Learning Forum

[ahmedshafie]

for external flow over a car , for calculating drag coefficient we project area on axis of flow , calculate projected area , and this is the area used in refrence values ? am i wrong ?nis it the same area for calculating lift ? or we should use projection normal to flow direction ?.forces calculation is independent of areas ? only cd and cl is dependent in the way fluent calculate them ?n

[Kalyan Goparaju]

Hello, nFor drag coefficient, the general convention is to use the frontal area. As you rightly said, you can project the car body onto the plane perpendicular to the flow direction (for eg - YZ plane if the flow is in x direction, with y axis pointing towards the roof the car) and obtain the frontal area, which you can use as a reference value to calculate the drag coefficient. nYou are also correct that, for lift, the top area of the car needs to be used as reference. So, you can project the car body onto the plane parallel (for eg - XZ plane if the flow is in the x direction, with y axis pointing towards the roof of the car) to the flow direction and obtain the top area. nThe reference areas are used to non-dimensional the calculated forces (which do not require any reference area to be calculated). nThanks,nKalyann

[ahmedshafie]

thanks alot for your generous answer ,nnIs the same thing applied for length ? For drag coeff will be in same direction of flow while for lift will be normal to it ?n

[Kalyan Goparaju]

Length is not a quantity that is used in calculating the lift and drag coefficients. n