6,146 research outputs found
Boundary element methods in the prediction of the acoustic damping of ship whipping vibrations
Damping of ship whipping vibrations following a slam due to wave
impact is traditionally assumed to be primarily of material or structural
origin. However, several mechanisms of energy dissipation to the
surrounding water exist, including gravity and acoustic waves. Neither
transports much energy for the lowest frequency modes, in which
the acoustic wavelength may be an order or magnitude greater than
the ship length whereas the gravity wavelength is at least an order of
magnitude shorter than the ship beam. However, the acoustic damping
ratio increases as the fourth power of frequency, becoming significant
for higher frequency modes
An Experimental Investigation of Ride Control Algorithms for High-Speed Catamarans Part 1: Reduction of Ship Motions
Ride control systems are essential for comfort and operability of high-speed ships, but it remains an open question what is the optimum ride control method. To investigate the motions of a 112-m high-speed catamaran fitted with a ride control system, a 2.5-m model was tested in a towing tank. The model active control system comprised two transom stern tabs and a central T-Foil beneath the bow. Six ideal motion control feedback algorithms were used to activate the model scale ride control system and surfaces in a closed-loop control system: heave control, local motion control, and pitch control, each in a linear and nonlinear version. The responses were compared with the responses with inactive control surfaces and with no control surfaces fitted. The model was tested in head seas at different wave heights and frequencies and the heave and pitch response amplitude operators (RAOs), response phase operators, and acceleration response were measured. It was found that the passive ride control system reduced the peak heave and pitch motions only slightly. The heave and pitch motions were more strongly reduced by their respective control feedback. This was most evident with nonlinear pitch control, which reduced the maximum pitch RAO by around 50% and the vertical acceleration near the bow by about 40% in 60-mm waves (2.69 m at full scale). These reductions were influenced favorably by phase shifts in the model scale system, which effectively contributed both stiffness and damping in the control action
The influence of the centre bow and wet-deck geometry on motions of wave piercing catamarans
The effects of tunnel height and centre bow length on the motions of a 112-m wave-piercer catamaran with an above-water centre bow were investigated through model tests. Five alternative centre bow configurations were considered, and multiple series of model tests were conducted in regular head sea waves. The results showed that both heave and pitch increased over a wide range of wave encounter frequency as the wet-deck height of the catamaran model increased. However, increasing the length of the centre bow showed an increase in the pitch but a decrease in the heave for a limited range of wave encounter frequency near the heave and pitch resonance frequencies of the catamaran model. The positions of minimum vertical displacement were found to be aft of the longitudinal centre of gravity, between 20% and 38% of the overall length from the transom. Increase in the wet-deck height and consequently the archway clearance between the main hulls and centre bow also resulted in an increase in the vertical displacement relative to the undisturbed water surface in the centre bow area. The results also indicated the vulnerability to wet-deck slamming for the different bow and wet-deck designs
The effect of centre bow and wet-deck geometry on wet-deck slamming loads and vertical bending moments of wave-piercing catamarans
An experimental study was performed to determine the influence of centre bow length and tunnel height on the magnitude of the wave slamming loads and bending moments acting on a 112 m Incat wave-piercer catamaran vessel. A 2.5 m hydroelastic segmented catamaran model was tested in regular head sea waves at a high model speed in multiple test series, whilst five centre bow (CB) and wet-deck configurations were considered, designated here as the parent, low, high, long and short CBs. The model global motions, centre bow slam loads, accelerations, and slam induced vertical bending moments of the catamaran model in waves were measured. It was found that the slamming force, the centre bow entry force and slam induced bending moment all increase as the centre bow length increases. Increasing the wet-deck height increased the motions but reduced the maximum slam load in moderate waves. It was seen that the short CB was the best design for the alleviation of slam loads. The high CB was the second best choice for operation in moderate waves but it was the worst configuration in terms of heave and pitch motions among various CB configurations tested
Origins and Development of Fresh Water Lake, Dominica, West Indies, and Exploratory Study of Traces of Catastrophic Events in its Sediments
Clinical anxiety promotes excessive response inhibition
BACKGROUND: Laboratory tasks to delineate anxiety disorder features are used to refine classification and inform our understanding of etiological mechanisms. The present study examines laboratory measures of response inhibition, specifically the inhibition of a pre-potent motor response, in clinical anxiety. Data on associations between anxiety and response inhibition remain inconsistent, perhaps because of dissociable effects of clinical anxiety and experimentally manipulated state anxiety. Few studies directly assess the independent and interacting effects of these two anxiety types (state v. disorder) on response inhibition. The current study accomplished this goal, by manipulating state anxiety in healthy and clinically anxious individuals while they complete a response inhibition task. METHOD: The study employs the threat-of-shock paradigm, one of the best-established manipulations for robustly increasing state anxiety. Participants included 82 adults (41 healthy; 41 patients with an anxiety disorder). A go/nogo task with highly frequent go trials was administered during alternating periods of safety and shock threat. Signal detection theory was used to quantify response bias and signal-detection sensitivity. RESULTS: There were independent effects of anxiety and clinical anxiety on response inhibition. In both groups, heightened anxiety facilitated response inhibition, leading to reduced nogo commission errors. Compared with the healthy group, clinical anxiety was associated with excessive response inhibition and increased go omission errors in both the safe and threat conditions. CONCLUSIONS: Response inhibition and its impact on go omission errors appear to be a promising behavioral marker of clinical anxiety. These results have implications for a dimensional view of clinical anxiety
Recommended from our members
Enhancement of regression of cervical intraepithelial neoplasia II (moderate dysplasia) with topically applied all-trans-retinoic acid: a randomized trial.
BackgroundRetinoids enhance differentiation of most epithelial tissues. Epidemiologic studies have shown an inverse relationship between dietary intake or serum levels of vitamin A and the development of cervical dysplasia and/or cervical cancer. Pilot and phase I investigations demonstrated the feasibility of the local delivery of all-trans-retinoic acid (RA) to the cervix using a collagen sponge insert and cervical cap. A phase II trial produced a clinical complete response rate of 50%.PurposeThis randomized phase III trial was designed to determine whether topically applied RA reversed moderate cervical intraepithelial neoplasia (CIN) II or severe CIN.MethodsAnalyses were based on 301 women with CIN (moderate dysplasia, 151 women; severe dysplasia, 150 women), evaluated by serial colposcopy, Papanicolaou cytology, and cervical biopsy. Cervical caps with sponges containing either 1.0 mL of 0.372% beta-trans-RA or a placebo were inserted daily for 4 days when women entered the trial, and for 2 days at months 3 and 6. Patients receiving treatment and those receiving placebo were similar with respect to age, ethnicity, birth-control methods, histologic features of the endocervical biopsy specimen and koilocytotic atypia, and percentage of involvement of the cervix at study. Treatment effects were compared using Fisher's exact test and logistic regression methods. Side effects were recorded, and differences were compared using Fisher's exact test.ResultsRA increased the complete histologic regression rate of CIN II from 27% in the placebo group to 43% in the retinoic acid treatment group (P = .041). No treatment difference between the two arms was evident in the severe dysplasia group. More vaginal and vulvar side effects were seen in the patients receiving RA, but these effects were mild and reversible.ConclusionsA short course of locally applied RA can reverse CIN II, but not more advanced dysplasia, with acceptable local side effects.ImplicationsA derivative of vitamin A can reverse or suppress an epithelial preneoplasia, lending further support to the notion that chemoprevention of human cancer is feasible
- …