Unit Width Research Articles

Elastic Molecular Crystals: Their Deformation-Induced Reversible Unit Cell Changes with Specific Poisson Effect

Abstract Detailed investigation of macroscopic deformation and nanoscopic structural changes in flexible organic crystals poses challenges for investigators. Herein, applied stress and subsequent relaxation of elastic organic crystals resulted in reversible macroscopic crystal deformation. X-ray diffraction with a curved stage-jig revealed reversible nanoscopic structural unit cell changes in the crystal structure under the bending stress and relaxation. The crystal lattice changed quantitatively under the applied macroscopic stress-strain (%). This method enables quantitative monitoring of the dynamic nanoscopic structural changes in detail associated with crystal deformation through the use of standard laboratory X-ray diffraction analysis. Importantly, the developed method offers a way of quantitatively measuring reversible structural changes, without synchrotron X-ray analysis. Moreover, the analysis derives Poisson’s ratio, i.e., the ratio of the change in the width per unit width of materials. It is important in materials science, and normally has a positive value in the range of 0.2–0.5. However, the crystals show not only the “Poisson effect” but also the unusual “negative Poisson effect”. This novel approach for investigation generates unprecedented opportunities for understanding dynamic nano-structural unit cell changes in flexible organic crystals.

A comprehensive study of the liquid transfer from the front to the back of a vertical perforated sheet

AbstractPerforations contribute to liquid redistribution in corrugated sheet packings. We focus on a simplified but relevant experimental configuration where a vertical perforated flat sheet is supplied with liquid on its front side. We examine how the perforations irrigate the back of the plate. We successively consider a single perforation, a spanwise row of perforations, and a staggered array of perforations. We quantify the liquid transfer through a single row of perforations and find that the transferred flow rate per unit perforation diameter varies linearly with the supply flow rate per unit width. We also analyze the spreading of the rivulets leaking from the perforations, their merging into a continuous wavy film, and the leveling of this film as it falls down the plate. The spreading and the merging exhibit a power‐law behavior in agreement with theoretical models. The leveling exhibits exponential decay behavior.

On some aspects of flow characteristics of the bifurcated channel – An experimental approach

ABSTRACT The purpose of this research is to get an overall idea about the physical process of discharge and sediment distributions in channel bifurcations. A physical model has been used for the experimental study of channel bifurcation corresponding to the river Kangsabati in the field. The study describes sediment distribution of different sediment sizes for a bifurcation angle of 37° (the angle between the tip of the nose and the symmetrical line of a bifurcation). A total of three different sizes of sediment (0.190 mm, 0.260 mm, and 0.275 mm) have been used to investigate the influence of sediment distribution over the branches. For each sediment size, four different upstream discharges (0.002, 0.004, 0.006, and 0.008 m3/sec) have been used. From the experiments, a set of data on s1 /s 2 and q 1/q2 (where q 1, q 2, and s 1, s 2 are the discharge and sediment transport rate per unit width through respective bifurcated branches) have been collected. These data have been set to the nodal point relation (the relation between the ratio of downstream discharges and the ratio of the downstream sediment transport rates). The study so far performed is based on experimental data obtained from the physical model of channel bifurcation, and the results have been discussed in the present paper.

Nonlinear hydrodynamic analysis and optimization of oscillating wave surge converters under irregular waves

To evaluate the mean annual capture width ratio (CWR) of the oscillating wave surge converters (OWSCs) under irregular waves, the time-domain dynamic equation was derived by considering the nonlinear hydrostatic stiffness, drag, and nonlinear power take-off (PTO) system. A Python code was developed to solve the response and capture performance with 4th-order Runge–Kutta integration. The wave spectrum was corrected according to the water depth in shallow water. Unlike the inflexible drag coefficients in steady flow, the drag coefficients in irregular waves (calibrated with OpenFOAM) are strongly affected by OWSC thickness. Multi-objective genetic algorithm (MOGA) optimization of OWSC geometric sizes, internal water filling, and PTO parameters was conducted for two objective functions: maximizing the mean annual CWR and minimizing the structural mass per unit width. The optimized result presents a slender OWSC that neutrally balances these two objectives. The effects and local sensitivities of the width, thickness, axis depth, water filling, PTO stiffness, damping, and friction were comprehensively discussed. The results show that the axis depth has the greatest positive influence on the CWR, and the increase of thickness creates significant economical disadvantage due to a heavier structure. However, inertia adjustment by filling water does not benefit the mean annual CWR.

