Friction measurement at low sliding speed using a servohydraulic tension-torsion machine

A method is presented for the determination of the coefficient of friction at low sliding speeds down to 3 nm/s. It is also possible to vary the contact pressure. The maximum average contact pressure is 40 MPa in the present test configuration, where a servohydraulic tension-torsion machine is used. The method, involving test equipment, measurement and the principles of evaluation, is exemplified by a study where the variation of the coefficient of friction with sliding velocity contact pressure and surface roughness is investigated for the material combination steel and concrete. The measurements have been performed as a 2³ factorial design. An error analysis was performed and the relative measurement uncertainty when determining the coefficient of friction was estinated to be 1.2 percent.     

超导磁力与静压液膜力复合轴承的静动特性分析

以新一代液体火箭发动机涡轮泵为应用前景,提出了一种带小孔节流的超导-液体静压复合推力轴承.该复合轴承由6块圆形超导瓦和6块带有小孔节流的圆形液体静压推力瓦构成,依靠涡轮泵系统自带的低温介质(如液氢液氧等),可以实现超导磁斥力与流体静压力的复合.基于解耦方法分析了复合轴承的静动特性,重点研究不同液膜厚度下复合轴承的承载力和刚度随节流孔径、液腔直径等的变化规律.在设计工作点附近,超导推力瓦和静压推力瓦的承载力大体相当,而后者的刚度则是前者的300倍以上.理论结果表明该复合结构既可以保证启动阶段无接触摩擦,又能在工作阶段保持较高刚度以抗冲击,对设计高可靠性火箭涡轮泵的轴系结构具有参考价值.     

煤仓内煤散料流动状态与力学行为影响因素

针对煤仓内煤散料流动问题及其力学行为,采用三维颗粒流模拟程序PFC3D建立了某型号煤仓与某种煤散料的离散元模型,简述了其力学模型与求解步骤,模拟分析了煤仓内煤散料卸料流动状态。通过分析水平向侧压力、颗粒速度场和接触力场,重点讨论了煤仓下部锥体内壁面摩擦系数、锥仓倾角和卸料口径等对煤散料颗粒流动状态和力学行为的影响。结果显示,深仓卸料流动为整体流动与中心流动混合状态,煤仓内壁摩擦系数、锥体倾角和卸料孔开口半径均对煤散料流动和水平侧压力有较大影响。     

The reduction of skin friction by riblets under the influence of an adverse pressure gradient

In this paper we address the effectiveness of riblets on skin friction reduction under the influence of an adverse pressure gradient. The measurements were taken in a wind tunnel. Skin friction was observed with a drag balance which has a reproducibility of better than 1%. The accuracy of the balance is estimated to be less than 1% for the case of zero-pressure gradient and at most 3% for a pressure gradient. The data on skin friction reduction at zero pressure gradient were consistent with previous results and amount to 5% at dimensionless riblet width of s ⁺ = 13. We find that at all adverse pressure gradients the skin friction reduction by riblets persists. At moderate pressure gradients the reduction increases somewhat to 7%. The velocity profile which is also measured, exhibits the characteristic shape for a boundary layer with an adverse pressure gradient and agrees well with theory. From the velocity profiles measured at two stations we estimated with the help of a momentum balance the skin friction and skin friction reduction. The results differ from the drag-balance data. Due to the poor accuracy of the momentum balance method which we estimate in our case, we conclude that the results obtained with this method are less reliable than those obtained with the drag balance. This throws some doubt on previous results on drag reduction under the influence of a pressure gradient which were based on the momentum balance method.     

An experimental concept to measure opacities under solar-relevant conditions

Recent solar abundance models (Asplund 2009) use a significantly lower abundance for C, N, O compared to models used roughly a decade ago. Although the models used now are much more sophisticated than before, a discrepancy still exists between the abundances in the models and the abundances determined by helioseismic measurements. Agreement can be obtained by ad hoc adjustments to the opacity of high-Z (Z > 2) elements ranging from a few percent in the solar interior to as much as 30 just below the convection zone (CZ). Although many of the opacity models are thought to agree within a few percent, a recent element-by-element study (Blancard 2012) indicates a larger disagreement between models for certain elements. Experimental opacity measurements for these elements in the regimes of interest will provide valuable information to help resolve these discrepancies. We will present an experimental platform designed to measure the opacity of C, N, and O and discuss the achievable parameter regime. We will also briefly discuss how this platform can be extended to include other high-Z elements.     

