Department of Mechanical and Mechatronic Engineering

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Browsing Department of Mechanical and Mechatronic Engineering by browse.metadata.advisor "Bekker, A"

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    Human responses to wave-induced motion, slamming and whipping.
    (Stellenbosch : Stellenbosch University, 2023-11) Engelbrecht, MC; Bekker, A; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: Ship motion is known to a ect comfort, rest and productivity. On ships, these factors could in uence fatigue-related accidents. Discomfort on cruise ships could adversely a ect customer satisfaction. The crew and passengers of vessels, live and work in this dynamic environment, often for months on end. Research regarding human comfort, as a result of impulsive wave slamming is specifically limited. Wave slamming is the random impact of waves against the ship hull leading to the phenomenon called whipping which causes vibration to propagate through the ship structure. This study investigated the effects of impulsive vibration on activity disturbance in the operational context of a slamming-prone vessel. Metrics from whole-body vibration standards were investigated as a possible means to quantify comfort. A possible threshold of the onset of discomfort was determined and compared to likely responses to whole-body vibration as documented in whole-body vibration standards, maritime standards and ship classification society documents. Subjective data was obtained from daily diary surveys conducted willingly by people on board. Additional human responses were reaped from instantaneous slam observations which were conducted from the Bridge during designated ship manoeuvres. Full-scale vibration measurements were recorded at several locations across the ship for the entire duration of the voyage and expressed in terms of metrics as provided in standards. Subjective and objective data were correlated. Subjective results show that slamming was prevalent on most days during investigated voyages. Sleeping was observed to be the activity most disturbed. Metrics such as vibration dose value and root-mean-square value had displayed positive strong correlations with the human response data. An hourly vibration dose value threshold of approximately 0.43 m/s1:75 and 0.47 m/s1:75 resulted from both the surveys and instantaneous slam ratings. This corresponds to the threshold where 50% of the sample population felt discomfort. The determined threshold for root-mean-square value of 0.03 m/s2 is much lower than guideline values provided in ISO 2631-1 for discomfort in public transport applications.
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    Simulation and testing of centrifugal pump cavitation.
    (Stellenbosch : Stellenbosch University, 2024-02) Van der Spuy, SJ; Bekker, A; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: Cavitation, the formation of vapour filled bubbles in areas of low pressure, can be deleterious to centrifugal pump operation. Cavitating flow may cause efficiency loss, damage to impeller surfaces, and excessive vibration in pump systems. Some cavitation is unavoidable and even harmless, however, preventing cavitation will in general enhance pump reliability, and efficiency. In this research question, computational fluid dynamics was used to simulate cavitation inception using single‑phase, steady‑state flow, in a centrifugal pump. The pump was a low specific speed radial flow centrifugal pump that is used for the conveyance of water or other products. Simulations were performed using Cadence (NUMECA) Fine Turbo 16.1 and AutoGrid 5 meshing software. The simulated performance characteristics were compared to experimental performance measurements. The cavitation model determined the net positive suction head (NPSH) of cavitation inception (NPSHi) in the post‑processing stage of the simulation. The cavitation model is based solely on the inlet pressure and minimum static pressure in the impeller to determine cavitation inception. The simulated NPSHi values were 0.903 m, 1.53 m, and 2.33 m, at shaft speeds of 800 RPM, 1000 RPM, and 1200 RPM respectively. This was compared against scaled NPSH3 values of 0.281 m, 0.475 m, and 0.722 m respectively. As expected, the simulated NPSHi values were consistently higher than NPSH3 values. Vibration sensors were employed to measure cavitation inception in the pump, but due to stall occurring in the impeller, the point of cavitation inception could not be distinguished above the noise caused by backflow vibration. Broadband excitation was measured on the pump, which was consistent with other test setups where backflow was present. Backflow was also confirmed by the simulations. Experimental performance measurements showed that for accurate performance simulation, a model containing only the impeller and diffuser was insufficient. Adherence to the predicted NPSHi will prevent operating pumps under damaging cavitating conditions, however, the correspondence of these values with true cavitation inception is tenuous.
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    A study of the longitudinal open-water performance of an ice-class ship.
    (Stellenbosch : Stellenbosch University, 2024-02) Steyn, Barend Johannes; Bekker, A; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    ENGLISH ABSTRACT: The hull and propeller of vessels engaged in operations in open water and ice are exposed to the encumbering effects of marine biofouling and the abrasive scouring of ice, both affecting ship resistance and performance. This thesis investigates the long term performance implications of such an operational profile, characterised by extensive open-water operations interspersed with ice navigation through a longitudinal study of the open-water performance of the Polar Supply and Research Vessel, the SA Agulhas II, with a specific focus on the hull and propeller condition. This study is enabled by a repository of measurement data and complementary information on the SA Agulhas II, spanning an equivalent period of ten years, the outcome of an extensive full-scale measurement campaign initiated in 2012. Key voyages within this period are selected for which the open-water performance is determined before, after, and intermittently during ice navigation. A tailored ship performance analysis method is formulated for the SA Agulhas II, utilising the available data to quantify the performance impact arising from the condition of the hull and propeller. Within the analysis method, three constituent elements are developed. First, a performance correction method, to differentiate between the effects of the degraded hull and propeller from those of environmental and operational disturbances, achieved through a constructed deterministic ship performance model. Second, a performance quantification method, to evaluate the performance losses, and third, a data conditioning method, to compile and filter the dataset to reduce uncertainty in the results. The efficacy of the formulated analysis method to exhibit the performance losses due to hull and propeller degradation is assessed through the review of four qualitative and quantitative measures, including an uncertainty analysis. The performance analysis reveals a cyclic pattern in the ship’s hull and propeller conditions, marked by progressive deterioration and subsequent near-baseline performance for the pre- and post-ice periods with each voyage. This cyclicality is attributed to the resurgence of biofouling growth between voyages, subsequently removed by ice scouring during ice navigation. Protracted docking periods in a temperate port contribute to biofouling growth, further exacerbated by anti-fouling coating degradation due to ice-scouring. Consequently, pre-ice operational penalties manifest, resulting in speed losses of up to 19% or power increases of 121%, driven by complete anti-fouling coating loss. Ecologically, the open-water performance results, recorded during operation in intermittent open-water passages after initial ice contact, indicate a gradual and near complete removal of the nonnative biofouling within a 200 km stretch into the marginal ice zone.