Minyee Jiang, Malarie Vanyo, Jason Updegraph, Evan Lee Naval Surface Warfare Center at Carderock May 12, 2010 STAR Aerospace & Defense Conference 2010
Introduction CFD Validation and Simulation for RIB In regular waves Varying reentry positions Lifeboat launching simulation from CD-Adapco Used for impact point position study
CFD Validation Task Title: CFD tools evaluation for marine applications Objective: Investigate, verify and validate computational tools for the design of advanced combatant craft undergoing hydrodynamic loadings and craft motions. CFD Code used in this project: STAR-CCM+
Numerical Tool and Method STAR-CCM+ A Reynolds Averaged Navier-Stokes (RANS) code developed by CD- Adapco, designed to solve wave interaction problems, applicable to surface ships and submarines. VOF Volume of fluid (VOF) is one of the approaches for accurately computing free surface flows and breaking waves. Grid cells near the free surface are split into fractions and computed appropriately as to whether they are air or water.
Volume of Fluid Free Surface Capturing air Incoming waves and current water
Geometries and Mesh Surface Mesh Volume Mesh
Boundary conditions Embedded Mesh allows inner region to rotate Inflow inlet BC Outflow BC Pressure outlet Wave direction
Regular Wave Conditions for CFD Wave Frequency Hz Model Scale Significant Wave Ht 1/80 Slope (in) 0.370 5.6 0.477 3.4
Grid Study Grid sizes represent half domain only (1.3M cells) Start with coarse grid in the free surface region and near the boat (2.4M cells) Refine the grid in the free surface region (4.1M cells) Further grid refinement near the boat All the values in the following slides are non-dimensional Number of cells 1.3M 2.4M 4.1M Drag relative to 4.1M cell value 0.955 0.996 1.0
Time Step Size Study (during reentry) Choosing an intermediate solution, while boat is airborne, to restart the calculation with different time step sizes to evaluate the effect of time step size. Time step size dt = 0.0025s dt = 1e-4 s dt = 2e-5 s Drag relative to dt=0.0025s 1 1.07 1.09
Animation samples Volume fraction Free surface
Average Peak Heave Average these values To be the Average Peak
Average Peak Heave (in) Average Peak Heave 1.2 Heave 1 0.8 0.6 0.4 0.2 heave-cfd-370 heave-measured-370 0 5 10 15 20 25 30 Model Speed (knots)
Average Peak Pitch (deg) Average Peak Pitch 1 Rib Pitch Angle 0.8 Deviate from the general trend 0.6 0.4 pitch-cfd-370 pitch-measured-370 0 10 20 30 Model Speed (knots)
Average Peak CG Acceleration (g's) Average Peak CG Acceleration 1.5 Rib Acceleration @ c.g. 1 0.5 Under predict at higher speeds 0 acc@ cg-cfd-370 acc@ cg-measured-370-0.5 5 10 15 20 25 Model Speed (knots)
Average Peak Acceleration (g's) Average Peak Acceleration 1.5 Rib Acceleration at Bow and Stern 1 F370-bow-cfd F370-bow-measured F370-stern-cfd F370-stern-measured Under predict at higher speeds 0.5 0-0.5 0 10 20 30 Model Speed (kts)
Average Peak Drag (lb) Average Peak Drag 1.2 Rib Drag 1 0.8 0.6 0.4 Under predict 0.2 0 drag-cfd-370 drag-measured-370-0.2 10 20 30 Model Speed (knots)
Testing the effect of varying reentry positions (Only changing the boat position relative to the waves) (1) Wave ends at 5m (2) Wave ends at 7.5m (3) Wave ends at 10m
acceleration at c.g. (g's) non-dimensional Comparison of Landing Positions Acceleration at c.g. 2 1.5 Impact Comparison Due to Landing Position Acceleration at c.g. 1 acc-cg-hit1 acc-cg-hit2 acc-cg-hit3 0.5 0-0.5-1 -1.5-2 0 0.5 1 1.5 Time (second)
Comparison of Landing Positions Acceleration at Stern Stern Acceleration (g's) 2 Impact Comparison Due to Landing Position Stern Acceleration 1.5 acc-stern-hit1 acc-stern-hit2 acc-stern-hit3 1 0.5 0-0.5 0.2 0.4 0.6 0.8 1 1.2 Time (second)
Comparison of Landing Positions Acceleration at Bow Bow Acceleration (g's) non-dimensional 2 Impact Comparison Due to Landing Position 1.5 Bow Acceleration acc-bow-hit1 acc-bow-hit2 acc-bow-hit3 1 0.5 0-0.5-1 0 0.5 1 1.5 Time (second)
Simulation of Flow and Motion of Lifeboats Hans Jørgen Mørch*, Milovan Perić**, Eberhard Schreck**, Ould el Moctar***, Tobias Zorn*** *CFD Marin, Tvedestrand, Norway **CD-adapco, Nürnberg Office, Germany ***Germanischer Lloyd, Hamburg, Germany www.cd-adapco.com Milovan.Peric@de.cd-adapco.com
Demonstration Free fall in flat water: Experiment 1 2 3 4 5 6
Effects of Hit Point Initial wave position varied by 20 m (drop from 32 m height). Following wave (180 ) Wavelength ca. 220 m, wave height 13.5 m, water depth 33.5 m The questions to be answered: When is the load on the structure the highest? When are accelerations the highest?
Effects of Hit Point and Wave Incidence, I Vertical acceleration at center of gravity for different hit points, following wave (180 incidence).
Effects of Hit Point and Wave Incidence, II Vertical acceleration at center of gravity, different hit points (head wave, 0 incidence)
Conclusion STAR-CCM+ predicts the average pitch and heave peaks for RIB reasonably, using averaged peak load, but it under predicts the drag and acceleration at high speeds. Wave profile compatibility is very important for accurately predicting the impact loads for a small combatant craft in waves Since the landing position affects the impact load and acceleration, use of the wave profile from the model test for time history comparison is recommended