• A new approach for strength and stiffness prediction of discontinuous fibre reinforced composites (DFC)

      Shah, S.Z.H; Choudhry, Rizwan, Saeed; Mahadzir, Shuhaimi; University of Derby; Universiti Teknologi, Malaysia (Elsevier, 2020-12-03)
      A new modelling methodology for strength and stiffness prediction of discontinuous fibre-reinforced composites (DFC) is proposed. This has been validated for both thermoplastic and thermoset, prepreg based, carbon fibre reinforced, random DFC laminates having high volume fraction, by implementing it in a commercial FE solver. The methodology involves explicit generation of internal architecture of DFC through an algorithm which is efficient (faster model generation and solution), easily customizable and scalable. It captures many of the realistic features of the DFC such as variation in volume fraction, interlacing of strands, random orientation and thickness variation of strands. Thus, the model accounts for the natural mechanical property variation, which is characteristic of random DFCs and was found to be conservative in terms of prediction of tensile strength and stiffness for all the validation cases considered. It is generic in the sense that it can be easily extended to generate preferentially aligned and hybrid DFC laminates.
    • Exploring educational advantage in the UK via graduate employment of joint honours degrees by examining pre-university tariff and degree classification

      Pigden, Louise; Moore, Garford; University of Derby (Emerald, 2019-12-17)
      In the UK, the majority of university students specialise and study just one subject at bachelor degree level, commonly known in the UK as a single honours degree. However, nearly all British universities will permit students if they wish to study two or even three subjects, so-called joint or combined honours degrees, internationally known as a double major. The purpose of this paper is to explore the relationship between graduate employment, pre-university educational attainment and degree classification achieved. The study also explored student choice with respect to university prestige. The authors analysed the complete data set provided from the Higher Education Statistics Agency Destination of Leavers from the Higher Education survey, and combined this with data from the POLAR4 quintiles, Universities and Colleges Admissions Service (UCAS) tariff points and degree classification. The data were analysed to establish whether there was a difference in the choices and highly skilled graduate employment of the joint honours students, focussing particularly on Russell Group and Post-92 Universities, in order to build on previous published work. For any UCAS tariff band, the higher the POLAR4 quintile the higher the rate of highly skilled destination. Russell Group outperform the Post-92 graduates in their rates of highly skilled destinations, for any tariff band and for both joint and single honours degrees. Higher POLAR4 quintile graduates are more likely to study at the Russell Group, with this effect increasing the higher the UCAS tariff. With the exception of first class honours graduates from Post-92 universities, joint and single honours from the Russell Group have a higher rate of highly skilled destination than Post-92 in the next higher degree classification. Low POLAR4 quintile students with high UCAS tariffs are “under-matching” and there is an impact on their graduate employment as a result. This study adds new insights into joint honours degrees and also reinforces the literature around educational advantage and achievement prior to university, and the impact on graduate employment. Educational disadvantage persists over the course of a university degree education, from the perspective of gaining graduate employment. Higher quintile graduates are proportionately more likely to achieve the highest degree classifications, and proportionately less likely to achieve the lowest classifications, than graduates from the lower quintiles. Joint honours graduates are less likely to achieve a first class honours degree than single honours, and this will affect their rate of highly skilled destination.
    • A review on recent developments of thermoelectric materials for room-temperature applications

      Solemaini, Zohreh; Zoras, Stamatis; Ceranic, Boris; Shahzad, Sally; Cui, YUANLONG; University of Derby; University of Sheffield (Elsevier, 2019-12-13)
      Wearable thermoelectric generators (TEGs) emerge as a viable renewable energy source, which directly convert the heat dissipated from human skin into electricity. Extensive reviews have been conducted on the efficiency of thermoelectric materials (TE) as the dominant element of TEGs. TE materials are categorised as inorganic, organic, and hybrid. Each of these reviews focused on either a specific type of TE materials, or on a certain specification (i.e. flexibility) of them. However, less attention has been paid to comprehensively review all these types without taking into account a certain specification. Therefore, the purpose of this paper is to summarize the progress and current state-of-the-art research on the three types of TE materials respecting their TE properties and efficiency at 300K, which is the operating temperature of wearable TEGs. Concerning the inorganic TE materials, the results show that Bi0.4-xSb1.6+xTe3 and Bi2Te2.7Se0.3 are the most optimal TE materials, which exhibit the greatest efficiencies at room temperature. In addition, it is remarkably more efficient to replace polymer based TE composites with carbon based TE composites in the organic and the hybrid types. In total, this comprehensive review paves the way for researchers to find out the most suitable TE materials at room temperatures.
    • Numerical analysis of flow in the gap of a simplified tractor-trailer model with cross vortex trap device