Submarine landslide morphometrics and slope failure dynamics along a mixed carbonate-siliciclastic margin, north-eastern Australia

Comparatively little work has been carried out on the morphology and distribution of submarine landslides on mixed carbonate-siliciclastic margins. The morphometric analysis of 84 open slope submarine landslides on the Great Barrier Reef (GBR) margin of north-eastern Australia provides useful insights into slope failure dynamics and frequency distribution of landslides on mixed margins. Our analysis has revealed that the slope area affected by failures (12.6% of the margin) is similar to siliciclastic-dominated passive margins, although the total volume of remobilized sediment (73 km3) is comparatively small. Landslide scars lie at shallower depths to the south of the margin (mean of 576 m vs 1517 m to the north) and there is good correlation between the depth at origin and depth at termination for the GBR landslides. The cumulative frequency distribution of volume, area and total length of the GBR landslides does not fit to common distributions (e.g., power law, logarithmic or exponential) for the entire dataset. Still, the cumulative frequency distribution of landslide dimensions can be statistically explained either by a power law similar to other passive margins, or by a lognormal distribution similar to some siliciclastic margins. Morphometric characteristics, such as the volume of sediment released per unit width and the probability function of volume distribution suggest that slope failures mainly involved relatively unconsolidated sediments. We find that the disintegration by debris flows was the dominant process along the entire GBR margin and that their spreading efficiency and mobility was relatively low. Margin stratigraphy, fluid overpressure at the base of the slope, and detachment surfaces at the boundary between different lithologies that separate sedimentary cycles may have preconditioned the slope to fail. This compilation provides a robust morphometric framework that allows comparison with existing and future slope failure databases, and lays the foundation for performing numerical simulations to assess the landslide-generated tsunamigenic hazards along the GBR margin.

Sentinel-1 Satellite Radar Images: A New Source of Information for Study of River Channel Dynamics on the Lower Vistula River, Poland

The amount of sediments transported by a river is difficult to estimate, while this parameter could influence channel geometry. It is possible to derive the bedload transport rate per unit width of the river channel by measuring the migration distance of bedform profiles over time and thickness of bedload layer in motion. Other possible methods include instrumental measurements using bedload traps and empirical formulas. It is possible to use remote-sensing techniques to measure the dynamics of bedform movements and geometries. Landsat images and aerial photographs have been used for this. A new source of remote-sensing information is radar satellite images. Sentinel-1 images have a temporal resolution of 2–3 days and spatial resolution of 25 m at middle latitudes, which make them usable on large rivers. The research area is the 814–820 km reach of the Lower Vistula River, where seven alternate sandbars were selected. The bank lines of the sandbars were delineated on Sentinel-1 images sensed during two low-flow periods of 4 August–26 September 2018 and 1 July–31 August 2019, when discharges at low flow were similar. From water stage observations at gauges, water elevations were assigned to every bank line of the alternate sandbars. The following morphometric parameters were calculated: alternate sandbar centers, volumes and longitudinal profile. Average daily movement of the sandbars in the period 4 August 2018–1 July 2019 was calculated as 0.97 m·day−1. A similar alternate sandbar movement velocity was obtained from a study of Sentinel-2 optical satellite images and hydro-acoustic measurements on the Lower Vistula River. Having depth of bedload in motion and alternate sandbar shift velocities, it was possible to calculate the rate of bedload transport according to the Exner approach formula. Rate of bedload transport was estimated as qb = 0.027 kg·s−1·m−1. This study shows a novel use of Sentinel-1 images to study the 3D geometry and movement rate of sandbars.

On fluid flow regime transition in rough rock fractures: Insights from experiment and fluid dynamic computation

The influence of roughness, aperture and asperity irregularity on fluid flow regimes in rough rock fractures was investigated by performing coupled triaxial water flow tests and fluid dynamic computation. Ten sets of fabricated curved wedges were developed to obtain different fracture surface roughness by splitting under compression. Three fluid flow regimes were identified in mated rock fractures: pre-linear flow at low flow velocity, linear Darcy’s flow at the medium flow velocity and post-linear flow at high flow velocity. In pre-linear flow regime, the increasing rate of flow rate increased with water pressure gradient, but it decreased in post-linear flow regime. The pre-linear flow is ascribed to the slippage effect of water-fracture interfaces, while the post-linear flow is mainly due to inertial effects. The critical hydraulic gradient for fluid flow regime transition from pre-linear to linear flow increased with the increase of confining stress. The numerical modeling shows that the asperity irregularity influences the flow regime. For low-speed flow in fracture under slip boundary condition, the slip flux per unit width in the fracture with triangular asperity is largest, while that of rectangular asperity is smallest. For high-speed flow in fracture with non-slip boundary condition, the rectangular asperity element causes more severe nonlinearity when compared to the fractures of trapezoidal and triangular asperity elements.

Обоснование параметров балластных ем- костей мобильного причала для перевалки лесоматериалов

The use of a mobile small-sized berth, the design of which was proposed by the authors in the previous paper, significantly increases capacity and reduces the cost of timber transshipment. When towing the berth over large distances, its ballast tanks are empty, which ensures that draft and water resistance to movement are minimized. At the point of operation the leveling ballast tank is filled with water until the top surface of the berth is leveled. In this position, the berth is moved locally in case of significant changes of the water level in the reservoir. Due to manufacturability and minimization of the volume of the leveling tank, it is designed as a part of the inner space at the berth stern, separated by a vertical wall. The permutation ballast tank is filled with water to submerge the berth until it leans on the leveled coastal slope. Water is pumped out of the tank to make the berth float during its local permutations. In this case, a significant trim should not be allowed. The lower point of the longitudinal section of the permutation tank was provided at the intersection of the berth bottom and the vertical, which bisects the waterline upon completion of the alignment. Algorithms have been developed for determining the position of the vertical wall of the leveling tank and the position and shape of the walls of the permutation tank. The distance of the vertical wall from the lower point of the berth and the coordinates of the points of the profiles of the left curved and right walls of the permutation tank were determined for a conditional berth of unit height and width. The coordinates were used to derive empirical dependencies. The values of the calculated metacentric heights verified the longitudinal and transverse stability of the berth in all positions. Experimental validation on a physical model of the berth confirmed the correctness of the results. Transition to the correct dimensions of the longitudinal profile from the conditional is assumed with the proportions retained. For citation: Posypanov S.V., Chuprakov V.O. Justification of Ballast Tank Parameters of a Mobile Berth for Timber Transshipment. Lesnoy Zhurnal [Russian Forestry Journal], 2022, no. 1, pp. 143–154. DOI: 10.37482/0536-1036-2022-1-143-154

ALMA Observations of NGC 6334S. II. Subsonic and Transonic Narrow Filaments in a High-mass Star Formation Cloud

