[Physics] Obtaining the maximum velocity of a flow and drawing the velocity profile

flowfluid dynamicshomework-and-exercises

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I am able to do everything here apart from the last two lines, I am pretty certain that the corresponding fluid flow is $w(x,t)=\frac{\Lambda}{4vt}*\frac{x}{\sqrt 4vt}*exp(-\frac{x^2}{4vt})$. However I am uncertain as to how to calculate the maximum velocity, I think it is when x tends to infinity. Furthermore i am uncertain of how to obtain the velocity profile.

Best Answer

1- the maximum velocity is the maximum value of $\omega$ considered as a function of $x$. Just derive it twice with respect to $x$ and set the condition for a maximum (I will put my solution at the end, in case you want to try on your own)

2- the velocity profile is $\omega(x, t=t^*)$ for a fixed time $t^*$. It's just the value of the function $\omega$ at a given time, which is now just a function of $x$.

$\textbf{Solution}$:

1- $${d\omega \over dx}={\Lambda\over (4\nu t)^{3\over 2}}e^{-{x^2\over (4\nu t)}}\left(1-{x^2\over 2\nu t}\right) $$ which is equal to zero for $$x=\pm\sqrt{2\nu t}$$

Thus the max velocity is $$\omega(x=\sqrt{2\nu t}, t)={\Lambda\over 4\nu t\sqrt{2}}e^{-{1\over 2}} $$

(and the minimum one is the same with a minus sign, which is the other solution of the derivative).

2- the velocity profile for $t=1s$ is the following function of $x$: $$\omega({x, t=1s})={\Lambda x \over (4\nu)^{3\over 2}}e^{-{x^2\over (4\nu)}}$$

the velocity profile for $t=2s$ is

$$\omega({x, t=2s})={\Lambda x \over (8\nu)^{3\over 2}}e^{-{x^2\over 8\nu)}}$$

and so on.. You just have the value of $t$ inside your $\omega(x, t)$ formula. At each time there is a different profile.

Here is a sketch of different profiles at different times (I set $\Lambda=1$ and $4\nu =1 $). The bigger the lobe, the smaller the times. Selected times are $t=0.5, 1, 2$. Y axis is $\omega$:

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