Editing v0.31:Flow

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{{quality|Masterwork|02:44, 08 November 2010 (UTC)}}
Reading from the raw files you will find that all water and magma/lava in the game are called flows. This however adds a certain amount of confusion since when you trying to [[power]] a [[water wheel]] you also need to know if your flow of water is flowing. For the purpose of clarity water and magma will instead be referred to as '''fluids''', and '''flow''' will be saved for a fluid that is in motion. 

'''Flow''' is a game mechanic used to simulate the motion of '''fluids'''. The two fluids that exist in dwarf fortress currently are '''[[water]]''' and '''[[magma]]'''. You can identify fluids that are flowing by looking for a tile that is blinking between {{Tile|≈|1:0:1}} and {{Tile|~|1:0:1}} tiles. If you have turned on SHOW_FLOW_AMOUNTS you will see the fluid '''depth''' instead and will not be able to easily tell if the game considers a tile to be flowing or not. Flow is typically present any time a fluid is in motion, but there are some exceptions which confuse things a bit.
*Note: In the current release flow does not seem to appear in magma. Magma follows the same rules of fluid motion and flow, it simply doesn't have flow in the sense that would allow it to power a water wheel.

==Basic Fluid Motion==
'''[[Water]]''' and '''[[magma]]''' both move in much the same way following a fairly simple set of rules. The only difference between the motion of [[magma]] and water is that magma behaves differently with regards to [[pressure]].

Fluids move mostly as one might expect: they will fall straight down if they can, or else they will spread out to the sides. Fluids can flow diagonally on the same z-level, but will never move sideways and down at the same time. Under basic fluid motion, fluid never moves back up, but it can appear to do so if pressure is involved.

Here is a quick example of how fluids can move to adjacent tiles. Also as water moves to an adjacent tile flow is generated in both tiles. This flow will remain for a short time before reverting back to being non-flowing water. In the first example of falling water all of the water is removed from the source tile, so flow only appears below where the water has moved to. In the other two examples flow will appear in both tiles. 

                        '''Before (side view)'''
                        ▒7▒       ▒7▒       ▒
                        ▒ ▒       ▒2▒       ▒7
                        ▒▒▒       ▒▒▒       ▒▒▒▒▒

                        '''After (side view)'''  
                        ▒ ▒       ▒2▒       ▒
                        ▒7▒       ▒7▒       ▒43
                        ▒▒▒       ▒▒▒       ▒▒▒▒▒

*1. Fluids move down 
*2. Fluids spread out to the sides

These rules are incomplete, however, without consideration of [[pressure]].

==Fluids under pressure aka. Teleportation==

Magma, which has no natural pressure, flows according to the rules of basic fluid motion. Water, however, can move by pressure when it falls down on top of full 7/7 water. In addition, pumps create pressure in both water and magma, and water entering the map at from a stream or river follows pressure as well.

Fluids moving under '''[[pressure]]''' do not just move to adjacent tiles, they also trace a path through other full tiles of fluid trying to move to more distant tiles. Fluids moving under [[pressure]] can effectively teleport through other tiles that are already filled with fluid. When teleporting, fluids still only generate flow in their source and destination tiles, NOT in any tiles they skipped over to get to their destination. Also fluids that are teleporting will not push objects in tiles they skip over.

                ▒7▒ ▒      ▒~▒ ▒       
   '''Before'''       ▒7▒ ▒      ▒~▒ ▒
                ▒777▒      ▒<nowiki>~~~</nowiki>▒                
                ▒▒▒▒▒      ▒▒▒▒▒       

                ▒ ▒ ▒      ▒ ▒ ▒
   '''After'''        ▒7▒7▒      ▒~▒≈▒       ≈   flowing fluid
                ▒777▒      ▒<nowiki>~~~</nowiki>▒       ~   non-flowing fluid
                ▒▒▒▒▒      ▒▒▒▒▒

This is a simple u-bend example of how water moves under pressure. However with regards to flow, it does something strange. Notice that flow only occurs in the destination tile here (since the source tile has been removed). The fluids in the bottom of the u-bend are unaffected, and furthermore any objects that happen to be here are not pushed either.

When a fluid tries to move by pressure, it tries to trace a path through full 7/7 fluids going down, and horizontally, but ''not'' diagonally.  In this way it is like basic flow, except that pressure works faster; fluid from the source is teleported to the open space at the end, rather than having to wait for open space to open up at the source via normal flow. This is why, for example, diagonal squeezes in channels can make water flow slower, and why rivers and streams on the map are usually full of 7/7 water until close to the edge of the map where the rules of basic fluid motion are draining the water off the map while pressure teleports new water from the source all the way down to the end.

