Clustering Kelvin Helmholtz Instabilities¶
Pipeline description¶
This example illustrates the capabilities of TTK to perform advanced statistical analysis of collections of datasets, based on their structural representations, along with the possibility to interactively explore the outcome of the analysis, with linked views (between the selection in the planar view  top right  and the flow visualization  top left).
This example considers an ensemble of 32 periodic, 2D Kelvin Helmholtz Instabilities in computational fluid dynamics, obtained with various simulation parameters (different solvers, different numerical schemes, different interpolation orders, etc.). The scalar field of interest is the "Enstrophy". It is an established measure of vorticity. Its prominent maxima denote the center of strong vortices. Two example members from the ensemble are show on the above screenshot (left). Strong vortices can be visualized with the dark green regions. The simulation parameters as well as the ground truth classification are provided as metadata for each entry of the database and are carried along the entire pipeline. See this publication for further details.
The goal of this example is to classify the 32 members of the ensemble into two classes, whether they describe the beginning or the end of the turbulence. This task is particularly challenging for traditional clustering pipelines since turbulent flows are highly chaotic and two flows belonging to the same ground truth class can be drastically different visually (as shown on the above screenshot  left). The common denominator between two turbulent flows in the same ground truth class is the distribution of energies of their vortices (i.e. the number and strengths of their vortices), which describes the turbulence of the flow.
In this context, topological data representations are particularly relevant to extract such subtle structural features. In particular, the persistence diagram involving the saddlemaximum pairs of the "Enstrophy" (second column, above screenshot) nicely captures the number of vortices as well as their individual strengths. Thus, in the reminder of this example, we will use this persistence diagram as a descriptor of each turbulent flow and we will proceed to a kmeans clustering directly in the Wasserstein metric space of persistence diagrams. For visualization purposes, we will compute a 2D layout of the ensemble (right most columns, above screenshot) to inspect the resulting classification.
First, the database of turbulent flows is loaded from disk with the CinemaReader module (line 6 of the Python script below). Then an SQL query is performed with CinemaQuery to select a relevant subset of this database (line 9). Finally the module CinemaProductReader is used to read the actual regular grids corresponding to the result of the previous SQL query. From this point on, the entire set of 32 turbulent flows will be organized as a vtkMultiBlockDatSet and each of these 32 members will be processed by the rest of the analysis pipeline.
Then for each of the 32 members of the ensemble, the first step consists in marking periodicity boundary conditions with the TriangulationManager (line 21). Next, the "Enstrophy" field of each member is normalized (between 0 and 1) with the ScalarFieldNormalizer to ease their comparison later. Finally (line 28) the PersistenceDiagram is computed (for the saddlemaximum pairs) to represent each of the 32 ensemble members by a diagram which encodes the number and the strengths of the vortices via the persistence of the maxima of "Enstrophy".
Next, the clustering of the persistence diagrams in the Wasserstein metric space is performed with the module PersistenceDiagramClustering (line 32).
For visualization purposes, we will then compute a 2D layout of the ensemble, where each ensemble member will be represented by a point and where the 2D distance between 2 points will encode the Wasserstein distance between their diagrams. This will provide an intuitive planar overview of the ensemble. For this, we will first compute a matrix of Wasserstein distances with the module PersistenceDiagramDistanceMatrix (line 39). The resulting distance matrix is visualized at the bottom of the middle column in the above screenshot. There, it can be seen that the Wasserstein distance already identifies two major clusters (large blue submatrices of low Wasserstein distances). Next (line 67), the module DimensionReduction is used to compute a 2D layout via multidimensional scaling. Finally, the resulting table is turned into a 2D point cloud which is ready to be visualized with TableToPoints (line 75). Then, the output is stored to a simple CSV file (line 81).
In the above screenshot, the resulting point cloud is shown in the 2 views at
the bottom right corner of the screenshot. The first view (left) shows the
point cloud colored by cluster identifier computed by the pipeline. The second
view (right) show the same point cloud, colored by the ground truth class.
There, one can directly visualize that the two classifications are identical
and that, therefore, this topological clustering pipeline succeeded.
For reference, a traditional pipeline based on the L2distance between the "Entrophy" fields is also provided in this example. For that, the module LDistanceMatrix is used (line 45) to compute a matrix of the L2 distances between each scalar field of the ensemble. The resulting distance matrix is visualized at the top of the middle column in the above screenshot. There, it can be seen that the L2 distance between the scalar fields fails at identifying any clear clusters (there are no large blue submatrices). Next (line 49), the module DimensionReduction is used to compute a 2D layout via multidimensional scaling. The resulting table is turned into a 2D point cloud with TableToPoints (line 55). Finally, the kmeans algorithm is run on this 2D point cloud with the module KMeans. Then, the output is stored to a simple CSV file (line 83). The resulting clustering can be visualized with the two views at the top right corner of the above screenshot. The groundtruth classification is provided by the color coding of the points in the second view (right) while the classification computed with this traditional pipeline is shown in the first view (left). There, it can bee seen that the coloring of the two point clouds differ, indicating an incorrect classification by the traditional kMeans algorithm. In particular, since all the metadata associated with each ensemble member travels down the analysis pipeline, one can select points in these planar views to inspect the corresponding datasets and persistence diagrams (left two columns of the screenshot). In particular, two members (red and yellow spheres) incorrectly marked as belonging to different classes are visualized on the left. There, one can see that although the two flows have the same "profile" of vortices (number and strengths), these are located in drastically different spots of the field, due to the chaotic nature of turbulence, hence explaining the reason of failure of traditional clustering pipelines.
ParaView¶
To reproduce the above screenshot, go to your ttkdata directory and enter the following command:
paraview states/clusteringKelvinHelmholtzInstabilities.pvsm
Python code¶
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To run the above Python script, go to your ttkdata directory and enter the following command:
pvpython python/clusteringKelvinHelmholtzInstabilities.py
Inputs¶
 khi.cdb: a cinema database containing 32 regular grids describing periodic, 2D Kelvin Helmholtz Instabilities (computational fluid dynamics). The scalar field of interest is the "Enstrophy" (an established measure of vorticity). The simulation parameters as well as the ground truth classification (two classes: beginning or end of the turbulence) are provided as metadata for each entry of the database and are carried along the entire pipeline. See this publication for further details.
Outputs¶

W2clusteringAndW2dimensionReduction.csv
: 2D point cloud representing the input ensemble (1 line = 1 member of the ensemble). The fieldClusterId
denotes the class identifier computed with a kmeans clustering (with k=2) directly performed in the Wasserstein metric space of persistence diagrams. After that, the 2D layout of the points is computed by multidimensional scaling of the matrix of Wasserstein distances between persistence diagrams. With this technique, the output classification perfectly matches the groundtruth classification. 
L2dimensionReductionAndClustering.csv
: 2D point cloud representing the input ensemble (1 line = 1 member of the ensemble). The fieldClosestId(0)
denotes the class identifier computed with a standard kmeans clustering (with k=2) obtained after a 2D projection of the point cloud. In particular, the 2D layout of the points is computed by multidimensional scaling of the matrix of L2 distances between the Enstrophy fields. With this technique, the output classification does not match the groundtruth classification.