Leandro Franco de Souza

The temporal and spatiotemporal linear stability of a viscoelastic planar jet flow is investigated for the linear (LPTT), quadratic (QPTT), and exponential (EPTT) variants of the Phan-Thien–Tanner constitutive model. The baseflow follows a hyperbolic-tangent velocity profile, with viscoelastic base-state stresses computed numerically via a Runge–Kutta marching procedure. The linearized equations are discretized using a Chebyshev pseudospectral collocation method and cast into a generalized eigenvalue problem, whose predictions are verified against high-order numerical simulations with relative differences below 1.43%. For the temporal analysis, the sinuous mode is found to be more unstable than the varicose mode in all cases. The three PTT variants exhibit distinct sensitivities to the extensibility parameter ε, and the slip parameter ξ: increasing ε produces nearly no change in growth rate for the LPTT model, a monotonic decrease for the QPTT model, and a non-monotonic response for the EPTT model, while increasing ξ is consistently destabilizing across all models. Spatiotemporal analysis reveals that both ε and ξ suppress absolute instability, with the EPTT model being the most sensitive to this suppression. The ordering of the models in terms of the extent of the absolutely unstable parameter region is LPTT > QPTT > EPTT models, with the Oldroyd-B model limit (ε=ξ=0) yielding the largest absolutely unstable region, this ordering is explained by the progressively stronger attenuation of base-state elastic stresses by the linear, quadratic, and exponential stress functions, respectively. A critical Weissenberg number map in the β×Wi plane is constructed for each model, providing a theoretical criterion for predicting the convective-to-absolute instability transition in PTT jet flows.

The incompressible flow around a circular cylinder, positioned at the center of an unconfined planar counterflow, is studied by means of numerical solutions of the conservation equations and linear stability analysis. The flow is completely defined by the Reynolds number (Re)—based on the cylinder radius, the strain rate defining the counterflow, and the kinematic viscosity. For very low values of Re, the flow is steady, two-dimensional, and fully attached to the cylinder wall. Increasing Re above Res≈16.86, the flow separates, giving rise to two symmetric, counter-rotating recirculation regions on each side of the cylinder. Further increasing Re leads to a progressive enlargement of the recirculation regions and the appearance of multiple recirculation centers, akin to Moffatt eddies. However, the convective acceleration imposed by the counterflow limits their size. An oscillatory mode becomes linearly unstable at Rec≈4146. This mode gives rise to a sinuous meandering of the wake flow on each side of the cylinder, being analogous to the well-known von Kármán instability. The frequency of this mode is directly proportional to the strain rate defining the counterflow.

Resumo O artigo apresenta uma pesquisa empírica sobre a oscilação das indenizações por danos morais ao longo do tempo em ações que discutem negativação ou protesto indevido. A pesquisa investigou se os valores das indenizações por danos morais nas sentenças dos últimos anos foram aumentados para acompanhar a inflação. Foi adotado o método quantitativo, com a coleta de sentenças de primeiro grau proferidas pelo Tribunal de Justiça de São Paulo (TJSP) entre 2014 e 2023. Utilizou-se mineração de texto (text mining) com Large Language Model (LLM) para identificar as sentenças que condenaram ao pagamento de indenização por danos morais e, em caso positivo, o valor da condenação. Os dados obtidos foram analisados por meio de ferramentas estatísticas. A média das condenações em cada ano foi comparada à variação do Índice Nacional de Preços ao Consumidor Amplo (IPCA), do salário mínimo e do Índice Geral de Preços - Mercado (IGP-M). Os resultados demonstraram que a média das indenizações sofreu pouca variação, não acompanhando a inflação. Apresentou-se como hipótese explicativa a ausência de ajuste pela inflação sobre a âncora usada nas sentenças, que seriam os valores das indenizações adotados pelo mesmo juiz em casos anteriores. Concluiu-se que as indenizações, nas sentenças de primeiro grau do TJSP, vêm perdendo valor real, recomendando-se a adoção de algum mecanismo de ajuste periódico.

