DOCSLIB.ORG

Passive Flow Separation Control on an Airfoil-Flap Model the Effect of Cylinders and Vortex Generators

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Passive Flow Separation Control on an Airfoil-Flap Model the Effect of Cylinders and Vortex Generators Master of Science Thesis Passive Flow Separation Control on an Airfoil-Flap Model The Effect of Cylinders and Vortex Generators D.P. Jansen August 2012 Faculty of Aerospace Engineering · Delft University of Technology Passive Flow Separation Control on an Airfoil-Flap Model The Effect of Cylinders and Vortex Generators Master of Science Thesis For obtaining the degree of Master of Science in Aerospace Engineering at Delft University of Technology D.P. Jansen August 2012 Faculty of Aerospace Engineering · Delft University of Technology Delft University of Technology Copyright ¥c Aerospace Engineering, Delft University of Technology All rights reserved. Delft University Of Technology Department of Aerodynamics The undersigned hereby certify that they have read and recommend to the Faculty of Aerospace En- gineering for acceptance a thesis entitled “Passive Flow Separation Control on an Airfoil-Flap Model” by D.P. Jansen in partial fulfillment of the requirements for the degree of Master of Science . Dated: August 2012 Head of department: Prof.dr.ir.drs. H. Bijl Supervisor: Dr.ir. L.L.M. Veldhuis Reader: Dr.ir. B.W. van Oudheusden Reader: Ir. W.A. Timmer Acknowledgements This report is written as a thesis for the degree of Master of Science at the Faculty of Aerospace Engi- neering, Department of Aerodynamics, at Delft University of Technology. I would like to take the opportunity to thank all the people who have helped me in accomplishing this graduation research. I would like to thank Dr. ir. L.L.M. Veldhuis, my supervisor, for his full support, enthusiasm and sharing of his in depth knowledge on experimental research and the field of flow separation control. I could not have wished for a better advisor for this project. I also would like to thank Dipl. ing. S. Bernardy and L. Molenwijk for their willingness in answering my questions and their assistance in performing the wind tunnel experiments. Then I would like to thank my parents for their motivation and advice in times most needed. Special thanks goes to R. Peacock for his sincere curiosity and help on the project. Finally I also want to mention Kim-Lan, my girlfriend, because without her love and unlimited support I definitely would not have achieved this result. Delft, The Netherlands D.P. Jansen August 2012 v D.P. Jansen vi MSc. Thesis Summary In the on-going search for improvements in high lift device performance, the field of flow separation control has seen explosive growth in recent years. This is due to a continuously improved understanding of fluid mechanics, the development in experimental and computational techniques and the variety of flow control mechanisms existing today. These multiple mechanisms can be differentiated into active and passive flow control techniques. In this research two passive flow separation control methods are tested on their ability to postpone flow separation on an airfoil with trailing edge flap model of the Extra EA-400. The two methods are the application of vortex generators on the airfoil and the positioning of vortex producing cylinders in the slot of the airfoil flap system. The 2D airfoil flap model was tested by experimental and numerical investigation. The flap deflection was ◦ ◦ ◦ set at δf = 45 , δf = 50 and δf = 55 to test the flap at moderate to highly separated flow conditions. Measurements in the experiment were done by pressure holes and probes and oil flow visualization. Numerical calculations were performed in FLUENT as part of the ANSYS 13.0 package, which was also used to create the grid. The model is a RANS analysis with the k − ω SST turbulence model and low Re near wall treatment. It was found in the experiments that both the vortex generators and the cylinders were able to delay flow separation and thereby increase the lift of the airfoil flap system. Both devices are able to create vortices that entrain flow into the boundary layer, which gets re-energized. The cylinders proved to be superior compared to the vortex generators in the ability to postpone the point of flow separation. With respect to size and positioning the cylinders also showed a larger bandwidth than the vortex generators at which the devices were effective. The results from the experiments indicated that the reduced frequency for the cylinder vortex shedding resulting in the largest delay in flow separation, matched the theoretical predictions found in literature. Especially at high flap deflections significant lift gains were obtained up to ◦ 6% for the vortex generators and 18% for the cylinders at a deflection of δf = 55 . At the flap deflection ◦ of δf = 45 the flow separation delay capabilities of