In the past three years, Professor Bakhtiari-Nejad has been the adjunct professor at the Department of Mechanical Engineering, University of Maryland, Baltimore County. Formerly he was professor of Mechanical Engineering at Amirkabir University of Technology. Professor Bakhtiari-Nejad is one of the founder of the Iranian Society of Acoustic and Vibration, and the chairman of the first ISAV conference at Amirkabir University of Technology in 2010. He has a PhD, MS and BS in Mechanical Engineering, and a BS in Electrical Engineering, from Kansas State University, USA, from 1975 to 1983.

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Recent Innovations on the Modeling and Control of Industrial Flexible Vibrating Structures

 

Flexible structures are used in many industrial parts and equipment. Drill strings, marine risers, helicopters blades and turbine and ship propellers are type of flexible structures which are widely used in important and critical industrial application these days. Exact modeling of these structures is required for understanding their behavior for better design and in many cases to design controllers for optimal performances. Structural modeling can be linear, nonlinear, deterministic or stochastic. Fractional modeling of continues systems is one of the method that has taken attention of many researchers in recent years. 

Oil drilling system is an important part of oil extraction in deep wells. Drill string is one of the important components of the rotary drilling mechanism. Dynamic modelling of coupled axial-torsional vibration of a drill string for investigation on the length increment effect on stick-slip instability is important. During the drilling process, the rotary table rotates with the nominal angular speed because of its high mass moment of inertia. During the drilling, drill string may be exposed to severe level of vibrations because of bit/rock interactions and of course, its low structural stiffness. Self-exited torsional vibration, also referred as stick-slip vibration, is one of the most common challenging problems of drilling operation which decreases the drilling performance by increasing the drilling time and costs and prevents the drilling to be performed with desired axial and torsional speeds. Reducing torsional vibrations of the drill string has been a major concern for petroleum operators. Non-causal stable inverse method is utilized for generating desired state trajectories and obtaining feed-forward control input from the given desired output. The total control input is designed as the sum of feed-forward (nominal control input) and feed-back control inputs which stabilizes tracking error between states and their corresponding trajectories by pole placement. For different values of desired angular and axial bit speeds proper controller in bit speed regulation is possible. In addition, output regulation was performed by applying conventional controllers used in literature such as sliding-mode and classical inverse dynamics to the proposed model. Corresponding results revealed very low performance of such controllers which made the system unstable because of ignoring the non-minimum-phase zeros in the model. Non-Causal Stable Inversion Control Method for Suppressing Coupled Axial-Torsional Vibrations of the Drill String 

Another important part in oil extraction from deep seas water marine riser. Harsh environments on the offshore marine riser make it a very critical industrial flexible structure. In this problem, it is important to reduce the exerted tension at the top end of riser which may lead to produce fatigue problems. It is necessary to model and control the vibration of riser. The hydrodynamic modeling of the system is needed for dynamic motion responses to reduce the top end angle and horizontal displacement of the flexible marine riser in addition to reduce the transverse vibration of the riser. Additionally, internal fluid in the riser effect its performance and can be considered as an internal disturbance. Since marine risers are subjected to severe time- varying hydrodynamic loads, fatigue failure is not a far-fetched phenomenon. Thus, it may be helpful to initially investigate the dynamic behavior of the system. Proper controller can help to reduce transverse vibration in addition to reduce the upper end angle and upper horizontal displacement of the riser. Lyapunov based controller can be designed in a way to attenuate the top end angle and reduce vibration. Active boundary controller is an effective tool in this study which is practical and implementable with existing instrumentation. An adaptive sliding mode control method in the presence of parametric uncertainties to reduce vibration of an elastic marine riser is used. The controller is applied on the top end of the riser to suppress end deflection. Parameters adaptation is performed in a way that disturbance adverse effects are removed. 

