Alternating vortex shedding in the wake of structures can cause unwanted vibrations. In the design phase of such structures, it is vital to obtain good predictions of the fluid forces acting on the structure. Current methods rely on computational fluid dynamic simulations (CFD), with a too high computational cost to be effectively used in the design phase or for control. In this work we show that it is possible to build an accurate, flexible and low-computationalcost mathematical model using nonlinear system identification techniques.