In this thesis, an initial study is carried out about the ultrasonic vibration propagation system used on the pipes that transport sea water inside cruise ships. These vibrations are used to keep the internal surfaces of the pipelines free from the formation of the so-called biofouling; an organic encrustation that occurs on any material in contact with sea water. The study aims to be a starting point for the implementation of a closed-loop control on the production of ultrasonic vibrations, realized through piezoelectric transducers applied to the outside of the ducts. The following topics will be covered: - First, an overview is made on the theoretical concepts that govern the vibrational mechanics of structures, the propagation of sound waves and finally also the phenomenon of cavitation associated with these vibrations. - Then, the two different simulations performed will be presented; the first using ANSYS software, in which the physical model of a sample pipeline is implemented; a hollow cylinder with the following dimensions: 3 [m] in length, 1,5 [m] in diameter and 0,008 [m] in thickness. Through this simulation, the mechanical characteristics of the vibrations of the structure are obtained, that is, the different modes of vibration associated with their natural frequencies, in the ultrasonic frequency range used; in addition, a series of frequency response functions (FRF) are carried out, which provide the vibrational response of the structure subject to a harmonic forcing of constant amplitude, with respect also in this case to a preset ultrasonic frequency range; the forcing used is intended to represent the pressure exerted by the piezoelectric transducer in the control conditions currently used in the real system, i.e., in an open-loop situation. - The second implementation is carried out in Simulink, using a state space model of the geometry, created by the ANSYS software, based on the previously implementation, which represents the system as SIMO, the only input is represented by the harmonic pressure also used for the FRFs, and as output the flexural deformation of the structure in four different preset points. Through this modelling, a control system is implemented using the theoretical pressure generated inside the pipeline, keeping it within the preset range. The pressure generated by the vibrations is controlled to avoid problems related to cavitation, which if stimulated by high pressures can cause damage to the pipes. As a solution, two types of control are implemented, both based on a hysteresis regulator, which, through the value of the error between the estimated pressure and the reference signal, evaluates the input to be delivered to the system; comparing if the error is positive or negative, and if it is within the desired range. The two implementations differ only based on the input supplied to the system, in one case it will use two values for the forcing, the maximum positive and negative amplitude of the pressure, and in the second case, in addition to these two values, the use of the zero pressure is allowed.
In questa tesi viene effettuato uno studio iniziale sul sistema di propagazione delle vibrazioni ultrasoniche utilizzato sulle tubazioni che trasportano acqua marina all'interno delle navi da crociera. Queste vibrazioni sono utilizzate per mantenere le superfici interne delle condotte, libere dalla formazione del così detto biofouling; un’incrostazione organica che si verifica su ogni materiale in contatto con l’acqua marina. Lo studio vuole essere una base di partenza per l'implementazione di un controllo ad anello chiuso sulla produzione di vibrazioni ultrasoniche, realizzate attraverso dei trasduttori piezoelettrici applicati all'esterno delle condotte. Verranno trattati i seguenti argomenti: - Innanzitutto, viene effettuata una panoramica sui concetti teorici che governano la meccanica vibrazionale delle strutture, la propagazione delle onde sonore ed infine anche il fenomeno della cavitazione associata a queste vibrazioni. - In seguito, verranno presentate le due diverse simulazioni effettuate; la prima utilizzando il software ANSYS, in cui viene implementato il modello fisico di una condotta campione; un cilindro cavo con le seguenti dimensioni: 3 [m] di lunghezza, 1,5 [m] di diametro e 0,008 [mm] di spessore. Attraverso questa simulazione vengono ottenute le caratteristiche della meccanica delle vibrazioni della struttura, cioè i diversi modi di vibrare associati alle frequenze naturali, nell’intervallo di frequenze ultrasoniche utilizzato; in più, viene effettuata una serie di frequency response function (FRF), che forniscono la risposta vibrazionale della struttura soggetta ad una forzante armonica di ampiezza costante, rispetto anche in questo caso ad un range di frequenze ultrasoniche preimpostato; la forzante utilizzata vuole