Drag Losses Research Articles

Power Losses of Oil-Bath-Lubricated Ball Bearings—A Focus on Churning Losses

This study investigates the power losses of rolling element bearings (REBs) lubricated using an oil bath. Experimental tests conducted on two different deep-groove ball bearings (DGBBs) provide valuable insights into the behaviour of DGBBs under different oil levels, generating essential data for developing accurate models of power losses. Observations of the oil bath dynamics reveal the formation of an oil ring at high oil levels, as observed for planetary gear trains, leading to modifications in the oil flow behaviour. The experiments demonstrate that oil bath lubrication generates power losses comparable to injection lubrication when the oil level is low. However, as the oil level increases, so do the power losses due to increased drag within the bearing. This study presents a comprehensive model for calculating drag losses. The proposed drag power loss model accounts for variations in oil level and significantly improves loss predictions. A comparison of existing models with the experimental results shows good agreement for both bearings, demonstrating the effectiveness of the developed model in accounting for oil bath height in loss calculations.

Study on the Shear Degradation Law of Polymer Drag Reducing Agents

Abstract The use of drag reducing agents in oil pipeline transportation can reduce driving energy consumption and has certain energy-saving benefits. However, irreversible degradation occurs when polymer drag reducing agents are sheared through pumping. In order to study the viscosity loss of polymer drag reducing agents caused by mechanical shear during the pumping process, the combination of methods such as a rotational rheometer and an indoor drag reducing loop testing system was used to test and analyze the rheological and drag reducing properties of the drag reducing agent solution, and to study the shear degradation law of polymer drag reducing agent solution under different conditions. The results showed that the shear stress of the polymer drag reducing agent solution increased with the increase in shear rate, and the viscosity increased with the increase of shear stress, showing shear thickening. When the polymer drag reducing agent solution is not sheared, the drag reduction rate increases and then decreases with the increase of concentration, and reaches the maximum value at 20 mg·L−1. However, after shear, the drag reduction performance decreases rapidly, but the high concentration still maintains a good drag reduction rate. The research results provide a theoretical basis for improving the performance of polymer drag reducing agents under different operating conditions and play a guiding role in further improving and perfecting the polymer drag reducing technology.

Results in Engineering Experimental investigation of drag loss and plate separation behavior of wet clutches under external forces

Understanding the drag loss behavior of wet clutches is crucial for the development of low-loss clutch systems. For non-horizontal installation positions and non-stationary applications, external forces caused by the inclination or vehicle dynamics act on the plates. It has yet to be investigated how external forces influence the plate separation process and distribution and, subsequently, the drag loss behavior. Therefore, this study aimed to fundamentally investigate wet clutches' drag loss and plate separation behavior under the effect of external forces. The investigations were conducted in three steps. First, fundamental knowledge of the plate separation and drag loss behavior was gained. The effects of relevant design and operating parameters on the drag loss behavior were determined then. Separation springs were finally tested to avoid the influence of external forces. The drag loss and plate separation behavior were investigated on a drag torque test rig based on different clutch systems. The external forces were applied by inclining the test rig. The plate separation behavior was determined through image-based measurement of the plate movements. It was found that the plates separate under external forces and that the drag torque curve shows characteristics similar to wet clutches operated without external forces applied. The utilization of the set total clearance depends on the inclination angle. Hence, increasing the set total clearance does not generally imply lower drag losses, but increasing the inclination angle leads to higher drag losses. It was confirmed that the drag loss behavior is not influenced by external forces when using spacing elements. The findings of this study extend the existing body of literature and may support the development process of brake and clutch systems.

Experimental and numerical analysis of 3D end effects in Naval Magnetohydrodynamic propulsion motors

This study explores the impact of end electric current on the performance of a marine magnetohydrodynamic (MHD) propulsion motor or thruster using a comprehensive 3D numerical simulation. The investigation focuses on various operating parameters, such as electromagnetic and drag forces, efficiency, losses, current, and different pressure components. To validate the numerical findings, a scaled-down experimental setup is utilised. The results demonstrate that certain operating parameters of MHD motors are not affected by the three-dimensional characteristics of their channels. These parameters, including fluid velocity, can be accurately calculated using the analytical model based on the effective electric current of the motor. Conversely, other operating parameters of MHD motors are significantly influenced by the end effects in their channels. For example, the overall efficiency, which depends on the total electric current of the motor, should be calculated using a more precise simulation method.

