Transient Growth Research Articles

Suppression of the tonoplast sugar transporter, StTST3.1, affects transitory starch turnover and plant growth in potato.

Transitory starch and vacuolar sugars function as highly dynamic pools of instantly accessible metabolites in plant leaf cells. Their metabolic regulation is critical for plant survival. The tonoplast sugar transporters (TSTs), responsible for sugar uptake into vacuoles, regulate cellular sugar partitioning and vacuolar sugar accumulation. However, whether TSTs are involved in leaf transient starch turnover and plant growth is unclear. Here, we found that suppressing StTST3.1 resulted in growth retardation and pale green leaves in potato plants. StTST3.1-silenced plants displayed abnormal chloroplasts and impaired photosynthetic performance. The subcellular localization assay and the oscillation expression patterns revealed that StTST3.1 encoded a tonoplast-localized protein and responded to photoperiod. Moreover, RNA-seq analyses identified that starch synthase (SS2 and SS6) and glucan water, dikinase (GWD), were downregulated in StTST3.1-silenced lines. Correspondingly, the capacity for starch synthesis and degradation was decreased in StTST3.1-silenced lines. Surprisingly, StTST3.1-silenced leaves accumulated exceptionally high levels of maltose but low levels of sucrose and hexose. Additionally, chlorophyll content was reduced in StTST3.1-silenced leaves. Analysis of chlorophyll metabolic pathways found that Non-Yellow Coloring 1 (NYC1)-like (NOL), encoding a chloroplast-localized key enzyme that catalyzes the initial step of chlorophyll b degradation, was upregulated in StTST3.1-silenced leaves. Transient overexpression of StNOL accelerated chlorophyll b degradation in tobacco leaves. Our results indicated that StTST3.1 is involved in transitory starch turnover and chlorophyll metabolism, thereby playing a critical role in normal potato plant growth.

Age-specific sensitivity analysis of stable, stochastic and transient growth for stage-classified populations.

Sensitivity analysis in ecology and evolution is a valuable guide to rank demographic parameters depending on their relevance to population growth. Here, we propose a method to make the sensitivity analysis of population growth for matrix models solely classified by stage more fine-grained by considering the effect of age-specific parameters. The method applies to stable population growth, the stochastic growth rate, and transient growth. The method yields expressions for the sensitivity of stable population growth to age-specific survival and fecundity from which general properties are derived about the pattern of age-specific selective forces molding senescence in stage-classified populations.

A data-driven optimal disturbance procedure for free-stream turbulence induced transition

The investigation of free-stream turbulence induced transition by means of simple and effective numerical methods traditionally represents a major challenge in the aerodynamic field. In this work, a data-driven algorithm aimed at obtaining optimal forcing and response concerning free-stream turbulence induced boundary layer transition is introduced. The method, referred to as Data-driven Optimal Disturbance (DOD) in the following, relies on dynamic mode decomposition to compute the linear matrix responsible for disturbance evolution in the streamwise direction and opens the possibility for optimal disturbance analysis in an equation-free manner. The procedure has been applied to high-fidelity large-eddy simulation (LES) results concerning zero pressure gradient flows. Four different combinations of turbulence intensity Tu and integral length scale Lg have been adopted as boundary conditions to investigate the sensitivity of the transition route to the free-stream turbulence properties. Overall, DOD applied within the transitional region identifies highly energetic turbulent scales embedded in the free-stream as the optimal forcing inducing the formation of streaky structures within the boundary layer. Furthermore, streaky structures characterized by the same spanwise wavelength observed in the LES results are identified by DOD as the boundary layer response to the optimal forcing. Finally, the amplification of disturbances provided by DOD along the streamwise direction clearly resembles the well-established transient growth. Thus, DOD appears a useful tool to analyze the free-stream turbulence induced transition of boundary layers by a simple equation-free algorithm merely based on data analytics.