      charles, terrance; Yang, Zhiyin; lu, yiling; University of Derby (World Academy of Science, Engineering and Technology, 2019-12)
      Heavy trucks are aerodynamically inefficient due to their un-streamlined body shapes, leading to more than of 60% engine power being required to overcome the aerodynamics drag at 60 m/hr. There are many aerodynamics drag reduction devices developed and this paper presents a study on a drag reduction device called Cross Vortex Trap Device (CVTD) deployed in the gap between the tractor and the trailer of a simplified tractor-trailer model. Numerical simulations have been carried out at Reynolds number 0.51×106 based on inlet flow velocity and height of the trailer using the Reynolds-Averaged Navier-Stokes (RANS) approach. Three different configurations of CVTD have been studied, ranging from single to three slabs, equally spaced on the front face of the trailer. Flow field around three different configurations of trap device have been analysed and presented. The results show that a maximum of 12.25% drag reduction can be achieved when a triple vortex trap device is used. Detailed flow field analysis along with pressure contours are presented to elucidate the drag reduction mechanisms of CVTD and why the triple vortex trap configuration produces the maximum drag reduction among the three configurations tested.
    • Creep damage of BGA solder interconnects subjected to thermal cycling and isothermal ageing

      Depiver, Joshua Adeniyi; Mallik, Sabuj; Amalu, Emeka H; Harmanto, Dani; University of Derby (IEEE Xplore, 2019-12)
      Solder joints of electronic components are the most critical part of any electronic device. Their untimely failure during the system’s operation often culminates in catastrophic failure of the device. The determination of creep damage in electronic component solder joint is vital to the prediction of crack initiation and prevention of premature failure. This paper presents the creep damage in solder joints in a ball grid array (BGA) soldered on a printed circuit board (PCB) and subjected to thermal cycling as well as isothermal ageing. ANSYS 19.0 package is employed to model the isothermal gaining at -40, 25, 75 and 150℃ temperatures for 45 days. Standard temperature cycle profile is used to simulate the effect of the coefficient of thermal expansion mismatch on the bonded materials in the BGA component. The solders studied are lead-based eutectic solder alloy and lead-free SAC396, SAC387, and SAC305. Based on the results obtained for the stress, strain and strain energy of the solders, the research investigation advises on the most effective solder for achieving improvement in the thermo-mechanical reliability of solder joints in BGA soldered on PCB.
    • Multiple geophysical techniques for investigation and monitoring of sobradinho landslide, Brazil

      Hussain, Yawar; Cardenas-Soto, Martin; Martino, Salvatore; Moreira, Cesar; Borges, Welitom; Hamza, Omar; Prado, Renato; Uagoda, Rogerio; Rodríguez-Rebolledo, Juan; Silva, Rafeal; et al. (MDPI AG, 2019-11-26)
      Geophysical methods have a varying degree of potential for detailed characterization of landslides and their dynamics. In this study, the application of four well-established seismic-based geophysical techniques, namely Ambient Noise Interferometry (ANI), Horizontal to Vertical Spectral Ratio (HVSR), Multi-Channel Analysis of Surface Waves (MASW) and Nanoseismic Monitoring (NM), were considered to examine their suitability for landslide characterization and monitoring the effect of seasonal variation on slope mass. Furthermore, other methods such as Ground Penetrating Radar (GPR) and DC Resistivity through Electrical Resistivity Tomography (ERT) were also used for comparison purpose. The advantages and limitations of these multiple techniques were exemplified by a case study conducted on Sobradinho landslide in Brazil. The study revealed that the geophysical characterization of the landslide using traditional techniques (i.e., GPR, ERT and MASW) were successful in (i) the differentiation between landslide debris and other Quaternary deposits, and (ii) the delineation of the landslide sliding surface. However, the innovative seismic based techniques, particularly ambient noise based (HVSR and ANI) and emitted seismic based (NM), were not very effective for the dynamic monitoring of landslide, which might be attributed to the short-time duration of the data acquisition campaigns. The HVSR was also unsuccessful in landslide site characterization i.e., identification of geometry and sliding surface. In particular, there was no clear evidence of the light seasonal variations, which could have been potentially detected from the physical parameters during the (short-time) ambient noise and microseismic acquisition campaigns. Nevertheless, the experienced integration of these geophysical techniques may provide a promising tool for future applications
    • The evolution of turbulence characteristics in the wake of a horizontal axis tidal stream turbine

