When i plot force magnitude on a wall surface, i get a range of force value from 0 to some positive number and there is no negative number plotted. Is it not possible to plot negative value of forces or is it because Star-cd can't extract negative value of aerodynamic force? I think a force pushing on the surface should be positive value and a force lifting the surface should be negative value and negative value of force should exist to represent effect of suction due to low pressure in high velocity regions.

Negative values of force are possible. You may have extracted the wrong forces, there are quite a number of options and it can be confusing.

Isn't force MAGNITUDE = sqrt(Fx^2+Fy^2+Fz^2). If so, then you should look at the individual components.

yes, magnitude should be that (all positive) but i need to plot force magnitude in normal direction of the surface, not simply Fx or Fz or Fz but a combination of all .. somehow, a force component in the normal direction of surface? Is it possible?

Hi,

The force normal to the surface is caused by the pressure. If you want to have a negative value, you should plot the total pressure.

Force = pressure * area

the area is the cell area on which the pressure is being ploted. you should be able to load this value manually in the post register in addition to the pressure value.

eg. pressure in register 4 and area in register 5. put the new value in register 6 then plot it.

you should get a negative value.

cheers

should the area typed in be the area for each range of pressure in the legend or the area of the entire surface?

The area should be the surface on which the pressure is being plotted. You can maipulate this with in the post register.

Klick on OPERATE then....FUNCTION

Choose LOAD CELL DATA...then AREA...then choose the register you want to put the data into.

Cheers!