Heat induced transition of a stable vortex street R.N. Kieft, C.C.M. Rindt * , A.A. van Steenhoven Faculty of Mechanical Engineering, Eindhoven University of Technology, WH-3.129, P.O. Box 513, Eindhoven 5600 MB, The Netherlands Received 10 January 2001; received in revised form 8 August 2001 Abstract A combined numerical and experimental investigation is presented showing the effect of heat on the stability of a horizontal vortex street for Re D ¼ 75 and Ri D between 1 and 2. Detailed 2D-HiRes PV experiments are compared with 2D numerical results. The results show that an early transition to 3D of the vortex street takes place for Ri D > 1. Furthermore, the location at which the 2D wake becomes essentially 3D turns out to be dependent on Ri D . For an increasing addition of heat (increasing Ri D ), this location is shifted into the direction of the cylinder. By applying 3D measuring techniques, such as 3D-PTV and 3D visualisation techniques, the transition process and the behaviour downstream of the transition point are investigated. Ó 2002 Elsevier Science Ltd. All rights reserved. Keywords: Cylinder; Transitional; Wakes 1. Introduction and problem definition The stability of wake flows is of interest for several reasons. From a fundamental point of view the predic- tion of the transition from a 2D to a 3D state was and still is a big issue for many researchers. Besides, from a more practical point of view also a strong interest exists in the (in)stability of wake flows. Considering for ex- ample heat exchange equipment, a transition of the wake inherently means an increase of exchange rate between the wake flow and its surrounding environment. Consequently, heat dissipating components positioned in this wake will probably experience an increased cooling efficiency. The stability of the wake is strongly dependent on the Reynolds number ðRe D Þ. For moderate Reynolds num- bers ðRe D < 150Þ and no heat addition, the shed vortex structures form a strongly linked pattern or vortex street. This vortex street turns out to be very stable. For 50 < Re D < 100 it even remains stable and 2D until the vortices are fully dissipated [1]. For higher Reynolds numbers ðRe > 150Þ, several investigations show that the vortex street starts to transit, triggered by spanwise disturbances. Depending on Re D these disturbances re- sult in an a or b mode instability [2]. For 50 < Re D < 150 a vortex street transition can be exhilarated by external sources, for example, by adding a vibration to the cylinder. For frequencies close to the natural vortex shedding frequency and for sufficiently large amplitudes, the vortex structures become de- formed. This can eventually lead to an early breakdown of the vortex street [3–5]. Also addition of heat will have an effect on the vortex street stability. Considering the upward flow around a heated cylinder, it turns out that heat stabilises the vortex street characteristics [6–8]. First a slight increase in the vortex shedding frequency can be observed but a further increase of heat addition results in the formation of two steady twin vortices. For a horizontal vortex street behind a heated cylinder it is found that addition of heat results in a disturbed vortex street eventually giving rise to an early transition [9]. The present investigation focuses on the early tran- sition of the horizontal vortex street due to the addition of heat. The problem to be investigated (Fig. 1(a)) consists of a horizontal heated cylinder with constant temperature T 1 which is exposed to a uniform horizontal International Journal of Heat and Mass Transfer 45 (2002) 2739–2753 www.elsevier.com/locate/ijhmt * Corresponding author Tel.: +31-40-247-2978; fax: +31-40- 243-3445. E-mail address: c.c.m.rindt@tue.nl (C.C.M. Rindt). 0017-9310/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII:S0017-9310(01)00356-8