Static Coefficient Research Articles

Wellbore Stability Analysis Based on the Combination of Geomechanical and Petrophysical Studies

Determining the physical and mechanical properties of rocks is crucial for planning drilling operations. Ignoring geomechanical studies and tools has led to millions of dollars in additional costs related to drilling, meshwork optimization, production optimization, casing collapse, and other issues. These costs can be managed with a thorough geomechanical study. Additionally, some costs may not be apparent in the short term but will become significant over the life of the field. Long-term reservoir management is essential to control these costs. Issues such as reservoir compression, rock cover fractures, and gas storage, if neglected in the short term, can lead to substantial and irreparable damage to the reservoir over time. The "geomechanical model" is fundamental to geomechanical studies, particularly regarding wellbore stability. Given the importance of reservoir geomechanics and petrophysics, this research investigates wellbore stability analysis by combining geomechanical and petrophysical studies through the construction of a one-dimensional geomechanical model of the well. First, the dynamic geomechanical coefficients of the rock for the studied depth range were calculated from wells and petrophysical charts. Then, the static geomechanical coefficients of the rock were estimated based on laboratory-developed relationships. Subsequently, rock mechanics test results were used to estimate rock resistance parameters. The pore pressure was estimated using the compressibility and Eaton methods for the studied range from Well No. 5. Comparing the results of these two methods with RFT test data indicated that the compressibility method is more accurate than Eaton’s method. By estimating the in situ and induced stresses in the well's studied area, the lower and upper limits of the safe mud weight were calculated using the Mohr-Coulomb and Hooke-Brown criteria. Comparison of the results showed that the Mohr-Coulomb criterion is more accurate and, moreover, more conservative than the Hooke-Brown method.

Effect of load and pulling speed on the static coefficient of friction of cross-country skis on snow

Effect of load and pulling speed on the static coefficient of friction of cross-country skis on snow

Transferability of Model-Based Static Coefficient of Friction

The accurate determination of static coefficients of friction (COFs) is crucial in engineering design, yet standard reference values often show considerable variability. As a result, engineers frequently need to perform experimental COF measurements to ensure the reliable transferability of model-based COFs to real-world components. However, the effectiveness of cost-efficient laboratory tests, typically conducted on standardized samples, in reflecting actual component performance is often questioned as it is not trivial to transfer and scale the tribosystem. This study addresses this issue by conducting friction coefficient experiments on interference fits and flange connections, comparing the results with laboratory-based COF tests. The findings reveal a strong correlation when the tribological conditions of the real assembly are replicated in the lab. This research offers a method to enhance the accuracy and transferability of COF values from lab tests to practical applications, providing engineers with a more reliable approach to friction testing.

Investigation the yarn to yarn static and kinetic friction based on the response surface method

One of the most significant physical characteristics of textiles is friction, which has a direct bearing on how they behave mechanically. This characteristic has been researched in a variety of methods thus far, demonstrating its significance. The static and kinetic coefficients of friction in yarn to yarn interaction have been examined in this paper by considering a number of variables, including yarn count (33, 50, 57, and 65 Tex), yarn movement speed (30, 45, and 60 mm/min), angle of yarn placement (0, 45, and 90 degrees), and surface reaction force (5, 10 and 15 g). To this aim, 32 different experiments were designed using the central composite design approach in order find the static and kinetic coefficients of friction. The outcomes were then statistically analyzed. The relationship between the friction coefficients and the aforementioned parameters was investigated as well. Next, a model with 93% and 85% accuracy for the static and kinetic coefficients of friction, respectively, was obtained using the response surface method. Additionally, the relationship between the friction coefficients and each of the aforementioned parameters was assessed.

