Rais Ahmad

Modeling of phased array transducers

Phased array transducers are multi-element transducers, where different elements are activated with different time delays. The advantage of these transducers is that no mechanical movement of the transducer is needed to scan an object. Focusing and beam steering is obtained simply by adjusting the time delay. In this paper the DPSM (distributed point source method) is used to model the ultrasonic field generated by a phased array transducer and to study the interaction effect when two phased array transducers are placed in a homogeneous fluid. Earlier investigations modeled the acoustic field for conventional transducers where all transducer points are excited simultaneously. In this research, combining the concepts of delayed firing and the DPSM, the phased array transducers are modeled semi-analytically. In addition to the single transducer modeling the ultrasonic fields from two phased array transducers placed face to face in a fluid medium is also modeled to study the interaction effect. The importance of considering the interaction effect in multiple transducer modeling is discussed, pointing out that neighboring transducers not only act as ultrasonic wave generators but also as scatterers.

Pipe Wall Damage Detection in Buried Pipes Using Guided Waves

It is well known that cylindrical guided waves are very efficient for detecting pipe wall defects when pipes are open in the air. In this paper it is investigated how efficient the guided waves are for detecting pipe wall damage when the pipes are embedded in the soil. For this purpose guided waves were propagated through pipes that were buried in the soil by placing transmitters on one end of the embedded pipe and receivers on the other end. Received signals for both defect-free and defective pipes were recorded. Then the received signals were subjected to wavelet transforms. To investigate whether embedding the pipe in the soil makes it more difficult to detect the pipe wall defects, the same set of defective and defect-free pipes were studied before and after burying them in the soil. In both cases the defective pipes could be easily identified. Interestingly, contrary to the intuition, it was observed that under certain conditions defective pipes could be identified more easily in buried conditions. For example, the difference between the strengths of the initial parts of the received signal from defect-free and dented pipes was found to be greater for the buried pipes. Some qualitative justification for easier detection of buried dented pipes is provided.

Ultrasonic field computation in the presence of a scatterer of finite dimension

A recently developed semi-analytical technique called DPSM (Distributed Point Source Method) is improved and used to model the ultrasonic field in a fluid generated by an ultrasonic transducer and scattered by a solid plate of finite dimension. Earlier works on the ultrasonic field modeling by the DPSM technique have been limited to homogeneous fluids or nonhomogeneous media with infinite interfaces. This is the first attempt to model the complete ultrasonic field consisting of incident, reflected, transmitted and diffracted fields by a finite scatterer of any shape or size. No closed form analytical solution exists for ultrasonic field computation in presence of a scatterer and an ultrasonic transducer, both of which can have finite dimensions and any shape. Finite element solution for wave propagation analysis is very time consuming; hence, the semi analytical technique used here appears to be the method of choice for solving such practical problems. The paper shows how the scattered field varies as the acoustic properties and dimensions of the scatterer change.

Theoretical computation of acoustic pressure generated by ultrasonic sensors in the presence of an interface

DPSM (Distributed Point Source Method) is a computational technique that can be used to model the pressure field generated by ultrasonic acoustic transducers. This technique involves discretization of the transducer face of any geometrical shape, into a number of elemental surface areas. Point sources are placed at the centroids of the elemental surface areas. The strengths of the point sources are proportional to the surface areas. Pressure field at a given point is the cumulative effect of the pressure fields generated by all point sources. The accuracy of the computational technique depends on the sensor surface discretization. In this paper, circular transducers are modeled using the DPSM technique. This technique is applied to calculate the pressure field distribution in non- homogeneous fluids with interface. The non-homogeneous fluid is composed of two fluid half-spaced with the interface in front of the transducer face parallel or inclined to the transducer face. Pressure fields in both fluids for normal and angular incidence of the ultrasonic beam have been calculated using DPSM technique.

Impact of temperature and precipitation on propagation of intestinal schistosomiasis in an irrigated region in Ethiopia: suitability of satellite datasets

Objective  To assess the suitability of satellite temperature and precipitation datasets for investigating the dependence of Schistosoma mansoni disease transmission on meteorological conditions in an irrigated agricultural region in Ethiopia.

Influence of Water Flow through Pipe Networks for Damage Detection using Guided Waves

Researchers have been trying to develop techniques for early forecasting of the degradation process in pipe networks. Different Non Destructive Testing Techniques are used for detecting damages in a variety of materials and structures. Guided wave technique is a suitable non-destructive technique which can be used for pipe inspection by generating cylindrical guided waves. In this research, steel pipes are inspected using cylindrical guided waves. The purpose of this paper is to investigate the influence of flowing water through the pipe on the guided wave propagation in the pipe wall. Investigations are also carried out when the pipes are in open air which gives the traction-free boundary condition. It is also investigated whether the direction of the flow influences the propagation of the guided waves. Experimental V(f) curves are extracted from the received signals for defect free and defective pipe specimens and compared to study the effect of water flow on the strength and other characteristics of various guided wave modes.

