Torque Coefficient Research Articles

Research on Effect of Ship Speed on Unsteady Hydrodynamic Performance of Bow Thrusters in Berthing and Departure Directions

With the continuous development of the shipping market, bow thrusters have become more important for ship maneuvering. Therefore, the performance of bow thrusters is studied in this paper. In order to obtain an unsteady performance of the bow thruster under different ship speed conditions, the SST k-ω turbulence model is adopted to predict the hydrodynamics of the bow thruster. With the ship’s speed increasing gradually, the variation characteristics of hydrodynamic coefficients and the flow field distribution at key positions are analyzed. The results show that with an increase in ship speed to three knots, the thrust coefficient and torque coefficient of the bow thruster decrease by 2.69~4.07% and 2.34~3.08%. In addition, the blade vibration amplitude intensifies. In the departure direction, the propeller load is more susceptible to being influenced and decreases by an additional 2.34~4.16% compared with that in the berthing direction. Meanwhile, it is found that the velocity distribution is asymmetrical. The inlet velocity at the bow side is faster, which results in the maximum peak pressure being about three times the minimum peak pressure. In addition, the pressure’s nonuniformity in the tunnel increases gradually with the increase in ship speed. Compared with the pressure distribution in the berthing direction, the pressure distribution before and after the propeller is more uniform, which is consistent with the results of hydrodynamic change and velocity distribution. The research in this paper has a certain reference significance for understanding the hydrodynamic performance of bow thrust operation.

Computational Fluid Dynamics (CFD) Evaluation of a Horizontal-axis Wind Turbine with a Bionic Blade

With the world's energy supply becoming increasingly scarce, wind power is gaining popularity as a clean source of renewable energy which is good for the environment. Thus, it is an alternative to burning fossil fuels such as coal, which may have an environmental impact during operations due to the emissions of unwanted gases. Wind turbines are the devices used to produce power by moving a turbine's propeller-like blades around a rotor, which spins a generator that generates energy as air passes through them. However, existing wind turbines still suffer from low conversion efficiencies. Therefore, in this current study, the bionic blade design has been used to enhance the performance of a horizontal axis wind turbine utilizing a computational simulation approach while seeking to improve its efficiency. The blade profile of the model used was based on the NACA 0012 airfoil with the modified angle of attack. The hybrid shear stress transport (SST) k-omega was used as the turbulence model. The computational procedures involved the use of various inlet velocities while maintaining a constant rotational speed. The results show that the bionic design was found to improve the overall performance of the standard NACA0012 blade design. Moreover, both power and torque coefficient outputs increase as the tip speed ratio (TSR) increases. However, the torque decreases as the TSR increases. In terms of power coefficient, the highest conversion efficiency was about 28% and it was achieved at 3.7 TSR.

A Novel Wind Energy Gathering Structure for the Savonius Wind Turbine and Its Parameter Optimization Based on Taguchi’s Method

An auxiliary structure can significantly improve the wind-trapping capacity of the Savonius wind turbine. In this study, a novel auxiliary structure called a wind energy gathering structure (WEGS) is proposed, and its five parameters, namely the lengths of the shrinkage and diffusion tubes, the length of the centerboard, the length of the throat, the length of the wind board, and the shrinkage and diffusion angles, are investigated using computational fluid dynamics (CFD) and Taguchi’s method. Meanwhile, Taguchi’s method and ANOVA reveal that among the studied parameters, the shrinkage and diffusion angles, the length of the centerboard, and the lengths of the shrinkage and diffusion tubes have a more significant influence on the performance of the WEGS. At a tip speed ratio (TSR) value of 1 and a wind speed of 7 m/s, the optimized combination of the WEGS parameters obtained by Taguchi’s method improves the mean torque coefficient of the turbine by 42.1%. Moreover, at other TSRs (0.6–1.2), the turbine with the WEGS also outperforms an open turbine in terms of aerodynamic (increases of 20.1–53%) and lifetime performance.

