Guidelines for Traversability of Roadside Slopes (2019) / Chapter Skim
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44 5.1 Introduction This chapter provides details of the vehicle encroachment simulations performed under this project. These simulations were performed to determine the influence of various roadside and encroachment parameters on the kinematics of the vehicles as they traverse the sloped terrains.
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Simulation Analysis 45 normal operation (except for aerodynamic forces)
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46 Guidelines for Traversability of Roadside Slopes element analysis codes are extremely popular in crash simulations and other types of analyses requiring determination of loads and deformations. In this research, however, such levels of detail for the vehicle or the terrain were not needed.
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Simulation Analysis 47 On the other hand, CarSim, which is a vehicle dynamics code, has about 30 pre-built vehicle models in different vehicle classes. These pre-built vehicle models can be modified with considerably less effort to represent different vehicle makes and models.
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48 Guidelines for Traversability of Roadside Slopes for some of the more severe foreslope configurations with steeper foreslopes and flat bottom, such a contact may occur. If a contact between the vehicle body and the terrain occurs during an encroachment event, it can significantly change the dynamics of the vehicle.
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Simulation Analysis 49 Where, K_total = K_terrain • TP––– • [1 + Mu • Vn] , K_terrain = stiffness coefficient of the terrain, Mu = damping coefficient of the terrain for normal penetration of point P, Vn = velocity of point P normal to the terrain surface, n = unit vector normal to the terrain surface penetrated by point P, U = friction coefficient of the body-to-terrain interface, and T = tangential unit vector in the direction of the sliding point P on the terrain surface.
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50 Guidelines for Traversability of Roadside Slopes After selecting the vehicle makes and models, the research team proceeded with developing vehicle dynamics models of these vehicles. A problem faced by the research team was the unavailability of these vehicles for taking the non-destructive measurements for developing the vehicle dynamics model.
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Simulation Analysis 51 of the Kia Rio model. These values were taken from the 4N6XPRT vehicle database described in Chapter 4.
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52 Guidelines for Traversability of Roadside Slopes Prior to discussing the details of the wrapper program, it will be helpful to have an overview of how various inputs are organized for running a CarSim analysis. There are five main components of a CarSim input (for each simulation case)
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Figure 5.6. TTI's CarSim wrapper program main flowchart.
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54 Guidelines for Traversability of Roadside Slopes the program can be run to perform CarSim analyses with user provided input files, skipping input generation altogether. This flexibility was built into the program to allow running various selective small-scale studies to evaluate the effects of the different parameters, such as terrain friction, driver perceptionreaction time (PRT)
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Figure 5.7. TTI's CarSim wrapper program inputs generation module flowchart.
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56 Guidelines for Traversability of Roadside Slopes The road files are generated by a separate subroutine in TTI's wrapper program. The flowchart of this subroutine is shown in Figure 5.8a.
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Simulation Analysis 57 The subroutine then calculates the total number of event files needed based on the number of parameters read. It then generates these event files in a format needed by the CarSim solver.
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58 Guidelines for Traversability of Roadside Slopes Figure 5.9. Flowchart of TTI's code for applying body-to-terrain contact with CarSim.
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Simulation Analysis 59 simulation output for each case, there was a need to generate an overall simulation output table with key outcomes recorded for each simulation case and the associated terrain and driver input parameters. The output module of the wrapper program was coded to generate this aggregate output table that facilitates the use of bulk simulation results in further statistical analysis.
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60 Guidelines for Traversability of Roadside Slopes Label Description Time Simulation time (s) XCG_SM X-coordinate of the vehicle's sprung mass CG in global coordinates (m)
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Simulation Analysis 61 coefficient between the tire and the tire testing machine's surface. During a simulation, the absolute lateral friction force from the test plots is adjusted based on the local terrain friction coefficient before being applied to the vehicle.
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62 Guidelines for Traversability of Roadside Slopes The friction ellipse method was coded into the wrapper program, which interacts with CarSim during run time to determine if the vehicle is traversing a terrain that is marked as soil, and if so, calculates and applies lateral forces to the tire using the friction ellipse method. The effective lateral friction coefficient, µsoil, is determined using the formulation shown in Figure 5.11.
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Simulation Analysis 63 speeds of 45, 55, and 65. Encroachment angles of 10, 20, and 30 degrees were used.
