6dof Model

[Josephina]

Modeland simulate point mass and six-degrees-of-freedom dynamics of fixed or variable mass atmospheric flight vehicles. Define representations of the equations of motion in body, wind, and Earth-centered, Earth-fixed (ECEF) coordinate systems. Transform between coordinate systems and perform unit conversions to ensure model consistency.

I am currently trying to learn to simulate structures dropping into fluid with the 6DOF model. There are two tutorials on this topic out there: One from ANSYS and one from CFD-Ninja on youtube. These are basically the same tutorials, the only difference is that in the first tutorial, the structure is a quadratic box and a sphere in the second.

Both of these models have in common, that there is a mesh zone around the structure, which gets assigned passive 6DOF properties in the dynamic mesh settings in Fluent setup. This seems to be quite important, as all of my efforts to get a dynamic mesh to work properly without such a mesh zone have lead to negative volume cells immediately .

Therefore, I really would like to know how to create such mesh zones around a rigid wall object. I fooled around a little bit with inflation in meshing and I got results which were optically quite similar, but I need the mesh zone as a choosable sub-part of the mesh, so that I can assign 6DOF properties to it in the Fluent setup.

Also I would appreciate some insights of why this particular mesh zone is needed for these type of simulations. All i can see is, that it seems to be important for the process of remeshing during the rigid bodys motion. In the tutorials, the background of this is not explained.

I already watched your tutorials on youtube with great interest, but they dont help me regarding my question on how to create the mesh zone around the rigid structure. As I wrote before, I get negative cell volumes immediately when I try to set up a dynamic mesh without this zone.

As most Lapua Ballistics users already know, Lapua Ballistics applies the 6DOF calculation to model flight paths for Lapua projectiles. 6DOF is also the calculation model used by Lapua Product Development. So what are the advantages of 6DOF, and what does it mean for a shooter requiring accuracy?

The modified point-mass calculation (sometimes referred to as 4DOF) is more advanced than the 3D model, as the calculation includes three translational degrees of freedom and one rotational degree of freedom (spinning). The aerodynamic model includes also e.g. the contribution of lift force. These are some of the same forces that the 6DOF model calculates, however the point-mass model applies a simplified calculation which does not take into account the actual attitude of the bullet, but merely includes an algebraic approximation for the attitude (pitch/yaw angles) of the projectile. Due to some simplifications involved the model is not able to give information concerning the flight stability.

The easy answer is: the most accurate results. As with the modified point-mass model, the 6DOF calculation includes three translational degrees of freedom and three rotational degrees of freedom which give the position and attitude as a function of time. Simplified, 6DOF tracks both the pitch, yaw and roll as well as the up/down, left/right and forwards/backwards movement of the bullet.

The big difference to the other calculation models is that the 6DOF models the actual trajectory path of the projectile at all times during flight, instead of just calculating points on the trajectory curve. The stability properties of the bullet are obtained during the trajectory integration since all the rotational degrees of freedom are present in simulations. Also, the stability analysis of the Lapua ballistics app is more advanced than most other calculators, as it considers both dynamic (Sd) and gyroscopic (Sg) stability factors, not only the latter as many other ballistic calculators do. The Sd is especially important in long range shooting for analyzing the transonic stability of the bullet.

The explanation lies in the background information of the app data. Every 6DOF-calculation is based on an aerodynamic values table that is calculated for every Lapua bullet. The table includes several aerodynamic coefficients and bullet specifics: Drag coefficient derived from Doppler radar data, Normal force coefficient slope, pitching moment coefficient slope, roll damping moment coefficient, two stability derivate, Magnus force coefficient slope and Magnus force moment. All those coefficients must be functions of the Mach number.

I am going to work on BlueROV2 Heavy soon and am trying to simulate it in Matlab/Simulink by following the model presented in the following thesis.

