The exergetic method is a new aerodynamic assessment tool mainly used for the analysis of future aircraft configurations. One of its major components is the mechanical exergy, representing the amount of recoverable energy of an aerodynamic system: its analysis and comprehension is key for the optimization of an airframe (drag reduction). The mechanical exergy field is studied by following a systematic approach. Firstly, the potential flow around a cylinder is analyzed. Then, a Van Der Vooren transformation is used in order to obtain the potential flow around an airfoil. Finally, a 2D CFD RANS solution of the same airfoil is analyzed in order to add the viscous effects. This study shows that the only recoverable mechanical exergy is that inside the viscous wake and not the mechanical exergy available on the inviscid region.

1 Introduction

The modern exergetic method for external aerodynamic applications was recently developed by Arntz [1]. It is based on the second principle of the thermodynamics: the “exergy” concept is exploited in order to split the total energy of the airflow into a recoverable part (Exergy) and non-recoverable part (Anergy). This theory was later used by the authors in order to split the drag coefficient curve of a classical airfoil into its exergy and anergy components [2]. The mechanical exergy is one of the key elements to be analyzed because it indicates the total amount of energy that can still be recovered, i.e., the amount of drag that can be still reduced by using a suited exergy recovery device (like BLI-boundary layer ingestion). A first step into this direction was already done for a classical airfoil [3], where it has been found that only the exergy available inside the wake seemed to be recoverable. However, a more physical insight is required in order to validate that key observation and this is the main objective of the present work.

2 The approach

A 2D potential flow around a cylinder will be studied first. The mechanical exergy flow field will be analyzed along with its distribution on several survey lines placed downstream of the body. This field serves to prove that the net mechanical exergy available on potential flows is zero, even though it may be non-zero locally.

Then a Van Der Vooren transformation is used in order to obtain the potential flow around a 12% thickness airfoil (the net mechanical exergy still being zero). Finally, a 2D CFD RANS solution of this airfoil is performed in order to add the viscous effects. It will be proven that the net mechanical exergy downstream of the airfoil is no longer zero. The net value outside of the wake it is still being zero (even though it may vary locally). Only the mechanical exergy inside the wake (associated with the viscous effects) is non-zero.

References

[1] Arntz, A., “Civil Aircraft Aero-thermo-propulsive Performance Assessment by an Exergy Analysis of High-fidelity CFD-RANS Flow Solutions”, Fluids mechanics, Université de Lille 1, 2014.

[2] Aguirre, M. and Duplaa, S., “Exergetic Drag Characteristic Curves,” AIAA Journal, (Accepted for publication with revisions in December 2018).

[3] Aguirre, M., Duplaa, S. and Carbonneau, X., “2D Flow Field Analysis by the Exergetic Method” , AIAA Applied Aerodynamics Conference, 17-21 june 2019, Dallas, Texas, United States (Submitted for publication in October 2018).