Paper:
This paper presents a novel physics-inspired deep learning approach for point cloud processing motivated by the natural flow phenomena in fluid mechanics. Our learning architecture jointly defines data in an Eulerian world space, using a static background grid, and a Lagrangian material space, using moving particles. By introducing this Eulerian-Lagrangian representation, we are able to naturally evolve and accumulate particle features using flow velocities generated from a generalized, high-dimensional force field. We demonstrate the efficacy of this system by solving various point cloud classification and segmentation problems with state-of-the-art performance. The entire geometric reservoir and data flow mimics the pipeline of the classic PIC/FLIP scheme in modeling natural flow, bridging the disciplines of geometric machine learning and physical simulation.

Review:
The paper is about using classical PIC/FLIP scheme in Computational Fluid Dynamics for solving the learning problem of 3D object detection and segmentation. In general, there are extrinsic CNNs like the Vox net etc. which look for global features which the authors refer to as Eulerian formulation of the data representation, and there are intrinsic CNNs like the GCN(graph convolutions), Point nets etc. which look for localized neighborhood information which the authors refer to as Lagrangian formulation. The authors acknowledge that hybridizing the extrinsic CNNs and intrinsic CNNs is not new and several works are cited. The key contribution is to look at this problem from the perspective of PIC/FLIP scheme which has been used in CFD for decades. 
The idea is very nice, well describes and quite novel in my opinion. I really liked the adoption of classical CFD approaches in learning. This provides a very interesting perspective to 3D deep learning. 
However, the papers struggles to demonstrate why the 3D deep learning community would adopt this approach. The results are not that conclusive. The algorithm works (understood well from the ablation study). However, the performance of the proposed approach is at best comparable to some of the state-of-the-art methods such as PointCNN or SE-Net. The authors need to clarify what potential advantages could there be other than accuracy (if any) such that the community uses the proposed method. 
Also, the grids used in the study are too low to make any conclusive remarks on what happens when dealing with higher resolutions of grid. The authors themselves acknowledge the limitation of not being able to go higher in resolution of grid. Interestingly, such limitations of CFD has recently motivated the community to explore deep learning based fast and agile surrogates for computationally tractable approaches.  
Some of the new works in 3D object recognition and segmentation such as  Deep SDF(https://arxiv.org/abs/1901.05103), AtlasNet(https://arxiv.org/abs/1802.05384),  Deep Level Sets (https://arxiv.org/abs/1901.06802), occupancy networks (https://arxiv.org/pdf/1812.03828v1.pdf), http://openaccess.thecvf.com/content_cvpr_2018/html/Yu_PU-Net_Point_Cloud_CVPR_2018_paper.html, Adaptive O-CNN (https://dl.acm.org/citation.cfm?id=3275050), https://arxiv.org/abs/1805.12254, 3D Point Capsule Networks, http://t.cvlibs.net/publications/Niemeyer2019ICCV.pdf etc. can be compared with or at least contrasted in the related works.
In summary, I really liked the algorithmic idea, but skeptical about its practical relevance from the results.