Upgrade to Pro — share decks privately, control downloads, hide ads and more …

Wake of an accelerating foil in stratified environment

5b9ffdc3cae26470b5a3278b3c2ce4aa?s=47 Alex Liberzon
November 07, 2012

Wake of an accelerating foil in stratified environment

M.Sc. thesis seminar of Hadar Ben Gida, supervised by Alex Liberzon (Tel Aviv University) and Roi Gurka (Ben Gurion University) , performed at the Turbulence Structure Laboratory

5b9ffdc3cae26470b5a3278b3c2ce4aa?s=128

Alex Liberzon

November 07, 2012
Tweet

Transcript

  1. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   Hadar Ben-Gida Advisors: Roi Gurka Alex Liberzon u∞ (y,t)
  2. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   2/27   Outline     Ø  Introduction •  Stably stratification •  Accelerating flows Ø  Research goals Ø  Experimental setup Ø  Results •  Wake characterization •  Velocity profiles •  Momentum thickness •  Life cycle of a stratified wake Ø  Conclusions and future work  
  3. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   3/27   Introduc.on     Ø  Flows with a background density gradient (increase in density with depth) Ø  For example, flows in the ocean thermocline layer Ø  Underwater vehicles operate most efficiently in the thermocline Atlantic ocean characterization Flow types in the ocean
  4. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   4/27   Introduc.on     Ø  Studied behind towed spheres and cylinders (Lin and Pao 1979; Boyer 1989; Chomaz 1993; Spedding 1996) ) 10 ( Re 3 O D U = = ∞ ∞ ν ) 1 ( O ND U Fr = = ∞ ∞ dz d g N ρ ρ − = ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ sec rad Ø  Stratification effect ‘pancakes’ Wake evolution of a sphere in a stratified medium shown from above (Spedding et al. 1996) Stratified wake evolution behind a body
  5. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   5/27   Introduc.on     Ø  Two perspectives: •  Body is held fixed and the flow accelerates •  Body accelerates in stagnant fluid   Ø  For accelerating wings: (Freymuth 1985) (Sarpkaya 1991) ν 5 . 1 5 . 0 Re c a a = ∞ ∞ = t U c A p U∞ - Final steady velocity of the wing at the end of the acceleration period t∞ - Time at the end of the acceleration period
  6. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   6/27   Research   Goals   Engineering Perspective: Underwater vehicles Ø  Commercial - make detailed maps of the seafloor for oil gas industry Ø  Military - surveillance and monitoring missions Ø  Research - measuring the ocean thermocline layer properties Tend to accelerate/decelerate during their missions! Fluid Mechanics Perspective: How buoyancy forces affect the initial growth of von Kármán vortices behind a hydrofoil accelerated from rest?
  7. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   7/27   Experimental   Setup   20cm Acceleration - U(x, t) y x z ρ(y) 50cm 20cm Camera for the wake Double- Pulsed Laser Terminal steady speed - U∞ Shift Mechanism Pressure Inlet Camera: 4007x2672 pixel2 CCD 11bit , 4Hz Laser: Nd:YAG, double- head 532nm 60mJ/Pulse Aquarium Trigger Hydrofoil NACA 0015  
  8. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   8/27   Experimental   Setup     20cm Acceleration - U(x, t) y x z ρ(y) 50cm 20cm Camera for the wake Double- Pulsed Laser Terminal steady speed - U∞ Shift Mechanism Pressure Inlet Camera: 4007x2672 pixel2 CCD 11bit , 4Hz Laser: Nd:YAG, double- head 532nm 60mJ/Pulse Aquarium Trigger NACA 0015   Hydrofoil