A Review of Computational Fluid Dynamics Approaches Used to Investigate Lubrication of Rolling Element Bearings

Abstract Optimizing bearing performance is based on effective lubrication, especially in high-speed machinery, where minimizing churning and drag losses is of significant importance. Over the past few decades, extensive research has been conducted into the better understanding of different aspects of bearing lubrication. These investigations have employed a combination of experimental methods and advanced computational fluid dynamics (CFD) models. This article provides a comprehensive overview of critical aspects of bearing lubrication, with a specific emphasis on recent advances in CFD models. Lubricant flow and distribution patterns are discussed while examining their impact on drag and churning losses. An extensive discussion is provided on the meshing strategies and modeling approaches used to simulate various flow phenomena within bearings. In addition, relevant trends and impacts of cage design on bearing lubrication and fluid friction have been explored, along with a discussion of prevailing limitations that can be addressed in future bearing CFD models.

Bioinspired Zwitterionic Nanowires with Simultaneous Biofouling Reduction and Release.

Marine biofouling is a complex and dynamic process that significantly increases the carbon emissions from the maritime industry by increasing drag losses. However, there are no existing non-toxic marine paints that can achieve both effective fouling reduction and efficient fouling release. Inspired by antifouling strategies in nature, herein, a superoleophobic zwitterionic nanowire coating with a nanostructured hydration layer is introduced, which exhibits simultaneous fouling reduction and release performance. The zwitterionic nanowires demonstrate >25% improvement in fouling reduction compared to state-of-the-art antifouling nanostructures, and four times higher fouling-release compared to conventional zwitterionic coatings. Fouling release is successfully achieved under a wall shear force that is four orders of magnitude lower than regular water jet cleaning. The mechanism of this simultaneous fouling reduction and release behavior is explored, and it is found that a combination of 1) a mechanical biocidal effect from the nanowire geometry, and 2) low interfacial adhesion resulting from the nanostructured hydration layer, are the major contributing factors. These findings provide insights into the design of nanostructured coatings with simultaneous fouling reduction and release. The newly established synthesis procedure for the zwitterionic nanowires opens new pathways for implementation as antifouling coatings in the maritime industry and biomedical devices.

Using Gaussian process regression for building a data-driven drag loss model of wet clutches

Drag loss calculation models support designing low-loss wet clutch systems and determining drivetrain efficiency. Available numerical models require high computational effort, whereas analytical models provide only limited accuracy. In contrast, data-driven models unite the advantages of fast and effective drag loss prediction and, thus, enable practice-oriented development. Therefore, this study aimed to build a data-driven model of the wet clutches’ drag losses, enabling predictions with low computational effort and, at the same time, sufficient accuracy while taking seven relevant influencing parameters into account. The dataset used originates from prior research projects that experimentally investigated the wet clutches’ drag loss behavior under consideration of various influencing parameters. Different machine learning algorithms were considered for model building, and it was found that Gaussian process regression best met the requirements of flexibility and interpretability. Four sub-models were built to predict four characteristic drag loss values based on the model input parameters of mean diameter, diameter ratio, number of gaps, clearance, dynamic viscosity, oil level, and groove design. Subsequently, the predicted characteristic drag loss values can be used as support points for approximating the drag loss curve. The sub-models show high R² values, ranging from 0.84 to 0.95, indicating a high model performance. The mean relative error of the sub-models was used for evaluating the models’ performance from an engineering perspective, ranging from 4.6 % to 14.2 %. The drag loss model was also validated based on documented knowledge of wet clutches’ drag loss behavior. The model enables the prediction of the drag loss curve within a few split-seconds and, simultaneously, with sufficient accuracy.

