When do objects take damage from being inside the Whirlpool option of the Control Water spell?

Question

The control water spell allows you to create a whirlpool which includes the following in its description:

[...] The first time each turn that an object enters the vortex, the object takes 2d8 bludgeoning damage; this damage occurs each round it remains in the vortex.

I understand that an object takes damage on any turn that it enters the vortex, even if it does so multiple times in a single round. But when exactly is "each round"; when does the re-occurring damage for remaining in the vortex actually happen; when exactly does an object inside of a whirlpool take the 2d8 bludgeoning damage?

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Note, that if they instead had said "each turn" then these objects would be taking damage multiple times per round. I hesitate a guess that some of the unusual wording here comes from the fact that objects don't even have turns of their own so they couldn't have used the wording they usually do for creatures.

Answer 1

The relationship between rounds and turns covers this

A round is the six second period during which "all things" happen during combat.
A turn is that place in the initiative order that a given creature takes actions, bonus actions, etc. The initiative order remains the same from round to round.¹

The Order of Combat
The game organizes the chaos of combat into a cycle of rounds and turns. A round represents about 6 seconds in the game world.
During a round, each participant in a battle takes a turn. The order of turns is determined at the beginning of a combat encounter, when everyone rolls initiative. Once everyone has taken a turn, the fight continues to the next round if neither side has defeated the other. (Basic Rules, p. 71)

Objects aren't covered, as you note, in the turns/rounds rule section. That means that to be consistent with the rhythm of the round/turn cycle, it will be useful to match the object to someone's turn in the first round where damage occurs. Do the rules specify this? No. Nothing specifies this.

Whirlpool. {Objects only}
You cause a whirlpool to form in the center of the area. The whirlpool forms a vortex that is 5 feet wide at the base, up to 50 feet wide at the top, and 25 feet tall. Any creature or object in the water and within 25 feet of the vortex is pulled 10 feet toward it. {snip}

The first time each turn that an object enters the vortex, the object takes 2d8 bludgeoning damage; this damage occurs each round it remains in the vortex.

There are two ways that an object will take the bludgeoning damage by being within 25 feet of the vortex

1. On the caster's turn.
   This is identified in the spell description as an if/then condition of where the object is when the vortex begins. If the caster's turn is when the object first takes damage, use that initiative point on subsequent rounds.
2. On the turn of any creature's who moves that object within range.
   This can include the caster's influence on the object during a different round than the initiating round. When this happens, use that initiative count on subsequent rounds.

How to make it work

The first time on 'whomever's' turn that the object takes 2d8 bludgeoning damage is the initiative ordered turn in that round that the object then follows until the whirlpool has destroyed it or the whirlpool ends.

Each round thereafter, apply that damage on the same turn - the same spot in the initiative order in the subsequent round(s).

Why do this? KISS principle ...

... and using the mechanics already present for running combat using rounds and turns. Also, it then does what the spell specifies by accounting for the rounds and turns.

Can you do it another time? If you want to do it differently as a DM, go ahead, but you may create confusion or additional complications if you do that. There may be an occasion where narrative necessity calls for the DM to do that damage to an object at some other point in a round, but that's going to be a situational call by a given DM.

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¹ Initiative

Initiative determines the order of turns during combat. When combat starts, every participant makes a Dexterity check to determine their place in the initiative order.

The DM ranks the combatants in order from the one with the highest Dexterity check total to the one with the lowest. This is the order (called the initiative order) in which they act during each round. The initiative order remains the same from round to round.

Answer 2

It's unclear

The game uses the word 'round' in several ways, two of which have bearing on this question. It is used to measure time-- the time between one initiative count and that same count in the next round is '1 round'. It is used to denote the entity within which turns and initiative counts happen-- 'the next round' uses the term this way.

If we take the first definition, the object takes damage each round (unit of time) it remains in the vortex. This means it is important to note the initiative count on which objects enter or leave, because that is relevant to measuring the total time in that sort of round. The damage in this case is applied at each completed round, which will typically but not always be the initiative count they enter on-- in this version leaving and re-entering the vortex doesn't reset the timer; an object which enters the vortex on initiative count 7, leaves on count 3, and re-enters on count 1 would then take damage on count 5 of the next round. Note that there are infinitely many counts within each round, so if an object is out of the vortex at the end of a second-definition round no finite amount of initiative counts will make up the difference. Note also that math with infinities is weird, and so there is a case to be made that missing any finite number of initiative counts does not reduce the time spent in the vortex to less than 1 round, and so the only thing that matters is whether an object is in the vortex at the end of a second-definition round, at which point it takes damage at on an indeterminable initiative count. You don't have to do math that way, though, and I recommend you just don't.

If you are using the second-definition 'round', then the object takes damage each entire 'round' entity it spends within the whirlpool. It takes the damage not on any initiative count but rather outside of rounds, triggered by the ending of a qualifying round. In this interpretation, entering on initiative count 25006 and remaining until count 2 of the following round deals no damage (except via entry), even though more than 1 round of first-definition round time has elapsed, because no round was spent entirely within the whirlpool.

The first definition has the advantage that it doesn't broach the illusion that rounds are an abstraction rather than something real within the game world. The second definition has the advantage that the spell is guaranteed not to do more than one instance of damage each second-definition round.

Answer 3

Each round is when everybody has had their turn

As you say, objects don’t have turns so it must have been put in the whirlpool and this must have happened on somebody’s turn (since it’s always somebody’s turn). So the round is complete when that person’s turn is completed again.