If you are near a large academic library (preferably at a university), you can get on-site access to many commercial databases. Ebsco's Education Research Complete gets 18 hits for the search: (Flipping or Inverting) and classroom*. Their default search is author, title, journal name, abstract, and subjects; and you can also limit to "scholarly/peer reviewed" either before OR after you do a search. You can also put the entire search string all into one box, even if you use the advanced search that has several boxes. You can also "Select a field" TX all text, and use "proximity search" to find words/phrases close together (at least in one place in each full text article). For example, the TX search: (Flipping or Inverting) n20 classroom* gets 47 hits in peer reviewed. Proximity search is tricky, and varies greatly among databases so I often don't risk trying to group the words into a set. But it seems to work in Ebsco; flipping n20 classroom* gets 29 peer reviewed and inverting n20 classroom* gets 19.
Some smaller universities, and many colleges will NOT have Education Research Complete, but most will at least have Academic Search Premier (also Ebsco), which gets 14 hits (6 peer reviewed) for the default search: (Flipping or Inverting) and classroom*. TX (Flipping or Inverting) n30 classroom* gets 37 peer reviewed.
Virtually all universities will have Proquest's Dissertations and Theses Full text. The searches will be very similar to Ebsco, except for Full Text proximity search use w/20, w/30 etc instead of n20, n30, etc. Even if you are not near a large university, you can get a little taste of this searching in Elsevier's ScienceDirect, which is all of that huge academic publisher's journals - and covers far more than just "science". It gets 21 hits for the default full text search: (inverting or flipping) w/30 classroom*. But ScienceDirect allows free SEARCH only - they want $30+ per article, unless you are on campus or logged into a proxy.
If you are far from a college or university, most public libraries will have some general interest databases such as Proquest full text journals or Ebsco's Masterfile. You can even get a little free fulltext from the web version of ERIC, which gets 20 hits (4 free online) for: (Flipping or Inverting) and classroom*. ERIC has a much cruder search engine than Ebsco, and it may be much wiser to go with exact phrases: flipping classrooms or inverting classrooms gets 3 hits; flipping classroom or inverting classroom gets 19.
Google Scholar will be very frustrating unless you ar on campus where a library subscribes to many of its indexed journal articles. Unless you are a student/staff with proxy login from off campus, I would advise trying a regular web search, adding site:edu, or site:ac.uk, etc. For example Google web search gets 12 unique (or "about 45") hits for: "flipping classrooms" site:edu. You can try search.usa.gov, which searches STATE as well as federal websites and reports; but it gets only 3 hits for: "flipping classrooms"
