Shear-Improved Smagorinsky Model for Large-Eddy Simulation of Wall-Bounded Turbulent Flows

A shear-improved Smagorinsky model is introduced based on recent results concerning shear effects in wall-bounded turbulence by Toschi et al. (2000). The Smagorinsky eddy-viscosity is modified subtracting the magnitude of the mean shear from the magnitude of the instantaneous resolved strain-rate tensor. This subgrid-scale model is tested in large-eddy simulations of plane-channel flows at two different Reynolds numbers. First comparisons with the dynamic Smagorinsky model and direct numerical simulations, including mean velocity, turbulent kinetic energy and Reynolds stress profiles, are shown to be extremely satisfactory. The proposed model, in addition of being physically sound, has a low computational cost and possesses a high potentiality of generalization to more complex non-homogeneous turbulent flows.Comment: 10 pages, 6 figures, added some reference

Spectral imbalance and the normalized dissipation rate of turbulence

The normalized turbulent dissipation rate $C_\epsilon$ is studied in decaying and forced turbulence by direct numerical simulations, large-eddy simulations, and closure calculations. A large difference in the values of $C_\epsilon$ is observed for the two types of turbulence. This difference is found at moderate Reynolds number, and it is shown that it persists at high Reynolds number, where the value of $C_\epsilon$ becomes independent of the Reynolds number, but is still not unique. This difference can be explained by the influence of the nonlinear cascade time that introduces a spectral disequilibrium for statistically nonstationary turbulence. Phenomenological analysis yields simple analytical models that satisfactorily reproduce the numerical results. These simple spectral models also reproduce and explain the increase of $C_\epsilon$ at low Reynolds number that is observed in the simulations

Decay of scalar variance in isotropic turbulence in a bounded domain

The decay of scalar variance in isotropic turbulence in a bounded domain is investigated. Extending the study of Touil, Bertoglio and Shao (2002; Journal of Turbulence, 03, 49) to the case of a passive scalar, the effect of the finite size of the domain on the lengthscales of turbulent eddies and scalar structures is studied by truncating the infrared range of the wavenumber spectra. Analytical arguments based on a simple model for the spectral distributions show that the decay exponent for the variance of scalar fluctuations is proportional to the ratio of the Kolmogorov constant to the Corrsin-Obukhov constant. This result is verified by closure calculations in which the Corrsin-Obukhov constant is artificially varied. Large-eddy simulations provide support to the results and give an estimation of the value of the decay exponent and of the scalar to velocity time scale ratio

Contained rupture of an aortic arch aneurysm in a patient with syphilitic aortitis. A case report

Syphilitic aortitis is a rare complication of tertiary syphilis, which can lead to aortic aneurysm formation, aortic valvular insufficiency, and ostial coronary stenosis. Syphilis has re-emerged worldwide over recent decades and vascular surgeons should be aware of its cardiovascular manifestations. Atypical clinical presentation, such as hemoptysis and a computed tomography angiography pattern of a thicker aneurysmal wall with ulcer-like aneurysm projections, should raise suspicion of syphilitic aortic aneurysm. An early diagnosis and appropriate surgical and medical therapies significantly contribute to successful treatment and favorable prognosis. Herein is reported the case of an 82-year-old male patient, positive for syphilis infection, with impending aortic arch aneurysm rupture treated with a hybrid arch repair. After 7 months, the patient was brought to the emergency room in cardiac arrest. Unsuccessful cardiopulmonary resuscitation maneuvers were performed, and an autopsy showed cardiac tamponade due to rupture of the ascending aorta

Endovascular Treatment of Aortic Arch Aneurysms

AbstractIntroductionThe aim of this study was to review our clinical experience with endovascular treatment of aortic arch aneurysms using different commercially available grafts (Gore, Talent, Endomed, Cook).MethodsFrom 1999 to 2004, 97 patients received endovascular treatment for diseases of the thoracic aorta. In 30 cases (26 males, 4 females) the aortic arch was involved.The left subclavian artery was overstented (Ishimaru zone ‘2’) in 18 cases (60%). Only in the first three cases had the subclavian artery been revascularized. The left common and subclavian arteries were covered (zone ‘1’) in 6 (20%) cases—all had the carotid artery reconstructed, either simultaneously (five cases) or as a staged procedure (one case). Finally, the whole aortic arch was over-stented (zone ‘0’) in 6 (20%) cases, with simultaneous (five cases) or staged (one case) grafting of the supra-aortic vessels from the ascending aorta.ResultsPerioperative mortality was 2/30 (7%), due to graft migration (zone ‘2’) and intra-operative stroke (zone ‘0’), respectively. One minor stroke was observed. No cases of paraplegia were recorded. Three type I endoleaks were observed. Two resolved at 6 months follow-up; one zone ‘0’ graft is still being followed. There was one surgical conversion for endograft failure 2 weeks after implantation. Thus, the technical success rate was 87% (26/30) cases. The mean follow-up time was 23±17 months. No new onset endoleaks or aneurysm-related deaths were recorded.ConclusionsCurrently available grafts may be deployed in the aortic arch in most instances. De-branching of the aortic arch with surgical revascularization for zone ‘0’ and ‘1’ seems to be adequate to obtain a satisfactory proximal landing zone

