Abstract
The characteristics of three-dimensional flow structures (scars and striations) resulting from the interaction between a heterostrophic vortex pair in vertical ascent and a clean free surface are described. The flow features at the scar-striation interface (a constellation of whirls or coherent vortical structures) are investigated through the use of flow visualization, a motion analysis system, and the vortex-element method. The results suggest that the striations are a consequence of the short wavelength instability of the vortex pair and the helical instability of the tightly spiralled regions of vorticity. The whirls result from the interaction of striations with the surface vorticity. The whirl-merging is responsible for the reverse energy cascade leading to the formation and longevity of larger vortical structures amidst a rapidly decaying turbulent field.
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Abbreviations
- A c :
-
Area of a vortex core (Fig. 6b)
- AR :
-
Aspect ratio of the delta wing model
- B :
-
base width of delta wing
- b 0 :
-
initial separation of the vortex couple
- d 0 :
-
depth at which the vortex pair is generated
- c :
-
average whirl spacing in the x-direction
- E :
-
energy density
- Fr :
-
Froude number (\(\left( {{{ = V_0 } \mathord{\left/ {\vphantom {{ = V_0 } {\sqrt {g b_0 } }}} \right. \kern-\nulldelimiterspace} {\sqrt {g b_0 } }}} \right)\))
- g :
-
gravitational acceleration
- L :
-
length of the scar band
- L ko :
-
length of the Kelvin oval
- N w :
-
number of whirls in each scar band
- P c :
-
Perimeter of a vortex core
- q :
-
surface velocity vector
- r c :
-
core size of the whirl ( = 2A c/P c)
- Re :
-
Reynolds number ( = \(\left( {{{ = V_0 } \mathord{\left/ {\vphantom {{ = V_0 } {{{v = \Gamma _0 } \mathord{\left/ {\vphantom {{v = \Gamma _0 } {2\pi v}}} \right. \kern-\nulldelimiterspace} {2\pi v}}}}} \right. \kern-\nulldelimiterspace} {{{v = \Gamma _0 } \mathord{\left/ {\vphantom {{v = \Gamma _0 } {2\pi v}}} \right. \kern-\nulldelimiterspace} {2\pi v}}}}} \right)\))
- Rnd :
-
a random number
- s :
-
inboard edge of the scar front (Fig. 6 a)
- t :
-
time
- u :
-
velocity in the x-direction
- υ:
-
velocity in the y-direction
- V b :
-
velocity imposed on a scar by the vortex couple (Fig. 6 a)
- V 0 :
-
initial mutual-induction velocity of the vortex couple (=Γ0/2πb 0)
- V t :
-
tangential velocity at the edge of the whirl core
- w :
-
width of the scar front (Fig. 6 a)
- z :
-
complex variable
- z k :
-
position of the whirl center
- α :
-
half included angle of V-shaped scar band
- β :
-
wave number
- Г m :
-
initial mean circulation of the whirls
- Г 0 :
-
initial circulation of the vortex pair
- Г w :
-
circulation of a whirl
- γ min :
-
minimum survival strength of a whirl
- Δt :
-
time step
- gDz :
-
increment of z
- gDζ :
-
change in vorticity
- δ :
-
cut-off distance in velocity calculations
- ɛ :
-
critical merging distance
- κ :
-
curvature of the surface
- λ :
-
wavelength
- ν :
-
kinematic viscosity
- ω :
-
angular velocity of a whirl core
References
Batchelor, G. K. 1967: An introduction to fluid dynamics. Camb. Univ. Press
Couder, Y.; Basdevant, C. 1986: Experimental and numerical study of vortex couples in two-dimensional flows. J. Fluid Mech. 173, 225–251
