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.
Investigation on the effectiveness of helical strakes in suppressing VIV of flexible riser
Abstract The effectiveness of the strakes in suppressing the vortex-induced vibration (VIV) of a long flexible cylinder is investigated by varying the pitch (p) and height (h) of the strakes. Measurements of cross-flow (CF) vibration amplitude, in-line (IL) and CF frequency responses, and hydrodynamic forces are presented for low mass flexible cylinders with helical strakes. Cylinders with aspect ratio of 162 were applied with constant pre-tension to restrict the vibration in IL and CF directions. Six cases with about 180 runs were executed under subcritical Reynolds number (Re = 0.144to 1.384) of uniform flow. Calculation on the laminar boundary layer thickness around a circular cylinder was performed as a benchmark in deciding the height of strakes. Present study shows that at certain strakes’ height which is larger than the laminar boundary layer thickness, significant VIV mitigation is found. However, the suppression effectiveness of helical strakes on flexible cylinder is less notable compared to a rigid cylinder. The change on pitch of strakes affects the occurrence of lock-in region and prevents the frequency to switch into higher mode. On the other hand, increase on the height of strakes narrows the lock-in region and plays the most major role in suppressing the vibration of a cylinder. The hydrodynamics forces indicate an unexpected phenomenon where the bare cylinder possesses higher force coefficients compared to cylinders with helical strakes. However, large drag penalty is still found in a particular case at higher reduced velocity. The most effective configuration of strakes in terms of the dynamic responses is p = 10D and h = 0.15D model. However, model of p = 10D and h = 0.10D performs better in reducing the hydrodynamic forces.
High-Speed Infrared Radiometers*
The paper describes high-speed infrared radiometers utilizing thermistor infrared detectors. Instruments with 4- and 8-in. diam Cassegrain mirror optics are described. The method of operation and the calibration of these radiometers is discussed.The radiometers are uniformly sensitive from 0.7 to 35 μ. They have a response time of 16 msec and a sensitivity of the order of 10−10 w/cm−2 corresponding to a blackbody temperature differential of 0.01°C.The radiometers are used for quantitative measurements of infrared radiation and remote temperature determinations.