Marwan Hassan

Finite element based model predictive control for active vibration suppression of a one-link flexible manipulator

This paper presents a unique approach for active vibration control of a one-link flexible manipulator. The method combines a finite element model of the manipulator and an advanced model predictive controller to suppress vibration at its tip. This hybrid methodology improves significantly over the standard application of a predictive controller for vibration control. The finite element model used in place of standard modelling in the control algorithm provides a more accurate prediction of dynamic behavior, resulting in enhanced control. Closed loop control experiments were performed using the flexible manipulator, instrumented with strain gauges and piezoelectric actuators. In all instances, experimental and simulation results demonstrate that the finite element based predictive controller provides improved active vibration suppression in comparison with using a standard predictive control strategy.

Numerical Estimation of Fluidelastic Instability in Tube Arrays

This study investigates unsteady flow in tube bundles and fluid forces, which can lead to large tube vibration amplitudes, in particular, amplitudes associated with fluidelastic instability (FEI). The fluidelastic forces are approximated by the coupling of the unsteady flow model (UFM) with computational fluid dynamics (CFD). The CFD model employed solves the Reynolds averaged Navier-Stokes equations for unsteady turbulent flow and is cast in an arbitrary Lagrangian-Eulerian form to handle any motion associated with tubes. The CFD solution provides time domain forces that are used to calculate added damping and stiffness coefficients employed with the UFM. The investigation demonstrates that the UFM utilized in conjunction with CFD is a viable approach for calculating the stability map for a given tube array. The FEI was predicted for in-line square and normal triangle tube arrays over a mass damping parameter range of 0.1-100. The effect of the P/d ratio and the Reynolds number on the FEI threshold was also investigated.

Evaluation of the Integrity of Steam Generator Tubes Subjected to Flow Induced Vibrations

Flow-induced vibrations (FIV) continue to affect the operations of nuclear power plant components such as heat exchanger tube bundles. The negative effect of FIV is in the form of tube fatigue, cracking, and fretting wear at the supports. Fretting wear at the supports is the result of tube/support impact and friction. Fluidelastic and turbulence forces are the two main excitation mechanisms that feed energy into the system causing these violent vibrations. To minimize this effect all support clearances must be kept at a very small value. This paper investigates the consequences of losing the effectiveness of a particular support as a result of corrosion or excessive fretting wear. A full U-bend tube subjected to both fluidelastic and turbulence forces is utilized in this work. The performance of countermeasures such as the installation of additional flat bars in the U-bend region is thoroughly investigated. The investigation utilized both deterministic and probabilistic techniques.

A Probabilistic Assessment Technique Applied to a Cracked Heat Exchanger Tube Subjected to Flow-Induced Vibration

Flow-induced vibration is a common phenomenon in shell-and-tube heat exchangers. The resulting vibration can lead to component failure by fretting wear due to tube-to-tube support impact or by fatigue. Due to manufacturing considerations, many parameters such as support clearance, alignment, and friction at the supports are not exactly known and are represented by statistical distributions. This makes the use of deterministic equations inaccurate. This paper presents a methodology that can be used during component operation to monitor known flaws and ensure safe operation. The methodology incorporates Monte Carlo simulations to predict remaining service life of a vibrating heat exchanger tube with a small circumferential through-wall crack next to the tube sheet. Vibration excitation includes turbulence and low-level fluid-elastic forces. Leakage calculations are made on the through-wall crack as it grows to fracture. A Weibull distribution is given for the time-to-fracture and for the time for the leak rate to reach a threshold value. This statistical information can then be used to assess the remaining service life and whether LBB criteria will be met.

Design of a Magnetic Resonance Imaging Compatible Metallic Pressure Vessel

High-pressure measurements in most scientific fields rely on metalvessels, a consequence of the superior tensile strength of metals.Magnetic resonance imaging in conjunction with metallic pres-sure vessels has recently been introduced. Magnetic resonanceimaging with compatible metallic pressure vessels is a very gen-eral concept. This paper outlines the specifics of the developmentand design of these vessels. Metallic pressure vessels not onlyprovide inherently high tensile strengths and efficient temperaturecontrol, they also permit optimization of the radio-frequencyprobe sensitivity. The design and application of magnetic reso-nance imaging compatible pressure vessels is illustrated througha rock core holder fabricated using nonmagnetic stainless steel.Water flooding through a porous rock at elevated pressure andtemperature is shown as an example of its applications. High-pressure magnetic resonance plays an indispensable role in sev-eral scientific fields; this work will open new avenues of investiga-tion for high-pressure material science magnetic resonanceimaging. [DOI: 10.1115/1.4023728]

Numerical Characterization of the Area Perturbation and Timelag for a Vibrating Tube Subjected to Cross-Flow

