Jamming transition in a two-dimensional open granular pile with rolling resistance

We present a molecular dynamics study of the jamming/unjamming transition in two-dimensional granular piles with open boundaries. The grains are modeled by viscoelastic forces, Coulomb friction and resistance to rolling. Two models for the rolling resistance interaction were assessed: one considers a constant rolling friction coefficient, and the other one a strain dependent coefficient. The piles are grown on a finite size substrate and subsequently discharged through an orifice opened at the center of the substrate. Varying the orifice width and taking the final height of the pile after the discharge as the order parameter, one can devise a transition from a jammed regime (when the grain flux is always clogged by an arch) to a catastrophic regime, in which the pile is completely destroyed by an avalanche as large as the system size. A finite size analysis shows that there is a finite orifice width associated with the threshold for the unjamming transition, no matter the model used for the microscopic interactions. As expected, the value of this threshold width increases when rolling resistance is considered, and it depends on the model used for the rolling friction.Comment: 9 pages, 6 figure

Identification of rolling resistance as a shape parameter in sheared granular media

Using contact dynamics simulations, we compare the effect of rolling resistance at the contacts in granular systems composed of disks with the effect of angularity in granular systems composed of regular polygonal particles. In simple shear conditions, we consider four aspects of the mechanical behavior of these systems in the steady state: shear strength, solid fraction, force and fabric anisotropies, and probability distribution of contact forces. Our main finding is that, based on the energy dissipation associated with relative rotation between two particles in contact, the effect of rolling resistance can explicitly be identified with that of the number of sides in a regular polygonal particle. This finding supports the use of rolling resistance as a shape parameter accounting for particle angularity and shows unambiguously that one of the main influencing factors behind the mechanical behavior of granular systems composed of noncircular particles is the partial hindrance of rotations as a result of angular particle shape.Comment: Soumis a Physical Review E; Statistical, Nonlinear, and Soft Matter Physics http://link.aps.org/doi/10.1103/PhysRevE.84.01130

Rolling resistance of electric vehicle tires from track tests

Special low-rolling-resistance tires were made for DOE's ETV-1 electric vehicle. Tests were conducted on these tires and on a set of standard commercial automotive tires to determine the rolling resistance as a function of time during both constant-speed tires and SAE J227a driving cycle tests. The tests were conducted on a test track at ambient temperatures that ranged from 15 to 32 C (59 to 89 F) and with tire pressures of 207 to 276 kPa (30 to 40 psi). At a contained-air temperature of 38 C (100 F) and a pressure of 207 kPa (30 psi) the rolling resistances of the electric vehicle tires and the standard commercial tires, respectively, were 0.0102 and 0.0088 kilogram per kilogram of vehicle weight. At a contained-air temperature of 38 C (100 F) and a pressure of 276 kPa (40 psi) the rolling resistances were 0.009 and 0.0074 kilogram per kilogram of vehicle weight, respectively

A model for investigating the influence of road surface texture and tyre tread pattern on rolling resistance

The reduction of rolling resistance is essential for a more environmentally friendly road transportation sector. Both tyre and road design can be utilised to reduce rolling resistance. In both cases a reliable simulation tool is needed which is able to quantify the influence of design parameters on the rolling resistance of a tyre rolling on a specific road surface. In this work a previously developed tyre/road interaction model is extended to account for different tread patterns and for losses due to small-scale tread deformation. Calculated contact forces and tyre vibrations for tyre/road interaction under steady-state rolling are used to predict rolling losses in the tyre. Rolling resistance is calculated for a series of different tyre/road combinations. Results are compared with rolling resistance measurements. The agreement between simulations and measurements is generally very good. It is found that both the tyre structure and small-scale tread deformations contribute to the rolling losses. The small-scale contribution depends mainly on the road roughness profile. The mean profile depth of the road surface is identified to correlate very well with the rolling resistance. Additional calculations are performed for non-traditional rubberised road surfaces, however, with mixed results. This possibly indicates the existence of additional loss mechanisms for these surfaces