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64 Guidelines for Traversability of Roadside Slopes Figure 5.13. Results of the sensitivity analyses for determining PRT.
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Simulation Analysis 65 5.7 Evaluation of Friction Model and Lateral Coefficient As mentioned previously, the researchers used the friction ellipse model to apply soil furrowing forces to the vehicle's tires. A key parameter for this model is the lateral friction coefficient, which controls the lateral force applied to the vehicle as it side-slips on the soil terrain.
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66 Guidelines for Traversability of Roadside Slopes For each of the cases in Figure 5.15, all 24 simulation runs were performed to compare the results. In examining the results of the simulations, the researchers evaluated the variations in lateral tire forces resulting from the tire-to-terrain contact when different friction models or coefficients were used.
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Simulation Analysis 67 When the speed is increased, or if the driver input is more aggressive, the differences between the CarSim friction model and friction ellipse model become more prominent. An example is shown in Figure 5.17 for a vehicle starting with non-tracking conditions, initial yaw rate of 15 degrees/s, initial speed of 35 mph, constant steer angle, and full ABS brakes applied throughout the simulation.
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68 Guidelines for Traversability of Roadside Slopes Figure 5.17. Lateral tire forces for small car and pickup truck with 35 mph initial speed, non-tracking initial conditions, constant steer angle, and full ABS brakes.
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Simulation Analysis 69 Results of the simulations demonstrate that the maximum lateral friction coefficient value can be used to adjust the maximum lateral tire force during side-slipping, which implies that it can successfully act as a surrogate for soil furrowing forces. 5.7.2 Selection of Appropriate Lateral Friction Coefficient Using the crash data, it is difficult to determine the probability of rollover on a flat surface when a vehicle leaves the roadway.
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70 Guidelines for Traversability of Roadside Slopes Results of the simulation indicate that a small increase in the maximum lateral friction coefficient beyond 2.0 results in significant increase in the number of rollovers, which is unrealistic when compared to the estimates of rollovers on flat terrains. With the maximum lateral friction coefficient of 2.0, the percentage of rollovers is more acceptable.
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Simulation Analysis 71 slower speeds and angles of 25 mph and 5 degrees to higher speeds and angles of 75 mph and 30 degrees, respectively. The simulation matrix included both tracking and non-tracking vehicle encroachments.
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72 Guidelines for Traversability of Roadside Slopes • Marginal Rolls. The vehicle does not roll over, but has a roll angle of greater than 55 degrees.
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Simulation Analysis 73 To highlight the key trends in rollovers, the others category was not plotted. Similarly, spinouts were not plotted in the first plot.
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74 Guidelines for Traversability of Roadside Slopes Influence of Shoulder Width/Type on Simulation Outcomes (%)
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Simulation Analysis 75 Influence of Slope Width on Simulation Outcomes (%) 8-ft 16-ft 32-ft Infinite Returns 35.10 31.27 29.15 24.93 Overturns 12.36 17.96 17.34 9.86 Marginal Rolls 0.30 0.45 0.54 0.35 Spinouts 14.26 14.12 16.69 19.56 Others 37.98 36.19 36.28 45.31 Total Simulations 10800 10800 10800 10800 Figure 5.24.
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76 Guidelines for Traversability of Roadside Slopes Influence of Encroachment Speed on Simulation Outcomes (%) 25 mi/h 35 mi/h 45 mi/h 55 mi/h 65 mi/h 75 mi/h Returns 41.42 40.89 33.93 26.65 20.33 17.44 Overturns 0.88 3.65 10.94 21.69 24.74 24.39 Marginal Rolls 0.01 0.11 0.61 0.43 0.53 0.76 Spinouts 2.54 8.81 23.61 22.19 20.79 19.00 Others 55.15 46.54 30.90 29.03 33.61 38.40 Total Simulations 7200 7200 7200 7200 7200 7200 Figure 5.25.
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Simulation Analysis 77 Influence of Encroachment Angle on Simulation Outcomes (%)
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78 Guidelines for Traversability of Roadside Slopes In conclusion, the results of the simulations showed trends that are intuitive with some interesting insights that have been described. For developing the slope traversability guidelines, these results were weighted according to their probabilities of occurrence in the real world.
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