Clearance Height Research Articles

Aerodynamic characteristic of limited micro-scale Taylor–Couette flow at hyper-rotate speed

To pursue improvements in the power-to-weight ratio, cost-effectiveness ratio, and reliability in the energy industry, rotating machinery has been developed with higher rotational speeds and smaller sizes, resulting in complex aerodynamic behavior of the rotational shearing flow between rotator and stator. This study reports experimental measurements of the aerodynamic characteristics of a limited micro-scale Taylor–Couette flow (TC flow) at hyper-rotational speeds for the first time. The maximum rotational speed and eccentricity ratio are 6.0 × 104 rpm and 0.8, respectively, and the minimum average clearance height is 100 μm. The static pressure of the shearing flow is measured using a high-response micro-manometer. A large eddy simulation has been employed to unveil flow fields of the gas film between the stator and the rotator. The results show that the micrometer-scale clearance height and end leakage due to limited length dominate the micro-scale Taylor–Couette flow. Decreasing the scale of the clearance height reduces the angular distance between the peak and bottom pressures compared to that observed in the TC flow with a large-scale clearance. Furthermore, ambient pressure cannot fully permeate into central section of the eccentric Taylor–Couette flow with a micrometer-scale clearance and end leakage, and the pressure upstream near the region the minimum size of clearance decreases along the axial end. Owing to the difference in the constraint degree at both ends along the axial direction, the peak value occurs near the region of Z/L = −0.32. The maximum pressure of the limited micro-scale Taylor–Couette flow increases with increasing rotational speed and eccentricity ratio, whereas it decreases with increasing clearance height. Within the scope of the parameters in this study, the maximum pressure is 0.025 MPa. Decreasing the size of gas film restrains the instability of the shearing flow, whereas increasing the rotational speed strengthens the turbulence. In this study, the shearing flow at the middle section (Z/L = 0) is in a nearly laminar state at δ < 0.012, whereas it is highly turbulent near the end of the clearance and in the ambient environment. The end leakage induces additional axial flow, which is conspicuous at a large eccentricity ratio. At hyper-rotational speeds, the micro-scale Taylor–Couette flow with end leakage exhibits distinctive patterns that have never been reported in classic Taylor–Couette flows with large clearances. This work of this study is significant because it can provide theoretical guidance and technical support for the design of actual dynamic pressure gas bearings and floating ring sealing flow.

A combined approach of design for Six Sigma, generic product development process and ergonomics-safety philosophy to reduce musculoskeletal disorder problems

Purpose The purpose of this paper is to decrease musculoskeletal disorder (MSD) problems using a combined approach including design for Six Sigma, generic product development process, and ergonomic-safety philosophies. Design/methodology/approach A total of 550 students voluntarily participated in this study. Kolmogorov–Smirnov and t-tests were used to analyze the collected anthropometry. A cause-and-effect diagram, Pareto’s chart, a brainstorming session, and a priority matrix were also used to extract the 22 design parameters of the product. Moreover, Cronbach’s alpha values were determined to verify the validity of qualitative and quantitative data. Findings Results showed that students are at a high risk of developing MSD issues. The most serious MSD problems among the students were neck pain (74%), back pain (73%), muscle and joint pain (67%), knee pain (57%), leg pain (54%), etc. Besides, all anthropometrics of males are higher than females except Hip Breadth. Among 12 anthropometries, two measurements and one measurement showed statistical significance at p < 0.01 and p < 0.05, respectively. Six design parameters of the mounted desktop (Seat Height, Desk Height, Seat to Desk Clearance, Seat Width, Seat Depth, and Backrest Height) are the most crucial design parameters to design and develop the product. Practical implications The proposed design of the mounted desktop can be used in different tertiary educational institutes to reduce MSDs among students. Originality/value To the best of the authors’ knowledge, this is the first attempt that introduces a combined approach to reduce MSD issues by applying Six Sigma, a generic product development process, and ergonomics-safety philosophy to design and measurement of 22 design parameters of mounted desktop for university students.

