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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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Collaboration diagram for Disk of sorts:A simple disk.
Its drag splits the disk velocity relative to the water, v, along the disk axis n (the direction of the Force input; z when the force is zero): v_n = (n·v)n, v_t = v - v_n, and F = -½ρ C_n A_n |v_n| v_n - ½ρ (C_t 2 A_n + C_e A_t) |v_t| v_t, with C_n = 1.2 on the face area (the annulus π(D² − d²)/4 outside the cable, ENV-0103), C_t = 0.005 skin friction on both faces and C_e = 1.2 on the edge area A_t = (D − d) T (environment::disk_hydro::DragForce, MARE-0098; owner ruling 2026-10-21, ENV-0122: C_t alone on the edge area before).
With CastWake set the disk casts a lumped wake for the other objects (environment::wake::BodyWake and environment::disk_hydro::DiskCaster, ADR 0006, MARE-0127): at each rebuild, in PreOdeFcn, its drag at the current-only free stream U (no waves) gives the momentum deficit M = F·ê_U/(½ρ|U|²), its frontal area normal to ê_U is π(D² − d²)/4·|n·ê_U| + D·T·√(1 − (n·ê_U)²) (face annulus outside the cable plus edge silhouette, D = DiskD, T = DiskT, d = D the cable diameter; ENV-0103), and the source sits at the rear face, (T/2)|n·ê_U| + (D/2)√(1 − (n·ê_U)²) downstream of the centre. The disk reads its own flow without its own field. The lumped wake starts as a step at the rear face (x_start = 0): another SimObject that crosses the rear-face plane sees a jump in its inflow, which can collapse the step size or fail the Newton iteration of an implicit integrator such as BDF (MARE-0152). Without CastWake nothing changes.
A disk inside a net's volume (a sinker or a float in a cage) sees the net's lumped wake only, which starts at the net's rear face, not the net's private field: no wake of the front panels (MARE-0364, documented limitation, owner ruling 2026-10-11; wake groups would lift it).
Its buoyancy is that of a thin disk's circle segment below the wave surface over its centre, over the disk's vertical half-extent (D/2)√(1 − n_z²) + (T/2)|n_z| (owner ruling R66; exact for a vertical disk, an approximation when tilted or flat), and in waves it takes the Froude-Krylov force Rho V_wet a_w (owner ruling R67). Along its axis n its inertia carries the added mass of a circular disc, m_a = (1/3) Rho DiskD³ (Ca = 2/π on (4/3)π(D/2)³, the exact potential-flow value (Newman, Marine Hydrodynamics, 1977); DNV-RP-C205 (2021) Table A-2 gives the same), of its wet fraction f: M = m I + f m_a n nᵀ, and in waves it takes the matching excitation f m_a (n·a_w) n (owner ruling R104). None across the axis (the edge-on added mass of a thin disk is small).
On the seabed the disk keeps its weight and buoyancy and takes the soil load of the environment's shared SeafloorForces on top (owner ruling R72, MARE-0219): the disk is a rigid cylinder D by T along its axis n, at rest in the soil's frame, so the load follows the disk's own velocity, not the one relative to the water (MARE-0393), and grows with the penetration, so the disk rests on the seabed instead of chattering at it or sinking through it. The soil parameters are the environment's (Bathymetry soil); the disk has none. The load's moment is dropped: the disk is a point mass.
| Name | Width | Description |
|---|---|---|
| Force | 3 | ... |
| Name | Width | Description |
|---|---|---|
| Position | 3 | Position of the disk. |
| Velocity | 3 | Velocity of the disk. |
| Name | Width | Description |
|---|---|---|
| Rho | 1 | Water density [kg/m³], for the drag and the buoyancy (Default: 1025). |
| DiskD | 1 | Diameter of the disk [m] (Default: 1). |
| DiskT | 1 | Thickness of the disk [m] (Default: 0.1). |
| DiskMass | 1 | Mass of the disk [kg] (Default: 10). |
| D | 1 | Diameter of the cable through the disk [m], which reduces both drag areas (Default: 0.01). |
| DiskMesh | 1 | Visualisation builds only. The mesh file the disk is drawn with, scaled by DiskD, DiskD and DiskT along its axes (Default: fhDisk.mesh). |
| DiskMaterial | 1 | Visualisation builds only. Material of the drawn disk (Default: firebrick2). |
| CastWake | 1 | Register and build the lumped wake; the other wake parameters (Wake*, ExternalWakeUpdate) are listed in fhsim_environment's doc/user/wake-parameters.md. (Default: false) |
| WakeGroup | 1 | With CastWake: the name of a fhsim_environment WakeGroup SimObject (3.1.0) to cast one wake with, instead of a wake of its own; the group's wake parameters apply (fhsim_environment doc/user/wakes.md, "Wake groups"). (Default: empty, none) |
| Name | Width | Description |
|---|---|---|
| Position | 3 | Position of the disk (Default: "0,0,0"). |
| Velocity | 3 | Velocity of the disk (Default: "0,0,0"). |
This SimObject is referred to as Net/Disk
1.9.8