bc2612, right parenthesis, equals, start color #0c7f99, v, start subscript, 1, end subscript, end color #0c7f99, left parenthesis, x, comma, y, right parenthesis, start bold text, i, end bold text, with, hat, on top, plus, start color #bc2612, v, start subscript, 2, end subscript, end color #bc2612, left parenthesis, x, comma, y, right parenthesis, start bold text, j, end bold text, with, hat, on top
we can measure this effect near a point left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis by looking at the change in start color #bc2612, v, start subscript, 2, end subscript, end color #bc2612 as start color #0c7f99, x, end color #0c7f99 changes.
start fraction, \partial, start color #bc2612, v, start subscript, 2, end subscript, end color #bc2612, divided by, \partial, start color #0c7f99, x, end color #0c7f99, end fraction, left arrow, start color gray, start text, space, S, u, g, g, e, s, t, s, space, c, o, u, n, t, e, r, c, l, o, c, k, w, i, s, e, space, r, o, t, a, t, i, o, n, space, i, f, space, p, o, s, i, t, i, v, e, space, end text, end color gray

## Combining both components

Putting these two components together, the rotation of a fluid flowing along a vector field start bold text, v, end bold text, with, vector, on top near a point left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis can be measured using the following quantity:
start fraction, \partial, start color #bc2612, v, start subscript, 2, end subscript, end color #bc2612, divided by, \partial, start color #0c7f99, x, end color #0c7f99, end fraction, left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis, minus, start fraction, \partial, start color #0c7f99, v, start subscript, 1, end subscript, end color #0c7f99, divided by, \partial, start color #bc2612, y, end color #bc2612, end fraction, left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis
When you evaluate this, a positive number will indicate a general tendency to rotate counterclockwise around left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis, a negative quantity indicates the opposite, clockwise rotation. If it equals 0, there is no rotation in the fluid around left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis. If you are curious about the specifics, this formula gives precisely twice the angular velocity of the fluid near left parenthesis, x, start subscript, 0, end subscript, comma, y, start subscript, 0, end subscript, right parenthesis.
Some authors will call this the "two-dimensional curl" of start bold text, v, end bold text, with, vector, on top. This isn't standard, but let's write this formula as if "2d-curl" was an operator.
start text, 2, d, negative, c, u, r, l, end text, start bold text, v, end bold text, with, vector, on top, equals, start fraction, \partial, start color #bc2612, v, start subscript, 2, end subscript, end color #bc2612, divided by, \partial, start color #0c7f99, x, end color #0c7f99, end fraction, minus, start fraction, \partial, start color #0c7f99, v, start subscript, 1, end subscript, end color #0c7f99, divided by, \partial, start color #bc2612, y, end color #bc2612, end fraction

## Example: Analyzing rotation in a 2d vector field using curl

Problem: Consider the vector field defined by the function
\begin{aligned} \quad \vec{\textbf{v}}(x, y) = \left[ \begin{array}{c} \cos(x+y) \\ \sin(xy) \end{array} \right] \end{aligned}
Determine whether a fluid flowing according to this vector field has clockwise or counterclockwise rotation at the point
\begin{aligned} \quad p &= \left(0, \dfrac{\pi}{2} \right) \\ \end{aligned}
Step 1: Compute the start text, 2, d, negative, c, u, r, l, end text of this function.
start text, 2, d, negative, c, u, r, l, end text, start bold text, v, end bold text, with, vector, on top, equals

Step 2: Plug in the point left parenthesis, 0, comma, pi, slash, 2, right parenthesis.
start text, 2, d, negative, c, u, r, l, end text, start bold text, v, end bold text, with, vector, on top, left parenthesis, 0, comma, pi, slash, 2, right parenthesis, equals

Step 3: Interpret. How does the fluid tend to rotate near this point?

Let's watch a sample of particles in this fluid flow: