# Define Fluid-Structure-Interactions

Virtual Wind Tunnel for ultraFluidX supports the P-FSI approach available in AcuSolve (based on the eigenmodes of the structure). Each part can be defined as flexible and the corresponding op2-file (containing the eigenmodes of the structure) can be assigned to each part individually. OptiStruct can be used to compute the eigenmodes up front.

The eigenmodes computed in OptiStruct are based on the location and orientation of the structural model. If the location of the model in Virtual Wind Tunnel for ultraFluidX is not the same as the one used to compute the eigenmodes, the P-FSI job does not work correctly.

Certain functions are not supported yet via the Virtual Wind Tunnel for ultraFluidX workspace. If any changes in the .vpar file were required, the Virtual Wind Tunnel for ultraFluidX job has to be submitted manually via the command line. The .vpar file can be generated by clicking Export in the Run dialog.
• Change the number of used modes (use less modes than present) by entering the following line to the .vpar file: flex_comp_1_modes = 6
The number of modes considered for the simulation is set by default to the number of present modes in the eigenmode file.
This will cause the usage of the first six eigenmodes of the eigenmode file corresponding to flexible component 1. If the number of modes is set to a higher value than the present modes in the op2 file, then the maximum number of modes is chosen. The pressure loads onto the parts are ramped over several time steps, to avoid a “shock” of the pressure loads onto the structure. Per default the ramping is linear between the time steps eight and 16 from zero to the full value. This yields the following default entries in the .vpar file:
mult_lower 	= 8		// loads have no effect on the structure till here
mult_upper	= 16		// loads have full effect on the structure from here
• Change the default time step interval for ramping in case it is to large or if it starts too late by editing the .vpar file (the .vpar file can be generated by clicking Export in the Run dialog).
• Monitor results, for example displacement of the structure over time, by defining monitor points.
For each monitor point, add the following lines to the .vpar file:
time_history_1_name 	= “point name”	  // the name of monitor point
time_history_1_coord	= ( 3.1, 0.0, 0.223 )	  // the (x,y,z) coordinates
time_history_1_output	= ( “x_displacement” )	  // output type for the report

Define a second or third monitor point by replacing the one in the above commands by two or three. Multiple outputs can be chosen, separated by comma. The possible candidates are x_displacement, y_displacement, z_displacement, displacement, x_velocity, y_velocity, z_velocity, velocity and pressure. The displacement options refer to the mesh displacement at the given coordinate point.

• Create animations by editing the .vpar file:
animation		= True	      // activate animation
saved_sates	= 50	     // keep the last 50 output files
outputFrequency	= 2	     // write output at every 2nd iteration
Activating the animation mode will generate one animation of a cross section through the entire body with the velocity magnitude as contour plot. Additionally, for each flexible part, three separate animations will be generated: one animation for the displacement (surface contour plot), one for the pressure (surface contour plot) and one animation showing a cross section cut with the velocity magnitude.