Performance analysis of air-cooled turbine based on source terms method

Abstract Investigating turbine blade cooling and blade tip clearance leakage is crucial for reducing turbine losses and enhancing overall engine performance. Therefore, this paper uses the E3 two-stage high-pressure turbine as a case and employs the source term method to model the film cooling configuration, validating its efficacy and examining the performance parameters and flow field characteristics of air-cooled turbines under varying blade tip clearances. The outcomes demonstrate that the source term approach accurately forecasts the comprehensive performance parameters of air-cooled turbines, with blade tip clearance height predominantly impacting turbine performance in high-velocity regions. For small blade tip clearance, air-cooled turbines exhibit diminished efficiency with a decreasing expansion ratio in high-velocity regions, but they demonstrate an inverse behavior in low-velocity regions. Conversely, under conditions of larger blade tip clearances, the cooling effectiveness of air-cooled turbines is more pronounced in regions characterized by low expansion ratios and low speeds.

A metaheuristic-based optimization model for flight procedure design

In recent years, the continuous increase in air traffic has led to a saturation of available airspace. The new localizer performance with vertical guidance (LPV) instrument approach procedures (IAPs) offer additional capacity at aerodromes and in airspaces. To support their implementation, a specifically developed model focused on the improved IAP design process and optimized solution generation is proposed. The model provides a fully automated search of the solution space by employing the bee colony optimization metaheuristic algorithm. The resulting list of alternatives is ranked by the objective function score, which is affected by the balancing values of the complexity indicators and the achieved obstacle clearance height. The model’s capabilities are successfully validated against the challenging operational environment of Podgorica Airport. The model was initially developed to conduct IAP feasibility studies; however, it can also be used in the initial stages of airport planning projects or for IAP criteria validation.

Development of End-Reduced Pre-Flex Girder

Bridges serve to connect spaces where natural terrain or structures restrict the movement of people or vehicles. Various bridge construction methods have been proposed, and the application of preflex construction methods has recently increased. The end-reduced preflex girder is a method that applies preflex construction and compensates for the decrease in structural performance and long-term deflection of the bridge. However, in the existing preflex construction method, compliance with the minimum clearance height regulations in Korea, necessitates an increase in the end plan height, which requires an additional embankment that narrows the driver's view. This study was conducted to verify and evaluate the performance of an end-reduced preflex girder that can increase the structural strength and secure the minimum clearance height by attaching a shear reinforcement plate to the bottom of the lower flange, enabling an economical design.

Study on nonuniform tip clearance in mixed flow compressor

In compressed air energy storage system, the inevitable axial vibration during the frequent start-stop of compressor leads to circumferential nonuniform tip clearance formation, which significantly impacts compressor performance. This paper utilizes numerical simulation to study the overall performance and internal unsteady flow field of a mixed flow compressor with 4 different eccentricities and compares them with the original model. The results indicate that the eccentricity of blade tip clearance has little effect on pressure ratio and isentropic efficiency of the mixed flow compressor, but has an important impact on its stall margin. Specifically, when the eccentricity is 70 % of the blade tip clearance, the compressor stall margin is reduced by 5.96 %. The circumferential distribution of tip relative leakage aligns with the circumferential variation of tip clearance height. The average tip relative leakage increases gradually with the increase of eccentricity, and its growth rate also increases gradually with the increase of eccentricity. There are two peaks and valleys in the radial force distribution of a single blade, while the axial force of a single blade is consistent with the change in circumferential tip clearance height. These findings can provide valuable guidance for subsequent compressor design and operation.

Soft ankle exoskeleton to counteract dropfoot and excessive inversion.

Wearable exoskeletons are emerging technologies for providing movement assistance and rehabilitation for people with motor disorders. In this study, we focus on the specific gait pathology dropfoot, which is common after a stroke. Dropfoot makes it difficult to achieve foot clearance during swing and heel contact at early stance and often necessitates compensatory movements. We developed a soft ankle exoskeleton consisting of actuation and transmission systems to assist two degrees of freedom simultaneously: dorsiflexion and eversion, then performed several proof-of-concept experiments on non-disabled persons. The actuation system consists of two motors worn on a waist belt. The transmission system provides assistive force to the medial and lateral sides of the forefoot via Bowden cables. The coupling design enables variable assistance of dorsiflexion and inversion at the same time, and a force-free controller is proposed to compensate for device resistance. We first evaluated the performance of the exoskeleton in three seated movement tests: assisting dorsiflexion and eversion, controlling plantarflexion, and compensating for device resistance, then during walking tests. In all proof-of-concept experiments, dropfoot tendency was simulated by fastening a weight to the shoe over the lateral forefoot. In the first two seated tests, errors between the target and the achieved ankle joint angles in two planes were low; errors of <1.5° were achieved in assisting dorsiflexion and/or controlling plantarflexion and of <1.4° in assisting ankle eversion. The force-free controller in test three significantly compensated for the device resistance during ankle joint plantarflexion. In the gait tests, the exoskeleton was able to normalize ankle joint and foot segment kinematics, specifically foot inclination angle and ankle inversion angle at initial contact and ankle angle and clearance height during swing. Our findings support the feasibility of the new ankle exoskeleton design in assisting two degrees of freedom at the ankle simultaneously and show its potential to assist people with dropfoot and excessive inversion.

