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Computational Fluid-Dynamics Investigations of Vortex Generators for Flow-Separation Control Florian Von Stillfried

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Computational Fluid-Dynamics Investigations of Vortex Generators for Flow-Separation Control Florian Von Stillfried Computational fluid-dynamics investigations of vortex generators for flow-separation control by Florian von Stillfried Doctoral Thesis Royal Institute of Technology School of Engineering Sciences Linn´eFLOW Centre Department of Mechanics SE - 100 44 Stockholm Akademisk avhandling som med tillst˚and av Kungliga Tekniska h¨ogskolan i Stockholm framl¨agges till offentlig granskning f¨or avl¨aggande av teknologie doktorsexamen onsdagen den 16 maj 2012 kl. 10:15 i F3, Lindstedtsv¨agen 26, Kungliga Tekniska h¨ogskolan, Stockholm. ©Florian von Stillfried, maj 2012 US–AB, Stockholm 2012 ii Abstract Many flow cases in fluid dynamics face undesirable flow separation due to ad- verse pressure gradients on wall boundaries. This occurs, for example, due to geometrical reasons as in a highly curved turbine-inlet duct or on flow-control surfaces such as wing trailing-edge flaps within a certain angle-of-attack range. Here, flow-control devices are often used in order to enhance the flow and delay or even totally eliminate flow separation. Flow control can e.g. be achieved by using passive or active vortex generators (VGs) for momentum mixing in the boundary layer of such flows. This thesis focusses on such passive and active VGs and their modelling for computational fluid dynamics investigations. First, a statistical VG model approach for passive vane vortex genera- tors (VVGs), developed at the Royal Institute of Technology Stockholm and the Swedish Defence Research Agency, was evaluated and further improved by means of experimental data and three-dimensional fully-resolved computa- tions. This statistical VVG model approach models those statistical vortex stresses that are generated at the VG by the detaching streamwise vortices. This is established by means of the Lamb-Oseen vortex model and the Prandtl lifting-line theory for the determination of the vortex strength. Moreover, this ansatz adds the additional vortex stresses to the turbulence of a Reynolds-stress transport model. Therefore, it removes the need to build fully-resolved three- dimensional geometries of VVGs in a computational fluid dynamics mesh. Usu- ally, the generation of these fully-resolved geometries is rather costly in terms of preprocessing and computations. By applying VVG models, the costs are reduced to that of computations without VVGs. The original and an improved calibrated passive VVG model show sensitivity for parameter variations such as the modelled VVG geometry and the VVG model location on a flat plate in zero- and adverse-pressure-gradient flows, in a diffuser, and on an airfoil with its high-lift system extracted. It could be shown that the passive VG model qualitatively and partly quantitatively describes correct trends and tendencies for these different applications. In a second step, active vortex-generator jets (VGJs) are considered. They were experimentally investigated in a zero-pressure-gradient flat-plate flow at Technische Universit¨at Braunschweig, Germany, and have been re-evaluated for our purposes and a parameterization of the generated vortices was conducted. iii Dependencies of the generated vortices and their characteristics on the VGJ setup parameters could be identified and quantified. These dependencies were used as a basis for the development of a new statistical VGJ model. This model uses the ansatz of the passive VVG model in terms of the vortex model, the additional vortex-stress tensor, and its summation to the Reynolds stress ten- sor. Yet, it does not use the Prandtl lifting-line theory for the determination of the circulation but an ansatz for the balance of the momentum impact that the VGJ has on the mean flow. This model is currently under development and first results have been evaluated against experimental and fully-resolved computational results of a flat plate without pressure gradient. Descriptors: flow-separation control, vane vortex generator, vortex generator jet, zero-pressure-gradient turbulent boundary layer, adverse-pressure-gradient turbulent boundary layer, statistical modelling, turbulence, Reynolds stress- transport model, computational fluid dynamics iv Preface This doctoral thesis is written within the area of fluid mechanics and mainly investigates computational studies regarding vortex generators. In particular, this thesis examines statistical vortex-generator models which were applied in wall-bounded turbulent flows. One main focus of this work was a thorough eval- uation study of passive vortex-generator models and its application in zero- and adverse-pressure-gradient flat-plate boundary-layer flows, in asymmetric diffuser flow, as well as on an airfoil with a deployed short-chord flap. Another main focus of this contribution was to parameterize and model the vortices from vortex-generator-jet experiments that were carried out at Technische Univer- sit¨at Braunschweig, Germany, and to determine the model parameter depen- dencies on the experimental vortex-generator-jet setup parameters. As a result, a new formulation for a