cfd validation for surface combatant 5415 at straight-ahead and 20 degree static drift conditions

562 Pages·2015·38.47 MB·English

CFD VALIDATION FOR SURFACE COMBATANT 5415 AT
STRAIGHT-AHEAD AND 20 DEGREE STATIC DRIFT
CONDITIONS

By

S. Bhushan1, H. Yoon2, F. Stern2, E. Guilmineau3, M. Visonneau3, S. Toxopeus 4, C. Simonsen5,
S. Aram6, S-E Kim6 and G. Grigoropoulos7

IIHR Technical Report # 493

IIHR—Hydroscience & Engineering
College of Engineering
The University of Iowa
Iowa City, IA 52242

February 2015

1 Center for Advance Vehicular Systems, Mississippi State University, US
2 IIHR- Hydroscience and Engineering, University of Iowa, US
3 CNRS/Centrale Nantes, Nantes, France
4 Maritime Research Institute Netherlands (MARIN)
5 FORCE Technology, Denmark
6 Naval Surface Warfare Center Carderock Division, West Bethesda, Maryland, US
7Laboratory for Ship and Marine Hydrodynamics, National Technical Univ. of Athens, Greece
Table of Contents

ABSTRACT
1. INTRODUCTION
2. OVERVIEW OF EXPERIMENTAL VALIDATION VARIABLES AND CFD
SUBMISSIONS
2.1. Experimental Facility, Model, Flow Conditions, Data and Uncertainties
2.2. CFD Submissions
2.3. Analysis Methods
3. CFD CODES AND SETUP
3.1. CFDShip-IOWA
3.2. ISIS-CFD
3.3. ReFRESCO
3.4. STARCCM+ (FORCE)
3.5. NavyFOAM
3.6. STARCCM+ (NTUA)

4. STRAIGHT-AHEAD CONDITION
4.1. CFDShip-Iowa V4.5
4.1.1. Vortex Structures, Onset and Separation Analysis
4.1.2. Effect of Grid Resolution and Turbulence Model
4.1.3. Validation of Forces
4.1.4. Validation of Wave Elevation and Profiles
4.1.5. Validation of Planar Variables
4.1.6. Validation of Volume Variables
4.2. ReFRESCO
4.2.1. Vortex Structures, Onset and Separation Analysis
4.2.2. Effect of Grid Resolution and Turbulence Model
4.2.3. Validation of Forces
4.2.4. Validation of Wave Elevation and Profiles
4.2.5. Validation of Planar Variables
4.2.6. Validation of Volume Variables
4.3. ISIS-CFD
4.3.1. Vortex Structures, Onset and Separation Analysis
4.3.2. Effect of Grid Resolution and Turbulence Model
4.3.3. Validation of Forces
4.3.4. Validation of Wave Elevation and Profiles
4.3.5. Validation of Planar Variables
4.3.6. Validation of Volume Variables
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4.4. NavyFOAM
4.4.1. Vortex Structures, Onset and Separation Analysis
4.4.2. Effect of Grid Resolution and Turbulence Model
4.4.3. Validation of Forces
4.4.4. Validation of Wave Elevation and Profiles
4.4.5. Validation of Planar Variables
4.4.6. Validation of Volume Variables
4.5. STARCCM+
4.5.1. Effect of Grid Resolution and Turbulence Model
4.5.2. Validation of Forces
4.5.3. Validation of Planar Variables