Lubrication of a rotating circular thrust bearing with a casson lubricant

Lubricants today are subjected to increasing mechanical shearing forces. This has resulted in an increasing interest in materials having variable viscosity. The problem of rotating circular thrust bearing is investigated with Casson fluid as a shear thinning lubricant. The pressure and load capacity of the thrust bearing is calculated when the feeding is done from the centre of the plates to the periphery. Also, moment of friction acting on the plates is calculated for different values of Casson number and for various values of ratio of inside to outside radius. It has been found that there is a decrease in the value of load capacity of the Casson lubricant. Also, it has been observed that there is an appreciable increase in the values of moment of friction for Casson fluid.     

Wave behavior in horizontal annular air–water flow

In this study, non-intrusive pressure drop, liquid base film thickness distribution, and wave behavior measurements have been obtained for 206 horizontal annular two-phase (air–water) flow conditions in 8.8, 15.1, and 26.3 mm ID tubes. Reliable wave velocity measurements are available for 185 of these flow conditions, while 131 flow conditions allow for reliable wave frequency measurements. The wave velocity is found to be predicted to within 9% by gas friction velocity and 6% by an optimized correlation of similar structure. Wave frequency can also be predicted with a simple correlation to within 5% for the 8.8 and 15.1 mm tubes, but a separate relation is required to explain 26.3 mm frequency data. The differences in wave behavior between the annular and wavy-annular/wavy regimes are also discussed.     

Full-field and point strain measurement via the inner surface of a rolling large lug tyre

Vehicle safety and performance can be dramatically improved if force or friction measurement of the tyre-terrain interface is known. Since the tyre-terrain interface is responsible for the majority of forces acting on the vehicle, this region has received a lot of attention in vehicle dynamics. Direct measurement of the tyre-terrain interface is difficult since it is hidden by the tyre and terrain. A lot of research has been conducted on the inside of tyre using accelerometers or strain gauges with research more focussed on passenger car tyres and very little work performed on agricultural tyres with larger lugs. This study performs strain measurements using point measurement, from strain gauges, and full field measurement, using a stereo camera measurement system, of the inside of an agricultural tyre on a drum test rig during vertical and lateral loading. Results show similar trends when compared to results on passenger car tyres, however the mounting of the strain gauge relative to the lug is shown to play a large role in the developed strain. Linear relationships between the applied tyre force and strain were obtained in different direction with R² values above 0.97.     

Dynamics of rotating conveying mud drill string subjected to torque and longitudinal thrust

In the view of fluid-structure interactions and rotor dynamics, this paper models the lateral vibration of a vertical downward rotating elastic drill string conveying mud subjected to supporting stabilizers, bit torque and longitudinal thrust. The dynamic model involves the rotational inertia of the drill string tube cross section, the gyroscopic effect caused by rotation, the damping due to friction with the surrounding fluid, the gravity force and mud buoyancy. Damped natural frequency, stability and resonance of the drill string system are determined by quadratic eigenvalue problem and investigated at influences of the stabilizer, rotational angular speed, mud flowing velocity, bit torque and thrust. As a result, the drill string can lose stability both at simultaneous and separate influences of the mud conveying, bit torque and thrust, whereas the rotation, stabilizer and gravity of the drill string can improve system stability; the rotational angular speed causing system resonance decreases with the increase of the mud flowing velocity, bit torque and thrust.     

Evaluation of thrust measurement techniques for dielectric barrier discharge actuators

Despite its popularity in the recent literature, plasma actuators lack a consistent study to identify limitations, and remedy thereof, of various thrust measurement techniques. This paper focuses on comparing two different experimental techniques commonly used to measure the global, plasma-induced thrust. A force balance is used to make a direct measurement of the thrust produced, which is then compared with a control volume analysis on data obtained through particle image velocimetry. The local velocity measured by particle image velocimetry is also validated with a fine-tip pressure probe. For the direct thrust measurements, the effect of varying the actuator plate length upon which the induced flow acts is investigated. The results from these tests show that the length of the actuator plate is most influential at higher voltages with the measured thrust increasing as much as 20?% for a six times reduction in the length of the plate. For the indirect thrust measurement, the influence of the control volume size is analyzed. When the two methods are compared against each other, good agreement is found when the control volume size has a sufficient downstream extent. Also, the discharge length is optically measured using visible light emission. A linear correlation is found between the discharge length and the thrust measurements for the actuator configurations studied. Finally, the energy conversion efficiency curve for a representative actuator is also presented.     

