Effects of Atmospheric Turbulence Unsteadiness on Ship Airwakes and Helicopter Dynamics

Abstract

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. ABLs are different from typical engineering boundary layers, such as one over a flat plate. Although 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 was observed that the energy increase in an unsteady ABL is higher than the increase found under a steady sheared ABL, suggesting a degradation in controller performance under realistic atmospheric inflow and resulting possibly in an increase in pilot workload.