Computational Fluid Dynamics Methodology to Estimate the Drag Coefficient of Balls in Rolling Element Bearings

The emergence of electric vehicles has brought new issues such as the problem of rolling element bearings (REBs) operating at high speeds. Losses due to these components in mechanical transmissions are a key issue and must therefore be taken into account right from the design stage of these systems. Among these losses, the one induced by the motion of rolling elements, known as drag loss, becomes predominant in high-speed REBs. Although an experimental approach is still possible, it is difficult to isolate this loss in order to study it properly. A numerical approach based on CFD is therefore a possible way forward, even if other issues arise. The aim of this article is to study the ability of such an approach to correctly estimate the drag coefficient associated with the motion of rolling elements. The influence of the numerical domain extension, the mesh refinement, the simplification of the ring shape, and the presence of the cage on the values of the drag coefficient is presented. While it seems possible to compromise on the calculation domain and mesh size, it appears that the other parameters must be taken into account as much as possible to obtain realistic results.

Experimental investigation of drag loss behavior of dip-lubricated wet clutches for building a data-driven prediction model

Fundamental knowledge of wet clutches’ drag loss behavior is essential for designing low-loss clutch systems. In contrast to the widely investigated injection lubrication, more comprehensive knowledge is needed on the drag loss behavior of dip-lubricated wet clutches. In the development phase, data-driven models allow drag loss predictions with low computational effort and, at the same time, sufficient accuracy. Therefore, this study aimed to deepen and expand knowledge of the drag loss behavior of dip-lubricated wet clutches based on experimental investigations. Moreover, the investigations were designed and conducted so that the generated data and findings can be used in further research for building data-driven prediction models. The investigations were conducted on two clutch systems from automotive and industrial applications. The practice-relevant parameters of clearance, oil level, oil viscosity, and plate shape were investigated based on a mixed-level full factorial design. The evaluation shows that a reduction in drag loss can be achieved primarily by increasing the clearance, reducing the oil viscosity, and choosing waved plates. The obtained drag loss behavior can be traced back to the form of oil displacement from the gaps. The displacement process, in turn, is influenced by the operating and geometry parameters. Although the flow in the gaps develops differently for dip and injection lubrication over differential speed, the study shows comparable integral effects of the influencing parameters for both types of lubrication. The generated datasets contain the investigated parameters as features and characteristic drag loss values as targets. The findings can support the selection and configuration of the machine learning algorithm and the validation of the trained models. The described procedure can serve as a template for generating and analyzing datasets for data-driven modeling of wet clutches’ drag losses.

A methodology for image-based measurement of plate movement in disengaged wet clutches

The drag loss behavior of a disengaged wet clutch is influenced, among other things, by the movement of the plates. Therefore, knowledge about the plate movement is essential for investigating and optimizing the drag loss behavior. This paper presents a methodology for image-based measurement of plate movement in disengaged wet clutches. A drag torque test rig is equipped with a camera to create the image series. The oil displacement from the measuring zone is crucial to obtain permanent optical access to the clutch pack. The rough plate positions are determined by segmentation using thresholding and template matching. Using the Canny edge detector significantly improves the accuracy of the position evaluation. The plate positions are finally converted into a metric unit based on the real plate thicknesses. The clearances are calculated from the determined positions of two adjacent plates. In the ideal case, an evaluation accuracy in the range of a few micrometers can be achieved. The image evaluation methodology is universally applicable to different clutch sizes, friction systems, plate types, and plate numbers. The methodology enables researchers to generate fundamental knowledge and derive design guidelines based on this, for example. In the development phase, it can also be used to optimize the design and operating parameters.

Effects of Riblet Dimensions on the Transitional Boundary Layers Over High-Lift Turbine Blades

Abstract Substantial research exists in the literature on reducing the profile loss of transitional boundary layers over low-pressure turbine (LPT) blades via different mechanisms such as freestream turbulence, upstream wakes, and surface roughness. These mechanisms have proven to be beneficial in mitigating the separation bubble-related losses in ultra-high-lift blade designs, despite an increase in the loss due to increased turbulent wetted area (TWA). In this work, we adopt a strategy of employing surface roughness in the transitional regime to minimize the separation bubble-related losses and flush-mounted riblets downstream to further mitigate the skin-friction drag and boundary layer losses due to an increase in the TWA. Several high-fidelity scale-resolving simulations are performed on this “rough-ribbed blade surface” to discern the effect of varying the riblet spacing (s+) and height (h+). The streamwise evolution of skin-friction coefficient, boundary layer integral parameters, and shape factor are compared and contrasted among riblets of different dimensions. The instantaneous flow features and second-order statistics such as the Reynolds stress, turbulent kinetic energy, and its production are analyzed for different test cases to determine the impact of riblets on these quantities. When compared to the roughness alone configuration, the scalloped shape riblets with s+ = 17 and h+ = 22 reduced the net skin-friction drag by 7.3% and the trailing edge momentum thickness by 14.5%, thereby demonstrating the efficacy of riblets in reducing the mixing losses under adverse pressure gradients. Through an analysis of flow blockage introduced by the application of riblets, the deleterious effects of increasing the riblet height along with the necessity of optimizing the riblet ramp are highlighted.

