Rn René Kieft

On the wake structure behind a heated horizontal cylinder in cross-flow

This paper describes a numerical and experimental study of the effect of heat input on the behaviour of the vortices shed from a horizontal cylinder in a horizontal cross-flow. The Reynolds number ( Re D ) is fixed at 75, while the Richardson number ( Ri D ) is varied between 0 and 1 (corresponding to forced and mixed convection, respectively). In this parameter regime the wake consists of a double row of alternately shed vortices. A rather unexpected effect of the induced heat is the downward motion of the shed vortex structures. Detailed experiments and numerical simulations show that this effect is caused by the difference in strength between the two vortex rows. An analysis of the vorticity sources present during the formation process shows that the thermally induced baroclinic vorticity production is mainly responsible for this.

Heat induced transition of a stable vortex street

A combined numerical and experimental investigation is presented showing the effect of heat on the stability of a horizontal vortex street for ReD=75 and RiD 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 RiD>1. Furthermore, the location at which the 2D wake becomes essentially 3D turns out to be dependent on RiD. For an increasing addition of heat (increasing RiD), 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.

The wake behaviour behind a heated horizontal cylinder

The behaviour of vortex structures shed from a heated cylinder is experimentally investigated by means of 2-D particle tracking velocimetry. Within this investigation the ReD number was chosen to be 73. The RiD number, the dimensionless number which presents the relative importance of the induced heat, varies between 0 and 1. The experiments were carried out in a large towing tank where the disturbances caused by boundary layers could be minimised. The results show that for small RiD numbers the induced heat results in a deflection of the vortex street in negative y-direction. Within the vortex street a linking of two subsequently shed vortices occurs where the vortex shed from the lower half of the cylinder rotates around the vortex shed from the upper half. These phenomena are assumed to be caused by a strength difference between the vortices shed from the upper half of the cylinder and the lower half. For RiD=1 the effect of the induced heat and buoyancy becomes even more pronounced resulting in a more upwards directed vortex street.