Muhammad, in short, this is a vector analysis problem. And to summarize the method used to solve it, you want to compute the longitudinal wind speed, hitting the truck from ahead, and the transverse wind speed, hitting the truck from the side.

The longitudinal component of relative wind is what causes the truck to burn more fuel. The transverse component will primarily be a nuisance, potentially causing the truck to want to steer off course. Or in very severe wind, maybe even fall over on its side!

1. What will be the direction of drag force? For example if the flow is at 30 degree yaw angle? Would the drag force be along x-axis or it will be along the flow streamline which are at 30 degree?

I think you are asking, if the truck in moving, and relative wind comes from 30 degrees, then what are the longitudinal and transverse components of that wind?

Let's say that relative wind is 60 km/h, from 30 degrees. That would make the relative longitudinal wind component 60 * cos(30), or 52 km/h. And the transverse component will be 60 * sin(30), or 30 km/h.

2. to compute drag coefficient, I need projected area which is 10m^2 for yaw angle 0 degree. Do I need to compute projected area for different yaw angle by taking plane in that direction or I only need projected area along X-axis as my model is along this axis in all cases?

Again, break this down into a vector problem, and all is simple. So, for longitudinal drag, it's going to be that 52 km/h wind pushing on the frontal area. For the stability-related transverse wind, it will have to be the 30 km/h component pushing on the much larger area, broadside.

Unless the wind is really, really strong, the forward motion of the truck will usually be such that the longitudinal component of relative wind is gong to be significantly greater than any transverse component. But in a long tractor-trailer truck, that transverse component can be problematic, just because the area is so big.

In case you aren't familiar with these relative wind terms. Relative wind is the wind felt by a moving vehicle. Imagine you are standing up, in a convertible car, while the car is moving. That wind you feel, speed and direction, that's relative wind.

True wind is the actual wind speed, irrespective of moving car. So, you're standing in that convertible car, stopped at a red light. The wind you feel then is true wind.

Hi Albert,

Thank you for your detailed explanation. I have divided my velocity into the longitudinal and transverse components. Now I am confident that the way I was performing the calculations is right.

I have one more confusion related to Ansys Fluent solver if you can provide any help on that. I have created another question here is the link for that

https://www.researchgate.net/post/Why_the_pressure_is_negative_in_wake_region_behind_the_vehicle

The problem I am having is that on some faces (such as rear face of trailer which s in wake region) the static pressure is negative but when I measure pressure force on those faces along X-Axis, on some faces the force is positive and on some the force is negative.

For example the static pressure is negative on both faces i.e. rear face of tractor and front face of the trailer but when I compute drag force on these faces the rear face of tractor has positive force and front face of trailer has negative force. I am confused with sign convention used by Ansys fluent here.

If you can recommend any website or document which can explain sign convention used by Ansys that would be a great help. I have tried help guide of Ansys and could not find any information on sign convention.

Thanks again for all your help and time. I really appreciate this.

Muhammad,

For example the static pressure is negative on both faces i.e. rear face of tractor and front face of the trailer but when I compute drag force on these faces the rear face of tractor has positive force and front face of trailer has negative force. I am confused with sign convention used by Ansys fluent here.

I'm not familiar with Ansys Fluent, but the sign convention they use seems to be logical enough. The rear face of the tractor, when the tractor is moving forward, will experience what amounts to a partial vacuum, compared with ambient pressure. The front face of the trailer will also be in this partial vacuum.

As to drag, let's see what makes sense. A partial vacuum in the back of the trailer wants to slow the trailer down, right? Think of it like someone sucking the tractor backwards, with a big vacuum pump. So, it takes more fuel to fight this partial vacuum, and they call this positive drag. Logical.

The opposite is the case for the front of the trailer. You can test this out yourself, although it is quite dangerous. If you are driving behind a big truck with a blunt back, and you get up really close to it, to can essentially coast along, using very little throttle. So, negative drag means that the partial vacuum in from of you is actually pulling you along, as opposed to slowing you down.

Thanks Albert,

Your point makes a perfect sense. I was thinking exactly the same. But someone was arguing with me on creation of vacuum and momentum transfer in the wake region. I am still a learner in this field so could not really explain him well.

His question was :

The particle of air are still there in the wake region and they are striking with the rear wall of the trailer and transmitting their momentum into that wall. Which supposed to be pushing that face in forward direction which means that the force should be -ve on that face which wasn't.

Is the answer related to the net force between vacuum suction pulling backward and momentum force pushing the wall forward?

Thanks again for your help!