Hi Kauthar,
The required input parameters for radial flow models are described here, modeling is considered here.
The interstitial column velocity is calculated from the flow rate (that you specify for the column inlet) and the cylindrical geometry. This computation additionally relies on the column height. The computed velocity decreases towards the outlet and is defined as
with v being some constant and r is the radial position. The constant is calculated as described above via
where L is the column height, F_{in} is the inlet flow rate and \varepsilon the column porosity.
Alternatively, you can specify the velocity ceofficient v directly and instead not specify the column height. If height is provided, the input of v will be ignored.
The derivation of the above velocity for cylinder coordinates and solely in radial direction can be found here
Best,
Jan