Date of Award

Summer 7-2020

Access Type

Dissertation - Open Access

Degree Name

Doctor of Philosophy in Aerospace Engineering

Department

Aerospace Engineering

Committee Chair

J. Gordon Leishman

First Committee Member

Ebenezer Gnanamanickam

Second Committee Member

Anastasios S. Lyrintzis

Third Committee Member

John A. Ekaterinaris

Fourth Committee Member

Jeff R. Brown

Abstract

The objective of this research was to better characterize the complex, three dimensional, unsteady aerodynamic flows produced by the superstructure of a ship, which is referred to as an airwake. This problem is relevant and important because the turbulent airwake significantly and adversely affects the ability for rotorcraft to operate safely from the decks of Navy ships. To this end, a series of wind tunnel measurements were performed on the SFS2 simplified frigate shape, which has a flight deck at its stern. The measurements were performed with and without a simulated atmospheric boundary layer (ABL), which included the aerodynamic scaling of its thickness, velocity profile, and turbulence. The ABL was simulated using Cowdrey grid method, which comprises sets of horizontal rods placed in the wind tunnel upstream of the test section. Two wind tunnels were used, in part to cover a wind range of Reynolds numbers based on ship length in the range of 0.6–6.2 million. The first was a 1:235 scale SFS2 ship model in the Boundary Layer Wind Tunnel (BLWT), and the other was a 1:90 scale SFS2 in the Low-Speed Wind Tunnel (LSWT). The measurements were performed using a combination of hot-wire anemometry and particle image velocimetry measurements, especially time-resolved particle image velocimetry (TR-PIV) in the LSWT in various streamwise and crosswise planes. The TR-PIV experiments were also supported by surface oil flow visualization, which was used to help interpret the off-surface flow measurements. The airwake was seen to comprise large regions of unsteady flow separation, dominant vortical flows, and significant wall-normal flows, especially over the region of flight deck regions, which was caused, in part, by the shedding of the vortices and turbulence from the upstream funnel and superstructure of the ship. The turbulence intensities were found to be particularly high over the flight deck. The results also suggested the existence of asymmetric, intermittent flow in the near-wall regions of the deck, and bistable movements were observed in the recirculation region behind the hangar and behind the stern of the ship. The measurements also showed the development of shear layers at the corners of the flight deck on both the port and starboard sides, and sets of counterrotating vortices at the edges of the flight deck. These results were found to be affected by the presence of the ABL, but were not strongly affected by the Reynolds number. An energy spectrum analysis was also performed, showing dominant frequencies in the regions where the shear layer was developed behind the funnel and above the flight deck. Proper Orthogonal Decomposition and Spectral Proper-Orthogonal Decomposition was used to extract the dominant energy modes from the TR-PIV measurements to better quantify the complex unsteady flow structures exhibited in the airwake. The application of Spectral Proper Orthogonal Decomposition revealed that the physically relevant coherent structures in the airwake were low frequency modes near the flight deck and at large scale of the order of the length of the deck. Although the concentration of the energetic modes in the airwake were at low frequencies, the overall energy content was still broadband. At lower frequencies, large-scale structures were observed in both the streamwise and wall normal directions in the near-wall regions of the flight deck, which help to explain the production of the intense zones of unsteady upwash/downwash over the flight deck that are known to affect rotorcraft that operate in the airwake.

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