FINITE ELEMENT SURFACE MODEL FOR FLOW AROUND VERTICAL WALL ABUTMENTS

A two-dimensional finite element surface model is developed to determine velocities, depths, and turning angles around vertical wall abutments. The model solves the Reynolds-averaged turbulent flow equations along a horizontal plane passing through the average water surface. This approach is an improvement over the depth-averaged flow models where dispersion terms reflecting vertical effects are neglected. In the model, vertical gradient effects are accounted for through the use of power law for the vertical distribution of the longitudinal velocity; a similar treatment is applied to lateral turbulent shear stresses. The model is capable of computing the dynamic pressure distribution, which in turn is converted to water elevation values. The model, being two dimensional, is computationally efficient and practical to use. The numerical model was successfully verified using experimental data from vertical wall abutments and groins with protrusion ratios (ratio of protrusion length perpendicular to direction of flow to total channel width) of 0·1, 0·2 and 0·3. The results show the occurrence of a high intensity velocity zone close to the upstream abutment nose similar to those observed experimentally. The effects of roughness, depth, and energy slope on the intensity of flow field is investigated and an analytical expression is developed. Numerical experiments indicate that grain roughness affects flow field around the abutment nose by controlling the magnitude of the lateral velocity component and by controlling the lateral extent of the affected zone. Velocity amplification at the abutment nose is found to be mainly related to the protrusion ratio and to the friction factor, and can be up to 1·75 times the approach velocities for protrusion ratios of 0·3. For a protrusion ratio of 0·3, for a typical range of roughness values the increase in nose velocities due to friction factor alone was found to be up to 20 percent.     

Motorcycle brake squeal: experimental and numerical investigation on a case study

Since, nowadays, the NVH performance of a vehicle is one of the most important priorities for the market, the noise radiating from disc brakes is considered a source of considerable discomfort and customer dissatisfaction. Squeal is an example of noise, caused by vibrations induced by friction forces, in which the vibration modes of the brake disc are coupled to those of the friction pads or of the caliper. In this work a case study, in which the squeal phenomenon was detected after changing the supplier of the disc pads, is presented. A test bench was purposely developed to investigate the squeal phenomenon; tests at different rotating speeds and pressures in the brake circuit were carried out employing different friction pads. The experimental apparatus appeared capable of reproducing the phenomenon observed in real practice and to investigate the effect of operating parameters and different components on the onset of instability. Friction tests and geometric analysis of the friction pads were also carried out to complete the investigation. At the same time, a finite element (FE) complex eigenvalue parametric analysis was performed on the brake assembly. The different propensity of the pads to squeal was attributed to differences in their geometry and in their friction coefficients. The FE analysis, confirmed the experimental observations and indicated possible design improvements to increase the stability of the system.     

Critical heat flux of liquid helium (I) in forced convective boiling

For liquid helium (I) flowing through a uniformly heated vertical tube of 1 mm i.d., critical heat flux (CHF) of forced convective boiling has been measured under the conditions of a pressure of 0.199 MPa with vapor-to-liquid density ratio, of 0.409, length-to-diameter ratios from 25 to 200, mass velocities from 11 to 108 kg m^?2 s^?1, and inlet subcooling enthalpy from ?3.5 to +7.0 kJ kg^?1. The experimental results thus obtained are analyzed revealing that the high pressure character, which has already been observed for water and Freons at extremely high pressures, is certainly found at high mass velocities for liquid helium too, though its appearance is restricted within very narrow limits due to the phenomenon that the wall temperature situation identifying the CHF condition fades away when the mass velocity becomes high.     

涡轮膨胀机止推轴承－转子系统的振动分析

为了确保涡轮膨胀机止推轴承在受到瞬时冲击时不致发生损坏事故，研究了止推轴承－转子系统的轴向瞬态过程，计算了止推轴承的线性油膜刚度系数和阻尼系数，并对线性和非线性两种情况下的轴向瞬态响应作了分析与比较，为止推轴承；转子系统的可靠性评估提供了科学依据。     

Der Einfluß der Prandtlzahl auf den Wärmeübergang und Druckverlust künstlich aufgerauhter Strömungskanäle

The influence of the Prandtl number on heat transfer and pressure drop characteristics of artificially roughened test sections has been investigated experimentally in the Prandtl number range from 3 to 180. For integral roughenesses and fully roughened test sections the efficiency η=ε _Nu /ε _ζ can be described by the Prandtl number and the roughness parameter \(k_{\text{S}}^ + = Re{\text{(}}k_{\text{S}} /d_{\text{h}} )\sqrt \zeta /8\) . The relation between the efficiency η, the Prandtl numberPr and the roughness parameterk _s ⁺ can be expressed by the following empirical relation: $$\eta = \log \frac{{Pr^{{\text{0,33}}} }}{{k_{\text{S}}^{ + {\text{ 0,243}}} }} - 0,32 \cdot 10^{ - 3} k_{\text{S}}^ + {\text{ log }}Pr + {\text{1,25}}{\text{.}}$$ With this relation for the heat transfer and friction characteristics of smooth and rough channels it is possible to calculate the increase of heat transfer for rough channels by means of pressure drop measurements which are necessary to determine the friction factor ζ and the equivalent sand roughness depth; provided that heat transfer and friction characteristics of the respective smooth channel are known.     

