N.W.M. Ko

Coaxial jets of different mean velocity ratios, part 2

This second part of the investigation presents the flow characteristics of coaxial jets of mean velocity ratios (inner to outer) 0·15 −1 −1 is also made. The inner potential core length, and hence the elongation function, is shown to play an important role in correlating data from coaxial jets of different λ −1 . At low λ −1 , coalescence of the coflowing-wake-vortices and their evolution into another train of vortices that resembles the wake-vortices found in the annular jet emphasizes the role of the coaxial jet as an intermediary between the single jet and the annular jet.

Experimental investigation of the structure interaction in an excited coaxial jet

Abstract On the basis of the conditional sampling measurement technique that incorporates the phase information between the flow signals, the development of the flow structures in the initial region of a coaxial jet with mean velocity ratio Λ −1 (inner to outer) of 0.3 under acoustic excitation at a frequency of 1.45 kHz and a forcing level of 107 dB were investigated. It was found that a preferred jet column mode for excitation existed. Vortical structures were found in the mixing layers of the jet. Interactions between these structures near the end of the outer potential core of the coaxial jet resulted in the formation of azimuthal structures. Also, the dominant structures beyond the end of the inner potential core were found to be generated from the interactions between the noncoalesced vortices in the outer mixing layer. Their dominance persisted for several diameters downstream of the end of the inner potential core. Results also revealed the importance of the relative positions of the vortices in the development of the coaxial jet.

Some covariance measurements in a subsonic jet

This paper presents covariance and phase velocity measurements in a subsonic cold jet. By using a digital technique, iso-contour maps of the correlation covariance, inside the potential cone and mixing region, yield information concerning the time averaged location of vortices and their velocity of convection.

Basic sound generation mechanisms in inviscid vortex interactions at low Mach number

The sound generation mechanisms during finite core vortex interactions at low Mach number are investigated in the present study. The theoretical deductions show clearly that the basic sound generation mechanisms are associated with the vortex core deformation and the vorticity centroid dynamics, independent of the vortex system. Such deductions are substantiated by numerical experiments with the interactions of two-dimensional vortices, vortex pairs and vortex rings. Detailed discussions on the similarities and differences between the sound generation processes of the two-dimensional and axisymmetric vortex systems are given. The relative importance of the two sound generation mechanisms in these vortex systems, their characteristics and interactions, which are hardly found in existing literature, are also examined. The present findings have also generalized and substantiated the previous results of the authors on the topic.

Responses of teachers to aircraft noise

Abstract Acoustic measurements of aircraft noise in 139 schools in Hong Kong have been carried out. The schools are located under and very near the flight-paths of aircraft coming in and leaving the international airport, Kai Tak. Coupled with the acoustic measurements, measurements of the subjective responses to this aircraft noise of 2100 Chinese teachers in these schools have been made. It is found that the subjective responses of the teachers correlate well with the Noise and Number Index. Besides the effect of annoyance, it is further found that the more serious effect of aircraft noise is the disruption of verbal communication, resulting in speech and teaching interference during lessons.

Interaction of flow structures within bistable flow behind two circular cylinders of different diameters

Abstract The bistable flow of two circular cylinders of diameter ratio 2:1 was investigated. Based on the quantitative and qualitative measurements, two modes of flow are observed. Interactions of the adjacent gap-side vortices with the outer vortices and the pairing of outer vortices in both modes form the important mechanisms for the development of flow structures and wakes further downstream.

Interaction of flows behind two circular cylinders of different diameters in side-by-side arrangement

Abstract Experiments were performed to study the fluid dynamics of flow past two cylinders of diameter ratio 1:2 in a side-by-side arrangement with a gap spacing of T / D = 1.5. Based on hot-wire measurements and flow visualization results, the most frequent mode of large-scale flow structures in the wake consists of two vortex streets developed from the two cylinders. Behind the big cylinder the “first” (outer) and “second” (inner) streets are isolated. The former has a higher convection velocity, which is the same as that of the small cylinders vortices, while the latter convects with a lower velocity. The big and small vortex streets interact in two stages, resulting in lateral transport of fluid across the entire widths of the two wake regions. A locked repetitive pattern of four big vortices with 10 small vortices is observed. A third spectral component behind the gap portion of the small cylinders wake region is the consequence of the first-stage interaction. The less frequent mode does not suggest interaction of the big and small vortices.

Further measurements in the initial region of an annular jet

Abstract This paper describes further the results of spectral and correlation measurements relevant to coherent structures in the initial region of a basic annular jet associated with the jet vortices and the wake induced vortices. Evidence has been found that for the wake induced vortices the first mode constituent ( m = 1) is more dominant in the region where their pressure intensity is high. Further downstream the first mode constituent becomes only slightly higher than that of the axisymmetric ( m = 0) mode. For the jet vortices during their generation and formation near the nozzle exit the first mode constituent is slightly more dominant than the axisymmetric constituent. Further downstream the axisymmetric constituent becomes more dominant. Nevertheless, fairly similar modal and basic characteristics of these two types of vortices are found.

Covariance measurements in the initial region of an annular jet

Abstract This paper describes the results of correlation measurements in the initial region of a basic annular jet, which is under the effect of the jet vortices and of the wake induced vortices. Evidence has been found that the wake induced vortices are excited by the disturbances associated with the wake vortices. For these wake induced vortices the azimuthal constituent (m = 1) is dominant. The similarity observed between the behaviour of the jet vortices and that of the wake induced vortices suggests that the jet vortices are also excited by these disturbances of the wake vortices, having the same Strouhal number. Thus the jet vortices are also dominated by the azimuthal constituent (m = 1). However, evidence has been found that within the first two outer diameters an axisymmetrical constituent (m = 0) is also present within the jet vortices. Comparison of the wake induced vortices with the jet vortices shows that these two helical waves are out of phase and are axially separated.

Further investigation in the responses of firemen to noise

Abstract A further study consisting of acoustic and subjective measurements of 552 Chinese firemen at 12 fire stations in Hong Kong has been carried out. Annoyance with aircraft and traffic noise conditions, as expressed by the firemen, was found to correlate well with the acoustic measurements. For aircraft noise the correlation of annoyance with the Number and Noise Index (NNI) was slightly better than with the dB(A) peak value. For traffic noise the similar correlation with the mean sound pressure levels which exceeds 10 % of the sampling period (L10) was slightly better than with the Noise Pollution Level and the Traffic Noise Index. The correlation of the arousal due to the aircraft and traffic noise was similarly found to depend on the NNI and L10 values. However, traffic noise was responsible for more disturbance than aircraft noise. The study demonstrated the desirability of adopting indoor acoustic measurements instead of outdoor measurements for any survey of this kind.