Application of the eN method of linear boundary layer stability theory to a natural laminar flow engine nacelle

A stability analysis of the laminar boundary layer on a natural laminar flow engine nacelle flight tested on the DLR VFW614/ATTAS is performed. Since the flow on this nacelle is effectively two-dimensional, only the flow transition due to Tollmien-Schlichting waves is considered. The e^N method of the local linear boundary layer stability theory is applied to determine the limiting N factors N_LIM for various flight Mach numers Ma and chord Reynolds numbers Rec making use of the SALLY code. The flight trials covered a flight Mach number regime 0.26 < Ma < 0.59 and the nacelle fan cowl chord Reynolds number range was 5.5x10hoch6 < Rec < 8.9x10hoch6. Key objective of the analysis was the determination of the effect of flight and engine operating conditions on the calculated limiting N factor. The results of the investigation show that the type of external surface static pressure distribution on the nacelle fan cowl has a marked effect on the magnitude of the limiting N factor. A comparison of the limiting N factors for the VFW614/ATTAS natural laminar flow nacelle flight tests with those from flight trials of a natural laminar flow wing glove on the VWF614/ATTAS and the LFU205 shows for the nacelle the value of N_LIM with N_LIM = 9-10 to be lower by DeltaN_LIM = 4-5 than for the wing glove cases for the same flight Mach number and transition Reynolds number ranges