The function replaces all the vowels in a given input string with the letter "u".

[CODE]:

```python
def f(x):
    vowels = ['a', 'e', 'i', 'o', 'u']
    result = ""
    for char in x:
        if char.lower() in vowels:
            result += "u"
        else:
            result += char
    return result
```