Unsteady and three-dimensional fluid dynamic instabilities

2014 - 2015XIV n.s

Efficient stability analysis of flows using complex mapping techniques

Global linear stability analysis of open flows leads to difficulties associated to boundary conditions, leading to either spurious wave reflections (in compressible cases) or to non-local feedback due to the elliptic nature of the pressure equation (in incompressible cases). A novel approach is introduced to address both these problems. The approach consists of solving the problem using a complex mapping of the spatial coordinates, in a way that can be directly applicable in an existing code without any additional auxiliary variable. The efficiency of the method is first demonstrated for a simple 1D equation modeling incompressible Navier–Stokes, and for a linear acoustics problem. The application to full linearized Navier–Stokes equation is then discussed. A criterion on how to select the parameters of the mapping function is derived by analyzing the effect of the mapping on plane wave solutions. Finally, the method is demonstrated for three application cases, including an incompressible jet, a compressible hole-tone configuration and the flow past an airfoil. The examples allow to show that the method allows to suppress the artificial modes which otherwise dominate the spectrum and can possibly hide the physical modes. Finally, it is shown that the method is still efficient for small truncated domains, even in cases where the computational domain is comparable to the dominant wavelength

Tumor Suppressors and Cell-Cycle Proteins in Lung Cancer

The cell cycle is the cascade of events that allows a growing cell to duplicate all its components and split into two daughter cells. Cell cycle progression is mediated by the activation of a highly conserved family of protein kinases, the cyclin-dependent kinases (CDKs). CDKs are also regulated by related proteins called cdk inhibitors grouped into two families: the INK4 inhibitors (p16, p15, p19, and p18) and the Cip/Kip inhibitors (p21, p27, and p53). Several studies report the importance of cell-cycle proteins in the pathogenesis and the prognosis of lung cancer. This paper will review the most recent data from the literature about the regulation of cell cycle. Finally, based essentially on the data generated in our laboratory, the expression, the diagnostic, and prognostic significance of cell-cycle molecules in lung cancer will be examined

Acoustic impedance and hydrodynamic instability of the flow through a circular aperture in a thick plate

We study the unsteady flow of a viscous fluid passing through a circular aperture in aplate characterized by a non-zero thickness. We investigate this problem by solving theincompressible Linearized Navier-Stokes Equations (LNSE) around a laminar base flow,in both the forced case (allowing to characterize the coupling of the flow with acousticresonators) and the autonomous regime (allowing to identify the possibility of purelyhydrodynamical instabilities). In the forced case, we calculate the impedances and discussthe stability properties in terms of a Nyquist diagram. We show that such diagramsallow us to predict two kinds of instabilities: (i) a conditional instability linked to theoverreflexion of an acoustic wave but requiring the existence of an conveniently tunedexternal acoustic resonator, and (ii) a purely hydrodynamic instability existing even in astrictly incompressible framework. A parametric study is conducted to predict the rangeof existence of both instabilities in terms of the Reynolds number and the aspect ratio ofthe aperture. Analysing the structure of the linearly forced flow allows to show that theinstability mechanism is closely linked to the existence of a recirculation region withinthe thickness of the plate. We then investigate the autonomous regime using the classicalnormal-mode ansatz. The analysis confirms the existence of the purely hydrodynamicalinstability in accordance with the impedance-based criterion. The spatial structure ofthe unstable eigenmodes are found to be similar to the structure of the correspondingunsteady flows computed using the forced problem. Analysis of the adjoint eigenmodesand of the adjoint-based structural sensitivity confirms that the origin of the unstabilitylies in the recirculation region existing within the thickness of the plate

Bifurcation scenario in the two-dimensional laminar flow past a rotating cylinder

The aim of this paper is to provide a complete description of the bifurcation scenario of a uniform flow past a rotating circular cylinder up to Re=200 . Linear stability theory is used to depict the neutral curves and analyse the arising unstable global modes. Three codimension-two bifurcation points are identified, namely a Takens–Bogdanov, a cusp and generalised Hopf, which are closely related to qualitative changes in orbit dynamics. The occurrence of the cusp and Takens–Bogdanov bifurcations for very close parameters (corresponding to an imperfect codimension-three bifurcation) is shown to be responsible for the existence of multiple steady states, as already observed in previous studies. Two bistability regions are identified, the first with two stable fixed points and the second with a fixed point and a cycle. The presence of homoclinic and heteroclinic orbits, which are classical in the presence of Takens–Bogdanov bifurcations, is confirmed by direct numerical simulations. Finally, a weakly nonlinear analysis is performed in the neighbourhood of the generalised Hopf, showing that above this point the Hopf bifurcation is subcritical, leading to a third range of bistability characterised by both a stable fixed point and a stable cycle

Apoptotic epitope-specific CD8+ T cells and interferon signaling intersect in chronic hepatitis C virus infection