The top end of the marine riser is connected to a ship or a floating platform. Propeller to control the position and speed of the ship can be considered as a rotating beam or plate based on the aspect ratio. Modeling and dynamic study of rotating propeller are needed for the hydro elastic analyses of the ship propeller.  Low aspect ratio rotating structures can be modeled as a rectangular cantilever rotating plate. A blade with an adjustable stagger angle is a type of blade that can be used in this type of propeller and partially rotate around its long axis. In fact, while a hub rotates, simultaneously the angle of the blade root section can change. The objective of the research in this area was to develop and analyze a model of rotating cantilever orthotropic plate with adjustable stagger angle. The present study is the first attempt to demonstrate the effects of time-varying stagger angle for this class of problems in the literature. A stable motion in the adjustable stagger angle configuration gives a more complicated response. It can be observed that in this dynamic situation, the results are affected by Coriolis, both in quality and quantity. The nonlinear classical plate theory has been selected as the structural model. 

The aeroelastic analysis of rotating beam or plate can also be considered. Peters’ theory is employed to model aerodynamic pressure that distinguishes this study from previously reported research. Since the Peters’ aerodynamic model was originally developed to provide lift and moment which is only applicable to structural model based on beam theories. In this study, using the basic concept of the Peters’ aerodynamic model in addition to utilizing the Fourier series, the pressure distribution is derived which make Peters’ model applicable to structural model based on the plate theory. By using data from an experiment carried out at Duke University, the theoretical results are evaluated. Furthermore, using a novel approach, the differences in structural models obtained from the plate and beam theories can be divided into two distinctive parts which are responsible for differences in bending and torsional behaviors of the structure, separately.

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Dr Hamed Haddad Khodaparast

Dr Hamed Haddad Khodaparast is an Associate Professor in the College of Engineering at Swansea University. Dr Haddad has extensive research experience in structural dynamics and methods development for stochastic model identification in aeroelastics. In Liverpool, he developed efficient codes for perturbation-based stochastic model updating procedures that were used for determination of irreducible variability in a population of nominally identical test pieces, such as cars off a production line. His codes were used by the German Aerospace Centre (DLR) on stochastic model updating of an aircraft-like structure. He also collaborated with the CFD group at Liverpool to examine the problem of uncertainty propagating through large CFD-based models. In 2015, he was awarded the Royal Academy of Engineering Industrial Secondment (RAEng IS) and Sêr Cymru National Research Network (NRN) awards and worked with Airbus in the area of gust load predictions in the presence of nonlinearity and uncertainty. During the industrial fellowship, he developed numerical tools that are capable of demonstrating the effects of structural uncertainties and nonlinearities on the prediction of unsteady aerodynamic loads. He is currently working on problem of structural joint modelling and identification in an EPSRC Programme grant EP/R006768/1, entitled Digital twins for improved dynamic design. He is also an active collaborator on Semi Aeroelastic Hing (Albatross-one) project in Airbus, working on the problem of gust load alleviation using movable winglet

 

Worst case gust loads estimation and alleviation

 

Unsteady loads calculations play an important part across much of the design and development of an aircraft and have an impact upon the concept and detailed structural design, aerodynamic characteristics, weight, flight control system design, control surface design and performance. They determine the most extreme stress levels and estimate fatigue damage and damage tolerance

for a particular design. For this purpose, load cases due to dynamic gusts and manoeuvres are applied to detailed structural models in order to determine the worst values for a range of different Interesting Quantities (IQs) e.g. load factors, shear stresses, etc. There may be 1000s of IQs that need to be considered in the certification of large commercial aircraft.

The presentation starts with an introduction to gust load modelling and the process of worst-case gust load prediction in industry. The use of surrogate modelling for fast prediction of worst-case gust loads in a realistic aeroelastic model of aircraft structure will be demonstrated. The presentation will then continue with an introduction of a simplified aeroelastic model representative of an elastic wing with a folding wingtip. It will be demonstrated how the presence of a movable wingtip will contribute to the root bending moment of the wing and hence gust load alleviation. A parametric study will be also carried out to evaluate the potential of this technique for gust load alleviation. Finally, the problem of input gust estimation using in-flight measured data will be introduced. To this end, it will be shown that how the methods developed for the force determination in structural dynamics can be extended to estimate the input gust velocity in aeroelastic problem. The effects of noise on the measured data and modelling errors will be also demonstrated.