Measurement of Thrust and Lift Forces Associated With Drag of Compliant Flapping Wing for Micro Air Vehicles Using a New Test Stand Design

Compliant wing designs have the potential of improving flapping wing Micro-Air Vehicles (MAVs). Designing compliant wings requires a detailed understanding of the effect of compliance on the generation of thrust and lift forces. The low force and high-frequency measurements associated with these forces necessitated a new versatile test stand design that uses a 250 g load cell along with a rigid linear air bearing to minimize friction and the dynamic behavior of the test stand while isolating only the stationary thrust or lift force associated with drag generated by the wing. Moreover, this stand is relatively inexpensive and hence can be easily utilized by wing designers to optimize the wing compliance and shape. The frequency response of the wing is accurately resolved, along with wing compliance on the thrust and lift profiles. The effects of the thrust and lift force generated as a function of flapping frequency were also determined. A semi-empirical aerodynamic model of the thrust and lift generated by the flapping wing MAV on the new test stand was developed and used to evaluate the measurements. This model accounted for the drag force and the effects of the wing compliance. There was good correlation between the model predictions and experimental measurements. Also, the increase in average thrust due to increased wing compliance was experimentally quantified for the first time using the new test stand. Thus, our measurements for the first time reveal the detrimental influence of excessive compliance on drag forces during high frequency operation. In addition, we were also able to observe the useful effect of compliance on the generation of extra thrust at the beginning and end of upstrokes and downstrokes of the flapping motion.     

碟形弹簧支承圆形瓦推力轴承热动力润滑性能分析

对碟形弹簧支承圆形瓦推力轴承在稳态运行时的热动力润滑性能进行了分析研究,考察了弹簧弯曲刚度,载荷和转速对轴承的油膜厚度为分布,压力分布,温度分布,功耗及油膜压力中心位置的影响,并提出了温度因子概念,研究表明,较小的弹簧弯曲刚度和载荷有利提高轴承的热动力润滑性能,油膜温度随温度因子的增大而增大,当轴承转速在较大的范围内变化时,温度因子基本为常母,油膜温度敢基本不变,油膜压力中心位于瓦几何中心的上游区     

碟形弹簧支承圆形瓦推力轴承 热动力润滑性能分析

The steady-statethermohydrodynamic lubrication p erformance of thrust bearing with plate-spring supported circular pads was stud ied.Ananalysis of influence of spring flexure rigidity,load, and angular speed on the distribution of film thickness, film pressure, filmtemperature,power l oss, and pressure center location was carried out.It is shown that low flexure r igidity of spring and load ishelpful in enhancing the lubrication performance o f thrust bearing. A concept of temperature factor is presented as well. Thetemp erature factor keeps almost unchanged in a wide range of angular speed, the cor responding change in the filmtemperature is negligible. Moreover,the pressure c enter of oil film is located at the upriver range of the circular pad.     

An improved constant volume cycle model for performance analysis and shape design of PDRE nozzle

An improved constant volume cycle(CVC) model is developed to analyze the nozzle effects on the thrust and specific impulse of pulse detonation rocket engine(PDRE).Theoretically, this model shows that the thrust coefficient/specific impulse of PDRE is a function of the nozzle contraction/expansion ratio and the operating frequency. The relationship between the nozzle contraction ratio and the operation frequency is obtained by introducing the duty ratio, by which the key problem in the theoretical design can be solved. Therefore, the performance of PDRE can be accessed to guide the preliminary shape design of nozzle conveniently and quickly. The higher the operating frequency of PDRE is, the smaller the nozzle contraction ratio should be. Besides, the lower the ambient pressure is, the larger the expansion ratio of the nozzle should be. When the ambient pressure is 1.013 × 105 Pa, the optimal expansion ratio will be less than 2.26.When the ambient pressure is reduced to vacuum, the extremum of the optimal thrust coefficient is 2.236 9, and the extremum of the specific impulse is 321.01 s. The results of the improved model are verified by numerical simulation.     

Another Proof of Stability for Global Weak Solutions of 2D Degenerated Shallow Water Models

We consider non-linear viscous shallow water models with varying topography, extra friction terms and capillary effects, in a two-dimensional framework. Water-depth dependent laminar and turbulent friction coefficients issued from an asymptotic analysis of the three-dimensional free-surface Navier–Stokes equations are considered here. A new proof of stability for global weak solutions is given in periodic domain Ω = T², adapting the method introduced by J. Simon in [15] for the non-homogeneous Navier–Stokes equations. Existence results for such solutions can be obtained from this stability analysis.