Tunable Electronic, Optoelectronic, and Photocatalytic Properties of MoS2 and GaS Monolayers in the MoS2/GaS Heterostructure

AbstractThe electronic structure calculations show that the GaS and MoS2 monolayer and MoS2/GaS hetero‐bilayer systems are semiconducting materials. It is also justified by the density of states. In the hetero‐bilayer system, the band gap is reduced. It causes an increase in the absorption, compared to their monolayers, of incident light in the visible region. This may be because of the charge transfer and the interlayer interaction between these monolayers in the heterostructure form. The calculated higher extinction coefficient causes the fast absorption of light, hence, the increased absorption coefficient. The heterostructure system makes the refractive index and reflection coefficient of GaS and MoS2 monolayers tuneable. The static reflection coefficient is in the following order GaS < MoS2/GaS < MoS2. It means that the MoS2/GaS heterostructure system can increase the static refractive index of GaS and decrease that of MoS2 monolayers simultaneously. The calculated band edge positions show that the MoS2 and GaS monolayers have excellent water oxidation and reduction capability, respectively. These predictions show that the proposed heterostructure may be a potential candidate for nanoelectronics and optoelectronics.

The effectiveness of chalk as a friction modifier for finger pad contact with rocks of varying roughness

The application of chalk (magnesium carbonate) in rock climbing is common practice as climbers attempt to improve their grip by removing moisture from their hands with the aim of increasing friction at the finger pad-rock interface. This novel work investigated the effectiveness of chalk as a friction modifier on four different rocks (sandstone, granite, dark limestone and light limestone) typically found in areas of the UK where the sport of climbing is undertaken, with varying surface roughness. The static coefficient of friction was measured for dry and wet fingertip conditions with and without chalk, under normal (‘grip’) forces of 5, 10 and 15 N. Results showed that the effectiveness of chalk as a friction modifier is dependent on a number of factors such as moisture level and the gradient of the asperity at the rock surface, however, in general chalk applied to dry fingertips had a more positive effect on the static coefficient of friction than in simulated sweaty conditions. During lab tests, chalk was also seen to be beneficial by making the static coefficient of friction more consistent across most test conditions. The results of this study, and the explanation of friction mechanisms involved, provides guidance for the use of chalk with consideration of the type of rock which is being climbed.

Frictional Properties of Biomimetic Micro-Hexagonal-Textured Surfaces Interacting with Soft Counterfaces under Dry and Wet Conditions.

Biomimetic micro-hexagonal-textured surfaces have sparked interest for their application in fields that demand high friction and adhesion, such as micro-robotics and biomedicine. Despite extensive research conducted on this specific microstructure, its friction behavior against soft counterfaces remains a topic that has not been fully investigated yet. This study examines how micro-hexagon textures behave when they come into contact with engineered and biological materials like gelatin and chicken skin in dry and wet conditions. The results show clearly that under dry contact conditions, flat surfaces generate higher friction compared to hexagon micropattern surfaces. Under wet conditions, hexagon micropattern surfaces generate higher friction compared to flat surfaces. In wet conditions specifically, the static coefficient of friction is up to 13 times greater than that of a flat specimen against glass, up to 11 times greater against gelatin, and up to 6 times greater against chicken skin. For the dynamic coefficient of friction, the patterned surface demonstrates a maximum increase by a factor of 28 against glass, 11 against gelatin, and 5 against chicken skin. These results further develop our knowledge of these hexagon micropattern surfaces and pave the way for their utilization in future technological advancements in which soft and wet counterfaces are to be considered, such as in biomedical applications that can benefit from increased friction in wet conditions for better control and stability.

Characterization of friction between metal-polymer material pairs with different speeds and surface pressures

Abstract Engineering design requires accurate knowledge of the friction conditions of material pairs. The friction coefficient (static or dynamic) depends on many parameters in addition to the pairs of materials used (e.g. surface roughness, temperature, humidity, surface pressure, etc.). In this study, based on our own measurements, we present the friction conditions of an aluminum (Al 6082-T6) – polypropylene homopolymer surface pair under different surface loads (5-15 MPa) and different relative speed differences (4-8–48-96 mm/min). The results of the experiments show that the static and dynamic friction coefficients decrease with the increase of surface load. As relative speed increases, the dynamic coefficient of friction increases, while the static coefficient of friction can be considered constant.

