Investigation into far-field drag calculation methods and pseudo total enthalpy generation for airplane CFD

Abstract

In this study drag calculations were performed for an airplane cruising at transonic speed in a flow-field using two far-field methods. Subjected flow-fields in the calculations were computational fluid dynamics (CFD) results of Reynolds-averaged Navier–Stokes simulation (RANS) around a wing. The objectives were to establish effective means to utilize far-field drag methods and wake flow phenomena as well as report pseudo total enthalpy which peculiarly appeared in the CFD simulation results. The drag calculation was completed using four wake integration equations. The first originated from the equation of the momentum conservation law itself, the second was based on enthalpy variation, the third was based on entropy variation, and the last was a method for induced drag calculation. Drag values resulting from the integrations were compared to those via a near-field method commonly used in CFD. Through the computation and comparison of wake integration drag values, it was proved that the both far-field methods were able to predict a comparably accurate drag value compared to that of the near-field method. In addition, strange inappropriate total enthalpy (pseudo total enthalpy) generation was found in some regions where a flying airplane never affects flow behavior. In the wake integrations, the pseudo total enthalpy generation seems to compensate for the drag value via the first equation. The pseudo total enthalpy generation depends on far-field boundary locations. In summary, wake integrations are promising to provide useful information for accurate drag prediction.