Journal ArticleDOI
The forced mixing layer between parallel streams
D. Oster,Israel Wygnanski +1 more
TLDR
The effect of periodic two-dimensional excitation on the development of a turbulent mixing region was studied experimentally in this article, where controlled oscillations of variable ampli- tude and frequency were applied at the initiation of mixing between two parallel air streams.Abstract:
The effect of periodic two-dimensional excitation on the development of a turbulent mixing region was studied experimentally. Controlled oscillations of variable ampli- tude and frequency were applied at the initiation of mixing between two parallel air streams. The frequency of forcing was at least an order of magnitude lower than the initial instability frequency of the flow in order to test its effect far downstream. The effect of the velocity difference between the streams was also investigated in this experiment. A typical Reynolds number based on the velocity difference and the momentum thickness of the shear layer was l04.It was determined that the spreading rate of the mixing layer is sensitive to periodic surging even if the latter is so small that it does not contribute to the initial energy of the fluctuations. Oscillations at very small amplitudes tend to increase the spreading rate of the flow by enhancing the amalgamation of neighbouring eddies, but at higher amplitudes the flow resonates with the imposed oscillation. The resonance region can extend over a significant fraction of the test section depending on the Strouhal number and a dimensionless velocity-difference parameter. The flow in the resonance region consists of a single array of large, quasi-two-dimensional vortex lumps, which do not interact with one another. The exponential shape of the mean-velocity distribution is not affected in this region, but the spreading rate of the flow with increasing distance downstream is inhibited. The Reynolds stress in this region changes sign, indicating that energy is extracted from the turbulence to the mean motion; the intensity of the spanwise fluctuations is also reduced, suggesting that the flow tends to become more two-dimensional.Amalgamation of large coherent eddies is resumed beyond the resonance region, but the flow is not universally similar. There are many indications suggesting that the large eddies in the turbulent mixing layer at fairly large Re are governed by an inviscid instability.read more
Citations
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Journal ArticleDOI
Micro-electro-mechanical-systems (mems) and fluid flows
Chih-Ming Ho,Yu-Chong Tai +1 more
TL;DR: The micromachining technology that emerged in the late 1980s can provide micron-sized sensors and actuators that can be integrated with signal conditioning and processing circuitry to form micro-electromechanical-systems (MEMS) that can perform real-time distributed control.
Journal ArticleDOI
The control of flow separation by periodic excitation
TL;DR: In this article, a review of the control of flow separation from solid surfaces by periodic excitation is presented, with an emphasis on experimentation relating to hydrodynamic excitation, although acoustic methods as well as traditional boundary layer control, such as steady blowing and suction are discussed in order to provide an appropriate historical context for recent developments.
Journal ArticleDOI
Vortex induction and mass entrainment in a small-aspect-ratio elliptic jet
Chih-Ming Ho,Ephraim Gutmark +1 more
TL;DR: In this article, a passive technique of increasing entrainment was found by using a small-aspect-ratio elliptic jet, which was several times greater than that of a circular jet or a plane jet.
Journal ArticleDOI
Combustion instability related to vortex shedding in dump combustors and their passive control
Klaus C. Schadow,Ephraim Gutmark +1 more
TL;DR: In this paper, a review summarizes recent research program related to the driving mechanism of dump combustor instability, including experimental research in dump combustors using gaseous fuel, and the emphasis here is on vortex shedding as a driving mechanism.
References
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Journal ArticleDOI
On density effects and large structure in turbulent mixing layers
Garry L. Brown,Anatol Roshko +1 more
TL;DR: In this article, Spark shadow pictures and measurements of density fluctuations suggest that turbulent mixing and entrainment is a process of entanglement on the scale of the large structures; some statistical properties of the latter are used to obtain an estimate of entrainedment rates, and large changes of the density ratio across the mixing layer were found to have a relatively small effect on the spreading angle.
Journal ArticleDOI
Orderly Structure in Jet Turbulence
S. C. Crow,F. H. Champagne +1 more
TL;DR: In this paper, the authors show that a large-scale orderly pattern may exist in the noiseproducing region of a round subsonic jet by observing the evolution of orderly flow with advancing Reynolds number.
Journal ArticleDOI
Vortex pairing : the mechanism of turbulent mixing-layer growth at moderate Reynolds number
C. D. Winant,F. K. Browand +1 more
TL;DR: A mixing layer is formed by bringing two streams of water, moving at different velocities, together in a lucite-walled channel as mentioned in this paper, where dye is injected between the two streams just before they are brought together, marking the vorticitycarrying fluid.
Journal ArticleDOI
On the inviscid instability of the hyperbolictangent velocity profile
TL;DR: In this paper, the Rayleigh stability equation of inviscid linearized stability theory was integrated numerically for amplified disturbances of the hyperbolic-tangent velocity profile.
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