Embodiments of the invention provide a method of determining pressure difference across a tube arising from fluid flow within the tube, comprising: obtaining three- dimensional time dependent fluid velocity data at a plurality of points along the tube processing the three-dimensional time dependent fluid velocity data to determine: i) a flow rate (Q) of the fluid through the tube ii) the kinetic energy (K) of the fluid flow through the tube iii) an advective energy rate (A) of the fluid flow through the tube and iv) a viscous dissipation rate (V) pertaining to the fluid flow and calculating the pressure difference in dependence on all of the flow rate (Q), kinetic energy (K), advective energy rate (A), and viscous dissipation rate (V). Further embodiments are also described.