the cylinders are less effective. The frequency tests, oil flow visualization and boundary layer measurements were used to understand the performance and working principles of the cylinders. The numerical simulations showed difficulties in reproducing the results for the cylinders obtained from the experiment. While for the attached airfoil flow the numerical simulations showed good agreement with the experiment, the flow separation on the flap at high deflections was not accurately modelled for both the baseline and cylinder configuration of the airfoil flap. The vortices created in the simulation showed a different frequency than found in the experiment and only minor effects of the cylinder were observed. Separation on the flap was very slightly delayed, although no significant mixing effects of the vortices were visible in the boundary layer. vii D.P. Jansen viii MSc. Thesis Contents Acknowledgements v Summary vii List of Figures xii List of Tables xvii Nomenclature xxi 1 Introduction 1 1.1 The need for high lift systems . 1 1.2 Limits in high lift performance . 3 1.3 Flow separation in high lift performance . 5 1.3.1 Plain airfoil characteristics . 5 1.3.2 Flapped airfoil characteristics . 6 1.4 Flow separation control . 8 1.4.1 Active flow separation control techniques . 8 1.4.2 Passive flow separation control techniques . 10 1.5 Opportunity of research . 13 1.6 Layout of the report . 13 2 Experimental model setup 15 2.1 Introduction . 15 2.2 The wind tunnel and model . 15 2.3 Passive flow separation methods . 18 2.3.1 Vortex generators . 19 ix Contents 2.3.2 Cylinders . 20 2.4 Measurement techniques . 23 2.4.1 Pressure measurements and calculation of Cl and Cd ................. 23 2.4.2 Frequency measurements . 25 2.4.3 Oil flow visualization . 26 3 Experimental results 27 3.1 Introduction . 27 3.2 Baseline model . 27 3.3 Vortex generators . 31 3.4 Cylinders . 34 ◦ 3.4.1 Flap deflection δf = 55 ................................. 34 ◦ 3.4.2 Flap deflection δf = 50 ................................ 42 ◦ 3.4.3 Flap deflection δf = 45 ................................. 43 3.4.4 Comparison of δf effects on cylinder performance and further investigation . 47 3.5 Conclusion experimental investigation . 52 4 Numerical investigation 55 4.1 Introduction . 55 4.2 General setup . 55 4.3 The numerical domain . 56 4.4 Model validation . 61 ◦ 4.4.1 Low flap deflection δf = 15 , no flow separation . 61 ◦ 4.4.2 High flap deflection δf = 45 .............................. 62 ◦ 4.4.3 Very high flap deflection δf = 55 ........................... 63 4.4.4 Conclusion on baseline model validation . 63 4.4.5 Influence of tunnel wall at large flap deflection . 63 4.4.6 Modelling laminar zones in CFD . 65 4.5 Results for baseline and cylinder configurations . 67 4.5.1 Baseline and cylinder velocity field . 68 4.5.2 Vortex development . 68 4.5.3 Vortex influence on the flap . 74 4.6 Conclusion on numerical investigation . 76 5 Conclusion and Recommendations 79 5.1 Conclusion . 79 5.2 Recommendations . 80 D.P. Jansen x MSc. Thesis Contents Bibliography 81 A Test matrix for the wind tunnel experiments 83 B Additional pictures on the wind tunnel and model layout 85 C Output file example of pressure measurements from Profmeasure 89 MSc. Thesis xi D.P. Jansen Contents D.P. Jansen xii MSc. Thesis List of Figures 1.1 General high lift system with single slotted trailing edge and leading edge flap. 2 1.2 Lift polar showing the effects of a trailing edge and leading flap deflection on a plain airfoil. 2 1.3 Circular-arc mean lines A, B, C obtained from the Joukowski transformation. Figure from Smith [1975]. 3 1.4 Flow around a cylinder with a very high circulation, such that the two stagnation points coincide. Figure from Smith [1975]. 4 1.5 Visualization of flow separation on a flap. Figure from Little et al. [2009]. 4 1.6 Visualization of a laminar separation bubble. 5 1.7 Visualization of flow separation. 6 1.8 Lift polar showing the general effect of increasing trailing edge flap deflection. 7 1.9 Triple-slot flap system of the B727-200. Figure from Rudolph [1996]. 8 1.10 PIV visualization of flow separation postponement on a wind turbine blade by applying synthetic jets at α = 16 ◦ and Re = 1 .6 · 10 5. Figure from Maldonado et al. [2010]. 9 1.11 Components of an electrodynamic synthetic jet. Figure from Cattafesta [2011]. 9 1.12 Working principle of boundary layer suction. Figure from Boermans [2008]. 10 1.13 Schematization of a piezoelectric flap. Figure from Veldhuis and van der Jagt [2010]. 10 1.14 Schematization of a DBD plasma actuator. Figure from LeBeau [2007]. 11 1.15 Vortex generators displayed in different setups. Figures from von Stillfried et al. [2010]. 11 1.16 Effect of counter-rotating micro VG’s on RMS anemometer output from hot-film sensors ◦ on flap upper surface with α = 8 and M∞ = 0 .2. Figure from Lin [1999]. 12 1.17 Lift-enhancing tabs as used by Lee et al.
