Is Visa required to obtain a very short term research grant (15 days) from a US University?
If the grant is only to reimburse you for your travel expenses (airfare, lodging, meals, etc), then you should be fine with the visa waiver.  You are not considered to be employed by the host institution and do not need a work visa.  (Source: Personal experience - I've been responsible for arranging travel reimbursements for many visa-waiver visitors.)

You will probably need to document your expenses, so save receipts for airfare, lodging, meals, ground transportation, and so on.