A temporal linear stability analysis of three-dimensional viscoelastic fluid flow between two parallel plates is performed using the linear and exponential forms of the Phan–Thien–Tanner (LPTT and EPTT) constitutive models. The study systematically investigates the influence of key dimensionless parameters—Reynolds number (Re), Weissenberg number (Wi), viscosity ratio (βnn), spanwise wavenumber (β), elongational parameter (ε), and mobility parameter (ξ)—on the onset of flow instability. Neutral stability curves in the α−Re plane are constructed to delineate stable and unstable regions for a wide range of these parameters. The results show that, for both models, two-dimensional waves (β=0) are found to be the most unstable mode. An increase in the Weissenberg number consistently destabilizes the flow, while a higher viscosity ratio (larger solvent fraction) has a strong stabilizing effect. The influence of the elongational parameter, ε, is shown to be model-dependent, exhibiting a destabilizing effect in the LPTT model and a more complex, dual role in the EPTT model that reverses from destabilizing at low Wi to stabilizing at high Wi. The mobility parameter, ξ, is consistently destabilizing for both models. A direct comparison reveals that the LPTT model consistently predicts a larger unstable region than the EPTT model, highlighting that the choice of the constitutive stress function is a critical factor in determining flow stability characteristics.

This paper presents a methodology for estimating the pressure field from experimental velocity fields using physics-informed neural networks (PINNs). The proposed model estimates pressures directly from raw velocity data, without requiring boundary conditions or regular meshes. Manufactured solutions for a lid-driven cavity problem were employed to evaluate the model's performance, demonstrating reasonable accuracy and minimal dependence on data refinement. Time-averaged vector velocity fields of single-phase flows across a tube bundle, obtained through spatial filter velocimetry, were applied to predict the pressure field from real experimental data. The results confirmed that convective effects dominate the momentum transport equations in tube bundles, and the effects of flow separations became more pronounced as the Reynolds number increases. While inherent uncertainties in the experimental data led to higher deviations compared to the manufactured solutions, data refinement and geometry had a minimal impact on the model's convergence. The proposed method highlights the high versatility of PINNs in pressure field estimation compared to traditional numerical methods, enhancing the extraction of information from experimental data and narrowing the gap between simulations and experiments.

Detecting leaks in hermetically sealed cooling equipment is crucial for the refrigeration industry, as manufacturing defects may lead to refrigerant loss or even render cooling systems completely inoperable. Currently, in the refrigeration sector, leak verification in hermetic compressors is typically performed through manual human inspections, which can potentially cause worker fatigue and exhaustion. In this working paper, we tackle the problem of automatically detecting manufacturing defects in hermetic compressors using computer vision-based solutions. More specifically, we design an AI-based approach that integrates optical flow algorithms, image processing filtering, and neural networks to detect bubbles when the compressors are submerged in a water tank. By experimentally analyzing the leakage detection problem on preliminary real image compressor videos, our computer system was able to automatically detect failures in damaged compressors in an accurate and efficient way.

This study reports the development of a numerical method to simulate two-phase flows of Newtonian fluids that are incompressible, immiscible, and isothermal. The interface in the simulation was located and reconstructed using the geometric volume of fluid (VOF) method. The implementation of the Piecewise-Linear Interface Calculation (PLIC) scheme of the VOF method was performed to solve the three-dimensional (3D) interface transport during the dynamics of two-phase flows. In this method, the interface is approximated by a line segment in each interfacial cell. The balance of forces at the interface was accounted for using the continuum interfacial force (CSF) model. To solve the Navier-Stokes equations, meshless finite difference schemes from the HiG-Flow computational fluid dynamics software were employed. The 3D PLIC-VOF HiG-Flow algorithm was used to simulate several benchmark two-phase flows for the purpose of validating the numerical implementation. First, the performance of the PLIC implementation was evaluated by conducting two standard advection numerical tests: the 3D shearing flow test and the 3D deforming field test. Good agreement is obtained for the 3D interface shape using both the 3D PLIC-VOF HiG-Flow algorithm and those found on the scientific literature. In addition, the absolute error of the volume tracking advection calculation obtained by our 3D PLIC-VOF HiG-Flow algorithm was found to be smaller than the one found in the scientific literature for both the 3D shearing and 3D deforming flow tests. Lastly, the 3D bubble rising problem was simulated for different fluid densities and viscosity’s ratios. Again, good agreement is obtained for the 3D interface shape using both the newly implemented algorithm and the OpenFOAM, DROPS and NaSt3D software. The bubble’s rise velocity and evolution of the bubble’s center of mass is also computed with the 3D PLIC-VOF HiG-Flow algorithm and found to be in agreement with these software.