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FhSim
3.1.0
Marine systems simulation
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FhSim
3.1.0
Marine systems simulation
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Class containing a net object.
Collaboration diagram for Net structure object:A lumped-mass net made of triangular net panels (NetElement3N) and two-node cable elements (NetCableElement) that share global nodes. Each node carries a position and a velocity state. The net is read from a separate net definition file, which is refined into smaller elements before the simulation starts (see the General tag below). Forces on the nodes come from the panels (twine tension, hydrodynamic load, weight in water, damping), the cables (tension, drag, weight in water, attached spheres and disks), seafloor contact on the original (unrefined) nodes, and the external force input ports.
The hydrodynamic load on each panel is one evaluation of a panel load law (fhsim_environment/hydrodynamics/PanelLoadLaw.h, chosen by HydroModel), shared equally between the panel's three nodes. The law sees the panel's deformed area and normal, its twine diameter, its solidity, its bar directions and bar and knot data, and the water velocity relative to the panel, the mean of its node velocities. The water velocity is the waked current at the panel centroid (GetWakedCurrent with the net's wake query; see Wake casting) plus the wave particle velocity averaged over the panel's nodes (owner ruling R62), both through NetFlowInteraction, always handed to the law as the free stream: it never contains the panel's own induction, which the default law adds itself. The solidity is the structure's, by SolidityModel (fhsim_environment/hydrodynamics/NetSolidity.h); the law never computes it. See doc/user/net_hydrodynamics_methods.md for the laws.
For flow in the panel plane every harmonic of KF2012 Eq. 14 vanishes at an inflow angle of 90 degrees, so the screen laws (LocalThroughFlow, ScreenKF2012, ScreenMF2022) carry only TwineCrossFlow's twine friction there (MARE-0111). Only TwineCrossFlow also gives cross-flow drag on the bars.
With CastWake the net casts its wake (src/net/NetFlowInteraction.h: fhsim_environment's BodyWake over its PanelSourceSet, the net's panels through src/net/NetWakePanels.h; plan §B1, §B4, §B6, ADR 0002, ADR 0006). It keeps two fields. The private field, a GriddedWake or, when WakeEvaluation is Direct, a direct sum over the panels, holds one elementary source per panel and is sampled only by the net's own panels and cable elements; it is not registered. With WakeEvaluation None there is no private field: the panels do not shade each other. The lumped field, one source made from the net's total panel force and frontal extent, is registered in the environment under the SimObject name, and every other object sees only it. In FinalSetup the net registers the lumped field; both stay empty until the first rebuild, and nothing samples the flow in FinalSetup. Rebuilds run in PreOdeFcn only, at the start of a major step, so OdeFcn stays const and every stage of a step sees the same fields.
Each rebuild marches the panels in flow order: it samples the private field of the panels upstream at each centroid, evaluates that panel's load law at the resulting inflow (the ambient current and the other objects' lumped wakes, without waves) and adds the panel's momentum deficit, with the panel's solidity, as an elementary source. The private field lies in a frame aligned with the area-weighted mean relative flow, with its origin at the area-weighted mean of the panel centroids, that translates with the source velocity u_s, the area-weighted mean of the panel velocities from the node velocities low-pass filtered with WakeFilterTime. It covers the structure and reaches WakeLength, or one panel diameter, beyond the rear panel. The lumped source then takes the momentum deficit M = F·ê_U/(½ρ|U|²) of the sum F of the panel forces of that build (lift and the cable elements left out), the frontal area W·H of the panels' nodes across the flow, widened when M/(W·H) exceeds 0.96 so that M is kept (logged), and sits at the centre of the rear face, where its wake starts. Both fields are blended in from the previous ones over WakeBlendTime (smoothstep, C1 in time). The builder's resolution warning goes to the FhSim log, and so does one line per rebuild (with the lumped source), freeze, re-arm and skipped rebuild.
