Date of Award

5-2021

Document Type

Thesis - Open Access

Degree Name

Master of Science in Aerospace Engineering

Department

Aerospace Engineering

Committee Chair

Dr. J. Gordon Leishman

First Committee Member

Dr. Ebenezer Gnanamanickam

Second Committee Member

Dr. Richard Prazenica

Abstract

The three-dimensional, unsteady, and turbulent airwake produced by a scaled model of a generic Navy ship (Simple Frigate Shape No. 2) was investigated in a low-speed wind tunnel. Stereoscopic, time-resolved particle image velocimetry (TR-PIV) measurements were made at six different crosswise planes over the flight deck region of the ship model, with and without the effect of a simulated atmospheric boundary layer (ABL).

Spatiotemporal analyses of the TR-PIV measurements were performed using modal decomposition, and the modes were decomposed further based on the frequency contents of their time dynamics. This approach allowed an inspection of the individual scales of the airwake and provided a quantitative estimate of energy distributions across the scales of vortical motions and fluctuations. The funnel wake above the superstructure, the flow recirculation region lee of the hangar door, and the flight deck vortices near the ship's stern were examined. Insights into their large-scale motions were provided, including the effects of the bistable flow recirculation region, and the organized turbulence lee of the funnel. Then, clear discrepancies and surprising similarities were noted between airwakes with and without the influence of ABL. Notably, the large-scale eddies were anisotropic, and regions of fluctuation varied with flow scales, underscoring the need for a spatiotemporal understanding of the flow field beyond single-point statistics.

It was observed that the airwake energy at higher-frequencies (well below the previous upper limit of 2 Hz) decayed monotonically, suggesting potential simplifications in future reduced-order models (ROM). Nonetheless, a deeper understanding of the rotorcraft/ship airwake coupled flow field is likely a prerequisite to effective ROMs for the airwake.

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