Effect of bridge height on airflow and aeolian sand flux near surface along the Qinghai-Tibet Railway, China

In this work, we studied the near-surface flow field structure of railway bridges with different heights through field investigation and wind tunnel simulation experiments. Meanwhile, we simulated the distribution of sand accumulation around a bridge via CFD software based on the sand accumulation around the Basuoqu bridge in the Cuona Lake section of the Qinghai–Tibet Railway. Results show that the sand around this railway bridge is mainly from the lake sediment on the west side of the railway and the weathered detritus on the east side. The height of the railway bridge in the sandy area affects the distribution of the near-surface flow field and the variation in speed on both sides of the bridge. The wind speed trough on both sides of the 6 m high bridge is higher, and the horizontal distance between the wind speed trough and the bridge section is 1.5 times that of the 3 m high bridge. Wind speed attenuates in a certain range on the windward and leeward sides of the bridge, forming an aeolian area; under the beam body, it is affected by the narrow tube effect, forming a wind erosion area. The height of the bridge determines its sand transport capacity. Under certain wind conditions, the overhead area at the bottom of the 3 m high bridge and its two sides do not have the sand transport capacity, so sand accumulates easily. Nevertheless, the sand accumulation phenomenon gradually disappears with the increase in bridge clearance height. The objectives of this study are to reveal the formation mechanism of sand damage for railway bridges, provide theoretical support for the scientific design of railway bridges in sandy areas, and formulate reasonable railway sand prevention measures to ensure the safety of railway running, which have certain theoretical significance and practical value.

Heat transfer coefficient evaluated for a designed jacketed vessel for parameter optimization in coal water slurry preparation using dual impellers

ABSTRACT This study examines the heat transfer coefficient during coal water slurry preparation in a jacketed vessel, focusing on key parameters influencing heat transfer. A dual impeller system is utilized to enhance mixing and heat transmission capacity. Using the Ant Colony Optimization (ACO) method, the equipment is designed, comprising a cylindrical flat-bottom tank with an impeller diameter of 0.079 m, a height of 0.3 m, and four uniformly positioned detachable flat baffles around the vessel’s perimeter. Experiments are conducted using the L27 orthogonal array at three different levels, varying impeller speed, clearance height to tank diameter ratio, slurry height to tank diameter ratio, and distance between the two impellers. Two categories of dual impeller experiments, propeller and Rushton, are examined. Statistical analysis indicates that impeller speed and Z/T ratio significantly influence heat transfer coefficient, while C/T ratio and distance between impellers have a lesser impact. In both propeller and Rushton experiments, impeller speed and Z/T ratio have the most substantial effect on heat transfer coefficient, contributing 62.907% and 59.47%, respectively, in propeller and Rushton experiments. These findings underscore the importance of impeller speed and Z/T ratio in enhancing heat transfer coefficient during coal water slurry preparation in a jacketed vessel.

Effects of natural flood management woody dams on benthic macroinvertebrates and benthic metabolism in upland streams: Importance of wood‐induced geomorphic changes

