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

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