Permeability Study Of Open Graded Friction Course (OGFC) Based On 3D Flow Simulation Model

Open Graded Friction Course(OGFC)is one of the most effective practices that may scale back rainfall quantity in road networks.The performance of this asphalt has been based on the permeability of its layer.The estimation and evaluation of permeability and hydraulic conductivity are essential for the design and quality assessment of OGFC which directs rain runoff to the drainage system.There are two typical measures to assess permeability,the constant head permeability test,and the falling head permeability test.However,many recent studies have shown that the calculated permeability coefficient from laboratory experiments is usually greater than that of field tests because the constant head permeability test in the laboratory only considers water flow in the vertical direction and neglects the water flow in the transverse directions.Some researchers started to utilize a new approach to evaluate permeability and investigate water flow in the OGFC mixture through a microstructure model and computational simulation.However,few studies indicated horizontal and vertical permeability evaluation by using software programs such as ABAQUS,ANSYS CFX,ANSYS FLUENT,FLOW 3D,etc.Furthermore,the permeability evaluation in OGFC has many factors that must be considered for the mix design including the air voids content,the size and shape of the aggregate particles,the binder and additive type,and the thickness of the specimens.Therefore,this study aimed to evaluate the permeability and hydraulic conductivity of OGFC in horizontal and vertical directions and study the most effective factors that may enhance the permeability in OGFC.To achieve this objective,horizontal and vertical permeability simulations were conducted on twelve cubic specimens of OGFC with different geometry and various porosities.The twelve cubic specimens were modeled by Design Modeler in Ansys Workbench 19.2 and the aggregates and mastic binder were considered as asymmetric solid particles.In order to study the effect of aggregate size on permeability,four of these specimens were simplified to solid particles with a small size of aggregate,whereas the remaining eight specimens were modeled with a large size of aggregate.Furthermore,eight of these specimens were modeled with a thickness of 30mm,while the other four specimens were modeled with a thickness of 45mm to study the influence of thickness on permeability.Each one of these specimens was imported into Ansys Fluent and then meshed according to the particle shape of the specimens.The twelve meshed specimens were set up into Ansys Fluent to simulate the flow model.Through Ansys Fluent,the laminar flow model was utilized for the twelve specimens using computational fluid dynamics(CFD).For each simulation,the Reynolds numbers,permeability(K),and hydraulic conductivity(k)were calculated in horizontal and vertical directions.The study found that the permeability in the horizontal direction is greater than in the vertical direction.The permeability in specimens with the small size of aggregates has the lowest values,while it increased in the specimens with large size of aggregates.In addition,specimens with a large size of aggregates and larger thickness of(45mm)have the highest values of permeability.Moreover,the air voids are not the only factor that controls the permeability of the asphalt mixture;however,the size of aggregates and the thickness of the specimens are more effective factors to the permeability.The higher the percentage of air voids are,the higher permeability in OGFC asphalt.In addition,the increase in the size of the aggregates and the thickness led to an increase in the permeability of the mix.The reason for this is that the large size of aggregates results in less void distribution,which enhances permeability.The study also found that as the diameter and percentage of air voids increased,the velocity of the flow inside the voids decreased,while the pressure increased as the diameter and percentage of voids increased.Moreover,the velocity of the flow increased as the size of the aggregates increased,whereas the pressure of the flow decreased.Under low inlet pressure,the velocity and Reynolds number were very low in vertical and horizontal directions of the flow and the flow can be described as laminar.The results demonstrate that the permeability evaluation values obtained using Ansys Fluent are in good agreement with the measurement based on previous studies.