Effects of helical strakes on the vortex-induced vibration over a thermowell in a pipe flow
A thermowell is a device to protect a thermometer used for measuring temperature in many industrial application. The thermowell is exposed to vortex-induced vibration when the flow speed in a pipe is large so that shedding frequency due to Karman vortex streets downstream of the thermowell corresponds with its natural frequency. The vortex-induced vibration can cause fracture of the thermowell structure by cyclic loading. This phenomenon has long been a research topic in the field of marine engineering. Now, it can be a concern in metrology area. The present study investigates the suppressing effect of the vortex-induced vibrations of the thermowell by means of helical strakes. Flow visualizations can show that the helical strakes can suppress the vortex-induced vibration by generating three-dimensional shedding vortices with short spanwise length. The three-dimensional shedding vortices does not give resonance to the thermowell effectively because the vortices generate asynchronous forces along the thermowell. In this study, flow visualizations of the thermowell vibrations and resonant frequency measurements were attempted to investigate the suppressing effect of the helical strakes for safely using the thermowell in operating conditions with high flow speeds.
Computational study on aerodynamic mitigation of wind-induced, large-amplitude vibrations of stay cables with strakes
Abstract Modifications of circular cylinder surfaces, such as strakes and helical wires, effectively mitigate Karman vortex-induced vibrations normal to flow and have been applied to the reduction of large-amplitude vibrations of stay cables in bridges, which occur under wind oblique to a cable with or without rainfall. This aerodynamic control method cannot be fully effective without understanding the behavior of the flow around and the associated forces on oblique cables. To address this issue, flow around a yawed cylinder with various strake patterns was studied using three-dimensional detached eddy simulation (DES) at Reynolds number of 1.4×10 5 . Results demonstrated that strake patterns strongly influence the development of flow structures around a yawed cylinder and therefore the associated forces on the cylinder. The results suggest that particular strake patterns can mitigate large-amplitude and low-frequency vibrations of stay cables induced by oblique wind.
Strouhal numbers in the wake of two inline cylinders
AbstractThe dominant vortex frequencies
fs in the wake of two tandem circular cylinders of identical diameter d have been measured simultaneously using two hot wires placed behind each cylinder. Measurements were conducted over the Reynolds number
Re (≡U∞d/ν, where U∞ and
ν are the free-stream velocity and the kinematic viscosity of fluid, respectively) =800–4.2×104 and the cylinder centre-to-centre spacing L/d=1–15. The Strouhal number
St (≡fsd/U∞) exhibits a strong dependence on L/d and Re. For L/d<(L/d)c, which is a critical value and ranges between 3.5 and 5, there is no vortex street formed in the gap between the cylinders, and St measured behind the downstream cylinder drops rapidly for increasing L/d. For L/d>(L/d)c, co-shedding occurs, that is, vortices are shed from the upstream as well as the downstream cylinder, and their frequencies are found to be identical.
St climbs with increasing
L/d, approaching a constant between 0.18 and 0.22 for L/d>10. The
St–Re relationship is classified into four categories, based on their behaviours, which are associated with distinct flow physics—category 1: for 1≤L/d<2, the shear layers separated from the upstream cylinder roll up behind the downstream cylinder; category 2: for 2≤L/d≤3, there is a transition from the shear layer rollup behind to reattachment on the downstream cylinder; category 3: for 35 is characterized by co-shedding only. The present measurements reconfirm the previous observation of a bi-stable flow at the transition from the reattachment to co-shedding regime. It is found for the first time that another bi-stable flow occurs at the transition from category 1 to 2, that is, the stable reattachment co-exists with the stable rollup (behind the downstream cylinder) of shear layers separating from the upstream cylinder, resulting in two distinct vortex-shedding frequencies even at the same Re and L/d. The St behaviour is further discussed along with flow visualization using the laser-induced fluorescence technique.