Predicting, Understanding and Avoiding the Ekofisk Rotor Instability Forty Years Later

LectureThis famous machine is re-examined to assess how well (or not) current design and analytical methods have evolved to avoid shaft whip instability. In addition to reviewing the compressors history and design evolution, the rotordynamic performance of a newly configured machine, based on todays technology, is compared against the original design

Understanding Link Dynamics in Wireless Sensor Networks with Dynamically Steerable Directional Antennas

Abstract. By radiating the power in the direction of choice, electronicallyswitched directional (ESD) antennas can reduce network contention and avoid packet loss. There exists some ESD antennas for wireless sensor networks, but so far researchers have mainly evaluated their directionality. There are no studies regarding the link dynamics of ESD antennas, in particular not for indoor deployments and other scenarios where nodes are not necessarily in line of sight. Our long-term experiments confirm that previous findings that have demonstrated the dependence of angleof-arrival on channel frequency also hold for directional transmissions with ESD antennas. This is important for the design of protocols for wireless sensor networks with ESD antennas: the best antenna direction, i.e., the direction that leads to the highest packet reception rate and signal strength at the receiver, is not stable but varies over time and with the selected IEEE 802.15.4 channel. As this requires protocols to incorporate some form of adaptation, we present an intentionally simple and yet efficient mechanism for selecting the best antenna direction at run-time with an energy overhead below 2 % compared to standard omni-directional transmissions.

Predicting, Understanding and Avoiding the Ekofisk Rotor Instability Forty Years Later

LectureThis famous machine is re-examined to assess how well (or not) current design and analytical methods have evolved to avoid shaft whip instability. In addition to reviewing the compressors history and design evolution, the rotordynamic performance of a newly configured machine, based on todays technology, is compared against the original design

Torsional Natural Frequencies: Measurement Vs. Prediction

TutorialExcessive torsional vibration can cause damage or failure to rotating equipment trains thereby resulting in costly shutdowns. A comprehensive torsional vibration analysis is the typical method of designing a torsional system that avoids such problems. Requirements of a torsional system design are commonly based on the API Standards (API 617, 2002 and API 684, 2005). These standards require torsional natural frequencies (TNFs) to have at least 10% separation margin (SM) from any excitation frequency. If the recommended SM cannot be achieved, then the torsional system design must be shown to be acceptable by stress analysis. The validity of the predicted TNF and any stress analysis is dependent on the accuracy of the model. Some degree of uncertainty is always present within the analytical data, the modeling techniques, and the assumptions for excitation and damping. This paper provides an uncertainty study of more than ten (10) torsional systems. Major sources of uncertainty in torsional modeling are identified. The effect of variation in mass-elastic data is examined, and a comparison between measured and predicted TNFs for numerous cases is presented. Based on the studies and measurements, a reasonable SM range is presented, and 5% SM for measured TNFs is proposed

A predictive index of axillary nodal involvement in operable breast cancer.

We investigated the association between pathological characteristics of primary breast cancer and degree of axillary nodal involvement and obtained a predictive index of the latter from the former. In 2076 cases, 17 histological features, including primary tumour and local invasion variables, were recorded. The whole sample was randomly split in a training (75% of cases) and a test sample. Simple and multiple correspondence analysis were used to select the variables to enter in a multinomial logit model to build an index predictive of the degree of nodal involvement. The response variable was axillary nodal status coded in four classes (N0, N1-3, N4-9, N > or = 10). The predictive index was then evaluated by testing goodness-of-fit and classification accuracy. Covariates significantly associated with nodal status were tumour size (P < 0.0001), tumour type (P < 0.0001), type of border (P = 0.048), multicentricity (P = 0.003), invasion of lymphatic and blood vessels (P < 0.0001) and nipple invasion (P = 0.006). Goodness-of-fit was validated by high concordance between observed and expected number of cases in each decile of predicted probability in both training and test samples. Classification accuracy analysis showed that true node-positive cases were well recognised (84.5%), but there was no clear distinction among the classes of node-positive cases. However, 10 year survival analysis showed a superimposible prognostic behaviour between predicted and observed nodal classes. Moreover, misclassified node-negative patients (i.e. those who are predicted positive) showed an outcome closer to patients with 1-3 metastatic nodes than to node-negative ones. In conclusion, the index cannot completely substitute for axillary node information, but it is a predictor of prognosis as accurate as nodal involvement and identifies a subgroup of node-negative patients with unfavourable prognosis

