Felix Kramer

Turbulent Drag Reduction by Oscillating Riblets

A novel approach using laterally oscillating riblets is investigated to reduce the turbulent drag of wall-bounded flows. The new method is intended to combine the eect of the wellknown stationary riblets with the strong eect of lateral wall-oscillations. Experimental investigations in this study show only minor eects for the parameters investigated. DNS results demonstrate that the secondary flow induced by the riblet motion has a strong influence on the amount of drag reduction. The influence is not straightforward since stronger oscillations can lead to a higher drag under certain conditions and to a lower drag under dierent conditions. It is shown that the oscillating riblets are able to induce a similar lateral motion as an oscillating wall.

Discrete Adjoint based Sensitivity Analysis for Optimal Flow Control of a 3D High-Lift Configuration

Active flow control techniques are used to delay or prevent the turbulent flow separation on the flap of a high-lift configuration. Effective separation control and thus lift enhancement can be achieved by finding the optimal set of actuation parameters, which may be in very large number. In this paper, a consistent unsteady discrete adjoint RANS solver in three-dimensions is developed towards the objective of finding the optimal set of actuation parameters. The adjoint code is applied to the sensitivity analysis of a practically relevant three-dimensional high-lift configuration. The sensitivities with respect to actuation parameters based on the consistent adjoint approach are compared with finite differences. The effect of frozen turbulence assumption on the accuracy of actuation sensitivities is studied.

Influence of Wave-Like Riblets on Turbulent Friction Drag

This article reports on a numerical and experimental study of the turbulent drag on riblet surfaces, where the trapezoidal riblet grooves were formed in a wave-like sinusoidal or zigzag pattern. The aim was to enhance the drag-reducing capabilities of conventional, straight riblet grooves by an additional contribution that originates from the induced oscillating lateral flow component. By means of a comprehensive parameter study in an oil channel at Re between 10,000 and 30,000 and DNS simulations at Re τ =180, suitable waveform parameters are sought, with which wave-like riblets produce a drag reduction larger than that of their straight counterparts. For a riblet cross-section shape that is known to be optimal for straight grooves, no such beneficial drag modification could be demonstrated. With a riblet groove cross-section different from the optimum shape, an augmented attainable drag reduction in comparison to straight riblet grooves was found within a certain range of the waveform amplitude. The improvement amounts up to 1.3%-points in terms of drag reduction. Wave-like riblets with reduced riblet height never outperformed the drag reduction of straight riblet grooves of optimal cross-section form, but exhibit a similar drag reduction in the best cases investigated. It is shown that this favourable influence on the riblet-modified turbulent drag persists under a mild misalignment of the riblets to the main flow direction.

Wavy riblets for turbulent drag reduction

A combined numerical and experimental study is carried out to investigate the capability of sinusoidal riblet patterns to reduce turbulent drag. In contrast to conventional straight riblets, sinusoidal riblets are able to induce a small amount of lateral motion in the ow. Therefore, they are thought to emulate the eect of lateral wall oscillations and hence to achieve a higher reduction of turbulent drag than straight riblets. This parameter study shows that lateral velocity oscillations are successfully induced. Positive eects of small geometric amplitudes are weak and could not be determined outside the uncertainty range of the measurements or simulation statistics. Larger amplitudes lessen the overall drag reduction which is mainly due to increasing pressure drag.

Optimal Design of Active Flow Control for a Complex High-Lift Configuration

This paper presents the optimal design of an active flow control mechanism for an industry relevant complex high-lift configuration. To control the flow, a large number of synthetic jet actuators are placed on the wing and flap faces. The actuation parameters at these faces are considered as control variables. The optimal set of actuation parameters that yield maximum mean-lift is evaluated by combining an unsteady discrete adjoint RANS solver with a gradient based optimisation algorithm. The adjoint solver is developed by employing Algorithmic Differentiation (AD) techniques. Numerical results have shown that optimisation has resulted in reasonable improvement in the mean-lift compared to the initial actuated flow. This study demonstrates the robustness, accuracy and applicability of AD based unsteady adjoint solver for large scale industrial applications.

Experimental Investigation of Oscillating Riblets for Turbulent Drag Reduction

Direct shear stress measurements were carried out on a riblet test plate with rectangular riblet grooves in a fully turbulent oil channel flow. The band-shaped riblets were tilted in spanwise direction by means of an electric drive up to an amplitude of 30° and a frequency of 4 Hz. Those oscillating riblets were intended to combine the drag reducing effect of conventional, stationary riblets with that of a laterally oscillating wall. Shear stress measurements on oscillating riblets with a cross-section optimal for stationary riblets yielded improvements in the drag-reducing capability of order of the measurement accuracy. For elongated riblets, a measurable increase in the drag reduction was obtained at certain values of the dimensionless oscillation period. Those results are in qualitative congruence to results from DNS simulations previously published.However, the maximum drag reduction of stationary riblets could not be increased by oscillating riblets. By 3D-PIV measurements, the successful generation of an additional spanwise velocity component in the near-wall region was proven.

Discrete Adjoint Based Optimal Active Control of Separation on a Realistic High-Lift Configuration

This paper presents a framework for the optimal active separation control mechanism on a realistic high-lift configuration. To control the separation, synthetic jet actuation is applied on the pressure and suction side of a 3D wing with slats, flaps and flap track fairings. Flow control is realised by varying the parameters of actuation like amplitude, frequency, phase shift and blowing angles. An optimal set of actuation parameters that delay the separation and enhance the aerodynamic performance is found by combining a gradient based optimisation algorithm with a discrete adjoint Unsteady Reynolds-averaged Navier Stokes (URANS) solver. A detailed analysis of the sensitivities with respect to the actuation parameters is presented. Optimisation has yielded a noticeable increase in the lift compared to the initial actuated flow.

A TWO-LEVEL HYBRID APPROACH FOR OPTIMAL ACTIVE FLOW CONTROL ON A THREE-ELEMENT AIRFOIL

In this paper we present a two-level approach that combines an adjoint-based gradient search method with an evolutionary algorithm for optimal active flow control. The suggested method effectively combines the advantages of both approaches and achieves a good compromise between the computational effort and the degree of freedom used in optimization. In the first level, a global optimization is performed with few design parameters using an evolutionary algorithm. In the second level, the global optimal solution from the first level is taken as the initial setting for the adjoint based local optimization using a large number of design parameters. The unsteady discrete adjoint solver required for the second level is developed based on Algorithmic Differentiation techniques for the unsteady incompressible flowsgoverned by Unsteady Reynolds-Averaged Navier Stokes (URANS) equations. In this way, the discrete adjoint solver is robust and has exactly the same functionality with the underlying URANS flow solver. The applicability of the two-level method is demonstrated by finding the optimal parameters of the active flow control mechanism on a three element airfoil configuration at a Reynolds number of Re = 10 and an angle of attack of AoA = 6◦. Numerical results have shown that the hybrid approach completely suppressed the separation and very significantly increased the mean-lift coefficient by 67% compared to the un-actuated baseline flow.

Optimal Separation Control on the Flap of a 2D High-Lift Configuration

Flow separation on the flap of a high-lift device degrades the overall aerodynamic performance and hence results in a drop in the lift coefficient. However, by employing the active flow control techniques, separation can be delayed and thus the lift can be enhanced. In these methods, the flow is controlled by varying the parameters of actuation. In the present work, the optimal set of actuation parameters is found using the gradient-based optimisation algorithms combined with an accurate and robust discrete adjoint method for unsteady RANS. Numerical results are presented for the optimal separation control on the flap of a high-lift configuration over a large time interval.