What's more, unlike basic flow, the path pressure traces can even go back up--but never higher(*) than the z-level of the first 7/7 tile on the path it was tracing. So it may appear that pressure 'pushes fluids up', but in fact it's only teleporting fluid to a level even or lower.

Thus the result is that pressure movement of fluids (especially water) is common and doesn't create very much flow. However rivers and streams still seem to have some kind of flow that powers water wheels, called natural flow.

(*) There's one way to push a fluid higher than its starting level, but it might be considered a bug on the flow mechanics and probably will be changed in following versions since allows for what could be considered exploits ''(see '''[[magma piston]])'''''.
Water falling down on top of full 7/7 water isn't the only thing that will teleport water, a ''natural'' wall of any material{{verify}} '''[[cave-in|falling]]''' onto both water ''or'' '''[[magma]]''' will teleport each tile of displaced fluid to open space directly above it:
        Start                             Step 1                            Step 2                Legend:
 ▒ ▒ ▒        ▒ ▒ ▒                ▒ ▒ ▒        ▒ ▒ ▒                ▒ ▒ ▒        ▒ ▒ ▒            |   support            
 ▒ | ▒        ▒ | ▒     Collapse   ▒   ▒        ▒   ▒      Fluid     ▒   ▒        ▒   ▒
 ▒ ▒ ▒        ▒ ▒ ▒   -----------> ▒ 7 ▒        ▒ ≈ ▒   -----------> ▒232▒        ▒<nowiki>~~~</nowiki>▒            ≈   flowing fluid
 ▒▒7▒▒        ▒▒~▒▒     Support    ▒▒▒▒▒        ▒▒▒▒▒     Spreads    ▒▒▒▒▒        ▒▒▒▒▒            ~   non-flowing fluid
 ▒▒▒▒▒        ▒▒▒▒▒                ▒▒▒▒▒        ▒▒▒▒▒                ▒▒▒▒▒        ▒▒▒▒▒
when done in an u-bend example the pushing above original fluid level can be easily appreciated, although it breaks the laws of regular fluid physics:
        Start                             Step 1                            Step 2     
 ▒ ▒ ▒ ▒      ▒ ▒ ▒ ▒              ▒ ▒ ▒ ▒      ▒ ▒ ▒ ▒              ▒ ▒ ▒ ▒      ▒ ▒ ▒ ▒
 ▒ | ▒ ▒      ▒ | ▒ ▒              ▒   ▒ ▒      ▒   ▒ ▒              ▒   ▒ ▒      ▒   ▒ ▒
 ▒ ▒ ▒ ▒      ▒ ▒ ▒ ▒   Collapse   ▒ 7 ▒ ▒      ▒ ≈ ▒ ▒    Fluid     ▒   ▒ ▒      ▒   ▒ ▒
 ▒ ▒ ▒ ▒      ▒ ▒ ▒ ▒ -----------> ▒ 7 ▒ ▒      ▒ ≈ ▒ ▒ -----------> ▒464▒ ▒      ▒<nowiki>~~~</nowiki>▒ ▒
 ▒▒7▒▒7▒      ▒▒~▒▒~▒   Support    ▒▒▒▒▒7▒      ▒▒▒▒▒~▒   Spreads    ▒▒▒▒▒7▒      ▒▒▒▒▒~▒
 ▒▒7777▒      ▒▒<nowiki>~~~~</nowiki>▒              ▒▒▒777▒      ▒▒▒<nowiki>~~~</nowiki>▒              ▒▒▒777▒      ▒▒▒<nowiki>~~~</nowiki>▒
 ▒▒▒▒▒▒▒      ▒▒▒▒▒▒▒              ▒▒▒▒▒▒▒      ▒▒▒▒▒▒▒              ▒▒▒▒▒▒▒      ▒▒▒▒▒▒▒
this is the basic principle that the '''[[magma piston]]''' exploit, if you want to prevent a future fix or simply want to simulate regular physics fluid behaviour, you can do something like this:
 z-level   Start                  Step 1                 Step 2
   z+0   ▒ ▒   ▒ ▒              ▒ ▒   ▒ ▒              ▒ ▒   ▒ ▒
   z-1   ▒ |   ▒ ▒              ▒     ▒ ▒              ▒     ▒ ▒
   z-2   ▒ ▒ ▒ ▒ ▒   Collapse   ▒ 7 7 ▒ ▒    Fluid     ▒     ▒ ▒
   z-3   ▒ ▒ ▒ ▒?▒ -----------> ▒ 7 7 ▒?▒ -----------> ▒44544▒?▒
   z-4   ▒▒777▒▒7▒   Support    ▒▒▒7▒▒▒7▒   Spreads    ▒▒▒7▒▒▒7▒
   z-5   ▒▒777777▒              ▒▒▒7▒777▒              ▒▒▒7▒777▒
   z-6   ▒▒▒▒▒▒▒▒▒              ▒▒▒▒▒▒▒▒▒              ▒▒▒▒▒▒▒▒▒