      Ahmadi, Mohammad H.B.; Yang, Zhiyin; University of Derby (Elsevier BV, 2019-11-21)
      The evolution of turbulence characteristics downstream of a laboratory-scale three-bladed horizontal axis turbine is investigated in this study. Large eddy simulation (LES) coupled with the Actuator Line Modelling (ALM) is used to simulate the flow. The numerical results compare well against experimental data, which shows that the LES/ALM technique is a powerful tool for simulating tidal stream turbines. The present study aims to obtain a better understanding of the turbulence characteristics of flow in the turbine wake by removing deterministic velocity fluctuations stemmed from the turbine rotation. Large eddy simulation is able to provide high-resolution spatial and temporal information needed for this work. The filtering process helps to have a clearer view of the flow structures downstream by tracking the streamwise variations of turbulence intensity and turbulent kinetic energy and, reveals a transition zone started shortly behind the turbine with a peak in the turbulence intensity. This study introduces turbulence intensity and turbulent kinetic energy as quantitative criteria to split the turbine wake into distinct regions. This research shows that to investigate and explain the influence of different upstream and operation conditions on the flow characteristics in the turbine wake, a well understanding of flow characteristic changes in the transition zone is necessary.
    • Buckling and strength analysis of panels with discrete stiffness tailoring

      Culliford, Lucie; Choudhry, Rizwan Saeed; Butler, Richard; Rhead, Andrew T.; University of Bath; University of Derby (Elsevier, 2019-11-05)
      Continuous variation of stiffness across flat plates has been shown, theoretically, to improve buckling performance by up to 60%. However, steered fibre manufacturing methods cannot achieve the minimum radius of curvature required for improvement whilst maintaining a high deposition rate. An alternative concept, Discrete Stiffness Tailoring (DST), which varies stiffness within a ply through discrete changes of angle, is compatible with high rate deposition methods such as Advanced Tape Laying. Through the simple example of redistribution of the material in a quasi-isotropic [±45/90/0]2S laminate whilst maintaining ply percentages, DST is shown both experimentally and theoretically to improve buckling stress by at least 15% with no indication of failure in regions of discrete angle change (seams). However, the reduced tensile strength of seams obtained by virtual and experimental testing means that increased buckling performance in the principle load direction needs to be balanced against loss of transverse strength
    • Invoking humanism in modernity: architecture and spectacle in fascist Italy

      Tracada, Eleni; Temple, Nicholas; University of Derby; University of Huddersfield (Routledge, 2019-11-01)
      The influence of Fascism on intellectual, artistic and architectural developments in interwar Italy has been the subject of intense debate. This has given rise to contested views about the combined impact of modernism and historical precedents on Fascist ideology, the arguments often clouded by disputes concerning the patrimony of art in Italy and whether Fascism should cultivate its own distinctive aesthetic. 2 However, many of the leading voices of Italian cultural life during the Fascist regime refused to discriminate between different aesthetic choices, believing that “Italian cultural traditions precluded aesthetic regulation.” 3 The debate becomes most revealing when considered in the context of the origins of Fascism. The eminent Italian philosopher and historian Benedetto Croce believed, for example, that Fascism could be traced back almost exclusively to the futurist movement, both in its artistic aspirations and in political activism … in the resolution to go down to the piazza, to impose one’s own feelings, to shut the mouths of those who are dissenting, to be unafraid of commotions and riots; in the eagerness of the new, in the desire to break every tradition, in the exaltation of youth, which was proper to futurism.” 4
    • Separated boundary layer transition under pressure gradient in the presence of free-stream turbulence

