curl {oce} | R Documentation

## Curl of 2D vector field

### Description

Calculate the z component of the curl of an x-y vector field.

### Usage

    curl(u, v, x, y, geographical=FALSE, method=1)

### Arguments

`u` | matrix containing the 'x' component of a vector field
`v` | matrix containing the 'y' component of a vector field
`x` | the x values for the matrices, a vector of length equal to the number of rows in `u` and `v`.
`y` | the y values for the matrices, a vector of length equal to the number of cols in `u` and `v`.
`geographical` | logical value indicating whether `x` and `y` are longitude and latitude, in which case spherical trigonometry is used.
`method` | A number indicating the method to be used to calculate the first-difference approximations to the derivatives. See “Details”.

### Details

The computed component of the curl is defined by dv/dx - du/dy and the estimate is made using first-difference approximations to the derivatives. Two methods are provided, selected by the value of `method`.

- For `method=1`, a centred-difference, 5-point stencil is used in the interior of the domain. For example, dv/dx is given by the ratio of v[i+1,j]-v[i-1,j] to the x extent of the grid cell at index j. (The cell extents depend on the value of `geographical`.) Then, the edges are filled in with nearest-neighbour values. Finally, the corners are filled in with the adjacent value along a diagonal. If `geographical=TRUE`, then `x` and `y` are taken to be longitude and latitude in degrees, and the earth shape is approximated as a sphere with radius 6371km. The resultant `x` and `y` are identical to the provided values, and the resultant `curl` is a matrix with dimension identical to that of `u`.

- For `method=2`, each interior cell in the grid is considered individually, with derivatives calculated at the cell center. For example, dv/dx is given by the ratio of 0.5*(v[i+1,j]+v[i+1,j+1]) - 0.5*(v[i,j]+v[i,j+1]) to the average of the x extent of the grid cell at indices j and j+1. (The cell extents depend on the value of `geographical`.) The returned `x` and `y` values are the mid-points of the supplied values. Thus, the returned `x` and `y` are shorter than the supplied values by 1 item, and the returned `curl` matrix dimensions are similarly reduced compared with the dimensions of `u` and `v`.

### Value

A list containing vectors `x` and `y`, along with matrix `curl`. See “Details” for the lengths and dimensions, for various values of `method`.

### Development status.

This function is under active development as of December 2014 and is unlikely to be stabilized until February 2015.

### Author(s)

Dan Kelley and Chantelle Layton

### Examples

    library(oce)
    ## 1. Shear flow with uniform curl.
    x <- 1:4
    y <- 1:10
    u <- outer(x, y, function(x,y) y/2)
    v <- outer(x, y, function(x,y) -x/2)
    C <- curl(u, v, x, y, FALSE)

    ## 2. Rankine vortex: constant curl inside circle, zero outside
    rankine <- function(x, y)
    {
        r <- sqrt(x^2 + y^2)
        theta <- atan2(y, x)
        speed <- ifelse(r < 1, 0.5*r, 0.5/r)
        list(u=-speed*sin(theta), v=speed*cos(theta))
    }
    x <- seq(-2, 2, length.out=100)
    y <- seq(-2, 2, length.out=50)
    u <- outer(x, y, function(x,y) rankine(x,y)$u)
    v <- outer(x, y, function(x,y) rankine(x,y)$v)
    C <- curl(u, v, x, y, FALSE)
    ## plot results
    par(mfrow=c(2,2))
    imagep(x, y, u, zlab="u", asp=1)
    imagep(x, y, v, zlab="v", asp=1)
    imagep(x, y, C$curl, zlab="curl", asp=1)
    hist(C$curl, breaks=100)

[Package oce version 0.9-18 Index]