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import numpy
from matplotlib import pyplot, cm
from mpl_toolkits.mplot3d import Axes3D
def build_up_b(rho, dt, dx, dy, u, v):
b = numpy.zeros_like(u)
b[1:-1, 1:-1] = (rho * (1 / dt * ((u[1:-1, 2:] - u[1:-1, 0:-2]) / (2 * dx) +
(v[2:, 1:-1] - v[0:-2, 1:-1]) / (2 * dy)) -
((u[1:-1, 2:] - u[1:-1, 0:-2]) / (2 * dx)) ** 2 -
2 * ((u[2:, 1:-1] - u[0:-2, 1:-1]) / (2 * dy) *
(v[1:-1, 2:] - v[1:-1, 0:-2]) / (2 * dx)) -
((v[2:, 1:-1] - v[0:-2, 1:-1]) / (2 * dy)) ** 2))
# Periodic BC Pressure @ x = 2
b[1:-1, -1] = (rho * (1 / dt * ((u[1:-1, 0] - u[1:-1, -2]) / (2 * dx) +
(v[2:, -1] - v[0:-2, -1]) / (2 * dy)) -
((u[1:-1, 0] - u[1:-1, -2]) / (2 * dx)) ** 2 -
2 * ((u[2:, -1] - u[0:-2, -1]) / (2 * dy) *
(v[1:-1, 0] - v[1:-1, -2]) / (2 * dx)) -
((v[2:, -1] - v[0:-2, -1]) / (2 * dy)) ** 2))
# Periodic BC Pressure @ x = 0
b[1:-1, 0] = (rho * (1 / dt * ((u[1:-1, 1] - u[1:-1, -1]) / (2 * dx) +
(v[2:, 0] - v[0:-2, 0]) / (2 * dy)) -
((u[1:-1, 1] - u[1:-1, -1]) / (2 * dx)) ** 2 -
2 * ((u[2:, 0] - u[0:-2, 0]) / (2 * dy) *
(v[1:-1, 1] - v[1:-1, -1]) / (2 * dx)) -
((v[2:, 0] - v[0:-2, 0]) / (2 * dy)) ** 2))
return b
def pressure_poisson_periodic(p, dx, dy):
pn = numpy.empty_like(p)
for q in range(nit):
pn = p.copy()
p[1:-1, 1:-1] = (((pn[1:-1, 2:] + pn[1:-1, 0:-2]) * dy ** 2 +
(pn[2:, 1:-1] + pn[0:-2, 1:-1]) * dx ** 2) /
(2 * (dx ** 2 + dy ** 2)) -
dx ** 2 * dy ** 2 / (2 * (dx ** 2 + dy ** 2)) * b[1:-1, 1:-1])
# Periodic BC Pressure @ x = 2
p[1:-1, -1] = (((pn[1:-1, 0] + pn[1:-1, -2]) * dy ** 2 +
(pn[2:, -1] + pn[0:-2, -1]) * dx ** 2) /
(2 * (dx ** 2 + dy ** 2)) -
dx ** 2 * dy ** 2 / (2 * (dx ** 2 + dy ** 2)) * b[1:-1, -1])
# Periodic BC Pressure @ x = 0
p[1:-1, 0] = (((pn[1:-1, 1] + pn[1:-1, -1]) * dy ** 2 +
(pn[2:, 0] + pn[0:-2, 0]) * dx ** 2) /
(2 * (dx ** 2 + dy ** 2)) -
dx ** 2 * dy ** 2 / (2 * (dx ** 2 + dy ** 2)) * b[1:-1, 0])
# Wall boundary conditions, pressure
p[-1, :] = p[-2, :] # dp/dy = 0 at y = 2
p[0, :] = p[1, :] # dp/dy = 0 at y = 0
return p
##variable declarations
nx = 41
ny = 41
nt = 10
nit = 50
c = 1
dx = 2 / (nx - 1)
dy = 2 / (ny - 1)
x = numpy.linspace(0, 2, nx)
y = numpy.linspace(0, 2, ny)
X, Y = numpy.meshgrid(x, y)
##physical variables
rho = 1
nu = .1
F = 1
dt = .01
# initial conditions
u = numpy.zeros((ny, nx))
un = numpy.zeros((ny, nx))
v = numpy.zeros((ny, nx))
vn = numpy.zeros((ny, nx))
p = numpy.ones((ny, nx))
pn = numpy.ones((ny, nx))
b = numpy.zeros((ny, nx))
udiff = 1
stepcount = 0
while udiff > .001:
un = u.copy()
vn = v.copy()
b = build_up_b(rho, dt, dx, dy, u, v)