       z-3 Top View           z-4 Top View           z-3 Top View (Step 1) z-3 Top View (Step 2)
         ▒▒▒▒▒▒▒                ▒▒▒▒▒▒▒                ▒▒▒▒▒▒▒                 ▒▒▒▒▒▒▒
         ▒     ▒                ▒▒▒▒▒▒▒                ▒     ▒                 ▒44444▒
         ▒ ▒▒▒ ▒▒▒              ▒▒777▒▒▒▒              ▒ 777 ▒▒▒               ▒45554▒▒▒
  Axis   ▒ ▒ ▒ ▒?▒              ▒▒777▒▒7▒              ▒ 7 7 ▒?▒               ▒55555▒?▒   Axis   
 --------▒ ▒▒▒ ▒▒▒--------------▒▒777▒▒▒▒--------------▒ 777 ▒▒▒---------------▒45554▒▒▒--------
  Cut    ▒     ▒                ▒▒▒▒▒▒▒                ▒     ▒                 ▒44544▒      Cut    
         ▒▒▒▒▒▒▒                ▒▒▒▒▒▒▒                ▒▒▒▒▒▒▒                 ▒▒▒▒▒▒▒
basically it involves dropping a giant ''(minimum size 3x3x1)'' donut/cylinder/tube of natural walls onto the fluid pool, given that it will teleport a donut/cylinder/tube of fluid in the same way ''(step 1)'' but after it spreads ''(step 2)'' it'll seem that the fluid actually went through the opening in the middle of the donut/cylinder/tube like a real fluid should behave, of course one exception it won't be pushed through the right tube like it should, you'll probably want to close the tile marked with a question mark "?" so it can give the impression of real fluid mechanics.

==Natural Flow==
Many water sources such as [[river]]s and [[brook]]s are constantly flowing with '''natural flow'''. This is different from other flow effects in that it is always considered to be flowing water. This remains true even when the water flows into a complete dead end channel or even when blocked off with a floodgate. Any channels that link up to a naturally flowing source will soon become naturally flowing water as long as they remain on the same z-level. Diagonal steps have no effect on natural flow although they can be used to change [[pressure]].

Trying to move natural flow up or down to a different z-level may have unpredictable results but in most cases this will break the natural flow effect resulting in still water that can only be made to flow by artificial means.

==Fluid Depth==
Fluids can have a depth anywhere from 1 to 7. To see the depth of a tile of fluid you can look at it with {{k|k}} which will reveal the depth in the text at the right. Alternatively you can enable [[Technical_tricks#Tiles|SHOW_FLOW_AMOUNTS]] which will replace the {{Tile|≈|1:0:1}} and {{Tile|~|1:0:1}} tiles with a numerical representation of the depth at all times. Turning on SHOW_FLOW_AMOUNTS does come with the drawback that you will no longer be able to see if a tile is flowing or not.

==Obstructions==
Water can be stopped by most solid tiles. These include [[wall]]s and [[building]]s as well as closed [[floodgate]]s, [[door]]s, and [[hatch]]es. Exceptions are vertical [[grate]]s, vertical [[bars]], and [[fortification]]s, which will allow fluids to pass freely. 

==Evaporation==
Fluids that remain at a depth of 1/7 for long enough will evaporate. Evaporated fluids are simply removed from the game. In '''hot''' or '''scorching''' environments, [[murky pool]]s can evaporate at greater depths.

==Bugs==
*Flows may cause items to disappear {{Bug|895}}
*Smears and blood will multiply when in the presence of flows {{Bug|296}}

{{Category|Physics}}