5. STATIC DRIFT 20 DEGREE CONDITION
5.1. CFDShip-Iowa V4.5
5.1.1. Mean Vortex Structures, and Onset and Separation Analysis
5.1.2. Effect of Grid Resolution and Turbulence Model
5.1.3. Validation of Forces and Moments, and Unsteady Analysis
5.1.4. Validation of Wave Elevation and Profiles
5.1.5. Validation of Planar Variables
5.1.6. Validation of Volume Variables
5.1.7. Instantaneous Vortex Structures, and Onset, Separation and Progression Analysis
5.1.8. Instability Analysis and TKE Budget
5.2. STARCCM+ (FORCE)
5.2.1. Validation of Forces and Moments
5.2.2. Validation of Wave Elevation
5.2.3. Validation of Planar Variables
5.3.ReFRESCO
5.3.1. Mean Vortex Structures, and Onset and Separation Analysis
5.3.2. Validation of Planar Variables
5.3.3. Validation of Volume Variables
5.4. ISIS-CFD
5.4.1. Effect of Grid Resolution and Turbulence Model
5.4.2. Validation of Forces and Moments, and Unsteady Analysis
5.4.3. Validation of Wave Elevation and Profiles
5.4.4. Mean Vortex Structures, and Onset and Separation Analysis Validation of Planar
Variables
5.4.5. Validation of Volume Variables
5.4.6. NavyFOAMEffect of Grid Resolution and Turbulence Model
5.4.7. Validation of Forces and Moments, and Unsteady Analysis
5.4.8. Validation of Wave Elevation and Profiles
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5.4.9. Mean Vortex Structures, and Onset and Separation Analysis
5.4.10. Validation of Planar Variables
5.4.11. Validation of Volume Variables
5.5. STARCCM+ (NTUA)
5.5.1. Effect of Grid Resolution and Turbulence Model
5.5.2. Validation of Forces and Moments, and Unsteady Analysis
5.5.3. Validation of Planar Variables

6. CONCLUSION AND FUTURE RESEARCH
6.1. CFDShip-Iowa V4.5 Conclusions and Future Work
6.1.1. Straight-ahead Condition
6.1.2. Static Drift 20 Degree Condition
6.1.3. Overall Conclusions and Future Work
6.2. ISIS-CFD Conclusions and Future Work
6.2.1. Straight-ahead Condition
6.2.2. Static Drift 20 Degree Condition
6.2.3. Overall Conclusions and Future Work

7. OVERALL CONCLUSIONS AND RECOMMENDATIONS
7.1. Vortex Structures, and Onset, Separation and Progression Analysis
7.1.1. Straight-ahead Condition
7.1.2. Static Drift 20 Degree Condition
7.2. Effect of Grid Resolution and Turbulence Model
7.2.1. Straight-ahead Condition
7.2.2. Static Drift 20 Degree Condition
7.3. Forces/Moments and Wave Elevation
7.3.1. Straight-ahead Condition
7.3.2. Static Drift 20 Degree Condition
7.4. Planar and Volume Variables
7.4.1. Straight-ahead Condition
7.4.2. Static Drift 20 Degree Condition
7.5. Instability Analysis and TKE Budget
7.6. Recommendations Future Research

REFERENCES
TABLES AND FIGURES - CHAPTER 2
TABLES AND FIGURES - CHAPTER 4
TABLES AND FIGURES - CHAPTER 5

APPENDIX I ADDITIONAL RESULTS FOR STRAIGHT-AHEAD CONDITION
(available upon request from authors)
I.1. CFDShip-IOWA (contact S. Bhushan/F. Stern)
I.2. ISIS-CFD (contact M. Visonneau)
I.3. ReFRESCO (contact S. Toxopeus)
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APPENDIX II ADDITIONAL RESULTS FOR STATIC DRIFT 20 DEGREE
CONDITION (available upon request from authors)
II.1. CFDShip-IOWA (contact S. Bhushan/F. Stern)
II.2. ISIS-CFD (contact M. Visonneau)
II.3. ReFRESCO (contact S. Toxopeus)
II.4. STARCCM+ (NTUA) (contact G. Grigoropoulos)

APPENDIX III ONSET AND PROGRESSION OF VORTICAL STRUCTURES, AND
TURBULENCE BUDGET FOR STATIC DRIFT CONDITION: CFDSHIP-IOWA (contact
S. Bhushan/F. Stern)
APPENDIX IV. FREE-SURFACE FOR STATIC DRIFT CONDITION: STAR-CCM+
(FORCE) (contact C. Simonsen)

