Page 95 - Thermal Hydraulics Aspects of Liquid Metal Cooled Nuclear Reactors

Rod bundle and pool-type experiments in water serving liquid metal reactors 69

Moreover, they studied the mixing between two subchannels by using a salt tracer.
Among other things, they found that the contribution of these vortex streets to cross-
flow mixing is approximately of the same order of magnitude as the mixing by tur-
bulence alone. The vortices are formed by the presence of an inflection point in
the average, axial velocity profile, showing similarities with instability that leads to
vortex streets or turbulence. The vortex streets alternate on each side of the gap
and interact as if they consist of connected gears. In this way, alternating, laterally
aligned flows are formed capable of transporting energy from one side to the other.
It should be noted that, due to its oscillatory and regular nature, flow pulsations cannot
be detected from a time-averaged signal.

3.1.3.4 Fluid-structure interaction
Although this may sound counterintuitive, a flow, which is aligned in parallel with the
rods in a rod bundle (like the ones we have in nuclear reactors), causes these rods to
move in the lateral direction. This type of FSI is called flow-induced vibrations and
canbefurtherclassifiedasinstability-inducedexcitationsorextraneouslyinducedexci-
tations. The first excitation refers to the presence of instabilities in the flow such as peri-
odic flow pulsations or vortex shedding behind a bluff body, the second to fluctuations
in the flow or pressure by, for example, turbulence or externally applied flow oscilla-
tions (Paı ¨doussis, 2004). Paı ¨doussis (1966) showed that a flexible rod, pinned on both
sides, bends and oscillates, depending on the stress applied to the rod and the velocity of
theflow.Fig.3.1.8showsthecriticalvelocities atwhichbucklingandoscillationsoccur
as a function of this stress. Frequencies are recorded as well by using the eye.

25
20
w co
15

u 10
w co
12 u co

8 u cb

4

0
0 40 80 120 160
G

Fig. 3.1.8 Critical velocities for buckling (u cb ), oscillations (u oc ), and frequencies (ω co )asa
function of the stress Γ applied to the rod.
(Adopted from Paı ¨doussis, M.P., 1966. Dynamics of flexible slender cylinders in axial flow part
2. Experiments. J. Fluid Mech. 26(4), 737–751.)