Analysis of film cooling and flow resistance characteristics of turbine blades with thermal barrier coatings

This paper employs numerical simulation to investigate the influence of thermal barrier coatings' (TBCs) thickness and surface roughness on the cooling and flow resistance characteristics of turbine blades. The results indicate that the application of TBCs significantly enhances the surface cooling efficiency of the blades. Turbine blades coated with a 0.4 mm thickness of TBC compared to blades without thermal barrier coatings, the average cooling efficiency of the blade surface increases by 12.3 %, and the maximum temperature drop at the leading edge(LE) is 317.8 K. However, the small increment in TBCs thickness leads to an increase in aerodynamic losses in the vane passage. The static pressure coefficient continuously decreases in the suction side(SS) within the interval 0.3 < x/C < 1.0. The average flow coefficient of the film holes exhibits distinct variations in different regions of the blade. As surface roughness increases, the cooling efficiency on the SS and pressure side(PS) of the blade decreases, while the heat transfer enhancement at the LE and cooling efficiency improve. Compared to a smooth coated surface, when the surface roughness height increases to 20 μm, the blade surface cooling efficiency decreases by 1.12 %, and the average temperature rise is 10.3 K. Simultaneously, the energy loss coefficient and total pressure loss coefficient in the vane passage rise with the increase in surface roughness, while the variation in the average flow coefficient of the film cooling holes remains relatively small.

Control of Flow and Acoustic Fields Around an Axial Fan Utilizing Plasma Actuators

Abstract Small axial fans, commonly employed for cooling electronic equipment, are frequently housed within narrow ducts, where intense tonal sound with duct resonance can occur, particularly when the blade passing frequency or its harmonic frequency aligns with the duct's resonance frequency. To mitigate resonant sound, this study proposes a flow control using dielectric barrier discharge plasma actuators, which induce a flow along the generation of plasma in air. The control effects on flow field, acoustic radiation, and aerodynamic characteristics are evaluated through direct aeroacoustic simulations and experiments conducted at different flow rates. The computational results reveal that swirling flow occurs in the inflow due to fan rotations at low flow coefficients. This swirling flow is weakened by utilizing plasma actuators, which are arranged to induce flows in the circumferentially reverse direction compared to fan rotations. This control method weakens the resonant sound at low and intermediate flow coefficients, while intensifying it at high flow coefficients, all at the same rotational speed. Moreover, the static pressure coefficient decreases and increases at low and high flow coefficients, respectively, with the latter attributed to an increase in the relative inflow angle induced by the control. Experimental findings demonstrate that the acoustic resonance was reduced by the control at both low and high flow rates, achieved by adjusting the rotational speed to maintain the same flowrate and static pressure rise as in the baseline case.

Gas–liquid mass-transfer characteristics during dissolution and evolution in quasi-static and dynamic processes

This study aims to understand the comprehensive behavior of the gas–liquid flow and dissolution–evolution mass transfer. A quasi-static closed-tank experiment was designed to measure the static mass-transfer coefficients of the dissolution and evolution processes using the diffusion equation. After a detailed uncertainty analysis, a dynamic ventilated-pipe experiment with different-sized orifice plates was designed to illustrate the relationship between the hydrodynamic parameters, physical structure, and gas–liquid mass-transfer characteristics. The results showed that, as the static pressure and liquid-level height increase, both the dissolution and evolution coefficients exhibit increasing trends. However, when the physical condition reaches the initial state after pressurization and depressurization, the gas absorbed by the solution cannot completely evolve from the solution; that is, the dissolution rate is always greater than or equal to the evolution rate. For the equal-diameter pipe, as the gas flow rate increases, the concentration increment decreases slightly after reaching the peak, owing to the reduction in mass-transfer time caused by the increase in liquid flow rate. In particular, the maximal dissolved concentration, an increment of 210.9 %, occurred in the double large-orifice plate with the ventilated condition, far exceeding the maximum value in the quasi-static process. Moreover, the concentration under the layout of two small-orifice plates decreases slightly, and the larger gas content enables the solution to have more gas nuclei, making it easier to induce the gas evolution. The current study provides guidance for the gas–liquid-mixture transportation and improvement of the dissolved efficiency.

