Large Eddy Simulations and Low-order Models of Tethered Coaxial Turbines

Tethered Marine Hydro Kinetic devices reduce the specific cost of energy production, while tapping the renewable energy potential of oceans. Specifically, tethered, coaxial-counter rotating turbines offer the advantage of producing zero net torque to avoid tether twisting and entanglements. The optimization of single and multiple coaxial devices requires insights into wake behavior, which can be obtained from numerical simulations under varying operating conditions and inlet turbulence. However, the turbines will be deployed in ocean current turbine (OCT) farms rather than as individual units, necessitating the development of low-order models to characterize their wake behavior. We have developed a low-order wake model based on the concept of an Equivalent Single Rotor (ESR), defined by its induction factor, that extracts the same relative power from the mean flow as a dual-rotor coaxial turbine, while injecting the same amount of turbulence into the wake. Validation for this concept and corresponding low-order models is achieved by comparing the wake structure from large eddy simulations (LES) of a coaxial turbine with results from a single rotor simulation with a turbine induction factor corresponding to the ESR condition. Analysis of turbulence is also presented, as inlet turbulence is varied in order to observe its effect on peak turbulence, transition point from near wake to far wake, and decay rate of turbulence for the coaxial configuration. The insights gathered via LES can be extended to capture wake interactions between multiple turbines in an OCT farm.