Normann Krimmelbein

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, nlinear stability code were coupled to predict the laminarA¢Â�Â�turbulent transition due to TollmienA¢Â�Â�Schlichting and ncrossflow instabilities using the eN method based on the two-N-factor approach. The coupled system was designed to nbe applied to three-dimensional aircraft configurations which are of industrial relevance. The transition prediction nmethodology provides two different approaches which are available to be used in different flow situations. Both napproaches are described and tested in detail. The application of the complete coupled system to a two-dimensional ntwo-element airfoil configuration and a three-dimensional generic full aircraft configuration is described and ndocumented in this paper. The prediction of the laminarA¢Â�Â�turbulent transition lines was done in a fully automatic nmanner. It will be shown that complex aircraft configurations can be handled without a priori knowledge of the ntransition characteristics of the specific flow problem. The computational results are partially compared to nexperimental data. This article is the first of two companion papers: the first dealing with the transition prediction nmethodology and the second dealing with the practical application of the coupled system.

Numerical Aspects of Transition Prediction for Three- Dimensional Configurations

The feasibility to predict transition for three dimensional configurations is presented by means of a coupled program system consisting of a 3D Navier-Stokes solver, a transition module and a stability code. The assumption to use inviscid streamlines as integration paths for the N-factor calculation makes it possible to use linear stability theory in a straightforward way for three dimensional flows. The developed transition module has been adapted to be used with sequential and parallel computations to account for the increased computational demand for three dimensional configurations. Detailed investigations have been carried out, to show the ability of the Navier-Stokes code to provide data of three dimensional boundary layers of high accuracy needed for the stability analyses. Applications of the transition prediction method using the 2N-factor method for the case of an inclined prolate spheroid shows reasonably good results compared to experiments. First application of the transition prediction tool on a generic transport aircraft show promising results for the ability to predict transition on complex geometries.

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.

RANS Simulation and Experiments on the Stall Behaviour of an Airfoil with Laminar Separation Bubbles

Measurements and simulations are presented of the flow past a tailplane research airfoil which is designed to show a mixed leading-edge trailing-edge stall behaviour. The numerical simulations were carried out with two flow solvers that introduce transition prediction based on linear stability theory to RANS simulations for cases involving laminar separation bubbles. One of the methods computes transition locations across laminar separation bubbles whereas the other assumes transition onset where laminar separations occur. For validation of the numerical methods an extensive measurement campaign has been carried out. It is shown, that the methodology mentioned first can simulate the size of laminar separation bubbles for angles of attack up to where the separation bubble and the turbulent separation at the trailing edge are well behaved and steady in the mean. With trailing edge separation involved, the success of the new numerical procedure relies on the diligent choice of a turbulence model. Cases with large 3D flow structures inside the turbulent trailing edge separations in windtunnel experiments for high angles of attack are compared and analysed along with 2D and 3D steady RANS calculations that model the measurement section of the windtunnel.

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

RANS Simulation and Experiments on the Stall Behaviour of a Tailplane Airfoil

Measurements and simulations are presented of the flow past a tailplane research airfoil which is designed to show a mixed leading-edge trailing-edge stall behaviour. The numerical simulations were carded out with two flow solvers that introduce transition prediction based on linear stability theory to RANS simulations for cases involving laminar separation bubbles. One of the methods computes transition locations across laminar separation bubbles whereas the other assumes transition onset where laminar separations occur. For validation of the numerical methods an extensive measurement campaign has been carded out. It is shown, that the methodology mentioned first can simulate the size of laminar separation bubbles for angles of attack up to where the separation bubble and the turbulent separation at the trailing edge are well behaved and steady in the mean. With trailing edge separation involved, the success of the new numerical procedure relies on the diligent choice of a turbulence model. Finally, for flows with increased unsteady behaviour of both, separation bubble and turbulent separation, which were observed at higher angles of attack in the experiment between maximum lift and leading-edge stall, steady state prediction methods for transition can no longer be applied and time-accurate methods have to be developed in a further step.

Coupling of a Reynolds Stress Model with the γ−Reθt Transition Model

A γ−Reθt transition transport model has been coupled with the SSG/LRR-ω differential Reynolds stress model to form a new transition and turbulence model containing nine transport equations. The aim...

Streamline-Based Transition Prediction Techniques in an Unstructured Computational Fluid Dynamics Code

An unstructured flow solver has been equipped with transition prediction techniques based on different streamline-based approaches, applying the eN method for the estimation of the points of transition onset. The integration paths for the N-factor integration can be approximated using lines parallel to the direction of the oncoming flow for some configurations. If arbitrary three-dimensional geometries are to be computed, the local flow direction can be taken into account. The calculation of the laminar boundary-layer data can either be carried out applying a suitable laminar boundary-layer method or by direct determination from the field solution of the unstructured computational fluid dynamics code. The development and characteristics of the two streamline-based transition prediction techniques, their elements, and properties are described. The focus is put on the latest achievements in the development activities of the two approaches and on their application to various configurations, most of them of i...

Transition Prediction for Three-Dimensional Configurations

A computational method for automatic transition prediction for general three-dimensional configurations is presented. The method consists of a coupled program system including a 3D Navier–Stokes solver, a transition module, a boundary layer code and a stability code. The transition module has been adapted to be used with parallel computation to account for the high computational demand of predicting flows around three-dimensional configurations. A comprehensive investigation on general computational and parallel performance identifies the numerical effort for the transition prediction method. The procedure has been validated comparing numerical results with experiments for the flow around an inclined prolate spheroid. Feasibility studies on generic transport aircraft show the code’s capability to predict transition lines on general complex geometries.