Thanks Claus! It was useful.
For the executable file, I made a batch file that opens command prompt, reads input files and runs the flow3d solver, then copy and paste a file and finally runs the postprocessor. In other words, when you click on the .bat file, it automatically reads the inputs and generates the outputs. The format of both input and output file is the name of simulation; however, you can open them with any text editor program.
For simplicity, I only have 1 design variable which is in the input file (I don't have any stl file), and 1 eval_ResultsValue. Sobol with 4 Variants was used. I could see that the code was running 4 times (command prompt opened 4 times, and cpu usage was increased 4 times). but the code couldn't read from the output file and shows "nan" in the eval_ResultValue column for all of the cases. Do you happen to know how I can solve this problem?