Best Practices for Maneuvering STAR Global Conference - Berlin 2017 Timothy Yen, PhD Marine and Offshore Technical Specialist Priyanka Cholletti Advanced Application Engineer Carlo Pettinelli Engineering Manager, Italy Realize innovation.
. Agenda Overview Steady Drift Rotating Arm Planar Motion Mechanism Free Running Page 2
. Agenda Overview Steady Drift Rotating Arm Planar Motion Mechanism Free Running Page 3
Maneuvering This is the deliberate dynamic motion by marine vessels to change its direction by altering the position of control surfaces or powering or both. It is important to be able to predict the hydrodynamic forces imparted on the vessel during maneuvers in order to ensure the intrinsic stability of its form and to adequately design the control surfaces. Adequate maneuvering is important to the designer for Safe ship operations Obstacle avoidance Man overboard Contract requirements Regulatory requirements Page 4
Objective We will recreate the tests that are used to isolate the coefficients that can then be applied to equations-of-motion to predict the ships motion. By controlling the different flow conditions we can determine how particular orientations or ship motion rates impact the forces on ship. The results can be used to derive the coefficients. This presentation will review some of the important settings for the successful setup of maneuvering simulations Page 5
. Agenda Overview Steady Drift Rotating Arm Planar Motion Mechanism Free Running Page 6
Steady Drift In a steady drift test we are investigating the ship forces and moments due to a constant yaw angle relative to the incoming flow. Efficient simulations require: Consistent meshes for consistent results at different yaw angles Quick way to alter the ship angles Approach: Cylindrical refinement zone for near field wake Reports driven test parameter setup Equilibrium DFBI Page 7
Boundary Conditions The setup of the far-field boundary conditions are the same for a standard calm water resistance simulation. For review: Inlets Velocity boundary conditions with VOF flat waves for velocity and volume fraction Outlets Pressure outlet to define hydrostatic pressure Lateral boundaries Velocity inlets to recreate an open water condition or No-slip walls to model tow tank blockage Page 8
Yaw Angle Orientation Changing drift angle parametrically An expression report can be created to define the drift angle for the current simulation. This expression report can then be applied to a Transform Mesh Operation to rotate the hull. This parameterization approach ensures consistency and allows for easier automation. Page 9
Body Fixed Reference In all cases the forces and moments recorded need to be defined in the ship s frame of reference. 1. It is necessary to define a custom coordinate system at the ship s CG location called CGpivot. 2. CGpivot is applied to the DFBI to define the initial orientation. This coordinate system is the direction that the hull s motion will be free to move around. Page 10
Body Fixed Reference, cont d 3. Finally, the coordinate system must be rotated to the same drift angle. Be sure to specify itself as the reference system so that it is rotated over the LCG location. Page 11 Note: Alternative route using global parameters and coordinate system exporting
Mesh Refinements Refinements are necessary to accurately capture the wake behind the hull and thus the wavemaking drag. In order to create a setup that doesn t require adjustments we must create a wake refinement that is large enough to accommodate the hull for all yaw orientations. Similarly, a circular refine zone should be created around the hull in order to resolve the nearfield flow field. The circular zone is utilized to maintain the consistency of the refinement regardless of the yaw angle. Page 12
Results Drift Angle: 0 degrees Page 13
Results Drift Angle: 5 degrees Page 14
Results Drift Angle: 10 degrees Page 15
Results Drift Angle: 15 degrees Page 16
Results Drift Angle: 20 degrees Page 17
Overset Setup The overset setup is an alternative method to resolving the static drift angle without requiring remeshing in the background mesh. The methodology may also be desirable for consistency with other maneuvering simulations such as PMM that require an Overset setup. Overset requires an adequate buffer around the ship. Nominally it should be at least 4 to 5 cells for interpolation between Regions. The Overset Region can be rotated to the desired drift angle about CGpivot. Note that expressions are not accepted. Macros are required for full automation. Page 18
. Agenda Overview Steady Drift Rotating Arm Planar Motion Mechanism Free Running Page 19
