Skip to main content
    • Aa
    • Aa

MTV measurements of the vortical field in the wake of an airfoil oscillating at high reduced frequency


We present an experimental investigation of the flow structure and vorticity field in the wake of a NACA-0012 airfoil pitching sinusoidally at small amplitude and high reduced frequencies. Molecular tagging velocimetry is used to quantify the characteristics of the vortex array (circulation, peak vorticity, core size, spatial arrangement) and its downstream evolution over the first chord length as a function of reduced frequency. The measured mean and fluctuating velocity fields are used to estimate the mean force on the airfoil and explore the connection between flow structure and thrust generation.

Results show that strong concentrated vortices form very rapidly within the first wavelength of oscillation and exhibit interesting dynamics that depend on oscillation frequency. With increasing reduced frequency the transverse alignment of the vortex array changes from an orientation corresponding to velocity deficit (wake profile) to one with velocity excess (reverse Kármán street with jet profile). It is found, however, that the switch in the vortex array orientation does not coincide with the condition for crossover from drag to thrust. The mean force is estimated from a more complete control volume analysis, which takes into account the streamwise velocity fluctuations and the pressure term. Results clearly show that neglecting these terms can lead to a large overestimation of the mean force in strongly fluctuating velocity fields that are characteristic of airfoils executing highly unsteady motions. Our measurements show a decrease in the peak vorticity, as the vortices convect downstream, by an amount that is more than can be attributed to viscous diffusion. It is found that the presence of small levels of axial velocity gradients within the vortex cores, levels that can be difficult to measure experimentally, can lead to a measurable decrease in the peak vorticity even at the centre of the flow facility in a flow that is expected to be primarily two-dimensional.

Corresponding author
Email address for correspondence:
Linked references
Hide All

This list contains references from the content that can be linked to their source. For a full set of references and notes please see the PDF or HTML where available.

D. G. Bohl & M. M. Koochesfahani 2004 MTV measurements of axial flow in a concentrated vortex core. Phys. Fluids. 16 (9), 4185.

R. K. Cohn & M. M. Koochesfahani 1993 Effect of boundary conditions on axial flow in a concentrated vortex core. Phys. Fluids. A 5 (1), 280.

R. K. Cohn & M. M. Koochesfahani 2000 The accuracy of remapping irregularly spaced velocity data onto a regular grid and the computation of vorticity. Experiments Fluids 29, S61.

J. O. Dabiri , S. P. Colin , J. H. Costello & M. Gharib 2005 Flow patterns generated by oblate medusan jellyfish: field measurements and laboratory analyses. J. Exp. Biol. 208, 1257.

P. Freymuth 1988 Propulsive vortical signature of plunging and pitching airfoils. AIAA J. 26, 881.

C. P. Gendrich & M. M. Koochesfahani 1996 A spatial correlation technique for estimating velocity fields using molecular tagging velocimetry (MTV). Experiments Fluids 22, 67.

C. P. Gendrich , M. M. Koochesfahani & D. G. Nocera 1997 Molecular tagging velocimetry and other novel applications of a new phosphorescent supramolecule. Experiments Fluids 23, 361.

J. Katz & D. Weihs 1978 Behavior of vortex wakes from oscillating airfoils. J. Airc. 15 (12), 861.

M. M. Koochesfahani 1989 Vortical patterns in the wake of an oscillating airfoil. AIAA J. 27, 1200.

J. Lighthill 1975 Mathematical Biofluiddynamics, SIAM.

H. Liu & K. Kawachi 1999 A numerical study of undulatory swimming. J. Comput. Phys. 155, 223.

W. J. McCroskey 1982 Unsteady airfoils. Annu. Rev. Fluid. Mech. 14, 285.

R. Ramamurti & W. Sandberg 2001 Simulation of flow about flapping airfoils using finite element incompressible flow solver. AIAA J. 39, 253.

M. Rosén , G. R. Spedding & A. Hedenström 2004 The relationship between wingbeat kinematics and vortex wake of a thrush nightingale. J. Exp. Biol. 207, 4255.

G. R. Spedding , A. Hedenström & M. Rosén 2003 Quantitative studies of the wakes of freely flying birds in a low-turbulence wind tunnel. Experiments Fluids 34, 291.

K. Streitlien & G. S. Triantafyllou 1998 On thrust estimates for flapping airfoils. J. Fluids Struct. 12, 47.

G. S. Triantafyllou , M. S. Triantafyllou & M. A. Grosenbaugh 1993 Optimal thrust development in oscillating foils with application to fish propulsion. J. Fluids Struct. 7, 205.

T. Von Kármán & W. R. Sears 1938 Airfoil theory for non-uniform motion. J. Aeronaut. Sci. 5 (10), 379.

M. C. Wilder , D. S. Mathioulakis , D. R. Poling & D. P. Telionis 1996 The formation and internal structure of coherent vortices in the wake of a pitching airfoil. J. Fluids Struct. 10, 3.

T. Y. Wu 1971 Hydromechanics of swimming of fishes and cetaceans. Adv. Appl. Mech. 11, 1.

Recommend this journal

Email your librarian or administrator to recommend adding this journal to your organisation's collection.

Journal of Fluid Mechanics
  • ISSN: 0022-1120
  • EISSN: 1469-7645
  • URL: /core/journals/journal-of-fluid-mechanics
Please enter your name
Please enter a valid email address
Who would you like to send this to? *