def velocity(avg_slice_df, u_keys, norm_u_keys, u_reference):
    """Normalize the flow velocity vector by a reference velocity.

    Args:
        avg_slice_df (DataFrame): Averaged slice dataframe.
        norm_u_keys (list): Keys to assign to normalized velocity components.
        u_reference (float): Reference velocity used for normalization.
    """
    dataframe = avg_slice_df.copy()
    dataframe[norm_u_keys] = dataframe[u_keys]/u_reference
    return dataframe