1. A method for autonomously adjusting the flow of a fluid through a valve or flow control device into a production pipe of a well in a hydrocarbon reservoir, said method comprising the steps of: flowing the fluid through an inlet of the valve or the flow control device so as to form a flow path through the valve or the flow control device passing by an un-biased freely movable tapering shaped body which is designed to move freely relative to the inlet and thereby reduce or increase a flow-through area of the valve or the flow control device, wherein the inlet is substantially aligned with a longitudinal axis of the tapering shaped body, and a flow of the fluid over a surface of the tapering shaped body is radially offset from the longitudinal axis of the tapering shaped body, wherein the tapering shaped body is moved with a force due to a fluid pressure difference between opposite sides of the tapering shaped body, the fluid pressure difference being created according to the Bernoulli effect and any stagnation pressure created over the tapering shaped body, whereby the flow control device, depending on the composition of the fluid and its properties, autonomously adjusts the flow of the fluid based on a pre-estimated flow design.