Replace vowel with next does not work

I have a string input and want to replace every vowel in that string with its next vowel.

However, it does not seem to work correct. I have tried various input like:

venky -> vinky // Correct
ui    -> uo    // Incorrect, expected ao
ai    -> ao    // Incorrect, expected eo

The vowel order is

char[] vowels = { 'a', 'e', 'i', 'o', 'u' };

My code is:

package test;
import java.util.Scanner;

public class Problem1 {
    public static void main(String args[]) {
        Scanner r = new Scanner(System.in);
        String str, res;
        char ch;
        int i, j;
        char[] vowels = { 'a', 'e', 'i', 'o', 'u' };

        System.out.println("Enter a string");
        str = r.nextLine();
        res = str;

        for (i = 0; i < str.length(); i++) {
            ch = str.charAt(i);

            for (j = 0; j < 4; j++) {
                if (ch == 'u') {
                    res = str.replace(ch, 'a');
                    break;
                } else if (ch == vowels[j]) {
                    res = str.replace(ch, vowels[j+1]);
                    break;
                }
            }
        }

        System.out.println("String after replacing the vowels with next vowel : "
            + res);
    }   
}