Rotating Arm In a rotating arm test we are towing a vessel at a fixed radius and rotation rate. This allows us to isolate the forces due to a constant yaw rate of change at a constant surge (travel) speed. CFD allows for infinite clean water, there is no wake to run into during testing Efficient simulations require: Efficient modification of rotating arm operating parameters Convenient post-processing Approach: Similar mesh refinement setup as the static drift cases Moving reference frame setup using Reports driven test parameters Overset as an alternative approach to implement additional drift angles Page 20
Moving Reference Frame Utilizing a moving reference frame has several benefits over using the built-in Rotating Arm DFBI motion option: A stationary domain simplifying post-processing Ability to restart at a higher forward speed for a more efficient speed sweep Utilization of Equilibrium DFBI for improved motions convergence Prevents the requirement for overall domain rotation and translation Arm Axis CG Setup of Moving Reference Frame can be parametrically driven and automated using reports. Note that the Axis Origin of the arm runs through the LCG of the vessel so that it s effective being towed at that point. Page 21
Rotating Arm Parameters The parameters for the rotating arm operating conditions are going to be run by Expression Reports. Rotating Radius specifies the radius of the rotating arm. Here it is presented in multiples of the LOA Test Speed specifies the constant surge speed of the vessel Rotation Rate computes the rotational speed applied to the moving reference frame Page 22
Rotating Arm Result Page 23
. Agenda Overview Steady Drift Rotating Arm Planar Motion Mechanism Free Running Page 24
Planar Motion Mechanism (PMM) This mechanism was developed in the 1960s to evaluate marine vehicles in long narrow tow tanks for maneuvering coefficients with out the expense of rotating arm facilities. The PMM also allows for deriving higher order coefficients due to yaw and sway accelerations. The mechanism consists of up to two oscillating motions (sway and yaw) superposed on the steady translation of the carriage. SWAY YAW Page 25
Test Motions Two primary types of motions are usually implemented in order to derive non-linear coefficients: Pure Sway Ψ = 0 In-phase velocity-dependent terms and Added mass terms and Pure Yaw and Yaw-rate dependent terms and Yaw-acceleration terms and Page 26
CFD Methodology CFD offers the advantage of easily accommodating hull motions like heave and pitch that may not be possible or practical at all experimental facilities. Additional drift angles can be applied and a large test suite can be investigated. Efficient simulations require: Parametrically driven PMM parameters Mean course fixed coordinate systems for post-processing This allows a camera to follow the mean path of the hull like on a carriage Approach: To use Overset to simulate 3+3 DOF motion of the Hull Region and 1DOF motion of Background Region Utilize built-in PMM motions features Create a nested coordinate system and motion to achieve a carriage-fixed like post-processing scenes Page 27
Motion and Topology The PMM motions will utilize the built-in Planar Motion Carriage functionality to translate and rotate the Region to fulfill the specified motions. We will follow a two Region strategy to separate the response hull motions from the primary carriage translation. This prevents the large vertical displacements motions at the farfield boundaries that cause artifacts on the freesurface. Prescribed Motions Only: Surge Overset Region Farfield Region Prescribed Motions: Surge Sway Yaw DFBI Response: Pitch Heave (Roll) Page 28
PMM Parameters Like in the other tests, motions are driven parametrically by Expression Reports. PMM Drift Angle specifies an additional drift angle PMM Sway Amplitude specifies the maximum transverse displacement of the CG from the mean path PMM Sway Frequency specifies the oscillation frequency of the maneuver. PMM Test Vel specifies the mean velocity of the hull. Analogous to the carriage speed in experiments. Page 29
Specifying PMM Motion The PMM motion is specified by altering the default DFBI set up to follow the Planar Motion Carriage The Reports are then assigned to the motion settings Specifies a Pure Yaw motion where hull is always tangential to the ship path. Unchecked means it s a pure sway motion. Note that upon initialization the hull is orientated at t=0 to be tangential to the path +/- any Additional Drift Angle Page 30
