B737200 From Malta, joined Feb 2005, 225 posts, RR: 2
Reply 2, posted (4 years 8 months 1 day 9 hours ago) and read 1824 times:

Its actually a fluids problem but I got stuck on the mathematical part.

The problem has water flowing out of a tank through an orifice whilst water is also flowing into the tank.

I ended up with an integral which I solved using a given result and I ended up with the above equation. It should be correct since my calculator worked it out and the answer matched but our lecturer is a bit fussy so I'm not sure if he'd except that in an exam.

I'll take a look at the link you sent, it seems somewhat familiar, maybe we did it last year in our maths unit. Hopefully something like that would be in the equation sheet we get, good luck trying to remember all the funny mathematical solutions to stuff that MIGHT come out. I get the feeling he is going to test us on mathematics as much us he is going to test us on our fluid's knowledge.

What nudged me towards the iteration method was the fact that the question states the we are to give an approximate answer; of course using a series solution would also yield and approximate value so maybe this is it.

Thanks, series totally slipped my mind, maybe its because I can't stand them

Yellowstone From United States of America, joined Aug 2006, 3071 posts, RR: 4
Reply 4, posted (4 years 8 months 1 day 1 hour ago) and read 1647 times:

Quoting B737200 (Reply 2): What nudged me towards the iteration method was the fact that the question states the we are to give an approximate answer; of course using a series solution would also yield and approximate value so maybe this is it.

What you want to use here is something called Newton's method.

First, rewrite your equation so that it's equal to zero. f(a) = a + ln(a) - 2.64 = 0

Next, you pick some value of a, call it a_0, that is decently close to the right answer. Then, calculate the following, where f ' (a_0) is the derivative of the function f evaluated at a_0.

a_1 = a_0 - ( f(a_0) / f ' (a_0) )

Calculate a_2 in the same fashion, replacing a_0 with a_1. Repeat until a_n converges.

Hydrogen is an odorless, colorless gas which, given enough time, turns into people.

B737200 From Malta, joined Feb 2005, 225 posts, RR: 2
Reply 6, posted (4 years 8 months 15 hours ago) and read 1532 times:

Firstly thanks for all the input, I wasn't expected this much help for my little mathematical conundrum.

Quoting Yellowstone (Reply 4): What you want to use here is something called Newton's method.

I was actually studying it this week, when I was trying the fluids problem I hadn't really gone over my maths note yet. Seems to bring a bit of order to my "lets just throw in some values" method.

Quoting Fabo (Reply 5): Why repeat? If I understood right, you can use limit. You should get to about 1.96467 if I am not mistaken

By using a limit you mean with Newton's method? The version we learned didn't use that, I'll look around but if I misunderstood you I'll be on a wild goose chase for nothing.

Quoting Fabo (Reply 5): I will try figuring out how to isolate a, but do not expect much from me right now...

Don't worry if you're busy, I understand and thanks for your input.

Quoting BMI727 (Reply 3): Perhaps looking at the differential equations for a mixing problem would be of help.

I derived a differential equation. The one I derived was:

Q_out - Q_in = A_tank * (dh/dt) where Qs are volume flow rates, A is area and h is the height of water level in the tank.

On working it out using a given result printed under the question itself I ended up with the 2.64 = lna +a.