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ABSTRACT
Collaboration is described on verification and validation of CFD for surface combatant 5415 for
static drift =0 and 20 degrees using recent experimental data: forces and moment; wave
elevations; and tomographic PIV planar, volume and vortex core velocity, vorticity and TKE.
Also studied are vortex onset, separation and instability mechanisms and TKE budget. Results
were obtained from five different solvers; four used finite volume discretization with VoF, and
one used finite-difference with level-set. Simulations were performed on solution-based adapted
grids with hexahedral cells with sizes ranging from 2.5M to 102M for =0 and 4.6M to 250M for
=20. Most used k- based URANS turbulence models, whereas two used DES. The =0 results
agreed with the data for the onset of the primary vortices, but showed large variation in their
progression and decay. The primary vortices exhibit cross-flow/open-type separation identified
by converging surface streamlines caused by cross-flow pressure gradients. The separation
occurs at the local shear stress divergence maxima due to adverse axial pressure gradient. All
solvers predict resistance within 2% of the data. The wave elevation predictions compared well
with the data with no significant differences between VoF and level-set. The finite-difference
solver provided better vortex decay predictions than finite-volume solvers. URANS turbulence
models predicted premature decay of vortices due to inherent large dissipation and inability to
account for streamline curvature. DES turbulence model predicted too strong vortical structures
and low resolved turbulence. The =20 results agreed with the data for the onset of primary
vortices, but showed large variations for their progression and decay, and for the leeward sonar
dome separation bubble and breaking waves. The many separations were open-, closed- or open-
closed types. The closed separations satisfied the topological rules for an isolated body or body
intersecting a wall. The forces and moment were predicted within 4.5% of the data. The wave-
elevation predictions compared well with the data near the hull, but displayed larger errors away
from the hull. Level-set and VoF provided similar predictions except for breaking waves. DES
performed better than URANS for the prediction of vortex strength and TKE. The primary
vortices exhibit helical instability and the separations on the leeward sonar-dome surface show
shear-layer and Karman-like instabilities. TKE budget reveal that the production occurs at the
vortex inception and is transported by pressure or turbulent fluctuations. The study indicates the
need for more accurate turbulence closures. URANS predicts excessive decay of the vortices due
to modeling errors. DES is promising but is subject to limitations, such as grid sensitivity and
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shear stress depletion. Several future research topics are identified. Improved RANS models such
as RSTM but their capability is limited due to inherent inability to model large-scale turbulent
coherent structures. Hybrid RANS/LES models with turbulence trigger, physics based blending
and explicit LES modeling, which addresses the limitations of DES. Automatic grid refinement
approaches for refinement along the vortex core. Effects of blockage on forces and moment
predictions for static drift.

