Additional Findings from the Common Research Model Natural Laminar Flow Wind Tunnel Test

Abstract

An experimental investigation of the Common Research Model with Natural Laminar Flow (CRM-NLF) took place in the National Transonic Facility (NTF) at the NASA Langley Research Center in 2018. The 5.2% scale semispan model was designed using a new natural laminar flow design method, Crossflow Attenuated NLF (CATNLF). CATNLF enables laminar flow on typical transport wings with high sweep and Reynolds number by reshaping the wing airfoils to obtain specific pressure distribution characteristics that control the crossflow growth near the leading edge. The CATNLF method also addresses Tollmien-Schlichting transition, attachment line transition, and Görtler vortices. During the wind tunnel test, data were acquired to address three primary test objectives: validate the CATNLF design method, characterize the NTF laminar flow testing capabilities, and establish best practices for laminar flow wind tunnel testing. The present paper provides both experimental and computational data to understand the CRM-NLF laminar flow characteristics, as well as address the three primary test objectives. The effects of angle of attack and Reynolds number on the CRM-NLF laminar flow extent are studied, and the dominant transition mechanism is evaluated at a variety of test conditions. Critical N-factors are calculated for the NTF environment, and a discussion on best practices for laminar flow wind tunnel testing is provided. The CRM-NLF in the NTF provided initial confirmation of the ability of the CATNLF method to suppress crossflow growth and enable significant extents of laminar flow on transport wings with high sweep and Reynolds numbers.

Determination of Critical N-Factors for the CRM-NLF Wing

“…3, 4, 5 and 6) and from heating layer method (orange lines in Figs. 3, 4, 5 and 6), are very similar in location and also match those transition locations derived by other authors [2,6] (magenta lines in Figs. 3, 4, 5 and 6).…”

“…In this respect, the calibration of the critical N-factors is an aspect of major importance in this paper. In contrast to the data from [2,5] in the present approach also critical Tollmien-Schlichting N-factors from incompressible LST are presented for the NTF, data that have not been published before. In addition, in the present paper the calibration process is described in detail.…”

“…The purpose of this paper is to provide an answer to one of the many questions that arose in the discussions around the CATNLF approach, its suitability for an effective NLF design of a long-range transport aircraft wing with large sweep and the question if it is appropriate to execute the necessary LST analysis in a compressible manner as was done in the underlying approach the CRM-NLF wing design is based on. Compressible LST is favored by NASA [2] and Boeing [5], whereas incompressible LST is the trusted approach at DLR [4] and Airbus. One major point in the discussions which were held mainly on workshops and conferences in the US was the question if the execution of a compressible LST analysis (in contrast to an incompressible analysis) is needed so that an appropriate CATNLF design as achieved for the CRM-NLF wing can be found in the design process.…”

“…To support and extend the existing evaluation of the critical Tollmien-Schlichting and cross flow N-factors for the CRM-NLF at the National Transonic Facility [2,5], in the present work transition lines have been extracted from TSP images for different angles of attack and stability computations using incompressible and compressible linear stability theory (LST) have been carried out.…”

“…To verify and validate a design method for airfoil shapes that lead to pressure distributions delaying transition on wings with large sweep angles and at high Reynolds number flows (CATNLF, Crossflow Attenuated Natural Laminar Flow [1]) a wind tunnel test campaign in the National Transonic Facility (NTF) at the NASA Langley Research Center has been conducted [2]. The experiments, also aiming at evaluating best practices for laminar flow testing and the general suitability of the National Transonic Facility for laminar flow investigations, where carried out for the flow around a semi-span model of the Common Research Model with Natural Laminar Flow (CRM-NLF).…”