Abstract We present a study of narrow filaments toward a massive infrared dark cloud, NGC 6334S, using the Atacama Large Millimeter/submillimeter Array. Thirteen gas filaments are identified using the H13CO+ line, while a single continuum filament is revealed by the continuum emission. The filaments present a compact radial distribution with a median filament width of ∼0.04 pc, narrower than the previously proposed “quasi-universal” 0.1 pc filament width. The higher spatial resolution observations and higher density gas tracer tend to identify even narrower and lower mass filaments. The filament widths are roughly twice the size of embedded cores. The gas filaments are largely supported by thermal motions. The nonthermal motions are predominantly subsonic and transonic in both identified gas filaments and embedded cores, which may imply that stars are likely born in environments of low turbulence. A fraction of embedded objects show a narrower velocity dispersion compared with their corresponding natal filaments, which may indicate that turbulent dissipation is taking place in these embedded cores. The physical properties (mass, mass per unit length, gas kinematics, and width) of gas filaments are analogous to those of narrow filaments found in low- to high-mass star-forming regions. The more evolved sources are found to be farther away from the filaments, a situation that may have resulted from the relative motions between the young stellar objects and their natal filaments.

Observer‐based adaptive controller design for chaos synchronization using Bernstein‐type operators

AbstractThis article uses the Bernstein‐type operators as the universal approximator to present an observer‐based robust adaptive controller for chaos synchronization. The lumped uncertainties including, un‐modeled dynamics and external disturbances, are modeled with this potent mathematic tool. It is shown that using the Bernstein‐type operators as basis functions and tuning the polynomial coefficients by the adaptive laws calculated in the stability analysis, uniform ultimate boundedness of the observer estimation error and synchronization error can be assured. To analyze the performance of the proposed controller scheme in terms of transient response behavior and robustness, the Duffing–Holmes oscillator is considered as the simulation testbed. A set of two different experiments are conducted to evaluate the efficiency of the introduced control approach. The performance of the proposed approach is also compared with RBFNN as a powerful approximation method. Unlike neural network/fuzzy methods, which require system states as inputs to estimate functions and construct a regressor vector, the proposed method is not dependent on the system states. Furthermore, there are more adjustable parameters in the regressor vector of the RBFNN (such as the width and center of the Gaussian units), and assigning the best values for these parameters is tedious and time‐consuming work due to the repeated trial and error.

Research on Optimization of Climate Responsive Indoor Space Design in Residential Buildings

This paper first analyzes the climate characteristics of five typical cities in China, including Harbin, Beijing, Shanghai, Shenzhen and Kunming. Then, based on Grasshopper, Ladybug and Honeybee analysis software, according to the indoor layout of typical residential buildings, this research extracts design parameters such as the depth and width of different rooms and their window-to-wall ratios etc., to establish a climate responsive optimization design process with indoor lighting environment comfort, with heating and cooling demand as the objective functions. Meanwhile, based on Monte Carlo simulation data, ANN (Artificial Neural Network) is used to establish a prediction model to analyze the sensitivity of interior design parameters under different typical cities’ climatic conditions. The study results show that the recommended values for the total width and total depth of indoor units under the climatic conditions of each city are both approximately 14.97 m and 7.88 m. Among them, under the climatic conditions of Harbin and Shenzhen, the design parameters of residential interiors can take the recommended value of UDI optimal or nZEB optimal. While the recommended values of window-to-wall ratios for the north bedroom, master bedroom and living room in Shanghai residential interiors are 0.26, 0.32 and 0.33, respectively. The recommended value of the window-to-wall ratio of the master bedroom in Kunming residences is 0.36, and that of the remaining rooms is between 0.15 and 0.18. The recommended values of window-to-wall ratios for the master bedroom and living room in Beijing residences are 0.41 and 0.59, respectively, and that for the remaining rooms are 0.15. The multi-objective optimization process based on parametric performance simulation used in the study can effectively assist architects in making energy-saving design decisions in the preliminary stage, allowing architects to have a case to follow in the actual design operation process.

