Research

My background is in Applied Mechanics and my main research activities and contributions are mostly related to mechanics and how mechanics are applied to various applications such as composites, noise and vibration, medicine, bio-fouling, design and manufacturing, nanotechnology, and more recently noise and vibration. The research is ongoing with immediate research plans for the mitigation of aircraft fly-past noise for schools and buildings under the flight path and also silent propellers for drones.

My main research contribution and impact is in Noise and Vibration as reflected in my recent grants, invited and plenary talks as well as the intellectual properties that have been filed and implemented. My secondary research contribution and impact is for the mechanics in medicine, for which I have established my niche in the applications of mechanics for the understanding and planning of medical conditions and clinical applications.

Noise and vibration
The early contribution is in terms of structural intensity. Structural intensity is a vector quantity and it is defined as power flow per unit cross sectional area. Research on structural intensity in the past before our endeavor only focused on vibration of plates and other structures in the general area of structural mechanics. Our group is among the pioneers to extend the applications to not only structural mechanics, but also impact mechanics, fracture mechanics, rotating disk, thermal induced vibration as well as biomechanics. YC Tse, a researcher in IHPC under my supervision, was one of the four winners of the 1st IASS (International Association of Space Structures) Hangai Prize in 2003. The paper was “YC Tse, XD, Xu, HP Lee and C Lu, Design of Stiffened Plate Structures using the Structural Intensity Concept”.

The next research contribution is in terms of the mechanics of Quartz Crystal Microbalance. A Quartz Crystal Microbalance (QCM) determines minute mass by measuring the change in frequency of a quartz crystal resonator. The resonance is disturbed by the addition or removal of a small mass due to particle, liquid droplet or film deposition at the surface of the acoustic resonator. The initial research on this topic started off with a NUS Academic Research Grant on “Quartz Resonator Sensors for Chemical and Biological Applications”. The work was further extended with other PhD students (Lu Feng and Zhuang Han) co-supervised by me. The works were reported in a couple of reputable Journals under the Institute of Physics such as Sensors and Actuators A – Physical, Smart Materials and Structures, Journal of Physics D – Applied Physics, Langmuir, and Analytical Chemistry. We are among the early researchers developing a mechanics model for modeling the behaviors of solid-liquid interface of QCM.

My more recent research efforts are in noise and vibration. The main objective of the research program is to establish an innovative solution with suitably designed sonic-crystal window structures in the form of periodic array of glass panels which will ensure a good balance of natural daylighting, noise mitigation and natural ventilation. A patent had been filed for this project. A featured article related to the work was recently reported in Straits Times.
http://www.straitstimes.com/singapore/too-noisy-sonic-crystal-windows-to-the-rescue

Another outcome from the L2NIC project is the use of smartphone for noise mapping. We have come up with a methodology for accurate calibration of smartphones for them to be used as sound level meters for the noise mapping. The accuracy was found to be within 1 dB compared to a type 1 sound level meter. Preliminary results of noise mapping can be found in the following website. The idea is being extended to temperature and humidity maps using wireless sensors as well as the GPDS data available from the smartphones.
http://worldnoisemap.com

The other outcome from the L2NIC project is the use of jagged edge and flat-tip jagged edge for both the louver windows and propeller blades for enhanced destructive noise interference and therefore enhancing the noise mitigation. We have designed propeller blades with suitably designed flat-tip jagged edge for drones. Serrated edge or jagged edge is not new but suitably designed flat-tip jagged edge is new for this application. I am also currently exploring the use of hybrid composite blades for drones for noise mitigation.

Another research is on the design of noise barriers for the mitigation of construction noise funded by the Land Transport Authority (LTA) innovation grant. The site test results proved that the jagged-edge noise barrier design consistently achieved a better performance, as compared to the existing straight-edge noise barrier design. The noise reduction was up to 5.0 dBA, equivalent to 30% reduction to the human ears. It had resulted in the filing of a patent and LTA is implementing it. Contractors for (Thomson East Coast Line) TELC5 and TELC6 have implemented/are implementing this new design across their sites. The outcome of the project had been won the Ministry of National Development (M&D) Minister’s R&D award (Merit Award) 2017. The other ongoing research project is on the acoustic performance of nature fibre and hybrid fibre composites. A recent finding is the use of natural fibre composite or hybrid composite in ear cups (or earmuffs) for enhanced mitigation of low frequency noise below 250 Hz. We are also exploring the use of natural fibre and hybrid fibre composites as propeller blades for drones to reduce the operation noise.

