Learning Forum

[venkyvenkatesh374]

Hello All., Am working on a project where am supposed to find the velocity of a bullet fired in open air/atmosphere.

I've used Explicit dynamics to set up this problem. Initial condition I've given force on back surface of bullet.

In solution information I've given directional velocity. After solving I've got results which is uniform velocity upto 1sec.

My doubt is Once the bullet reaches max velocity It has to drop down., To find this out should I increase the analysis time step ( If I increase the time, the computtaional time taken is very very long ). Can someone please help if am doing anything wrong here.

[Ashish Khemka]

Hi,

Unfortunately this question is beyond the scope for ANSYS employees to answer on a public forum, so hopefully other users can chime in.

Regards,

Ashish Khemka

[Tonmoy]

Hello Venkatesh,

Could you clarify more about the problem you are trying to model? As far as I can understand, you are prescribing motions at the back surface of the bullet and expecting it to drop down after some time – the part I don’t understand is that why it has to drop down if there is no external force or friction acting upon it!

If you’re trying to understand the behavior of a bullet due to gravity load, then model the bullet as a rigid body (if we don’t care about the deformations it’ll have during the flight) and apply the gravity in the Z direction using *LOAD_BODY_Z. For more information you can take a look here: https://www.dynasupport.com/howtos/general/gravity-load.

However, (though unrealistic) if you want to understand the behavior of the bullet under the friction of the atmosphere then I believe, you’d need to do a CFD analysis. I’m not an expert about it but I think, you can do some research about achieving this using ALE or SPH element. To know about the intricacies to perform it using LS-Dyna, you can find some useful conference articles in here: https://www.dynalook.com/. I think initially starting from a “ballistic protection” will be much easier as previously people used Dyna a lot to do this kind of simulations. One person who did a lot of these kinds of simulations that I can think of is Varmint Al, an ex scientist and engineer of LLNL. A massive collection of his experience with these type of modeling and Dyna are available in his website: https://www.varmintal.com/aengr.htm.

I hope these suggestions will help to answer your question to some extent. Happy modeling. Thanks!

 

- Tonmoy.

[venkyvenkatesh374]

Hello Tonmoy,

Thanks for the time taken to reply for my query. 

My aim is to find out what will be the max velocity reached by a particular shape bullet ( material - Stuctural steel) when fired from a cylinder with 90psi. so I've set this up model in explicit dynamics,

My Boundary conditions are,

Pressure 90psi at horizontal direction, standard earth gravity in -Z direction. Analysis time i have given it to 0.05sec.

My aim is to obtain a curve in inverted "V" shape that specifies initial vel, max vel at mid and final vel while bullet drops down. 

 

My question is for an time period of 0.05sec am getting a graph slope line with incresing vel., will i be able to achieve inverted "V" shape if i increase the time step. 

[Tonmoy]

Hello Venkatesh,

If I understand correctly, you’ve specified a termination time of 0.05 seconds, which may not be enough to see the trend we are expecting. Though this isn’t my area of expertise and I didn’t do something like that before, I would suggest you to consider using *MAT_RIGID to model the structural steel. This would help reduce the solution time. Additionally, you could impose more boundary conditions on the model, particularly since we’re interested in the system’s behavior in the x-z plane. Therefore, we can constrain everything except for the X, Z and RY. The timestep size is generally determined by the CFL limit in LS-Dyna, and I don’t see changing it will improve anything in this case.

If I were you, I would initially start with a small amount of impact pressure, for example 1 psi or 5 psi, at the beginning of the simulation. I would then set the termination time in *CONTROL_TERMINATION to be very long, allowing enough time to observe the expected behavior. Once satisfied, I would gradually increase the impact pressure, starting from 10 psi, then 20 psi, and so on, sequentially.

This type of simulation is quite complex, involving both time and space considerations. Without any friction to reduce the velocity, the bullet might travel a significant distance, such as 5 or 10 km, before we can see the expected change. You can resort to MPP dyna or check if there is an alternative to do it (like changing initial condition or so).

 

With Regards, Tonmoy.

 

 

[venkyvenkatesh374]

Hii Tonmoy,

I agree 0.05 termination wouldn't give the desired result., also as suggested by you let me reduce the force for smaller values and then try simulating the problem. 

Also just for your clarification, am not simulating in LS dyna., Am solving this in Explicit dynamics.