Load more

Recommended publications
  • Chapter 4: Immersed Body Flow [Pp
    MECH 3492 Fluid Mechanics and Applications Univ. of Manitoba Fall Term, 2017 Chapter 4: Immersed Body Flow [pp. 445-459 (8e), or 374-386 (9e)] Dr. Bing-Chen Wang Dept. of Mechanical Engineering Univ. of Manitoba, Winnipeg, MB, R3T 5V6 When a viscous fluid flow passes a solid body (fully-immersed in the fluid), the body experiences a net force, F, which can be decomposed into two components: a drag force F , which is parallel to the flow direction, and • D a lift force F , which is perpendicular to the flow direction. • L The drag coefficient CD and lift coefficient CL are defined as follows: FD FL CD = 1 2 and CL = 1 2 , (112) 2 ρU A 2 ρU Ap respectively. Here, U is the free-stream velocity, A is the “wetted area” (total surface area in contact with fluid), and Ap is the “planform area” (maximum projected area of an object such as a wing). In the remainder of this section, we focus our attention on the drag forces. As discussed previously, there are two types of drag forces acting on a solid body immersed in a viscous flow: friction drag (also called “viscous drag”), due to the wall friction shear stress exerted on the • surface of a solid body; pressure drag (also called “form drag”), due to the difference in the pressure exerted on the front • and rear surfaces of a solid body. The friction drag and pressure drag on a finite immersed body are defined as FD,vis = τwdA and FD, pres = pdA , (113) ZA ZA Streamwise component respectively.
    [Show full text]
  • Chapter 4: Immersed Body Flow [Pp
    MECH 3492 Fluid Mechanics and Applications Univ. of Manitoba Fall Term, 2017 Chapter 4: Immersed Body Flow [pp. 445-459 (8e), or 374-386 (9e)] Dr. Bing-Chen Wang Dept. of Mechanical Engineering Univ. of Manitoba, Winnipeg, MB, R3T 5V6 When a viscous fluid flow passes a solid body (fully-immersed in the fluid), the body experiences a net force, F, which can be decomposed into two components: a drag force F , which is parallel to the flow direction, and • D a lift force F , which is perpendicular to the flow direction. • L The drag coefficient CD and lift coefficient CL are defined as follows: FD FL CD = 1 2 and CL = 1 2 , (112) 2 ρU A 2 ρU Ap respectively. Here, U is the free-stream velocity, A is the “wetted area” (total surface area in contact with fluid), and Ap is the “planform area” (maximum projected area of an object such as a wing). In the remainder of this section, we focus our attention on the drag forces. As discussed previously, there are two types of drag forces acting on a solid body immersed in a viscous flow: friction drag (also called “viscous drag”), due to the wall friction shear stress exerted on the • surface of a solid body; pressure drag (also called “form drag”), due to the difference in the pressure exerted on the front • and rear surfaces of a solid body. The friction drag and pressure drag on a finite immersed body are defined as FD,vis = τwdA and FD, pres = pdA , (113) ZA ZA Streamwise component respectively.