1. INTRODUCTION
Prediction of 3D separation including onset and progression of vortical structures 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, shafts/struts etc.; free-
surface/Froude number (Fr) effects; and high Reynolds numbers (Re).
The calm water straight-ahead condition has been central topic of CFD Workshops: most
recently Gothenburg 2010 (Larsson et al. 2014). Most attention has focused on low Fr = 0.142
large block coefficient C = 0.81 tanker hull forms (KVLCC2), which display 3D separation at
B
the stern characterized by 2 co-rotating axial vortices with hook-shaped axial-velocity contours
in the nominal wake plane (symmetric with respect to center plane). As concluded in Bhushan et
al. (2014) URANS underpredicted the main and secondary vortex sizes and overpredicted
turbulent kinetic energy (TKE). DES on 305 M grid predicted unsteady vortical structures with >
95% resolved turbulence levels. They overpredicted both axial velocity, and vortical and
turbulent structures. Overall, best DES predictions were found to be comparable to best URANS
predictions. Note that poor predictions for the turbulence variables which could be partially due
to dated experimental wind-tunnel data. Higher Fr and smaller C = 0.51 surface combatants
B
(5415) have also received attention due to complexity of the onset and progression of sonar
dome and forebody keel keel vortices and inherent free-surface effects (Fr = 0.28).
Underresolved DES and URANS agree on onset of sonar dome and fore body keel vortices;
however, show differences on their progression/interaction and experimental towing-tank mean
flow data is too sparse for validation. Overall conclusions are that anisotropic URANS models
are better than the isotropic models for the prediction of onset and progression of vortical
structures, but are too dissipative even for grids with more than 10M cells, and results do not
show improvements even with 4th order convection schemes. Hybrid RANS/LES models are
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promising in providing the details of the flow topology, but show modeled stress depletion and
grid induced separation in the boundary layer issues for bluff bodies and underresolved
turbulence issues for slender bodies. Detailed validation of the CFD results was not possible as
the available experimental data was dated, sparse, and/or lacking turbulence measurements.
The calm water static and dynamic maneuvering conditions have been the topic of
Maneuvering Simulation Workshops, including URANS simulations methods: most recently
SIMMAN 2008 (Stern et al., 2011). For static drift, comparison errors E for surge X and sway Y
forces and yaw moment N are relatively small about 5%D for small drift angle β ≤ 10 degree,
but large about 10%D for larger β = 20 degree. For dynamic maneuvers, the trends were similar
for smaller vs. for larger sway and yaw rates. Note that for calm water straight-ahead condition
E = 0.1%D, which is significantly smaller than the validation uncertainty, i.e., root-mean-square
of EFD and numerical uncertainties. Local flow analysis used towing tank PIV measurements for
pure sway and yaw at moderate rates, which showed good agreement for the mean flow, but with
under-estimation of detailed vortex structures. Overall conclusion was that large E is due to
insufficiency of turbulence models and/or grid resolution. Sakamoto et al. (2012a,b) provides
results of a URANS for 5415, including V&V and diagnostics for forces/moment, hydrodynamic
derivatives and local flow for both static and dynamic maneuvers.
Vortical and turbulent structures for KVLCC2 (Fr=0) at =0, 12 and 30 degrees were
investigated using DES and URANS (Xing et al., 2012). The =0 degrees results were presented
in G2010 (Larsson et al., 2014). The =12 degree results were compared with those from the
Tokyo 2005 CFD Workshop. The primary focus was an exploratory study for =30 degree
building on precursory study of unsteady wave-induced separation for a NACA24 surface-
piercing foil (Xing et al., 2007). One shear layer, one Karman-like vortex shedding, and three
helical mode instabilities were identified. The after-body side vortex, fore-body side vortex, and
after-body bilge vortex exhibit all characteristics for vortex breakdown. For helical instability in
the wake of the vortex breakdowns, Strouhal number using the ship length or the distance along
vortex core shows the same trend with delta wings. The turbulent kinetic energy peaks near the
separation point at the bow and on the vortex core of all vortices. For the after-body side vortex,
turbulent kinetic energy reaches local maximum right after the vortex breakdown and intensifies
along the vortex core further downstream. In addition to the three steady vortices previously
identified, the simulations at =12 degree show unsteady aft-body hairpin and after-body side
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vortices. Fr effects for the Wigley hull with  up to 60 degrees were also investigated, but the
analysis was limited due to the use of coarse grids (Pinto-Heredero et al., 2010).
Bhushan et al. (2011) performed scalability studies for CFDShip-Iowa for large grids (up to
that date) using both URANS and DES for 5415. The simulations included half domain barehull
5415 at straight-ahead condition for up to 300M grid, and full domain 5415 with bilge keels at
static drift  = 20 degree for up 250M grid. The straight-ahead results were presented in the
G2010 workshop. DES for β = 20 degree results were motivated to resolve large drift angle
errors at SIMMAN using DES with finer grids and analyze the vortical structures, instabilities,
and turbulent structures for 5415 at large drift angle as previously done for KVLCC2 (Xing et
al., 2012). The fine grid simulations with high levels of resolved turbulence, and less dependence
on turbulence modeling, displayed reduced errors for forces and moment, i.e., errors reduced to
less than 5%D compared to 10%D reported at SIMMAN. The unsteady vortex separation and
instabilities were similar to those observed for KVLCC2 at =30 degree. The β = 20 degree
results along with additional grid studies and analysis are included in the present study. Bhushan
et al. (2011) results guided the concurrent towing-tank tomographic PIV measurements used
herein for validation of simulations (Yoon et al., 2014a&b).
Within a collaborative exercise, the flow around the bare hull DARPA SUBOFF in rotational
motion was computed (Toxopeus et al. 2012). Primary aim was to predict the loads on the hull as
a function of drift angle and rotation rate, to predict the vortices shed from the smooth hull and to
cross-compare the different computations to better understand the influence of grid topology,
grid resolution and computational settings. Based on verification and validation of the solutions
modeling errors were identified, and were attributed to turbulence models or settings. The study
showed that with sufficiently fine grids and the selection of modern turbulence models, the flow
around the hull could be predicted well. Results obtained with the standard SST model as
published by Menter (1994) were found to be inferior to results with corrections to this model or
with more advanced turbulence models (e.g. RSM models).
Also of relevance are recent EFD and CFD for the high-speed Delft catamaran (DC),
including static drift condition (Milanov et al., 2010; Broglia et al., 2012; Dogan et al., 2013).
The objective of the present research is CFD verification and validation for surface
combatant 5415 for  = 0 and 20 degree conditions, including analysis of the numerical methods,
turbulence and free-surface modeling and grid resolution; onset and progression of vortex
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structures; mean flow, turbulence and organized fluctuations; and wave patterns and breaking.
This research is collaboration amongst NATO and partner countries under the auspices of the
NATO AVT-183 Reliable Prediction of Separated Flow and Onset and Progression for Air and
Sea Vehicles. The sea facet specifically focused on EFD and CFD for 5415, KVLCC2 (Maksoud
et al., 2015; Eloot et al., 2015) and DC (Broglia et al., 2015), of which the present report
documents the CFD results for 5415. The experiments were performed at IIHR currently with the
submission of the simulation results by the collaborators on this test case (Yoon et al., 2014a&b).
The CFD and EFD studies are integrated such that the global flow visualization in CFD
complements EFD for the identification of the vortical structures, EFD provides global and local
measurements for CFD validation, and lastly once validated CFD fills in the sparse EFD
enabling detailed diagnostics. Summaries of this report and Yoon et al. (2014b) report will be
contributed to the AVT-183 final report.