Study on the influence of the fully enclosed barrier on the vortex-induced vibration performance of a long-span highway–railway double-deck truss bridge

Long-span highway–railway double-deck truss bridges are mostly located in urban centers, where noise pollution and traffic safety issues have a great impact. The fully enclosed barrier has excellent sound insulation and windproof performance and has been widely used in double-deck truss bridges in recent years. However, the large volume and the low air permeability rate will affect the aerodynamic characteristics of the bridge, resulting in vortex-induced vibration (VIV). To analyze how the fully enclosed barrier influences the highway–railway bridge VIV performance, this study analyzes the Huangjuetuo Yangtze River Bridge, combined with the wind tunnel test and the numerical calculation method to study different variations of the static three-force coefficient, the change of VIV response, and its influence mechanism after setting the fully enclosed barrier. The results show that the static three-force coefficient of the double-deck truss bridge changes significantly, the drag coefficient increases, and the absolute values of the lift coefficient and the moment coefficient decrease after the fully enclosed barrier is set. The aerodynamic performance of the bridge is significantly reduced after the fully enclosed barrier is set, and the amplitude and range of the VIV response are increased. Vertical bending VIV increased by an average of 58.5%, and torsional VIV increased by an average of 21.9%. Considering driving comfort and safety, attention should be paid to the double-deck truss bridge with a fully enclosed barrier.

RMSE POWER COEFFICIENT OF WIND TURBINES WITH INVERSE TOQUE-SPEED CONTROL

The Blade Element Momentum (BEM) method stands out among various turbine modelling techniques due to its integration of airfoil aerodynamics with momentum theory, allowing detailed force calculations on blade elements. While traditional methods like BEM and Wake Models are efficient for initial turbine design, advancements in computational power have enabled the use of Computational Fluid Dynamics (CFD) for more accurate simulations. Although these models are precise, they are computationally intensive, leading to the continued use of simpler empirical methods, such as the static power coefficient approach, in power system studies. Future research aims to validate and implement dynamic estimation models to account for wind variability and turbine control dynamics. Traditional constant torque-speed control strategies maintain a constant generator speed by using a fixed electromagnetic torque command and adjusting the pitch angle when wind speed exceeds the rated value. However, if the pitch control does not respond quickly enough, rotor acceleration can occur, leading to increased power beyond the generator’s rating. To mitigate this, the pitch control mechanism must react promptly to adjust the turbine's torque profile, ensuring convergence to the rated operating point and avoiding prolonged generator overload. To reduce the burden on the pitch-control system, an inverse torque-speed control approach is proposed, where the electromagnetic torque is dynamically adjusted based on real-time rotor speed data. This method aims to achieve rated power operation with improved responsiveness and reduced stress on the pitch-control mechanism.

Selected Physical, Mechanical and Chemical Properties of Fresh Fruit Bunches for Processing of Palm Oil

Data on engineering properties of palm fruits enable the design of machinery for optimal production of palm oil. Specific physical, chemical, and mechanical characteristics of fresh fruit bunches are examined in this study. Fresh palm fruit's selected mechanical, chemical, and physical characteristics were determined using standard formulas and equations. One hundred fresh fruit bunch samples were used to ascertain the physical characteristics of the fruit bunches. While the chemical properties were ascertained using suggested standard equations and formulae, the fresh fruit bunches were tested for their strength properties under compression when loaded under an INSTRON Universal Testing Machine. Using the Microsoft Excel program, statistical analysis was performed on the collected data. The average length, breadth, thickness, geometric mean, sphericity, surface area, true mass, true volume and true density mean for palm fruits were 3.35±0.48 cm, 1.92±0.32 cm, 2.24±0.33 cm, 2.42±0.25 cm, 0.73±0.08 cm, 18.61±3.62 cm2, 7.99±1.81 g, 7.01±1.72 cm3, 1.17±0.24 g/cm3 respectively. The average static coefficient of frictions for palm fruits were 0.46±0.13, 0.64±0.18, 0.57±0.21 and 0.59±0.22 while the average angle of reposes was 24.48±6.08, 31.62±9.14, 29.26±6.80, and 29.91±9.11 on wood, mild steel, glass and stainless-steel surface respectively. The energy at break, force at break, deformation at break, strain at break, and stress at break for the palm fruits were 6.87±0.95 J, 2.22±0.80 kN, 14.58±3.02 mm, 0.36±0.08 mm/mm and 123.46±44.31 MPa respectively. The moisture, ash, fat, crude fibre, crude protein, and carbohydrate contents (proximate composition) of the palm fruits were 22.80±0.02%, 0.52±0.01%, 46.47±0.06%, 18.66±0.01%, 2.4±0.21%, and 9.14±0.30% respectively. The sodium, potassium, calcium, magnesium, iron, phosphorus and zinc contents of the palm fruits were 58.75±0.07 ppm, 94.20±0.14 ppm, 68.45±0.64 ppm, 5.21±0.00 ppm, 1.22±0.00 ppm, 1.60±0.00 ppm and 55.70±0.28 ppm respectively. This study has provided relevant information which will help engineers to develop a more efficient palm fruit extraction machines that will in turn increase the productivity and profitability of the farmers and food processors. These measured engineering properties of palm fruits help in development of palm fruit processing machines.

