Real-time simulation and visualization techniques for combustion processes and general fluids Marek Gayer (supervisor Pavel Slavík) C omputer G raphics G roup Department of Computer Science and Engineering Faculty of Electrical Engineering Czech Technical University in Prague The defense of the PhD thesis April, 27, 2006 Prague, Czech Republic

2CTU 2006 Outline of the presentation Introduction and motivation for visualization of dynamic processes and objectives of the thesis Presentation of major thesis contributions –Acceleration of simulation –Higher quality of images achieved by the use of GPU –Interactive visualization of dynamic processes with tree based data structures  Fluid Simulator States (FSS)  FSS Tree  Unsteady Data Sets (UDS) Tree Conclusion and future work Live demonstration of the dynamic visualization

3CTU 2006 Thesis objectives Design and implement techniques that together could be used to form a solution allowing interactively visualize dynamic processes – namely for pulverized coal combustion processes in real time (2D, namely for education). Contributions of the thesis should be reusable in general projects regarding fluids No ambitions for reaching precision or compete professional combustion software

4CTU 2006 Objective: Interactive visualization of dynamic processes (e.g. fluid processes) Understanding of a dynamic process is possible only with a dynamic (real-time) and interactive visualization Dynamic processes are described by complex differential equations (slow to compute) Combustion processes which our thesis deals with is a use case of solving this problem

5CTU 2006 Introduction and motivation to coal combustion modelling and visualization Both for the ecological and economical reasons Finding optimal boiler configurations –To reduce pollution –Combustion optimization –To find a way for optimal fuel preparation How can visualization help

6CTU 2006 Modelling and visualization of combustion by existing CFD software Most often: solving complex differential equations (e.g. Navier-Stokes) Coal combustion as a CFD application Current solutions and systems: Precise, robust Slow, complex, expensive, non real- time => unsuitable e.g. for education (ANSYS, Fluent => 1 frame = several minutes - hours) + -

7CTU 2006 Thesis contributions leading to dynamic, real-time visualization of dyn. processes 1. Speeding up the computation of data for consecutive visualization 2. Use HW acceleration for quality yet fast visualization of the data 3. Pre-calculate parts of simulation, then run simulation accelerated so dynamic real-time visualization can be achieved

8CTU 2006 Problem 1: The classical simulation of combustion by existing solvers is slow Solution / contribution of the thesis: –Fast fluid simulator based on Euler and continuity equation allowing real- time interactivity –Original virtual coal particle system using simplified combustion model, suitable for both the simulation and visualization

9CTU 2006 The Fluid Simulator Dividing boiler area to structured grid cell arrays containing: –Velocities –Masses/Pressures –O 2 concentrations –Temperatures State update Principle of local simulation

10CTU 2006 Virtual coal particle system Used for both simulation and visualization of the combustion process Virtual particle system approach Simplified combustion and heat transfer computation

11CTU 2006 Sample visualization - cell characteristics

12CTU 2006 Sample visualization – coal particles

13CTU 2006 Problem 2: How to visualize simulated data fast and with good quality? Solution / contribution of the thesis: –High quality bicubic spline contours visualization –Non-jagged isolines –Computed parallel by pixel or vertex shaders on GPU’s of the current graphics accelerators

14CTU 2006 Real-time Visualization with Bicubic Spline Patches Interpolation Better quality then linear Complex computations with 4x4 and 4x1 vector matrices (2x) Non real-time when not accelerated – (graphics hardware natively supports only linear interpolation) Our contribution utilizing Modern GPU’s paralell processing gains real-time performance

15CTU 2006 Utilization of GPU shaders Vertex & fragment programs (shaders) Works on common current mainstream graphics hardware Texture mapping interpolation core running on GPU (high quality per-pixel / per-vertex) Optional per-vertex subdivision - pixels between vertices are linear interpolated

16CTU 2006 Side by side comparison with linear texture interpolation

17CTU 2006 Non-Jagged Isolines (contour lines) Simple modification of mapping textures: Simple, fast (no additional computations at all!) Smooth, non jagged Blurry on certain places with low gradients + - +

18CTU 2006 Problem 3: How to gain real-time visualization even with slow simulation? Contribution / solution of the thesis: –Fluid Simulator States –Fluid Simulator States Tree –Unsteady Data Sets Tree

19CTU 2006 Storing results for real-time replaying Used when computation is too slow for real-time simulation and visualization Results are stored on hard disk, then real-time replayed –Data sets for selected characteristics –AVI and MPEG files –Limited interaction Our contribution: HIERARCHICAL TREES OF PRE- COMPUTED DATA STRUCTURES (FSS, UDS Trees)

20CTU 2006 Extending Fluid Simulator with Fluid Simulator States (FSS) Simulation is divided into two phases: –Storing phase - fluid simulator states for each time step are saved on HDD –Replaying phase - simulation runs accelerated with pre-computed fluid simulator states; we can interactively set parameters of visualization Except first frame, no other data are saved (e.g. particles, characteristics) State files are stored in binary files

21CTU 2006 Simulation system architecture

22CTU 2006 First frame (complete state) Second Frame. Collection of cascading FSS states for accelerated datasets generation... Frame N - 1 Last frame (complete state)

23CTU 2006 Unsteady simulation datasets (UDS) Stores one or more characteristics for selected time part Can allow real-time replaying of results Full interactivity in visualization part After storing, no interactivity in simulation part possible No additional changes to already computed data and configuration

