# Polar to cartesian coordinates for stream function

1. Jun 18, 2011

### xzi86

1. The problem statement, all variables and given/known data
Consider a velocity field where the radial and tangenetial components of velocity are V_r=0 and V_theta=cr, respectively, where c is a constant. Obtain the equations of the streamlines.

2. Relevant equations
x=rcos(theta)
y=rsin(theta)

3. The attempt at a solution
I know how to obtain the equations of the streamlines. I don't know how to translate the polar coordinates into cartesian. Since V_r=0, wouldn't there be no x-component? But looking at the textbook solution, there is an x component. Any help would be appreciated. Thanks.

2. Jun 18, 2011

### tiny-tim

hi xzi86!

(try using the X2 icon just above the Reply box )

Vr = 0 means there is no radial component …

in which (x,y) direction is the radial direction?

3. Jun 18, 2011

### Karlx

x=x(r,$\theta$)=rcos$\theta$
y=y(r,$\theta$=rsin$\theta$

You are given

V$_{r}$=$\frac{dr}{dt}$=0

V$_{\theta}$=$\frac{d\theta}{dt}$=cr

How do you calculate

V$_{x}$=$\frac{dx}{dt}$

and

V$_{y}$=$\frac{dy}{dt}$ ?