NUMERICAL STUDIES ON REDUCING DRAG IN SEMI-TRAILER ASSEMBLY BY USING DIFFUSER

: Recently, the fuel price has been continuous to increase, as result of those producers of heavy vehicles to resort the impatient aerodynamic design of heavy vehicles such as trucks, wagons, tractors as well as trailers. In this study make comparsion analysis of the drag coefficient for semi-trailer with four models which are three types of diffuser and another typ without diffuser.(Malviya, et al, 2008) In this study, compute the drag force on the four models of the semi-trailer, and take the optimisation design of diffuser and compute the effect side wind with semi-trailer without diffuser. In this paper the drag coefficient has decresed to by diffuser.


‫والتطبيقية‬ ‫األساسية‬ ‫للعلوم‬ ‫األسمرية‬ ‫الجامعة‬ ‫مجلة‬
Thus in any velocities higher than 30MPH, the drag force is most important teradata force, it uses about 50% of the car be used power of the engine to overtake this teradata.
In the UK the goods operator has 600 tractor units and two times as many semitrailers. This flotilla would yearly envelop about 56 million miles which price practically £19250000 in fuel. Thus, even a small part attained in fuel reserve could mean reserves of thousands of pounds. In this paper involves the impact of use the diffuser onto semi-trailer in terms of drag force, lift and side force coefficient. The analysis has been made through Computational Fluid Dynamics (CFD). (Malviya, Mishra, et al,2009)

2-Theory:
One of the most widely tool, which used for solving problems involving fluid flows, is computational fluid dynamics. On the other hand, computational fluid dynamics is a cheaper method compared to traditional wind tunnel experiments, but it needs a great amount of computing power to achieve a minute result.
The basic equations of the compute fluid dynamic are the Navier-Stokes equations, which describe the physics of flowing, and the motion of a fluid in both laminar and turbulent flow. It is derived through the application of Newton's second law to fluid motion on the assumption that the viscous stresses are commensurate with the rates of deformation as well as for incompressible flow. So the Navier-Stokes equations are (Kundu, 2013): The solution to the Navier-Stokes equations is a description of the velocity of the fluid at a given point in space and time that for 1D and the three dimensional continuity equation is (Magoul s, 2011): As a result of the velocity and pressure fields around the velocity, that it will impact the magnitudes and directions of various aerodynamic forces and moments, for example drag force, lift forces, side forces, rolling moment, pitching moment and yawing moment (Malviya et al., 2009). So, to enable comparison between the forces acting on different objects, which is called a aerodynamic coefficients, it will describe two coefficients:

2-1 Drag coefficients ( ):
The drag coefficient is a dimensionless amount, which to a large extent relies on the frontal surface area ( ) due to it work in reverse the direction of travel (Malviya et al., 2009); as result of this, it will impact on the fuel efficiency of the vehicle.

‫والتطبيقية‬ ‫األساسية‬ ‫للعلوم‬ ‫األسمرية‬ ‫الجامعة‬ ‫مجلة‬
The drag force is partially generated by evenly distributed pressure at the surface and parcel by frictional forces as the flow passes over the surface. The drag force could be defined by equations (Bruno & Ismir, 2012).

( )
Here, ρ is the density of air, (v) is the vehicle speed, is the coefficient of drag and A is the vehicle frontal area. It notes the drag force heighten relative to the square of vehicle speed (V).
The power (P) required to overcome this drag is proportional to the cube of the speed (Malviya et al, 2009).

2-2 Lift coefficients ( ) :
The lift is a dimensionless coefficient, which connects the lift created by an aerodynamic body, the dynamic pressure of the fluid flow around the body and a reference area associated with the body (Barnard, 2001).

