The Langley Stability and Transition Analysis Code (LASTRAC) : LST, Linear and Nonlinear PSE for 2-D, Axisymmetric, and Infinite Swept Wing Boundary Layers

Summary Predicting transition onset remains a daunting task evenwith the state-of-the-art computing facilities. At the lowestfidelity, transition may be predicted by using a prescribedN-factor and a simple linear stability or parabolized sta-bility theory. Depending on the accuracy requirement, theN-value may come from empirical correlations with exist-ing wind tunnel or flight experiments, or from a commonlyused value such as 10. The major problem of this approachlies in the N-value itself. According to past experiences,the transition N-value may vary from as small as 2 or 3for a noisy facility to as high as 15 or 20 in flight. Thus,for a new configuration, determining the N value is itself adifficult task. Despite this uncertainty, N-factor correlationremains the most viable method for transition predictiondue to its simplicity. LASTRAC provide both LST- andPSE-based N-factor correlation capability.For a given mean flow, a possible unstable frequencyrange is suggested by LASTRAC and by using the max-imizing N-factor option, users may obtain an unstableparameter range of disturbance frequency and spanwisewavelength with minimum effort and little prior knowledgeof the mean flow under consideration. Further linear calcu-lations using either quasi-parallel LST or nonparallel PSEmay be launched to obtain an envelop of N-factors formedby a broad range of unstable modes. The N-factor envelopethen can be used for transition correlations or predictions.LASTRAC also provides the capability to compute dis-turbance evolution based on an absolute amplitude. Non-linear PSE calculations may be performed for a numberof unstable modes with a finite amplitude. We show sev-eral test cases in which the boundary layer is perturbedwith a given amplitude and transition is captured by us-ing the nonlinear PSE option. Coupled with the receptivitymodel which will be incorporated in the near future, LAS-TRAC offers a transition prediction tool that may be used tocompute transition onset without any modeling or N-factorassumptions.It is also demonstrated that LASTRAC may be used forparametric studies of several supersonic laminar flow con-trol concepts. We presented two such techniques, thermaland distributed roughness control. The test cases shown inthis paper cover a broad range of flow configurations. In ad-dition to the traditional N-factor method, LASTRAC offersa comprehensive set of options based on the state-of-the-artnumerical methods that may be used for the stability calcu-lations and transition predictions in an integrated fashion.