Space-time/frequency/scale representation of the turbulence near the wall

International audienceIt is proved that the Window Average Gradient (WAG) scheme designed to detect singularities in the turbulence is approximately equivalent to the Hilbert transform of the "Mexican Hat" wavelet. Several identities are derived between these schemes and their validity are theoretically and experimentally shown through the fluctuating wall shear stress and velocity data taken in the turbulent boundary layer. The instantaneous amplitude-frequency representation of WAG at the large scale is also considered. These results indicate that the wall turbulence is significantly regular. It is shown that the near wall singularities involve in the large scale frequency shift key process and that the corresponding instantaneous phase consists of discontinuous line segments

Spectral characteristics of the Near-Wall turbulence in an unsteady channel flow

International audienceThe modulation characteristics of the turbulent wall shear stress and longitudinal intensities in the inner layer are experimentally investigated in an unsteady channel flow wherein the centerline velocity varies in time in a sinusoidal manner. The fluctuating wall shear stress and velocity signals are temporally filtered and subsequently phase averaged. It is shown that the outer structures corresponding to the low spectrum range have a constant time lag with respect to the centerline velocity modulation. The inner active structures, in particular those with a frequency band containing the mean ejection frequency of the corresponding steady flow dominate the dynamics of the near-wall unsteady turbulence. The structures respond to the imposed shear oscillations in a complex way, depending both on their characteristic scales and the thickness of the oscillating shear zone in which they are embedded

Contrôle actif dual d'écoulement turbulent

Un contrôle actif de type dual, reposant sur l'interaction entre deux schémas de contrôle (boucle ouverte par soufflage localisé en espace et périodique en temps + boucle fermée), est ici étudié dans un canal turbulent. Deux type de contrôle par asservissement seront analysés, Ad-Hoc et suboptimal en présence d'un soufflage sinusoïdal en temps et localisé sur la paroi inférieure. On montre alors qu'une stratégie de contrôle duale, formée d'une manipulation sinusoïdale et suboptimale, peut s'avérer être une méthode de contrôle efficace afin de réduire la contrainte pariétale turbulente et améliorer la contrôlabilité du système

Réduction de traînée par forçage électromagnétique transverse (Simulations Numériques directes)

Des simulations numériques directes d'écoulement turbulent en canal plan soumis à un forçage électromagnétique transverse oscillant permettent de mieux comprendre les mécanismes de réduction de traînée. L'écoulement pariétal transverse généré par ce forçage rabat contre la paroi les replis des nappes de vorticité. Par ce fait, les fluctuations de la composante normale de la vorticité sont réorientées suivant la direction transverse. Le cycle de régénération de la turbulence est alors perturbé par la diminution de la production de fluctuations de vorticité longitudinale entraînant une diminution de l'intensité des éjections et balayages

Right ventricular involvement in left ventricular non-compaction cardiomyopathy

Background: Left ventricular non-compaction cardiomyopathy (LVNC) features extensive trabeculations. Involvement of the right ventricle (RV) has been reported; however, distinction from normal RV trabeculation is difficult. This study aimed at assessing RV morphology and function in LVNC by cardiac magnetic resonance (CMR) and transthoracic echocardiography (TTE).Methods: Dimensional and functional parameters were assessed according to guidelines. Novel CMR parameters were RV end-diastolic (ED) trabeculated area, RV ED trabeculated volume, and RV ED non-compacted to compacted (NC/N) ratio in short axis (SAX) as well as in four-chamber view (4CH).Results: Twenty patients with LVNC and 20 controls were included. RV size and function were comparable in LVNC and controls and exhibited a good correlation between TTE and CMR. Although RV trabeculated area, RV trabeculated volume, and RV ED NC/C ratio in SAX as well as in 4CH were larger in LVNC, there was a major overlap with values in controls. RV ED NC/C ratio in SAX correlated with LV ED NC/C ratio (not in 4CH). Quantitative assessment of RV non-compaction was not feasible in TTE.Conclusions: Right ventricle size and function in LVNC can be measured by CMR and TTE, while RV trabeculation can only be quantified by CMR. RV myocardium displays more trabeculations in LVNC; however, overlap with normal individuals is extensive, not allowing separation of patients with LVNC from controls

Pericardial effusion unrelated to surgery is a predictor of mortality in heart transplant patients

Background: Hemodynamically irrelevant pericardial effusion (PeEf) is a predictor of adverse outcome in heart failure patients. The clinical relevance of a PeEf unrelated to surgery in heart transplant patients remains unknown. This study assesses the prognostic value of PeEf occurring later than 1 year after transplantation. Methods: All patients undergoing heart transplantation in Zurich between 1989 and 2012 were screened. Cox proportional hazard models were used to analyze mortality (primary) and hospitalization (secondary endpoint). PeEf time points were compared to baseline for rejection, immunosuppressants, tumors, inflam­mation, heart failure, kidney function, hemodynamic, and echocardiographic parameters. Results: Of 152 patients (mean age 48.3 ± 11.9), 25 developed PeEf. Median follow-up period was 11.9 (IQR 5.8–17) years. The number of deaths was 6 in the PeEf group and 46 in the non-PeEf group. The occurrence of PeEf was associated with a 2.5-fold increased risk of death (HR 2.49, 95% CI 1.02–6.13, p = 0.046) and hospitalization (HR 2.53, 95% CI 1.57–4.1, p = 0.0002). Conclusions: This study reveals that the finding of hemodynamically irrelevant PeEf in heart trans­plant patients is a predictor of adverse outcome, suggesting that a careful clinical assessment is war­ranted in heart transplant patients exhibiting small PeEf

A comparative study of turbulence models in a transient channel flow

Open Access funded by Engineering and Physical Sciences Research Council Under a Creative Commons license The authors would like to acknowledge the financial support provided by the Engineering and Physical Sciences Research Council (EPSRC) through the Grant No. EP/G068925/1.Peer reviewedPublisher PD