On the unsteadiness of ship airwakes subject to atmospheric boundary-layer inflow from a helicopter operation perspective

In the present work, unsteady effects present in a ship airwake are further analyzed. The effort is performed in the context of simulation of helicopter launch and recovery operations under a realistic atmospheric inflow. A ship airwake is formed as a combination of the natural wind speed and ship motion, and the incoming flow is turbulent due to the presence of an atmospheric boundary layer (ABL). On a helicopter-ship dynamic interface simulation, accounting for the effects of an ABL can be important. Atmospheric boundary layers are different than typical engineering boundary layers, such as one over a flat plate. While in a time-averaged sense such boundary layers are comparable, real ABLs contain important unsteady features that set them apart. This work separates and quantifies effects inherently due to the unsteady atmosphere, and effects due to a sheared profile without atmospheric turbulence. Two cases are compared: (1) a realistic time-resolved ABL, and (2) a steady sheared velocity profile. Using a frequency-domain analysis of the control input sticks, it is observed that the energy increase in an unsteady ABL is considerably higher than the increase found under a steady sheared ABL, suggesting increased pilot workload at the relevant frequencies.