Determination of structural reliability of a reinforced concrete slab under fire Load

Four reinforced concrete rooms are used in this study, with varying floor areas, openings, and fuel loads. These rooms are office, Classroom, computer, and chemical laboratories of institutional buildings. The time versus Eurocode (EC) parametric fire relationship is determined and compared with ISO 834 and ASTM 119. The performance of the slab is evaluated based on the EC target reliability index of 3.8 for 50 years design period using the First Order Reliability Method (FORM). The probability of failure and structural reliability of each fire compartment at different fire duration is determined. Variables used were concrete compressive strength, a reduction factor of concrete compressive strength, steel yield strength, concrete cover, slab depth, unit width, and area of steel reinforcement provided. The reliability is determined at each 30 minutes fire duration for all fire compartments. The starting point is ambient temperature, then after every thirty minutes until the time when the maximum gas temperature is reached. The fire compartments were able to show fire resistance for two hours and then started to drop over time. Yield strength reduction factor, concrete cover, and area of reinforcement respectively were the most significant variables to determine the reliability index. The structure was able to withhold fire until two hours with a reliability of more than 3.8after that, the slab reliability was less than the target reliability and will need a repair.

Rationally Regulating the Mechanical Performance of Porous PDMS Coatings for the Enhanced Icephobicity toward Large-Scale Ice.

Ice accumulation on various surfaces in low-temperature and high-humidity environments is still a major challenge for several engineering applications. Herein, we fabricated a kind of PDMS coating with the introduction of porous structures under the surface by a two-step curing and phase separation method. The coatings with no further surface modification showed good hydrophobicity and icephobicity, and the typical ice adhesion strength was down to 40 kPa with a water contact angle of 116.5°. More than that, the porous PDMS coatings showed extraordinary icephobicity, especially toward large-scale ice (>10 cm2). In this case, the large-scale ice layer can be rapidly removed under a small external deicing force in a form of interface crack propagation rather than whole direct fracture. It was confirmed that by regulating the pore size and porosity of PDMS coatings properly, the stiffness mismatch between coatings and ice can be controlled to induce the initiation of interfacial cracks. On this basis, under the condition of a large-scale icing area, a small external deicing force can cause an increased surface stress concentration, and the formed interface cracks can propagate quickly, resulting in the ice layer falling off easily. In addition, under the influence of the size effect, ice can be removed without an additional force, and the minimum external force (per unit width) can be only 60 N/cm. This paper proposes that prefabricating a large number of microcracks at the interface can significantly weaken the bonding between ice and coatings, that is, reduce the fracture toughness. The new coatings have a remarkable effect toward large-scale icing.

Analysis of upward- and downward flame spread over vertical installed polyethylene-insulated electrical wires

Upward- (UFS) and downward (DFS) flame spread along a vertical installed electrical wire has been examined experimentally to gain better understandings of the heat transfer mechanism and burning behavior over electrical wires. UFS is considered as a much more hazardous fire scenario than DFS, however, UFS over electrical wires was very limited addressed. Fundamentally, the controlling heat transfer boundary over the preheating zone, hence the flame spread behaviors are quite different between DFS and UFS. This problem is more complicated for an electrical wire, for which the metal conduction is involved additionally, playing an important role in the heat transfer process. Thirteen types of polyethylene-insulated electrical wires with Cu- and NiCr-core were used to investigate the effect of metal core thermal conduction and wire configuration for both UFS and DFS. During the flame spread process, the pyrolysis length of UFS was enlarged, while only portion of virgin material of DFS were burnt as the result of distinct dripping. By further concerning the curvature effect, the dependence of the flame length on heat release rate per unit width (i.e. perimeter for cylinder) can be well explained by non-dimensional analysis. It turns out that the growth of flame length will be restrained as radius decreases significantly, and can be categorized in to two regions, one is controlled by laminar molecular diffusion followed the other controlled by turbulent entrainment. The variation of flame spread rate (FSR) with wire configuration and metal core thermal conduction are well interpreted by a theoretical model, which describes the heat recirculation through flame-insulation-core comprehensively based on thermodynamics theories, and attempts to consider the effects of temperature gap between core and insulation at pyrolysis front, burning fraction and melting heat. The validity of this new model is well confirmed by measured FSR. An important finding is that a highly conductive core will trigger an acceleration of FSR over the wire, primarily because heat flux in gas phase is more sensitive to the change of core thermal conductivity than heat conduction through core. Even though the heat diffusion length increases at the same time, however, the heat flux in gas phase grows faster than the corresponding heat loss through core, thereby compensate the corresponding heat loss. This work could facilitate understanding of heat transfer mechanism and burning behavior not only on the electrical wires, but also on the composite material with high thermal-conductive media.