WakeUpdate sets the schedule. Freeze builds at WakeBuildTime, rebuilds up to WakeIterations more times WakeIterInterval apart while the structure deforms, until the change between two rebuilds, the larger of the largest change of the velocity ratio R = 1 - d of the private field at any panel centroid and the relative change |ΔM|/M of the lumped source, is below WakeTolerance (owner ruling R121), and then holds the wakes; a round that runs out of iterations is logged "without converging". The lumped M depends on the wakes upstream (R87), which reach one row of a farm further per rebuild, so an N-row farm needs WakeIterations ≥ N (the default 4 covers up to four rows, owner ruling R130). It re-arms, with a log line, when the filtered source velocity, or the filtered ambient current at the structure (a moored cage under a turning or tidal current), drifts from its value at the freeze by more than WakeRearmTolerance times the mean relative flow speed at the freeze (MARE-0136). Periodic rebuilds every WakePeriod, relaxing each panel's momentum deficit and the lumped M by WakeRelaxation. Off builds once. A rebuild never starts while the previous blend runs (lead ruling R22): it waits, and the wait is logged. Between rebuilds the wakes move at the source velocity of the last rebuild, so under a manoeuvre a panel can move into its own frozen wake once its velocity has changed by more than about x_start per rebuild interval; use Periodic with a short WakePeriod for manoeuvres. For a free panel thrown upstream at 1 m/s in a 0.5 m/s current and slowed by drag, the states differ from CastWake off by 2.3 % at WakePeriod 1 s, 1.5 % at 0.25 s and not at all at 0.05 s (NetStructure_WakeManoeuvre*_in.xml; MARE-0116).
A panel's own inflow is the waked current at its centroid with the net's wake query (GetWakedCurrent with the net's BodyWake::Query(), held per step, see below) plus the node-averaged wave particle velocity (R62): the other objects' lumped wakes and the net's private field, which includes the wake of the panels upstream and never the panel's own, since the kernel starts x_start downstream of the source; the net's own lumped field is skipped. The cable elements read the flow with the same query. Other SimObjects that query the flow see the lumped wake only, so a separate SimObject inside the net's volume sees no internal wake (owner answer QF8; MARE-0364, documented limitation, pinned by NetWake.BodyInsideTheNetSeesNoWakeOfItsFrontPanels_MARE0364), and GetWakeDeficit shows the lumped wakes. The lumped wake starts as a step at the rear face (x_start = 0, owner answer QF7): the deficit is zero upstream of the rear-face plane and the full lumped deficit (about 0.29 at C_T 0.5) on it, so a separate SimObject that crosses that plane (a sinker tube, a bottom ring, a bridle node) sees a jump in its inflow, a state-dependent discontinuity in its OdeFcn that can collapse the step size or fail the Newton iteration of an implicit integrator such as BDF (MARE-0152). The analytical Jacobian holds no wake term: the wakes are frozen with respect to the states, and their spatial gradient at a panel is left out, as the current's is. With the default ExternalWakeUpdate Step the other objects' wakes and the net's own private field are composed once per integrator step, in PreOdeFcn after the net's own rebuild, at the step's start centroids and midpoints, and held for every RK stage and Jacobian evaluation of the step (fhsim_environment's environment::wake::ExternalWakes, owner decision R46, owner ruling R135), so the right-hand side holds no wake gradient within a step either; the ambient current and the waves stay live. During a rebuild blend the held samples take the blend weight of each evaluation's time, so the right-hand side stays C1 in time through the blend (owner ruling R136). A clamped current (a depth profile beyond its layers, OUT_OF_RANGE_INACCURATE) is used as it is, and where a current field fails the sum of the others is kept (owner ruling R122). The caster checks the current lookups every step, also while its wakes are frozen: while one fails it casts no lumped wake, logged once; once no blend runs it publishes an empty lumped field and the private field of that partial current, both at once without a blend and not counted (it cannot end a Freeze round), and builds nothing more until the lookup recovers, when a rebuild is due at once in every WakeUpdate mode (MARE-0336, MARE-0339).
In waves the wave particle velocity is evaluated once per node and averaged over a panel's three nodes (a cable element's two) on top of the current and the wakes at its centroid (midpoint), owner ruling R62, MARE-0210. The wave field is not linear in position, so this differs from the value at the centroid where a panel spans a noticeable part of a wave length (about 10 % for a 5 cm panel in a 0.3 m wave, by (kh)²); it costs a wave evaluation per node instead of per panel and cable (2100-panel cage in a 50-component sea, 0.1 s RK45: the waves added 0.33 s instead of 0.59 s).
SeafloorForces shared resource must be available.The SimObject takes a parameter File (or BinaryFile) that names the net definition file, looked up relative to the directory of the main input file (fhsim_marine_elements_issue_0120). NetElements and CableElements hold one child element per element, named El1, El2, ... in sequence. Node numbers are zero-based.
