Numerical study of heat flux developed on an aero-spike structure in hypersonic flow

Several past studies have analyzed the topic of blunt body aerodynamics, and have gone in depth, detailing the reduction in drag caused by the effect of aero-spike structures that are attached to blunt shaped bodies in hypersonic flow, like re-entry capsules and missiles. The results have showed the formation of a bow shock in front of the spike, which creates a re-circulation region in front of the main body thus protecting it from the oncoming flow. Even though this leads to a reduction in drag on the main body, the heat flux generated on the surface of the aero-spike is immense. This study has been carried out to understand the flow properties over the aero-spike in detail and to arrive at the best possible configuration that could be put to practical use for missiles and re-entry vehicles and manages to keep the heat flux on both the body as well as the aero-spike to a minimum. Initial studies were carried out on a blunted cone-flare in hypersonic flow at Mach No = 6 (Temperature = 234 K; Pressure = 1064 Pa; Altitude = 100km) to clearly grasp the flow physics at high Mach numbers and also to validate the Computational Fluid Dynamics (CFD) solver used. The results obtained were encouraging, with good agreement with the available experimental data. A test model based on Apollo re-entry capsule was designed, with a cylindrical aero-spike attached to its front. Computational analysis was done for this configuration, with the commercially available Fluent CFD software. The initial results found good similarity with the past studies, showing a drag reduction of more than 60% when the aero-spike is attached to the capsule. The dimensions of the aero-spike cylinder were varied uniformly as a function of the radius of the capsule. This showed some interesting results, with the drag decreasing gradually up to a minimum value as the spike radius is increased, and then increasing as the radius is increased further. This radius was kept constant and the length of the spike was varied next, which yielded similar uniform results for drag reduction. It was clear that the variation in the dimensions of the aero-spike was modifying the area of the flow re-circulation region in front of the capsule which in turn leads to a variation in drag. The flow separation point on the aero-spike was plotted as a function of the spike radius, and this showed a linear variation. When the aero-spike is attached, the heat flux generated on the capsule decreases by about 25% of the initial value. The heat flux generated on the entire aero-spike structure is obtained. As expected, the maximum heat flux is developed at the front blunt body of the spike and then varies along the surface of the spike. The boundary layer thickness along the length of the spike is calculated using the available CFD tools. It is evident that for different aero-spike dimensions, the boundary layer thickness values at the various locations differ significantly for the same flow properties. By adjusting the spike geometry, it is possible to predict the exact heat flux that is developed at different locations on the spike as well as the re-entry capsule. And by knowing the locations at which thermal load is maximum, suitable heat resistant systems can be installed at the critical locations exclusively, leading to a reduction in the over-all operational cost and better safety margins.