Test wake body

A kinematic sphere-like drag body that casts a lumped wake for the other objects.

Collaboration diagram for Test wake body:

A sphere-like drag body moving at a constant velocity, casting a lumped wake.

A test and demonstration model of the generic wake-casting interface (environment::wake::WakeCastingObject, environment::wake::LumpedWakeCaster; phase F of the net-hydrodynamics programme). The body moves at the constant TowVelocity from its initial Position; zero holds it fixed. Its drag is the quadratic law F = ½ ρ C_d A |u| u on the relative flow u = u_water − TowVelocity, with A = π D²/4, the frontal area of a sphere in any direction. The drag does not act on the motion: the body is kinematic.

With CastWake set, the body registers one lumped source under its name in FinalSetup and rebuilds it in PreOdeFcn on the schedule of the Wake* parameters: the free stream at the centre is the ambient current with every other registered wake (never its own, never the waves); the momentum deficit is M = F·ê_U / (½ ρ |U|²) = C_d A for this law; the source sits at the rear face, D/2 downstream of the centre, where its wake starts. The body's centre is therefore never inside its own wake, and its own field is excluded from the flow it samples, so this also holds if the relative flow reverses between rebuilds (the wake frame does not rotate). The wake acts on the current part of every other object's flow.

Example Configuration (XML)

<Lib LibName="fhsim_environment" SimObject="TestWakeBody" Name="Sphere"
     Diameter="1" DragCoefficient="0.5" CastWake="true" WakeUpdate="Freeze" />

Input ports

None.

Output ports

Port	Size	Description
Position	3	Centre [m], NED.
WaterVelocity	3	Waked water velocity at the centre, including waves [m/s], NED.
Force	3	Drag at WaterVelocity [N], NED.

States

Position (3) [m], NED: set its initial value with the initial condition <Name>.Position.

Configuration Parameters

Name	Width	Description
Diameter	1	Diameter D [m], > 0. (Default: 1)
DragCoefficient	1	Drag coefficient C_d on the frontal area [-], >= 0; a test value, not a measured sphere coefficient. (Default: 0.5)
Rho	1	Water density [kg/m^3], > 0. (Default: 1025)
TowVelocity	3	Constant velocity of the body [m/s], NED. (Default: 0, 0, 0, fixed)
CastWake	1	Register and build the lumped wake. (Default: false)
WakeUpdate	1	Off, Freeze or Periodic. (Default: Freeze)
WakeBuildTime	1	Time of the first build [s]. (Default: 0)
WakeIterations	1	Freeze rebuilds after the first. (Default: 4)
WakeIterInterval	1	Time between Freeze rebuilds [s]. (Default: 5)
WakeTolerance	1	Freeze converges when |dM|/M between two builds is below it [-]. (Default: 0.01)
WakePeriod	1	Time between Periodic rebuilds [s]. (Default: 10)
WakeRelaxation	1	Periodic relaxation alpha in (0, 1]: M = alpha M_new + (1 - alpha) M_old. (Default: 1)
WakeBlendTime	1	Cross-fade of each rebuild [s]; must not exceed WakePeriod (Periodic) or WakeIterInterval (Freeze). (Default: 2)
WakeRearmTolerance	1	Relative drift of the source velocity or of the ambient current at the centre that re-arms Freeze [-]. (Default: 0.1)
WakeRearmDistance	1	Freeze re-arms when the centre is this fraction of sqrt(frontal area) away from the frozen wake frame (R50) [-]. (Default: 0.1)
WakeFilterTime	1	Low-pass time constant of the source velocity and of the ambient current the re-arm watches [s]. (Default: 5)
WakeExpansion	1	Wake expansion rate k [-]. (Default: 0.05)
WakeCutoff	1	Kernel cutoff deficit, in (0, 1). (Default: 0.001)

This SimObject is referred to as TestWakeBody