Saturday, 28 January 2017

fluid dynamics - Where can I check a solution to 3D Navier Stokes?


A few years ago I developed a solution to the Navier-Stokes equations and as of yet have not been able to locate a similar version of the solution. I would like to know if anyone has seen a solution like this or can spot any significant errors.


The version of the equations I worked with are as follows, where I set $\nu = 1$:


$\partial_tu+u\partial_xu+v\partial_yu+w\partial_zu = -\partial_xp + \nu(\partial_{xx}u+\partial_{yy}u+\partial_{zz}u)$


$\partial_tv+u\partial_xv+v\partial_yv+w\partial_zv = -\partial_yp + \nu(\partial_{xx}v+\partial_{yy}v+\partial_{zz}v)$


$\partial_tw+u\partial_xw+v\partial_yw+w\partial_zw = -\partial_zp + \nu(\partial_{xx}w+\partial_{yy}w+\partial_{zz}w)$


$\partial_xu+\partial_yv+\partial_zw=0$




The original solution I came up with is (recognizing that $\sin^2(x)+\cos^2(x)=1$, which is important to keep expanded in order to easily see the cancellations):


$p =-(\sin^2(x) + \cos^2(x))e^{-\frac{1}{2}(|y|+|z|+|t|)}$



$u =\sin(x)e^{-\frac{1}{2}(|y|+|z|+|t|)}$; $v =\cos(x)e^{-\frac{1}{2}(|y|+|z|+|t|)}$; $w =\cos(x)e^{-\frac{1}{2}(|y|+|z|+|t|)}$


I have checked to see if this works several times, but have always wondered if I made a mistake, the relevant derivations are below, where $cos(x)=c_x$ and $sin(x) =s_x$.






$\partial_x{u} =c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_x{v} =-s_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_x{w} =-s_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$


$\partial_y{u} =\frac{-1}{2}s_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_y{v} =\frac{-1}{2}c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_y{w} =\frac{-1}{2}c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$


$\partial_z{u} =\frac{-1}{2}s_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_z{v} =\frac{-1}{2}c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_z{w} =\frac{-1}{2}c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$




$\partial_t{u} =\frac{-1}{2}s_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_t{v} =\frac{-1}{2}c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_t{w} =\frac{-1}{2}c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$







$\partial_{xx}{u} =-s_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_{xx}{v} =-c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_{xx}{w} =-c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$


$\partial_{yy}{u} =\frac{1}{4}s_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_{yy}{v} =\frac{1}{4}c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_{yy}{w} =\frac{1}{4}c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$


$\partial_{zz}{u} =\frac{1}{4}s_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_{zz}{v} =\frac{1}{4}c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\partial_{zz}{w} =\frac{1}{4}c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$




$\sum{} =\frac{-1}{2}s_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\sum{} =\frac{-1}{2}c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$; $\sum{} =\frac{-1}{2}c_xe^{-\frac{1}{2}(|y|+|z|+|t|)}$






$u\partial_x{u} =s_xc_xe^{-(|y|+|z|+|t|)}$; $u\partial_x{v} =-s^2_xe^{-(|y|+|z|+|t|)}$; $u\partial_x{w} =-s^2_xe^{-(|y|+|z|+|t|)}$


$v\partial_y{u} =\frac{-1}{2}s_xc_xe^{-(|y|+|z|+|t|)}$; $v\partial_y{v} =\frac{-1}{2}c^2_xe^{-(|y|+|z|+|t|)}$; $v\partial_y{w} =\frac{-1}{2}c^2_xe^{-(|y|+|z|+|t|)}$



$w\partial_z{u}=\frac{-1}{2}s_xc_xe^{-(|y|+|z|+|t|)}$; $w\partial_z{v}=\frac{-1}{2}c^2_xe^{-(|y|+|z|+|t|)}$; $w\partial_z{w}=\frac{-1}{2}c^2_xe^{-(|y|+|z|+|t|)}$




$\sum{}=(s_xc_x-s_xc_x)e^{-(|y|+|z|+|t|)}$;$\sum{}=-(s^2_x + c^2_x)e^{-(|y|+|z|+|t|)}$;$\sum{}=-(s^2_x + c^2_x)e^{-(|y|+|z|+|t|)}$






I have played a little bit with the thought that there are actually three versions of the solution, one oriented for each axis as illustrated below:


Velocity matrix


Where the pressure component can be added as follows:


velocity with pressure


Note: Just an addendum, as pointed out in the comments, my earliest version of this was sans the absolute value symbols, which allows for the solution to approach infinite values in negative coordinates. My initial thought was that the absolute values were a sufficient constraint, but as pointed out in the comments this results in a discontinuity in some of the derivatives when values are set to zero. I have not yet explored if this is a real singularity or a coordinate one, since the solution appears singularity free in the positive domain.




Answer



Solutions of the form


$$ cos(x_i)e^{-x_j}$$


are common specific solutions of the Navier-Stokes equations in simplified (not simple) problems. These are however problems where inertia is ignored, which you include. (Please note that I am using index notation, with $i,j\in\{1,2,3\}$). $x_j$ is then the wall normal direction. This is actually quite well related to the remarks of Jaime.


I did not check your solutions, and I am not very familiar with the literature on this topic, but it is interesting that the same trick also works in three dimensions. The only thing is, that you found three separate solutions, and a linear combination of the three is most likely no longer a solution due to the non-linearity of the equations.


You will probably also find that a solution in the form


$$ u_n(\vec{x})=cos(k_nx_i)e^{-l_nx_j}$$


also satisfies the equations. In the cases without inertia, you can write the solution as a linear combination and you get:


$$ u=\sum_n u_n(\vec{x})$$


where $k_n$ and $l_n$ take control of the different sizes of flow features.



No comments:

Post a Comment

Understanding Stagnation point in pitot fluid

What is stagnation point in fluid mechanics. At the open end of the pitot tube the velocity of the fluid becomes zero.But that should result...