The phrase "strike upon" refers to discovering or encountering something unexpectedly. There are several synonyms that can be used in place of this phrase to add variety to your writing. For example, "stumble upon" can be used to convey the idea of discovering something by chance. "Chance upon" or "come across" are also useful synonyms that imply a fortuitous encounter. If you want to convey a sense of suddenness or surprise, you might use "happen upon" or "encounter unexpectedly." Each of these phrases is a great way to vary your writing and add a touch of specificity to your descriptions.