Experimental analysis of oil flow and drag torque generation in disengaged wet clutches

Fundamental knowledge of the oil flow in a disengaged wet clutch is essential for optimizing the cooling performance and the drag losses. However, no fundamental information on the oil flow and drag torque generation is available for dip-lubricated wet clutches. Therefore, the oil flow and drag torque generation in the sub-millimeter gap of a dip-lubricated wet clutch was experimentally investigated for three practically relevant oil levels. To enable optical access to the gap, transparent components were used. Further, a high-speed camera was used to capture the oil flow in the gap and grooving. Independent of the set oil level, the gap is oil-filled at low differential speeds, resulting in a single-phase flow. The drag torque increases approximately linearly with increasing differential speed due to the fluid shearing. In certain regions of the waffle grooving, air bubbles form locally. The air bubbles preferably occur in the grooves oriented in the radial direction, while the grooves oriented in the peripheral direction are filled with oil. Above a certain differential speed, the oil is continuously displaced from the gap, starting from the inside, due to the increasing centrifugal force. Consequently, the drag torque increases in a degressive manner until a maximum value is finally reached. The ongoing displacement of oil from the gap eventually results in a decrease in the drag torque. A steady drag torque is generated only when the oil is almost entirely displaced from the gap. Since the oil displacement from the gap already commences at a low differential speed, the cooling performance is limited for dip-lubricated wet clutches. The continuous displacement of oil from the gap can be held up, among other things, by increasing the oil level.

Performance evaluation with turbulent flow and heat transfer characteristics in rectangular cooling channels with various novel hierarchical rib schemes

Turbulators, such as ribs, dimples, and pin-fins, play a vital role in the internal cooling efficiency of turbine blades. As a typical turbulator, various rib configurations using a uniform arrangement scheme have indicated high heat transfer enhancement but the friction loss is simultaneously subject to a great increase. In this work, a novel hierarchical arrangement scheme of ribs is developed aiming to improve the cooling efficiency. Adopting the uniform scheme as a baseline, the hierarchical scheme is implemented for six representative rib configurations (including transverse ribs, angled ribs, V-shaped ribs, inverted V-shaped ribs, M-shaped ribs, and inverted M-shaped ribs) and evaluated for its feasibility and generality. For different cooling designs, turbulent flow and heat transfer of the ribbed cooling channel are studied by three-dimensional numerical simulations based on the finite volume method with a constructed turbulence model. It is found that for all rib configurations, the hierarchical scheme can remarkably reduce the friction loss as desired, especially for the inverted V-shaped rib with a reduction of up to 50%. Due to the occurrence of flow separation, secondary flows offered by transverse ribs are characterized by a two-dimensional recirculation vortex behind the rib. For other rib configurations, secondary flows present a typical three-dimensional characteristic including the downwash flows along the inclined rib leg and the longitudinal vortices. The usage of the hierarchical scheme with small ribs strongly suppresses these secondary flows, which contributes to the significant decrease in form drag loss. Meanwhile, using the hierarchical scheme produces a slight heat transfer deterioration commonly, which is because the constrained secondary vortices weaken the turbulent mixing and convection heat transfer. Significantly, for the two W-shaped ribs, the limited secondary vortices but fully developed under the hierarchical scheme achieve a higher heat transfer enhancement. Finally, for all considered ribs, the hierarchical scheme can improve the overall performance factor of (Nu/Nu0)/(f/f0)1/3 by more than 10%, and up to 21.15% for the V-shaped rib. Adjusting design variables, including the decreasing ratio of the rib size and the initial rib size, the hierarchical scheme still provides even higher performance enhancement.

