噫 yi, pronounced with a natural rising tone, with slight falling at the end (like you'd sometimes do when saying "ew"). This is not restricted to "ew" specifically though, just that it covers the situations for "ew", and people do use it frequently.
Pinyin i + pinyin e in sequence (one syllable), often with a rising tone, sometimes with a low tone that is like the third tone, or a falling tone that's like the fourth. This awkward way of spelling it is because you can't spell this syllable in Pinyin; ie is totally different. If you can read IPA, it is roughly [iə]. There is no character to represent this syllable either. Again, this is not restricted to "ew" but covers it. Sometimes this is broken into two syllables, yi3 (y')e2/4
Some other more generic exclamations, like a permutation of ei/ai + ya/you/y'e/e, with the tone sequence of either 3-2 or 2/1-neutral. Most of these are more generic exclamations, so for most of them, just saying them flat does not get the meaning across, but I think in context and when you need to express the idea, the meaning would get across.