AbstractNatural Flood Management (NFM) aims to reduce flood hazard by working with nature and is gaining prominence worldwide. One particular NFM technique involves the use of channel‐spanning woody dams that maintain a clearance height above baseflow. These dams function by increasing channel roughness during high flows and by forcing excessive water onto the floodplain. Whether these dams provide additional benefits to nature remains unclear. While there are many existing studies on natural in‐stream wood structures, very few have documented the impact of NFM woody dams in particular. This study adopted a multidisciplinary approach and a Before–After Control–Impact (BACI) research design to assess whether NFM woody dams installed in a small upland catchment had driven changes in benthic macroinvertebrate assemblages and benthic metabolic activities through the geomorphic changes that they had created. Statistical results indicate that macroinvertebrate density, richness, and diversity did not show any difference between stream reaches with and without NFM woody dams. The metrics were generally not related to grain‐size parameters and volumes of sediments eroded or deposited. However, individual genera such as Baetis and Rhithrogena became more dominant in the control reach towards the end of the study period, likely due to the higher flow velocities and coarser sediments there resulting from the lack of flow resistance in the absence of NFM woody dams. Rates of benthic respiration (but not rates of photosynthesis) were consistently significantly higher in woody dam reaches than in control reaches, likely due to the presence of patches of finer sediments in the former.

Effect of the Visual Illusion on Stepping-Over Action and Its Association with Gaze Behavior.

An adequate foot clearance height while stepping over an obstacle is important for safety in daily life. In the present study, we examined whether visual illusions affect foot clearance during a stepping-over action, and whether this is further influenced by gaze behavior. Twelve participants stepped over an obstacle placed four meters away under conditions of three different obstacle characteristics: white, horizontal, or vertical lines. We measured the participants' foot clearances during the step-over action and their gaze behavior during the approaching phase. Participants stepped significantly higher over the obstacles in the vertical lines (illusion) condition. The duration of gaze fixation on the obstacle positively correlated with increased foot clearance in the vertical condition, suggesting that the effect of the visual illusion on foot clearance was enhanced by prolonged gaze fixation. Conversely, prolonged fixation negatively correlated with foot clearance in the white (control) condition, implying that a cautious perception of an obstacle may contribute to efficient stepping-over action.

Investigation on the Dynamic Pressure Effect of Clearance Oil Film at a Stepped Hydrostatic Thrust Bearing Working in Heavy Loading Duty

The dynamic pressure effect of the clearance oil film of stepped hydrostatic thrust bearing is studied by taking the double rectangular cavity oil cushion as an example. According to the hydrodynamics theory, the dynamic pressure of lubricating oil film in different clearance height regions is theoretically deduced and calculated. The dynamic pressure effect of the clearance oil film in the stepped hydrostatic thrust bearing is studied through the combination of theoretical calculation, simulation, and experimental verification. It is found that the experimental value of the dynamic pressure is between the theoretical value and the simulated value, which is basically not affected by the load. In the speed range of 0 r/min–200 r/min, compared with the viscosity of lubricating oil, the speed is the main factor affecting the dynamic pressure of the oil film of the stepped hydrostatic thrust bearing. The dynamic pressure value of the clearance oil film increases in a stepped fashion along the radial direction of the double rectangular cavity oil cushion. The dynamic pressure value has an obvious upward trend at the junction of the circumferential right oil cavity and the sealing edge and then decreases to 0 after reaching the peak value.

Effect of moving end wall on tip leakage flow in a compressor cascade with different clearance heights

Numerical simulations have been carried out to investigate the effect of the moving end wall on the tip leakage flow structure in a compressor cascade with four different tip clearance heights. A detailed comparison of the leakage flow’s loss; mass flow rate; and the tip leakage vortex and its roll-up, trajectory, size, and interaction with the passage flow was made. The results show that the performance of the cascade changes more severely as the tip clearance height increases with the moving end wall than with the stationary end wall. The roll-up of the tip leakage vortex has been studied by analyzing the pathline structure and vorticity distribution. An alternative explanation is proposed for the initial roll-up mechanism of the tip leakage vortex. The vorticity transport is responsible for the initial roll-up of the vortex, whereas the leakage jet/passage flow shearing is responsible for the vortex development. The smaller the clearance height is, the easier it is for the moving end wall to alter the vorticity transport from the gap to the tip leakage vortex.

