Double-bar notation for second-order tensors4

I found it interesting to represent a tensor with the notation normally used by my professor. Looking for respect to the mathematical symbol I found this code from this post, very interesting.

I would like to insert this command below,I modified the code but the result with '\underline' is not pleasant

\newcommand*{\rttensor}{\underline{\underline{#1}}}
\overline{u'_{i}u'_{j}u'_{i,j}} = \rttensor{T}


but the result is not satisfactory. The command with \bar is aesthetically superior, but I can not find a way to put it underneath

\newcommand*{\rttensortwo}{\bar{\bar{#1}}}

\overline{(\rho'u'_{i})} = \rttensortwo{T}


I think another possible solution could be this, but I have some doubts about it and I have to look more carefully.

others have had experience with this?

MWE

\documentclass{article}
\usepackage{amsmath}
\newcommand*{\rttensor}{\underline{\underline{#1}}}
\newcommand*{\rttensortwo}{\bar{\bar{#1}}}

\begin{document}

$\overline{u'_{i}u'_{j}u'_{i,j}} = \rttensor{T}$
\par\bigskip
$\overline{(\rho'u'_{i})} = \rttensortwo{T}$
\end{document}


I found a problem with the position of the double bar in a certain type of tensor

\documentclass{article}
\usepackage{amsmath}
\usepackage{stackengine}

\newcommand*{\barbI}{\stackunder[1.0pt]{$#1$}{\rule{1.0ex}{0.095ex}}} %% One bar
\newcommand*{\barbII}{\barbI{\barbI{#1}}}%% double bar
\newcommand{\TRII}{\ensuremath{ \barbII{T_{R}}}} %% Tensore di Reynols

\begin{document}

$\TRII$ \quad\text{Tensore di Reynolds}\\

$\barbI{T}$\\

\end{document}


-------------------UPDATE-----------------

\documentclass{article}
\usepackage{amsmath}
\usepackage{stackengine}

\newcommand*{\barbI}{\stackunder[1.0pt]{$#1$}{\rule{0.9ex}{0.095ex}}}
\newcommand*{\barbII}{\barbI{\barbI{#1}}}
\newcommand{\TRII}{\ensuremath{\barbII{\emph{T}}_{\!R}}}

\begin{document}

\begin{enumerate}
\item $\barbII{T}_{R}$
\item   $\TRII$ < ----- Optimal Solution
\end{enumerate}

\end{document} • I took the liberty to add in the minimal working example (MWE). In the future please provide one when posting a question here on StackExchange. – Troy Feb 26 '18 at 11:52
• Note that the {\bar{\bar{#1}}}syntax embodied in your \rttensortwo definition only seems to properly align the bars with Lualatex, but not with pdflatex nor xelatex. – Steven B. Segletes Feb 26 '18 at 12:07

Do you mean something like \barbelow in this MWE, which can be recursively used? Taken from my answer at \bar below symbol

\documentclass{article}
\usepackage{stackengine}
\begin{document}
\newcommand\barbelow{\stackunder[1.2pt]{$#1$}{\rule{.8ex}{.075ex}}}
$$\overline{A} \bar{A} \barbelow{A} \barbelow{\barbelow{A}}$$
\end{document} • @Steven B. Segletes your answer is Excellent, I do not understand why in my texstudio it signals the command \barbelow in red as if it were an error, but when to compile the pdf it's all ok. I would like to clarify this before closing and accepting your answer – Antonio Feb 26 '18 at 18:50
• @Antonio I don't use texstudio and so I am not quite sure. From a LaTeX perspective, it is essential to include stackengine as a used package, and to define \barbelow (best in the preamble, though I did not do so) according to LaTeX syntax. Perhaps this question, tex.stackexchange.com/questions/299628/…, would help with the red highlighting. – Steven B. Segletes Feb 26 '18 at 18:54
• @StevenB.Segletes yes, I read it is a problem related of the syntax checking in TeXstudio. it was very useful to read the post you suggested. – Antonio Feb 26 '18 at 20:12