Hi there @qianyi & @mlauber71!
Regarding flow variables in Column Filter node. From KNIME version 4.1 new flow variables types have been introduced. Specifically this means you can control number of column in Column Filter (and nodes alike) dynamically and flow variable of type array (collection/list) is required. You can check this workflow to see how it is done in GroupBy node.
@qianyi can you explain a bit more what are you trying to accomplish?
Br,
Ivan