Comprehensive ground coverage analysis of large-scale fixed-tilt bifacial photovoltaic plants

In the design of large-scale photovoltaic plants, decisions on array pitch and tilt play an essential role in life-cycle economics. These design issues have been studied extensively in traditional monofacial systems. However, bifacial systems, which are in increasing demand, are being studied and face challenges in system modeling, particularly in characterizing rear irradiance. Studies considering bifacial panels in plant layouts are recent and must be extensive. Existing studies are based on computationally intensive software and are limited to specific configurations. This study presents the development and validation of an analytical model of a multirow bifacial photovoltaic plant against Bifacial Radiance and DUET ray-tracing software. Comprehensive ground coverage and array tilt analyses are carried out for all ranges of configuration parameters (clearance height, albedo, collector length, bifaciality, or yield loss) and latitudes of up to 60° using this model. The results of extensive simulations were used to train the artificial neural networks. These mathematical architectures allow easy and fast execution of the proposed model with a slight accuracy loss and are provided in the supplementary material. In-depth analysis and developed tools will facilitate decision-making to obtain photovoltaic layouts more efficiently in terms of energy and economy for large-scale bifacial plants.

Biomechanical Insights for Developing Evidence-Based Training Programs: Unveiling the Kinematic Secrets of the Overhead Forehand Smash in Badminton through Novice-Skilled Player Comparison

Badminton, a dynamic racquet sport demanding agility and power, features the overhead forehand smash as a pivotal offensive shot. Utilizing 3D motion analysis, this research delves into the intricate biomechanical facets underpinning this pivotal shot, with a dual focus on both novice and proficient players. Through a comparative analysis of these two player cohorts, the investigation aims to elucidate the fundamental factors influencing the quality of the forehand smash. Our findings reveal that skilled players exhibit significant improvements in smash quality, including a 60.2% increase in shuttlecock speed, reduced clearance height, and flight angle at release. These enhancements are associated with specific determinants, such as consistent positioning, racket angle at impact, and range of motion (ROM) in various joints. More crucially, full-body tension-arc formation and a four-segment whip-like smash contribute to these improvements. Unique to the whip-like smash is the rapid trunk and shoulder rotations in early whip-like control inducing passive elbow flexion and wrist over-extension, enhancing the stretch-shortening cycle (SSC) effect of muscles for a more powerful smash. Emphasizing this uniqueness and the determinants simplify smash learning, potentially boosting training effectiveness. This research contributes to a deeper understanding of badminton’s biomechanics and offers practical implications for coaches and players to enhance their forehand smashes, especially among beginners.

The UNSW tip clearance flow noise test rig

A new test rig to measure the far-field sound, surface pressure fluctuations and flow-field in tip clearance flows has been designed at UNSW. The test rig is a part of the UNSW anechoic wind tunnel (UAT) and has been designed to allow studies of the tip clearance flow and sound radiation across a vast parameter space—tip clearance height, Reynolds number, angle of attack, and the tip geometry. This paper will present and discuss the design, characterization, and capabilities of this new test rig, along with some far-field sound results for a tip clearance flow formed by placing a stationary cambered airfoil adjacent to a wall. The measurements were performed for 51 different clearance heights at free-stream velocity of 30 m/s. The geometric angle of attack in the measurements was 5.6 and the clearance height Reynolds number ranged between 2000 and 102,000. The far-field sound was measured using a 64-microphone spiral phased array and the array output was beamformed to reveal the tip clearance noise sources. It is expected that the test rig will aid in a better understanding of the sound radiated by tip clearance flows which may be used to devise noise control strategies in the future.

Enhanced Bifacial Gain with Optimized Mountings in Photovoltaic Systems

A new design of mounting system targets to reduce the backside shading of ground‐mounted fixed‐tilt bifacial photovoltaic (PV) systems and improve the bifacial gain has been deployed in a large‐scale bifacial PV plant and tested against conventional (nonoptimized) substructures to quantify experimentally, as well as by modeling the improved yield and bifacial gain of bifacial PV plants. This work examines the impact of design changes in purlins, beams, piles, and frames on the bifacial gain. The results show that the optimized mounting systems reduced the backside shading in bifacial PV systems and increased the yearly bifacial gain by at least 0.13% absolute, which is confirmed by simulations and experiments. Depending on the geometrical configuration, low ground coverage ratios, higher albedo, and higher clearance height can lead to a yearly bifacial gain improvement as high as 1% absolute. The quantification of bifacial gain improvement in this study is useful for future studies to determine the profitability of the new design, which comes with additional costs compared to nonoptimized mounting systems.

Overground gait adaptability in older adults with type 2 diabetes in response to virtual targets and physical obstacles.

