The Prediction of Rough Wall Fluxes to the ExoMars Vehicle

Abstract

Ablative heat shields roughen during planetary entry due to the ablation process. This results in enhanced surface fluxes when compared with those obtained from a smooth vehicle. For example, it has been experimentally determined that the turbulent convective flux to a roughened blunt body in hypersonic flow can be up to three times greater than for the hydraulically smooth equivalent case. As such, the potential level of augmentation is significant and has required careful consideration in the prediction of the heat fluxes for the design of the ExoMars thermal protection system (TPS). The fidelity and maturity of current predictive methodologies are insufficient when one considers the potential impact this phenomenon has on TPS design. Commonly, aerothermal databases are derived using methods to determine smooth wall fluxes which are post-processed by correlation to account for rough surface augmentation. The visibility of experimental data used for the construction of these correlations is typically low due to the fact that they originate from strategic weapons programmes. As such, it is difficult to assess the applicability of these correlations to any specific application. The work performed on the ExoMars project to date concerns the modification of the predictive methodology to account for surface roughness, and validation of the methodology against legacy data and a specific set of tests performed in the DLR H2K wind tunnel. Significant progress has been made in this area to support the design of the ExoMars TPS. The roughness model of Krogstad has been implemented into the FGE Navier-Stokes code TINA, with development work including a blending with Van-Driest's roughness model at low k+ (dimensionless roughness height) as well as a modification to account for compressibility. Experiments have been performed at Mach 7 and Mach 5.3 on a PEEK model, and the heat fluxes inferred from infra-red camera measurements of surface temperature. The data clearly shows augmentation of the fluxes due to the rough surface and has been rebuilt in order to verify the Krogstad model.