Comparison of aerodynamics of vertical-axis wind turbine with single and combine Darrieus and Savonius rotors

Vertical-axis wind turbines (VAWT) with Darrieus and Savonius rotors were studied based on the developed specialized package of computational aerodynamics. The flow structure around the Savonius and Darrieus rotors was numerically reconstructed, considering the mutual influence. From this reconstruction, the main stages of the vortex structure formation during the flow of the turbulent wind around the rotors were identified. Qualitative and quantitative assessments of the influence of the Savonius rotor on the total aerodynamic and energy characteristics of a VAWT were carried out. It is shown that the main contribution to the torque of the VAWT is due to the Darrieus rotor, mainly in the windward section of the trajectory. The Savonius rotor accounts for only a few percent of the total torque produced by the installation. The interaction of the Darrieus rotor blades with macrovortices from the Savonius rotor in the leeward part of the trajectory leads to a sharp drop in the torque coefficient. In the absence of a Savonius rotor, the vortices separated from supported tower are much smaller both in size and intensity. Therefore, their interaction with the blades of the Darrieus rotor does not lead to a significant change in the aerodynamic characteristics. A power characteristics comparison of two VAWTs showed that the torque coefficient is higher for wind turbines with only single Darrieus rotor. The developed approaches and techniques make it possible to reproduce real aerodynamic processes of flow most reliably around bodies of arbitrary shape and calculate their aerodynamic characteristics.

High-Precision Numerical Investigation of a VAWT Starting Process

For both conventional and renewable energy conversion processes, computational fluid dynamics (CFD) has been used to address more energy-related challenges in recent decades. Using CFD to investigate vertical-axis wind turbines has become more common in recent years. The main goals of this application have been to more accurately predict the turbine’s performance and to comprehend the complicated nature of the complex turbulent flow. The vertical-axis wind turbine (VAWT) simulation for energy-generating applications has several intricate components. One of them is the study of the chaotic flow that occurs during the first stages of the starting process, and which greatly influences overall effectiveness. In this article, the performance of the wind turbine was increased using a passive flow control approach. The numerical research was carried out using Large Eddy Simulation for four alternative tip speed ratios in both cases, the classic and the optimized case, equipped with a vortex trap on the extrados of the blades. The power and torque coefficient variations, as well as the velocity magnitude contours, show that the starting process may begin with a significant improvement in efficiency when flow control is used.

Effect of Blade Gap Ratio on Turbine Performance in Drag-Based Vertical-Axis Wind Turbines

The aim of this study was to improve the performances of three-bladed vertical-axis wind turbines with gap distance. For this purpose, turbine design conditions such as a gap ratio between the blades and the addition of blades were changed to provide performance increase, and an aerodynamic performance-enhancing performance setup placed in front of the vertical-axis wind turbine was also used. Therefore, in this study, the effects of gap distance and blade addition on wind turbine performance were investigated, especially by keeping the aspect ratio of the vertical-axis wind turbine constant. The investigation of the effects of turbine design parameters, such as the gap distance and additional blade number, on turbine performance was carried out using the ANSYS Fluent program with a numerical analysis method validated with experimental data. The turbine gap ratio, at which the maximum torque coefficient of a three-bladed vertical-axis wind turbine is obtained, was determined to be 40% without performance setup and 30% with performance setup. It was determined that the average torque coefficient value obtained from the turbine with the addition of turbine blades and with the performance setup increased by approximately 66% compared to the torque coefficient value obtained from the three-bladed turbine without the performance setup. It was also determined that the maximum power coefficient obtained from the vertical-axis wind turbine with optimum turbine design parameters with the performance setup increased by approximately 46% compared to the maximum power coefficient obtained from the version of the turbine without the performance setup.

Effects of propeller cavitation on ship propulsion performance in off-design operating conditions

The propeller cavitation is closely related with its hydrodynamic performance. When the propeller works in the cavitating flow, the propeller hydrodynamic performance, in terms of thrust and torque coefficients, is reduced due to the propeller cavitation. When the high-speed and high-power ship sails in adverse sea, the varying loads on propeller will cause severer cavitation on propeller. The reduction in propulsion performance caused by cavitation will further affect the propulsion system, impacting the ship sailing performance and safety. The VP1304 propeller cavitation CFD simulation are conducted to validate the CFD method. Furthermore, the influence of propeller advance ratio and cavitation numbers on the performance of low-speed high-power series propeller in cavitating flow are studied and illustrated. This influence of cavitation has also been transferred to the propulsion system and this process are calculated by the ship propulsion system numerical model. Additionally, the sailing performance of ship propulsion system under off-design conditions, where the cavitation is more likely to occur, has been investigated in this article. Under severe heavy load conditions, it can even result in a complete loss of propulsion capability, posing significant risks. Therefore, it is necessary to further optimize control strategies for the propulsion system to mitigate the hazards caused by cavitation.

Research on Drillpipe Stick-slip Vibration and Its Suppression Mechanism in Deep Well Drilling

Abstract During drilling deep wells, stick-slip vibration(SSV) is a reciprocating vibration form of drill bit adhesion, sliding, re-adhesion, and re-sliding. If not suppressed, it can cause serious drilling accidents, such as accelerated wear of drilling equipment, which lead to decreased drilling safety, reduced drilling efficiency, and prolonged drilling cycle. This article establishes a SSV model of the drill string system, considering the three degrees of freedom of the rotary table/top drive, hammer, and drill bit, and analyzes the SSV characteristics of the drill bit during deep well drilling. The SSV analysis of the drill string system was carried out for well XX-X-4, and the sensitivity of rotational speed, viscous damping, drilling pressure, drill bit length, and friction torque coefficient was analyzed. The results showed that the friction torque coefficient and rotational speed are the main factors affecting stick reduction efficiency, while the length of the drill collar is a secondary factor. The WOB and viscous damping have the least impact on stick reduction efficiency. The parameters were optimized to reduce SSV as the target. This article has certain guiding significance for deep well drilling operations.

Enhancing Savonius Rotor Performance with Zigzag in Concave Surface-CFD Investigation

Savonius, a type of vertical-axis wind turbine (VAWT), is suitable as an appropriate small-scale energy conversion apparatus for regions with relatively low wind speeds, such as Indonesia; however, it exhibits sub-optimal efficiency. One potential approach to improving the efficiency of Savonius turbines is to increase the drag force on the concave surface of the blades. In this case, the dissimilarity in the forces experienced by the two blades can be increased, resulting in a corresponding increase in torque. This investigation aims to assess and compare the power coefficient (Cp), torque and drag coefficient (Cd) of the conventional Savonius rotor with the zigzag pattern implemented in the middle area of the concave surface of the blades at low wind speeds. The efficiency can be achieved by implementing the k-ω shear stress transfer (SST) turbulent model and 3D computational fluid dynamics simulation at tip speed ratio (λ) 0.4-1 with a velocity inlet of 4, 5, and 6 m/s. The study results show that using the zigzag pattern on the concave surface led to an 18.8% boosted in Cp of at λ = 0.8 and an inlet velocity (U) = 5 m/s compared to the standard Savonius rotor model. In this case, the efficiency of the Savonius wind turbine may be enhanced by incorporating a zigzag pattern in the middle of the concave surface of the Savonius rotor.

Methodology for Real-Time Torque Estimation in a Ship Propulsion Digital Twin

Abstract The safe operation of ships requires the condition of propulsion components to be maintained. Digital twins are a promising alternative for intelligent monitoring of these complex systems. Digital twins require models which ensure that the digital representation is able to mimic the behavior of the physical system. Alternate modeling solutions must be found when intellectual property restrictions or lack of available information limit the usability of physics-based models. This paper considers such a case where a system model of the propulsion system requires a real-time capable model of the propeller hydrodynamic torque. The creation of a data-driven hydrodynamic torque model based on full-scale, operational measurements is discussed. The described method focuses on the significant challenges associated with data cleaning and preparation while also evaluating whether well-known machine learning methods are suited for this application. The methods use speed-over-ground, heading, course, rotational speed, and propeller pitch as inputs. The outputs of the models are the single quadrant propeller torque coefficient and the amplitude of harmonic torsional excitation. These outputs are then combined to create a holistic prediction of the torque. Results indicate that both a polynomial least-squares fit and a shallow neural network predict the mean and the amplitude of harmonic components of the torque well. This prediction can be used to isolate the hydrodynamic torque when more than one torque source is present or to simulate what-if scenarios in a digital twin environment.

Effects of endplate designs on the performance of Savonius vertical axis wind turbine

In aerodynamic studies, endplates or wingtip devices play a crucial role in enhancing the performance of airfoil blades and minimizing losses. This is particularly important for wind turbines, especially the drag-type Savonius turbine. This paper aims to investigate the effects of different endplate designs. Both experimental works and numerical simulations were conducted on a Savonius rotor with an aspect ratio of 1. A total of eight different endplate geometries and sizes were investigated. The mechanical torque and rotational speed were measured experimentally under various loads with an incoming wind speed of 5.89 m/s. The coefficient of torque (CT) and coefficient of power (CP) were calculated in both the experiments and simulations. The simulation was first validated using data from the literature, and flow characteristics were then scrutinized. The results indicate that the endplates greatly impact the turbine performance, with improvements of up to 299.7 % compared to a turbine without endplates from the experiment. The endplates help prevent flow leakage near the blade edges. In addition to tip losses reducing turbine performance, aerodynamic parasitic drag also contributed to a decrease in CP for the full endplate. The relationship between the aerodynamic parasitic drag with TSR is also reported. By implementing a well-designed endplate, these adverse effects can be mitigated.