Structural Health Monitoring of Steel Pipes under Different Boundary Conditions and Choice of Signal Processing Techniques

Guided wave technique is an efficient method for monitoring structural integrity by detecting and forecasting possible damages in distributed pipe networks. Efficient detection depends on appropriate selection of guided wave modes as well as signal processing techniques. Fourier analysis and wavelet analysis are two popular signal processing techniques that provide a flexible set of tools for solving various fundamental problems in science and engineering. In this paper, effective ways of using Fourier and Wavelet analyses on guided wave signals for detecting defects in steel pipes are discussed for different boundary conditions. This research investigates the effectiveness of Fourier transforms and Wavelet analysis in detecting defects in steel pipes. Cylindrical Guided waves are generated by piezo-electric transducers and propagated through the pipe wall boundaries in a pitch-catch system. Fourier transforms of received signals give information regarding the propagating guided wave modes which helps in detecting defects by selecting appropriate modes that are affected by the presence of defects. Continuous wavelet coefficients are found to be sensitive to defects. Several types of mother wavelet functions such as Daubechies, Symlet, and Meyer have been used for the continuous wavelet transform to investigate the most suitable wavelet function for defect detection. This research also investigates the effect of different boundary conditions on wavelet transforms for different mother wavelet functions.

Cylindrical guided waves for damage detection in underground pipes using wavelet transforms

This paper investigates if cylindrical guided waves can be effectively used for pipe wall defect detection in soil-embedded pipes. For this purpose guided waves are propagated through pipes that are buried in the soil by placing transmitters on one end of the pipes and the receivers on the other end. Received signals for both defect-free and defective pipes are subjected to wavelet transforms. It is found that when a Continuous Wavelet Transform (CWT) based algorithm is applied to analyze the received signals then it is very easy to make distinction between damaged and undamaged pipes. To investigate whether embedding the pipe in the soil makes it more difficult to detect the pipe wall defects, the same set of defective and defect-free pipes are analyzed before and after burying them in the soil. In both cases the defects are easily detected after analyzing the wavelet transformed signals. Interestingly it can be detected more easily for the buried pipes because the difference between the received signal strengths from defect-free and defective pipes is found to be greater for the buried pipes. For soil embedded pipes the ultrasonic energy scattered by the defect is absorbed by the surrounding soil making the energy reaching the receiver significantly weaker than that for the defect-free soil embedded pipe.

Influence of water flow on pipe inspection

From various studies by different investigators it has been now well established that a number of cylindrical guided wave modes are sensitive to the pipe wall defects. Several investigations by these authors and other researchers showed that the strengths of the guided waves propagating through a pipe that is placed in air are reduced when the pipe wall defects are encountered. This reduction is expected because the pipe wall defects (gouge, dent, removed metal due to corrosion etc.) alter the pipe geometry, hampering the free propagation of guided wave modes. When water flows through the pipes, the guided wave technique becomes more challenging because the flowing water absorbs part of the propagating acoustic energy. Flowing water may also induce some standing modes. The propagating cylindrical guided wave modes become leaky modes in presence of the flowing water, in other words energy leaks into water. Therefore, the energy detected by a receiver, placed at a large distance from the transmitter, is reduced even for a defect free pipe. Further reduction in the signal strength occurs in presence of defects.

Application of Gabor transform on cylindrical guided waves to detect defects in underground pipes

Defects in underground pipes are detected by applying Gabor transforms on experimental guided wave signals and comparing the experimental group velocity plots with the theoretical group velocity dispersion curves. Gabor transform, which is a powerful signal processing tool, maps a signal into a two-dimensional space of time and frequency. Thus it provides information about both when and at what frequency a signal arrives. Focus of this paper is to study the applicability of cylindrical guided waves to detect defects in underground pipes using Gabor transform. Cylindrical guided waves are generated by piezo-electric transducers. Guided waves are propagated through pipes that are buried in the soil by placing transmitters on one end of the pipes and the receivers on the other end. The recorded signals are then processed using 2-D Gabor Transform or Short Time Fourier Transform (STFT). Gabor transform converts the time-amplitude signal into a time frequency signal which reveals the group velocities hidden in the signal. These experimentally obtained group velocities are then compared with the theoretical velocities for cylindrical pipes embedded in the soil. From the comparison of the theoretical and experimental group velocities, an effort has been made to identify which modes are propagating through the embedded defective pipes and which modes are having difficulty to propagate through the defective pipe wall. From this knowledge pipe wall defects can be detected.