Imran Afgan

Large eddy simulation of the flow around single and two side-by-side cylinders at subcritical Reynolds numbers

The flow around single and two side-by-side infinite cylinders is numerically modelled using dynamic Smagorinsky large eddy simulation (LES). For the single cylinder, the Reynolds number based on the diameter and the free stream velocity is 3900. A complete sensitivity study was conducted based on the extrusion length in the span-wise direction, the grid refinement at the wall, the convection scheme, and the sub-grid scale (SGS) model. It was found that the mean solution is not influenced by the extrusion length beyond 4 diameters or by 1% up-winding. However, coarsening the mesh in the wall normal direction or switching off the sub-grid scale model led to drastic effects on the recirculation length and on the underlying velocity field. The two side-by-side cylinders were tested for a range of pitch to diameter ratios (T/D=1.0,1.25≤T/D≤5.0) at a Reynolds number of 3000. For the intermediate pitch to diameter ratios (1.25≤T/D≤1.75), multiple shedding frequencies were detected with a biased wake flow deflec...

Orthogonal Blade-Vortex Interaction on a Helicopter Tail Rotor

The orthogonal blade-vortex interaction (BVI) has been simulated using unsteady Reynolds-averaged Navier-Stokes equations. The cases investigated are relative to an interaction between a lifting blade at high angle of attack and an orthogonal vortex that travels either head-on or at 45 degrees to the leading edge...

Stochastic response of the laminar flow past a flat plate under uncertain inflow conditions

The present study aims at analysing the sensitivity of two-dimensional flow past a flat plate to uncertain inflow conditions in the laminar flow regime. Both the Reynolds number and angle of incidence are treated as random inflow variables. The methodology consists of a stochastic collocation method based on generalised polynomial chaos (gPC) theory coupled with standard deterministic numerical simulations. With respect to the two random inputs, sensitivity analysis of global integral parameters such as Strouhal number, drag and lift coefficients and the time-averaged flow fields is performed, resulting in the construction of their response surfaces. The stochastic response of the full spectrum of the drag coefficient is also obtained. It is noticed that integral parameters are sensitive to the two random parameters. There is a peak in the probability density function (PDF) of mean drag coefficient. Two additional high frequencies are predicted in the spectrum of drag coefficients. They are about two and four times the primary vortex shedding frequency respectively, corresponding to first and second harmonics of the primary frequency. For the flow fields, the analysis demonstrates that the most probable solutions are significantly different from the deterministic ones and the solution sensitivity is localised near the regions transitioned to large scale fluid structure movements. Non-linear coupling between the two uncertainties is also studied thanks to the Sobols decomposition. The angle of incidence is found to be the most influential variable to the mean flow fields.

Laminarization of Internal Flows Under the Combined Effect of Strong Curvature and Rotation

The laminarization phenomenon for the flow under the combined effect of strong curvature and rotation is discussed based on numerical predictions of large-eddy simulation (LES). Initially, the laminarization process is presented for the fully developed flow inside a spanwise rotating straight square duct. LES predictions over a wide range of rotation numbers (Ro=0-5) show that the turbulent kinetic energy decreases monotonically apart from 0.2<Ro<0.5. Subsequently, a spanwise rotating U-duct flow is considered with Ro=±0.2. The interaction of curvature and Coriolis induced secondary flows enhances the turbulence for the negative rotating case, whereas this interaction ensues strong laminarization for the positive rotating case. Finally, the laminarization is presented in the impeller of a typical centrifugal compressor, rotating at a speed of Ω =1862 rpm (Ro=0.6). The resulting LES predictions are observed to be better than those of Reynolds-averaged Navier-Stokes (PANS) in the regions where turbulence is significant. However, for the regions dominated by strong laminarization, RANS results are seen to approach those of LES and experiments.