The simple flow rate can be prescribed at the inlet. Its fixed value is equal to massFlowRate.

p:

 boundaryField
 {
    ".*_inlet"
    {
     type            zeroGradient;
    }
 }

U:

 boundaryField
 {
    ".*_inlet"
    {
        type            flowRateInletVelocity;
        massFlowRate    6.8;
        value           $internalField;
    }
  }