No Flow Portal is a strange effect where a portal immersed in whatever medium — a liquid, like a body of water, or a different kind of atmosphere — or exposed to conditions which would otherwise affect the other side of the portal, for some reason doesn't let it through even when the portal is working. The Hero can swim into the portal on one side and walk out of it the other side, yet the body of water he swam in remains as stolidly fixed as if it were up against glass. The hero could even cause an explosion next to a portal, and the people on the other side won't even feel a breeze.
This trope isn't restricted to water, though water is one of the easiest ways to show it in action in fiction. Different atmospheric conditions can be strangely shy about crossing the portal barrier. This is obvious enough with two completely different atmospheres, but for a few more observant viewers, this trope may manifest itself in a case where the atmospheres are the same but one is blowing across or even into the portal on one side and having no effect on the other side. What you'd expect to happen in the former is that the air would get sucked through the portal as the wind creates an area of lower pressure in the other world. Even sand may fall victim to this trope - Sand Is Water, for a given definition of 'water' - though in this case the portal is more likely to be just partly submerged in the sand rather than completely smothered by it.
The opposite of this trope, naturally, would be where the conditions do affect things on the other side of the portal. A portal which lead straight to a Lethal Lava Land, for instance, would be pretty toasty on the other side if Convection Schmonvection was done away with. If combined with Time Travel, this trope's aversion is usually closely allied with San Dimas Time.
Sometimes, an inexplicably fierce whirlwind sucks in everything on one side of the portal. This is quite common in fantasy works where the portal in question leads to somewhere nefarious and supernatural, though it'll always be less about thermal currents and fluid dynamics and more about looking impressive - how else to make a portal look foreboding and scary?
For those of us who like to dwell on this sort of thing, it often leads to a Fridge Logic moment when you wonder why biological matter (which itself contains a lot of fluids, including water) can pass through but a body of water can't. MS T3k Mantra is usually enough to dismiss it.
- In Watchmen, a superheroine has her mind wiped and is trapped in a marriage with an abusive and worthless husband. The friend who rescues her to restore her memory delivers an ultimatum to the "husband" who is in on the deception: walk through that door and disappear, and you'll never hear from us again. He walks, only to find, very briefly, he is at the bottom of the Pacific Ocean being crushed by thirty thousand feet of water. The transition between states is not detailed, nor is the mechanism that prevents six square miles of water under serious pressure suddenly gushing into a bedroom in the USA.
- Dungeon Keeper Ami: Depends on the type of portal:
- Jadeite's portals, don't prevent things on the other side from going through, as shown with the first one seen, where a jet of lava rushes out.
- Underworld Portal installations, don't cause atmosphere exchange between the locations they connect, shown when one connects to a very hot location, from a cool underground dungeon, and the temperature of the destination is only noticed when the portal is exited.
- Justified and ZigZagged in Pacific Rim. It's repeated throughout the movie that humans tried to close the Breach with nuclear weapons but they never worked. This is because the Breach only opens when it recognizes Kaiju DNA. Of course, this doesn't stop other things (radio signals, the Kaiju Hive Mind, and water of the Pacific Ocean) to flow through the portal at different times in the movie.
- The Binary Chute that leads Supergirl to Earth opens up at the bottom of Lake Michigan. If it wasn't to a No Flow Portal, Argo City would be flooded.
- Averted in Goblin Slayer, where at one point the protagonist opens a portal to the bottom of the ocean in order to crush a powerful enemy with the resulting blast of water.
- Averted in David Weber and Linda Evans Hell's Gate series, if only in passing. At least one portal connects two points that are very disparate in terms of altitude. While the situation has mostly stabilized by the time of the story, the initial gale scoured the area down to the bedrock and the vegetation has yet to recover. There is also still a constant wind blowing from both sides of the portal.
- In Spin, the enormous portal connecting the Indian Ocean to a distant alien planet not only doesn't let water pass through, it doesn't let anything pass through unless it contains sentient passengers. A manned boat will pass through; the same boat floating by itself won't. Justified in that the portal is to some extent, a sentient, or at least reasoning entity, and can thus decide what goes through and what doesn't.
- In Terry Goodkind's Sword of Truth series, there is a sentient magical portal called The Sliph. It requires the passenger to have both Additive and Subtractive magic. Also justified in that the Silph was constructed with a person as the base and is actually alive.