Integrated Plug High-Strength Geocell Reinforcement in Foundation Design for Square Footing

This paper develops an analytical model to calculate the ultimate bearing capacity of the integrated plug high-strength geocell (IPGC)-reinforced foundation under a square footing. The high strength and stiffness of the geocell wall and the typical failure of the integrated plug-in joint tearing were considered. The ultimate bearing capacity of the IPGC-reinforced foundation was calculated in two separate parts. The ultimate bearing capacity of an unreinforced foundation was calculated using the modified Terzaghi equation. The increased bearing capacity of the IPGC was calculated as the function of the tearing force of the geocell wall, the height and the diameter of a geocell, the empirical static earth pressure coefficient, and the vertical additional stress coefficient under uniformly distributed rectangular loading. The results showed that the maximum error between the experimental and the theoretical results is less than 18%. The ultimate bearing capacity of IPGC-reinforced foundations decreases with larger geocell diameters. When the diameter of the geocell exceeds 1.8 times the foundation width, the confinement effect of IPGC becomes negligible. The findings of this study offer a robust analytical equation for predicting IPGC-reinforced foundations, along with valuable insights into the efficacy of IPGC reinforcement in enhancing foundation stability.

Explicit frictional stick–slip dynamics of elastic contact problem incorporating the LuGre model

Dynamical contact problem with friction is of continuous concern and have been investigated over the years. The effective and accurate description of the development of the friction force is the key to this problem. However, the stick–slip phenomenon, as typical dynamical behaviour, has rarely been considered when developing contact algorithms for frictional contact problems of elastic bodies. The conventionally used Coulomb’s model, with differentiating the static and kinetic coefficient of friction, can lead to inaccurate results due to errors generated when switch between the stick or slip states. In this work, a three-dimensional mortar-based frictional contact algorithm is proposed in an explicit scheme. The proposed algorithm incorporates the LuGre model to account for dynamical frictional behaviours such as stick–slip. The effectiveness and accuracy of the proposed algorithm are validated through several simplified two-dimensional cases. The sticking time ratio and the evolution of the global coefficient of friction with time of a three-dimensional contact problem with a randomly rough contact interface are investigated as an application.

Virtual wall: Numerical model on interaction between trees and building façade to quantify heat exchange and microclimate in urban context

When simulating a building's energy demand, the tree surroundings should be accounted in their shade and reshaping microclimate, just representing an envelope layer as a virtual wall. This study delves into the effects of such tree-induced virtual walls on building energy dynamics, with a dataset derived from 24 distinct samples of building façades and adjacent trees within a campus environment. An experiment was designed to validate microclimate simulation that captures variations in sheltered radiation and temperature reductions. The study introduces and calculates the dynamic and static equivalent heat transfer coefficients and thermal resistances for the virtual wall. The findings reveal significant correlations between the calculated coefficients and resistances, and various factors such as seasonality, weather conditions, and façade orientation. The equivalent thermal resistance value for south façade is highest on sunny days, reaching up to 3.7 m2·K/W, and on cloudy days, east façade has the highest equivalent heat transfer coefficient and thermal resistance values, at around 8.5 W/m2·K and 10.3 m2·K/W, respectively. Moreover, the study reveals that the tree's impact is primarily determined by its height and canopy area, with the equivalent heat transfer coefficient negatively correlated with tree height and canopy area, and positively correlated with building height.