Crow, S. C. 1970: Stability theory for a pair of trailing vortices. AIAA J. 8, 2172–2179
Dritschel, D. G. 1986: The nonlinear evolution of rotating configurations of uniform vorticity. J. Fluid Mech. 172, 157–182
Kochin, N. E.; Kibel, I. A.; Roze, N. V. 1964: Theoretical hydrodynamics. Interscience Publishers
Lamb, H. (Sir) 1945: Hydrodynamics. Dover Publications, (6th ed.), 221–224
Langmuir, I. 1938: Surface motion of water induced by wind. Science 87, 119–123
Leeker, R. E., Jr. 1988: Free surface scars due to a vortex pair. M. Sc. Thesis, Naval Postgraduate Sch., Monterey, CA
Marcus, D. L.; Berger, S. A. 1989: The interaction between a counter-rotating vortex pair in vertical ascent and a free surface. Phys. Fluids A1, 1988–2000
Merzkirch, W. 1974: Flow visualization. New York: Academic Press
Merzkirch, W. 1987: Techniques of flow visualization. AGARD-AG-302
Moore, D. W.; Saffman, P. G. 1973: Axial flow in laminar trailing vortices. Proc. Roy. Soc. A333, 491–508
Ohring, S.; Lugt, H. J. 1989: Two counter-rotating vortices approaching a free surface in a viscous fluid. R & D Rep. No. DTRC-89/013, David Taylor Res. Center, Bethesda, MD
Peltzer, R. D.; Garrett, W. D.; Smith, P. M. 1987: A remote sensing study of a surface ship wake. Int. J. Remote Sensing 8, 689–704
Rayleigh, L. 1916: On the dynamics of revolving fluids. Scientific Papers 6, 447–453, Cambridge Univ. Press
Rosenhead, L. 1930: The spread of vorticity in the wake behind a cylinder. Proc. Roy. Soc. A127, 590–612
Sarpkaya, T. 1983: Trailing vortices in homogeneous and density stratified media. J. Fluid Mech. 136, 85–109
Sarpkaya, T. 1985: Surface signatures of trailing vortices and large scale instabilities. Proc. Colloquium on Vortex Breakdown (Sonderforschungsbereich 25), Aachen, Germany (ed. Staufenbiel, R. W.) 145–187
Sarpkaya, T. 1986: Trailing-vortex wakes on the free surface. Proc. 16th Symp. on Naval Hydrodynamics, Washington D.C., 38–50
Sarpkaya, T. 1989: Computational methods with vortices — 1988 Freeman Scholar Lecture. J. Fluids Eng. 111, 5–52
Sarpkaya, T.; Elnitsky, J.; Leeker, R. E. 1988: Wake of a vortex pair on the free surface. Proc. 17th Symp. on Naval Hydrodynamics, Washington D.C., 53–60
Sarpkaya, T.; Henderson, D. O. 1984: Surface disturbances due to trailing vortices. Naval Postgraduate School Tech. Rep. No. NPS-69-84-004, Monterey, CA
Sarpkaya, T; Henderson, D. 1985: Surface scars and striations. AIAA Paper No. 85-0445, 1985
Sarpkaya, T.; Johnson, S. K. 1982: Trailing vortices in stratified fluids. Tech. Rep. No. NPS-69-82-003, Naval Postgraduate School, Monterey, CA
Scott, J. C. 1982: Flow beneath a stagnant film on water — the Reynolds ridge. J. Fluid Mech. 116, 283–296
Telste, J. G. 1989: Potential flow about two counter-rotating vortices approaching a free surface. J. Fluid Mech. 201, 259–278
Widnall, S. E. 1975: The structure and dynamics of vortex filaments. Ann. Rev. Fluid Mech. 7, 141–165
Yu, D.; Tryggvason, G. 1990: The free-surface signature of unsteady, two-dimensional vortex flows. J. Fluid Mech. 218, 547–572
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Sarpkaya, T., Suthon, P. Interaction of a vortex couple with a free surface. Experiments in Fluids 11, 205–217 (1991). https://doi.org/10.1007/BF00192746
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DOI: https://doi.org/10.1007/BF00192746