A method for the field assessment of rolling resistance properties of manual wheelchairs, Computer Methods in Biomechanics and Biomedical Engineering

This article presents an examination and validation of a method to measure the field deceleration of a manual wheelchair (MWC) and to calculate the rolling resistances properties of the front and rear wheels. This method was based on the measurements of the MWC deceleration for various load settings from a 3D accelerometer. A mechanical model of MWC deceleration was developed which allowed computing the rolling resistance factors of front and rear wheels on a tested surface. Four deceleration sets were conducted on two paths on the same ground to test the repeatability. Two other deceleration sets were conducted using different load settings to compute the rolling resistance parameters (RPs). The theoretical decelerations of three load settings were computed and compared with the measured decelerations. The results showed good repeatability (variations of measures represented 6–11% of the nominal values) and no statistical difference between the path results. The rolling RPs were computed and their confidence intervals were assessed. For the last three sets, no significant difference was found between the theoretical and measured decelerations. This method can determine the specific rolling resistance properties of the wheels of a MWC, and be employed to establish a catalogue of the rolling resistance properties of wheels on various surfaces.The authors would like to thank the French National Research Agency (ANR) for its financial support to the SACR-FRM project (ANR-06-TecSan-020) and to the CERAH for the loan of all the manual wheelchairs evaluated in this work

