Hi,

I am a PhD Student currently running a drag force analysis of tractor-trailers for different deflector design on the cab Roof. I have define name selection for each face of tractor trailer (such as front, rear, sides roof etc) separately in design modular. I am using Ansys Fluent 17.0 for the simulation and K-E model. When I am trying to calculate drag force on faces, the values are okay but the sign convention does not seem to be right to me. As you can see in figure 1 the pressure contour is negative on the rear face of the tractor trailer and positive on front face. But when I calculate forces on these faces it is giving me a positive answer for both faces( see figure 2). I checked the area of both faces from surface integral option (see figure 3) then I computed static pressure from area-weighted average and I got negative pressure for rear face and positive pressure for front face (figure 4). when I multiply this pressure with the above computed areas it is giving me almost the same forces which I have computed in figure 2 but why the sign is positive there when pressure on rear face clearly have a negative sign. The same problem is happening on the gap between tractor and trailer area. I am wondering if I have done something wrong or it's Ansys which is confused with the direction of face area may be.

Please help me out here as I am stuck in my research at this point. I have read Ansys guide but could not find any possible explanation. I hope a lot of experts are working in the field of aerodynamics would be able to provide a solution. I am happy to provide any further detail if required.