CXCR4 Promotes Bortezomib Resistance and Regulates Metabolic Reprogramming, Activating Both Glycolytic and Oxphos Pathways in Multiple Myeloma

CXCR4 overexpression in multiple myeloma (MM) correlates with aggressive features and poor prognosis. Our previous work has demonstrated that CXCR4 induces proliferation and protect MM cells from drug-induced apoptosis. In the current study we addressed the role of CXCR4-mediated signals in MM cell metabolism and drug response, using MM cell lines with low CXCR4 versus exogenously overexpressed CXCR4, and cells with acquired resistance to bortezomib (Bort) established in our laboratory. MM cells with overexpressed CXCR4 exhibited a higher expression of numerous components of the CXCR4 signaling, including pro-survival MEK and PI3K/AKT/mTOR pathways, with constitutively phosphorylated AKT, pS6, and 4E-BP1 proteins. Given the role of mTOR in glycolysis regulation, elevated levels of key enzymes including HK2, LDHA1, and PDK1 were detected in cells with up-regulated CXCR4, indicating increased glycolytic activity. Importantly, CXCR4-overexpressing MM cells demonstrated reduced responsiveness to 2-DG and DCA, known inhibitors of glycolysis. In contrast to Bort-sensitive cells, CXCR4-expressing resistant cells demonstrated only transient growth inhibition rather than apoptosis in response to inhibition of glycolysis with less mitochondrial depolarization, and growth recovery, once the glycolysis inhibitors were removed. These results suggest that MM cells with high CXCR4 expression and those resistant to Bort, are less dependent on glycolysis and may activate alternative metabolic pathways that support cell adaptation and survival. Importantly, increased mitochondrial mass, enhanced mitochondrial membrane potential, and augmented ATP production were found in both CXCR4-high and Bort-resistant MM cells, suggesting that CXCR4 can induce mitochondrial fitness and associated metabolic pathways including oxidative phosphorylation (OXPHOS) in MM. In addition to energy production, OXPHOS coupled with electron transport chain (ETC) generates reactive oxygen species (ROS). Indeed, MM cells with high CXCR4 possessed elevated basal ROS levels. Notably, Bort treatment induced distinct molecular responses in CXCR4-low sensitive versus CXCR4-high Bort-resistant cells. In sensitive cells, Bort reduced AKT/mTOR activity with a concomitant decrease in mRNA levels of HK2 and LDHA, suggesting Bort to mediate an anti-glycolytic effect. In contrast, expression levels of glycolytic genes were not affected by Bort in resistant cells. Furthermore, Bort reduced ATP levels and mitochondrial polarization in sensitive cells, while increasing both in CXCR4-high resistant MM cells, therefore indicating a metabolic switch to OXPHOS in resistant cells upon Bort exposure. Analysis of molecular function profile revealed a distinct proteomic signature in purified mitochondria from MM cells with overexpressed CXCR4, and cells with acquired resistance to Bort, associated with enhanced expression of proteins involved in the metabolic processes and antioxidant activity. Thus, abundant proteins involved in respiratory activity were upregulated in CXCR4-high resistant cells, including ETC Complex II, III and IV subunits. Furthermore, an increase in mitochondrial ribosomal proteins, metabolite transporters, proteasome subunits, outer membrane proteins involved in mitochondria stabilization, ROS detoxification proteins, and factors regulating mitochondrial dynamics (both fission and fusion) was observed in Bort resistant cells with high CXCR4. Altogether, these results suggest that CXCR4-regulated chemoresistance is driven by empowered mitochondria with increased OXPHOS, stabilized membrane, and enhanced ROS detoxifying capacity, while CXCR4 low cells are vulnerable to mitochondrial oxidative stress. In approval, using a selective and potent OXPHOS inhibitor, IACS010759 monotherapy or in combination with Bort effectively targeted Bort-resistant cells, inducing overwhelming oxidative stress and uncompensated mitochondrial damage. Collectively, our findings indicate that CXCR4-mediated chemoresistance in MM cells involves AKT/mTOR signaling, promoting both glycolysis and mitochondrial OXPHOS, and enabling metabolic plasticity with OXPHOS shift upon Bort treatment. These results delineate the novel mechanisms in CXCR4 activity and suggest OXPHOS as a potential target for therapeutic intervention in MM. Figure 1View largeDownload PPTFigure 1View largeDownload PPT Close modal

Long-Term Pulmonary Dysfunction by Hyperoxia Exposure during Severe Viral Lower Respiratory Tract Infection in Mice.

