DES and RANS of Unsteady Free-Surface Wave Induced Separation

vortex and turbulence interactions. First identified and studied using a surface-piercing foil mounted on the floor of a hydraulic flume. This building block geometry facilitates identification of the salient features; since, the foil profile was designed for limited separation for the deep, no-wave, and twodimensional (2D) condition. Tests were conducted for Froude number ( gc Uc / = Fr ) range 0. 20.48 with average Reynolds number ( ν / Re c U c = ) = 7x10 (where Uc=carriage speed, c=chord length, and ν=kinematic viscosity). Wave profile photographs and needlepoint, dye injection flow visualization was used to determine the length and depth of the separation region. Separation was defined by a region of highly disturbed free-surface flow, which initiated just beyond the wave trough and extended to the foil trailing edge for all but the highest Fr ranging from separation starting point x/c=. 7 at small Fr=. 2, to maximum x/c=. 42 at medium Fr=. 25, and x/c=. 6 at Fr=. 37. Beyond the separation starting point, the wave profile was nearly constant. The depth of separation was defined by reversed axial flow and upward cross flow, which was observed close to the foil surface in a wedge shaped region gradually expanding from the separation starting point to a maximum depth near the foil trailing edge with magnitude similar to the wave height. Stratford’s laminar separation criterion showed good agreement with separation starting point data. A general-purpose unsteady Reynolds-averaged Navier-Stokes (URANS) research code CFDSHIPIOWA developed for ship hydrodynamics application is extended for detached eddy simulation (DES) capability. CFDSHIP-IOWA uses surface-tracking free-surface model, k-ω turbulence model, and high performance computing. Both 2 and 3 order upwind biased scheme for spatial derivatives were applied for URANS while 3 order upwind biased scheme used for DES. DES extensions are based on the blended k-ω model by modifying the length scale in the k equation and validated with surface piercing NACA 0024 benchmark, including IIHR towing-tank EFD data and concurrent URANS. Domain and grid convergence studies were conducted for 2 order RANS. 3 order RANS was also studied on a coarse grid and a 3 order DES was conducted on both coarse and medium grids. Statistical analysis of the results, including time history, running mean, and FFT of total drag and side forces, mean and RMS of wave elevations and pressure on foil surface, and unsteady 3D separation flow pattern are presented. Results show fairly good agreement EFD validation data for mean, RMS, and FFT frequencies for wave elevations and surface pressure; however, many modeling and numerical issues remain. Seemingly credible flow features of unsteady wave-induced separation have been simulated for the first time, which will be used to guide future PIV measurements. More recently, [2-4] used a similar geometry (surface-piercing NACA 0024) for complementary towing-tank (3x3x100m) experimental fluid dynamics (EFD) and steady RANS computational fluid dynamics (CFD). Wave profiles, mean far field wave elevations, and mean and RMS near field wave elevations and surface-pressure measurements were made for c=1.2m and deep draft d=1.5m foil and Fr=0.19, 0.37, and 0.55 and Re=(0.822, 1.52, 2.26) x 10. EFD results similar to[1], except differences foil geometry and restricted water and foil bottom effects. Steady RANS solutions show good agreement wave profiles and surface pressure, but only fair agreement wave elevations due to poor resolution short waves for low Fr and separation region for medium Fr. RANS solutions also provide details of separated flow pattern, but data not yet available for validation