General Mechanical

General Mechanical

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Load ramping and time-steps

    • Lauri Kauhanen
      Subscriber

      Hi, 

      Is there any way to move the ramp up of a load to later time steps? I have an analysis build to introduce loads in different timesteps (1st own weigth, 2nd hydrostatic pressure etc.) The problem is that hydrostatic pressure is only activated after first step has converged, and thus the new load is applied with full effect at the start of second step. The analysis doesn't converge due to excessive strain (over 15% limit) and adding substeps doesn't really help in this context as the load is 100% in those as well.

      The hydrostatic pressure object doesn't allow tabular data to be edited, as far as I know. So is there a workaround to move the ramp-up to start a step later

       

      Thanks 

      Lauri

    • peteroznewman
      Subscriber

      Hi Lauri,

      Try removing Standard Earth Gravity in Step 1.  Most of the stress in the structure will come from the Hydrostatic pressure.  The self-weight of the structure may be insignificant compared to the weight of the fluid. What is the weight of the structure compared with the weight of the fluid?

      Please show the Tabular Data for the Hydrostatic pressure and the Details window for this load and some graphics for the structure.

      Regards,
      Peter

    • Lauri Kauhanen
      Subscriber

      Hi, Thanks for the reply

      Yes I know that I could move the hydrostatic pressure to the first step, but that is not really the result I'm after. 

      Tabular data table below. The acceleration values are greyed out. 

      It seems a bit like an oversight that hydrostatic pressure should always be on the first step, even if one would want to check the stresses on an empty tank before filling or wanted to pretense the bolts before filling, while wanting to view the combined result also. All would be fine if I could edit the acceleration to zero in the now inactivated step.

      Sorry that I don't post pictures of the design, but that should not be relevant to the case. I rather wanted to know whether this is a bug, or if there is some reason behind this restriction.

      Thanks

    • peteroznewman
      Subscriber

      Read the Ansys Help page in the Mechanical Application on Hydrostatic Pressure. 

      https://ansyshelp.ansys.com/account/secured?returnurl=/Views/Secured/corp/v232/en/wb_sim/ds_Hydrostatic_Pressure_Loads.html?q=hydrostatic

      There you will see this note:

      During a multiple step analysis, tabular data is visible for this boundary condition. This information is read-only but you can use the context menu (right-click) features of the Tabular Data display to activate or deactivate the loading per step.

      That means you can deactivate the Hydrostatic Pressure load in Step 1 or any number of steps.

    • Lauri Kauhanen
      Subscriber

      Hi,

      Yes that is exactly what I have done so far. But deactivating earlier steps doesn't move the ramp up start to the first active step. That is the key to my problem from the beginning. 

      But I guess we'll have to live with bad convergence for now.

      BR

       

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