Viral-induced lower respiratory tract infection (LRTI), mainly by respiratory syncytial virus (RSV), causes a major health burden among young children and has been associated with long-term respiratory dysfunction. Children with severe viral LRTI are frequently treated with oxygen therapy, hypothetically posing an additional risk factor for pulmonary sequelae. The main goal of this study was to determine the effect of concurrent hyperoxia exposure during the acute phase of viral LRTI on long-term pulmonary outcome. As an experimental model for severe RSV LRTI in infants, C57Bl/6J mice received an intranasal inoculation with the pneumonia virus of mice J3666 strain at post-natal day 7, and were subsequently exposed to hyperoxia (85% O2) or normoxia (21% O2) from post-natal day 10 to 17 during the acute phase of disease. Long-term outcomes, including lung function and structural development, were assessed 3 weeks post-inoculation at post-natal day 28. Compared to normoxic conditions, hyperoxia exposure in PVM-inoculated mice induced a transient growth arrest without subsequent catchup growth, as well as a long-term increase in airway resistance. This hyperoxia-induced pulmonary dysfunction was not associated with developmental changes to the airway or lung structure. These findings suggest that hyperoxia exposure during viral LRTI at young age may aggravate subsequent long-term pulmonary sequelae. Further research is needed to investigate the specific mechanisms underlying this alteration to pulmonary function.

Grain growth competition during melt pool solidification — Comparing phase-field and cellular automaton models

A broad range of computational models have been proposed to predict microstructure development during solidification processing but they have seldom been compared to each other on a quantitative and systematic basis. In this paper, we compare phase-field (PF) and cellular automaton (CA) simulations of polycrystalline growth in a two-dimensional melt pool under conditions relevant to additive manufacturing (powder-bed fusion). We compare the resulting grain structures using local (point-by-point) measurements, as well as averaged grain orientation distributions over several simulations. We explore the effect of the CA spatial discretization level and that of the melt pool aspect ratio upon the selected grain texture. Our simulations show that detailed microscopic features related to transient growth conditions and solid–liquid interface stability (e.g. the initial planar growth stage prior to its cellular/dendritic destabilization, or the early elimination of unfavorably oriented grains due to neighbor grain sidebranching) can only be captured by PF simulations. The resulting disagreement between PF and CA predictions can only be addressed partially by a refinement of the CA grid. However, overall grain distributions averaged over the entire melt pools of several simulations seem to lead to a notably better agreement between PF and CA, with some variability with the melt pool shape and CA grid. While further research remains required, in particular to identify the appropriate selection of CA spatial discretization and its link to characteristic microstructural length scales, this research provides a useful step forward in this direction by comparing both methods quantitatively at process-relevant length and time scales.

On the Effect of Nucleation Undercooling on Phase Transformation Kinetics

We carry out an extensive comparison between Johnson–Mehl–Avrami–Kolmogorov (JMAK) theory of first-order phase transformation kinetics and phase-field (PF) results of a benchmark problem on nucleation. To address the stochasticity of the problem, several hundreds of simulations are performed to establish a comprehensive, statistically significant analysis of the coincidences and discrepancies between PF and JMAK transformation kinetics. We find that PF predictions are in excellent agreement with both classical nucleation theory and JMAK theory, as long as the original assumptions of the latter are appropriately reproduced—in particular, the constant nucleation and growth rates in an infinite domain. When deviating from these assumptions, PF results are at odds with JMAK theory. In particular, we observe that the size of the initial particle radius \(r_0\) relative to the critical nucleation radius \(r^*\) has a significant effect on the rate of transformation. While PF and JMAK agree when \(r_0\) is sufficiently higher than \(r^*\), the duration of initial transient growth stage of a particle, before it reaches a steady growth velocity, increases as \(r_0/r^*\rightarrow 1\). This incubation time has a significant effect on the overall kinetics, e.g., on the Avrami exponent of the multi-particle simulations. In contrast, for the considered conditions and parameters, the effect of interface curvature upon transformation kinetics—in particular negative curvature regions appearing during particle impingement, present in PF but absent in JMAK theory—appears to be minor compared to that of \(r_0/r^*\). We argue that rigorous benchmarking of phase-field models of stochastic processes (e.g., nucleation) needs sufficient statistical data in order to make rigorous comparisons against ground truth theories. In these problems, analysis of probability distributions is clearly preferable to a deterministic approach.

