Blasius equation

j4vier

im wreckin my head around an excercise , i know it should be easy but its bein a while since i had to handle these equations so i hope some1 from around here can help me out

basically the equation is in the form:

f=0.316Re ^ -0.25


Now i have a series of values for f and Re.

I am told to plot lnf vs lnRe and to see how the values i get compare to 0.316 and -0.25

my question is if i plot lnf vs lnRe should that be a straight line? would the slope represent the -0.25 of the previous equation and the intercept the 0.316?

if any1 can help me out would be great, thanks!

Mary-Ellen

Hey there,

Not so sure I understand the question
By "inf" do you mean integral of f otherwise what do you mean? :rolleyes:

You could try plot whatever you're looking for with some type of mathsie programming language too, matlab and maple are the ones I've used.

j4vier

sorry thats meant to be an L , as Lnf , natural log of f.

i did a couple of graphs already with excel but im a bit far from the true values .
dunno might be the usual experimental error :rolleyes:

Mary-Ellen

Tee hee about not recognising Ln, I can be a little ditsy:p

To be honest I'm not in the mood to use my head overly much but::

My initial thoughts would be

It depends on what you're gonna plot

if you plot "Lnf" it'll look different cause of the log on the right

but if you're plotting "f"
you get the inverse log of both sides to get the variable f on it's own
on the left and that leaves you with (0.316*Re^(-0.25)) on the right so essentially you're still plotting the original equation.

Apologise in advance if i'm wrong:D

j4vier

i dunno im more confused than before now

well i now for sure that LnF vs lnRe is the way to go cos im told to do so

if i remember right the -0.25 part will then no longer be exponential so i ll probably end up with a straight line or not?!?

i better leave it till the morning i think..

thanks for now mary ellen

Mary-Ellen

No bother,:)

I'll have a look again in the morning too

and if I'm around college tomorrow I'll pop it into maple for you

humbert

I don't know if I'm following this at all but....

f = a*x^b

ln(f) =ln(a*x^b) = ln(a) + ln(x^b) = ln(a) + b*ln(x)

which is in the form of f = mx + c

So I'd guess the slope is -0.25 and the intercept is ln(0.316)

j4vier

thats exactly what i was thinkin of
humbert ur 100% sure of that?
thx a mill this thing was startin to annoy me

humbert

They're just log laws. You can do a quick google and verify it yourself. I've never heard of this Blasius fellow though, so I can't guarantee that there isn't more to the question.

j4vier

ok thanks for the help!

dudara

j4vier wrote:

basically the equation is in the form:

f=0.316Re ^ -0.25

ln(f) = ln(0.316Re ^ -0.25)

ln(f) = ln(0.31) + ln(Re^-0.25)

ln(f) = ln(0.31) -0.25*ln(Re)

So yes, it looks like there should be a linear relationship, with a slope of -0.25 and an intercept of ln(0.31)

j4vier

thanks dudara
i know im on the right track now