# How to Sketch Streamlines

So how can we use the velocity components,

and the stream function to actually sketch what the streamlines are in an actual flow.

Well lets look at an example. Lets say we have velocity components such that u equals

4y+3, and v equals 2x+6, and remember when we are defining theses stream functions there

are in 2-D. So the first thing we want to know. Is this physically possible flow. What

that means is, is this in-compressible, and that for that to be the case du/dx plus dv/dy,

as to equal 0, and just by looking at it you can tell that is the case. Now lets introduce

our stream function. So by definition u is the derivative of the stream function with

respect to y, which is 4y plus 3. So we separate and integrate in order to find this part of

the stream function, and when we do that we come up with. That the stream function equals

2y^2+3y+f(x). Now lets do it for v. So v is the negative derivative of the stream function

with respect to x, which equals 2x+6. Again we separate and integrate, and this time it

is (2x+6) dx, but lets not forget that negative sign. So now our stream function equals negative

x^2-6x+f(y), and when we combine these two. We come up with a stream function that is

2y^2 plus 3y minus x^2 minus 6x plus a constant. So how do we go from there to actually drawing

stream lines. Well the first thing we are going to do is set this constant equal to

0, and in order to get different stream lines we change the number of the stream function

and change it to different constants. So we can set this equal to 0, we can set it equal

to 1. We can set it equal to 2. We can set it equal to any constant we want. So let me

show you what happens, and how you work this if we set this stream function equal to 0.

Now you have to solve this in terms of y, and so our y is going to equal minus 3/4 plus

or minus 1/4 times the square root of 9 plus 8, x^2 plus 48 times x. By the way I did not

figure this out on my own. I put it in a computer program, and let it solve for me. So now that

you have this equation, what do you do with it. Well you take something like excel, and

you have an x column, a y1 column, and a y2 column, and your x’s can go from any number

that you want generally you are going to want to start with 0. So what I did here is I went

from 0, 0.1 all the way to 1. Now you solve for your y1, such that you use the positive,

minus 3/4 plus 1/4, etc, and then you use it for the negative, and then you plot it.

What you will end up with is a plot that looks like this. So these are your streamlines,

and you can do it negative x’s, etc. These are your streamlines for our phi, excuse me

our streamline equals 0. Now instead of setting it equal to 0. You can use this stream function

to equal for example 2 or 4, and you solve it for y, except now your equation is going

to be for example 2 equals 2y^2 plus 3y, minus x^2, minus 6x. So you solve for y, and then

plot this particular series or families of streamlines

thanks! it was helpful! 🙂

thanks but your handwriting is awful sorry for saying that, i cant read the equation you write at 3:22

nice than

Thank you! That was really helpful!!

Thanks a lot.

Thank you..really helped me for my test tmw

The equations of u with (xi)…are having wrong sign notation. it should have negative sign with (xi).

u= – do (xi)/ do (y) & v = + do(xi)/ do(x)

u say fx is fucnction of x but then u take it as a constant?

thanks

Excellent video.This explained exactly what I needed in a very efficient manner.

why no one is explaining these eq. for 3 dimensions..??

do they exist..?

How do you solve for y? Anyone please help!

Worst video …. With no logics

I have a very complicated function. Setting c=0 is going to be my go to at all times lmao