def fill_out_df(out_df, y_df):
    """Appends output dataframe of each simulation to main output dataframe.

    Args:
        out_df (dataframe): main output dataframe.
        y_df (dataframe): output dataframe of each simulation.

    Returns:
        dataframe: main output dataframe containing results of all simulations.
    """    

    out_df = out_df.append(y_df, ignore_index=True)

    return out_df