Automatic Transition Prediction Research Articles

Validation of transition modeling techniques for a simplified fuselage configuration

A Reynolds-averaged Navier-Stokes solver has been equipped with two different transition prediction and modeling techniques for automatic transition prediction on general three-dimensional geometrical configurations. The first technique is a streamline-based approach that applies an automated local linear stability code, the eN-method and a simplified two-N-factor strategy and is applicable with any kind of turbulence model. The second technique is a transition transport equation approach and uses the γ-Reθ,t model. The standard γ-Reθ,t model has been extended by a modeling approach that captures transition due to crossflow instabilities. The extended model can be applied to three-dimensional configuration exhibiting crossflow transition whereas the original γ-Reθ,t model only captures streamwise transition mechanisms. The paper focuses on a three-dimensional inclined 6:1 prolate spheroid which can be considered as a simplified fuselage configuration. For a number of years this configuration has become a standard basic three-dimensional test case for transition prediction approaches and has been used in a number of international workshops and research activities. The focus of the paper is the validation of the two transition modeling approaches for six different angles of attack using two different turbulence models, a standard two-equation eddy-viscosity model and a differential Reynolds-stress model. The simulation results from the two approaches have been compared against each other and to the available experimental data. The comparisons highlight as many qualitative and quantitative similarities and agreements as they reveal quantitative differences and deviations of specific details.

A Surrogate-Based Method for Compressible Boundary-Layer Transition Prediction

To improve robustness and efficiency of automatic transition prediction in aerodynamic design, a reduced model of linear stability analysis is usually adopted, such as -envelope or -database method. Nevertheless, building such a model is challenging when it comes to compressible flows, as the transition mechanism is more complex and multiple flow parameters should be taken into consideration. To address this problem, this paper proposes an efficient surrogate-based method for compressible boundary layers that uses pretrained surrogate models to substitute linear stability analysis, concerning stability analysis of both Tollmien–Schlichting and Mack modes, as well as transition prediction of flow over arbitrary-shaped airfoils. The proposed method is demonstrated by stability analysis of compressible flat-plate boundary layers at a wide range of Mach numbers of . It is also validated by transition prediction of flow over a low-speed natural-laminar-flow (NLF) airfoil NLF-0416 and a transonic NLF airfoil NPU-LSC-72613. Besides, a sample partitioning method is presented to accelerate surrogate-model training with large samples. Results show that the predicted growth rates of perturbations, factors, and corresponding transition locations by our method of using surrogate-based stability analysis agree well with those by a standard method of solving full linear stability equations.

Automatic Transition Prediction in a Navier–Stokes Solver Using Linear Stability Theory

A structured Reynolds-averaged Navier–Stokes solver is directly coupled to a linear stability theory (LST) solver to include the effect of laminar–turbulent transition in the flow simulations. The flowfield variables of the flow solver are used to both find streamlines along which transition can be predicted and to provide the LST code with the required boundary-layer profiles. Instabilities included in the analysis are of the Tollmien–Schlichting and crossflow nature relevant to high-Reynolds-number flows in low turbulence environments. The coupling is fully automated and can therefore be used efficiently in the analysis and design of geometries with external flows. The Technical University of Braunschweig’s sickle wing with spanwise-varying crossflow and the natural laminar flow version of the Common Research Model are simulated under various conditions. Applications to these relevant three-dimensional test cases showcase the capability of the method to model the real flow physics. Advantages and challenges of the approach with regard to future design endeavors are discussed.

Aerodynamic Shape Optimization of Natural-Laminar-Flow Wing Using Surrogate-Based Approach

Aerodynamic shape optimization of a swept natural-laminar-flow wing in the transonic regime is still challenging. The difficulty is associated with reliable prediction of laminar–turbulence transition and reasonable compromise of viscous and wave drags. This paper proposes to use efficient global optimization based on surrogate models to address this problem. The Reynolds-averaged Navier–Stokes flow solver features automatic transition prediction via a full method, in which dual factors are used for Tollmien–Schlichting and crossflow instabilities, respectively. The optimizer is based on the kriging surrogate model and parallel infill-sampling method. The baseline natural-laminar-flow wing for short- and medium-range transport aircraft is designed at a cruise Mach number 0.75. Then, drag minimization with up to 42 design variables is carried out, and significant drag reduction (8.79%) has been achieved. A close examination of the optimal wing shows that the drag reduction mainly comes from shock-wave weakening on the upper surface and laminar flow extending via suppression of crossflow instability on the lower surface. Robustness of the optimal wing is investigated, and multipoint optimization is further exercised to improve the robustness to the Mach number variation. It is demonstrated that surrogate-based optimization is feasible and effective for aerodynamic shape optimization of transonic natural-laminar-flow wings.