Otolaryngology research
The nose is the guardian angel of the respiratory tract. It has several important physiological functions which include air-conditioning, filtrating the inspired air, and smell. It also plays an important defence function, as the nose is the first place where foreign pathogens and allergens contact the host. To serve these important functions, a functional or patent nasal passage is essentially needed. A better understanding of how the nose functions is important and related to the treatment of respiratory related medical conditions such as snoring, Obstructive Sleep Apnea (OSA), and the contraction of diseases such as SARS and Bird Flu. Nasal obstruction is also a common complaint which is difficult to quantify clinically. The etiologic factors for nasal obstruction include anatomic variations of the nose and various local and systemic diseases. Hence, objective assessment of the nasal airway will aid diagnosis, treatment, research and medico-legal documentation. The advantages of computational fluid dynamics (CFD) enable researchers to obtain detailed flow patterns in the human upper airway by reconstructing models from computed tomography (CT) and Magnetic Resonance Imaging (MRI) images, which has become a new reliable trend of nasal airway exploration. The research on otolaryngology and nasal airflow is an attempt to bridge the current gap by correlating the engineering simulations to actual physiological functions of the nasal airways. Several nasal cavity models have been created based on CT or MRI of patients. The initial phase of the research focused on the analysis of nasal blockade or Inferior Turbinate Hypertrophy on the aerodynamic pattern and physiological functions of the turbulent airflow. Subsequent studies extended the flow simulations to particle deposition related to drug delivery as well as the thermal effect. The study was then extended to other geometric effects such as septal deviation and septal perforation as well as the effect of various surgical procedures such as inferior turbinectomy and towards the later part of the research, on the effect of Functional Endoscopic Sinus Surgery (FESS) as well as nasal fractures. All the numerical simulations were examined and co-related to clinical observations and therefore most of the findings were in fact published in reputable clinical journals related to otolaryngology such as the Laryngoscope, Journal of Aerosol Medicine and Pulmonary Drug Delivery, American Journal of Rhinology and Allergy, Respiratory Physiology and Neurobiology and a few others.

The other aspect of the research is on the study of mucociliary functions. Numerical models and simulations based on Immersed Boundary Methods and Projection Method have been formulated and carried out to investigate the effects of temperature and viscosity of mucus layer on the mucociliary functions. Experimental studies have been carried out to establish a viable procedure for the sample cilia hair collection and culture. Human subject studies have been carried out to examine the effect of temperature on the mucociliary functions. We have also developed a technique based on Rhinomanometry and Spirometer which can objectively assess the possible impairment in breathing efforts caused by the wearing of N95 facemasks. This is the first time that such an objective measurement regarding the impairment of facemasks has been reported.

The research had led to a project on the effect of long duration wearing of N95 respirators and surgical facemasks funded by the MOH Communicable Disease grant. The objective was to examine the potential effect on breathing resistance due to long-duration wearing of N95 respirator and N95 facemask in response to the potential occurrence of infectious disease such as SARS and also the recurrence of haze. The project was based on my earlier work which was able to measure and quantify the increase of breathing resistance using acoustic rhinomanometry while wearing the typical N95 respirator. A surprised finding was that the flow resistance will still increase half an hour after the removal of facemask probably due to physiological factors.

Another aspect is the nasal trauma research. Although fractures of the nose are the most common fractures of the face, the current treatment outcome of nasal fractures with closed reduction and manipulation is associated with suboptimal results in only half of the patients. To date this remains a clinical problem that is poorly understood. In primary and secondary correction of nasal fractures, the key maneuver is reconstruction of septal alignment and integrity, which often requires open surgery. Preoperative planning for nasal fractures has a clinical blind spot – although a large component of the projecting external nose is cartilaginous, conventional CT and MRI scans fail to show these nasal cartilages well, neither in its entirety nor in 3D. Clinicians frequently have to rely on indirect methods to assess the septal cartilage via clinical and endoscopic evaluation to assess septal shape and dislocation. This is non-ideal and provides incomplete information. This collaborative research seeks to bridge the significant gaps in the current understanding of nasal septal fracture by studying the common septal fracture patterns and their respective causes as well as addressing the issues of the 3D anatomy of the normal nasal septal cartilage and its inter relationship with the bony septum. The works were published in the Journal of Laryngoscope. An external grant from Switzerland, CranioMaxilloFacial Clinical Priority Program was obtained for this project.

Head injury and protective technology research

Head injury is one of the main causes of death or permanent disability in everyday life. There is a need for biomechanical studies of head injury, its mechanisms and its tolerance to external loading. My initial research focus for this area started with the exploration for the use of Structural Intensity (power flow per unit cross sectional area) as a potential head injury criterion. The research has progressed with the study of head helmet interaction and a more recent endeavor in developing a more detailed finite element head model including the upper airway and nasal cartilages. The research findings were reported in Journal of Biomechanics, Computer Methods in Biomechanics and Bioengineering, International Journal of Impact Engineering, IEEE Transactions on Biomedical Engineering, and a few other related journals. There were a couple of collaborative projects funded by DSO National Laboratory and Temasek Defence Systems Institute. We were among the pioneers to construct a 3D Helmet model based on CT scans. The technique was later extended to the examination of internal defects or damages for composite sample after impact.