Dynamics of bypass transition behind roughness element subjected to pulses of free-stream turbulence

This study explores the dynamics of bypass transition of a zero pressure gradient boundary layer transitioning under the combined influence of an isolated roughness element with pulses of free-stream turbulence (FST). We consider a hemispherical roughness element placed over a flat plate, while the pulses of FST are introduced at the inlet, which is in contrast to continuous FST largely explored in the literature. For a fixed turbulence intensity and length scale, a series of eddy-resolving simulations are carried out to examine the effect of varying the pulsing frequency of FST. The flow behind the roughness element remains stable in the absence of FST for the subcritical Reynolds number Rek = 400 considered in this study. We observe that with the pulses of FST, the transition is triggered due to the interaction of the FST-induced Klebanoff streaks with the roughness-induced streamwise vortices. With an increase in the frequency of FST pulses, the boundary layer has less time to relax to its unperturbed state resulting in an earlier onset of transition. The transition onset predicted is in favorable agreement with the correlations proposed in the literature. We analyze the growth of disturbance kinetic energy, the shape of secondary instabilities over the streaks, and their phase speeds in detail. The FST pulse convecting over the roughness element triggers the inner varicose modes in its near-wake region. The varicose modes decay rapidly further downstream and the well-known sinuous instabilities (or the outer modes) trigger transition via transient growth associated with convective instabilities. Such clear identification of the sinuous and varicose instabilities is not usually observed in cases with continuous FST, highlighting the importance of our study in applications involving transition under intermittent turbulence.

Chemical and Microstructural Nanoscale Homogeneity in Superconducting YBa2Cu3O7-x Films Derived from Metal-Propionate Fluorine-free Solutions.

Research involved in developing alternative energy sources has become a necessity to face global warming. In this context, superconductivity is an appealing solution to enhance clean electrical energy provided that lower production costs can be attained. By implementation of chemical solution deposition techniques and high-throughput growth methods, low-cost nanostructured epitaxial cuprate superconductors are timely candidates. Here, we present a versatile and tunable solution method suitable for the preparation of high-performance epitaxial cuprate superconducting films. Disregarding the renowned trifluoroacetate route, we center our focus on the transient liquid-assisted growth (TLAG) that meets the requirement of being a greener chemical process together with ultrafast growth rates beyond 100 nm/s. We developed a facile, fast, and cost-effective method, starting from the synthesis of metal-propionate powders of Y, Ba, and Cu of high purity and high yields, being the precursors of the fluorine-free solutions, which enable the chemical and microstructural nanoscale homogeneity of YBa2Cu3O7-x (YBCO) precursor films. These solutions present endured stability and enable precise tunability of the composition, concentration, porosity, and film thickness. Homogeneous precursor films up to thicknesses of 2.7 μm through eight layer multidepositions are demonstrated, thus establishing the correct basis for epitaxial growth using the fast kinetics of the TLAG process. YBCO films of 500 nm thickness with a critical current density of 2.6 MA/cm2 at 77 K were obtained, showing the correlation of precursor film homogeneity to the final YBCO physical properties.

Compounds in cigarette smoke induce EGR1 expression via the AHR, resulting in apoptosis and COPD.