To step over an unexpected obstacle, individuals adapt gait; they adjust step length in the anterior-posterior direction prior to the obstacle and minimum toe clearance height in the vertical direction during obstacle avoidance. Inability to adapt gait may lead to falls in older adults with diabetes as the results of the effects of diabetes on the sensory-motor control system. Therefore, this study aimed to investigate gait adaptability in older adults with diabetes. Would diabetes impair gait adaptability and increase sagittal foot adjustment errors? Three cohorts of 16 people were recruited: young adults (Group I), healthy older adults (Group II), and older adults with diabetes (Group III). Participants walked in baseline at their comfortable speeds. They then walked and responded to what was presented in gait adaptability tests, which included 40 trials with four random conditions: step shortening, step lengthening, obstacle avoiding, and walking through. Virtual step length targets were 40% of the baseline step length longer or shorter than the mean baseline step length; the actual obstacle was a 5-cm height across the walkway. A Vicon three-dimensional motion capture system and four A.M.T.I force plates were used to quantify spatiotemporal parameters of a gait cycle and sagittal foot adjustment errors (differences between desired and actual responses). Analyses of variance (ANOVA) repeated measured tests were used to investigate group and condition effects on dependent gait parameters at a significance level of 0.05. Statistical analyses of Group I (n = 16), Group II (n = 14) and Group III (n = 13) revealed that gait parameters did not differ between groups in baseline. However, they were significantly different in adaptability tests. Group III significantly increased their stance and double support times in adaptability tests, but these adaptations did not reduce their sagittal foot adjustment errors. They had the greatest step length errors and lowest toe-obstacle clearance, which could cause them to touch the obstacle more. The presented gait adaptability tests may serve as entry tests for falls prevention programs.

A Computation Fluid Dynamics Methodology for the Analysis of the Slipper–Swash Plate Dynamic Interaction in Axial Piston Pumps

This paper proposes a CFD methodology for the simulation of the slipper’s dynamics of a swash-plate axial piston unit under actual operating conditions. The study considers a typical slipper design, including a vented groove at the swash-plate interface. The dynamic fluid–body interaction (DFBI) model is exploited to find the instantaneous position of the slipper, while the morphing approach is adopted to cope with the corresponding mesh distortion. A modular approach is adopted to ensure high-quality mesh on the entire slipper surface and sliding interfaces provide the fluid dynamic connection between neighboring regions. The external forces acting on the slipper are included by means of user-defined lookup tables with the simulation estimating the lift force induced by fluid compression. Moreover, the force produced by the metal-to-metal contact between the slipper and the swash plate is modeled through a specific tool of the software. The pressure signal over an entire revolution of the pump is taken as an input of the simulation and a variable time step is used to manage the high-pressure gradients occurring in the regions of inner and outer dead points of the piston. The weakly compressible characteristic of the fluid is considered by a specific pressure-dependent density approach, and the two-equation eddy-viscosity k-ω SST (shear stress transport) model is used to assess the turbulent behavior of the flow. Furthermore, the transitional model predicts the onset of transition, thus solving different equations depending on whether the flow enters a laminar or turbulent regime. In conclusion, the proposed methodology investigates the motion of the slipper in response to several external forces acting on the component. The numerical results are discussed in terms of variable clearance height, pressure distribution within the gap, and lift forces acting on the slipper under specific pump operations.

Numerical investigation of tip clearance flow in a variable geometry turbine with non-uniform partial clearance

Abstract In variable geometry turbine vanes, tip clearance height and shape vary with the rotation of the vane, which affect the aerodynamic performance significantly. However, these issues are rarely considered in published studies. The current paper investigated the flow field features of transonic variable geometry turbine vanes with non-uniform partial clearance induced by the vane rotating. The results show that: The influence of guide vane rotation on the clearance height and its distribution cannot be ignored. At the same turning angle, the maximum clearance difference is up to 0.79 mm (0.8 % vane height). The height and shape variation of the non-uniform clearance leads to the change in the leakage flow rate, secondary flow structure, and aerodynamic loss of the variable guide vane. Under the combined effect of pressure difference on both sides of the clearance, axial and circumferential non-uniformity of clearance height, the total pressure loss coefficient is up to 9.44 % when the turning angle is −10°. The effect of the pivot on the clearance flow was also analyzed. The pivot increases the pressure in the gap flow field and reduces leakage flow velocity. However, a backflow region appears at the suction side of the pivot, which increases the aerodynamic losses.