p = pressure_poisson_periodic(p, dx, dy)
u[1:-1, 1:-1] = (un[1:-1, 1:-1] -
un[1:-1, 1:-1] * dt / dx *
(un[1:-1, 1:-1] - un[1:-1, 0:-2]) -
vn[1:-1, 1:-1] * dt / dy *
(un[1:-1, 1:-1] - un[0:-2, 1:-1]) -
dt / (2 * rho * dx) *
(p[1:-1, 2:] - p[1:-1, 0:-2]) +
nu * (dt / dx ** 2 *
(un[1:-1, 2:] - 2 * un[1:-1, 1:-1] + un[1:-1, 0:-2]) +
dt / dy ** 2 *
(un[2:, 1:-1] - 2 * un[1:-1, 1:-1] + un[0:-2, 1:-1])) +
F * dt)
v[1:-1, 1:-1] = (vn[1:-1, 1:-1] -
un[1:-1, 1:-1] * dt / dx *
(vn[1:-1, 1:-1] - vn[1:-1, 0:-2]) -
vn[1:-1, 1:-1] * dt / dy *
(vn[1:-1, 1:-1] - vn[0:-2, 1:-1]) -
dt / (2 * rho * dy) *
(p[2:, 1:-1] - p[0:-2, 1:-1]) +
nu * (dt / dx ** 2 *
(vn[1:-1, 2:] - 2 * vn[1:-1, 1:-1] + vn[1:-1, 0:-2]) +
dt / dy ** 2 *
(vn[2:, 1:-1] - 2 * vn[1:-1, 1:-1] + vn[0:-2, 1:-1])))
# Periodic BC u @ x = 2
u[1:-1, -1] = (un[1:-1, -1] - un[1:-1, -1] * dt / dx *
(un[1:-1, -1] - un[1:-1, -2]) -
vn[1:-1, -1] * dt / dy *
(un[1:-1, -1] - un[0:-2, -1]) -
dt / (2 * rho * dx) *
(p[1:-1, 0] - p[1:-1, -2]) +
nu * (dt / dx ** 2 *
(un[1:-1, 0] - 2 * un[1:-1, -1] + un[1:-1, -2]) +
dt / dy ** 2 *
(un[2:, -1] - 2 * un[1:-1, -1] + un[0:-2, -1])) + F * dt)
# Periodic BC u @ x = 0
u[1:-1, 0] = (un[1:-1, 0] - un[1:-1, 0] * dt / dx *
(un[1:-1, 0] - un[1:-1, -1]) -
vn[1:-1, 0] * dt / dy *
(un[1:-1, 0] - un[0:-2, 0]) -
dt / (2 * rho * dx) *
(p[1:-1, 1] - p[1:-1, -1]) +
nu * (dt / dx ** 2 *
(un[1:-1, 1] - 2 * un[1:-1, 0] + un[1:-1, -1]) +
dt / dy ** 2 *
(un[2:, 0] - 2 * un[1:-1, 0] + un[0:-2, 0])) + F * dt)
# Periodic BC v @ x = 2
v[1:-1, -1] = (vn[1:-1, -1] - un[1:-1, -1] * dt / dx *
(vn[1:-1, -1] - vn[1:-1, -2]) -
vn[1:-1, -1] * dt / dy *
(vn[1:-1, -1] - vn[0:-2, -1]) -
dt / (2 * rho * dy) *
(p[2:, -1] - p[0:-2, -1]) +
nu * (dt / dx ** 2 *
(vn[1:-1, 0] - 2 * vn[1:-1, -1] + vn[1:-1, -2]) +
dt / dy ** 2 *
(vn[2:, -1] - 2 * vn[1:-1, -1] + vn[0:-2, -1])))
# Periodic BC v @ x = 0
v[1:-1, 0] = (vn[1:-1, 0] - un[1:-1, 0] * dt / dx *
(vn[1:-1, 0] - vn[1:-1, -1]) -
vn[1:-1, 0] * dt / dy *
(vn[1:-1, 0] - vn[0:-2, 0]) -
dt / (2 * rho * dy) *
(p[2:, 0] - p[0:-2, 0]) +
nu * (dt / dx ** 2 *
(vn[1:-1, 1] - 2 * vn[1:-1, 0] + vn[1:-1, -1]) +
dt / dy ** 2 *
(vn[2:, 0] - 2 * vn[1:-1, 0] + vn[0:-2, 0])))
# Wall BC: u,v = 0 @ y = 0,2
u[0, :] = 0
u[-1, :] = 0
v[0, :] = 0
v[-1, :] = 0
udiff = (numpy.sum(u) - numpy.sum(un)) / numpy.sum(u)
stepcount += 1
fig = pyplot.figure(figsize=(11, 7), dpi=100)
pyplot.quiver(X[::3, ::3], Y[::3, ::3], u[::3, ::3], v[::3, ::3]);
pyplot.show();
fig = pyplot.figure(figsize=(11, 7), dpi=100)
pyplot.quiver(X, Y, u, v);
pyplot.show();