def test_skew():
assert np.allclose(utils.skew(np.zeros(3)), np.zeros((3,3)))
assert np.allclose(utils.skew([1,2,3]), np.array([
[ 0, -3, 2],
[ 3, 0, -1],
[-2, 1, 0],
]))
def test_skew():
assert np.allclose(utils.skew(np.zeros(3)), np.zeros((3, 3)))
assert np.allclose(utils.skew([1, 2, 3]),
np.array([
[0, -3, 2],
[3, 0, -1],
[-2, 1, 0],
]))
def Morison_inertial_force(self, w):
r"""Calculate the Morison inertial force
"""
# XXX
rho = 1025
# Define the matrix which transforms the 6 rigid-body DOFs
# into the acceleration at each element
L = np.array([np.c_[np.eye(3), -skew(x)] for x in self.element_centres])
# For inertial force: calculate normal fluid acceleration for unit wave
a_fluid = 1j * w[:,newaxis,newaxis] * self.wave_velocity_transfer_function(w)
# Only normal accelerations count -- subtract the longitudinal
# component at each element: at = (a . t) t
t = self.element_axes[:,:,2]
at = np.einsum('wei,ei,ej->wej', a_fluid, t, t)
an = a_fluid - at
# Added mass force for each element (Nel x 3 x 6)
F_inertial = (self.Cm * rho *
(pi*self.element_diameters[newaxis,:,newaxis]**2/4) *
self.element_lengths[newaxis,:,newaxis] * an)
# Sum up to get total resultant force and moment
# A = sum [ L.T * F_added ]
F = np.einsum('eix,wei->wx', L, F_inertial)
return F
def structural_velocity_transfer_function(self, w, H):
"""Return the transfer functions from wave elevation to structure
velocity.
- H: array of transfer functions (RAOs)
Returns: array (frequency, element, xyz)
"""
H_us = np.zeros((len(w), len(self.element_diameters), 3),
dtype=np.complex)
for iel in range(H_us.shape[1]):
xs = skew(self.element_centres[iel])
for j in range(H_us.shape[0]):
# H may have more than 6 DOFs if model is flexible
H_us[j, iel, :] = (1j*w[j] * (H[j, 0:3] - dot(xs, H[j, 3:6])))
return H_us
def Morison_added_mass(self):
"""Calculate added mass matrix from Morison elements"""
# Define the matrix which transforms the 6 rigid-body DOFs
# into the acceleration at each element
L = np.array([np.c_[np.eye(3), -skew(x)] for x in self.element_centres])
# Only normal accelerations count -- subtract the longitudinal
# component at each element: at = (a . t) t
t = self.element_axes[:,:,2]
a = L
at = np.einsum('eix,ei,ej->ejx', a, t, t)
an = a - at
# Added mass force for each element (Nel x 3 x 6)
F_added = ((self.Cm - 1) * 1025 *
(pi*self.element_diameters[:,newaxis,newaxis]**2/4) *
self.element_lengths[:,newaxis,newaxis] * an)
# Sum up to get total resultant force and moment
# A = sum [ L.T * F_added ]
A = np.einsum('eix,eiy->xy', L, F_added)
return A
def total_drag(self, w, H_wave, S_wave, vc=None):
r"""Calculate the linearised forces on each element then sum
The total force and moment together are
$$ \boldsymbol{F} = \sum \boldsymbol{L}_i \boldsymbol{F}^_i$$
with
$$\boldsymbol{L}_i = \begin{bmatrix}
\boldsymbol{I} \\ \boldsymbol{\tilde{X}_i} \end{bmatrix}$$
The drag force on each element is
$$ \boldsymbol{F}_i = C \boldsymbol{A} (
\boldsymbol{u}_f - \boldsymbol{u}_s ) + C \boldsymbol{b} $$
where $C = \rho C_d D \mathrm{d}s / 2$ is a constant, and
$\boldsymbol{A}$ is the transformed linearisation matrix.
Using the fluid velocity transfer functions $\boldsymbol{H}_{uf \zeta}$,
$\boldsymbol{u}_f(t) = \boldsymbol{H}_{uf \zeta}(\omega)
\zeta_a e^{i\omega t}$, so this part goes on the RHS as a force
$C \boldsymbol{A} \boldsymbol{H}_{u_f \zeta} \zeta_a
e^{i\omega t} + C \boldsymbol{b}$.
The structural response is given by
$$ \boldsymbol{u}_s = i\omega
\begin{bmatrix} \boldsymbol{I} & -\boldsymbol{\tilde{X}} \end{bmatrix}
\boldsymbol{\Xi}(\omega) e^{i\omega t} $$
so the rest of the drag force goes on the LHS as a damping matrix
$$\boldsymbol{B}_v = C \boldsymbol{A} \boldsymbol{L}^{\mathrm{T}}$$
This function returns Fvc = sum (C Li bi), the steady part of the force,
and Fvv[freq] = sum (C Li Ai H_uf), which is the harmonic drag force for
unit wave amplitude.
NB this is very slow if w goes right to zero -- why?
"""
# XXX
rho = 1025
H_uf = self.wave_velocity_transfer_function(w)
A, b = self.linearised_drag_force(w, H_wave, S_wave, vc)
Bv = np.zeros((6,6)) # viscous drag damping
Fvc = np.zeros(6) # constant drag force
Fvv = np.zeros((len(w), 6), dtype=complex) # time-varying drag force
for iel in range(len(self.element_diameters)):
# Drag constant for each element
C = (0.5 * rho * self.Cd *
self.element_diameters[iel] * self.element_lengths[iel])
# Force acts at centre of each element
x = self.element_centres[iel]
L = np.r_[ np.eye(3), skew(x) ] # forces and moments about origin
# Damping effect of viscous drag force
Bv += C * dot(L, dot(A[iel], L.T))
# Applied viscous drag force
Fvc += C * dot(L, b[iel])
for iw in range(len(w)):
Fvv[iw] += C * dot(L, dot(A[iel], H_uf[iw, iel, :]))
return Bv, Fvc, Fvv