Research on economical mode switching control strategy of dual-motor four-wheel drive considering motor drag and mode switching transient loss

In existing research on economical-performance control strategies for 4WD EV, only vehicle control strategies obtained through static efficiency interpolation of the electric-drive-system are considered, without considering the influence of high-frequency mode switching on energy consumption, and the difference between the actual power loss of the motor step-torque transient response and the theoretical loss calculated by steady-state map interpolation in the mode switching process. The research presents an economical mode switching control strategy that considers the step-torque transient response power loss caused by motor drag and mode switching. An electric-drive-system torque response testing platform was built to study the influence of the actual power loss of the motor step-torque transient response on energy consumption during mode switching under transient conditions. A mode boundary division method was proposed to analyze how different mode division impact on energy consumption and mode switching frequency so as to find the relationship between different operating points and mode boundary areas, and the optimal mode boundary width in each characteristic range was optimized hierarchically to realize reduction in energy consumption by 3.37% and reduction in mode switching frequency by 63.13%. Among them, the optimal mode boundary width accounts for 83.33% of the vehicle energy consumption reduction rate.

A methodology for data-driven modeling and prediction of the drag losses of wet clutches

AbstractIn wet clutches, load-independent drag losses occur in the disengaged state and under differential speed due to fluid shearing. The drag torque of a wet clutch can be determined accurately and reliably by means of costly and time-consuming measurements. As an alternative, the drag losses can already be precisely calculated in the early development phase using computing-intensive CFD models. In contrast, simple analytical calculation models allow a rough but non-time-consuming estimation. Therefore, the aim of this study was to develop a methodology that can be used to build a data-driven model for the prediction of the drag losses of wet clutches with low computational effort and, at the same time, sufficient accuracy under consideration of a high number of influencing parameters. For building the model, we use supervised machine learning algorithms. The methodology covers all relevant steps, from data generation to the validated prediction model as well as its usage. The methodology comprises six main steps. In Step 1, the data is generated on a suitable test rig. In Step 2, characteristic values of each measurement are evaluated to quantify the drag loss behavior. The characteristic values serve as target values to train the model. In Step 3, the structure and quality of the dataset are analyzed and, subsequently, the model input parameters are defined. In Step 4, the relationships between the investigated influencing parameters (model input) and the characteristic values (model output) are determined. Symbolic regression and Gaussian process regression have both been proven to be suitable for this task. Lastly, the model is used in Step 5 to predict the characteristic values. Based on the predictions, the drag torque can be predicted as a function of differential speed in Step 6, using an approximation function. The model allows a user-oriented prediction of the drag torque even for a high number of parameters with low computational effort and sufficient accuracy at the same time.

Application of a Novel Quantitative Trait Locus Combination to Improve Grain Shape without Yield Loss in Rice (Oryza sativa L. spp. japonica).

Grain shape is one of the key factors deciding the yield product and the market value as appearance quality in rice (Oryza sativa L.). The grain shape of japonica cultivars in Korea is quite monotonous because the selection pressure of rice breeding programs works in consideration of consumer preference. In this study, we identified QTLs associated with grain shape to improve the variety of grain shapes in Korean cultivars. QTL analysis revealed that eight QTLs related to five tested traits were detected on chromosomes 2, 5, and 10. Among them, three QTLs-qGL2 (33.9% of PEV for grain length), qGW5 (64.42% for grain width), and qGT10 (49.2% for grain thickness)-were regarded as the main effect QTLs. Using the three QTLs, an ideal QTL combination (qGL2P + qGW5P + qGT10B) could be constructed on the basis of the accumulated QTL effect without yield loss caused by the change in grain shape in the population. In addition, three promising lines with a slender grain type were selected as a breeding resource with a japonica genetic background based on the QTL combination. The application of QTLs detected in this study could improve the grain shape of japonica cultivars without any linkage drag or yield loss.