Brain imaging in rabbits : preliminary results of CBF variation by different anaestethic drugs

In the last decades the animal model has been usually considered as a research tool, especially for functional studies and brain imaging. Although general anesthesia and sedation are fundamental requirements to perform nuclear imaging in veterinary patients, very few studies have been published on the effect of anesthesia itself, on brain perfusion. Brain SPECT in humans is widely applied to assess brain perfusion mainly in awake patients. The aim of the study was to evaluate general brain perfusion in rabbits through a non-invasive nuclear medicine technique, before and after the administration of different anesthetic protocols commonly used for veterinary patients. Ten male New Zeland White rabbits of 6 months of age were enrolled in the prospective study. Before SPECT examinations, the rabbits underwent CT studies of the skull in order to exclude any gross malformations or lesions and to acquire images for CT/SPECT fusion. 99mTc-HMPAO brain SPECT scans were acquired with a single head gamma camera: circular orbit, continuos rotation 10 seconds/step and 120 steps. During the first session 99mTc-HMPAO was IV injected in two groups of five awake rabbits, with a randomized selection. The first one was subsequently anesthetized with propofol and the other with dexmedetomidine. The same procedure was repeated three weeks later when the injection of the radiopharmaceutical was performed after the induction of general anaesthesia. The brain perfusion uptake index (BPUi%) was calculated as the percentage ratio between total counts in the brain and injected activity. Rabbits anesthetized with propofol showed exactly the same tracer distribution in both injection condition: awake or asleep. The radiopharmaceutical was concentrated in the brain but a generalized distribution was observed also in the facial muscles. On the contrary when dexmedetomidine was used, rabbits anesthetized after the 99mTc-HMPAO injection showed a distribution similar to propofol group, while when the radiotracer was injected after the anesthetic drug, a generalized reduction of the uptake was observed especially in extra-encephalic tissues. The average BPUi% values were about 1.6% for all rabbits anesthetized with propofol and for rabbits injected with 99mTc-HMPAO before dexmedetomidine administration. Animals injected with 99mTc- HMPAO after dexmedetomidine administration showed a lower value of BPUi% equal to about 1.25 %. Although the major limitation of our study is the small number of subjects analyzed, our results showed that when propofol is used as anesthetic drug, any difference in brain perfusion occurred if the radiotracer was injected prior or before anesthesia. On the contrary the vasocostriction of dexmedetomidine is responsible of a mild reduction of the IRU in the brain and a good inhibition of tracer uptake in other tissues. These preliminary data suggest that the use of propofol in uncooperative patients, that need sedation before brain perfusion studies, could not influence the CBF. On the other hand the results of the CBF in rabbits medicated with dexmedetomidine before tracer injection, suggest a possible neuronal protective proprieties of this drug

Contrast enhanced magnetic resonance imaging of the foot in horses using intravenous versus regional intraarterial injection of gadolinium

The use of contrast enhanced magnetic resonance imaging (MRI) for the detection of orthopedic pathologies in equine patients is poorly described. In few studies, enhanced MRI allowed to differentiate active lesions from chronic ones and to classify ambiguous lesions. The aim of this clinical prospective pilot study is to describe and compare the MRI lesions observed in horses with lameness localized to the foot using a single intravenous bolus dose of gadolinium contrast versus regional intraarterial bolus of contrast agent. Ten horses that underwent contrast enhanced MRI were included in the study. Gadolinium was injected intravenously in 3 patients and in 7 horses contrast agent was administered by intraarterial regional delivery. Regions of interest (ROI) were collected from both pre- and post- contrast images and ratios between pre- and post-contrast ROIs were calculated. No adverse reactions were noted after contrast agent injection. Injured structures that revealed greater increase in signal in post-contrast images were the deep digital flexor tendon (DDFT), the navicular spongiosa and the peritendinous tissues. Regional intraarterial administration of gadolinium provided higher ratio of contrast enhancement. Enhanced MRI using both intravenous or intraarterial injection of gadolinium, increased the diagnostic capability of MRI in horses with foot lesions. Nevertheless, regional intraarterial administration of gadolinium was considered the best choice due to the higher signal and lower volumes of contrast agent required