Despite the integral role that off-road vehicles play in combat and military operations, researchers do not currently have the ability to accurately estimate vehicle operational parameters such as forces, torques, and sinkage that a wheel or track on a vehicle experiences; investigate the interaction that occurs between a wheel/track and soft soil; and simulate a ground vehicle’s ability to navigate complex off-road terrain.

Researchers at the University of Wisconsin recently developed a modeling, simulation, and visualization framework, called Chrono, aimed at enabling high performance, physics-based, analysis of ground vehicle mobility.

Military maneuvers, involving tactical formations and movements of wheeled and tracked vehicles across a landscape, provide the edge in combat for military units. However, scientists and engineers, who most often design the vehicle-weapons systems capabilities used in combat operations, are generally not included in the tactical planning process and must design vehicles based on expected mobility challenges. It is difficult and expensive to evaluate a vehicle’s performance during a majority of military maneuvers using experiments. The Chrono framework provides an end-to-end mobility toolkit to simulate a variety of mobility models to investigate the performance of a vehicle during common military maneuvers, such as urban, muddy terrain, gravel slopes, and river fording operations.

A brick wall was modeled in Chrono to represent a common obstacle that military vehicles face in an urban environment. The model is composed of a rigid terrain with a wall composed of 150 bricks.

Six simulations were performed with the cohesion of the brick wall ranging from 5 to 30%. The tracked vehicle starts at zero velocity and ramps up to a steady state velocity of approximately 3.5 m/s. The tracked vehicle collides with the brick wall at approximately 3 s, causing the vehicle to lose velocity. For low levels of cohesion, the tracked vehicle is able to easily surmount the obstacle. As the cohesion increases, however, it is more challenging for the vehicle to destroy the wall. For example, the vehicle is completely immobilized when the cohesion of the brick wall is set to 30%.

The muddy terrain simulation consisted of a 3000-kg (6600-lb) HMMWV and four rigid 60-kg (130-lb) wheels with treads. Each wheel was rigidly attached to a fixed axle that was driven at a constant angular velocity of 1 rad/s. A bed of 200,000 objects of varying shape were created with a density of 1250 kg/m^3.

Using this model, the sinkage of each wheel along with the reaction forces at each wheel spindle can be measured. Further improvements will incorporate a suspension into the vehicle so that the forces between the wheel and the chassis can be computed. Additionally, a torque, rather than a constant angular velocity will be applied to the wheels so that the speed of the vehicle through the mud can be determined.

A set of simulations was performed to understand the vehicles’ ability to traverse muddy terrain at decreasing values of inter-particle cohesion. The wheels of the vehicle were rotated at 1 rad/s and the sinkage and forward velocity of the chassis were measured. As expected, for lower cohesion the height of the chassis is lower as the entire vehicle sinks into the ground.

Three simulations were performed in Chrono with the slope of the hill ranging from 14° to 20° and terrain composed of over 8000 unique rocks with a randomly generated polyhedral collision geometry. The wheeled vehicle starts at zero velocity and increases velocity on a flat rigid terrain due to a constant torque applied to the rear wheels. The wheeled vehicle hits the slope at approximately 2 s, causing the vehicle to lose velocity. For steep slopes, the wheeled vehicle cannot surmount the hill and gets stuck. As the slope decreases, the vehicle is able to make it farther and faster up the slope.

Although most vehicles can handle rivers, it is desired to design faster and more efficient vehicles. Speed is elusive because of the severe drag and wave making properties implicit in box-shaped bodies. More and more power does not assure more speed, except in small increments, but rather serves to make bigger and bigger waves. Additionally, vehicles operating in water have the potential to become unstable.

The river fording simulation consisted of 200,000 frictionless spheres with a density of 1000 kg/m^3 in a deep channel. The HMMWV is kinematically driven through the channel with the rigid wheels rotating at a constant speed of 1 rad/s. The chassis and wheel geometry are made up of triangle meshes.

Simulations were performed with various amounts of particles and particle sizes to better understand how the vehicle interacts with the fluid as it enters and exits the section of river. These simulations show that when the height of the water is below the chassis, the water does not resist the vehicles motion and the wheels cut through easily. As the water level increases and it comes into contact with the chassis, a large wave in front of the vehicle forms as the vehicles pushes through the water. If the level is too high the fluid would likely enter the engine and crew compartments.

This article was written for SAE Off-Highway Engineering by Daniel Melanz, Hammad Mazhar, and Dan Negrut of the University of Wisconsin.