Physical and Numerical Visualizations of Unsteady Separated Flows Separated Flows

Transcription

1 Physical and Numerical Visualizations of Unsteady Separated Flows Separated Flows Fathi Finaish Department of Mechanical and Aerospace Engineering Missouri University of Science and Technology Great Midwestern Regional Space Grant Consortia Meeting Cleveland, Ohio September 24-25, 2009

2 The Hubble Space Telescope snapped this image of a dying star that looks like a delicate butterfly 2

3 NASA releases new Hubble photos This image released by NASA on Wednesday (9/9/09) was taken by the refurbished Hubble Space Telescope. It shows stars bursting to life in the chaotic Carina Nebula. 3

4 Hubble Space Telescope Servicing Mission: The Soft Capture Mechanism Instrument replacement Battery replacement Gyro replacement Installing the Soft Capture Mechanism (SCM). 4

5 Jacopo Frigerio Lockheed Martin Space Systems Hubble Servicing Mission Program Chief Engineer developing the Soft Capture Mechanism Jacopo Frigerio is a Senior Staff Engineer and Project Manager with Lockheed Martin Space Systems Company s (LMSSC) Engineering Resources & Development organization. Jacopo has 14 years of engineering, design, project management and leadership experience at Lockheed Martin Space Systems Company (SSC), including 8 years in propulsion engineering and 7 years as a project manager leading teams in developing test, prototype and flight hardware. He is also a graduate of the Advanced Technical Leadership Program at SSC. Most recently he has been on a 1 year assignment in the Engineering Resources and Development organization, working to improve Workforce Development and Knowledge Management. 5

6 Unsteady Separated Flows Separated flows are encountered in many engineering applications Low and high angle of attack aircraft maneuvers Turbo machinery Flutter/Transonic flows Aircraft stall Rocket launches.. 6

7 An example Separated flows are encountered in many practical applications Low and high angle of attack aircraft maneuvers Turbo machinery Flutter Aircraft stall Rocket launches.. unsteady turbulent flow past a high-lift airfoil 7

8 Steady Attached vs. Unsteady Separated Flows Attached Flows: Traditional Aerodynamics Keep flow attached Maximize performance (Lift to Drag Ratio) Control boundary layer flow developments Separated Flows: New Directions Force flow separation Encourage subsequent vortex formation Control Vortex development and make use of it Enhance performance (maneuverability/agility, flow mixing) 8

9 Characteristics Unsteady Three-Dimensional Viscous Turbulent. Complicated flows physics Large parameter space Extensive requirement of computational resources 9

10 Limits Complex flow physics associated with these flows coupled with extensive requirement of computational resources are limiting even ambitious studies to spot investigations. The lack of validation sources prevented calibrations and economic use of computational resources. Thus, despite the availability of modern CFD tools, using them in computing separated flows remains to be difficult and expensive. 10

11 A few Examples 11

12 High Lift Systems Today CFD is used to design complex high lift systems; however, the prediction of CL max by direct computation is still difficult. Confluent boundary layers and flow separation are at the center of the difficulties that computer code developers and designers of high-lift airfoils must deal with. 12

13 The triple-slotted flap system used on the 737 Schematic of Geometry Configuration CFD Simulation 13

14 Multiple jets Flow Interactions near Ground JSF Vertical Takeoff CFD Simulation For multiple jets, an upward flow is created. 14

15 Unsteady Separated Flows: An Example

16 Development of an Accelerating Flow Over A High Angle of Attack Airfoil L.E T.E 16

17 Development of an Accelerating flow over a high angle of attack aircraft wing 17

18 Two-Dimensional & Corresponding Three- dimensional Flow development downstream of an accelerated wing model at high angle of attack L.E Vortex Development T.E Vortex Sheet Development 18

19 Development of a vortex sheet downstream of an accelerated wing at 20 degrees angle of attack 19

20 Turbulence 20

21 Turbulence Initial Stages 21

22 Evolution of Turbulent Spot 22

23 Evolution of Turbulent Spot 23

24 Evolution of Turbulent Spot 24

25 Applications and Limit of Modern CFD Codes in Predicting Separated Flow Developments: A Few Examples 25

26 Case Study A Computational Study of the Flow Fields Around Supersonic Airfoils at Low Mach Numbers 26

27 Acknowledgements The support for this projects was provided by NASA Calmar Research UMR 27

28 Motivation This work is motivated by the recent efforts on the development of Supersonic Business Jets Lack of available data on the development of separation bubbles over supersonic airfoils at low Mach numbers 28

29 Research Goals Develop computational procedure to analyze the development of separated bubbles over supersonic airfoils. Examine the influence of the bubbles on the aerodynamic performance of several airfoils at low Mach numbers. Conduct computational parametric study and compare produced results with the limited available experimental measurements 29

30 Laminar Separation Bubble (LSB) Physics of Formation Geometry Induced Adverse Pressure Gradient Reversal of flow in boundary layer Effect on Aerodynamic Performance Increase in drag Onset of Stall Promotion of Unsteady Flow 30

31 Schematic of Separation Bubble 31

32 Grid Generators Employed Surface Grid Generation Aerodynamic Grid and Paneling System (AGPS) Volume Grid Generation (Chimera Grid Tools) 32

33 Typical Grids Even Density- 325,440 Total Pts Generated around the Double Wedge Airfoil Sharp Leading Edge 4.23% Thick Max Thickness at Mid- Chord Top Front- 229,400 Total Pts 33