  9. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   9/27   Experimental   Setup   Laser light sheet   20cm Acceleration - U(x, t) y x z ρ(y) 50cm 20cm Camera for the wake Double- Pulsed Laser Terminal steady speed - U∞ Shift Mechanism Pressure Inlet Camera: 4007x2672 pixel2 CCD 11bit , 4Hz Laser: Nd:YAG, double- head 532nm 60mJ/Pulse Aquarium Trigger NACA 0015   Hydrofoil
  10. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   10/27   Experimental   Setup    
  11. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   11/27   Experimental   Setup   Late trigger case t = 5.1 sec Early trigger case t = 3.2 sec An example of the PIV images that were taken during the experiments of fresh water with two different triggers; late trigger/wake and early trigger/wake
  12. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   12/27   Experimental   Setup   32X32 pixels2 interrogation window with 50% overlap 20cm 50cm 35cm 6cm 2.9 cm 14cm 10cm Camera Field of View PIV Analysis Area 5cm 7.5cm 7.5cm 8cm 18cm Fluid level Hydrofoil location at t=0sec Hydrofoil location when PIV images were taken Aquarium
  13. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   13/27   Experimental   Setup   32X32 pixels2 interrogation window with 50% overlap 20cm 50cm 35cm 6cm 3.3cm 27cm 10cm 3.75cm Camera Field of View PIV Analysis Area 5cm 7.5cm 7.5cm 8cm 18cm Fluid level Hydrofoil location at t=0sec Hydrofoil location when PIV images were taken Aquarium
  14. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   14/27   Experimental   Setup   ρ(y) x Aquarium Tank 1 Tank 2 Fresh Water C1 = 0 M V = 15 L C(t) V(t) NaCl Solution C(t=0) = 0.5 M V(t=0) = 11 L q1 q2 = 2q1 V-1 V-3 V-2 H = 20 cm L = 50 cm W = 20 cm ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ − ⋅ ⋅ + = ) 0 ( 2 ) 0 ( ) 0 ( 1 V C C y A C C y W L A ⋅ = Ø  Modified Oster method min / 0 . 1 1 L q = min / 5 . 0 2 L q =
  15. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   15/27   Experimental   Setup   Ø  Density meter 1012.0 1014.0 1016.0 1018.0 1020.0 1022.0 1024.0 1026.0 1028.0 1030.0 0 0.05 0.1 0.15 0.2 ρ [kg/m3] Height [m] t=00:35 hr t=00:42 hr t=00:49 hr t=00:56 hr t=01:03 hr t=01:10 hr t=01:17 hr t=01:24 hr Density meter device Density gradients measured by the density meter
  16. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   16/27   Experimental   Setup   Normalized hydrofoil velocity variation with time passed since the trailing edge was moved y = 0.1508x + 0.289 R² = 0.9709 y = 0.1776x + 0.249 R² = 0.9788 y = 0.4238x + 0.3325 R² = 0.9781 0.00 0.20 0.40 0.60 0.80 1.00 1.20 0.00 1.00 2.00 3.00 4.00 5.00 U/U∞ t [sec] U∞=10.3 cm/sec - Run 1 U∞=10.3 cm/sec - Run 2 U∞=10.3 cm/sec - Run 3 U∞=10.3 cm/sec - Run 4 U∞=10.3 cm/sec - Run 5 U∞=11.9 cm/sec - Run 1 U∞=11.9 cm/sec - Run 2 U∞=11.9 cm/sec - Run 3 U∞=26.3 cm/sec - Run 1 U∞=26.3 cm/sec - Run 2 U∞=26.3 cm/sec - Run 3 U∞=26.3 cm/sec - Run 4 U∞=26.3 cm/sec - Run 5 ∞ ∞ − = U U a 025 . 0 0171 . 0 2
  17. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   17/27   Results     ∑ = ∞ ∞ = n i avg i U n U 1 , 1 300 , 5 Re = ∞ 0 . 2 = ∞ Fr sec / 0 . 10 , cm U avg = ∞ 07 . 0 = p A 400 , 1 Re = a 1 sec 1 . 1 − = N 2 sec / 5 . 1 cm aavg = ∑ = = n i i avg a n a 1 1
  18. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   18/27   Results     ωz c/U∞ x/c=0 x/c=7.5 Hydrofoil at rest Hydrofoil at U∞ Laminar wake (Re=9,000) behind a NACA 0012 airfoil at 0o angle of attack as being visualized by a dye (Neuhart and Pendergraft 1988) Water - late
