Description of laminar-turbulent transition of an airfoil boundary layer measured by differential image thermography using directed percolation theory

Transition from laminar to turbulent flow is still a challenging problem. Recent studies indicate a good agreement when describing this phase transition with the directed percolation theory. This study presents a new experimental approach by means of differential image thermography (DIT) enabling to investigate this transition on the suction side of a heated airfoil. The results extend the applicability of the directed percolation theory to describe the transition on curves surfaces. The experimental effort allows for the first time an agreement between all three universal exponents of the (1+1)D directed percolation for such airfoil application. Furthermore, this study proves that the theory holds for a wide range of flows, as shown by the various conditions tested. Such a large parameter space was not covered in any examination so far. The findings underline the significance of percolation models in fluid mechanics and show that this theory can be used as a high precision tool for the problem of transition to turbulence.Comment: 9 pages, 8 figure

Directed percolation in aerodynamics: resolving laminar separation bubble on airfoils

In nature, phase transitions prevail amongst inherently different systems, while frequently showing a universal behavior at their critical point. As a fundamental phenomenon of fluid mechanics, recent studies suggested laminar-turbulent transition belonging to the universality class of directed percolation. Beyond, no indication was yet found that directed percolation is encountered in technical relevant fluid mechanics. Here, we present first evidence that the onset of a laminar separation bubble on an airfoil can be well characterized employing the directed percolation model on high fidelity particle image velocimetry data. In an extensive analysis, we show that the obtained critical exponents are robust against parameter fluctuations, namely threshold of turbulence intensity that distinguishes between ambient flow and laminar separation bubble. Our findings indicate a comprehensive significance of percolation models in fluid mechanics beyond fundamental flow phenomena, in particular, it enables the precise determination of the transition point of the laminar separation bubble. This opens a broad variety of new fields of application, ranging from experimental airfoil aerodynamics to computational fluid dynamics.Comment: 8 pages, 11 figure

Data-driven repetitive control: Wind tunnel experiments under turbulent conditions

A commonly applied method to reduce the cost of wind energy, is alleviating the periodic loads on turbine blades using Individual Pitch Control (IPC). In this paper, a data-driven IPC methodology called Subspace Predictive Repetitive Control (SPRC) is employed. The effectiveness of SPRC will be demonstrated on a scaled 2-bladed wind turbine. An open-jet wind tunnel with an innovative active grid is employed to generate reproducible turbulent wind conditions. A significant load reduction with limited actuator duty is achieved even under these high turbulent conditions. Furthermore, it will be demonstrated that SPRC is able to adapt to changing operating conditions.Accepted Author ManuscriptTeam Jan-Willem van Wingerde

Wind tunnel measurements on the influence of turbulence on polars and flow separation of an airfoil

The influences of turbulence on the performance of the airfoil and on the appearing forces are of great interest in wind energy since fluctuations and gusts are common in the atmospheric boundary layer1. To analyse and understand the effects, measurements in laminar and turbulent inflow conditions on a DU91-W2-250 airfoil are performed in a Göttingen type wind tunnel. An adjustable and reproducible turbulent flow is generated with an active grid2. Different experimental methods are used and compared with each other. With this knowledge active and passive control strategies to alleviate loads can be envisaged. Polar lines are measured with 3 different methods. Two force sensors and a moment sensor are attached to the airfoil that is arranged in a closed test section with rotatable end plates and allow an analysis of the force dynamics under turbulent conditions. First results of the force balance under laminar conditions are in good agreement with the coefficients gained with wall pressure measurements in the wind tunnel (Figure 1a) and with comparative CFD simulations applying the DLR structured flow solver FLOWer3. The local pressure distribution on the airfoil (Figure 1b) is obtained by pressure tabs which are sampled with a 32-channel pressure scanner. This allows a direct measurement of the local flow separation and the stall effect on the suction side. The integrated pressure distribution yields lift and drag curves that are also in good agreement with the results of the other methods (Figure 1a). The side walls of the wind tunnel out of Plexiglas allow optical access to the turbulent flow around the airfoil. Measurements with a high speed stereo PIV system are possible to investigate the local flow separation with a high temporal resolution. This presentation is an overview of first measurements with the mentioned experimental methods in laminar and turbulent conditions

