Do we know the experiments conducted by Nazi doctors/scientists that WEREN'T useful but just plain cruel.

by J2quared
ibbity

One could make the argument that this applies to all concentration camp "scientific experiments," as they were conducted in a very unscientific manner and didn't result in any useful data. If you're looking for a description of what, specifically, the Nazi scientists did to their subjects with an accompanying explanation of their reasoning (such as it was) for doing those things, then this is probably a good resource for you. (Fair warning: some of the things discussed in that link are very nasty.) The source contains some things which are not relevant to your question, as it's set within the framework of Jewish ceremonial law (halacha) and ethics, but most of it is quite relevant.