The ExternalNodeMap tag of the net definition file gives each accessible node a tag, as <tag> = "<node number>". The main-file parameters NodesInputForce and NodesOutputPosAndVel pick the tags that get ports. With UseTagForExternalNodes = "true" (the default) a port is named after the tag, for example PWing1Force. With UseTagForExternalNodes = "false" it is named after the zero-based node number instead, for example ExternalForce8. Both forms are listed below; only one is registered. In the node-number form, two selected tags that name the same node (as PWing1 to PWing4 all name node 8 in Morenot520Simple.xml) give two ports with the same name.
| Name | Width | Description |
|---|---|---|
| <tag>Force | 3 | External force [N] added to the node of each tag in NodesInputForce. Registered when UseTagForExternalNodes is true. |
| ExternalForce<node> | 3 | As \<tag\>Force, named by node number. Registered when UseTagForExternalNodes is false. |
| Name | Width | Description |
|---|---|---|
| <tag>Force | 3 | The value of the matching \<tag\>Force input port [N], passed through. Registered when UseTagForExternalNodes is true. |
| ExternalForce<node> | 3 | As \<tag\>Force, named by node number. Registered when UseTagForExternalNodes is false. |
| <tag>Pos | 3 | Position [m] of the node of each tag in NodesOutputPosAndVel. Registered when UseTagForExternalNodes is true. |
| ExternalPos<node> | 3 | As \<tag\>Pos, named by node number. Registered when UseTagForExternalNodes is false. |
| <tag>Vel | 3 | Velocity [m/s] of the node of each tag in NodesOutputPosAndVel. Registered when UseTagForExternalNodes is true. |
| ExternalVel<node> | 3 | As \<tag\>Vel, named by node number. Registered when UseTagForExternalNodes is false. |
| BottomContact | 6 | Sum over the original nodes of the seafloor force on the net (components 0 to 2, N, NED) and of its moment about the NED origin (components 3 to 5, N m), at the last evaluation of the equations of motion (MARE-0064), low-pass filtered with the time constant BottomContactTimeConstant at each accepted step (MARE-0355). Zero while no node touches the seafloor and before the first accepted step. |
| VelNorm | 1 | Sum of the absolute values of all velocity state components of all nodes [m/s]. |
| StrainEnergy | 1 | Sum over the cable elements of \(EA(l - L_0)^2 / (2 L_0)\) [J], with the full (unreduced) axial stiffness, counting compression as well as tension. Panels do not contribute. |
The parameters both nets share (the net file, the panel load law and solidity, the threads, the wake), and the format of the net definition file, are on the page Net core: shared parameters and net file format. The parameters below are this SimObject's own.
| Name | Width | Description |
|---|---|---|
| UseTagForExternalNodes | 1 | Names the external node ports by tag (true) or by node number (false). See above (Default: true). |
| NodesInputForce | n | Comma-separated tags from ExternalNodeMap that get a force input port (Default: none; with NetSpec the specification's Connect nodes). |
| NodesOutputPosAndVel | n | Comma-separated tags from ExternalNodeMap that get position and velocity output ports (Default: none; with NetSpec the specification's Connect and Output nodes). |
| BottomContactTimeConstant | 1 | Time constant of the first-order filter on the BottomContact output [s]; 0 is no filter (Default: 0.01). |
| BottomContactDiameter | 1 | Diameter of the sphere that each original node presents to the seabed contact [m]; geometry only, the soil is the environment's (Default: 0.5). |
| NetDampingRatio | 1 | Damping ratio of the panels. Each panel's damping coefficient is 2 * NetDampingRatio * sqrt(m), where m is the twine mass plus the mass of the displaced water (Default: 1). |
| LineVisualization | 1 | Visualisation builds only. Draws each cable element as a line, green when shorter than its unstretched length and red otherwise, instead of drawing panels and cables (Default: false). |
| NumMeshPerLine | 1 | Visualisation builds only, when LineVisualization is false. Number of meshes per drawn twine line on the panels (Default: 1). |
There are two states per node after refinement, numbered from 1: state Pos_k belongs to zero-based node k - 1. Unset values are set to 0 (see FoldingPointRefinement for the exception). The z axis points down.
| Name | Width | Description |
|---|---|---|
| Pos_1...Pos_<N> | 3 | Position of each node [m]. |
| Vel_1...Vel_<N> | 3 | Velocity of each node [m/s]. |
This SimObject is referred to as Net/NetStructure
1.9.8