Compute mooring knockdown by a horizontal current

The current may be a depth-independent or depth-dependent, as specified by the u argument. The computation is iterated at most niteration times until the RMS deviation between computed z values falls below the product of convergenceCriterion and wire length. The returned result has an attribute named u that holds the value of that argument, and this is how a later call to plot() is able to display a velocity profile; see Examples 2 and 3.

knockdown(m, u = 1, convergence = 0.1, maxiteration = 100, debug = 0L)

Arguments

m: an object of the "mooring" class, usually created with mooring(), followed by a call to segmentize().
u: either a non-negative number indicating depth-independent velocity, or a function that returns that velocity as a function of depth (m) below the ocean surface.
convergence, maxiteration: criteria for stopping the iteration process. If the root-mean-squared difference in element angles drops below this convergence degrees, then convergence is declared and the iterative solution process stops. If not, then the iteration stops after maxiteration passes (and a warning is issued). In any case, the actual number of iterations is stored in an attribute named iterations, the RMS angle difference (in degrees) is stored as RMSAngleChange, and the RMS difference in element depth (in m) is stored RMSDepthChange.
debug: an integer value (or a numeric that can be converted to an integer) that controls debugging. If this is positive, then some debugging information may be printed by the function.

Value

knockdown returns a mooring object representing the deformed mooring, with x and z values updated, with the original values being saved as x0 and z0. In addition, the stress and angle of each element is stored in fields named tau and phi, the latter in radians. Attributes are added to the object to describe the solution in more detail. The u attribute stores the value supplied to knockdown(), and waterDepth stores the water depth that was supplied to the anchor() call. An overview of the iterative solution is provided in the attributes (see convergence).

Author

Dan Kelley

Examples

# Illustrate importance of drag on the wire.
library(mooring)
m <- mooring(anchor(), wire(length = 30), float("16in Viny"), waterDepth = 50)
ms <- segmentize(m)

# Example 1: no current
plot(ms)


# Example 2: uniform 0.5 m/s (approx. 1 knot) current
par(mfrow = c(1, 2))
k1 <- knockdown(ms, u = 0.5)
plot(k1, which = "velocity")
plot(k1)


# Example 3: 0.5 m/s surface current, exponential decay below
k2 <- knockdown(ms, u = function(depth) 0.5 * exp(-depth / 30))
par(mfrow = c(1, 2))
plot(k2)
plot(k2, which = "velocity")


# Example 4: as Example 3, but show knockdown and tension
# The red dashed line in the tension plot indicates the
# anchor weight.
par(mfrow = c(1, 2))
plot(k2, which = "knockdown")
plot(k2, which = "tension")