      Li, Huajun; Yang, Zhiyin; University of Derby; University of Sussex (AIP Publishing, 2019-10-14)
      Large-eddy simulation (LES) has been carried out to investigate the transition process of a separated boundary layer on a flat plate. A streamwise pressure distribution is imposed to mimic the suction surface of a low-pressure turbine blade, and the free-stream turbulence intensity at the plate leading edge is 2.9%. A dynamic subgrid scale model is employed in the study, and the current LES results compare well with available experimental data and previous LES results. The transition process has been thoroughly analyzed, and streamwise streaky structures, known as the Klebanoff streaks, have been observed much further upstream of the separation. However, transition occurs in the separated shear layer and is caused by two mechanisms: streamwise streaks and the inviscid K-H instability. Analysis suggests that streamwise streaks play a dominant role in the transition process as those streaks severely disrupt and break up the K-H rolls once they are formed, leading to significant three-dimensional (3D) motions very rapidly. It is also demonstrated in the present study that the usual secondary instability stage under low free-stream turbulence intensity where coherent two-dimensional (2D) spanwise rolls get distorted gradually and eventually broken up into 3D structures has been bypassed.
    • State-of-the-art review of 3DPV technology: structures and models

      Cui, Yuanlong; Zhu, Jie; Stamatis, Zoras; Chen, Xiangjie; Bi, Haixia; Qiao, Yaning; Soleimani, Zohreh; University of Derby; University of Nottingham; China University of Mining and Technology (Elsevier, 2019-10-05)
      Increasing energy conversion efficiency from sunlight to power is one of the key solutions for the world’s energy shortage and greenhouse gas reduction, but the conventional flat photovoltaic module without sun tracking mechanism has the low sunlight energy collection ability. This paper presents the state-of-the-art three-dimensional photovoltaic (3DPV) technology with high photovoltaic energy conversion efficiency, which is able to absorb off-peak sunlight and reflected light more effectively, thereby it can generate more power. At first, this paper is to catalogue and critique different 3DPV structures and models, as well as assess their characteristics. Afterwards, the main influence factors on the 3DPV structures and models including shape, height and spacing of the solar cells, latitude of the installation, optimal device design and shadow cast, are reviewed. Finally, the challenges and future technological developments of 3DPV structures and models are highlighted. This study demonstrated that the 3DPV technology can increase the captured sunlight approximately 15–30% in comparison with the conventional flat PV technology.
    • Creep response of various solders used in soldering ball grid array (BGA) on printed circuit board (PCB)

      Depiver, Joshua Adeniyi; Mallik, Sabuj; Amalu, Emeka H; University of Derby (Newsroom Limited, 2019-10)
      In electronics packaging, solder joints play a critical role by providing electrical, thermal and mechanical connections between the package and the printed circuit board (PCB). As the joint is both miniature and brittle, it is the weakest part of the assembly and thus susceptible to untimely damage. This paper presents the creep response of solder joints in a ball grid array (BGA) soldered on a PCB subjected to isothermal ageing in one experiment and temperature cycling in another test. The ageing is simulated in an ANSYS package environment at -40, 25, 75 and 150℃ temperatures applied for 45 days. The thermal cycling profile started from 22℃ and cycled between -40℃ and 150℃ with 15 minutes dwell time at the lowest and highest temperatures. The solders used in the investigation are lead-based eutectic solder alloy and lead-free SAC305, SAC387, and SAC396 solders. The research seeks to qualify these solders against strain and strain energy response for improved reliability in operation. The results show that the lead-free SAC387 accumulated the maximum strain and thus strain energy while the lead-based eutectic solder is found to accrue the least amount of the quantities. Further results show the distribution of damage in the BGA solder bump. Based on the results, it is proposed that lead-free SAC396 is the best replacement of the lead-based eutectic solder in the drive for the achievement of comparable thermo-mechanical reliability of assembled BGA on PCB.
    • Synthesis and characterization of tungsten and barium co-doped La2Mo2O9 by sol-gel process for solid oxide fuel cells

      Shao, Lingbo; Ji, Dongdong; Yang, Jie; Xie, Jinsong; Yin, Qiyi; Le, Huirong; Hefei University; University of Derby (Elsevier, 2019-09-30)
      Herein, we demonstrate the synthesis of W and Ba co-doped La2Mo2O9 (LBMWO) nanocrystalline powder by a sol-gel process. In all the compositions have general formulae La1.9Ba0.1Mo2–xWxO8.95 (x = 0–0.40). The crystal structure, microstructure and conductivity of LBMWO were characterized by X-ray diffraction, scanning electron microscopy and electrical impedance spectroscopy. In addition, the thermal and decomposition properties of the LBMWO gel were analyzed by differential scanning calorimetry - thermogravimetric. The results reveal that all LBMWO powders calcined at 700 °C have a cubic structure; the average crystallite size is about 48 nm. The unit cell parameter of LBMWO powders increases with increase in W content. The as-synthesized nanocrystalline LBMWO samples exhibit excellent sinterability and a relatively lower sintering temperature of 900 °C. A high relative density of ∼96% is achieved after sintering at 900 °C which is in good agreement with the results of the SEM. Moreover, W and Ba co-doping suppresses the phase transition and effectively stabilizes the β-phase at low temperature. At the same time, La1.9Ba0.1Mo1.85W0.15O8.95 exhibits high ionic conductivity, 3.07 × 10−2 S/cm at 800 °C. It is therefore concluded that co-doping can improve the properties of La2Mo2O9 electrolytes.
    • Antibacterial properties of silver nanoparticles grown in situ and anchored to titanium dioxide nanotubes on titanium implant against Staphylococcus aureus

