Sample Problems > Applications > Fluids

{ CHANNEL.PDE

This example is a modification of the LOWVISC.PDE problem, in which the

no-slip boundary has been placed at the bottom of the domain, with free flow

at the top.

The declared parameters in this problem are chosen for demonstration purposes,

and are not intended to represent any real conditions. The fluid is far more

viscous than water.

}

title 'Flow in 2D channel'

select errlim = 0.005

variables

u(0.1)

v(0.01)

p(1)

definitions

Lx = 5 Ly = 1.5

p0 = 1 { input pressure }

speed2 = u^2+v^2

speed = sqrt(speed2)

dens = 1

visc = 0.04

vxx = -(p0/(2*visc*(2*Lx)))*y^2 { open-channel x-velocity with drag at the bottom }

rball=0.4

cut = 0.1 { value for bevel at the corners of the obstruction }

penalty = 100*visc/rball^2

Re = globalmax(speed)*(Ly/2)/(visc/dens)

initial values

{ In nonlinear problems, Newton's method requires a good initial guess at the solution,

or convergence may not be achieved. You can use SELECT CHANGELIM=0.1 to

force the solver to creep toward a solution from a bad guess.

In our problem, the open channel velocity is a good place to start. }

u = vxx v = 0 p = p0*x/(2*Lx)

equations

u: visc*div(grad(u)) - dx(p) = dens*(u*dx(u) + v*dy(u))

v: visc*div(grad(v)) - dy(p) = dens*(u*dx(v) + v*dy(v))

p: div(grad(p)) = penalty*(dx(u)+dy(v))

boundaries

region 1

start(-Lx,0)

value(u) = 0 value(v) = 0 load(p) = 0

line to (Lx/2-rball,0)

to (Lx/2-rball,rball) bevel(cut)

to (Lx/2+rball,rball) bevel(cut)

to (Lx/2+rball,0)

to (Lx,0)

load(u) = 0 value(v) = 0 value(p) = p0

mesh_spacing=Ly/20

line to (Lx,Ly)

mesh_spacing=100

load(p) = 0

line to (-Lx,Ly)

value(p) = 0

line to close

monitors

contour(speed) report(Re)

plots

contour(u) report(Re)

contour(v) report(Re)

contour(speed) painted report(Re)

vector(u,v) as "flow" report(Re)

contour(p) as "Pressure" painted

contour(dx(u)+dy(v)) as "Continuity Error"

elevation(u) from (-Lx,0) to (-Lx,Ly)

elevation(u) from (0,0) to (0,Ly)

elevation(u) from (Lx/2,0)to (Lx/2,Ly)

elevation(u) from (Lx,0) to (Lx,Ly)

end