Numerical Model Parameters Choice of Helical Savonius Wind Rotor: CFD Investigation and Experimental Validation

Electrical power is essential for human beings welfare. The available wind as a clean and renewable source of energy has whetted extensive interest over decades. Savonius vertical axis wind rotor as an energy converter has the merit of being adequate for specific implementations owing to its lower cost and independency on wind direction. From this perspective, multiple studies have been conducted to boost its efficiency. This research work emphasizes on the helical Savonius wind rotor (HSWR). The basic objective is to investigate the impact of selecting the numerical model parameters on its aerodynamic and performance characteristics. Experimental tests were realized with a 3D printed HSWR in a wind tunnel. The experimental performances in terms of power, static and dynamic torque coefficients were addressed. Next, a numerical study was undertaken through Ansys Fluent 17.0 software. Grid, turbulence model and rotating domain size tests were examined. Good accordance was obtained, which validated the numerical model with an averaged error of 5%. The maximum power coefficient proved to be equal to 0.124 at a tip speed ratio of 0.73 and 0.1224 at a tip speed ratio of 0.69, respectively, numerically and experimentally

Gravimetric characteristics and friction parameters of common bean (Phaseolus vulgaris L.)

When designing appropriate machinery systems, equipment, and infrastructures for interacting with, cultivating, gathering, and agriculture-related processing, it is required to have an understanding of the engineering characteristics of agricultural products. This unpredictability makes it difficult to design or develop machines that can efficiently and effectively manage a wide range of product characteristics. Experimental analysis was used to accomplish the study's objective, which was to investigate the implications of variation on the gravimetric characteristics and frictional parameters of common bean (Phaseolus vulgaris L.) concerning the design of the threshing machine. The mean average values of gravimetric parameters were determined by analysing the experimental data: arithmetic mean diameter (7.042 ± 0.473 mm), geometric diameter (6.737 ± 0.463 mm), bulk density (781.20 ± 25.34 kg m-3), true density (1347.03 ± 143.0 kg m-3), porosity (41.385 ± 7.05%), width (6.316 ± 0.502 mm), thickness (4.962 ± 0.50 mm), projected area (49.194 ± 6.715 mm2), and volume of the seed (161.689 ± 3.778 mm3). The average moisture content values were found to be 11.214±1.185% on a dry basis, the static coefficient of friction varied between 0.276 and 0.386 on the surface of iron sheets, 0.294 to 0.435 on stainless steel, 0.317 to 0.434 on galvanized iron, 0.321 to 0.451 on medium density fiberboard, 0.319 to 0.480 on aluminum, 0.310 to 0.470 on painted sheets, 0.320 to 0.440 on glass, 0.333 to 0.447 on plastic, and 0.374 to 0.575 on rubber. Perforated sheet surfaces showed the highest static coefficients of friction, followed by rubber, plastic, plywood, glass, aluminum, galvanized iron, painted sheet, stainless steel, and iron sheet surfaces. These data are not only required for predicting loads in agricultural storage structures but are also needed to establish useful sources for the development of machinery for handling, cleaning, storing, transporting and drying, among other things.

Analysis of structural, electronic and optical properties of Er-doped rock salt AlN using ab-initio calculations.

This investigation includes the structural and optoelectronic characteristics of both pure and Er-doped rock salt aluminium nitride (AlN). Upon introducing Er doping into the AlN host, the calculations reveal a rise in the atomic parameter. Incorporating Er into the system leads to enhancements in the static dielectric coefficient ɛ1(0), static reflectivity R(0), as well as static refractive index n(0), at zero frequency. After doping, the peaks of imaginary dielectric tensor, extinction coefficient and absorption coefficient shift towards lower energy levels. Various exchange correlation potentials are incorporated to compare the results of electronic and optical characteristics. We employed the full potential linearized augmented plane wave (FP-LAPW) approach with WIEN2k code in conjunction with the density functional theory (DFT). To explore the optoelectronic characteristics of both pure as well as doped systems, three distinct exchange correlation potentials are utilized: the Perdew-Burke-Ernzerhof Generalized Gradient Approximation (PBE-GGA), Modified Becke Johnson Generalized Gradient Approximations (mBJ + GGA) and Hubbard potential (GGA + U).