%&program=xelatex
%&encoding=UTF-8 Unicode
\documentclass{article}
\usepackage{fontspec}
\usepackage{polyglossia}
\newfontfamily\skt[Language=Sanskrit,Script=Devanagari]{FreeSerif}
\newfontfamily\Skt[Script=Devanagari]{FreeSans}
\def\V{\addfontfeature{Mapping=velthuis-sanskrit}}
\frenchspacing
\renewcommand{\baselinestretch}{1.1}\selectfont
\parskip\baselineskip
\begin{document}

{\skt विकर्णनीय  द्वि-आधारी संक्रिया}

{\skt\V vikar.naniiya dvi-aadhaari sa.mkriyaa}

{\Skt विकर्णनीय  द्वि-आधारी संक्रिया}

{\Skt\V vikar.naniiya dvi-aadhaari sa.mkriyaa}

\end{document}