acceleration and deceleration of a moving body

JohnAB · April 9, 2014, 21:34

Hello,

I am using the overset method to simulate a very simple translation movement:
- my little cube (moving body, "overset" Region) has a translation motion along x.
The simulation works great for the steady state but my problem is that my speed is going straight from 0 to steady state and what I want is the cube to accelerate slowly from 0 to steady state, and then, after a given number of iterations, decelerate to 0.

How can I do that ?

In Regions > Overset, I can only enter the speed in the "Initial Conditions" tab. There I put [0;0;0] since I want to start from 0.
then, in Regions > Overset > Physics Values > Motion specification, I put User Defined Vertex. I figured it is the only one that could work, since translation can only do steady state (I have tried it and created a new translation in the Motions folder, but no way to put an acceleration/field function there...).
After that, I'm stuck, I tried ever User Defined Vertex Option, with no luck...

It would be nice if I could move forward and solve this.

Thanks!

me3840 · April 9, 2014, 23:11

You can put user-defined functions in motions, just type them in. Either create three scalar field functions (or fewer if some are zero) for your three components of velocity or just create a vector field function that has all three built in. Then just type in [$x,$y,$z] for the former or $$vectorFieldFunctionName for the latter.

JohnAB · April 11, 2014, 12:34

I got it to work, thank you!
one more thing, how do I add the deceleration in the same field function ?
I understand it is going to look very similar to acceleration, but what is the syntax that links the two expressions ? I tried a comma, going to next line, a semi-colon, etc.. and it did not work...

thanks for all your help!

me3840 · April 11, 2014, 15:38

I'm not entirely sure if this is what you're trying to achieve, but you can nest if statements. For example, consider this function that ramps up and down a scalar value of 10 for the first second and 5th second, respectively.

($Time<1.0)?10*$Time/1.0

($Time>4.0)?10*(1/($Time/5.0)):10.0)

JohnAB · April 11, 2014, 16:24

I did not know how to nest the statements so this is exactly what I needed! I am going to try it, thanks!

mna23 · August 4, 2021, 00:20

Hello Mr. John. I see that you have succeed to define the acceleration for flat wave accelleration from 0 m/s to x m/s. Can i kneow about the user field function that you have defined? i have the same problem too to have speed up for my ship simulation. It great if you reply this massege. Thanks. Regards.

April 9, 2014, 21:34	acceleration and deceleration of a moving body	#1
JohnAB Member Join Date: Nov 2012 Posts: 74 Rep Power: 14	Hello, I am using the overset method to simulate a very simple translation movement: - my little cube (moving body, "overset" Region) has a translation motion along x. The simulation works great for the steady state but my problem is that my speed is going straight from 0 to steady state and what I want is the cube to accelerate slowly from 0 to steady state, and then, after a given number of iterations, decelerate to 0. How can I do that ? In Regions > Overset, I can only enter the speed in the "Initial Conditions" tab. There I put [0;0;0] since I want to start from 0. then, in Regions > Overset > Physics Values > Motion specification, I put User Defined Vertex. I figured it is the only one that could work, since translation can only do steady state (I have tried it and created a new translation in the Motions folder, but no way to put an acceleration/field function there...). After that, I'm stuck, I tried ever User Defined Vertex Option, with no luck... It would be nice if I could move forward and solve this. Thanks!

April 11, 2014, 15:38		#4
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 25	I'm not entirely sure if this is what you're trying to achieve, but you can nest if statements. For example, consider this function that ramps up and down a scalar value of 10 for the first second and 5th second, respectively. ($Time<1.0)?10$Time/1.0($Time>4.0)?10(1/($Time/5.0)):10.0)

April 9, 2014, 23:11		#2
me3840 Senior Member Join Date: Nov 2010 Location: USA Posts: 1,232 Rep Power: 25	You can put user-defined functions in motions, just type them in. Either create three scalar field functions (or fewer if some are zero) for your three components of velocity or just create a vector field function that has all three built in. Then just type in [$x,$y,$z] for the former or $$vectorFieldFunctionName for the latter.

April 11, 2014, 12:34		#3
JohnAB Member Join Date: Nov 2012 Posts: 74 Rep Power: 14	I got it to work, thank you! one more thing, how do I add the deceleration in the same field function ? I understand it is going to look very similar to acceleration, but what is the syntax that links the two expressions ? I tried a comma, going to next line, a semi-colon, etc.. and it did not work... thanks for all your help!

April 11, 2014, 16:24		#5
JohnAB Member Join Date: Nov 2012 Posts: 74 Rep Power: 14	I did not know how to nest the statements so this is exactly what I needed! I am going to try it, thanks!

August 4, 2021, 00:20		#6
mna23 New Member Anonymous Join Date: Aug 2021 Posts: 1 Rep Power: 0	Hello Mr. John. I see that you have succeed to define the acceleration for flat wave accelleration from 0 m/s to x m/s. Can i kneow about the user field function that you have defined? i have the same problem too to have speed up for my ship simulation. It great if you reply this massege. Thanks. Regards.