34 325,440 Total Grid Points 34

35 Flow Visualizations as Depicted by The Flow Streamlines α = 3.8 o, Re = 5.8 X 10 6, M =

36 Visualizations of Velocity Profiles And Pressure Distribution α = 3.8 o, Re = 5.8 X 10 6, M =

37 Sample of Surface Pressure Distribution α = 2 o,re = 5.8 X 10 6, M =

38 Sample of Surface Pressure Distribution α = 3 o, Re = 5.8 X 10 6, M =

39 Comparison of Bubble over Different Airfoil Configurations 39

40 CFD simulation of separation bubble development and subsequent massive separation over a supersonic airfoil at low Mach number. Re = 5.8x10 6, M =

41 Conclusions Attached regions are well predicted (Good agreement outside of LSB region) Pressure plateau is not as prominent as experimental data suggests Fully Turbulence model implemented over entire domain correlated best with experimental results 41

42 Current Research Efforts Collection of reliable experimental data 3-D D Modeling Bubble Modification Techniques 42

43 Case Study A Computational Study of Three-Dimensional Flow Fields in A Mixing Chamber 43

44 Acknowledgment This project was funded by in part by Ruskin Research Laboratory ASHRAE TRP-1045 Verifying Mixed Air Damper Temperature and Air Mixing Characteristics 44

45 Motivation The purpose of the study was to determine whether mixed air dampers perform their intended functions of the mixing of two streams of air with dissimilar temperatures and CO concentration. 2 45

46 Schematic of the Flow Problem: A Typical Flow Through Air Handling Unit 46

47 Background Thermal stratification of the mixed outside and return air streams results in regions of hot and cold air on the conditioning coils. If the outside air is < 32 F, more serious conditioning coil freezeup and/or tube rupture could occur. 47

48 Research Goals Develop a CFD model that is useful for studying thermal mixing and flow development. Use the CFD model to analyze the relationship between thermal mixing and input parameters of damper blade angles and flow velocity ratio. Identify the flow developments in the mixing chamber that have primary influence on the thermal mixing. 48

49 Test Facility 49

50 Test Facility 50

51 Temperature Sensors at the Mixed Air Station 51

52 Comparison with Experimental Measurements 52

53 Temperature distribution for best mixing: Configuration 1 PLOT OF TEMPERATURE DISTRIBUTION [Best Case] Confiuration 1 Percent Outside Air: 15 Test No: 7 Blade Size of Damper: 6" Outside Air Temperature: 37.0 F Return Air Temperature: 76.9 F Thermal Mixing Effectiveness- Range: 0.89 Statistical: y (inches) x (inches)

54 Temperature distribution for least mixing in Configuration 1 PLOT OF TEMPERATURE DISTRIBUTION [Worst Case] Confiuration 1 Test No: 6 Percent Outside Air: 30 Blade Size of Damper: 4" Outside Air Temperature: 35.2 F Return Air Temperature: 74.3 F Thermal Mixing Effectiveness- Range: 0.62 Statistical: y(inches) x (inches)

55 View of Typical Grid 26 Blocks, 174,160 Grid Points 55

56 Cross-section section of Mixing Chamber 56

57 An example shows dimensionless Flow Speed of outside airflow in the mixing chamber. VOA=1,500 fpm, OA damper angle = 0 57

58 Three-dimensional flow computation in a mixing Chamber. Dimensionless flow speed distribution 58

59 Visualization of vortex development in the mixing chamber. VRA=1,500 fpm 59

60 Flow visualization of outdoor and return airflows in the mixing chamber. 60

61 Samples of Temperature Distribution in the mixing chamber VOA=VRA=1,500 fpm : fra=foa=0 61

62 Comparison of Temperature Distributions VOA=VRA=1,500 fpm

63 Conclusions The temperature distribution in the Mixing chamber is determined Primarily by the Velocity Ratio. Temperature Mixing Effectiveness Increases with Downstream Distance The Mixing Effectiveness Generally Increases with Increasing Velocity Ratio and with Increasing Damper Angle 63

64 Final Concluding Remarks 64

65 Final Conclusions The two examples presented demonstrate the application of modern CFD tools in simulating external and internal flow configurations dominated by separated flows. While the simulation results correlated well with corresponding experimental measurements, flow computations in the separated bubble (example 1) and the upstream flow region deviated considerably from experimental measurements. The second example reveals the complex flow interactions between the two airflow streams as dominated by flow separation and subsequent vortex developments downstream in the mixing chamber. There still need for further experimental measurements, in order to assess the simulation results. 65

66 Final Conclusions Obtaining reliable experimental measurement and visualizations of separated flows would be useful resource for assessing, calibrating, and economizing the use of modern CFD tools. Experimental data needs to be collected with careful attention paid toward the free stream and wall conditions, as these are required for successful validations of parallel computational efforts. 66

67 Outlook Exploring possibilities of utilizing separated flows in many engineering application may lead to new fruitful opportunities in the future. For instance, new flow control technologies and techniques to control massive flow separation may open up new opportunities in making effective use of separated flows in a wide range of engineering applications. However, understanding of these flows still limited. Experimental and computational studies that been conducted on unsteady separated flows revealed complex flow physics associated with these flows and the challenge to document their time dependent nature. 67

68 fiftywithtime.wmv 68