Space between caption and table in TeX

In lack of information, why you actually use scalebox, I assume, that your document (i) has two columns, and that (ii) you will be willing to use mdwtab packages which enables to design nice tables ...

With this my assumptions the MWE below generate:

documentclass[twocolumn]{article}
    usepackage{caption}
    usepackage{mdwtab}

    usepackage[showframe]{geometry}
    usepackage{lipsum}   

begin{document}
lipsum[1]
begin{table}[h]
    footnotesize
    setlength{tabcolsep}{3.5pt}
caption{Numerical example geometry}
label{tab_numerical_geo}
centering
begin{tabular}{|*{7}{Mc|}}
hlx{hvv}
d^A  & d^B  & d^C  & l^A_{12} & l^C_{12} & h^A & h^C 
hlx{vhv}
-0.4434 & 0.3455 & 0.7798 & 0.1023 & 0.1523 & 0.04 & 0.023 
hlx{vh}
end{tabular}
end{table}
lipsum[2-5]
end{document}

option showframe in geometry package is used only to show page layout. In real-life, it had to be omitted.

Addendum: If you don't like to use mdwtab you can stick with classic tabular elements: replace hlx{...} with hline and column type Ml with >{$}c<{$}. For this column type you had to load package array. Of course, obtained table (to my taste) is not so beautiful.

Regarding caption: if you use package caption, you will obtain better spacing between caption and table without any manual adjustment.

Some suggestions for typesetting the table are below. Without knowing your constraints it's difficult to be more concrete. To make sure the caption isn't getting too close to the top of the table, I've used a strut at the end of the caption.

    documentclass{article}
    usepackage{booktabs}
    begin{document}
    begin{table}
        caption{Numerical example geometrystrut}
        label{tab_numerical_geo}
        centering
        begin{tabular}{@{}ccccccc@{}}
        toprule
        $d^A$  & $d^B$  & $d^C$  & $l^A_{12}$ & $l^C_{12}$ & $h^A$ & $h^C$ 
        midrule
        $-0.4434$ & 0.3455 & 0.7798 & 0.1023 & 0.1523 & 0.04 & 0.023 
        bottomrule
        end{tabular}
    end{table}

    begin{table}
        caption{Numerical example geometrystrut}
        label{tab_numerical_geo2}
        catcode`!=active
        def!{phantom0}
        centering
        begin{tabular}{@{}lr@{}}
        toprule
        $d^A$       &$-0.4434$ 
        $d^B$       &0.3455    
        $d^C$       &0.7798  
        $l^A_{12}$  &0.1023       
        $l^C_{12}$  &0.1523    
        $h^A$       &0.04!!  
        $h^C$       &0.023!  
        bottomrule
        end{tabular}
    end{table}
    begin{table}
        caption{Numerical example geometrystrut}
        label{tab_numerical_geo3}
        catcode`!=active
        def!{phantom0}
        centering
        begin{tabular}{@{}lr@{qquad}lr@{}}
        toprule
        $d^A$       &$-0.4434$ &
        $d^B$       &0.3455    
        $d^C$       &0.7798   & 
        $l^A_{12}$  &0.1023       
        $l^C_{12}$  &0.1523   & 
        $h^A$       &0.04!!  
        $h^C$       &0.023!  
        bottomrule
        end{tabular}
    end{table}

    end{document}

Use package caption and resizebox:

documentclass[twocolumn]{article}
usepackage{caption}
usepackage{graphicx}
usepackage{booktabs}
usepackage{blindtext}
begin{document}
    blindtext
    begin{table}[!htb]
    caption{Numerical example geometrystrut}label{tab_numerical_geo}
    centering
    resizebox{linewidth}{!}{%
    begin{tabular}{@{}ccccccc@{}}toprule
    $d^A$  & $d^B$  & $d^C$  & $l^A_{12}$ & $l^C_{12}$ & $h^A$ & $h^C$    midrule
    $-0.4434$ & 0.3455 & 0.7798 & 0.1023 & 0.1523 & 0.04 & 0.023 bottomrule
    end{tabular}}
    end{table}

    blindtext
end{document}

Some suggestions:

• To create a bit more spacing between the caption and the tabular material, load the caption package and specify the desired value for the option skip; in the example below, I set skip=0.5baselineskip.

• Don't use a center environment inside a table; instead, use the centering macro.

• Since the material in the data row can, apparently, contain negative numbers, use an array environment instead of a tabular environment. Doing so will free you from having to type lots of $ symbols in the header row.

• If you must use a smaller font size, don't use scalebox, as doing so will create very "spindly-looking" output. Instead, use small (for a 10% linear reduction of the font size) or footnotesize (for a 20% linear reduction).

• For better spacing in the header and data rows, insert typographic struts.

documentclass{article}
usepackage[skip=0.5baselineskip]{caption}
%% define a few struts
%% (from code by Claudio Beccari in TeX and TUG News, Vol. 2, 1993)
newcommandTstrut{rule{0pt}{2.9ex}}         % "top" strut
newcommandBstrut{rule[-1.2ex]{0pt}{0pt}}   % "bottom" strut
newcommandTBstrut{TstrutBstrut}           % "top and bottom" strut

begin{document}

begin{table}
caption{Numerical example geometry}
label{tab_numerical_geo}
small % better than scalebox{0.9}{...}
centering
$begin{array}{|*{7}{c|}}
hline
d^ATBstrut & d^B  & d^C  & l^{A}_{12} & l^C_{12} & h^A & h^C 
hline
-0.4434TBstrut & 0.3455 & 0.7798 & 0.1023 & 0.1523 & 0.04 & 0.023 
hline
end{array}$
end{table}
end{document}

Try to add bigskip after the caption. It will add a blank line which will create a desired space.

documentclass{article} 
usepackage[utf8]{inputenc}
begin{document}
begin{table}[h]
caption{Numerical example geometry}
bigskip
label{tab_numerical_geo}
begin{center}
scalebox{0.9}{
...

Came with the same issue, I tried the vspace{5mm} option. Worked easily well for me.

You can put begin{center} your table spec end{center} after your caption in the begin{table}.