Chronic obstructive pulmonary disease (COPD) is a major cause of mortality worldwide, and pulmonary epithelial cell apoptosis is regarded as one of the most important factors in its pathogenesis. Here, we examined the molecular mechanisms of apoptosis caused by cigarette smoke (CS). In the normal bronchial epithelium cell line BEAS-2B, a CS extract markedly induced apoptosis together with transient early growth response 1 (EGR1) protein expression, which is activated over time via the aryl hydrocarbon receptor (AHR). The CS extract-induced apoptosis decreased cell count of BEAS-2B cells and was significantly reversed by knockdown of either EGR1 or AHR. In vivo, the CS extract caused alveolar wall destruction, mimicking COPD, 1week after intrathoracic injection. Bronchoalveolar lavage fluid (BALF) from the CS extract-treated mice contained massive numbers of apoptotic epithelial cells. Furthermore, it was found that aminoanthracene induced EGR1 expression and cell apoptosis. By contrast, the AHR antagonist stemregenin 1 (SR1) restored apoptosis upon CS treatment. These results suggest that aryl hydrocarbons, such as aminoanthracene, induce EGR1 expression via the AHR, resulting in cell apoptosis and that this can be prevented by administration of an antagonist of AHR.

Design, Mechanical Properties, and Dynamics of Synthetic DNA Filaments.

Over the past 40 years, structural and dynamic DNA nanotechnologies have undoubtedly demonstrated to be effective means for organizing matter at the nanoscale and reconfiguring equilibrium structures, in a predictable fashion and with an accuracy of a few nanometers. Recently, novel concepts and methodologies have been developed to integrate nonequilibrium dynamics into DNA nanostructures, opening the way to the construction of synthetic materials that can adapt to environmental changes and thus acquire new properties. In this Review, we summarize the strategies currently applied for the construction of synthetic DNA filaments and conclude by reporting some recent and most relevant examples of DNA filaments that can emulate typical structural and dynamic features of the cytoskeleton, such as compartmentalization in cell-like vesicles, support for active transport of cargos, sustained or transient growth, and responsiveness to external stimuli.

Tourism's long- and short-term influence on global cities' economic growth: The case of Hong Kong.

This research examines how tourism development has impacted economic growth in a global city–Hong Kong. A large body of research has investigated national tourism-led growth in developed and developing countries. However, many such studies have overlooked how policies aimed at fostering the development of tourism affect the local economic development of global cities. The Chinese and Hong Kong governments liberalized their visa policies with the launch of the Individual Visit Scheme in 2003. Such liberalization has led to significantly more tourist arrival from China. Our autoregressive distributed lag model of tourism-related data from 2003 to 2019 provides strong evidence that more tourism can spur short-run economic growth. Yet, such tourism can lead to uncertain effects on local economic development in the longer run. Hong Kong’s transient tourism-led growth has almost entered the stagnation stage of the Tourism Area Life Cycle model. During such stagnation, jurisdictions like Hong Kong can expect limited long-term economic growth from their tourist sector. Our findings thus sound a warning for global cities looking to tourism to sustain longer-term economic growth.

Global stability analysis and direct numerical simulation of boundary layers with an isolated roughness element

