mjhjmtu

I am trying to draw a cylindrical tank that has water in it, using tikz. Here is what I have so far:

documentclass[a4paper, 12pt]article
 usepackagepgfplots
 pgfplotssetcompat=1.15
 usepgfplotslibrarypolar
 usepgflibraryshapes.geometric
 usetikzlibrarycalc

begindocument

beginfigure[ht]

centering

begintikzpicture

node[draw, cylinder, rotate=90, shape aspect=4, minimum height=4cm, minimum 
width=8cm, fill = blue, fill opacity = 0.2, blue] ;

node[draw, cylinder, rotate=90, shape aspect=4, minimum height=6cm, minimum 
width=8cm, thick] (A) ;

draw[dashed, thick]
 let p1 = ($ (A.after bottom) - (A.before bottom) $),
 n1 = 0.5*veclen(x1,y1)-pgflinewidth,
 p2 = ($ (A.bottom) - (A.after bottom)!.5!(A.before bottom) $),
 n2 = veclen(x2,y2)-pgflinewidth
 in
 ([xshift=-pgflinewidth] A.before bottom) arc [start angle=0, end 
angle=180,
 x radius=n1, y radius=n2];

endtikzpicture

captionThe tank.
labelfig:tank

endfigure

enddocument

This is the result:

I want the blue part to be shifted down so that it is at the same level as the bottom of the black one.

Any suggestions to this solution, or to making my code more efficient, would be great!

you need to define anchors of your cylinders at their bottoms ...

documentclass[a4paper, 12pt]article
 %usepackagepgfplots
 %pgfplotssetcompat=1.15
 %usepgfplotslibrarypolar
 usepackagetikz
 usetikzlibrarycalc,
 positioning, 
 shapes.geometric

begindocument
 beginfigure[ht]
 centering
begintikzpicture[
 node distance = 0pt,
valj/.style args = #1/#2draw, cylinder, shape aspect=4, shape border rotate=90,
 fill=#2, fill opacity=0.2,
 minimum height=#1, minimum width=8cm,
 outer sep=0pt, anchor=bottom, % <--- added
 node contents=
 
 ]
node (A) [valj=60mm/white];
node (B) [valj=40mm/blue,
 above=of A.bottom]; % <--- positioning of cylinder
draw[dashed]
 let p1 = ($ (A.after bottom) - (A.before bottom) $),
 n1 = 0.5*veclen(x1,y1)-pgflinewidth,
 p2 = ($ (A.bottom) - (A.after bottom)!.5!(A.before bottom) $),
 n2 = veclen(x2,y2)
 in (A.before bottom) arc [start angle=0, end angle=180,
 x radius=n1, y radius=n2];
endtikzpicture
 captionThe tank.
labelfig:tank
 endfigure
enddocument

I would use tikz-3dplot for it, in particular if you are considering adding more 3d-like elements. The following has 4 parameters, one view angle which is set to 100, a radius R, the height of the water HW and the height of the cylinder HC.

documentclass[tikz,border=3.14mm]standalone
usepackagetikz-3dplot
usepgflibraryshapes.geometric
usetikzlibrarycalc
begindocument
tdplotsetmaincoords1000
begintikzpicture[tdplot_main_coords]
pgfmathsetmacroR4
pgfmathsetmacroHW4
pgfmathsetmacroHC6
% water
fill[blue!40] plot[variable=x,domain=0:180,smooth] (R*cos(x),R*sin(x),0)
-- 
plot[variable=x,domain=180:360,smooth] (R*cos(x),R*sin(x),HW)
-- cycle;
draw[blue] plot[variable=x,domain=0:360,smooth,samples=51]
(R*cos(x),R*sin(x),HW);
% "invisible" lined
draw[dashed] plot[variable=x,domain=180:360,smooth]
(R*cos(x),R*sin(x),0);
% visible cylinder lines
draw plot[variable=x,domain=0:180,smooth]
(R*cos(x),R*sin(x),0) 
-- 
plot[variable=x,domain=180:360,smooth]
(R*cos(x),R*sin(x),HC) -- cycle;
draw plot[variable=x,domain=0:180,smooth]
(R*cos(x),R*sin(x),HC); 
endtikzpicture
enddocument

The meaning of the view angle is illustrated by this animation.

documentclass[tikz,border=3.14mm]standalone
usepackagetikz-3dplot
usepgflibraryshapes.geometric
usetikzlibrarycalc
begindocument
foreach X in 0,10,...,350
tdplotsetmaincoords120+30*sin(X)0
begintikzpicture
pgfmathsetmacroR4
pgfmathsetmacroHW4
pgfmathsetmacroHC6
path[use as bounding box] (-1.1*R,-0.75*HC) rectangle (1.1*R,1.25*HC);
beginscope[tdplot_main_coords]
% water
fill[blue!40] plot[variable=x,domain=0:180,smooth] (R*cos(x),R*sin(x),0)
-- 
plot[variable=x,domain=180:360,smooth] (R*cos(x),R*sin(x),HW)
-- cycle;
draw[blue] plot[variable=x,domain=0:360,smooth,samples=51]
(R*cos(x),R*sin(x),HW);
% "invisible" lined
draw[dashed] plot[variable=x,domain=180:360,smooth]
(R*cos(x),R*sin(x),0);
% visible cylinder lines
draw plot[variable=x,domain=0:180,smooth]
(R*cos(x),R*sin(x),0) 
-- 
plot[variable=x,domain=180:360,smooth]
(R*cos(x),R*sin(x),HC) -- cycle;
draw plot[variable=x,domain=0:180,smooth]
(R*cos(x),R*sin(x),HC); 
endscope
endtikzpicture
enddocument

I have figured out a solution. In the square brackets containing the parameters for the first cylinder (the one drawing the water), we need only add xshift = -0.75cm. Since the height of the outer cylinder is 6cm and the height of the inner is 4.5cm, so the difference in height is 1.5cm. Since both cylinders are centered at the same spot, the height of the gaps above and below the blue are 0.75cm each, so we just have to shift it down that distance. I was at first curious as to yshift did not work, but then I realized that the cylinder is rotated in the beginning by 90 degrees, so that is probably the reason.