Sediment load determines the shape of rivers

Understanding how rivers adjust to the sediment load they carry is critical to predicting the evolution of landscapes. Presently, however, no physically based model reliably captures the dependence of basic river properties, such as its shape or slope, on the discharge of sediment, even in the simple case of laboratory rivers. Here, we show how the balance between fluid stress and gravity acting on the sediment grains, along with cross-stream diffusion of sediment, determines the shape and sediment flux profile of laminar laboratory rivers that carry sediment as bedload. Using this model, which reliably reproduces the experiments without any tuning, we confirm the hypothesis, originally proposed by Parker [G. Parker, J. Fluid Mech 89, 127-146 (1978)], that rivers are restricted to exist close to the threshold of sediment motion (within about 20%). This limit is set by the fluid-sediment interaction and is independent of the water and sediment load carried by the river. Thus, as the total sediment discharge increases, the intensity of sediment flux (sediment discharge per unit width) in a river saturates, and the river can transport more sediment only by widening. In this large discharge regime, the cross-stream diffusion of momentum in the flow permits sediment transport. Conversely, in the weak transport regime, the transported sediment concentrates around the river center without significantly altering the river shape. If this theory holds for natural rivers, the aspect ratio of a river could become a proxy for sediment discharge-a quantity notoriously difficult to measure in the field.

Experimental study of obstacle and bed roughness effects on behavior of turbidity current

Laboratory experiments were carried out to study the behavior of turbidity current under subcritical approach flow condition. In order to study the effects of solid obstacle and bed roughness on turbidity current, 24 experiments were conducted. Two trapezoidal macro-roughness elements with 0.01 m and 0.03 m height and three triangular obstacles with 0.1 m to 0.3 m height were used. The parameters such as front velocity, velocity profiles, body height, flow discharge, suspended load transport rate and efficiency of the obstacle were determined. It was found that under subcritical flow regime, the main portion of the turbidity current over the rough bed is controlled, if the obstacle height is 2–3 times of the body height. The growth of the front height depends on the inlet sediment concentration, the roughness elements and the obstacle height. The front velocity upstream of the obstacle over the smooth bed is independent of the obstacle height and significantly influenced by roughness elements. Simultaneous use of the bed roughness and obstacle significantly reduces the front velocity downstream of the obstacle. The flow discharge per unit width significantly decreases downstream of the obstacle. Simultaneous use of an obstacle and roughness elements decreases the flow discharge per unit width by about 90%. An obstacle with height equal to 0.75 of the initial height of the turbidity current with the smooth bed blocks about 65% of the suspended sediment. While, simultaneous use of the obstacle and the roughness elements blocks about 95% of the suspended sediment.

Hydraulic model experiment of energy dissipation on the horizontal and USBR II stilling basin

Flow conditions on overflow systems can result in construction failure, mainly due to the high flow energy. Stilling basin at downstream of the spillway is useful for reducing flow energy. It can reduce the destructive force of water flow. Controlling the hydraulic jump is an important part that includes the jump’s energy, length, and height. The physical hydraulic model was carried out with several series, by making a series of bottom lowering of horizontal and USBR II stilling basin. The experimental study is expected to represent flow behavior in the overflow system regarding flow conditions and energy dissipation. Based on the analytical calculation of flow velocity, the amount of flow energy that occurs at each control point is calculated. The control points are the starting point of the spillway, the chute way toe, and flow depth after the hydraulic jump. The energy loss can be calculated for each control point, while the efficiency of energy dissipation on stilling basin is calculated at the downstream flow depth after the hydraulic jump. Velocity calculated by dividing discharge per unit width by water depth which is based on the flow depth measurement data in the hydraulic model.