Background Translation Motion We now need to specify a motion model that will translate the Background region at the carriage speed of the Hull motion. This will allow the ship to sway about a fix longitudinal position relative to the background mesh Create a new Translation Motion under Tools Motion in the simulation tree Specify the x-direction translation with the Report defining the carriage speed Select the CGpivot coordinate system as the Managed Coordinate System This will translate the coordinate system by the carriage motion Apply this Translation to the Background Region under its Physics Values Page 31
Flow Visualization Visualization scenes typically center on the hull but since the PMC motion translates the hull longitudinally each Scene s camera needs to follow the correct Coordinate System In the scene s Attributes View specify the CGpivot Coordinate System The CGpivot CS is being managed by the Translation motion so it allows the camera to follow the hull s course made-good Page 32
Time Step Size Besides the physical parameters that govern time step size selection, the interaction of the mesh spacing and its overset motion need to be considered In overset applications to maintain robustness and accuracy it is suggested that the time step size allow for a motion CFL number In a Pure Sway case, Page 33
Result Page 34
. Agenda Overview Steady Drift Rotating Arm Planar Motion Mechanism Free Running Page 35
Free running This condition is when the hull is experiencing all 6 degree-of-freedom dynamically due to hydrodynamic pressures and forces. It s a dynamic combination of: Self-propulsion Maneuvering Control surfaces Control systems Simulations require: Control system implementation Propulsion forces from propeller or virtual disk Approach: Overset for 6DOF hull motion and 3DOF Background Region motion to follow hull path and yaw but ignore heave, pitch and yaw Overset for propeller domains and control surfaces Use Reports to implement a proportional control system Page 36
Domains Setup Use a multi-region Overset setup like the PMM case Prescribed Motions Only: Surge Sway Yaw Overset Region Farfield Region DFBI Motions: Surge Sway Heave Roll Pitch Yaw One Region for each control surface One Region for each propeller (if applicable eg Virtual Disk) Page 37
Background Mesh Motions A moving background Region is necessary to solve this simulation economically and avoid an impractically large background mesh for the vessel to operate within. The Background Region orientation and position needs to conform to the hull on the x-y plane. It will translate and yaw about the hull CG. It will not heave, roll or pitch. To implement this we need to translate a dummy coordinate system that stay parallel to the laboratory coordinate system but follows the hull s CG. Finally we create the real motion that translates like the hull but rotates about the hull CG. Page 38 Body Reports: Yaw Rate (Z Rot) in Lab CSys Vel X in Lab CSys Vel Y in Lab Csys Motion 1 Planar Translation Vel X Body Report Vel Y Body Report Reference Lab CSys Manage CSys: Planar Trans CSys Stays parallel with Lab Csys Init origin at CG Motion 2 Planar Translation & Turning Yaw Rate Body Report Vel X Body Report Vel Y Body Report Reference Planar Trans Csys Force background mesh to translate and rotate about ship CGx and CGy point
Control Surface Motions Rudders use Superposed motions relative to CG Rudder rotation rate specified by control system Current rudder angle is necessary for control system but there is no report. Workaround is necessary: Split the rudder into inboard and outboard surfaces Create unit normal vector field function $$Area/mag($$Area) Report the Surface Average Normal in X & Y Dir in Rudder Csys at initial conditions Compute a baseline angle constant at zero deflection atan2(i_dir, j_dir) Compute current angle at anytime by offsetting with zero deflection baseline angle Page 39
Control System Basic proportional feedback control system The P in a PID controller COURSE Page 40 HEADING - COURSE HEADING Requires some information: Max rudder speed: KNOWN Current rudder angle: COMPUTED Requested rudder angle: = Error x Kp Maximum rudder angle: KNOWN Proportional constant: TUNED PARAMETER Current heading: DFBI ORIENTATION REPORT Set course: USER SPECIFIED CFD RUDDER ANGLE Output rudder speed applied to superposed motion is computed as: 1. IF Request Angle < Max Angle IF (Request Angle Current Angle) / Δt > Max Speed THEN apply Max Speed ELSE apply (Request Angle Current Angle) / Δt 2. IF Request Angle > Max Angle Adjust Request Angle to Max Angle Goto 1 Note that angle is positive and negative. Maintain the sign of the requested angle Control system is implemented as a series of Expression Reports and then applied to DFBI Superposed Rotation motions.
Results 15 deg Zig Zag Page 41
Free running with manoeuvring Page 42
Carlo Pettinelli Engineering Manager, Torino Siemens PLM Italy DF PL S&SE GS CSS EU IT E-mail: carlo.pettinelli@siemens.com Realize innovation. Page 43