Manuel Keßler

Computational Aeroacoustics with Higher Order Methods

The Lighthill acoustic analogy in combination with two different higher-order CFD solvers is used to investigate the sound generation of two test cases. The flow around a cylinder at Re = 150 is analysed with a Discontinous Galerkin method and a counter-rotating open rotor (CROR) with a WENO scheme. The simulation of the cylinder is able to predict both aerodynamic and acoustic behaviour correctly, the vortex street behind the cylinder is responsible for the noise radiation similar to of an acoustic dipole. The analysis of the CROR focuses on the effect of using a higher-order method with a detailed comparison with a standard second order method. While global aerodynamic forces show only small differences, the better transport of vortices, especially of the blade tip vortex, is a benefit for the prediction of interaction noise of the two rotors. This paper includes different investigations on the new HLRS Cray Hermit cluster. The DG code was optimized for single-core usage while still maintaining its good parallel performance. The effect of node-pinning is studied with the CROR configuration which improved the computational time slightly.

Aeroacoustic Simulation of a Complete H145 Helicopter in Descent Flight

In the past years, the aeroacoustic noise emission of a helicopter became one of the most important, but also challenging issues in helicopter development. The blade vortex interaction phenomenon is one of the dominant phenomena characterizing the helicopters aeroacoustic footprint, which is insufficiently predicted by low fidelity computational methods. For a high fidelity noise prediction of a helicopter configuration, a multidisciplinary CFDCSD-CAA tool chain has been established at the Institute of Aerodynamics and Gas Dynamics of the University of Stuttgart. With higher order CFD computed noise generation at the near field and the noise convection using a Ffowcs-Williams Hawkings based CAA code, very good agreement to measured aeroacoustic noise in wind tunnel as well as free flight experiments of helicopters is achieved. However, the simulations had been limited to the main rotors geometry up to now, where some residual deviations to the experiment still exist. In this paper, we present a high fidelity aeroacoustic simulation of a complete helicopter configuration and the benefit compared to an isolated rotor simulation in predicting its aeroacoustic noise emission. Shading and reflection effects are clearly resolved, influencing the behaviour of the helicopters noise radiation. The simulated aeroacoustic noise emission of the helicopter lies within the experimental variation and shows therefore highly promising results for the next generation of aeroacoustic noise prediction.

A Parallel CFD Solver Using the Discontinuous Galerkin Approach

In the present paper a high-order Discontinuous Galerkin method is presented for the numerical simulation of the turbulent flow around complex geometries using unstructured grids. In order to close the Reynolds-averaged Navier-Stokes (RANS) system we use the Spalart-Allmaras turbulence model, the Wilcox K-ω turbulence model or a Detached Eddy Simulation technique. The paper includes some details of the code implementation. The excellent parallelisation characteristics of the scheme are demonstrated, achieved by hiding communication latency behind computation. Some results, like flows over a flat plate and around a sphere, which could not be predicted with an Unsteady Reynolds averaged Navier-Stokes calculation, are calculated with high accuracy and compared with theory and experiments.

Computation of Helicopter Phenomena Using a Higher Order Method

The enhancement of the structured Computational Fluid Dynamics solver FLOWer in the field of flux computation and its advantage to numerical helicopter simulations is presented. The improvement includes the replacement of the second order spatial scheme with a fifth order scheme for flow state reconstruction and the implementation of different Riemann solvers for flux computation. Aim of the implementation is to reduce the numerical dissipation and to achieve a high vortex preservation essential for numerical investigations of helicopter flows, such as rotor-fuselage interaction and noise emission of the rotor due to blade vortex interactions. For these phenomena which are sensitive to the rotor wake preservation, an investigation with the second order and the fifth order scheme is performed to compare the numerical results as well as the computational performance. The results show significant improvements in the rotor wake conservation, especially in case of an acoustic evaluation. It is shown that the Riemann solver has a high influence to the vortex conservation with low additional computational cost.

CFD Calculation of a Helicopter Rotor Hovering in Ground Effect

A helicopter rotor in hover conditions out of ground effect as well as in ground effect is studied numerically by means of an URANS approach. Tip vortex geometrical positions as well as thrust coefficients are scrutinized and compared to reference data in literature. Parameter variations are done in the rotor/ground plane distance in the range of h/R = 0.32...0.84 and collective angles of the rotor blades between 13° ... 17°. Generally, very good agreement with the experiment is achieved.

