4 results
Dissipative small-scale actuation of a turbulent shear layer
- B. VUKASINOVIC, Z. RUSAK, A. GLEZER
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- Journal:
- Journal of Fluid Mechanics / Volume 656 / 10 August 2010
- Published online by Cambridge University Press:
- 02 June 2010, pp. 51-81
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The effects of small-scale dissipative fluidic actuation on the evolution of large- and small-scale motions in a turbulent shear layer downstream of a backward-facing step are investigated experimentally. Actuation is applied by modulation of the vorticity flux into the shear layer at frequencies that are substantially higher than the frequencies that are typically amplified in the near field, and has a profound effect on the evolution of the vortical structures within the layer. Specifically, there is a strong broadband increase in the energy of the small-scale motions and a nearly uniform decrease in the energy of the large-scale motions which correspond to the most amplified unstable modes of the base flow. The near field of the forced shear layer has three distinct domains. The first domain (x/θ0 < 50) is dominated by significant concomitant increases in the production and dissipation of turbulent kinetic energy and in the shear layer cross-stream width. In the second domain (50 < x/θ0 < 300), the streamwise rates of change of these quantities become similar to the corresponding rates in the unforced flow although their magnitudes are substantially different. Finally, in the third domain (x/θ0 > 350) the inviscid instability of the shear layer re-emerges in what might be described as a ‘new’ baseline flow.
4 - Drops and bubbles
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- By S. Chandra, C. T. Avedisian, M. P. Brenner, X. D. Shi, J. Eggers, S. R. Nagel, M. Tjahjadi, J. M. Ottino, PH. Marmottant, E. Villermaux, B. Vukasinovic, A. Glezer, M. K. Smith, A. Lozano, C. J. Call, C. Dopazo, D. E. Nikitopoulos, A. J. Kelly, D. Frost, B. Sturtevant, M. M. Weislogel, S. Lichter, M. Manga, H. A. Stone, J. Buchholz, L. Sigurdson, B. Peck
- M. Samimy, Ohio State University, K. S. Breuer, Brown University, Rhode Island, L. G. Leal, University of California, Santa Barbara, P. H. Steen, Cornell University, New York
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- Book:
- A Gallery of Fluid Motion
- Published online:
- 25 January 2010
- Print publication:
- 12 January 2004, pp 42-53
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Summary
The collision of a droplet with a solid surface
The photographs displayed above show the impact, spreading, and boiling history of n-heptane droplets on a stainless steel surface. The impact velocity, Weber number, and initial droplet diameter are constant (values of 1 m/s, 43 and 1.5 mm respectively), and the view is looking down on the surface at an angle of about 30°. The photographs were taken using a spark flash method and the flash duration was 0.5 μs. The dynamic behavior illustrated in the photographs is a consequence of varying the initial surface temperature.
The effect of surface temperature on droplet shape may be seen by reading across any row; the evolution of droplet shape at various temperatures may be seen by reading down any column. An entrapped air bubble can be seen in the drop when the surface temperature is 24°C. At higher temperatures vigorous bubbling, rather like that of a droplet sizzling on a frying pan, is seen (the boiling point of n-heptane is 98°C) but the bubbles disappear as the Leidenfrost temperature of n-heptane (about 200°C) is exceeded because the droplet become levitated above a cushion of its own vapor and does not make direct contact with the surface. The droplet shape is unaffected by surface temperature in the early stage of the impact process (t≤0.8 ms) but is affected by temperature at later time (cf. t≥ 1.6 ms) because of the progressive influence of intermittent solid-liquid contact as temperature is increased.
3 - Patterns
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- By G. E. Koppenwallner, D. Etling, C.-W. Leong, J. M. Ottino, E. Villermaux, J. Duplat, P. D. Weidman, V. O. Afenchenko, A. B. Ezersky, S. V. Kiyashko, M. I. Rabinovich, E. Bodenschatz, S. W. Morris, J. R. De bruyn, D. S. Cannell, G. Ahlers, C. F. Chen, F. Zoueshtiagh, P. J. Thomas, G. Gauthier, P. Gondret, F. Moisy, M. Rabaud, M. Fermigier, P. Jenffer, E. Tan, S. T. Thoroddsen, B. Vukasinovic, A. Glezer, M. K. Smith, N. J. Zabusky, W. Townsend, R. A. Hess, N. J. Brock, B. J. Weber, L. W. Carr, M. S. Chandrasekhara
- M. Samimy, Ohio State University, K. S. Breuer, Brown University, Rhode Island, L. G. Leal, University of California, Santa Barbara, P. H. Steen, Cornell University, New York
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- Book:
- A Gallery of Fluid Motion
- Published online:
- 25 January 2010
- Print publication:
- 12 January 2004, pp 28-41
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Summary
Vortex flows paint themselves
The artistlike pictures of vortex flows presented here have been produced by the flow itself. The method of this “natural” flow visualization can be described briefly as follows: The working fluid is water mixed with some paste in order to increase the viscosity. Vortex flows are produced by pulling a stick or similar devices through the fluid or by injecting fluid through a nozzle into the working tank.
The flow visualization is performed in the following way: the surface of the fluid at rest is sparkled with oil paint of different colors diluted with some evaporating chemical. After the vortex structures have formed due to wakes or jets, a sheet of white paper is placed on the surface of the working fluid, where the oil color is attached to the paper immediately. The final results are artistlike paintings of vortex flows which exhibit a rich variety of flow structures.
Mixing in regular and chaotic flows
These photographs show the time evolution of two passive tracers in a low Reynolds number two-dimensional timeperiodic flow. The initial condition corresponds to two blobs of dye, green and orange, located below the free surface of a cavity filled with glycerine. The flow is induced by moving the top and bottom walls of the cavity while the other two walls are fixed. In this experiment the top wall moves from left to right and the bottom wall moves from right to left; both velocities are of the form Usin2(2πt/T), with the same U and the same period T, but with a phase shift of 90°.
Vibration-induced drop atomization and bursting
- A. J. JAMES, B. VUKASINOVIC, MARC K. SMITH, A. GLEZER
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- Journal:
- Journal of Fluid Mechanics / Volume 476 / 10 February 2003
- Published online by Cambridge University Press:
- 17 March 2003, pp. 1-28
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A liquid drop placed on a vibrating diaphragm will burst into a fine spray of smaller secondary droplets if it is driven at the proper frequency and amplitude. The process begins when capillary waves appear on the free surface of the drop and then grow in amplitude and complexity as the acceleration amplitude of the diaphragm is slowly increased from zero. When the acceleration of the diaphragm rises above a well-defined critical value, small secondary droplets begin to be ejected from the free-surface wave crests. Then, quite suddenly, the entire volume of the drop is ejected from the vibrating diaphragm in the form of a spray. This event is the result of an interaction between the fluid dynamical process of droplet ejection and the vibrational dynamics of the diaphragm. During droplet ejection, the effective mass of the drop–diaphragm system decreases and the resonance frequency of the system increases. If the initial forcing frequency is above the resonance frequency of the system, droplet ejection causes the system to move closer to resonance, which in turn causes more vigorous vibration and faster droplet ejection. This ultimately leads to drop bursting. In this paper, the basic phenomenon of vibration-induced drop atomization and drop bursting will be introduced, demonstrated, and characterized. Experimental results and a simple mathematical model of the process will be presented and used to explain the basic physics of the system.