# Homework Help: Fluid Dynamics question

1. Mar 6, 2017

### shreddinglicks

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution
I don't even know where to start. I don't understand the question.

2. Mar 6, 2017

### Staff: Mentor

Have you learned that the speed of the flow is proportional to the magnitude of the gradient of the stream function?

3. Mar 6, 2017

do you mean

V^2=u^2+v^2

where

4. Mar 6, 2017

### Staff: Mentor

Yes. But you need to express theta in terms of x and y to evaluate these derivatives.(except, of course, far upstream).

5. Mar 6, 2017

### shreddinglicks

So would it be better to use

and make y = rsin(theta)

6. Mar 6, 2017

### shreddinglicks

Wait, I see what you mean

7. Mar 6, 2017

8. Mar 6, 2017

9. Mar 6, 2017

### Staff: Mentor

I'm not going to check your math. I leave it to you to get the math correct.

10. Mar 6, 2017

### shreddinglicks

That's fine. I'm not here to learn math. Since I now have V^2 what do I do from here? I still don't understand the question I need to solve.

11. Mar 6, 2017

### Staff: Mentor

You need to show that, at the x and y corresponding to theta = 66.8 degrees and psi = 0, the speed is the same as at y = 0, x = infinity

Last edited: Mar 6, 2017
12. Mar 6, 2017

### shreddinglicks

So I know at a large value of -x and y= o that my V^2 is equal to 1.

would it be appropriate to sub in
x=rcos(theta)
y=rsin(theta)
r=x^2+y^2
and then plug in 66.8 = theta

13. Mar 6, 2017

### Staff: Mentor

You have to evaluate it at psi = 0.

14. Mar 6, 2017

### shreddinglicks

Bernoulli eq?

15. Mar 6, 2017

### Staff: Mentor

16. Mar 6, 2017

### shreddinglicks

The only thing I can think of that would relate the velocity eq and pressure would be that. Is that what I should be using?

17. Mar 6, 2017

### Staff: Mentor

No. You should be setting psi = 0 and theta = 66.8 degrees (in radians). This gives you an equation for y. Once you know y and theta, you know x.

18. Mar 6, 2017

### shreddinglicks

I see exactly what you mean. I must be losing my mind thinking psi is pressure. I did exactly what you said and got x and y. I plugged into the the V^2 equation and got 1 as my answer.

19. Mar 6, 2017

### shreddinglicks

Thanks for helping me. I can sleep easy tonight. You are the man Chestermiller!