Volker Hannemann

Numerical Investigation of Double-Cone and Cylinder Experiments in High Enthalpy Flows using the DLR TAU Code

In the framework of validation and further development of the DLR TAU code for hypersonic applications several laminar calculations were conducted participating in a RTO working group. The enthalpy in the generic test cases ranges from 5 to 22 MJ/kg addressing thermo-chemical relaxation processes in air and pure nitrogen. Strong bow shocks as well as boundary layer separation, shock-shock and shock-boundary layer interactions are present in the flow fields. The applied numerical models are discussed. The comparison with the experimental data shows the current capability of the DLR TAU code to predict the aero-thermodynamic loads and how model improvements can be supported by these experiments.

Numerical investigation of an effusion cooled thermal protection material

A spacecraft re-entering the earth’s atmosphere has to endure extreme heat loads on parts of its structure as a result of the surrounding hypersonic flow field. The design of such parts benefits from a close coupling of structural and fluid mechanics. A good prediction of the vehicle’s surface temperature is crucial for an optimal choice of materials to fulfill the technical requirements. The numerical analysis of the flow field with a CFD (Computational Fluid Dynamics) code has to take into account the high temperature effects occurring in hypersonics (like chemical reactions and vibrational excitation in thermo-chemical non-equilibrium) and to provide a local heat flux in the gas phase at a given surface temperature.

DLR τ -Code for High Enthalpy Flows

The development of an equilibrium chemistry model for the DLR τ-Code has extended its ability to simulate high enthalpy flows, such as those around reentry vehicles or within high speed test facilities. Details of the development of the chemistry model, and some test cases, are included in this paper. Also presented is an improved axisymmetry treatment for the code, aimed at producing accurate and noise-free solutions for common hypersonic applications such as blunt bodies and shock tunnels.

Numerical Study of the Subsonic Base Flow with a Side Support

The base flow of a generic rocket configuration is investigated numerically with different levels of turbulence modeling. At the nominal flow conditions, the comparison of numerical results with the experiments shows significant deviations in the vertical plane where a side support stands. A simulation of the open test section indicates two necessities of correction. On the one hand, an C p increase of 0.015 is necessary to correlate the measured plenum pressure with the inflow location of the numerical simulation. On the other hand, a − 0.32° angle of attack modification should be accounted for a justified comparison between the DES results and the experiments. A strong sensitivity towards such small angles of attack has been observed later in the experiments but not in respective RANS solutions. The DES results agree well with the experiment based on the above-mentioned corrections.

Computations of shock wave propagation with local mesh adaptation

The numerical simulation of discontinuous flow phenomena results in high demands related to the used computational grids. A high resolution of the grid is required to resolve shock waves and contact surfaces. This leads, especially for unsteady flows with moving structures, to grids with a large number of points. Local mesh adaptation allows to reduce the computational effort by refining the mesh only in regions where it is necessary. In the present paper a numerical simulation of the shock tunnel flow in the High Enthalpy Shock Tunnel G¨ottingen (HEG) is performed. Local grid adaptation is used to capture shocks and contact discontinuities. Of particular interest for the shock tunnel performance and the investigation of driver gas contamination is the shock reflection process and the interaction between the reflected shock, the boundary layer and the contact surface separating the test gas from the driver gas.

Hybrid RANS-LES Study of Transonic Flow in the Wake of a Generic Space Launch Vehicle

Transonic flow in the wake of a generic space launch vehicle is investigated using Improved Delayed Detached-Eddy simulations on quasi-structured, partially unstructured and hybrid grids. It is shown that a \(90^\circ \) segment of the full domain can already capture the main afterbody flow dynamics and give a good agreement with experimental results. In order to assess the effect of unstructured grids, simulation results on a grid with an unstructured recirculation region are compared to a quasi-structured and a true hybrid grid. Finally, a comparison between IDDES and zonal DES results are presented and discussed.

Accuracy in a Finite Volume Godunov Type Method

The standard Godunov type method used in computational fluid dynamics shows accuracy problems for low Mach number flows and for the kinetic energy at the highest wave numbers resolvable on a given grid. Both drawbacks become visible when simulating the decay of isotropic turbulence at the low Mach numbers typical for the respective experimental investigations. A modification to cure both problems is proposed by Thornber et al. [10] with a mathematical motivation in case of a special fifth order reconstruction. The theoretical results are repeated here. Numerical results are achieved for schemes not investigated in that literature, namely AUSMDV and AUSM + -up which includes already modifications for low Mach number flows. First experiences with Thornber’s modification confirm the positive influence in combination with AUSMDV even if the reconstruction is only of second order. In combination with AUSM + -up Thornber’s modification provides too little damping when used without subgrid scale modelling.

Evaluation of Turbulence Models in Predicting Hypersonic and Subsonic Base Flows Using Grid Adaptation Techniques

The flows behind the base of a generic rocket, at both hypersonic and subsonic flow conditions, are numerically studied. The main concerns are addressed to the evaluation of turbulence models and the using of grid adaptation techniques. The investigation focuses on two configurations, related to hypersonic and subsonic experiments. The applicability tests of different turbulence models are conducted on the level of two-equation models calculating the steady state solution of the Reynolds-averaged Navier-Stokes(RANS) equations. All used models, the original Wilcox k-ω, the Menter shear-stress transport (SST) and the explicit algebraic Reynolds stress model(EARSM) formulation, predict an asymmetric base flow in both cases caused by the support of the models. A comparison with preliminary experimental results indicates a preference for the SST and EARSM results over the results from the older k-ω model. Sensitivity studies show no significant influence of the grid topology or the location of the laminar to turbulent transition on the base flow field, but a strong influence of even small angles of attack is reported from the related experiments.

Implementation of the VOF method in the DLR TAU code

The DLR TAU code is enhanced with the volume-of-fluid method (VOF) for a reconstruction of the fluid interface. The compressible Reynolds-Averaged-Navier-Stokes equations are discretized by a finite volume technique using tetrahedra, pyramids, prisms and hexahedra. TAU subdivides the either structured or unstructured primary grid into tetrahedra on which the pre-processing solver computes the initial volume-of-fluid distribution. The initial distribution is defined to be rectangular, parabolical, spherical or a combination of the aforementioned. For the consideration of surface ten-sion forces, a continuum surface force method (CSF) is implemented. Due to the application on structured as well as on unstructured grids, the volume-of-fluid values are smoothed with a Laplacian type smoother for a stable calculation of the interface curvature. The continuum surface force algorithm is validated with a test case in which surface tension results in a transition from a cubical shape to a stable spherically shaped bubble in the absence of any external forces.

Simulation of Magnetohydrodynamic Effects on an Ionised Hypersonic Flow by Using the TAU Code

In the present investigation the DLR TAU code is extended to support future experimental investigations of magnetohydrodynamic effects in high temperature hypersonic flows. According to the conditions in the High Enthalpy Shock Tunnel Gottingen (HEG) the first steps in enhancing the TAU code are the implementation of a source term formulation of electromagnetic forces and the calculation of the electrical conductivity of air as a gas mixture in chemical non equilibrium. To verify the source term implementation a perfect gas study related to numerical simulations from Poggie and Gaitonde is conducted and shows reasonable agreement. Applied to the experimental conditions the model predicts a noticeable increase of the shock stand off distance.