Spurious Separation Research Articles

Unsteady Lattice Boltzmann Simulations of Corner Separation in a Compressor Cascade

Abstract Lattice Boltzmann simulations of corner separation flow in a compressor cascade are presented. The lattice Boltzmann approach is rather new in the context of turbomachinery, and the configuration is known to be particularly challenging for turbulence modeling. The present methodology is characterized by a quasi-autonomous meshing strategy and a limited computational cost (a net ratio of 5 compared to a previous finite-volume compressible Navier–Stokes simulation). The simulation of the reference case (4 deg incidence) shows a good agreement with the experimental data concerning the wall pressure distribution or the distribution of losses. A good description is also obtained when incidence angle is increased to 7 deg, with a span-wise development of the separation. Subsequently, the methodology is used to investigate the sensitivity of the flow to the end-wall boundary-layer thickness. A thinner boundary-layer results in a smaller corner separation, but not a complete elimination. Finally, the ingredients of the wall modeling are analyzed in details. On the one hand, the curvature correction term promotes transition to turbulence on the blade suction side and avoids a spurious separation. On the other hand, the addition of the pressure-gradient correction term allows a wider and more realistic corner separation.

Cross-validated Between Group PCA Scatterplots: A Solution to Spurious Group Separation?

Between group PCA (bgPCA) has been developed to summarize group differences in high dimensional spaces like in geometric morphometrics and microarray data where the number of variables is often larger than sample size. However, it has been very recently shown that this technique inflates apparent differences as seen in scatterplots and, in extreme cases, can even create differences where there are none, an effect that becomes more exaggerated as dimensionality increases. In this study, we explore whether leave-one-out cross-validated scatterplots, in which cross-validated scores are used to construct the final ordination instead of the conventional ones, can mitigate the issue. Using simulated data with both isotropic variation or covariance, and increasing the number of variables, we show that cross-validated bgPCs reduce but do not completely remove the distortion of mean differences. However, although scatterplots might still depict inaccurate relationships between group means and must therefore be interpreted with great caution, cross-validation largely solves the issue of spurious separation. Thus, cross-validated bgPCA offers a big improvement for faithfully summarizing overlap or separation among groups in high dimensional spaces and its results will be largely consistent with distance-based permutation tests of significance for group mean differences in the full data space.

Subsampling reveals that unbalanced sampling affects Structure results in a multi-species dataset

Studying the genetic population structure of species can reveal important insights into several key evolutionary, historical, demographic, and anthropogenic processes. One of the most important statistical tools for inferring genetic clusters is the program STRUCTURE. Recently, several papers have pointed out that STRUCTURE may show a bias when the sampling design is unbalanced, resulting in spurious joining of underrepresented populations and spurious separation of overrepresented populations. Suggestions to overcome this bias include subsampling and changing the ancestry model, but the performance of these two methods has not yet been tested on actual data. Here, I use a data set of 12 high-alpine plant species to test whether unbalanced sampling affects the STRUCTURE inference of population differentiation between the European Alps and the Carpathians. For four of the 12 species, subsampling of the Alpine populations-to match the sample size between the Alps and the Carpathians-resulted in a drastically different clustering than the full data set. On the other hand, STRUCTURE results with the alternative ancestry model were indistinguishable from the results with the default model. Based on these results, the subsampling strategy seems a more viable approach to overcome the bias than the alternative ancestry model. However, subsampling is only possible when there is an a priori expectation of what constitute the main clusters. Though these results do not mean that the use of STRUCTURE should be discarded, it does indicate that users of the software should be cautious about the interpretation of the results when sampling is unbalanced.

I ain't got no body: Developmental psychology must be embodied and enactive, as well as “social”

Although we agree with the authors' criticism of the reigning approach to children's sociocognitive development, we raise three further issues. First, “mind talk” is not, in fact, any different from the other aspects of the social world about which children learn. Second, there is no choice between either the “single mind” or the “social context.” Finally, there is a spurious separation between organism and environment.

Electronic symmetry breaking in polyatomic molecules. Multiconfiguration self-consistent field study of the cyclopropenyl radical C3H3

For equilateral triangle geometries (point group D3h), the C3H3 radical has a degenerate 2E″ electronic ground state. Although the 2A2 and 2B1 components separate in energy for C2v geometries, these two components should have identical energies for equilateral triangle structures. In fact, when approximate wave functions are used and the orbitals not required to transform according to the D3h irreducible representations, an energy separation between the 2A2 and 2B1 components is observed. At the single configuration self-consistent field (SCF) level of theory this separation is 2.8 kcal with a double-zeta basis set and 2.4 kcal with double-zeta plus polarization. It has been demonstrated that this spurious separation may be greatly reduced using multiconfiguration self-consistent field (up to 7474 variationally optimum configurations) and configuration interaction (up to 60 685 space and spin adapted configurations) techniques. Configurations differing by three and four electrons from the Hartree–Fock reference function are found necessary to reduce the 2A2-2B1 separation to below 0.5 kcal.

Compressible turbulent boundary layer with pressure gradient and heat transfer.

a = C f = Cp = g = h = H = Hi = Htr = The compressible turbulent boundary layer is expressed in terms of the momentum integral and moment-of-momentum integral equations for arbitrary pressure gradient. Upon simpli- fication of this equation set with a Mager-type transformation, the two coupled differential equations are solved simultaneously to provide boundary-layer momentum thickness, dis- placement thickness, skin-friction coefficient, and related quantities as a function of the body surface coordinate. The analysis includes surface heat-transfer effects. Results of the analysis are presented for adiabatic, non-adiabatic, two-dimensional, and axisymmetric flow- fields in a Mach number range between 2 and 8. Flows with favorable and adverse pressure gradients are also examined. The present analysis is compared with available experimental and theoretical results. The analysis is found to yield satisfactory results for flows with and without heat transfer. In addition, for flows in the pressure of large adverse pressure gradients in which the boundary layer is known to be well-behaved, spurious separation is not indi- cated; instead, rather good agreement with available experimental data is observed.