      Gunputh, Urvashi F.; Le, Huirong; Lawton, Kiruthika; Besinis, Alexandros; Tredwin, Christopher; Handy, Richard D.; University of Derby; Plymouth University (Taylor & Francis, 2019-09-30)
      Medical grade titanium alloy, Ti-6Al-4V, with TiO2 nanotubes (TiO2-NTs) grown on the surface and then decorated with silver nanoparticles (Ag NPs) is proposed to enhance the antimicrobial properties of the bone/dental implants. However, the decoration with Ag NPs is not consistent and there are concerns about the direct contact of Ag NPs with human tissue. The aim of this study was to achieve a more even coverage of Ag NPs on TiO2-NTs and determine their biocidal properties against Staphylococcus aureus, with and without a top coat of nano hydroxyapatite (nHA). The decoration with Ag NPs was optimised by adjusting the incubation time of the TiO2-NTs in a silver ammonia solution, and using biocompatible δ-gluconolactone as a reducing agent. The optimum incubation in silver ammonia was 7 min, and resulted in evenly distributed Ag NPs with an average diameter of 47.5 ± 1.7 nm attached to the surface of the nanotubes. The addition of nHA did not compromise the antimicrobial properties of the materials; high-resolution electron microscopy showed S. aureus did not grow on the composite with nHA and with >80% biocidal activity measured by the LIVE/DEAD assay, also limited lactate production. Dialysis experiment confirmed the stability of the coatings, and showed a slow release of dissolved silver (3.27 ± 0.15 μg/L over 24 h) through the top coat of nHA.
    • Value chain impacts of EU waste framework directive 2018/851 as a result of reporting substances of very high concern from 2021

      Takhar, Sukhraj; Liyanage, Kapila; College of Engineering and Technology, University of Derby; Assent Compliance, Canada (University of Cambridge, 2019-09-27)
      The EU Waste Framework Directive 2018/851 sets out requirements for producers and importers of products within the EEA, to report data on Substances of Very High Concern (SVHC) content within products into a new central European database to from 5th January 2021. The reporting requirements: (1) support chemical regulations that impose the need on industry to record the use of hazardous chemicals; (2) identification of products entering waste streams containing hazardous chemicals; (3) support circular economy initiatives within the EU. To meet these new reporting requirements industry will need to collect additional information from all supply chain actors, who as duty holders will also be required to report into the new EU SVHC database system. Failure to provide the required information may result in enforcement actions from the authorities, which could see products being restricted from the EEA. The new EU database system will be accessible to industry, regulators, NGOs and the general public. This paper explores the new requirements, together with feedback received from various stakeholders for collection of data and reporting into the new EU database system from 2021.
    • Geohazard assessment of landslides in south Brazil: Case study

      Hamza, Omar; De Vargas, Tiago; Boff, Fernando Eduardo; Hussain, Yawar; Sian Davies-Vollum, K.; University of Derby; Centro Universitário da Serra Gaúcha (FSG); University of Brasilia (Springer Science and Business Media LLC, 2019-09-10)
      Slope instability phenomena such as mudslides represent a major geohazard in Brazil, which has caused devastation in many states and affected the lives of people, particularly in self-built settlements on steep slopes. This paper presents and discusses slope stability issues encountered in Caxias do Sul in the State of Rio Grande do Sul, which exemplifies the existing situation of landslide risk assessment in southern Brazil. Local geology and ground conditions of the area in relation to slope instability were reviewed and gaps in information required for mitigating risk were identified, such as inadequate geotechnical information and lack of full inspection and continuous monitoring of active landslides. Although risk assessment has been developed for the city and regarded as a fundamental management tool in the mitigation of landslide hazards, the study showed that the risk assessment works are outdated and not effectively considered for the development of the city. With significant unplanned urban expansion (where houses have been self-built on very steep terrains without geotechnical assessment of the ground and slope conditions), new geohazard mapping is essentially required. Several key recommendations were provided for mitigating the destructive effect of landslides and improving their management in mountainous urban settings.
    • Understanding the implications of chemical regulations, circular economy and corporate social responsibility for product stewardship

