Nonintrusive Ground Force Decomposition Applied to Turbofans—Part I: Control Volume Development and Uncertainty

Abstract A control volume framework is derived to decompose forces produced by a turbofan with special considerations for application to experimental measurements. The rich history of aerodynamic force decomposition is reviewed. The parallel expanded streamtube (PES) method of Drela was chosen as the best fit for application to experimental data because it allows for realistic measurement types and locations, especially for optical methods. The PES method is extended here using a combination of the Townsend criterion and Prandtl’s turbulent mixing length model to decompose the exhaust stream into the core and bypass flow of a turbofan engine. The methodology uncertainty including random, bias, and control volume error is derived using the propagation of uncertainty technique. Finally, a simple analytically simulated data set is used to test the extended PES (EPES) method and compute the uncertainty. The total error is then presented along with the error influence coefficients assuming uncertain variables such as axial velocity, specific heat ratio, specific gas constant, static temperature, static density, freestream static pressure, freestream velocity, and measurement area. The largest sources of uncertainty were found to be the static temperature and density. In general, the EPES method showed approximately a 1% error in total force, a 2.5% error in core force, and a 2% error in bypass force. This work is applied to real data in Part II of this paper using the filtered Rayleigh scattering technique to gather inlet and exhaust values to measure ground force on a Honeywell TFE-731 turbofan at Virginia Tech’s TurboLab.