As far as I know, you have no control over alignment in Flow. Maybe there is some way by - example - using attached properties, but I doubt it. You can probably achieve good results by carefully sizing the Flow element and it's contents, but that is not possible in all situations + quite fragile.
An alternative would be to use the "Grid":http://qt-project.org/doc/qt-5/qml-qtquick-grid.html element.