Modelling aerodynamic forces and torques of spheroid particles in compressible flows

In the present study, we conduct numerical simulations of compressible flows around spheroid particles, for the purpose of refining empirical formulas for drag force, lift force, and pitching torque acting on them. Through an analysis of approximately a thousand numerical simulation cases spanning a wide range of Mach numbers, Reynolds numbers and particle aspect ratios, we first identify the crucial parameters that are strongly correlated with the forces and torques via Spearman correlation analysis, based on which the empirical formulas for the drag force, lift force and pitching torque coefficients are refined. The novel formulas developed for compressible flows exhibit consistency with their incompressible counterparts at low Mach number limits and, moreover, yield accurate predictions with average relative errors of less than 5%. This underscores their robustness and reliability in predicting aerodynamic loads on spheroidal particles under various flow conditions.

Mitigating thermal stratification in lakes/reservoirs through wind-powered air diffusers.

Thermal stratification can cause various water quality issues in large water bodies. To address this, a new wind-powered artificial mixing system is designed and experimentally tested for various Savonius rotor combinations (three-stage and four-stage rotors). These turbines directly utilize wind energy to draw air into the water column for aeration, bypassing the need for electrical conversion. The rotor performances were tested in terms of power and torque coefficients. Additionally, these rotors were tested for artificial mixing efficiencies in a specially designed water tank that can mimic thermal stratification typically observed in an actual water supply reservoir. Among the rotors, the three-stage rotor with a 60° phase shift was found to exhibit superior power and torque coefficients, achieving a power efficiency value of 0.14. As for the mixing efficiency, the four-stage rotor with a 45° phase shift excelled in mixing efficiency, reaching 95%. PRACTITIONER POINTS: A new wind-powered artificial mixing system is designed and tested for various Savonius rotor combinations. While keeping the total rotor height constant, the three-stage Savonius rotor class shows superior performance against the four-stage Savonius rotor class in terms of power and torque efficiency. Apart from the rotor performance results, the four-stage Savonius rotors show greater artificial mixing efficiency than the three-stage Savonius rotors. Single-pump/diffuser artificial destratification system exhibits better mixing efficiency than multiple-pump/diffuser systems.

A Comparison of a Three Blade and Five Blade Wind Turbine in Terms of the Mechanical Properties Using the Q-Blade Software

يمكن لتوربينات الرياح المنتشرة في مزارع الرياح على نطاق المرافق أن تدعم تلبية رغبات الطاقة المستقبلية وتقليل انبعاثات ثاني أكسيد الكربون عن طريق تقليل متطلبات الطاقة من الوقود الأحفوري. مع ارتفاع درجة حرارة الهواء على مدار اليوم، تزداد سرعة الرياح بسبب تدرجات درجة الحرارة، والتي تنتج تدرجًا في الكثافة / الضغط، مما يؤدي إلى حركة الهواء التي تواجهها توربينات الرياح. اعتمادًا على تضاريس الأرض، يمكن أن تواجه الرياح وتوجه في الوديان بين التلال وفوقها حيث تتدفق وتتبع منحنيات الأرض. تنتج هذه التضاريس زيادة في سرعة الرياح في القمم والتلال. في هذا العمل، تم تصميم شفرة دوار توربينات الرياح الأفقية الصغيرة للعمل في ظل سرعة الرياح المنخفضة، باستخدام برنامج (Q-Blade). استنادًا إلى نظرية عنصر الشفرة (BEM). مع الجنيح NACA3712. تم استخدام دوار ثلاثي الشفرات ودوار بخمس شفرات بناءً على نوع التوربين وحجم الدوار لتوليد الطاقة الميكانيكية من طاقة الرياح. تم إجراء مقارنة وتحليل قوة التوربين ومعامل القدرة ومعامل عزم الدوران عند سرعة رياح منخفضة ((1m/s-8m/s وتم الحصول على نتائج دقيقة للغاية. وجد أن أفضل أداء يمكن أن يعمل فيه دوار توربيني ثلاثي الشفرات بقدرة توربينية تبلغ (955W) كما تم الحصول على قدرة التوربين ((582W للدوار خماسي الشفرات. وجد أن تصميم توربينة رياح أفقية صغيرة بخمس ريش أفضل من التوربين بثلاث ريش ومناسب للعمل في المناطق ذات سرعة الرياح المنخفضة وبكفاءة عالية مقارنة بحجم التوربين.

