Alexej Pogorelov

Cut-cell method based large-eddy simulation of tip-leakage flow

The turbulent low Mach number flow through an axial fan at a Reynolds number of 9.36 × 105 based on the outer casing diameter is investigated by large-eddy simulation. A finite-volume flow solver in an unstructured hierarchical Cartesian setup for the compressible Navier-Stokes equations is used. To account for sharp edges, a fully conservative cut-cell approach is applied. A newly developed rotational periodic boundary condition for Cartesian meshes is introduced such that the simulations are performed just for a 72° segment, i.e., the flow field over one out of five axial blades is resolved. The focus of this numerical analysis is on the development of the vortical flow structures in the tip-gap region. A detailed grid convergence study is performed on four computational grids with 50 × 106, 250 × 106, 1 × 109, and 1.6 × 109 cells. Results of the instantaneous and the mean fan flow field are thoroughly analyzed based on the solution with 1 × 109 cells. High levels of turbulent kinetic energy and pressure fluctuations are generated by a tip-gap vortex upstream of the blade, the separating vortices inside the tip gap, and a counter-rotating vortex on the outer casing wall. An intermittent interaction of the turbulent wake, generated by the tip-gap vortex, with the downstream blade, leads to a cyclic transition with high pressure fluctuations on the suction side of the blade and a decay of the tip-gap vortex. The disturbance of the tip-gap vortex results in an unsteady behavior of the turbulent wake causing the intermittent interaction. For this interaction and the cyclic transition, two dominant frequencies are identified which perfectly match with the characteristic frequencies in the experimental sound power level and therefore explain their physical origin.

Cut-Cell Method Based Large-Eddy Simulation of a Tip-Leakage Vortex of an Axial Fan

The viscous flow around a rotating axial fan at a Reynolds number of 9.36 × 10 based on the outer casing diameter is investigated by large-eddy simulation (LES) with special focus on the tip-leakage flow region. A massively parallelized finite-volume flow solver for compressible flows based on hierarchical Cartesian grids is used. The immersed boundaries of the fan geometry are handled by a fully conservative cut-cell method. A 72◦ segment, which includes one of the five fan blades, is resolved with approx. 250 million cells, for which a rotational periodic boundary condition for Cartesian meshes has been developed. Results of the instantaneous and the mean fan flow field are discussed and compared to Reynolds-averaged Navier-Stokes (RANS) results of a 360◦ simulation. The main differences are observed for the turbulent kinetic energy in the wake region generated by the tip-gap vortex. Furthermore, the influence of the tip-gap size on the vortical structures is investigated. It is shown that a reduction of the tip-gap size leads to a change of the shape and size of the tip-gap vortex. Additionally, more separation and counter-rotating vortices are generated inside the tip-gap, which, however, result in a lower turbulent kinetic energy.

Impact of Periodic Boundary Conditions on the Flow Field in an Axial Fan

The flow field in a rotating low Mach number axial fan is studied by large-eddy simulations (LES). The Reynolds number based on the outer casing diameter and the rotational speed of the fan tip is 9.36 × 10. A parallelized finite-volume flow solver for unsteady compressible Navier-Stokes flows based on hierarchical Cartesian grids and a conservative cut-cell method to describe the complex fan geometry is used. For a simulation of a 72 fan section, which resolves one out of five blades, and for a flow rate coefficient of Φ = 0.165 and tip-gap size of s/Do = 0.01 an interaction of the turbulent wake caused by the decay of the tip-gap vortex with the neighboring blade is observed. For this critical case, the impact of the periodic boundary condition in the circumferential direction on the axial fan flow field is investigated. Therefore, besides the single-blade solution, a 360 full-fan computation is conducted using the same grid resolution. Numerical results of the flow field from both computations are compared and analyzed in detail. It is shown that the impact of prescribed periodicity for the investigated configuration and operating point is marginal such that the use of periodic boundary conditions is reasonable not only in the context of efficient flow analyses.

Large-Scale Simulations of a Non-generic Helicopter Engine Nozzle and a Ducted Axial Fan

Large-eddy simulations (LESs) of a helicopter engine jet and an axial fan are performed by using locally refined Cartesian hierarchical meshes. For the computations a high-fidelity, massively parallelized solver for compressible flow is used. To verify the numerical method, a coaxial hot round jet is computed and the results are compared to reference data. The analysis is complemented by a grid convergence study for both applications, i.e., for the helicopter engine jet and the axial fan. For the helicopter engine jet, additional computations have been performed for two different nozzle geometries, i.e., a simplified nozzle geometry that is consisting of a center body and divergent outer annular channel, and a complete engine nozzle geometry with four additional struts were used. The presence of the struts results in a different potential core break-down and turbulence intensity. Furthermore, for the axial fan configuration, computations have been performed at two different volume flow rates. The reduction of the volume flow rate results in an interaction of the tip-gap vortex with the neighboring blade which leads to a higher turbulent kinetic energy near and inside the tip-gap region.

Aeroacoustic Simulations of Ducted Axial Fan and Helicopter Engine Nozzle Flows

The flow and the acoustic field of an axial fan and a helicopter engine jet are computed by a hybrid fluid dynamics – computational aeroacoustics method. For the predictions of the flow field a high-fidelity, parallelized solver for compressible flow is used in the first step. In the second step, the acoustic field is determined by solving the acoustic perturbation equations. The axial fan is investigated at a Reynolds number of Re = 9. 36 × 105 for two tip-gap sizes, i.e., s∕D o = 0. 001 and s∕D o = 0. 01 at a fixed flow rate coefficient Φ = 0. 195. A comparison of the numerical results of the pressure spectrum and its directivity with measurements show a good agreement which confirms the correct identification of the sound sources and the accurate prediction of the acoustic duct propagation. Furthermore, the results show in agreement with the experimental data a higher broadband noise level for the larger tip-gap size. In the second application, jets from three different helicopter engine nozzles at a Reynolds number of Re = 7. 5 × 105 are investigated, showing an important dependence of the jet acoustic near field on the presence of the nozzle built-in components. The presence of the centerbody increases the OASPL compared to the clean nozzle, where the inclusion of struts reduces the OASPL compared to the centerbody nozzle owing to the increased turbulent mixing caused by the struts which lesses the length and time scales of the turbulent structures shed from the centerbody.

Large-Eddy Simulation of the Flow Field in a Rotating Axial Fan

Large-eddy simulation (LES) results of the turbulent flow field in a \(72^{\circ }\) segment of a rotating axial fan are discussed. A newly developed finite-volume flow solver which is based on non-boundary-fitted Cartesian grids is used to solve the three-dimensional flow equations for viscous compressible fluids. Computations are performed for two operating points at a constant Reynolds number of \(9.36 \times 10^5\) based on the outer casing wall diameter. The capability of the method to accurately resolve the main flow phenomena including the unsteady turbulent tip-gap vortices is demonstrated.