Sample Problems > Usage > 3D_domains > 3d_helix

{ 3D_HELIX_WRAPPED.PDE

This problem shows the use of the function definition facility of FlexPDE to

create a helix of square cross-section in 3D.

The mesh generation facility of FlexPDE extrudes a 2D figure along a straight

path in Z, so that it is not possible to directly define a helical shape.

However, by defining a coordinate transformation, we can build a straight rod

in 3D and interpret the coordinates in a rotating frame.

Define the twisting coordinates by the transformation

xt = x*cos(y/R);

yt = x*sin(y/R);

zt = z

In this transformation, x and y are the coordinates FlexPDE believes it is working

with, and they are the coordinates that move with the twisting.

xt and yt are the "lab coordinates" of the twisted figure.

The chain rule gives

dF/d(xt) = (dx/dxt)*(dF/dx) + (dy/dxt)*(dF/dy) + (dz/dxt)*(dF/dz)

with similar rules for yt and zt.

Some tedious algebra gives

dx/dxt = cos(y/R) dy/dxt = -(R/x)*sin(y/R) dz/dxt = 0

dx/dyt = sin(y/R) dy/dyt = (R/x)*cos(y/R) dz/dyt = 0

dx/dzt = dy/dzt = 0 dz/dzt = 1

These relations are defined in the definitions section, and used in the equations

section, perhaps nested as in the heat equation shown here.

Notice that this formulation produces the upward motion by tilting the bar in

the un-twisted space and wrapping the resulting figure around a cylinder.

We have added a cylindrical mounting pad at each end of the helix.

See "3d_helix_layered.pde" for a different approach to constructing a helix.

See "Usage/CAD_Import/helix_OBJimport.pde" for how to import a helix from an OBJ file.

}

title '3D Helix - transformation with no shear'

coordinates

cartesian3

select

ngrid=160 { generate enough mesh cells to resolve the twist }

variables

definitions

zlong = 60

turns = 4

pitch = zlong/turns { z rise per turn }

xwide = 4.5

zhigh = 4.5

Rc = 22 - xwide/2 { center radius }

alpha = y/Rc

zstub = 5*zhigh { rod pieces at each end }

sturn = Rc*2*pi { arc length per turn }

yolap = pi*Rc*zhigh/pitch

slong = turns*sturn { arc length of spring }

stot = slong + 2*sturn { add one turn at each end for rod }

xin = Rc-xwide/2

xout = Rc+xwide/2

xbore = Rc/2

{ transformations }

rise = pitch/(2*pi) { z-rise per radian }

c = cos(alpha)

s = sin(alpha)

xt = x*c

yt = x*s

zt = z-zlong/2

{ functional definition of derivatives }

dxt(f) = c*dx(f) - s*(Rc/x)*dy(f)

dyt(f) = s*dx(f) + c*(Rc/x)*dy(f)

dzt(f) = dz(f)

{ Thermal source }

Q = 10*exp(-(xt-Rc)^2-yt^2-zt^2)

z1 = -zstub

z2 = max( 0, min(zlong, pitch*y/sturn - zhigh/2))

z3 = max(0, min(zlong, pitch*y/sturn + zhigh/2))

z4 = zlong + zstub

initial values

Tp = 0.

equations

{ the heat equation using transformed derivative operators }

Tp: dxt(dxt(Tp)) + dyt(dyt(Tp)) + dzt(dzt(Tp)) + Q = 0

extrusion z = z1, z2, z3, z4

boundaries

Limited Region 1 { the spring }

layer 2

start(xin,yolap)

line to (xout,yolap)

line to (xout, slong-yolap)

line to (xin,slong-yolap)

line to close

Limited Region 2 { top rod overlap with coil }

surface 4 value(Tp)=0 {cold at the end of the rod }

layer 2 layer 3

start(xbore,slong-yolap)

line to (xout,slong-yolap) to (xout,slong+yolap) to (xbore,slong+yolap) to close

Limited Region 3 { top rod free of coil }

surface 4 value(Tp)=0 {cold at the end of the rod }

layer 2 layer 3

start(xbore,slong+yolap)

line to (xout,slong+yolap) to (xout,slong+sturn-yolap) to (xbore,slong+sturn-yolap)

to close

Limited Region 4 { bottom rod overlap with coil }

surface 1 value(Tp)=0 {cold at the end of the rod }

layer 1 layer 2

start(xbore,-yolap)

line to (xout,-yolap) to (xout,yolap) to (xbore,yolap) to close

Limited Region 5 { bottom rod free of coil }

surface 1 value(Tp)=0 {cold at the end of the rod }

layer 1 layer 2

start(xbore,-sturn+yolap)

line to (xout,-sturn+yolap) to (xout,-yolap) to (xbore,-yolap) to close

monitors

grid(xt,yt,zt) paintregions { the twisted shape }

plots

grid(xt,yt,zt) paintregions { the twisted shape again }

{ In the following, recall that x is really radius, and y is really azimuthal distance.

It is not possible at present to construct a cut in the "lab" coordinates. }

grid(x,z) on y=0

contour(Tp) on y=0 as "ZX Temp"

contour(Tp) on z=0 as "XY Temp"

elevation(Tp) from(Rc,0,0) to (Rc,slong,zlong) { centerline of coil }

end

3d_helix_wrapped

3d_helix_wrapped