Quantum of thermal conductance of nanofilms due to surface-phonon polaritons

Based on the Boltzmann transport equation, we demonstrate that the thermal conductance per unit width of a sufficiently thin polar nanofilm supporting the propagation of surface-phonon polaritons along its surfaces is independent of the material properties and is given by $12z(3){k}_{B}^{3}{T}^{2}/c{h}^{2}$, where ${k}_{B}$ and $h$ are the respective Boltzmann and Planck constants, while $c$ is the light speed in vacuum, $T$ is the temperature, and $z(3)$ is the Riemann zeta function. The huge propagation length of these energy carriers establishes that this quantization holds not only for temperatures much smaller than 1 K, as is the case of electrons and phonons, but also for those comparable to room temperature, which can significantly facilitate its observation and implementation in the thermal management of nanoscale electronics and photonics.

Improving stability of movement of machine section for soil preparation and seeding

The subject research is operation of a combined machine for soil preparation and seeding of sunflower and corn seeds. Technological process of machine operation with installed guides, passive rotating flat discs with flanges (which properly ensure movement of soil along the plough share to loosening-separating device), a sowing device, a seed tube, a furrow former, a rotor, a separating grid, a parallelogram mechanism, a spring, a share is described. Dynamic prerequisites for increasing uniformity of depth of groove formation and seed placement in depth in soil are considered. Values of length of links of parallelogram mechanism, initial angle of their installation and stiffness of spring, values of deviations of combined machine section from given depth of movement of plough share are determined. It is proved that with an increase in length of levers of parallelogram mechanism, maximum deviations of section increase. An increase in initial angle of inclination of levers of parallelogram mechanism causes an increase in maximum deflections. As spring stiffness increases, maximum deflections decrease. Relevance of study lies in ensuring stability of copying soil surface by working bodies of combined machine while depth of seeding remains unchanged along entire length of movement, which will make it possible to increase movement speed and unit width. Target group of consumers of information in the article - designers, specialists involved in development of tillage machines.

Geometry effects on flow characteristics of micro-scale planar nozzles

A numerical investigation of geometry effects on the flow characteristics associated with converging-diverging 2D planar micronozzles is reported. The classical N–S with linear slip model, DSMC method, and a hybrid N–S/DSMC based on the continuum breakdown concept are employed for the simulations. The various methods adopted are validated with available experimental data. The study addresses the issue of heterogeneous findings on optimized half divergence angle recommended by previous studies. The analysis is conducted for various throat dimensions (2–200 µm thoat height), divergence angles (5∘–30∘), and divergence length. The performance and flow characteristics are compared at two distinct operating conditions, i.e. when the flow is mostly subsonic and supersonic in the divergent section. Results show an optimum divergence angle and length, maximizing the performance for a specific operating condition and nozzle size. Literature data suggest various divergence angles for optimum performance. This is due to the differences in operating conditions and the size of the micronozzles used. The current study shows that differences in flow behavior are mainly due to the pressure difference across the nozzle. When the micronozzle is operating under low-pressure differences, the flow is primarily subsonic in the divergent section. However, it accelerates due to the effect created by the growing thick boundary layer. The flow at higher pressure differences is characterized by clear flow separation and the occupation of the separated flow to a considerable portion of the divergent section. The separated phenomenon becomes less pronounced as the pressure difference and nozzle size decreases. As the nozzle size decreases to the nanoscale, Re is very low, and the subsonic layer fully occupies the divergent nozzle section even at very high-pressure differences. The results show that the performance is significantly influenced by the wall thermal conditions. The study highlights the differences in flow behaviour between micro and nano-scale nozzles at various operating conditions. Based on numerical data obtained in this present work, a new correlation is proposed to predict thrust per unit width for micronozzles.