Self-occluding Candy-Plug: Implantation Technique to Obtain False Lumen Thrombosis in Chronic Aortic Dissections

Purpose: To describe the implantation steps of the latest generation of candy-plug device (third CP generation [CP III]) and to illustrate its possible pitfalls by discussing a case in whom this device was employed to occlude the false lumen (FL) of a chronic type B aortic dissection. Technique: A 69 year-old male patient who underwent a frozen elephant trunk arch repair due to residual type A aortic dissection developed a FL aneurysmal degeneration limited to the descending thoracic aorta. Two thoracic stent-grafts were deployed into the true lumen up to the celiac trunk origin. Then, the FL was occluded with a self-occluding CP III device (Cook Medical, Bloomington, Indiana), placed at the same level as the distal thoracic stent-graft. The distal un-stented sleeve was pushed upward to allow immediate occlusion of its central lumen, avoiding interference with reno-visceral arteries arising from the FL. Both intraoperative transesophageal echocardiography and follow-up computed tomographic angiography scan demonstrated complete FL thrombosis. Conclusion: The introduction of CP III with its self-occluding mechanism helped to shorten and standardize the procedure. However, adjunctive steps may be needed to immediately obtain FL occlusion and avoid hampering the perfusion of vessels arising from the FL

Extrinsic outflow graft flow obstruction in patients with HeartMate3 LVAD

Blood flow obstruction at the level of the outflow graft is a rare but severe complication of LVAD support. We present a series of five patients supported with HeartMate3 LVAD (Abbott Labs, Chicago, IL) that developed an outflow graft obstruction after 607–1250 days of support, during prolonged antithrombotic therapy. Three patients presented with severe symptoms of heart failure, were treated with endovascular stenting and experienced full recovery. Preoperative computed tomography angiography and intraoperative angiography together with intravascular ultrasound provided diagnosis and guided treatment. In two patients, outflow obstruction was an occasional finding at imaging without heart failure symptoms and a “watchful waiting” approach was adopted: delayed treatment in one of them was futile. This late adverse event is peculiar for its pathophysiology and not yet discussed among the mechanical circulatory support community

Delayed open conversion after endovascular abdominal aortic aneurysm: Device-specific surgical approach

none7Objectives: Despite several advances in endoluminal salvage for failed endovascular abdominal aortic repair (EVAR), in our experience an increasing number of cases necessitate delayed open conversion (dOC). Methods: EVAR patients requiring delayed (>30 days) conversion were prospectively collected in a computerized database including demographics, details of aortoiliac anatomy, procedural and clinical success, and postoperative complications. Results: Between 2005 and 2011, 54 patients were treated for aortic stent-graft explantation. Indications included 34 type I and III endoleaks, 13 type II endoleaks with aneurysm growth, 4 cases of material failures, and 3 stent-graft infections. All fit-for-surgery patients with type I/III endoleak underwent directly dOC. Different surgical approaches were used depending on the type of stent-graft. Overall 30-day mortality was 1.9%. Overall morbidity was 31% mainly due to acute renal failure (13 cases). Mean hospitalization was 6 days (range, 5-27 days). Overall survival at mean follow-up of 19 months was 78%. Conclusions: In recent years, the use of EVAR has increased dramatically, including in young patients regardless of their fitness for open repair. dOC after endovascular abdominal aortic aneurysm seems to be a lifesaving procedure with satisfactory initial and mid-term results. © 2013 European Society for Vascular Surgery. Published by Elsevier Ltd. All rights reserved.Marone, E.M.; Mascia, D.; Coppi, D.; Tshomba, Y.; Bertoglio, L.; Kahlberg, A.; Chiesa, R.Marone, ENRICO MARIA; Mascia, D.; Coppi, D.; Tshomba, Y.; Bertoglio, L.; Kahlberg, A.; Chiesa, R