    [Show full text]
  • Upwind Sail Aerodynamics : a RANS Numerical Investigation Validated with Wind Tunnel Pressure Measurements I.M Viola, Patrick Bot, M
    Upwind sail aerodynamics : A RANS numerical investigation validated with wind tunnel pressure measurements I.M Viola, Patrick Bot, M. Riotte To cite this version: I.M Viola, Patrick Bot, M. Riotte. Upwind sail aerodynamics : A RANS numerical investigation validated with wind tunnel pressure measurements. International Journal of Heat and Fluid Flow, Elsevier, 2012, 39, pp.90-101. 10.1016/j.ijheatfluidflow.2012.10.004. hal-01071323 HAL Id: hal-01071323 https://hal.archives-ouvertes.fr/hal-01071323 Submitted on 8 Oct 2014 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. I.M. Viola, P. Bot, M. Riotte Upwind Sail Aerodynamics: a RANS numerical investigation validated with wind tunnel pressure measurements International Journal of Heat and Fluid Flow 39 (2013) 90–101 http://dx.doi.org/10.1016/j.ijheatfluidflow.2012.10.004 Keywords: sail aerodynamics, CFD, RANS, yacht, laminar separation bubble, viscous drag. Abstract The aerodynamics of a sailing yacht with different sail trims are presented, derived from simulations performed using Computational Fluid Dynamics. A Reynolds-averaged Navier- Stokes approach was used to model sixteen sail trims first tested in a wind tunnel, where the pressure distributions on the sails were measured.
    [Show full text]
  • Airfoil Boundary Layer Separation Prediction [Pdf]
    Airfoil Boundary Layer Separation Prediction A project present to The Faculty of the Department of Aerospace Engineering San Jose State University in partial fulfillment of the requirements for the degree Master of Science in Aerospace Engineering By Kartavya Patel May 2014 approved by Dr. Nikos Mourtos Faculty Advisor ABSTRACT Airfoil Boundary Layer Separation Prediction by Kartavya Patel This project features a MatLab complied program that predicts airfoil boundary layer separation. The Airfoil Boundary Layer Separation program uses NACA 4 series, 5 series and custom coordinates to generate the airfoil geometry. It then uses Hess-Smith Panel Method to generate the pressure distribution. It will use the pressure distribution profile to display the boundary layer separation point based on Falkner-Skan Solution, Stratford’s Criterion for Laminar Boundary Layer and Stratford’s Criterion for Turbulent Boundary Layer. From comparison to Xfoil, it can be determined that for low angle of attacks the laminar flow separation point can be predicted from Stratford’s LBL criterion and the turbulent flow separation point can be predicted from Stratford’s TBL criterion. For high angle of attacks, the flow separation point can be predicted from Falkner-Skan Solution. The program requires MatLab Compiler Runtime version 8.1 (MCR) which can be downloaded free at http://www.mathworks.com/products/compiler/mcr/ . ACKNOWLEDGEMENTS I would like to thank Dr. Nikos Mourtos for his support and guidance throughout this project. I would also like to thank Hai Le, Tommy Blackwell and Ian Dupzyk for their contribution in previous project “Determination of Flow Separation Point on NACA Airfoils at Different Angles of Attack by Coupling the Solution of Panel Method with Three Different Separation Criteria”.
