I'm writing a piece in which I need string instruments to switch regularly between flautando and normal timbre. How do I denote this? Basically, I need a word which means "normal" with regard to where the bow falls.
Normale (norm.) or ordinario (ord.) are the standard methods for marking a return to regular articulation after using an alternative technique.
In doubt, you can always negate a specific playing instruction by mentioning it with a non prefix. In this case I'd probably write non flaut., if in your context ord. would not be clear enough.
Alternatively you could consider what particular sonic quality of “normal bowing” you want to contrast against flautando. Should it be the robust sound and attack? You might alternate between flautando and pesante. You want the opposite of the sweet airiness of a flautando? You could alternate between sul tasto and sul ponticello. Etc. etc..
In such cases, the notation arco is typically given to request that the performers play with standard technique.
So just as you specify flautando for some portions, simply specify arco for the other portions.