      Takhar, Sukhraj; Liyanage, Kapila; University of Derby (IOS Press, 2019-09-10)
      Chemical regulations exist to limit and control the amount of hazardous chemical substances being used by industry to mitigate potential risks. Increasing awareness of diminishing natural resources, increasing pollution, and reducing the amounts of harmful waste, has led towards pressure on industry to change from the traditional linear economic model (closed-loop manufacturing), towards the adoption of the Circular Economy (CE) activities. Corporate Social Responsibility (CSR) extends the relationship between industry and society, defining strategy and communications. The speed at which chemical regulations, CE and CSR adoption takes place will depend on: (1) the speed at which natural resources become more depleted; (2) increasing awareness of the effects of chemical substances by regulators and consumers; (3) increasing numbers of chemical substances being regulated as hazardous, and; (4) increasing consumer and societal pressures for change. Product Stewardship (PS) can be viewed as: (1) involving a wide range of functional areas (design, manufacture, purchasing, sales, support); (2) identifying health and safety, and environmental impacts of product(s); (3) ensuring adequate measures are in place to understand, control or limit impact(s) against a product throughout its lifecycle. The aim of this paper is to focus on using a literature review. The conclusions from this paper will attempt to outline a framework for PS to align with CE and CSR.
    • Thematic analysis of the learning experience of joint honours students: their perception of teaching quality, value for money and employability

      Pigden, Louise; Jegede, Franc; University of Derby (Taylor and Francis, 2019-09-04)
      The focus of the research was students’ perceptions of whether joint honours degrees represented a sound student learning experience and led to good academic and employability outcomes. In this minority mode of study for the United Kingdom, students study two academic subjects, rather than the more usual single subject. The qualitative methodology followed was a thematic analysis of self-administered questionnaire data, collected from joint honours students at four English universities in the United Kingdom. The principal results were that the students experienced difficulties in fitting into the two communities of learning for their respective subjects; in being adequately supported; in disparities in teaching, learning and assessment between their two subjects, and in a high workload. The major conclusion was that although the students perceived their learning experiences to have value by virtue of having studied two subjects, a number of issues stymied their achieving the best learning experience possible.
    • Experimental data of bio self-healing concrete incubated in saturated natural soil

      Souid, Adam; Esaker, Mohamed; Elliott, David; Hamza, Omar; University of Derby (Elsevier, 2019-08-22)
      he provision of suitable incubation environments is vital for successful implementation of bio self-healing concrete (bio-concrete). We investigated the effect of soil incubation to examine if the self-healing process can be activated in comparison with the conventional incubation environment (water). The data was collected from laboratory-scale experiments conducted on mortar specimens. The mortar was impregnated with Bacillus subtilis and this bacteria was encapsulated in calcium alginate for protection from the production process. The mortar specimens were mechanically cracked and then incubated within fine-grained fully saturated natural soil for about 4 weeks. The cracks were inspected before and after incubation by light microscopy to evaluate the healing ratio. The mineral precipitations on crack surfaces were examined by Scanning Electron Microscope (SEM) and Energy Dispersive X-Ray Spectrometry (EDX). The data reflects the efficiency of bio-concrete for certain structures such as tunnels and deep foundation, where concrete elements are exposed to ground conditions.
    • Variation of molecular mean free path in confined geometries

      Xie, Jianfei; University of Edinburgh (AIP Publishing, 2019-08-05)
      This paper aims to settle disputes in the literature about the spatial variation of the molecular mean free path (MFP) in confined geometries. The MFP of a gas is determined by using both molecular dynamics (MD) and the direct simulation Monte Carlo (DSMC) technique. In spatially-homogeneous cases, the numerical results exactly recover the kinetic theory predictions of a constant MFP. However, in microchannels, the MFP is found to vary near to the bounding walls and reduce at the surfaces to half of its bulk value as long as collisions between gas molecules and wall atoms are taken into account in the calculation of the MFP.