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.
Experimental investigation of flow-induced vibration on isolated and tandem circular cylinders fitted with strakes
Abstract The effect of varying the geometric parameters of helical strakes on vortex-induced vibration (VIV) is investigated in this paper. The degree of oscillation attenuation or even suppression is analysed for isolated circular cylinder cases. How a cylinder fitted with strakes behaves when immersed in the wake of another cylinder in tandem arrangement is also investigated and these results are compared to those with a single straked cylinder. The experimental tests are conducted at a circulating water channel facility and the cylindrical models are mounted on a low-damping air bearing elastic base with one degree-of-freedom, restricted to oscillate in the transverse direction to the channel flow. Three strake pitches (p) and heights (h) are tested: p=5, 10, 15d, and h=0.1, 0.2, 0.25d. The mass ratio is 1.8 for all models. The Reynolds number range is from 1000 to 10 000, and the reduced velocity varies up to 21. The cases with h=0.1d strakes reduce the amplitude response when compared to the isolated plain cylinder, however the oscillation still persists. On the other hand, the cases with h=0.2, 0.25d strakes almost completely suppress VIV. Spanwise vorticity fields, obtained through stereoscopic digital particle image velocimetry (SDPIV), show an alternating vortex wake for the p=10d and h=0.1d straked cylinder. The p=10d and h=0.2d cylinder wake has separated shear layers with constant width and no roll-up close to the body. The strakes do not increase the magnitude of the out-of-plane velocity compared to the isolated plain cylinder. However, they deflect the flow in the out-of-plane direction in a controlled way, which can prevent the vortex shedding correlation along the span. In order to investigate the wake interference effect on the strake efficiency, an experimental arrangement with two cylinders in tandem is employed. The centre-to-centre distance for the tandem arrangement varies from 2 to 6. When the downstream p=10d and h=0.2d cylinder is immersed in the wake of an upstream fixed plain cylinder, it loses its effectiveness compared with the isolated case. Although the oscillations have significant amplitude, they are limited, which is a different behaviour from that of a tandem configuration with two plain cylinders. For this particular case, the amplitude response monotonically increases for all gaps, except one, a trait usually found in galloping-like oscillations. SDPIV results for the tandem arrangements show alternating vortex shedding and oscillatory wake.
THE ATLANTIC SLAVE TRADE AND COLONIALISM: REASONS FOR AFRICA’S UNDERDEVELOPMENT?
It is probably an understatement to assert that Africa, the world’s second largest continent, is underdeveloped compared with other continents. Two major exogenous factors often cited for Africa’s development crisis are the so called ‘dual historical injustices’ – the Atlantic slave trade and colonialism. It is often pointed out that while the former almost completely arrested and retarded Africa’s demographical growth for more than three centuries, evoked a perennial state of acrimony and warfare and created a tense and unprecedented atmosphere of insecurity which hindered economic activities and stifled local entrepreneurship and technological drive and dislocated Africa’s socio-economic and political structures and institutions; the latter tied African countries to the apron strings of western imperial powers thereby rendering the pre-conditions necessary for socio-economic and political transformation unachievable. This paper contends that while the history of Africa’s underdevelopment may be traceable to the Atlantic slave trade and colonialism, African leaders had made enormous contributions to the continent’s underdevelopment and backwardness. The paper concludes that irresponsible and irresponsive leadership, vampirism and prodigality, corruption, lack of respect for democratic ideals, insecurity, endemic civil wars, nepotism, etc are the fundamental causes of Africa’s underdevelopment.