Accurate machine-learning-based prediction of aerodynamic and heat transfer coefficients for cylindrical biomass particles

Cylindrical biomass (straw) particles play a crucial role in the numerical simulations of biomass co-firing in coal-fired boilers, wherein the aerodynamic and heat transfer coefficients of these particles are particularly important. Despite the existence of models proposed thus far for various specific conditions, developing a universal model for cylindrical particles remains challenging in multiphase flow systems. In this paper, a total of 1092 validated computational fluid dynamics (CFD) datasets of cylindrical drag, lift, torque coefficients and average Nusselt number are acquired based on direct numerical simulations of OpenFOAM body-fitted meshes, and a hybrid algorithmic framework of N+1 convolutional neural network (CNN) machine learning optimality is established. where multiple single-tree models (referred to as N models) are combined with an additional CNN layer (referred to as +1 model).The framework inherits the advantages of a single-tree models while significantly improving the overall algorithmic generalisation performance, with the aim of allowing efficient use of time and minimal expertise in the fluid modelling process. The mean relative absolute errors for drag, lift, torque coefficient, and the average Nusselt number was evaluated, and the results demonstrated a reduction of 75.19 %, 89.48 %, 89.53 %, and 85.70 %, respectively, in the mean relative absolute errors compared to the extremely randomised tree single-tree model. We extended the model scope to different aspect ratios (1 ≤ Ar ≤ 30), angles of incidence (0° ≤ θ ≤ 90°), and Reynolds numbers (1 ≤ Re ≤ 4000), and conducted random predictions. Compared with traditional mathematical relationships, the proposed framework reduced the average relative error to 0.99 %. The simultaneous application of models for the prediction of unknown parameters was validated against relevant early literature, which demonstrated that the average relative error of drag coefficients for the predicted values remained within 2.07 %. The N+1 (CNN) hybrid algorithm framework exhibited high accuracy and excellent adaptability, furnishing a substantial quantity of data to support the Euler–Lagrange model of non-spherical biomass particle multiphase flow in industrial applications.

Predictive modelling of drag, lift and torque coefficients of cylindrical biomass particles under artificial intelligence

The escalating threats of climate change and fossil fuel depletion have greatly spurred the widespread utilization of renewable energy sources. Biomass energy stands out as the most abundant renewable energy resource. Among all biomass conversion technologies, direct combustion of biomass for heating and power generation represents the most common and cost-effective approach. Suspended combustion technology is widely employed in existing biomass combustion power plants worldwide. However, conventional modeling procedures often fail to adequately address the differences between coal and biomass combustion, particularly the significant impact of large-sized and highly non-spherical cylindrical straw biomass particles on their trajectories. This study utilizes OpenFOAM-body-fitted mesh direct numerical simulation (DNS) to generate a Computational Fluid Dynamics (CFD) dataset consisting of 300 data points for cylindrical particles, aimed at constructing a prediction model based on machine learning (N+1) for key aerodynamic parameters (drag coefficient, lift coefficient, and torque coefficient). The proposed model enables precise prediction of aerodynamic parameters for a wider range of different cylindrical particles and flow conditions, thus extending the existing Euler-Lagrangian model for non-spherical biomass multiphase flow.

Study on the Effect of Thermal Characteristics of Grease-Lubricated High-Speed Silicon Nitride Full Ceramic Ball Bearings in Motorized Spindles

Grease lubrication is cost-effective and low-maintenance for motorized spindles, but standard steel bearings can fail at high speeds. This study focuses on high-speed full ceramic ball bearings lubricated with grease. The coefficient of friction torque in the empirical formula is corrected by establishing the heat generation model of full ceramic ball bearing and combining it with experiments. A simulation model of grease flow is established to study the influence of grease filling amount on grease distribution. The simulation model of the temperature field of a full ceramic ball bearing is established to analyze the influence of rotating speed on bearing heat generation, and experiments verify the calculation results of the theoretical model. The results show that an optimal grease filling amount of 15~25% ensures even distribution without accumulation. Additionally, when the amount of grease is constant, the outer ring temperature increases with higher rotating speeds. The test results show that when the grease filling is 0.9~1.2 g, it accounts for about 9~12% of the volume of the bearing cavity, and the temperature of the outer ring is the lowest. At a rotation speed of 24,000 rpm, the outer ring temperature of the grease-lubricated bearing is 50.1 °C, indicating a reasonable range for use in motorized spindles. It provides a theoretical basis for the optimization design of macro-structural parameters of full ceramic ball bearings in the future, which can minimize heat generation and maximize bearing capacity.