-9de2-441c-8a17-655405d5fc2e/1/ThesisWu2018.pdf

I have a question about thruster force directions shown in Fig 4.2 on Page 40 of the thesis. I am confused about how the horizontal thrusters are oriented. I think horizontal thrusters 3 and 4 on the ROV are oriented such that when positive input is applied, they would move the ROV forward in the x direction. Similarly, when negative input is applied to thrusters 1 and 2, that will cause the ROV to move in the positive x-direction. It would also mean that the effective directions of forces applied by thrusters 3 and 4 on the ROV are in the directions opposite to those depicted in Fig 4.2. And same would apply for thrusters 1 and 2. However the thesis uses force directions as shown in that figure in calculating actual forces and torques on the ROV. Am I missing something here or misinterpreting things?

Also, are all vertical thrusters on the ROV oriented such that they produce forces in the same direction? Figure 4.2 in the thesis shows varying directions for forces generated by the vertical thrusters.

That said, physically speaking the installed propeller orientation/type in a thruster determines which way the water flows through it when the propeller turns in a given direction. The propellers are set to counter rotate (see Figure 3.4), which keeps vehicle torques balanced.

@EliotBR Thank you so much for your detailed reply, it cleared many things for me. When I originally drew my force diagram, I tried to abstract away propeller rotation and control signal by assuming that positive control input corresponds to forward direction as you have suggested, and as also what ArduSub does. Then looking at the thesis force diagram completely threw me off Again, thanks for the excellent explanation!

Based on how ArduSub assigns forces (positive input - forward direction), I modified the thrust configuration matrix in Eq 4.61. Then I simulated the model with constant positive input to thrusters 3 and 4, and constant negative input to thrusters 1 and 2, thus simulating forward movement of the ROV along the x-direction. All the vertical thrusters were at zero inputs. The initial depth of the ROV was 50m and it would rise slowly because of buoyancy in the absence of vertical thrust. What I observed is that the with only forward movement, ROV pitches down slightly with small oscillations around that average negative pitch. This is counterintuitive to me since I was expecting to see a slightly positive pitch angle since horizontal thrusters are slightly below the CG (as per the model in the thesis) and because of that they would produce a positive pitching moment. Does the physical ROV pitch up or down in water moving forward without any vertical thrust? As the control input increases beyond 30%, ROV basically becomes unstable and just keeps pitching extremely steeply, almost in place. I wonder if this is the expected behavior and you see large pitch angles on the real ROV with only horizontal thruster inputs for forward motion. Or if my thrust configuration is still messed up and if I am still not clear about it.

In order to figure this out I looked at SIM_Submarine code (I have not run the sim yet) and realized that the model used there is substantially different in many details. Is there any documentation about the model used in SIM_Submarine? Does SIM_Submarine exhibit any such pitch behavior as I am seeing?

As the control input increases beyond 30%, ROV basically becomes unstable and just keeps pitching extremely steeply, almost in place. I wonder if this is the expected behavior and you see large pitch angles on the real ROV with only horizontal thruster inputs for forward motion. Or if my thrust configuration is still messed up and if I am still not clear about it.

Thank you so much for a very detailed reply to my post; that helped immensely. And sorry for the late reply. I did some digging and found a paper that was referenced in the SITL+Gazebo sim forum (freefloating gazebo plugin).

The paper mentioned that since most values in the added mass matrix Ma and the Ca matrix are calculated empirically, they can be substantially wrong. As such, the freefloating Gazebo plugin disregards those matrices in the vehicle dynamics. I followed that paper and removed Ma and Ca matrices from the BlueROV2 Heavy model. Since then the model has been working very well.

I'm a user of krpano for years, but I wanted to upgrade my workflow and user experience of virtual tours. I am an interior designer and until now, I'm rendering my panoramic images in 3DS max with vray, putting the images together in krpano tools - create some hotspots to navigate - and watch the tour in a 3DOF headset, a browser on my desktop or mobile device.

But... as I mentioned before, I want to upgrade my workflow. I would like to make a more walktrough experience with for example the navigator Pro plugin in with a Meta Quest 2 or Pico 4. I do have 2 questions:

- At the moment I am not an Everpano user / license owner (only krpano). Is it worth the work and the money (for everpano and a new VR headset) to switch from 3DOF to 6DOF?

- As far as I know, I do not need the Everpano software for modeling, I only need it for de Navigator plugin, which feels like a waste of money. Are there alternatives to use for the experience I'm looking for?

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