Criminalizing Support for Terrorism: A Comparative Perspective

The student project Elba at KTH started in 2011 and is a project where a battery driven prototype car is developed. The vehicle competes in Eco Shell Marathon in Rotterdam, Holland. In this competition the vehicle that travels a certain distance including several starts and stops with the lowest energy consumption wins. To minimize the consumption is therefore very important, which requires knowledge of the vehicles energy losses. A vehicle's energy losses depends on several factors and can be divided into the powertrain efficiency and the driving resistance. The driving resistance can then be divided in to rolling and air resistance, if smaller losses like vibrations are ignored. This report seeks to develop, describe, and evaluate a couple of methods to measure these two parameters on behalf of KTH Transport Labs. The methods chosen are based on a literature study of earlier developed methods and a dialog with the clients. The measuring methods have been adapted to a low budget and simple measuring conditions such as a garage or a tarmac road. The methods are based on either pulling and free rolling, or only free rolling. The traveled distance, speed, acceleration and driving force are logged during the measurements. The measurements have been done at two different locations, in a garage at Stockholmsmässan in Älvjö and on a parking lot by Teknikringen 8 at KTH. To measure the different parameters an encoder, accelerometer and force transducer has been used. To computing the driving resistance a graphical user interface was developed in Matlab. The developed measuring methods compute both the rolling and air resistance coefficients which creates great demands on the environment for the measurement. This could make it hard to find a satisfying test track. The unreasonable results from the coast-down, were Elba were rolling freely, at Teknikringen 8 shows the difficulties when the measurement is unprotected from the wind. With small resistance, as in an energy efficient vehicle like Elba, the measurement is more sensitive to disturbances. This causes the need of preprocessing data before the calculations are done. The final finding is that a combination of the described methods is preferable. A measurement to calculate the rolling resistance coefficient is made by pulling the car. Then a coast-down is made according to any of the two suggested methods. This allows the cast down to be done on a different road surface than the one that the rolling resistance coefficient is calculated for. This means that the air resistance for instance could be measured indoors where there is no wind. The purpose of this project was to develop a method to measure rolling and air resistance for Elba or similar vehicles. Due to technical issues and poor measuring conditions no reliable values of the resistance have been resolved with the developed and chosen methods. If the measurements would be repeated with better equipment and better conditions, a good result would be possible to get.Studentprojektet Elba på KTH startade 2011 och är ett projekt där en batteridriven prototypbil avsedd att tävla i Eco Shell Marathon i Rotterdam, Holland tas fram. I denna tävling vinner det fordon som på utsatt tid kör en viss sträcka inkluderande flera starter och stopp med en så låg energiförbrukning som möjligt. Att minimera energikonsumtionen är alltså mycket viktigt vilket ställer krav på kunskap om fordonets förluster. Ett fordons energiförluster beror av flera faktorer såsom drivlinans verkningsgrad och färdmotståndet. Om mindre förluster så som vibrationer försummas kan färdmotståndet i sin tur delas upp i rull- och luftmotstånd. Denna rapport syftar till att ta fram, beskriva och utvärdera några metoder för att mäta dessa två parametrar på uppdrag av KTH Transport Labs. Metoderna som har valts att studeras är baserade på en litteraturstudie av tidigare framtagna metoder och samtal med beställaren. Valen utgår från en låg budget och enkla testmiljöer så som garage eller asfaltsväg. Metoderna består av att antingen dra och frirulla, eller enbart frirulla fordonet. Tillryggalagd sträcka, hastighet, acceleration och drivande kraft loggas under mätningen. Mätningar har genomförts vid två olika platser, i ett garage vid Stockholmsmässan i Älvsjö och på en parkering vid Teknikringen 8 på KTH. För att mäta de olika storheterna har en pulsgivare, accelerometer och kraftgivare använts. För behandling av data och beräkning av färdmotstånden har ett grafiskt användargränssnitt tagits fram i Matlab. De framtagna mätmetoderna beräknar både rull- och luftmotståndskoefficienten vilket ställer stora krav på mätmiljön. Detta kan göra det svårt att hitta en tillfredställande testbana. De orimliga resultaten från utrullningen, där Elba frirullas, vid Teknikringen 8 visar på svårigheterna då mätningen görs utan skydd från vinden. Med små färdmotstånd, som hos ett energisnålt fordon som Elba, blir beräkningarna känsligare för störningar. Detta medför att viss förbehandling av data krävs innan beräkningarna kan genomföras. Slutsatsen är att en kombination av de beskrivna metoderna är att föredra. Ett dragtest görs separat för att bestämma rullmotståndskoefficienten. Sedan appliceras någon av de två metoderna för att beräkna luftmotståndskoefficienten. Detta gör att utrullningen inte behöver ske på det underlag som rullmotståndskoefficienten söks för. Det innebär att luftmotståndet är möjligt att mätas inomhus där det är vindstilla. Syftet med detta arbete var att ta fram en metod för beräkning av rull- och luftmotstånd hos Elba eller andra liknande fordon. På grund av tekniska problem och bristfälliga mätmiljöer har inga tillförlitliga värden för motstånden tagits fram med de utvecklade och valda metoderna. Om mätningarna skulle återupprepas med bättre utrustning och med bättre förhållanden skulle dock trovärdiga resultat kunna uppnås

Rolling friction of a hard cylinder on a viscous plane

The resistance against rolling of a rigid cylinder on a flat viscous surface is investigated. We found that the rolling-friction coefficient reveals strongly non-linear dependence on the cylinder's velocity. For low velocity the rolling-friction coefficient rises with velocity due to increasing deformation rate of the surface. For larger velocity, however, it decreases with velocity according to decreasing contact area and deformation of the surface.Comment: 7 pages, 3 figure

On line estimation of rolling resistance for intelligent tires

The analysis of a rolling tire is a complex problem of nonlinear elasticity. Although in the technical literature some tire models have been presented, the phenomena involved in the tire rolling are far to be completely understood. In particular, small knowledge comes even from experimental direct observation of the rolling tire, in terms of dynamic contact patch, instantaneous dissipation due to rubber-road friction and hysteretic behavior of the tire structure, and instantaneous grip. This paper illustrates in details a new powerful technology that the research group has developed in the context of the project OPTYRE. A new wireless optical system based on Fiber Bragg Grating strain sensors permits a direct observation of the inner tire stress when rolling in real conditions on the road. From this information, following a new suitably developed tire model, it is possible to identify the instant area of the contact patch, the grip conditions as well the instant dissipation, which is the object of the present work