Numerical Investigation of Counter-Rotating Open Rotor Noise Emission in Different Flight Conditions

Noise emission of a 9×7 and 8×8 open rotor configuration in cruise and in take-off conditions is examined by 3D unsteady numerical simulations utilising the chimera method to represent rotor movement. The acoustic analysis has been carried out with a Ffowcs Williams-Hawkings code over one rotor revolution with a resolution of 360 time steps. To ensure covering of all sources while keeping numerical losses low different hull surfaces have been examined. The comparison of two configurations at different flight conditions shows two main noise generating effects: the single rotor emission and emission caused by interaction of the rotors. The single rotor emission can mostly be seen in the rotor plane whereas the interaction can be examined at an angle of 20 to 45 and 135 to 155 degrees to the rotating axis with approximately the same share of total noise in take-off conditions. In cruise conditions the single rotor emission prevails over the interaction. This can be explained by the transonic blade tip speeds during cruise. Due to the reduced tip speeds in take-off interaction noise contributes to the total noise with a higher share than in cruise conditions. The 8×8-configuration shows higher noise emissions by interaction since the rotor-rotor interactions occur simultaneously.Copyright

Detached Eddy Simulation Using the Discontinuous Galerkin Method

In the present paper the Discontinuous Galerkin (DG) method is applied for the discretisation of the Reynolds-Averaged Navier-Stokes (RANS) equations. Turbulence modelling is done with the Detached eddy simulation (DES) model based on the Spalart-Allmaras (SA) turbulence equation. In this method a turbulent lengthscale based on the cell width of the mesh is used for the filter width of the large eddy simulation (LES) region. As DG employs higher-order polynomial basis functions inside a cell an adaptation of the “cell width definition” is needed so that the turbulent viscosity is not too high in the LES region of the simulation. Hence, the cell width is adapted with the polynomial degree of the employed basis functions. Simulations of a backward facing step as well as a circular cylinder show that the adaptation is successful.

CFD-CSD-Coupled Simulations of Helicopter Rotors Using an Unstructured Flow Solver

Demands concerning geometrical detail arising in the simulation of certain flow phenomena in helicopter interactional aerodynamics can only be dealt with in an unstructured simulation environment. This work deals with the extension of the simulation capabilities at IAG to an unstructured flow solver. The new approach is validated against and compared to the standard structured flow chain in the context of isolated weakly CFD-CSD coupled simulations in wind tunnel conditions corresponding to the low-speed pitch up case of the GOAHEAD experiment. Generally, good agreement between the two respective toolchains is shown. All computations performed herein were conducted using the massively-parallel NEC Nehalem platform at HLRS.

Optimization and HPC-Applications of the Flow Solver FLOWer

Recent optimizations and HPC-applications of the flow solver FLOWer are presented in this paper. A graph partitioning method is introduced to the MPI communication, which reduces the number of messages as well as the total message size, leading to a run time speed-up of 20%. A numerical investigation of a finite wing shows the influence of the wind tunnel wall only in the wing root area and agrees well with experimental data for attached flow. Both a URANS and a Delayed Detached-Eddy Simulation (DDES) of the massively stalled wing reveal difficulties in matching the experimental behaviour of flow separation. Finally, a simulation of a model Contra-Rotating Open Rotor (CROR) at various operating conditions exhibit interaction effects, blade loadings and noise emissions which agree well with expectations and results from literature.

Adding Hybrid Mesh Capability to a CFD-Solver for Helicopter Flows

The enhancement of the so far structured Computational Fluid Dynamics solver FLOWer to enable the use of hybrid meshes and its advantage to numerical helicopter simulations is presented. The improvement is conducted by the implementation of unstructured grid handling into the existing code framework. The aim of the implementation is to reduce meshing effort in near body regions requiring the mapping of complex surfaces including boundary layer extrusion. Using the hybrid mesh approach, off-body regions can still be solved with structured meshes using computationally efficient higher order methods. This off-body region can be meshed automatically using Cartesian grids. The unstructured module features a second-order reconstruction scheme with an efficient GMRES implementation to solve linear systems of equations. Efficient high performance computation is ensured by multi-blocking and efficient load balancing considering the computational effort of the block according to the mesh type and numerical methods applied to. A forward facing step test case provides a reliable reproduction of different physical phenomena. An application-oriented complete helicopter simulation with particular use of unstructured body grids demonstrates the benefit of the hybrid mesh approach regarding our regular work flow.