simply create your datafile name from a write statement:
character*12 fnName
write(fnName,'(A,I2.2)') 'hello.dat_',timer
open(1, file=fnName)
do t=1,10
....
write(1,*) ...
end do
your filenames should look like: hello.dat_01, hello.dat_02,....
you can also use different format if you...