OpenFoam_compressibleInterFoam solver_calculation void fraction in the Outlet
 

Hello everyone
I use to compressibleInterFoam solver for two phase flow in horizontal channel.
i want to calculate void fraction (alpha) in the Outlet,
for this purpose, i add this formula in the controlDict :

functions {
alpha.wateroutlet
{
type patchExpression;
patches (
outlet1
outlet2
);
verbose true;
expression "alpha.water";
accumulations (
average
);
}

}

the error is :
- -> FOAM FATAL ERROR:
Parser Error for driver PatchValueExpressionDriver at "1.1-5" : "field alpha not existing or of wrong type"
"alpha"
^^^^^
- - |

can you help me?
Thank you