( ( A ( shock tube ) ) ( ( ( was therefore ) ( investigated ( with ( ( ( ( low ( density gas ) ) ( along ( the ( tube axis ) ) ) ) and ) ( ( high ( density gas ) ) ( along ( the walls ) ) ) ) ) ) ) . ) )
(ROOT (S (NP (DT A) (NN shock) (NN tube)) (VP (VBD was) (ADVP (RB therefore)) (VP (VBN investigated) (PP (IN with) (NP (NP (NP (JJ low) (NN density) (NN gas)) (PP (IN along) (NP (DT the) (NN tube) (NNS axis)))) (CC and) (NP (NP (JJ high) (NN density) (NN gas)) (PP (IN along) (NP (DT the) (NNS walls)))))))) (. .)))
A shock tube was therefore investigated with low density gas along the tube axis and high density gas along the walls.
(ROOT (S (NP (NNS Gases)) (VP (VBP have) (NP (DT the) (JJS lowest) (NN density))) (. .)))