( )
Here, is the lift force, ρ is the density of air, (v) is the vehicle speed, is the coefficient of drag and (A) is the vehicle frontal area. Table 1 shows the typical influence of drag on the fuel consumption of a typical 40 tonne semi-trailer. It may be noted from the relation that a decrease in drag has the ability to contribute to significant savings in fuel (Malviya et al., 2009). About 90% of the drag force acting on the vehicle, this is called the non-uniform pressure field, which it is contributed by drag. The rest of the drag force is called a viscous or friction drag, which is caused by what is called skin friction due to shear stress as surfaces. It can be decreasing the aerodynamic drag on the vehicle by making the body streamlined and thus decreasing the form drag; also it can be decreasing the drag by controlling the separation of flow where shape alteration is not possible (Malviya et al., 2008).  (Hucho, 1998)

3-Introduction to CDF:
Engineers in the area of aerodynamics have been using CFD for extended time, which is used to ascertain wind tunnel experiments, improving vehicle design in terms of drag and lift, also research of prominent flow attributes, CFD analysis decreasing the total of time and effort necessary in labs. (Fluent Inc(2007)

3-1-Fluent:
Fluent is the world's major supplier of commercial computational dynamic (CFD) software and services. Fluent progress full meshes exibility, inclusive capacity to solve problems using unstructured meshes that can be created about complex geometries with relative ease. All task needful to calculate a solution and exhibit the results are reachable in FLUENT through an interactive, menu-driven edge. (Fluent Inc (2007)).
2-Make the model geometry and grid.
3-Prepare the solver and physical models.

3-3-1-Model creative:
The dimensions of the model for semi-trailer that it have chosen to ANSYS ,length is 16,2m, width is 2.6m and the height of trailer is 4.3 m, wheels dimension is (0.3m ,R0.46m). (Malviya et al, 2009) as shown in figure (2) and all dimensions by millimetre:

Figure (2) Dimension of semi-trailer
The first step in CFD is to make a computer aided design CAD model, ANSYS bolsters number of CAD software's including AUTO-CAD, SolidWorks, and GAMBIT. Through this step, the model of semi-trailer in geometry was made in Solidworks, which was then imported in CFD, as showing in the table (1):

3-3-2-Numerical formulation:
The CFD is solving a group of partial differential equations with boundary conditions, which is predefined. In this simulation the equation for conservation of mass and momentum have been reached to resolve successively with two additional transport equations for steady turbulent flow. Pressure insertion was made by using the standard scheme. Discretisation of the momentum equation was made by using the second order upwind to apply higher precision of flow variables at each cell face. (Malviya, et al, 2009)

3-3-3Computational Domain
The different models of the semi-trailer formation to be tested were each imported into a three dimensional flow domain generated in geometry. The flow domain was composed of a rectangular

3-3-4-Mesh:
The second step in ANSYS is Mesh, which is the major step in ANSYS. We can control equality of mesh by control the size of elements. In this study has been used model of the sime-trialer as a reference for result that from Malviya at 2009 paper's. In the reference used about 2.2 million elements and the velocity at 19.5m/s, so I tried to make a three type of size for elements in order to get the best size of elements as shown in the

3-3-5-Boundary conditions:
In this report the velocity of the domain of the semi-trailer was defined as velocity inlet,which is constant 24.59m/s. The side face of domain behind the model was defined as pressure outlet was zero, the bottom face of domain flow was defined as moving wall by velocity 24.59m/s. The figure (7) shows the name of all face of the semi-trailer.  Pressures statics,Semi-trailer with diffuser 2 As shown in the charts the drag coefficient, which has decreased after adding a diffuser, but also it noticed that the lift coefficient has increased. Therefore the result of diffuser 2 was the best, and it was the optimal diffuser. As shown in the charts the drag coefficient, lift coefficient and side coefficient, which have increased after increasing the angle, because the area in the front of the flow had increased. However the increase in the drag coefficient of the semi-trailer with diffuser is less than the semi-trailer without diffuser. In this paper, the studied diffuser in semi-trailer when the velocity of semi-trailer was 55MPH by CFD analysis. Any decreasing in aerodynamic drag by 2%, it will improve in fuel economy approximately 1%. (Cummins MPG Guide, 2007) The diffuser used for semi-trailer found, that the drag coefficient decreased by 12.7%, as a result of that it will improve in fuel economy by about 6.37%, thus providing significant fuel savings and flexibility to freight operators.

Conclusion
 The results appear that the existence of a diffuser reduces the drag force on the semitrailer, however at the same time, it decrease the lift force. As result of that the diffuser is better for decrease drag force only.  The result appear that the existence of a diffuser increases the drag , lift and side force on the semi-trailer by increasing the semi-trailer at an angle, because the area in the front of the flow had increased.  From the three types of diffuser that it have studied in this paper. The diffuser 2 with the left and right faces of the diffuser designed as a curvature, which it was the optimisation.