Air-Wake Flow Dynamics on a Simplified Frigate Shape

Air-Wake Flow Dynamics on a Simplified Frigate Shape: An Experimental Study by Large-Scale Tomographic PTV

Rius Vidales, A.F.

Sciacchitano, A. (mentor)

Aerospace Engineering

Department of Aerodynamics

2016-12-14

: The Air-Wake produced by a ship superstructure is a complex, highly turbulent and unsteady three-dimensional flow, which contains large flow features that interact with the helicopter rotor during critical take-off and landing procedures. An international research effort (TTCP, AER-TP2) which involves experimental and numerical studies on simplified frigate shapes SFS has been conducted with the aim of understanding and simulating the ship air-wake. An unexpected asymmetry of the time-averaged air-wake flow has been noted when the model is oriented parallel to the free-stream direction. In this condition, it has been suggested that the flow features a bi-stable behavior which requires further investigation. During this research project the air-wake flow dynamics on the landing deck of a simplified frigate shape (SFSN) modified by the Netherlands Aerospace Laboratory has been studied using time-resolved large-scale Tomographic Particle Tracking Velocimetry with helium filled soap bubbles. The results show that the time-average flow-field at head-wind condition (ß=0º) on the SFSN present a slight asymmetrical stream-wise velocity distribution, where higher velocity deficits are located on the starboard-side of the model. The time-averaged air-wake structure suggests that the horse-shoe vortex structure from the funnel super-structure convects downstream and interacts with the flow-field on the landing deck. Moreover, the analysis revealed that, as expected, an arch vortex is present in the proximity of the step wall. A low order reconstruction led to an unambiguous identification of the recirculation region on the instantaneous measurements, where the velocity difference between both sides of the deck is calculated. Based on this difference, three flow-states (PS port-side, SB Starboard side and SY symmetric) have been identified. An alternation factor was calculated to determine the number of occurrences of each flow state and the consecutive time that the flow remained on a specific state. This result suggests that at head-wind condition the flow is stable for either (PS, SB) and unstable for (SY) which is considered to be a transitional state. This shows that the air-wake flow of the SFSN at headwind condition is bi-stable and that the bi-stability is restricted to the range ß=+/- 1º degree of yaw. The results from the probability based on the number of occurrence of each flow state show that at head-wind condition the flow state SB presents a higher probability than the flow state PS, which explains the slight asymmetry observed on the stream-wise time-averaged velocity distribution. Finally, the experimental results obtained during this research project contribute to the area of knowledge by characterizing, in spatial and time domains, this complicated bi-stable flow using a three-dimensional flow measurement technique. Moreover, the experimental campaign performed at the Low Speed Wind Tunnel of the German-Dutch Wind Tunnels show that it is possible to perform large-scale flow measurements using time-resolved Tomographic Particle Tracking Velocimetry with HFSB in an industrial wind tunnel facility. This opens new possibilities to the study of aerodynamic industrial applications and more specifically three-dimensional bi-stable bluff-body air-wakes.

Wind Tunnel Testing
Tomographic PTV
Bluff-body
Bi-stability
Simplified Frigate Shapes
Ship Air-Wakes
Aerodynamics

http://resolver.tudelft.nl/uuid:160dbf23-207f-4178-b67b-4f5115d8a26b

Student theses

master thesis

(c) 2016 Rius Vidales, A.F.