Coupled Inline-Cross Flow VIV Hydrodynamic Coefficients Database

Vortex Induced Vibrations (VIV) cause major fatigue damage to long slender bodies and have been extensively studied in the past decades. While most of the past research focused on the cross flow direction, it was recently shown that the inline motion in the direction of the flow has a major impact on the fatigue life damage due to its higher frequency (second harmonic) and more importantly, its coupling with the crossflow motion, which triggers a third harmonic stress component in the cross flow direction. In this paper, the coupled inline-crossflow VIV problem is addressed from semiempirical modeling of fluid forces. Extensive fine grid forced inline-crossflow VIV experiments were designed and carried out in the MIT towing tank. An inline-crossflow VIV hydrodynamics coefficients database was newly constructed using the experimental results and it is expected to be useful for other semi empirical programs predicting coupled inlinecrossflow VIV in the field. Several key hydrodynamic coefficients in the database, including lift force coefficients, drag force coefficients and added mass coefficients, were systematically analyzed. The coefficients in the crossflow and the inline directions were found to have strong dependency on the phase between the inline and crossflow motions.BP-MIT Major Program

Inline-Crossflow Coupled Vortex Induced Vibrations of Long Flexible Cylinders

The inline motion of long flexible cylinders caused by Vortex Induced Vibrations (VIV) has been long neglected due to its small amplitude compared to the cross-flow response amplitude. However, the inline motion has a major impact on fatigue life due to its higher frequency (second harmonic) and more importantly, because it triggers a third harmonic stress component in the crossflow direction along with a broad-band frequency stress component. We introduce an inline response prediction module to VIVA, a VIV response prediction program widely used in the offshore industry, to be able to consequently predict the higher harmonic and chaotic VIV response characteristics of flexible cylinders. Extensive forced inline and combined inline-crossflow experiments were employed to provide hydrodynamic coefficient databases for input to VIVA, in addition to existing crossflow hydrodynamic coefficients. The Norwegian Deepwater Programme (NDP) experimental data were used to validate this prediction methodology.BP-MIT Major Projects Progra

The Mechanics of Fast-Start Performance of Pike Studied Using a Mechanical Fish

A northern pike (Esox lucius) is capable of achieving a maximum instantaneous acceleration of 25g, far greater than that achieved by any manmade vehicle. In order to understand the physical mechanisms behind achieving such high accelerations, we have built a mechanical fish to emulate the motion of a pike, a fast-start specialist. A live pike bends its body into either a C-shaped or an S-shaped curve and then uncoils it very quickly to send a traveling wave along its body in order to achieve high acceleration. We have designed a mechanical fish whose motion is accurately controlled by servo motors, to emulate the fast-start by bending its body to a curve from its original straight position, and then back to its straight position. Furthermore, this mechanical fish is designed to be adjustable in swimming pattern, tail shape, tail rigidity, and body rigidity, making it possible to study the influence of all of these parameters on the fast-start performance. Peak accelerations of 2.0 m/s2 and peak velocities of 0.09 m/s are measured. Although the maximum accelerations and velocities observed in our mechanical fish are smaller than those of live fish, the form of the measured acceleration signal as function of time is quite similar to that of a live fish. The hydrodynamic efficiencies are observed to be around 12%, and it is shown that the majority of the thrust is produced at the rear part of the mechanical fish - similarly to the live fish. Copyright © 2011 by ASME

Re-Evaluation of VIV Riser Fatigue Damage

The paper describes a new characterization of the properties of the vortex-induced vibrations (VIV) of marine risers, which emerges from processing field and experimental data. We show that two currently employed assumptions: (a) that VIV is a statistically steady-state response containing one or several frequencies, and (b) that VIV consists of alternating dominant modes (mode-sharing), are inadequate. Instead, we find that the response either contains strong traveling wave components accompanied by high force harmonics; or consists of a chaotic wandering among several traveling and standing waves, associated with a wide-band spectrum; both types of response require careful consideration for correct fatigue evaluation. Topics: Fatigue damage, Pipeline risers, Vortex-induced vibrationBP-MIT Major Projects Progra

Modeling of Switching Power Losses in Cascaded H-Bridges with Unipolar PWM

This paper studies the extraction of an equivalent resistance for calculation of the switching power losses in cascaded H-bridge multilevel inverters with unipolar pulse width modulation. The concept of local and global rms currents and switching power losses are introduced in each H-bridge considering different frequencies of the inverter output and the switching network, i.e., transistor and diode. Energy conservation law has been utilized to show that the equivalent averaged local and global resistances for switching power losses are functions of load power factor angle, load rms current, and modulation index. This dependency is then reduced to only load rms current when the equivalent resistances are transformed to the inverter output branch based on the reflection rule. Consequently, a resistive model of switching power losses of the inverter is deduced, which depends neither on the knowledge of the physics of the switch, nor on complicated nonlinear equations of the semiconductor devices, and loop inductances. Double-pulse tests are conducted to acquire reliable data on the switching characteristics of the devices for different operating point

Load assessment and analysis of impacts in multibody systems

The evaluation of contact forces during an impact requires the use of continuous force-based methods. An accurate prediction of the impact force demands the identification of the contact parameters on a case-by-case basis. In this paper, the preimpact effective kinetic energy (Formula presented.) is put forward as an indicator of the intensity of the impact force along the contact normal direction. This represents a part of the total kinetic energy of the system that is associated with the subspace of constrained motion defined by the impact constraints at the moment of contact onset. Its value depends only on the mechanical parameters and the configuration of the system. We illustrate in this paper that this indicator can be used to characterize the impact force intensity. The suitability of this indicator is confirmed by numerical simulations and experimentsPostprint (author's final draft

Scleral buckling surgery for rhegmatogenous retinal detachment with subretinal proliferation

PurposeTo evaluate the outcome of scleral buckling surgery in patients with rhegmatogenous retinal detachment (RRD) with subretinal proliferation.MethodsIn this retrospective study, a chart review of all patients with RRD associated with subretinal proliferation who were primarily treated with scleral buckling procedure, from April 2007 to April 2014, was undertaken. Main outcome measures were anatomical retinal reattachment and visual acuity.ResultsForty-four eyes of 43 patients including 24 males and 19 females with a mean age of 26.5±13.1 years were evaluated. Immediately after the surgery, retina was reattached in all eyes. However, five eyes (11.3) needed additional surgery for retinal redetachment. Single surgery anatomical success rate was 88.7. Four eyes (9.1), needed pars plana vitrectomy for the treatment of redetachment associated with proliferative vitreoretinopathy and scleral buckle revision surgery was successfully performed in the other eye. Best corrected visual acuity improved from 1.5±0.9 logMAR before surgery to 1.1±0.7 logMAR after surgery (P2 lines was found in 23 eyes (52.2) and worsening of best corrected visual acuity of >2 lines was observed in 2 eyes (4.5).ConclusionsScleral buckling surgery is highly successful in eyes with RRD associated with subretinal proliferation. © 2015 Macmillan Publishers Limited All rights reserved