Numerical study of the interaction between cylindrical particles and shear-thinning fluids in a linear shear flow

In this paper, the interaction between cylindrical particles and shear-thinning non-Newtonian fluids in a linear shear flow is investigated using particle-resolved direct numerical simulation. The Carreau model is used to represent the rheological properties of shear-thinning fluids, and the numerical method is validated against previously published data. Then, the effects of Reynolds number (Re), aspect ratio (Ar), power-law index (n), Carreau number (Cu), and incident angle (α) on drag coefficient (CD), lift coefficient (CL), and torque coefficient (CT) of cylindrical particles are investigated. The numerical results show that the flow field structure and pressure distribution around the cylindrical particle in a shear flow are different from those in a uniform flow, and the particles in a shear flow generate extra CL and CT. Furthermore, comparing with Newtonian fluids, the shear-thinning properties of the non-Newtonian fluid change the viscosity distribution and significantly decrease the CD, CL, and CT of the particles. The variation laws and influencing mechanisms of CD, CL, and CT under different working conditions are discussed by dividing the total coefficients into pressure and viscous shear contributions. Predictive correlations of CD, CL, and CT are established by considering the effects of Re, Ar, n, Cu, and α. The findings indicate that both the shear flow mode and shear-thinning properties must be considered when evaluating relevant particle–fluid interactions, which provides important guidance for predicting and controlling the orientation and distribution of cylindrical particles in shear-thinning fluids. Meanwhile, the predictive correlations can be used for large-scale simulations of multiphase coupling.

Numerical studies on the performance of Savonius turbines for hydropower application by varying the frontal cross-sectional area

The Savonius turbine is a drag-based device that utilizes hydrokinetic energy in irrigation channels for small-scale hydropower generation. Since it is a drag-type device, the turbine blade frontal cross-sectional area influences the performance of the turbine blade. In this paper, the influence of taper on the turbine blade bottom, top, and middle sides on the Savonius turbine performance was computed numerically using the sliding mesh technique with a 0.5 m/s water inlet velocity. In this study, the effect of various blades with varying cross-sectional areas has been analyzed numerically using the sliding mesh technique, and the results were compared with the semicircular blade profile. The variation of torque coefficient with respect to rotor blade rotation was plotted, and pressure and velocity variation were analyzed to investigate the performance parameters. The results showed that the top small, middle high, and bottom small blade profiles performed better than the semicircular, inverted taper and top high, middle small, and bottom high blade profiles. It is observed that the maximum torque coefficient at a TSR of 0.7 is 0.35, and the maximum power coefficient at a TSR of 0.9 is 0.253.

Numerical investigation of the effects of increased and mixed chord lengths for a 4-bladed darrieus vertical axis wind turbine

Abstract Increasing amounts of study and research on vertical axis wind turbines (VAWTs) have shown that they are a competitive option in wind energy power generation. However, the VAWT’s primary drawback is low power efficiencies. Although there are several studies on the effects of solidity on Darrieus VAWT performances, few focus on the effect of the aerofoil chord length. Hence, in the present study, 2D numerical simulations are performed to explore the effects of different aerofoil chord lengths on the performance of a Darrieus VAWT. The simulation was first validated with the experimental data from the literature. The studied turbine is a 4-bladed VAWT fitted with NACA0021 blades with an original chord length, c of 85.8 mm and another with an increased chord length of 1.2 c (102.96 mm). Additionally, a modified rotor geometry with mixed chord lengths of c and 1.2 c to improve turbine performance is proposed and investigated. The coefficients of power (C P) and torque (C T) for tip speed ratios (TSRs) between 1.4 and 3.3 for each of the turbines are evaluated and comparatively analysed. All the data was obtained using the computational fluid dynamics (CFD) software ANSYS Fluent in conjunction with the shear stress transport (SST) k−ω turbulence model. The findings show that the turbine with 1.2 c chord length and hence larger solidity outperforms those with smaller chord lengths at low TSRs. However, their performances decrease significantly at TSRs above 2.5, resulting in up to 86.1% lower C P values. The mixed chord lengths case was successful at achieving significantly higher C P values at both TSR ranges with only a decrease of 3.03% in maximum C P at its optimum TSR.