Abstract
Collaboration is described on assessment of computational fluid dynamics (CFD) predictions for surface combatant model 5415 at static drift β = 0 deg and 20 deg using recent tomographic particle image velocimetry (TPIV) experiments. Assessment includes N-version verification and validation to determine the confidence intervals for CFD solutions/codes, and vortex onset, progression, instability, and turbulent kinetic energy (TKE) budget analysis. The increase in β shows the following trends. Forces and moment increase quadratically/cubically, and become unsteady due to shear layer, Karman and flapping instabilities on the bow. Wave elevation becomes asymmetric; its amplitude increases, but the total wave elevation angle remains same. The vortex strength and TKE increase by about two orders of magnitude, and for large β, the primary vortices exhibit helical mode instability similar to those for delta wings. Forces and moment for both β and wave elevation for β = 0 deg are compared within 4% of the data, and are validated at 7% interval. Wave elevation for β = 20 deg, and vortex core location and velocities for both β are compared within 9% of the data, and are validated at 12% interval. The vortex strength and TKE predictions show large 70% errors and equally large scatter and are not validated. Thus, both errors and scatter need reduction. TKE budgets show transport of turbulence into the separation bubble similar to canonical cases, but pressure transport is dominant for ship flows. Improved CFD predictions require better grids and/or turbulence models. Investigations of solution-adaptive mesh refinement for better grid design and hybrid Reynolds-averaged Navier-Stokes/large eddy simulation models for improved turbulent flow predictions are highest priority.
Highlights
Prediction of three-dimensional (3D) separation including onset and progression of vortices is a grand challenge for ship hydrodynamics due to relatively bluff hull forms with attendant thick boundary layer and wake; appendages such as bow bulb/ sonar dome and shafts/struts; free-surface/Froude number (Fr) effects; and high Reynolds numbers (Re)
This study focuses on the assessment of computational fluid dynamics (CFD) prediction capability for separated flow onset and progression induced by 5415 at b 1⁄4 0 deg and 20 deg using forces and moment, wave elevation, and mean and turbulent flow in the vortex core as the validation variables and adaptive grid verification studies
The prediction raises the question: what is the source of high turbulent kinetic energy (TKE) during far-wake progression? Is it maintained by the velocity fluctuation on the free-surface due to the impact of periodic breaking waves originating from the sonar dome? Or it is possible that TKE is convected along the core without any dissipation? Additional experiments and computations are required to investigate these issues
Summary
Prediction of three-dimensional (3D) separation including onset and progression of vortices is a grand challenge for ship hydrodynamics due to relatively bluff hull forms with attendant thick boundary layer and wake; appendages such as bow bulb/ sonar dome and shafts/struts; free-surface/Froude number (Fr) effects; and high Reynolds numbers (Re). This study focuses on the assessment of CFD prediction capability for separated flow onset and progression induced by 5415 at b 1⁄4 0 deg and 20 deg using forces and moment, wave elevation, and mean and turbulent flow in the vortex core as the validation variables and adaptive grid verification studies. The b 1⁄4 10 deg data showed similar vortices, except the SDTV strength reduced, both BKTV and ABKV strengths increased, TKE decreased for all the vortex cores, and FSVs were absent.
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