    [Show full text]
  • Turbulent Boundary Layer Separation (Nick Laws)
    NICK LAWS TURBULENT BOUNDARY LAYER SEPARATION TURBULENT BOUNDARY LAYER SEPARATION OUTLINE ▸ What we know about Boundary Layers from the physics ▸ What the physics tell us about separation ▸ Characteristics of turbulent separation ▸ Characteristics of turbulent reattachment TURBULENT BOUNDARY LAYER SEPARATION TURBULENT VS. LAMINAR BOUNDARY LAYERS ▸ Greater momentum transport creates a greater du/dy near the wall and therefore greater wall stress for turbulent B.L.s ▸ Turbulent B.L.s less sensitive to adverse pressure gradients because more momentum is near the wall ▸ Blunt bodies have lower pressure drag with separated turbulent B.L. vs. separated laminar TURBULENT BOUNDARY LAYER SEPARATION TURBULENT VS. LAMINAR BOUNDARY LAYERS Kundu fig 9.16 Kundu fig 9.21 laminar separation bubbles: natural ‘trip wire’ TURBULENT BOUNDARY LAYER SEPARATION BOUNDARY LAYER ASSUMPTIONS ▸ Simplify the Navier Stokes equations ▸ 2D, steady, fully developed, Re -> infinity, plus assumptions ▸ Parabolic - only depend on upstream history TURBULENT BOUNDARY LAYER SEPARATION BOUNDARY LAYER ASSUMPTIONS TURBULENT BOUNDARY LAYER SEPARATION PHYSICAL PRINCIPLES OF FLOW SEPARATION ▸ What we can infer from the physics TURBULENT BOUNDARY LAYER SEPARATION BOUNDARY LAYER ASSUMPTIONS ▸ Eliminate the pressure gradient TURBULENT BOUNDARY LAYER SEPARATION WHERE THE BOUNDARY LAYER EQUATIONS GET US ▸ Boundary conditions necessary to solve: ▸ Inlet velocity profile u0(y) ▸ u=v=0 @ y = 0 ▸ Ue(x) or Pe(x) ▸ Turbulent addition: relationship for turbulent stress ▸ Note: BL equ.s break down
    [Show full text]
  • Computational and Experimental Study on Performance of Sails of a Yacht
    Available online at www.sciencedirect.com SCIENCE DIRECT" EIMQINEERING ELSEVIER Ocean Engineering 33 (2006) 1322-1342 www.elsevier.com/locate/oceaneng Computational and experimental study on performance of sails of a yacht Jaehoon Yoo Hyoung Tae Kim Marine Transportation Systems Researcli Division, Korea Ocean Research and Development Institute, 171 Jang-dong, Yuseong-gu, Daejeon 305-343, South Korea ^ Department of Naval Architecture and Ocean Engineering, Chungnam National Un iversity, 220 Gung-doiig, Yuseong-gu, Daejeon 305-764, South Korea Received 1 February 2005; accepted 4 August 2005 Available online 10 November 2005 Abstract It is important to understand fiow characteristics and performances of sails for both sailors and designers who want to have efhcient thrust of yacht. In this paper the viscous fiows around sail-Hke rigid wings, which are similar to main and jib sails of a 30 feet sloop, are calculated using a CFD tool. Lift, drag and thrust forces are estimated for various conditions of gap distance between the two sails and the center of effort ofthe sail system are obtaiaed. Wind tunnel experiments are also caiTied out to measure aerodynamic forces acting on the sail system and to validate the computation. It is found that the combination of two sails produces the hft force larger than the sum of that produced separately by each sail and the gap distance between the two sails is an important factor to determine total hft and thrust. © 2005 Elsevier Ltd. AU rights reserved. Keywords: Sailing yacht; Interaction; Gap distance; CFD; RANS; Wind tunnel 1. Introduction The saihng performance of a yacht depends on the balance of hydro- and aero-dynamic forces acting on the huU and on the sails.
    [Show full text]
  • Lift Forces in External Flows
    • DECEMBER 2019 Lift Forces in External Flows Real External Flows – Lesson 3 Lift • We covered the drag component of the fluid force acting on an object, and now we will discuss its counterpart, lift. • Lift is produced by generating a difference in the pressure distribution on the “top” and “bottom” surfaces of the object. • The lift acts in the direction normal to the free-stream fluid motion. • The lift on an object is influenced primarily by its shape, including orientation of this shape with respect to the freestream flow direction. • Secondary influences include Reynolds number, Mach number, the Froude number (if free surface present, e.g., hydrofoil) and the surface roughness. • The pressure imbalance (between the top and bottom surfaces of the object) and lift can increase by changing the angle of attack (AoA) of the object such as an airfoil or by having a non-symmetric shape. Example of Unsteady Lift Around a Baseball • Rotation of the baseball produces a nonuniform pressure distribution across the ball ‐ clockwise rotation causes a floater. Flow ‐ counterclockwise rotation produces a sinker. • In soccer, you can introduce spin to induce the banana kick. Baseball Pitcher Throwing the Ball Soccer Player Kicking the Ball Importance of Lift • Like drag, the lift force can be either a benefit Slats or a hindrance depending on objectives of a Flaps fluid dynamics application. ‐ Lift on a wing of an airplane is essential for the airplane to fly. • Flaps and slats are added to a wing design to maintain lift when airplane reduces speed on its landing approach. ‐ Lift on a fast-driven car is undesirable as it Spoiler reduces wheel traction with the pavement and makes handling of the car more difficult and less safe.