Effects of pre-swirl radius on cooling performance of a rotor-stator pre-swirl system in gas turbine engines

The radius ratio of pre-swirl nozzles to supply holes ( r 1 / r 3 ) has a direct influence on the flow losses generated as well as the temperature drop in the pre-swirl cooling systems used in gas turbine engines. Deciding a proper radius ratio is a significant step for achieving the maximum cooling efficiency. In this paper, the relationship between the temperature drop and entropy increase of the cooling systems is first established theoretically, and from this a novel maximum temperature drop assumption is proposed to give an optimal r 1 / r 3 for the maximum cooling efficiency. To validate this method, the ideal r 1 / r 3 value for a realistic cover-plate pre-swirl system was calculated by this approach, and numerical simulations on five different r 1 / r 3 values of the system were conducted. Static pressure ratio and swirl ratio distributions, entropy production and entropy increase for the five configurations were compared and analyzed. The results show that the cooling performance of a pre-swirl system only depends on the entropy increase in the whole system. For the specific system geometry and design operating conditions investigated, the proposed method and CFD both show that the values of r 1 / r 3 ranging from 0.80–0.96 are the ideal nozzle radial locations, which minimise the wall drag and mixing loss occurred in pre-swirl cavity and rotating supply holes. In this range, the non-dimensional temperature drop is at least 8.6% and 22% higher than those with r 1 /r 3 = 0.72 and 0.64 respectively.

Biomimetic Research of Fish Mucus and Cargo Ships

The goal of this research was to determine if one could increase the efficiency of drag and energy loss on containership hulls in the water by utilizing the principles of biomimetics. All species of fish contain a coat of mucus which makes the surface of the fish slippery and reduces drag, adds external protection and many additional advantages. The mucus layer fills the gaps of a body to make a smooth and slippery surface for minimum drag while also providing hydrodynamic advantages by increasing velocity from which the surrounding fluid hits the surface of the fish. Increase in efficiency during movement on water was determined by the different travel speeds using 2 cargo ship model boats (one was coated with fish mucus on the hull and the other one was left uncoated). It was observed that the ship with fish mucus on its hull increased in speed and decreased the drag force, thus making the travel more efficient. The data obtained in this experiment supports the hypothesis that cargo ships would use less fuel and waste less energy with a fish mucus-like synthetic on their hulls.

Development of Transaxle Fluid for Electrification Vehicles: Design of Novel Additive Formulation

<div class="section abstract"><div class="htmlview paragraph">To achieve carbon neutrality by reducing carbon dioxide (CO<sub>2</sub>) emissions, vehicles with an internal combustion engine have started to be replaced by electrification vehicles such as hybrid electric vehicles (HEVs), plug-in HEVs (PHEVs), and battery EVs (BEVs) worldwide, which have motors in their transaxles (T/As). Reducing transmission torque loss in the transaxles is effective to reduce CO<sub>2</sub> emissions, and lowering the viscosity of lubrication fluids in T/As is a promising method for reducing churning and drag loss. However, lowering viscosity generally leads to thin oil films and makes the lubrication condition severe, resulting in worse anti-fatigue and anti-seizure performance. To deal with these issues, we made improvements on the additive formulation of fluid, such as the addition of an oil-film-forming polymer, chemical structure change of calcium detergents, and an increase of anti-wear additives including phosphorus and sulfur. As a result, we succeeded in developing a novel fluid with greater lubricity than a conventional automatic transmission fluid (ATF), despite the fact that the viscosity of the new fluid was lowered by around 50% compared with that of the ATF. In addition, the electrical insulating and anti-foaming performance of this fluid, which are required for T/A fluid in electrification vehicles, were equal to or greater than those of the ATF. The fuel economy in the Toyota Hybrid System (THS) was improved by 1.0% and more compared with the ATF because of the significant viscosity decrease in the developed fluid. This fluid will be widely used for HEVs, PHEVs, BEVs, and fuel cell electric vehicles (FCEVs) and can contribute to the realization of a carbon-neutral society.</div></div>

An engineering drag losses model for rolling bearings

Rolling bearing friction is an important measure of bearing performance. With increasing industrial trends of energy savings and CO2 footprint reduction, rolling bearing friction has become a very important parameter in the machinery design and selection process. Particular importance have high-speed applications like spindles or electrical vehicle traction drives and gearboxes. Since handling thermal effects has become an important problem to solve in these applications, oil lubrication is gaining interest due to its cooling capacity. Bearing friction is made of rolling, sliding and drag losses components in oil lubricated bearings. In this paper the emphasis is given to the drag losses component, which becomes the most critical in oil bath in medium and high-speed applications. An engineering model is derived for the calculation of this component for medium and potentially high speeds. The results are compared with experimental measurements (in-house and literature). In general, good agreement is found between the engineering model and the experimental results.