Perhaps you should try some spelling variations:
"discrete level simulation" » "discrete model simulations", "discrete event simulation", "discrete line simulation", "discrete least simulation"
"discrete model simulation" » "discrete modeling simulation", "discrete modelling simulation", "discrete models simulation"

Try widening your search to All Fields.

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• The words AND, OR and NOT may confuse the search; try adding quotes: "((\"discrete level simulation\") OR (\"discrete model simulation\"))".
• Removing quotes may allow a broader search: (( discrete level simulation ) OR ( discrete model simulation )).
• Adding a wildcard symbol may retrieve word variants: (("discrete level simulation") OR ("discrete model simulation"))*.