Automatic Transition Prediction for Three-Dimensional Configurations with Focus on Industrial Application

A computational method to predict transition lines for general three dimensional configurations is presented. The method consists of a coupled program system including a 3D Navier-Stokes solver, a transition prediction module, a boundary-layer code and a stability code. Focus is placed on the industrialization of the approach. For this, the transition prediction module has been adapted to be used for parallel computation to account for high computational demands for three dimensional configurations. Dierent calculation methods for the laminar boundary layer that are available in the transiton prediction module are presented. The method is validated against experimental data of the flow around an inclined prolate spheroid. Application examples are shown for dierent three-dimensional aircraft configurations and topics arising from these tests concerning the industrialization of the method are discussed.

Automatic Transition Prediction in Hybrid Flow Solver, Part 1: Methodology and Sensitivities

A hybrid Reynolds-averaged NavierA¢Â�Â�Stokes solver, a laminar boundary-layer code, and a fully automated local, linear stability code were coupled to predict the laminarA¢Â�Â�turbulent transition due to TollmienA¢Â�Â�Schlichting and crossflow instabilities using the eN method based on the two-N-factor approach. The coupled system was designed to be applied to three-dimensional aircraft configurations which are of industrial relevance. The transition prediction methodology provides two different approaches which are available to be used in different flow situations. Both approaches are described and tested in detail. The application of the complete coupled system to a two-dimensional two-element airfoil configuration and a three-dimensional generic full aircraft configuration is described and documented in this paper. The prediction of the laminarA¢Â�Â�turbulent transition lines was done in a fully automatic manner. It will be shown that complex aircraft configurations can be handled without a priori knowledge of the transition characteristics of the specific flow problem. The computational results are partially compared to experimental data. This article is the first of two companion papers: the first dealing with the transition prediction methodology and the second dealing with the practical application of the coupled system.

Automatic Transition Prediction in Hybrid Flow Solver, Part 2: Practical Application

This article is the second of two companion papers which document the concept and the application of a coupled computational fluid dynamics system which was designed to incorporate the prediction of laminarA¢Â�Â�turbulent transition into a hybrid Reynolds-averaged NavierA¢Â�Â�Stokes solver. Whereas the first part deals with the description of the transition prediction methodology and the sensitivities of the coupled system, the second part documents its practical application. The complete coupled system consists of the Reynolds-averaged NavierA¢Â�Â�Stokes code, a laminar boundary-layer code, and a fully automated local, linear stability code. The system predicts and applies transition locations due to TollmienA¢Â�Â�Schlichting and crossflow instabilities using the eN method based on the two-N-factor approach. The coupled system was designed to be applied to three-dimensional aircraft configurations which are of industrial relevance. The application of the coupled system to a wingA¢Â�Â�body configuration with a three-element wing consisting of slat, main wing, and flap is described and documented in this paper. The prediction of the laminarA¢Â�Â� turbulent transition lines was done in a fully automatic manner. It is shown that complex aircraft configurations can be handled without a priori knowledge of the transition characteristics of the specific flow problem.

Automatic Transition Prediction and Application to Three-Dimensional High-Lift Configurations

A Reynolds-averaged Navier-Stokes solver, a laminar boundary-layer code and two e N -database methods for the prediction of transition due to Tollmien-Schlichting and crossflow instabilities were coupled to perform Reynolds-averaged Navier-Stokes computations of three-dimensional, finite wings with automatic laminar-turbulent transition prediction. It will be shown, that the coupled system represents a Reynolds-averaged Navier-Stokes-based computational fluid dynamics tool that provides accurate values of the transition locations during the ongoing Reynolds-averaged Navier-Stokes computation automatically and fast without the need for the intervention by the code user. Thus, Reynolds-averaged Navier-Stokes computations of three-dimensional wing configurations with transition can be carried out without a priori knowledge of the transition characteristics of the specific flow problem. The coupling structure and the underlying algorithm of the transition prediction procedure and the functioning of the e N -database methods are described. The testing of the transition prediction procedure is described and documented. The computational results are compared with experimental data.

Automatic Transition Prediction and Application to Three-Dimensional Wing Configurations

Automatic Transition Prediction and Application to Three-Dimensional Wing Configurations

Automatic Transition Prediction and Applicationto High-Lift Multi-Element Configurations

A Reynolds-averaged Navier-Stokes (RANS) solver, a laminar boundary layer code and an e(N)-database method for transition prediction were coupled in order to perform RANS computations of two-dimensional high-lift multi-element systems with automatic laminar-turbulent transition prediction and transitional flow regions. It will be shown, that the coupled system represents a RANS-based CFD tool that provides accurated values of the transition locations during the ongoing RANS computation automatically and fast without the need for the intervention by the code user. Thus, RANS computations of two-dimensional high-lift multi-element configurations with transition can be carried out without a priori knowledge of the transition characteristics of the specific flow problem. The coupling structure and the underlying algorithm of the transition prediction procedure as well as the physical modelling of transitional flow regions and their generation in the RANS computational grid are described. The testing of the transition prediction procedure is described and documented. The computational results are compared to experimental data.