The author guidelines for the Journal of Computational Physics do not specify a word limit for the abstract, saying only that it should be 'brief'. But should I impose a word limit myself? I have heard that some indexing services can truncate overly-long abstracts, though I've not come across evidence that this is this case.
Yes, you should limit yourself because the goal of the abstract is to be short. Now, where to put the limit?
A good idea would be to look at recently published articles in this journal and note their length.
An abstract should sell your paper, introduce the family of constructs/theies used and may indicate the key narrative/outcomes. It should end after that so that one needs to read the paper. The better journals usually ask for stricter limits. (Journals of ) Pure sciences probably have editors wanting to allow you to use complete sentences and sensible sentences even in the abstract so skipped the abstract word limit out of politeness..