24CTU 2006 Detailed comparison against data sets Store method / Grid size FSS / 20*40 FULL / 20*40 FSS / 50*100 FULL / 50*100 Simulation time1214s1230s5128s5133s Write [MB/s] Replay time627s603s816s864s Read [MB/s] AVG Fps Disk space GB Total accelerationx 1.9x 2.0x 6.2x 5.9

25CTU 2006 Feature comparison of FSS against common unsteady data sets (UDS) Much less disk requirements (only fluid simulator states are being saved) Lower disk bandwidth Better scalability for large grids and/or tasks with many particles Same or even better acceleration resulting in better interactivity No seeking and skip frame ability

26CTU 2006 Forming data to tree cluster structure

27CTU 2006 Changing simulation parameters in each of the tree node

28CTU 2006 Comparison of FSS and UDS tree advantages against direct simulation Can run in orders faster Seeking and skip frame ability (UDS) Creating hierarchy of replay-able results Incremental step-by-step solution Constructing of paths of solution Interactivity is allowed only in the nodes of the tree UDS requires disk bandwidth and capacity

29CTU 2006

30CTU 2006 Reusability - possible applications Originally used for our fluid application for combustion processes Parts can be reused for other simulation and visualization applications –Fluid simulator –Contours visualization E.g.: simulation and animation of liquids, water and gaseous phenomena

31CTU 2006 Conclusion of major contributions Fast data generator for dynamic visualization –Fast & simple real-time fluid simulator –Particle system with simplified combustion engine –Interactive changes to the model during run Quality contours and contour lines visualization using pixel or vertex shaders on GPU Real-time data based structures for interactive visualization and accelerated simulation –Fluid Simulator States –Fluid Simulator States Tree –Unsteady Data Sets Tree

32CTU 2006 In general, create a more precise computational model or enhance and extend current equations Enhancements to the combustion and thermodynamics engine and fluid simulator Convert the whole project to 3D Create 3D FSS and UDS Trees Create more user-friendly user interface Dynamic visualization of other phenomena's Possible future research and work

33CTU 2006 The awards for the thesis contributions (fluid simulator, virtual particle system) 2003, CTU FEE at Prague Award of Dean of the Faculty of Electrical Engineering of the Czech Technical University in Prague for work Simulation and Visualization of Combustion Powered by Fluid Simulator, presented at conference CTU Poster , Brno University of Technology, Faculty of Electrical Engineering and Communication (BUT FEEC) - Award of Dean for the best work in the International Competition of student creative projects Student EEICT , CTU in Prague Price of the rector for placing between the best projects of PhD students, that were solved within scope of CTU internal grants and that were presented on conference CTU Workshop 2003

34CTU 2006 Thank you for your attention. ?????? Do you have any questions ?

35CTU 2006 Question 1 (doc. Pavel Zemčík) The presented thesis does contain description of a novel “simplified” fluid and combustion modeling/simulation method (Chapter 5); it also contains comparison of the results of the method with other simulation system and from that comparison it seems that the simulation results of the proposed method are not very precise (some of the global values are over 20% different). Given these facts, is the proposed modeling/simulation method still usable for investigation of e.g. the airflow?

36CTU 2006 Reply 1(Precision of the model) The level precision = weak part, because: Price for the interactive visualization; instead of 1 picture / minutes => several pictures / seconds Combustion experts from P.E. of faculty of M.E.CTU: for overview of dynamics is OK Not for professional design By further improvement and adding some equations could approach reality better. When using it for airflow only => less errors; no dynamic combustion code

37CTU 2006 Question 2 (doc. Pavel Zemčík) Has the proposed combustion process visualization system been evaluated from the point of view of “accuracy” of perception of the process by humans? Do any visualization parameters (e.g. way of shading of the particles as shown in Figure 10-2 versus 10-3, color assignment, etc.) affect the “understandability” of the image by humans?

38CTU 2006 Reply 2 (Particle visualization) For the grid, we use visualization methods, that are standard for other combustion systems (our offers better quality and performance) Particles - we have many possibilities to display –Using various palettes and colors (with scales) –Size of the particle, particle tracks –We can even use same methods as for the cells –Statistics Moving particles and particle tracks display important visualization parameter – the dynamics No tests of accuracy of perception of the users No user complaints regarding understandability Possitive feedback from student users

39CTU 2006 Question 3 (doc. Pavel Zemčík) The precision of the simulation process generally depends on the integration (simulation) time step. Does the proposed real-time simulation in general achieve optimum precision from this point of view? If not, does the system have any information about the precision of simulation (e.g. “now the simulation is too complex and to achieve real-time performance, the precision has been given up…”)?

40CTU 2006 Reply 3 (Time steps and precision) To achieve fast performance, we are always choosing the lowest time steps as possible The timestep is set manually Increased timestep, the results slightly more precise in most characteristics, but still the difference between the Fluent not consid. better The greater precision should be achieved by enhancing combustion and heat transfer equations, or replace fluid simulator with another one and use our proposed accelerating structures to keep it real-time. Or use some precise simulator as data source

41CTU 2006 Q4 - How the 2d results can be composed into a 3d visualization system ? (doc. Andrej Ferko) The 2D results represent the 3D boiler slice with selectable depth of the slice By linear interpolation, we can divide the voxels & convert it to another grid Alternatively, values of several slices of boiler could be computed and composed to display We can use existing visualization systems and methods for visualization (e.g. volume rendering, flow visualization methods spot noise, LIC) Last possibility – re-implement to 3D