Let me give a brief summary of the process involved in the FEA of lifting surfaces with the wing as an example: 1. The wing is modelled as a hollow surface body in Ansys, meaning only the exterior aerodynamic surface is specified. 2. Internal structure made of composites is modelled next. 3. Aerodynamics loads and Boundary conditions (fixed edge at the root of the wing) are specified. 4. The solution is obtained in terms of displacements, stresses and failure criteria etc.
As part of Step-3, aerodynamic loads are generated in the form of Cp values (Coefficient of Pressure) at discrete points on the exterior surfaces of the wing. This data is generated from XFLR5. Cp values are converted into pressure values and applied on the exterior surfaces of the wing. To do the same, the following formula is used:
Pressure values obtained from the LHS of the above expression are applied over the exterior surfaces of the wing. FEA process is completed and the solution is obtained.
Observe the crushing of the wing cross section in this figure where the top surface and bottom surface come close to each other and airfoil geometry is completely lost
However, it finally struck me that there has to be an internal air pressure that will support the cross section from the inside as what happens with hollow structures on earth, eg. a hollow can be placed on the ground doesn't get crushed into itself since the external and internal pressures cancel each other out.
Hence, we should not be using absolute pressure but rather gauge pressure (difference in external and internal pressure) should be used. Otherwise, it essentially means that there is a vacuum on the inside and air pressure on the outside.
Once this change was made, we no longer observe the "crushing" effect.
Finally, we still want to be sure if the internal air pressure would be equal to the static pressure at that altitude as has been assumed in the usage of gauge pressure. Our assumption is that the pressure inside the wing body would be equal to the static air pressure at that altitude from thermodynamic relations even if the wing is completely sealed. IS THIS ASSUMPTION CORRECT? ANY SUGGESTIONS WOULD BE HELPFUL?