Fluidelastic instability can have disastrous effects on the integrity of steam generators. Over the last five decades there has been a great deal of research done in an attempt to understand this phenomenon. These efforts have resulted in several theoretical models and design guidelines. The semi-analytical model of fluidelastic instability initially developed by Lever and Weaver is based on a single tube in a channel flow. The mechanism responsible for instability was found to be one of flow redistribution. While previous studies have been able to characterize the pressure and velocity within a tube bundle, the behaviour of the area of the channel has not yet been fully investigated.The current study aims to characterize the area of the channel surrounding the tube. Reynolds Averaged Navier Stokes (RANS) equations are cast in an Arbitrary Lagrangian Eulerian (ALE) form and are used to compute the flow conditions in a rigid tube bundle due to a single flexible tube vibrating in the transverse direction. The properties of the velocity field are used to determine the channel boundaries. Properties of the channel area such as area perturbation, mean area, and area phase are investigated for various reduced flow velocities. Dynamic simulations are conducted to determine the impact on the stability threshold for transverse fluid force cases using a mass damping parameter range of 10–200.Copyright

The Effect of Flat Bar Supports on Streamwise Fluidelastic Instability in Heat Exchanger Tube Arrays

Flow-induced vibration is an important criterion for the design of heat exchangers in nuclear, fossil, and chemical plants. Of the several known vibration excitation mechanisms, fluidelastic instability (FEI) is the most serious because it can cause tube failures in a relatively short period of time. Traditionally, FEI has been observed to occur in the direction transverse to the flow and antivibration bars have been used to stiffen the tubes against this motion. More recently, interest has increased in the possibility of FEI occurring in the streamwise direction, parallel to the flow. This is the subject of the present paper. Numerical simulations have been carried out to study the effects of tube-to-support clearance, tube sliding friction, tube-to-support preload, and ambient turbulence levels on the FEI threshold in the streamwise direction. As one would expect, increasing friction and tube preload against the support both tend to stabilize the tube against streamwise FEI. Importantly, the results also show that decreasing tube-support clearances destabilizes streamwise FEI while having little effect on transverse FEI. Increasing ambient turbulence levels also has the effect of destabilizing streamwise FEI.

Numerical Estimation of Fluidelastic Instability in Staggered Tube Arrays

This study investigates the unsteady flow and the resulting fluidelastic forces in a tube bundle. Numerical simulations are presented for normal triangle tube arrays with pitch-to-diameter (P/d) ratios of 1.35, 1.75, and 2.5 utilizing a 2-dimensional model. In this model a single tube was forced to oscillate within an otherwise rigid array. Fluid forces acting on the oscillating tube and the surrounding tubes were estimated. The predicted forces were utilized to calculate fluid force coefficients for all tubes. The numerical model solves the Reynolds-Average Navier-Stokes (RANS) equations for unsteady turbulent flow, and is cast in an Arbitrary Lagrangian-Eulerian (ALE) form to handle mesh the motion associated with a moving boundary. The fluidelastic instability (FEI) was predicted for both single and fully flexible tube arrays over a mass damping parameter (MDP) range of 0.1 to 200. The effect of the P/d ratio and the Reynolds number on the FEI threshold was investigated in this work.Copyright

Parametric Studies of the Turbulence Response of Heat Exchanger Tubes in Loose Supports

This paper reports the results of the numerical simulation of a loosely-supported heat exchanger tube excited by turbulence. The effect of support clearance, support geometry, and flow orientation are investigated. The finite element method is utilized to model the vibrations and impact dynamics. Three different friction models are examined to account for the tube/support friction forces. Issues regarding the efficiency and accuracy of the different techniques are discussed. Tube response and tube/support interaction parameters, such as the impact force, the contact ratio, and most importantly, the integrated product of the contact force and the sliding distance (work rate), are presented. The study indicates that some flow orientations and support geometries provide favourable support conditions for higher tube sliding motion against its support and therefore, potentially greater wear rates under service conditions.Copyright

The effect of upstream edge geometry on the acoustic resonance excitation in shallow rectangular cavities

The flow-excited acoustic resonance phenomenon is created when the flow instability oscillations are coupled with one of the acoustic modes of a confined duct, which in turn generates acute noise problems and/or excessive vibrations. In this study, the effect of the upstream edge geometry on attenuating these undesirable effects is investigated experimentally for flows over shallow rectangular cavities with two different aspect ratios of L/D = 1 and 1.67, where L is the cavity length and D is the cavity depth, for Mach number up to 0.45. The acoustic resonance modes of the cavity are self-excited due to the development of free shear layers over the cavity mouth. Twenty four different upstream cavity edges are investigated in this study, including round edges, chamfered edges, vortex generators, and spoilers with different sizes and configurations. The results for each upstream cavity edge are compared with the base case where sharp edge is used. Most of the spoiler edges are found to be effective in suppressing the pressure amplitude of the excited acoustic resonance. Hot-wire measurements that were taken along the lateral direction downstream of the spoilers reveal the existence of secondary vortices generated by the spoilers, orthogonal to the cavity shear layer, which results in suppressing the resonance. The performance of each spoiler depends on its specific geometry (i.e. thickness, height, and angle) and the size and strength of the orthogonal vortices that can be generated. A summary of the results is presented in this paper.