Vikram R. Melapudi

A Scalable Parallel Wideband MLFMA for Efficient Electromagnetic Simulations on Large Scale Clusters

The development of the multilevel fast multipole algorithm (MLFMA) and its multiscale variants have enabled the use of integral equation (IE) based solvers to compute scattering from complicated structures. Development of scalable parallel algorithms, to extend the reach of these solvers, has been a topic of intense research for about a decade. In this paper, we present a new algorithm for parallel implementation of IE solver that is augmented with a wideband MLFMA and scalable on large number of processors. The wideband MLFMA employed here, to handle multiscale problems, is a hybrid combination of the accelerated Cartesian expansion (ACE) and the classical MLFMA. The salient feature of the presented parallel algorithm is that it is implicitly load balanced and exhibits higher performance. This is achieved by developing a strategy to partition the MLFMA tree, and hence the associated computations, in a self-similar fashion among the parallel processors. As detailed in the paper, the algorithm employs both spatial and direction partitioning approaches in a flexible manner to ensure scalable performance. Plethora of results are presented here to exhibit the scalability of this algorithm on 512 and more processors.

A Wavelet Based Signal Processing Technique for Image Enhancement in Terahertz Imaging Data

Terahertz imaging is a relatively new technique for sub‐surface imaging using radiations in the spectral range between 0.1 to 10 THz. The technique has been used to image artificially induced inserts simulating disbonds in metal‐foam interfaces and has shown significant promise as a possible non destructive evaluation technique for evaluating the bonding quality of foam. The data in these cases is obtained by scanning across a surface on top of the foam coated on metal structures and collecting the time signal in a window of interest at each point in the scan plane. Proper data processing and visualization techniques become critical in being able to detect the disbonds and delaminations that, additionally, become convoluted due to the wide variety of artifacts and support structures that occur on the metal substrates. In this work we discuss a wavelet based signal enhancement algorithm that provides an effective scheme for visualizing the imaging data and provides a very high contrast between disbonded areas and normal substrate. The technique also shows promise as a first step towards automatic detection and classification of the disbonds. Some preliminary results, obtained on data collected using simulated disbonds that demonstrate the usefulness of the algorithm will be presented.

Microwave NDE for Reinforced Concrete

Nondestructive assessment of the integrity of civil structures is of paramount importance for ensuring safety. In concrete imaging, radiography, ground penetrating radar and infrared thermography are some of the widely used techniques for health monitoring. Other emerging technologies that are gaining impetus for detecting and locating flaws in steel reinforcement bar include radioactive computed tomography, microwave holography, microwave and acoustic tomography. Of all the emerging techniques, microwave NDT is a promising imaging modality largely due to their ability to penetrate thick concrete structures, contrast between steel rebar and concrete and their non‐radioactive nature. This paper investigates the feasibility of a far field microwave NDE technique for reinforced concrete structures.

Ray‐Rracing Model for Terahertz Imaging of SOFI Inspection

Significant advances made in the field of Terahertz Imaging have captured the attention of many applications including industrial NDE, biomedical imaging and homeland security. The NASA spray on foam insulation (SOFI) inspection is one such critical application were Terahertz imaging is used to detect disbonds and delamination between SOFI and the substrate metal (external tank). This paper discusses modeling aspects of the high frequency inspection system. The feasibility of using a simple ray‐tracing model is studied. The efficiency of the model is demonstrated by comparing the model prediction with experimental measurements and further using it in an iterative inverse problem scheme for defect reconstruction.

Imaging and modeling techniques for terahertz inspection of NASA-SOFI

Short‐pulsed terahertz imaging techniques have found application in recent years especially in the areas of nondestructive evaluation, homeland security and biomedical imaging. One such application involves the inspection of bonding between spray on foam insulation (SOFI) and the external tank in the NASA space shuttle. This work discusses a suite of image enhancement techniques that was developed to improve the probability of detection of voids and disbands SOFI. Physics based defect detection and profiling methods are detailed along with initial results. In addition a ray‐tracing model was developed to simulate the inspection process. Results comparing the model and experimental images will also be presented.

A GMR based Eddy Current Field Sensing System for NDE of Aircraft Structures

A new giant magnetoresistance (GMR) based eddy current field measurement system which can inspect components at rates comparable to magneto-optic imaging (MOI) sensor systems has been designed and developed. The main advantage of this system over the MOI is its ability to retain complete quantitative information about the magnetic field. Other advantages of the system include high bandwidth and sensitivity.

Ray Tracing Model for Terahertz Inspection of Spray On Foam Insulation (SOFI)

Terahertz imaging techniques are beginning to find many applications in areas such as nondestructive evaluation, biomedical imaging and homeland security. The NASA spray on foam insulation (SOFI) inspection is a critical application where terahertz imaging is used to detect disbonds and delamination between SOFI and the substrate metal (external tank). This paper discusses a ray-tracing model for the high frequency inspection system. Model based inversion techniques for the characterization of defects is also proposed

Electromagnetic‐Acoustic Modeling of Fields Induced by Gradient Pulses in Diffusion Tensor Magnetic Resonance Imaging

Diffusion tensor magnetic resonance imaging (DT‐MRI) has recently gained popularity because of its capabilities in axonography of the central nervous system. Fast imaging sequences are used to reduce motion induced distortion effects on the diffusion signal, resulting in low signal levels, loud acoustic noise, and occasional peripheral nerve stimulation. Eddy current induced by diffusion gradient pulses is also a challenge to DT‐MRI. Magnetic field associated with the eddy current is a major source of artifacts in scanner images. This paper introduces a finite element modeling of electromagnetic and acoustic fields in DT‐MRI sequences. The analysis involves three dimensional modeling of the scanner and its interaction with pulses applied to gradient coils. Efficient modeling of induced fields is essential in optimizing parameter settings and improving performance of this imaging modality.