Gravity (/GRAV) vs Imposed Acceleration (/IMPACC) load

Gravity (/GRAV) and Imposed Acceleration (/IMPACC) are two types of loads with similar behavior in some applications.  As a result, the confusion between the two is a common mistake in simulation set-up.

Imposed Acceleration (/IMPACC) is the total acceleration of a body.  No matter what happens this body is subjected to a specific acceleration.  This is a kinematic condition, so the entire motion of the body will only depend on it and on the initial conditions, but NOT on the other forces or loads. Hence, in Newton’s Second Law, the acceleration parameter on the right side will be fixed to a specific value. Any force applied on the left side (contact for instance) will be ignored.  For example:

Gravity (/GRAV) acts as an external force to the body and so represents an additional load.  The body’s acceleration is affected not only by gravity load, but also by other loads applied to the body.  Hence, in Newton’s Second Law, gravity /GRAV will create another Force on the left side of the equation and the acceleration could have any value the sum of the forces creates.  For example:

To demonstrate the different effects of these loads a simple model has been created that contains one ball falling from a specific height on a deformable floor.  The model can be seen in Figure 1.

Figure 1: Model layout

To model the ball properly, shell elements have been used and assigned with a fabric elastic orthotropic material law (/MAT/FABRI).  Also, a controlled volume (/MONVOL/AIRBAG1) has been created to keep the pressure inside the ball constant during the simulation.

The only loads applying to the ball are /GRAV or /IMPACC, one in each model.  This is also the only change between the provided models.  The target value of acceleration is 0.00981 mm/msec2 (or 9.81 m/s2).

The floor part is considered deformable with boundary nodes being constrained in all the degrees of freedom.  Finally, to model the interaction between floor and ball a linear contact interface has been created (/INTER/TYPE24).

The kinematics of the ball for the 2 models can be shown in Figure 2.

Figure 2: Animation of ball kinematics

As we can see in the first situation (/GRAV) the contact force makes the ball to bounce in the floor, while in the second (/IMPACC), ball movement is not affected by the contact force but rather enforces a great deformation of the floor.

In Figures 3 and 4 we can see the kinematic variables of the ball under Gravity (/GRAV) and Imposed Acceleration (/IMPACC) respectively.  A specific node at the ball surface has been created to capture these values.

Figure 3: Kinematics of ball under Gravity (/GRAV) load

Figure 4: Kinematics of ball under Imposed Acceleration (/IMPACC) load

As we can see in Figure 3, the value of acceleration is constant to gravity acceleration (0.00981 mm/msec2), only when gravity is the only force applied to the ball.  Between 170 and 200 msec the acceleration changes rapidly because of the contact.  Additionally at the first 20 msec, the acceleration value is not preserved because of the control volume initialization.

On the other hand, as Figure 4 showcases, the Imposed Acceleration doesn’t let the ball to have any other acceleration value during the whole simulation time.  To achieve this, external forces are applied to the system in a way that the ball’s acceleration is preserved.

Specifically, in Figure 4 we can see that the ball follows the equations of free fall with zero initial velocity.  These equations for displacement and velocity are explained below.

In Figures 5 and 6 we can see the different types of Energy during simulation for Gravity (/GRAV) and Imposed Acceleration (/IMPACC) load respectively.

Figure 5: System Energy under Gravity (/GRAV) load

Figure 6: System Energy under Imposed Acceleration (/IMPACC) load

In Figure 6, we can see clearly that a great amount of External Force is applied in the model after the contact to preserve the acceleration value and, as a result, deform the floor part.

All in all, gravity (/GRAV) and imposed acceleration (/IMPACC) loads have a completely different scope and way of application, even if in some cases they can produce equal results.  The former is a specific force, while the later a kinematic condition.