Archivo Digital UPM: No conditions. Results ordered -Date Deposited. 2022-05-26T13:58:35ZEPrintshttps://oa.upm.es/style/images/logo-archivo-digital.pnghttps://oa.upm.es/2010-12-01T12:51:17Z2016-04-20T14:09:33Zhttps://oa.upm.es/id/eprint/5316This item is in the repository with the URL: https://oa.upm.es/id/eprint/53162010-12-01T12:51:17ZLinear instability analysis of low-pressure turbine flowsThree-dimensional linear BiGlobal instability of two-dimensional states over a periodic array of T-106/300 low-pressure turbine (LPT) blades is investigated for Reynolds numbers below 5000. The analyses are based on a high-order spectral/hp element discretization using a hybrid mesh. Steady basic states are investigated by solution of the partial-derivative eigenvalue problem, while Floquet theory is used to analyse time-periodic flow set-up past the first bifurcation. The leading mode is associated with the wake and long-wavelength perturbations, while a second short-wavelength mode can be associated with the separation bubble at the tralling edge. The leading eigenvalues and Floquet multipliers of the LPT flow have been obtained in a range of spanwise wavenumbers. For the most general configuration all secondary modes were observed to be stable in the Reynolds number regime considered. When a single LPT blade with top to bottom periodicity is considered as a base flow, the imposed periodicity forces the wakes of adjacent blades to be synchronized. This enforced synchronization can produce a linear instability due to long-wavelength disturbances. However, relaxing the periodic restrictions is shown to remove this instability. A pseudo-spectrum analysis shows that the eigenvalues can become unstable due to the non-orthogonal properties of the eigenmodes. Three-dimensional direct numerical simulations confirm all perturbations identified herein, All optimum growth analysis based on singular-value decomposition identifies perturbations with energy growths O(10(5)).Nadir AbdessemedSpencer SherwinVassilios Theofilis2010-12-01T12:23:16Z2016-04-20T14:09:35Zhttps://oa.upm.es/id/eprint/5317This item is in the repository with the URL: https://oa.upm.es/id/eprint/53172010-12-01T12:23:16ZTransient growth analysis of the flow past a circular cylinderWe apply direct transient growth analysis in complex geometries to investigate its role in the primary and secondary bifurcation/transition process of the flow past a circular cylinder. The methodology is based on the singular value decomposition of the Navier-Stokes evolution operator linearized about a two-dimensional steady or periodic state which leads to the optimal growth modes. Linearly stable and unstable steady flow at Re=45 and 50 is considered first, where the analysis demonstrates that strong two-dimensional transient growth is observed with energy amplifications of order of 10(3) at U-infinity tau/D approximate to 30. Transient growth at Re=50 promotes the linear instability which ultimately saturates into the well known von-Kaacutermaacuten street. Subsequently we consider the transient growth upon the time-periodic base state corresponding to the von-Kaacutermaacuten street at Re=200 and 300. Depending upon the spanwise wavenumber the flow at these Reynolds numbers are linearly unstable due to the so-called mode A and B instabilities. Once again energy amplifications of order of 10(3) are observed over a time interval of tau/T=2, where T is the time period of the base flow shedding. In all cases the maximum energy of the optimal initial conditions are located within a diameter of the cylinder in contrast to the spatial distribution of the unstable eigenmodes which extend far into the downstream wake. It is therefore reasonable to consider the analysis as presenting an accelerator to the existing modal mechanism. The rapid amplification of the optimal growth modes highlights their importance in the transition process for flow past circular cylinder, particularly when comparing with experimental results where these types of convective instability mechanisms are likely to be activated. The spatial localization, close to the cylinder, of the optimal initial condition may be significant when considering strategies to promote or control shedding.Nadir AbdessemedAtul S. SharmaVassilios TheofilisSpencer Sherwin2010-10-28T08:29:47Z2016-04-20T13:49:29Zhttps://oa.upm.es/id/eprint/4714This item is in the repository with the URL: https://oa.upm.es/id/eprint/47142010-10-28T08:29:47ZOptimal growth of linear perturbations in low pressure turbine flowsThis paper presents a numerical algorithm for the linearized flow initial value problem involving complex geometries where analytical solution is impossible. The method centres around the calculation of an eigenvalue problem involving the linearised flow and its spatial adjoint, and yields the flow perturbations that grow the most in a prescribed time, the magnitude of that growth and the perturbations after the growth has occurred. Previous work has shown that classical stability analysis of flow past a low-pressure turbine blade gives only stable eigenvalues, which cannot explain transition to turbulence in this flow. The inital value problem for this fan blade is presented and demonstrates significant perturbation growth, indicating that this growth may be the facilitator for transition in this case.Atul S. SharmaNadir AbdessemedSpencer SherwinVassilios Theofilis