    [Show full text]
  • Studies of Mast Section Aerodynamics by Arvel Gentry
    Studies of Mast Section Aerodynamics By Arvel Gentry Proceedings of the 7th AIAA Symposium on the Aero/Hydronautics of Sailing January 31, 1976 Long Beach, California Abstract This paper summarizes the studies that were conducted with the objective of obtaining a mast-section shape with improved aerodynamic flow properties for use on the 12-Meter Courageous. TheDecember project 1999 consisted of a theoretical study defining the basic aerodynamics of the mast-mainsail combination followed by both analytical and experimental studies of various mast-section shapes. A new 12-Meter mast-section shape evolved from these studies that demonstrated significantly improved airflow patterns around the mast and mainsail when compared directly with the conventional 12-Meter elliptical section. This new mast shape was used in constructing the mast for Courageous used in the successful 1974 defense of the America's cup against the Australian challenger Southern Cross. 1. Introduction The mast has always been thought of as being an Bill Ficker, who skippered Intrepid in the 1970 defense, was undesirable but necessary appendage on a sailboat. It slated to be the skipper of the new aluminum Courageous. holds the sails up but contributes considerable drag and Ficker and David Pedrick (S&S project engineer on disturbs the airflow over the mainsail so that its efficiency Courageous) wondered if a new mast-section shape might is greatly reduced. This popular belief has led to a number lead to improved boat performance. This paper describes of different approaches in improving the overall efficiency the work done in attempting to answer that question. of a sailing rig.
    [Show full text]
  • Adaptive Separation Control of a Laminar Boundary Layer Using
    J. Fluid Mech. (2020), vol. 903, A21. © The Author(s), 2020. 903 A21-1 Published by Cambridge University Press doi:10.1017/jfm.2020.546 Adaptive separation control of a laminar https://doi.org/10.1017/jfm.2020.546 . boundary layer using online dynamic mode decomposition Eric A. Deem1, Louis N. Cattafesta III1,†,MaziarS.Hemati2, Hao Zhang3, Clarence Rowley3 and Rajat Mittal4 1Mechanical Engineering, Florida State University, Tallahassee, FL 32310, USA 2Aerospace Engineering and Mechanics, University of Minnesota, Minneapolis, MN 55455, USA https://www.cambridge.org/core/terms 3Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544, USA 4Mechanical Engineering, Johns Hopkins University, Baltimore, MD 21218, USA (Received 18 November 2019; revised 13 April 2020; accepted 22 June 2020) Adaptive control of flow separation based on online dynamic mode decomposition (DMD) is formulated and implemented on a canonical separated laminar boundary layer via a pulse-modulated zero-net mass-flux jet actuator located just upstream of separation. Using a linear array of thirteen flush-mounted microphones, dynamical characteristics of the separated flow subjected to forcing are extracted by online DMD. This method provides updates of the modal characteristics of the separated flow while forcing is applied at a rate commensurate with the characteristic time scales of the flow. In particular, online DMD provides a time-varying linear estimate of the nonlinear evolution of the controlled , subject to the Cambridge Core terms of use, available at flow without any prior knowledge. Using this adaptive model, feedback control is then implemented in which the linear quadratic regulator gains are computed recursively. This physics-based, autonomous approach results in more efficient flow reattachment compared with commensurate open-loop control.