statistical vortex-generator-jet model was derived and evaluated against experimental and fully-resolved computational results. This thesis is subdivided into two parts. In the first part, background on vortex generators, the governing equations, the basic concepts and methods for the statistical-modelling approach, and a short discussion of selected results are presented. The second part of this thesis includes in total four journal papers, three of them either published or accepted for publication, and which are adjusted to comply with the present thesis format, as well as one conference proceedings paper. A digital version of this thesis is available for download on the KTH library homepage on http://www.kth.se/en/kthb. Stockholm, May 2012 Florian von Stillfried v vi Contents Abstract iii Preface v Part I. Overview and summary Chapter 1. Introduction 1 Chapter 2. Background 4 2.1. PassiveVaneVortexGenerators 4 2.2. Active Vortex Generator Jets 7 Chapter 3. Governing Equations 10 Chapter 4. Vortex Generator Modelling 13 4.1. Existing Vane Vortex Generator Models 13 4.2. User Challenges of VG Models 15 4.3. Statistical Vortex Generator Models 16 4.4. Statistical Vane Vortex Generator Model 19 4.5. StatisticalVortexGeneratorJetModel 24 Chapter 5. Selected Results and Discussion 28 5.1. Original VVG Model in APG Flat-Plate Flow 28 5.2. Comparison of the VVG models 28 5.3. VVG Model Applications 32 5.4. Parameterization of VGJ Experiments 34 5.5. Evaluation of the Statistical VGJ Model 36 Chapter 6. Summary of Appended Papers 39 Chapter 7. Concluding Remarks and Outlook 43 Chapter 8. Papers and Authors’ Contributions 45 vii Acknowledgements 48 References 50 Part II. Papers Paper 1. Vortex-Generator Models for Zero- and Adverse-Pressure-Gradient Flows 57 Paper 2. Evaluation of a Vortex Generator Model in Adverse Pressure Gradient Boundary Layers 89 Paper 3. Evaluating and Parameterizing Round Vortex-Generator Jet Experiments for Flow Control 121 Paper 4. A Statistical Vortex-Generator-Jet Model for Turbulent Flow-Separation Control 157 Paper 5. Application of a Statistical Vortex Generator Model on the Short-Chord Flap of a Three-Element Airfoil 183 viii Part I Overview and summary CHAPTER 1 Introduction The demand to design more efficient flow-separation control systems such, for example, new high-lift configurations for future aircraft which enable better low-speed behaviour during takeoff and landing, increased safety, and less en- vironmental impact is becoming more important for the aircraft industry. At the same time, reduction of the complexity of existing flow-separation control systems is not only a trend but necessary in order to, for the example of a high-lift system, advance flight safety, reduce overall weight, lower fuel emis- sions, increase the operating distance, just to mention a few. For this case, it can be stated that the high-lift system has a meaningful impact on the total performance of the aircraft, economically as well as ecologically. Not only the aircraft industry faces such demands, other industries that develop and use fluid-mechanical processes are constantly in the need to improve products to either be and remain competitive, and/or to fulfil legal requirements as, for ex- ample, certification processes due to changing regulations on different markets throughout the world. Flow-separation control can be a very effective way for improving existing fluid-dynamical systems, and a powerful tool in the conceptual design pro- cess from the very beginning of a product-development cycle. The term “flow- separation control” in fluid dynamics is generally used when a wall-bounded fluid flow is modified by flow-separation-control devices such as, for exam- ple, vortex generators (VGs). The general benefit from applying VGs in wall- bounded flows is a possible delay and/or prevention of boundary-layer separa- tion and thereby, an increase of the overall system efficiency. In the limit of the operational envelope, fluid-dynamical systems may not perform properly without flow-separation control. When flow-separation control is necessary, equipping control surfaces with VGs is a common procedure; see figures 1.1 and 1.2. Such VGs mix the fluid near those surfaces and push higher-momentum-containing fluid closer towards the wall, and vice versa. This increases the near-wall velocity and the near-wall momentum, and consequently the stability of the flow in terms of separation delay and/or prevention. During a product-development phase, an increasing amount of computational analysis is used nowadays, and VGs generally have the disadvantage of being computationally costly and time-consuming when included in a detailed analysis. Computational grids often fully resolve VGs, leading to a large amount of additional nodes in their vicinity, which causes 1 2 1. INTRODUCTION Figure 1.1. Deployed flaps and spoilers uncover VGs close to the leading edge on a flap during landing. Figure 1.2. VGs against buffeting effects on a main wing during cruise flight. high computational costs. Therefore, VGs are often neglected in computational analyses. In a later stage during an experimental evaluation, VGs are, on the other hand, often included in for example wind- or water-tunnel investigations so that their impact can be studied thoroughly.
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