- In Philip José Farmer's World of Tiers series, portals are used as a narrative device for characters to transit between Tiers. The hero crosses through a ring portal only to find himself thirty thousand feet up and falling fast. As he began at ground level, the books are silent as to how the atmospheric pressure and temperature differentials are dealt with, so that the difference between the two does not cause a hurricane-force wind between the two levels connected by the portal.
- Averted in Reflex. The "jumping" teleportation ability that Davy uses actually functions by opening a "Davy-shaped doorway" that moves around him. He develops his jumping ability to the point that he can rapidly teleport between two points and essentially be in two places at once, which keeps the portal open. Davy uses this in the climax by 'twinning' to the ocean by a nearby beach, flooding the bad guys' facility.
- One episode of Angel has the titular character black-mailed into entering a Hell Dimension to save someone. He gears up before entering only for all the weapons to drop to the floor when he vanishes.
- In the first series of Primeval, neatly averted: a portal which opened in the Cretaceous sea allowed water to flow into the present and flood a basement, resulting in the Hesperornis getting through.
- Stargate SG-1 and Stargate Atlantis, over and over again. Having one end of a Stargate immersed in water doesn't result in it flooding the other end (or having said water disintegrated if it's the destination end), nor does it suck the air out of the room if the gate is opened into the vacuum of space. Somehow, the gate can differentiate between a specific object entering it and ambient pressure, only allowing the former through. Averted in the case of gravitational forces in "A Matter of Time", when the effects of the black hole cross through the wormhole. Also averted with air in one case, where a stargate is buried by a meteorite strike, so they open the wormhole, so that the resulting "kawoosh" carves out a small cave on the other side by disintegrating the rock in front of it (well, above, given that the gate is facing up). Then Teal'c goes through with a breathing mask in order to dig to the surface, while the gate stays open the maximum 38 minutes in order to provide enough air for him to breathe (at the same time, Teal'c has to avoid falling back into the open gate, as it will disintegrate him).
- This is generally played straight for portals in Dungeons & Dragons (A Wizard Did It, often literally), though Forgotten Realms plays with it by featuring portals that only lets the atmospheric conditions/water, etc. through, and blocks creatures and constructs from going through. Don't want air elementals to disturb that fresh air you're bringing in from the Elemental Plane of Air, after all.
- Played straight with the Pandora gates in Eclipse Phase. As they are sufficiently advanced, they use some sort of force field to keep the atmospheres on opposite sides of the portal from interacting.
- In Crash Bandicoot 3: Warped, Crash can take a Time Twister portal from the Warp Zone straight to the underwater levels and no water escapes from there to the present.
- Portal 2:
- Averted in the ending: A portal on the moon causes Wheatley, the space core, and various junk to be sucked out into space.
- Played straight however, with the fire pit at the end of Test Chamber 19 in the first game. If you put a portal as low as possible, and look through from the other end, the heat won't hurt you, even if you're right next to the other portal. Also, if you activate a cheat in the console that allows you to place a portal on any surface, it's possible to have one portal be half-submerged in Grimy Water and have it not come through.
- Sound also has issues traveling through Portals, though this is difficult to experience in game due to the size of the test chambers and general lack of objects that make noise, though in both games there are test chambers with radios large enough to experience this effect with.
- In Spyro 2: Ripto's Rage!, the one side of a portal from Aquaria Towers to Summer Forest is underwater and yet the Ghibli Hills level isn't flooded at all.
- In Spyro: Year of the Dragon, the portals to a chinese fireworks factory, an abandoned ghost ship and a Slippy-Slidey Ice World are all accessed only by underwater lake, yet the respective worlds are still dry and the lake hasn't emptied its water through them at all.
- Averted in this Awkward Zombie comic.
- Discussed in Concerned where two combine soldiers mention how their dead buddy used to say that they could place a teleporter at the bottom of the ocean and bring it with them off-world. Even though he did the math, they still didn't believe him. This is a reference to how the Combine in Half-Life did exactly that to drain the Earth's water for their other planets.
- Breach from Generator Rex can create several portals to basically everywhere. One episode has her teleporting a probe to the bottom of the Atlantic Ocean without flooding the room she's in with water.
- Invoked as a limitation of Rick's portal gun in Rick and Morty when the portals are unable to transport the telepathic cloud entity known as Fart because "he's gaseous" and apparently unable to pass through them. In the same scene that this is established, Rick uses a portal to flood the room with water, implying that only gasses are affected.
- In Transformers: Prime the Autobots space bridge can open into space without sucking the air from the base into a vacuum.