The most obvious error I see is that you change the varialbe name when you append the '_'. You save that result to filename. You then use filename1 in the next line, which means the underscore is not appearing in your final result.
You might want to consider using the function fullfile to create filename1. You can do it in a single line of code, and it will handle adding the slashes, etc. for you.
for i=1
filename1 = fullfile("D:","gavaDB",["cara" + num2str(i) + "_" + "abajo" + ".wrl"])
end