    [Show full text]
  • Flow Over Immerced Bodies
    Department of Mechanical Engineering AMEE401 / AUTO400 Instructor: Marios M. Fyrillas Aerodynamics Email: [email protected] HOMEWORK ASSIGNMENT #2 QUESTION 1 Clearly there are two mechanisms responsible for the drag and the lift, the pressure and the shear stress: The drag force and the lift force on an object can be obtained by: Dpcos d Aw sin d A Lpsin d A w cos d A LIFT: Most common lift-generating devices (i.e., airfoils, fans, spoilers on cars, etc.) operate in the large Reynolds number range in which the flow has a boundary layer character, with viscous effects confined to the boundary layers and wake regions. For such cases the wall shear stress, w , contributes little to the lift. Most of the lift comes from the surface pressure distribution, as justified through Bernoulli’s equation. For objects operating in very low Reynolds number regimes (i.e. Re 1) viscous effects are important, and the contribution of the shear stress to the lift may be as important as that of the pressure. Such situations include the flight of minute insects and the swimming of microscopic organisms. DRAG: Similarly, when flow separation occurs, i.e. a blunt body or a streamlined body at a large angle of attack, the major component of the drag force is pressure differential due to the low pressure in the flow separation region. On the ultimately streamlined body (a zero thickness flat plate parallel to the flow) the drag is entirely due to the shear stress at the surface (boundary layers) and, is relatively small.
    [Show full text]
  • Numerical Investigation of the Turbulent Wake-Boundary Interaction in a Translational Cascade of Airfoils and Flat Plate
    energies Article Numerical Investigation of the Turbulent Wake-Boundary Interaction in a Translational Cascade of Airfoils and Flat Plate Xiaodong Ruan 1,*, Xu Zhang 1, Pengfei Wang 2 , Jiaming Wang 1 and Zhongbin Xu 3 1 State Key Laboratory of Fluid Power and Mechatronic Systems, Zhejiang University, Hangzhou 310027, China; [email protected] (X.Z.); [email protected] (J.W.) 2 School of Engineering, Zhejiang University City College, Hangzhou 310015, China; [email protected] 3 Institute of Process Equipment, Zhejiang University, Hangzhou 310027, China; [email protected] * Correspondence: [email protected]; Tel.: +86-1385-805-3612 Received: 24 July 2020; Accepted: 27 August 2020; Published: 31 August 2020 Abstract: Rotor stator interaction (RSI) is an important phenomenon influencing performances in the pump, turbine, and compressor. In this paper, the correlation-based transition model is used to study the RSI phenomenon between a translational cascade of airfoils and a flat plat. A comparison was made between computational results and experimental results. The computational boundary layer velocity is in reasonable agreement with the experimental velocity. The thickness of boundary layer decreases as the RSI frequency increases and it increases as the fluid flows downstream. The spectral plots of velocity fluctuations at leading edge x/c = 2 under RSI partial flow condition f = 20 Hz and f = 30 Hz are dominated by a narrowband component. RSI frequency mainly affects the turbulence intensity in the freestream region. However, it has little influence on the turbulence intensity of boundary layer near the wall. A secondary vortex is induced by the wake–boundary layer interaction and it leads to the formation of a thickened laminar boundary layer.
    [Show full text]
  • The Dynamics of Static Stall
    16th Int Symp on Applications of Laser Techniques to Fluid Mechanics Lisbon, Portugal, 09-12 July, 2012 The Dynamics of Static Stall Karen Mulleners1∗, Arnaud Le Pape2, Benjamin Heine1, Markus Raffel1 1: Helicopter Department, German Aerospace Centre (DLR), Göttingen, Germany 2: Applied Aerodynamics Department, ONERA, The French Aerospace Lab, Meudon, France * Current address: Institute of Turbomachinery and Fluid Dynamics, Leibniz Universität Hannover, Germany, [email protected] Abstract The dynamic process of flow separation on a stationary airfoil in a uniform flow was investigated experimentally by means of time-resolved measurements of the velocity field and the airfoil’s surface pressure distribution. The unsteady movement of the separation point, the growth of the separated region, and surface pressure fluctuations were examined in order to obtain information about the chronology of stall development. The early inception of stall is characterised with the abrupt upstream motion of the separation point. During subsequent stall development, large scale shear layer structures are formed and grow while travelling down- stream. This leads to a gradual increase of the separated flow region reaching his maximum size at the end of stall development which is marked by a significant lift drop. 1 Introduction Stall on lifting surfaces is a commonly encountered, mostly undesired, condition in aviation that occurs when a critical angle of attack is exceeded. At low angles of attack a, lift increases linearly with a. At higher angles of attack an adverse pressure gradient builds up on the airfoil’s suction side which eventually forces the flow to separate and the lift to drop.
    [Show full text]