Ulf Bunge

Turbulence Modelling in Application to The Vortex Shedding of Stalled Airfoils

ABSTRACT A thorough investigation is conducted into some open questions remaining in the Detached–Eddy Simulation (DES) technique. Principle among these are the questions of grid fineness, and the extent of dependency on the background turbulence model inherent in DES. The investigation takes the case of the NACA0012 airfoil beyond stall, for which a large number of grid fineness and turbulence model combinations are computed. In order to investigate the influence of high resolution in the LES zone, a new highly–refined grid is constructed. The results of the investigation demonstrate a strong deterioration of the results due to the uneven grid fineness distribution, and the reasons for this in relation to a general grid sensitivity present in DES are discussed. However, the physical character of the DES calculations remains encouraging, and motivated by this, an analysis of the phenomenon of stochastic weak vortex shedding cycles is undertaken.

Numerical Study of Separation Control by Movable Flaps

The effect of movable flaps on the upper surface of an airfoil is studied by an unsteady numerical RANS simulation of the flowfield around static flaps at different flap angles. The self-adjusting flap remains in an equilibrium position if the sum of all resulting aerodynamic forces is zero. In this case, the lift increases significantly compared to the clean airfoil. The numerical results, however, indicate that in equilibrium position, the flap does not yield the optimal performance. Additionally, the effect of an oscillating flap as a means of flow-control is studied to investigate the effect of different excitation frequencies.

Method Used and Highlight Results Achieved in FLOMANIA

This chapter describes the method used by the Technical University of Berlin and it summarises the highlight results obtained with this method in the course of FLOMANIA. The main results are the successful implementation and calibration of DES for three different models and the improved representation of flow physics with these DES implementations.

Application-oriented synthesis of work

The current main chapter contains 14 contributions which cover all applications treated within the context of UNSI. Emphasis was placed on cross comparison of data in order to support potential scientists in their own work on validation/verification of all these test cases.

Summary of work carried out in the main tasks of the UNSI project

In the following 8 chapters, a synthesis of task related work is presented which reflects improvements and enhancements of specific scientific topics that have been tackled in the UNSI project.

Technical, partner reports containing method descriptions and concise presentation of important results

In the following 15 chapters, partners provide an overview about methods used and highlight results achieved. For application oriented results, the reader is referred to Chapter IV, while basic approaches and general information concerning the different tasks in UNSI are provided in Chapter III.

Turbulent Two-Dimensional Flow Around a Flexible Membrane Airfoil

The two-dimensional, incompressible flow at Reynolds number \( \operatorname{Re} = \frac{{U\infty \cdot c}}{v} = 1.3 \cdot {10^6} \) around an inextensible, flexible membrane airfoil (sail) with varying excess length e is examined solving the Reynolds-Averaged Navier-Stokes (RANS) equations on grids deforming according to the sail movement within fixed outer boundaries. Results are presented for fully turbulent conditions employing closure models of different degree of complexity in comparison to experimental and analytical results. Good agreement can be found for low angle of attack. However, for higher angle of attack, approaching onset of separation and beyond, the predictive accuracy varies significantly with the representation of turbulence in the presence of strong unsteady phenomena.

Simulation of Flow-Induced Oscillations of a Rectangular Bluff Body in Incompressible, Turbulent Flow

The incompressible flow around a rectangular body with a length to height ratio of L/H = 2 and its flow–induced oscillatory behavior is numerically investigated at different Re–numbers in a range between 1 to 6 · 104 . The body has one degree of freedom perpendicular to the mean–flow direction with a linear spring and linear damping. To compute the flow a finite–volume based Navier–Stokes CFD-code is used, which is enhanced by a finite–difference based algorithm to solve the vibration differential equation. Target is the numeric simulation of a incident flow velocity where resonance occurs and the exact determination of the physical mechanisms especially in the flowing medium. Particular substantial parameters of the total model, e.g. turbulence modeling, time step or grid, which exert influence on the quality of the simulation are examined. To achieve this aim simulations with steady and oscillating body are compared with experimental data and deviations are analyzed.Copyright