6.5. 3 Gridpoint Velocities
The gridpoint velocities may be assessed either by plotting the complete field of velocities or by selecting certain key points in the grid and tracking their velocities with histories/time solution. Steady-state conditions are indicated, if the velocity histories show horizontal traces in their final stages. If they have all converged to “near zero” (in comparison to their starting values), then absolute equilibrium has occurred. If a history has converged to a “non-zero” value, then steady plastic flow occurs at the grid point corresponding to the recorded history. If one or more velocity history plots show fluctuating velocities, then the system is likely to be in a transient condition. To confirm that continuing plastic flow is occurring, a plot of plasticity should be examined (figure 18). When the model is stable, the gridpoint velocities decrease to zero and the velocity vectors often appear random in direction. However, for unstable model, the gridpoint velocities have converged to a non-zero value; it is likely that steady plastic flow is occurring in the model. In this case, the velocity vectors show some systematic orientation.
Figure 18: Result of slope in terms of Gridpoint Velocities and velocity vector