rappresentare la pressione esercitata dal trasduttore piezoelettrico nelle condizioni di controllo attualmente utilizzate nel sistema reale, cioè in una situazione ad anello aperto. - La seconda implementazione viene effettuata in Simulink, utilizzando un modello state space della geometria creato dal software ANSYS, basato sull'implementazione precedentemente effettuata; il quale rappresenta il sistema come SIMO, in cui l'unico input è rappresentato dalla pressione armonica utilizzata anche per le FRF, e come output la deformazione flessionale della struttura in quattro diversi punti preimpostati. Attraverso questa modellizzazione viene implementato un sistema di controllo sulla pressione teorica generata all'interno della condotta, mantenendola dentro l’intervallo preimpostato. La pressione generata dalle vibrazioni viene controllata per non incorrere in problematiche legate alla cavitazione, che se stimolata da pressioni elevate può causare danni alle tubature. Come soluzione vengono implementati due tipi di controllo, entrambi basati su un regolatore ad isteresi, che, attraverso il valore dell'errore tra la pressione stimata e il segnale di riferimento, valuta l’input da erogare al sistema; comparando se l’errore è positivo o negativo, e se è all'interno dell'intervallo voluto. Le due implementazioni si differenziano solo in base all’input erogato al sistema, in un caso utilizzerà due valori per la forzante, l'ampiezza massima positiva e negativa della pressione, e nel secondo caso oltre a questi due valori si predispone l'utilizzo della pressione nulla.
Analysis and control of ultrasonic vibrations control for biofouling mitigation in seawater pipelines
Ciotti, Federico
2020/2021
Abstract
In this thesis, an initial study is carried out about the ultrasonic vibration propagation system used on the pipes that transport sea water inside cruise ships. These vibrations are used to keep the internal surfaces of the pipelines free from the formation of the so-called biofouling; an organic encrustation that occurs on any material in contact with sea water. The study aims to be a starting point for the implementation of a closed-loop control on the production of ultrasonic vibrations, realized through piezoelectric transducers applied to the outside of the ducts. The following topics will be covered: - First, an overview is made on the theoretical concepts that govern the vibrational mechanics of structures, the propagation of sound waves and finally also the phenomenon of cavitation associated with these vibrations. - Then, the two different simulations performed will be presented; the first using ANSYS software, in which the physical model of a sample pipeline is implemented; a hollow cylinder with the following dimensions: 3 [m] in length, 1,5 [m] in diameter and 0,008 [m] in thickness. Through this simulation, the mechanical characteristics of the vibrations of the structure are obtained, that is, the different modes of vibration associated with their natural frequencies, in the ultrasonic frequency range used; in addition, a series of frequency response functions (FRF) are carried out, which provide the vibrational response of the structure subject to a harmonic forcing of constant amplitude, with respect also in this case to a preset ultrasonic frequency range; the forcing used is intended to represent the pressure exerted by the piezoelectric transducer in the control conditions currently used in the real system, i.e., in an open-loop situation. - The second implementation is carried out in Simulink, using a state space model of the geometry, created by the ANSYS software, based on the previously implementation, which represents the system as SIMO, the only input is represented by the harmonic pressure also used for the FRFs, and as output the flexural deformation of the structure in four different preset points. Through this modelling, a control system is implemented using the theoretical pressure generated inside the pipeline, keeping it within the preset range. The pressure generated by the vibrations is controlled to avoid problems related to cavitation, which if stimulated by high pressures can cause damage to the pipes. As a solution, two types of control are implemented, both based on a hysteresis regulator, which, through the value of the error between the estimated pressure and the reference signal, evaluates the input to be delivered to the system; comparing if the error is positive or negative, and if it is within the desired range. The two implementations differ only based on the input supplied to the system, in one case it will use two values for the forcing, the maximum positive and negative amplitude of the pressure, and in the second case, in addition to these two values, the use of the zero pressure is allowed.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/187632