A use-name (alias) for Arwen?

It's clear from the LOTR text that casual visitors to Rivendell aren't necessarily told of Arwen's existence or introduced to her. Frodo wasn't and even Aragorn, who grew up there, didn't know she existed until he was grown up.

So, if someone was visiting Rivendell, and Arwen wanted to meet them, she might need a use-name or alias to use (just in case the visitors had heard the name of Arwen 'cause she is a noted beauty)

Sindaring for "Daddy's Girl" might be spot-on for meaning but seems pretty unlikely. :)

Any suggestions?

Maybe Aduial as in twilight (evening)?

I don't really follow why Arwen would need some special alias here. Do you mean for her to hide her identity? That doesn't seem likely. Also FWIW the reason Aragorn was unaware of Arwen at first is because Arwen was simply not there at the time.

At any rate, if your intention is to render her Quenya epithet, Undómiel, in Sindarin, then some options to consider would be Eluial, Giluial, and Tinuial, all essentially meaning "Star of Twilight" using different elements for "star" (these suggestions were once made by Röandil on VL).

As for why, she might want to get to know a visitor without them knowing who she is. That way, they will act more naturally than they would if they knew her lineage and position in Rivendell.

Aragorn was in Rivendell and 20 years old. Here is the exact quote from the Tale of Aragorn and Arwen:

"Often it is seen," said Aragorn, "that in dangerous days men hide their chief treasure. Yet I marvel at Elrond and your brothers; for though I have dwelt in this house from childhood, I have heard no word of you. How comes it that we have never met before? Surely your father has not kept you locked in his horde?"

(Emphasis added by me.) This isn't just a matter of her being elsewhere; he did not even know of her existence! And Aragorn was no passing visitor, either.

So, if she is at home, she may need a use-name to meet with strangers. It should not be too close to any poetic name she is known by because sharp-witted folks who know of her existence, and have heard her descriptions, would too easily make the connection.

I find it difficult to believe that Arwen would habitually engage in concealing her identity for such purposes, but there's no need to argue the point :)

Regarding Aragorn, I think his case is more indicative of general Elvish mentality than anything else. Note that for an Elf, 20 years are like ~38 days for a human, in terms of ageing (cf. NoMe, pp. 146-7). Of course comparing ageing to a general day-to-day experience of life is not quite equivalent, but still it may be perfectly normal for Elves to not mention someone for such a brief period of them being away, just as I can imagine a human going "Oh yeah, I forgot to mention, I have a daughter! She went to stay with her grandma a couple months ago..."

All fair points. And yet, for an entire community not to mention her in his presence for 20 years seems a bit more structured than a slip of the mind. . It's also possible that Aragorn, due to some foresight of Elrond's, was viewed as a special threat to his daughter's safety. That might be the main reason.

Remember when Frodo describes the feast in Rivendell? He mentions a pretty woman under a canopy off to the side. He has no idea who she is. That could be because he's a country bumpkin, or it could be for the reasons I describe.

I do think that daddy Elrond is overly protective of Arwen. Possibly more so after his wife is captured by goblins.

And possibly Arwen gets tired of meeting people who are in awe of her station in life AND her beauty. Maybe she would like to meet people as a person instead of as a princess in order to get a better idea of what they are really like. People today who have lots of money have the same problem - is this new acquaintance someone who likes them or likes their money? It's why some of them use stealth wealth.