Global stability analysis and direct numerical simulation (DNS) are used to study boundary layer flows with an isolated roughness element. The aspect ratio of the element ( $\eta$ ) is small, while the ratio of element height to displacement boundary layer thickness ( $h/\delta ^{*}$ ) is large. Both steady base flows and time-averaged mean flows are able to capture the frequencies of the primary vortical structures and mode shapes. Global stability results highlight that although the varicose instability is dominant for large $h/\delta ^{*}$ , sinuous instability becomes more pronounced as $Re_h$ increases for the thin geometry ( $\eta =0.5$ ), due to increased spanwise shear in the near-wake region. Wavemaker results indicate that $\eta$ affects the convective nature of the shear layer more than the type of instability. DNS results show that different instability mechanisms lead to different development and evolution of vortical structures in the transition process. For $\eta =1$ , the varicose instability is associated with the periodic shedding of hairpin vortices, and its stronger spatial transient growth indicated by wavemaker results aids the formation of hairpin vortices farther downstream. In contrast, for $\eta =0.5$ , the interplay between varicose and sinuous instabilities results in a broader-banded energy spectrum and leads to the sinuous wiggling of hairpin vortices in the near wake when $Re_h$ is sufficiently high. A sinuous mode with a lower frequency captured by dynamic mode decomposition analysis, and associated with the ‘wiggling’ of streaks, persists far downstream and promotes transition to turbulence. A new regime map is developed to classify and predict instability mechanisms based on $Re_{hh}^{1/2}$ and $d/\delta ^{*}$ using a logistic regression model. Although the mean skin friction demonstrates different evolutions for the two geometries, both of them efficiently trip the flow to turbulence at $Re_h=1100$ . An earlier location of a fully-developed turbulent state is established for $\eta =1$ at $x \approx 110h$ .

Kinetic Control of Ultrafast Transient Liquid Assisted Growth of Solution-Derived YBa2 Cu3 O7 -x Superconducting Films.

Transient liquid assisted growth (TLAG) is an ultrafast non-equilibrium growth process mainly governed by kinetic parameters, which are only accessible through fast in situ characterizations. In situ synchrotron X-ray diffraction (XRD) analysis and in situ electrical resistivity measurements are used to derive kinetic diagrams of YBa2 Cu3 O7- x (YBCO) superconducting films prepared via TLAG and to reveal the unique peculiarities of the process. In particular, diagrams for the phase evolution and the YBCO growth rates have been built for the two TLAG routes. It is shown that TLAG transient liquids can be obtained upon the melting of two barium cuprate phases (and not just one), differentiated by their copper oxidation state. This knowledge serves as a guide to determine the processing conditions to reach high performance films at high growth rates. With proper control of these kinetic parameters, films with critical current densities of 2-2.6 MA cm-2 at 77 K and growth rates between 100-2000nm s-1 are reached. These growth rates are 1.5-3 orders of magnitude higher than those of conventional methods.

Effect of waveform on turbulence transition in pulsatile pipe flow

Pulsatile flow in a straight pipe is a model system for unsteady internal flows in industrial engineering and physiology. In some parameter regimes, the laminar flow is susceptible to helical perturbations, whose transient energy growth scales exponentially with the Reynolds number (Re). In this paper, we link the transient growth of these perturbations to the instantaneous linear instability of the laminar flow. We exploit this link to study the effect of the waveform on turbulence transition by performing linear stability and transient growth analyses of flows driven with different waveforms. We find a higher-energy growth in flows driven with longer low-velocity phases as well as with steeper deceleration and acceleration phases. Finally, we perform direct numerical simulations and show that cases with larger transient growth transition faster to turbulence and exhibit larger turbulence intensities. However, these same cases are also more prone to relaminarisation once turbulence has been established. This highlights that, in pulsatile flows, the linear mechanisms responsible for turbulence transition are distinctly different from the nonlinear mechanisms sustaining turbulence.

Weak nonlinearity for strong non-normality

We propose a theoretical approach to derive amplitude equations governing the weakly nonlinear evolution of non-normal dynamical systems, when they experience transient growth or respond to harmonic forcing. This approach reconciles the non-modal nature of these growth mechanisms and the need for a centre manifold to project the leading-order dynamics. Under the hypothesis of strong non-normality, we take advantage of the fact that small operator perturbations suffice to make the inverse resolvent and the inverse propagator singular, which we encompass in a multiple-scale asymptotic expansion. The methodology is outlined for a generic nonlinear dynamical system, and four application cases highlight common non-normal mechanisms in hydrodynamics: the streamwise convective non-normal amplification in the flow past a backward-facing step, and the Orr and lift-up mechanisms in the plane Poiseuille flow.

