Flow characteristics comparison of PIV and numerical prediction results for an unmanned underwater vehicle positioned close to the free surface. (December 2022)

• Record Type: Journal Article
• Authors: Kilavuz, A.; Ozgoren, M.; Kavurmacioglu, L.A.; Durhasan, T.; Sarigiguzel, F.; Sahin, B.; Akilli, H.; Sekeroglu, E.; Yaniktepe, B.
• Is Part Of: Applied ocean research. Volume 129(2022)
• Publication Date: 2022-12
• Subjects: CFD -- Free surface -- PIV -- Turbulent flow -- Unmanned underwater vehicle; Ocean engineering -- Periodicals; 620.416205
• DOI: 10.1016/j.apor.2022.103399
• Languages: English
• ISSNs: 0141-1187

Abstract

Highlights: The influence of the free-surface flow characteristics around a micro UUV geometry was investigated experimentally and numerically. An immersion ratio of h/D = 0.75 causes asymmetrical flow structure formation in the wake region. Q-criterion and instantaneous vorticity isosurfaces show clearly unsteady 3D flow characteristics due to the free surface effect. Force and moment coefficients showed a decreased trend with the increase of Reynolds number and immersion ratio. The smaller wake size and the closer location to the rear of the stern yielded by increasing Reynolds number. Abstract: In the present paper, flow characteristics of an Unmanned Underwater Vehicle (UUV) with a commonly used Myring profile were investigated numerically and experimentally using Computational Fluid Dynamics (CFD) and the Particle Image Velocimetry (PIV) technique under the influence of free surface. The 3-D and two-phase flow simulation generated using the Volume of Fluid (VOF) were carried out using the Large Eddy Simulation (LES) turbulence model for high accuracy in both near free-surface and almost uniform flow conditions. Due to the presence of the free-surface effect, dynamics and unsteady instantaneous flow characteristics such as force and moment coefficients, streamlines topology, and pressure values on the body surface along with vorticity structures were found to be very chaotic and have irregular motion in the wake while the followable variation trend of the time-averaged properties was obtained to show critical immersion ratio. The immersion ratios of 0.75≤ h/D≤3.50 were examined at Reynolds numbers Re = 2.0 × 10 4 and 4.0 × 10 4 . Jet-like flow between the UUV body and the free surface of the water was detected at the immersion ratio of h/D = 0.75, which caused a substantial asymmetry in flow structures, resulting in highest drag and lift values. Increased surface disturbance at Re = 4.0 × 10 4 caused air introduction into via jet-like flow in h/D = 0.75, which caused positive lift. Hydrodynamic coefficients and isosurfaces shown that the free-surface effect decreased significantly up to h/D = 1.50 at constant Reynolds numbers. Further investigation of time-averaged velocity components, streamlines, vorticity and turbulence statistics revealed that h/D = 1.50 acted as a transitional immersion ratio as the flow structure changed significantly with Reynolds numbers. The utilized CFD approach yielded especially excellent agreement with the PIV measurements with the discrepancy which varies from 1% to 15% in near wake for streamwise velocity components to simulate the essential unmeasured flow features needed in the research and development process of UUVs when they move below the free surface. Graphical abstract: Image, graphical abstract .