A Warning message in AFT Arrow

Full message is:

Physical flow area is less than the sonic area at junction X. Sonic choking probably exists but was ignored in this solution because no CdA was specified.

AFT Arrow does not account for sonic choking at a junction unless the C_dA for Sonic Choking is specified. For several junctions, a physical area is specified by the user. For example, for an orifice the orifice area or diameter is specified. However, the real flow area is less than this physical area because of vena contracta and wall separation effects. In order to perform a proper sonic choking calculation, AFT Arrow needs to know the C_dA.

When this warning is displayed, AFT Arrow has determined that at the specified junction sonic choking was ignored because no C_dA was specified, but that the physical area was less than the sonic area. Thus sonic choking may exist even though AFT Arrow could not calculate it because the C_dA data was not supplied.

When this warning is displayed it is recommended you obtain the proper C_dA for the junction and run the model again.