The perpendicular distance between the flats of the section was 609.6 mm with the tube being rigidly mounted across the middle of the section. 1(a), the test-section was octagonal in shape with the walls made of 19.05-mm-thick clear acrylic. The experiments were performed in an open-circuit, subsonic wind tunnel equipped with a variable speed controller. Extensive hot-wire measurements and correlation analysis are performed to delineate the effect of the fins on the streamwise evolution of the mean and fluctuating velocity profiles and on the correlation length of vortex shedding. The effect of fins is examined by comparing the wake characteristics with those of a bare tube whose diameter is the same as the root diameter of the fins. different number of fins/unit tube length) are tested in a wind tunnel at two Reynolds numbers in the subcritical regime (2.61×10 4 and 4.98×10 4). Three tubes with similar fin height but different fin density (i.e. This paper focuses on some of these unresolved issues for the simple case of a finned tube exposed to cross-flow. These are the basic parameters that are needed to estimate the characteristics of unsteady fluid loading and sound generation by vortex shedding from finned tubes.
For example, it is not clear how the fins affect the streamwise development of the wake profile of turbulence intensity, or the correlation length of wake vortices. However, several unresolved issues still need to be investigated in order to improve our fundamental understanding of the effect of fin geometry on the mechanisms of sound generation and vortex-induced vibration of finned tubes. The above findings indicate clearly that coherent vortex shedding can occur in the wake of an isolated finned cylinder as well as inside arrays fabricated of finned tubes. Both of these effects are expected to be dependent on the fin pitch and height as well as on the spacing ratios between the tubes. These contradictions are not surprising since the fins are expected to influence not only the shedding phenomenon, but also sound attenuation (or acoustic damping) due to viscous losses between the fins. The increase in the sound pressure level due to the fins was as high as 20–30 dB. (1997) found the acoustic resonance of finned tube arrays to be much stronger than that occurring in bare tube arrays. While Kouba (1986) and Nemoto and Yamada, 1992, Nemoto and Yamada, 1994 suggested that the fins had no noticeable effect on the generation of noise in finned tube arrays, Nemoto et al.
(1991).įor the case of finned tube arrays, conflicting results have been reported.
Vortex-induced resonant vibration of a finned tube in water cross-flow has also been reported by Katinas et al. They found the spanwise scale of the wake vortices to be considerably larger than the pitch of the fins. More recently, similar features were observed by Hamakawa et al. (1975) reported that vortex shedding still occurs in the wake of finned tubes and found the shedding frequency to be correlated relatively well with the tube effective diameter, which is based on the projected frontal area of the cylinder. However, several studies of vortex shedding from finned tubes do not seem to support such expectations. Intuitively, if the fins were to be considered as “vortex spoilers”, or generators of streamwise vorticity, they ought to disturb the shed vortices, making them less coherent and three dimensional, and thereby weaken their effect as an excitation source. The majority of previous work dealt with bare tubes and very little attention has been given to finned tubes although various types of the latter are widely used in industrial applications such as air heaters, gas coolers and boilers (Reid and Taborek, 1994). The phenomenon of vortex shedding in the wake of isolated cylinders and inside tube arrays exposed to cross-flow has been investigated extensively, because it can cause large amplitude resonant vibrations or generate intolerable noise levels (Bloor and Gerrard, 1966 Blevins, 1984, Blevins, 1994 Weaver, 1993 Oengören and Ziada, 1998 Ziada and Oengören, 2000, Ziada and Oengören, 1992).