Guidelines for Cost-Effective Safety Treatments of Roadside Ditches (2021) / Chapter Skim
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95 CHAPTER 6. SIMULATION ANALYSIS METHODS INTRODUCTION To evaluate vehicle dynamics based on a combination of various ditch design parameters, vehicle encroachment conditions, vehicle types, and driver inputs, an extensive simulation effort was needed.
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96 quality, and vehicle trajectory during roadside encroachments. However, these codes do not have the option of modeling the contact between a vehicle's body and the terrain.
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97 VEHICLE BODY-TO-TERRAIN CONTACT In modeling a vehicle traversing roadside ditches, the vehicle's body-to-terrain contact is important and can significantly affect the kinematics of the vehicle due to terrain forces applied to the vehicle. TTI researchers have developed a vehicle body-to-terrain contact algorithm for CarSim separate from this research.
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98 which was based on a soil subgrade modulus of 40 lb/inch3 and a contact area of 100 inch2. The damping coefficient of the terrain for normal penetration (Mu)
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99 3. Runs CarSim in loop to perform analysis for all simulation cases.
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100 appropriate contact force needed to remove the penetration. The wrapper program submits the contact forces to the CarSim solver to be applied to the vehicle body points being tracked using the CarSim solver exchange variables.
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101 Figure 6.2. TTI's CarSim wrapper program main flowchart.
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102 Figure 6.3. Flowchart of the input generation module of TTI's wrapper program.
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103 Inputs Generation Module If the user provides a table listing all of the simulation input files (Simfiles) for the CarSim solver, the main program skips the inputs generation module.
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104 Figure 6.4. Flowcharts of the subroutines for generating CarSim (a)
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105 penetration is detected, no contact force is applied. However, if a vehicle body point penetrates the terrain, the program determines the contact forces needed to remove the penetration.
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106 simulation from running longer than needed and saves time when a large number of simulations need to be performed. A simulation is stopped if any of the following conditions is met: 1.
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107 Table 6.1. Simulation outcomes logged in the aggregate simulation results table.
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108 Table 6.2. Simulation output data saved for each simulation case.
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109 Note: Lateral force is plotted as function of slip angle for different vertical tire loads. Figure 6.6.
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110 tests. However, both terrains differed significantly from each other, and no general method for determining the length and friction coefficient of adjacent surface patches existed.
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111 SENSITIVITY STUDIES Prior to performing simulations of the entire simulation matrix, it was important to evaluate the sensitivity of some of the parameters. These parameters included the maximum lateral friction coefficient to incorporate forces due to soil-furrowing (µsoil)
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112 If the vehicle rolled or pitched more than 65 degrees, it was categorized as an overturn. If it had higher than a 55-degree roll or pitch, but did not overturn, it was categorized as marginal.
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113 Simulations were performed with the "panic steer, no brake" and "panic steer and brake" driver inputs only. These inputs are the only driver inputs that require inclusion of PRT.
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114 Figure 6.10. Results of the sensitivity analyses for determining perception-reaction time.
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115 sensitivity of the lateral friction coefficient value to the change in vehicle dynamics code (from older the HVOSM to newer and actively maintained CarSim) and the use of more recent vehicle models and modeling techniques.
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116 2. Tracking, panic return-to-road steer, and ABS brakes after P/R time of 1 s.
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117 Effectiveness of the Friction Ellipse Model The researchers compared the lateral tire forces applied to the vehicle during the simulation as a result of the tire's interaction with the terrain. Lateral tire forces for simulations performed with CarSim's default friction model were compared to the simulations performed with the friction ellipse model.
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118 Figure 6.13. Lateral tire forces for small car and pickup truck with 25-mph initial speed, tracking initial conditions, and panic return-to-road steer after 1 second P/R time.
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119 shown in Figure 6.15. Lateral tire forces are shown for the small car (left)
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120 Simulations performed with the maximum lateral friction coefficient of 1.2 did not result in any rollover on a flat terrain. Thus, there was a need to determine an appropriate value of the lateral friction coefficient that results in close to 10% rollovers on a flat terrain.
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121 VEHICLE MODELING AND VALIDATION The researchers developed a model for a 5,000-lb (2270 kg) pickup truck using mostly predefined vehicle properties and datasets available in CarSim.
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122 Ditch Traversal Tests Although the speed bump tests were designed to help researchers calibrate suspension properties of the vehicle model, the ditch traversal tests were considered more pertinent for this research because they allowed validation of the overall kinematic performance of the model. A 30-ft wide symmetric V-ditch with 5.5H:1V slopes was used in the testing, as shown in Figure 6.18.
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123 Table 6.3. Vehicle's initial speed during speed bump tests.
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124 braking system, aerodynamic loads, tire properties, suspension compliance coefficients, suspension auxiliary roll moments, and suspension bump stop properties. The overall geometric and mass properties for the vehicle model were obtained by measuring them from the test vehicle used in the speed bump and ditch traversal testing.
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125 Figure 6.19. Geometric and mass properties of the test vehicle.
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126 Note: The table shows coordinates of these points with respect the model's origin. Figure 6.20.
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127 Model Validation Once the model of the vehicle and the terrain were completed, the researchers performed CarSim simulations matching the test conditions and compared simulation results to test results. The details of this model validation are presented next.
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128 dissipated through the suspension spring and damper components in the simulation, which can result in slightly greater compression. Nevertheless, the differences in the test and simulation are not that significant.
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129 Figure 6.24. Speed bump traversal comparison for single speed bump with vehicle speed of 11.5 mph.
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130 Figure 6.26. Speed bump traversal comparison for single speed bump with vehicle speed of 30.8 mph.
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131 Figure 6.28. Speed bump traversal comparison for two speed bumps with vehicle speed of 21.1 mph.
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132 Ditch Traversal Although the speed bump tests were used to validate the response of individual suspension components, a more relevant validation of the overall ability of the model to accurately predict vehicle kinematics while traversing ditches (as related to use in this project) , relied on the ability to match the ditch traversal test data.
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133 Figure 6.31. Vehicle path comparison at 41.6 mph.
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134 Figure 6.33. Vehicle roll, pitch, and yaw comparison at 41.6 mph.
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135 the use of CarSim models. Among such validations are the validation of driver input, side slipping, rollover, and the like.
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136 correlation was obtained between the test and simulation results, thus validating the vehicle model for further use in the project. It was noted that not all models used in this research can be validated in a similar manner due to budgetary constraints.
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