Nonmodal and modal analyses of a flow through inhomogeneous and anisotropic porous channel

A study of nonmodal and modal stability analyses of a three-dimensional Poiseuille flow through a channel is performed where the bounding walls are partially covered by inhomogeneous and anisotropic porous layers. The evolution equations corresponding to normal velocity and normal vorticity are derived for fluid and porous layers to decipher the transient disturbance energy growth. The modal stability analysis reveals that the anisotropy parameter has a stabilizing impact, while the coefficient of inhomogeneity shows a peculiar behaviour on the most unstable shear mode. The nonmodal stability analysis predicts that the transient disturbance energy growth exists and attenuates with the increase in the value of the anisotropy parameter and larger values of the coefficient of inhomogeneity; however, it intensifies for smaller values of the coefficient of inhomogeneity.

Pancreatic tumor eradication via selective Pin1 inhibition in cancer-associated fibroblasts and T lymphocytes engagement

Cancer associated fibroblasts (CAFs) support tumors via multiple mechanisms, including maintaining the immunosuppressive tumor microenvironment and limiting infiltration of immune cells. The prolyl isomerase Pin1, whose overexpression in CAFs has not been fully profiled yet, plays critical roles in tumor initiation and progression. To decipher effects of selective Pin1 inhibition in CAFs on pancreatic cancer, here we formulate a DNA-barcoded micellular system (DMS) encapsulating the Pin1 inhibitor AG17724. DMS functionalized with CAF-targeting anti-FAP-α antibodies (antiCAFs-DMS) can selectively inhibit Pin1 in CAFs, leading to efficacious but transient tumor growth inhibition. We further integrate DNA aptamers (AptT), which can engage CD8+ T lymphocytes, to obtain a bispecific antiCAFs-DMS-AptT system. AntiCAFs-DMS-AptT inhibits tumor growth in subcutaneous and orthotopic pancreatic cancer models.

Practical design of a time-varying iterative learning control law using fuzzy logic

Iterative Learning Control (ILC) is an intelligent control algorithm that can effectively handle a tracking error in any system that operates in a repetitive manner. In practice, it is hardly possible to implement a single gain learning control law to improve the tracking performance due to the existence of large transient growth. To prevent the growth, this paper proposes a time-varying learning control design using the unique concept of fuzzy logic control to track the desired trajectory as well as the desired control input signal. The proposed control design is developed on both serial and parallel ILC configurations. The two configurations are initially constructed and implemented on a robotic manipulator with the use of a single gain learning control law. To avoid bad transients, the gain adjustment mechanism based on fuzzy logic control is introduced to vary the learning gain in each time step for enhancing the robustness of the system. According to the simulation and experiment on a robotic manipulator, both ILC structures with the proposed mechanism achieve the desired learning performance without bad transients.

Subharmonic eigenvalue orbits in the spectrum of pulsating Poiseuille flow

Spectral degeneracies where eigenvalues and eigenvectors simultaneously coalesce, also known as exceptional points, are a natural consequence of the strong non-normality of the Orr–Sommerfeld operator describing the evolution of infinitesimal disturbances in parallel shear flows. While the resonances associated with these points give rise to algebraic growth, the development of non-modal stability theory exploiting specific perturbation structures with much larger potential for transient energy growth has led to waning interest in spectral degeneracies. The appearance of subharmonic eigenvalue orbits, recently discovered in the periodic spectrum of pulsating Poiseuille flow, can be traced back to the coalescence of eigenvalues at exceptional points. We present a thorough analysis of the spectral properties of the linear operator to identify exceptional points and accurately map the prevalence of subharmonic eigenvalue orbits for a large range of pulsation amplitudes and frequencies. This information is then combined with solutions of the linear initial value problem to analyse the impact of the appearance of these orbits on the temporal evolution of linear disturbances in pulsating Poiseuille flow. The periodic amplification phases are shown to be heralded by repeated non-normal growth bursts that are intensified by the formation of subharmonic orbits involving the leading eigenvalues. These bursts are associated with the change of alignment of the perturbation from the decaying towards the amplified branch of the subharmonic eigenvalue orbits in a so-called branch transition process.