Try An experimental investigation of the design variables for NACA Submerged Duct Entrances
RM A7I30, 1948, Mossman, Randall
Or Theoretical investigation of submerged inlets at low speeds, TN 2323, 1951
A hint on searching thru the NACA reports: don't bother with their search. Open All Citations, and use your browser's search-on-page. Submerged Duct and Submerged Inlet are a subject of several papers.
STF