CD8(+) T cells specific to caspase-cleaved antigens derived from apoptotic T cells represent a principal player in chronic immune activation (CIA). Here, we found that both apoptotic epitope (AE)-specific and hepatitis C virus (HCV)-specific CD8(+) T cells were mostly confined within the effector memory (EM) or terminally differentiated EM CD45RA(+) cell subsets expressing a dysfunctional T-helper-1-like signature program in chronic (c)HCV infection. However, AE-specific CD8(+) T cells produced tumor necrosis factor (TNF)-α and interleukin-2 at the intrahepatic level significantly more than HCV-specific CD8(+) T cells, despite both populations acquiring high levels of programmed death-1 receptor expression. Contextually, only AE-specific CD8(+) T cells correlated with both interferon-stimulated gene levels in T cells and hepatic fibrosis score. Taken together, these data suggest that AE-specific CD8(+) T cells can sustain CIA by their capacity to produce TNF-α and be resistant to inhibitory signals more than HCV-specific CD8(+) T cells in cHCV infection

The flow past a freely rotating sphere

We consider the flow past a sphere held at a fixed position in a uniform incoming flow but free to rotate around a transverse axis. A steady pitchfork bifurcation is reported to take place at a threshold $Re^\mathrm{OS}=206$ leading to a state with zero torque but nonzero lift. Numerical simulations allow to characterize this state up to $Re\approx 270$ and confirm that it substantially differs from the steady-state solution which exists in the wake of a fixed, non-rotating sphere beyond the threshold $Re^\mathrm{SS}=212$. A weakly nonlinear analysis is carried out and is shown to successfully reproduce the results and to give substantial improvement over a previous analysis (Fabre et al. in J Fluid Mech 707:24–36, 2012). The connection between the present problem and that of a sphere in free fall following an oblique, steady (OS) path is also discussed

Stability and sensitivity analysis of the secondary instability in the sphere wake

The three-dimensional flow past a fixed sphere placed within a uniform stream is investigated. This paper focuses on the second bifurcation, which is responsible for the onset of the unsteadiness. Using the highly efficient Nek5000 parallel solver together with a recently developed numerical algorithm to stabilize and accelerate the numerical solution, it was possible to identify the three-dimensional eigenmode responsible for the second bifurcation. The characteristics of this eigenmode are analyzed in detail. The value of the critical Reynolds number ReIIcr=271.8, as well as the Strouhal number of the arising limit cycle, agree well with previous experimental and numerical investigations. To further assess the nature of the instability, an adjoint-based sensitivity analysis is carried out. The structure of the direct and adjoint modes are discussed, and then the core of the instability is localized. Finally, the sensitivity of the instability to a generic base flow modification is investigated

Linear stability and weakly nonlinear analysis of the flow past rotating spheres

We study the flow past a sphere rotating in the transverse direction with respect to the incoming uniform flow, and particularly consider the stability features of the wake as a function of the Reynolds number Re and the sphere dimensionless rotation rate Ω . Direct numerical simulations and three-dimensional global stability analyses are performed in the ranges 150⩽Re⩽300 and 0⩽Ω⩽1.2 . We first describe the base flow, computed as the steady solution of the Navier–Stokes equation, with special attention to the structure of the recirculating region and to the lift force exerted on the sphere. The stability analysis of this base flow shows the existence of two different unstable modes, which occur in different regions of the Re/Ω parameter plane. Mode I, which exists for weak rotations ( Ω0.7 ), is characterized by a larger frequency. Both modes preserve the planar symmetry of the base flow. We detail the structure of these eigenmodes, as well as their structural sensitivity, using adjoint methods. Considering small rotations, we then compare the numerical results with those obtained using weakly nonlinear approaches. We show that the steady bifurcation occurring for Re>212 for a non-rotating sphere is changed into an imperfect bifurcation, unveiling the existence of two other base-flow solutions which are always unstable

Identification of genes down-regulated during lung cancer progression: A cDNA array study

<p>Abstract</p> <p>Background</p> <p>Lung cancer remains a major health challenge in the world. Survival for patients with stage I disease ranges between 40–70%. This suggests that a significant proportion of patients with stage I NSCLC may actually be under-staged.</p> <p>Methods</p> <p>In order to identify genes relevant for lung cancer development, we carried out cDNA array experiments employing 64 consecutive patients (58 men and 6 women) with a median age of 58 years and stage 1 or stage 2 non-small-cell lung cancer (NSCLC).</p> <p>Results</p> <p>Basic cDNA array data identified 14 genes as differentially regulated in the two groups. Quantitative RT-PCR analysis confirmed an effective different transcriptional regulation of 8 out of 14 genes analyzed. The products of these genes belong to different functional protein types, such as extra-cellular matrix proteins and proteases (<it>Decorin </it>and <it>MMP11</it>), genes involved in DNA repair (<it>XRCC1</it>), regulator of angiogenesis (<it>VEGF</it>), cell cycle regulators (<it>Cyclin D1</it>) and tumor-suppressor genes (<it>Semaphorin 3B</it>, <it>WNT-5A </it>and retinoblastoma-related <it>Rb2/p130</it>). Some previously described differences in expression patterns were confirmed by our array data. In addition, we identified and validated for the first time the reduced expression level of some genes during lung cancer progression.</p> <p>Conclusion</p> <p>Comparative hybridization by means of cDNA arrays assisted in identifying a series of novel progression-associated changes in gene expression, confirming, at the same time, a number of previously described results.</p