Application of Particle Image Velocimetry -Theory and Practice- Course Notes

The main interest of today’s research in fluid mechanics is more and more directed to problems where unsteady and separated flows are predominant. For investigations of flow fields with pronounced spatial structures and/or rapid temporal or spatial changes (transition from laminar to turbulent flow, coherent structures, pitching airfoils in transonic flows with shocks, rotors, test facilities with short run time, etc.) new experimental techniques, such as Particle Image Velocimetry (PIV) are required which allow to capture the flow velocity of large flow fields instantaneously. An important feature of PIV is that for the first time, a reliable basis of experimental flow field data is provided for direct comparison with numerical calculations and hence, for validation of computer codes. During the last years an increasing number of scientists have started to utilize the PIV technique to investigate the instantaneous structure of velocity fields in various areas of fluid mechanics. A large number of diff rent approaches for the recording and evaluation of PIV images have been described in literature. This course, which is the 23rd course on PIV since 1993 organized by DLR, will mainly concentrate on those aspects of the theory of PIV relevant to applications. Besides giving lectures on the fundamental aspects, special emphasis is placed on the presentation of practical and reliable solutions of problems which are faced during the implementation of this technique in wind tunnels and other test facilities. During practice the participants will have the opportunity to carry out the recording and the evaluation of PIV images by themselves in small groups. Recent developments of the PIV technique such as 3D(t)-PIV/-PTV (tomographic and holographic PIV) and Stereo PIV, Time Resolved PIV and Micro-PIV will be discussed and demonstrated

Application of Particle Image Velocimetry - Theory and Practice - Course Notes

The main interest of today’s research in fluid mechanics is more and more directed to problems where unsteady and separated flows are predominant. For investigations of flow fields with pronounced spatial structures and/or rapid temporal or spatial changes (transition from laminar to turbulent flow, coherent structures, pitching airfoils in transonic flows with shocks, rotors, test facilities with short run time, etc.) optical experimental techniques, such as Particle Image Velocimetry (PIV) and Lagrangian Particle Tracking (LPT) are required which allow to capture the flow velocity of large flow fields instantaneously. An important feature of PIV and LPT is that a reliable basis of experimental flow field data is provided for direct comparison with numerical calculations and hence, for validation of flow simulation codes. During the last years an increasing number of scientists have started to utilize the PIV techniques to investigate the instantaneous structure of velocity fields in various areas of fluid mechanics. A large number of different approaches for the recording and evaluation of PIV images have been described in literature. This course, which is the 26th course on PIV since 1993 organized by DLR, will mainly concentrate on those aspects of the theory of PIV relevant to applications. Besides giving lectures on the fundamental aspects, special emphasis is placed on the presentation of practical and reliable solutions of problems which are faced during the implementation of this technique in wind tunnels and other test facilities. During practice the participants will have the opportunity to carry out the recording and the evaluation of PIV images by themselves in small groups. Matured developments of the PIV technique such as Stereo PIV, Time Resolved PIV, Micro-PIV and recent innovations in 3D(t)-PIV/3D-PTV (tomographic and digital holographic PIV, LPT Shake-The-Box) will be discussed and demonstrated

Application of Particle Image Velocimetry -Theory and Practice- Course Notes

The main interest of today’s research in fluid mechanics is more and more directed to problems where unsteady and separated flows are predominant. For investigations of flow fields with pronounced spatial structures and/or rapid temporal or spatial changes (transition from laminar to turbulent flow, coherent structures, pitching airfoils in transonic flows with shocks, rotors, test facilities with short run time, etc.) optical experimental techniques, such as Particle Image Velocimetry (PIV) are required which allow to capture the flow velocity of large flow fields instantaneously. An important feature of PIV is that a reliable basis of experimental flow field data is provided for direct comparison with numerical calculations and hence, for validation of flow simulation codes. During the last years an increasing number of scientists have started to utilize the PIV technique to investigate the instantaneous structure of velocity fields in various areas of fluid mechanics. A large number of different approaches for the recording and evaluation of PIV images have been described in literature. This course, which is the 24th course on PIV since 1993 organized by DLR, will mainly concentrate on those aspects of the theory of PIV relevant to applications. Besides giving lectures on the fundamental aspects, special emphasis is placed on the presentation of practical and reliable solutions of problems which are faced during the implementation of this technique in wind tunnels and other test facilities. During practice the participants will have the opportunity to carry out the recording and the evaluation of PIV images by themselves in small groups. Matured developments of the PIV technique such as Stereo PIV, Time Resolved PIV, Micro-PIV and recent innovations in 3D(t)-PIV/4D-PTV (tomographic and digital holographic PIV, Shake-The-Box) will be discussed and demonstrated