2. OVERVIEW OF EXPERIMENTAL VALIDATION VARIABLES AND CFD
SUBMISSIONS

2.1. Experimental Facility, Model, Flow Conditions, Data and Uncertainties
The experimental data used herein for 5415 with bilge keels at Fr=0.28 at straight-ahead and
static drift  =20deg conditions includes: resistance and moments; wave elevation and profiles;
flow and vorticity contours at several planes along the hull; and isosurfaces of vorticity at
selected 3D volumes along the primary vortices. The primary data in this study are the planar
and volume velocity and vorticity field. Yoon et al. (2014a) recently procured these datasets, and
preliminary results were presented in Egeberg et al. (2014). Yoon et al. (2014b) procured the
forces and moments previously. The wave elevation and wave cuts data are taken from Longo et
al. (2007), which are for the straight-ahead barehull 5415.
The hull form used in the tests was the DTMB 5512, a 1:46.6 scale, 3.048 m long, fiber-
reinforced Plexiglas hull, manufactured by the Naval Surface Warfare Center. The model is a
geosym of the DTMB 5415, a 1:24.8 scale, 5.72 m model. The full-scale hull form is a
preliminary design for a surface combatant for the US Navy. Characteristic features of the
DTMB 5415 hull include a transom stern and a sonar dome bow. The model was un-appended
except for bilge-keels, i.e., not equipped with shafts, struts, propellers, nor rudders. The model
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