ON THE INSTABILITY OF WAKES BEHIND STATIONARY CIRCULAR CYLINDERS

The study on the formation of vortex streets behind stationary circular cylinders is of substantial sense in engineering, since vortex streets play the leading role in introduction of vibration, generation of noise and wake instability, etc.. In the present paper, the Orr-Sommerfeld equation combined with measured velocity profiles is used to investigate the type of instability behind stationary cylinders, which determines the mechanism of vortex formation. The numerical calculations for Reynolds numbers of 56-140 000 imply that there is a range around the end of the dead water region in cylinder wakes where the instability belongs to the absolute type. Beyond the range, the flows show to be of the convective type. On the other hand, the flows tend to be of the convective instability when Reynolds number is below the subcritical value of forming vortex streets. All of the correspondent Strouhal numbers agree with experimental data very well. The formation of vortex streets behind stationary cylinders is proposed to be caused by an absolute instability in the near wake. There is always an absolute instability region for the Reynolds numbers from 56-140,000. Further, the experimental manipulation of vortex streets according to the stability analysis mentioned above is proved to be very effective.

WKBJ analysis in the periodic wake of a cylinder

The nature of the three-dimensional transition arising in the flow past a cylinder is investigated by apply- ing the Lifschitz-Hameiri theory along special Lagrangian trajectories existing in its wake. Results show that the yon K^rm^n street is unstable with regard to short-wavelength perturbations. The asymptotic analysis predicts the possible existence of both synchronous （as modes A and B） and asynchronous （as mode C） instabilities, each associated to specific Lagrangian orbits. The proposed study provides useful qualitative information on the origin of the different modes but no quantitative agreement between the growth rates predicted by the asymptotic analysis and by a global stability analysis is observed. The reasons for such mismatch are briefly discussed and possible improvements to the present analysis are sug- gested.

Molecular tagging techniques and their applications to the study of complex thermal flow phenomena

This review article reports the recent progress in the development of a new group of molecule-based flow diagnostic techniques, which include molecular tag- ging velocimetry （MTV） and molecular tagging thermometry （MTT）, for both qualitative flow visualization of thermally induced flow structures and quantitative whole-field mea- surements of flow velocity and temperature distributions. The MTV and MTT techniques can also be easily combined to result in a so-called molecular tagging velocimetry and ther- mometry （MTV＆T） technique, which is capble of achieving simultaneous measurements of flow velocity and temperature distribution in fluid flows. Instead of using tiny particles, the molecular tagging techniques （MTV, MTT, and MTV＆T） use phosphorescent molecules, which can be turned into long-lasting glowing marks upon excitation by photons of appropriate wavelength, as the tracers for the flow veloc- ity and temperature measurements. The unique attraction and implementation of the molecular tagging techniques are demonstrated by three application examples, which include： （1） to quantify the unsteady heat transfer process from a heated cylinder to the surrounding fluid flow in order to exam- ine the thermal effects on the wake instabilities behind the heated cylinder operating in mixed and forced heat convec- tion regimes, （2） to reveal the time evolution of unsteady heat transfer and phase changing process inside micro-sized, icing water droplets in order to elucidate the underlying physics pertinent to aircraft icing phenomena, and （3） to achievesimultaneous droplet size, velocity and temperature measure- ments of ＂in-flight＂ droplets to characterize the dynamic and thermodynamic behaviors of flying droplets in spray flows.

VANISHING OF THREE-DIMENSIONALITY IN THE WAKE BEHIND A ROTATIONALLY OSCILLATING CIRCULAR CYLINDER

The flow behind a three-dimensional rotationally oscillating circular cylinder was studied by a numerical method. The computations were performed at a Reynolds number of 260, which is at a level that the flow wake has developed into a three-dimensional state called Mode-B. The purpose of this paper is to examine the influence of various rotational amplitudes （0.1-0.7） on the wake instability of the flow, while the oscillation frequency is fixed to the value of that measured in the wake of a stationary cylinder. The results show that the rotation with sufficiently high amplitude brings the flow back to its nominal two-dimensional state. Moreover, it is found that the value of the time-averaged drag and the RMS value of the lift are larger than those of a stationary circular cylinder.

Linear stability analysis of interactions between mixing layer and boundary layer flows

The linear instabilities of incompressible confluent mixing layer and boundary layer were analyzed.The mixing layers include wake,shear layer and their combination.The mean velocity profile of confluent flow is taken as a superposition of a hyperbolic and exponential function to model a mixing layer and the Blasius similarity solution for a flat plate boundary layer.The stability equation of confluent flow was solved by using the global numerical method.The unstable modes associated with both the mixing and boundary layers were identified.They are the boundary layer mode,mixing layer mode 1（nearly symmetrical mode）and mode 2（nearly anti-symmetrical mode）.The interactions between the mixing layer stability and the boundary layer stability were examined.As the mixing layer approaches the boundary layer,the neutral curves of the boundary layer mode move to the upper left,the resulting critical Reynolds number decreases,and the growth rate of the most unstable mode increases.The wall tends to stabilize the mixing layer modes at low frequency.In addition,the mode switching behavior of the relative level of the spatial growth rate between the mixing layer mode 1 and mode 2 with the velocity ratio is found to occur at low frequency.