  19. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   19/27   Results     ωz c/U∞ x/c=0 x/c=7.5 Hydrofoil at rest Hydrofoil at U∞ Water - late ωz c/U∞ x/c=0 x/c=7.5 Hydrofoil at rest Hydrofoil at U∞ Stratification - late
  20. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   20/27   Results     Ωz c/U∞ x/c=0 x/c=7.5 Hydrofoil at rest Hydrofoil at U∞ Ωz c/U∞ x/c=0 x/c=7.5 Hydrofoil at rest Hydrofoil at U∞ ∑ = = Ω N i i z z y x N y x 1 ) , ( 1 ) , ( ω N - No. of PIV images Water - late Stratification - late
  21. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   21/27   Results     Local normalized streamwise velocity profiles (u’/U) of the ensemble wakes behind a hydrofoil accelerating through water and stratified layer (Re∞ =5,300). Squares - water; Asterisks - stratification x/c=0 Hydrofoil at U∞ Hydrofoil at rest ) ( ) , ( ) , ( ' x U y x u y x u + =
  22. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   22/27   Results     dy U u U u c x c y y ∫ = = ⎥ ⎦ ⎤ ⎢ ⎣ ⎡ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − ⋅ ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ = 2 0 ' 1 ' 1 ) ( θ Hydrofoil at U∞ Water (late) Stratification (late) Water (early) Stratification (early) Normalized momentum thickness with error bars along the ensemble wakes behind a hydrofoil accelerating through water and stratified layer (Re∞ =5,300)
  23. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   22/27   Results     dy dv dx du dx dv dy du y x Swirl ci − = = ) , ( λ ⇒ ∇ ) det( u  0.60 0.70 0.80 0.90 1.00 1.10 1.20 4.00 4.50 5.00 5.50 6.00 6.50 7.00 y/c x/c Water Stratification Vortices positions in the wake behind a hydrofoil at Re=5,300 0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 (b/Re∞ )·104 [cm] x/c Water Stratification Vertical distance between vortex cores, b, normalized to the steady state Reynolds number (5,300) in the wake behind a hydrofoil
  24. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   23/27   Results     Hydrofoil rest position for late wakes Hydrofoil at U∞ Hydrofoil rest position for early wakes 2D-NW NEQ 2D-FW Stratification   (late)   Stratification   (early)   Phase I Phase II Phase III Normalized momentum thickness with error bars along the ensemble wakes behind a hydrofoil accelerating through a stratified layer (Re∞ =5,300; Fr∞ =2.0)
  25. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   24/27   Conclusions     Ø  Stratification effects on the wake (compared to water): •  Vortices being stretch •  Lower spanwise vorticity values •  Less deficit and vertical extent of the wake •  Relatively short stratified wake life cycle DRAG reduction!   Higher efficiency for accelerating underwater vehicles   Smaller “finger tips”
  26. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   25/27   Future  Work     Ø  Change angle of attack (AoA) – how lift is being manifested by stratification? Ø  Repeat the experiments at higher accelerations Ø  Characterize the wake of a hydrofoil connected to a self-propelled body  
  27. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   26/27   Ø  Zach Taylor Ø  Hadar Traugott Ø  Gal Grinstein Ø  Turbulence Structure Laboratory team Ø  Funding agencies: Ø  Bi-National Science Foundation  
  28. Outline     Introduc.on     Research   Goals  

    Experimental   Setup   Results     Conclusions     Future  Work     Seminar  -­‐  Stra.fied  Wake  Behind  an  Accelera.ng  Hydrofoil    -­‐  Hadar  Ben-­‐Gida   07/11/12   27/27   Faculty of Engineering Department of Mechanical Engineering Faculty of Engineering School of Mechanical Engineering