In the present work, the behavior of a flexible disk rotating close to a fixed, a co-rotating, and a counter rotating flat-stabilizers in open air is investigated both experimentally and numerically.

1. rotating close to这里的close是副词吗？还是形容词呢？
【答】close to 是一个“复合介词”：close （副词）+ to （介词）= 靠近（某物）。我们常说：He sat close to me.   本句是介词短语作状语，修饰动名词rotating。

2.both限定词可以修饰experimentally和numerically吗？同时修饰investigated？

【答】both ...and ...是成对的并列连词，both 跟 and 连用，起强调作用，也可以不用。把两个副词连接起来，(both) experimentally and numerically。它们本身不修饰谓语动词，而是两个副词修饰谓语动词 is investigated。