Physical flow area is less than the sonic area
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 CdA 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 CdA.
When this warning is displayed, AFT Arrow has determined that the physical area was less than the sonic area at the specified junction, but sonic choking was ignored because no CdA was specified. Thus sonic choking may exist even though AFT Arrow could not calculate it. When a sonic CdA is specified, it is compared with the sonic area to determine whether sonic choking exists, as shown in Figure 1.
Figure 1: Flow is sonically choked through an orifice when the sonic flow area exceeds the sonic CdA
When this warning is displayed it is recommended you obtain the proper CdA for the junction and run the model again.
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