DIRECT NUMERICAL SIMULATION OF SINGLE ABLATIVE PARTICLE DYNAMICS IN NEAR-WALL COUETTE FLOW UNDER AERODYNAMIC LOAD

When hypersonic vehicles reenter the atmosphere, the surface thermal protection materials will ablate under the action of high temperature airflow. In the process, the ablative particles will entrance the high temperature airflow and affect boundary-layer transition and turbulence characteristics downstream. Those phenomena will also happen in an arc-heated wind tunnel when conducting material thermal response experiments. Therefore, it is a significant basic scientific problem to study the transport behavior of inertial ablative particles under aerodynamic load. In this article, we analyzed the flow condition and particle exfoliation process very near a hypersonic vehicle wall with dimensional theory. After a series of reasonable assumptions and simplifications, we modelled the ablative particle exfoliation and transport process as one spherical inertial particle in Couette flow and adopted the particle resolved-direct numerical simulation (PR-DNS) method to study it. As a result, the particle exfoliation and transport characteristics were revealed and a normalized expression of particle start-up velocity was obtained, which would provide theoretical basis for accurate prediction of particle mass loss in the future. The research findings show that as the particle fluid density ratio \rho _r increases, the particle inertia St increases, and the horizontal and normal velocities of particle decrease. The larger the particle diameter is, the larger the particle inertia St is, and the horizontal velocity of the particle decreases. However, the normal velocity and displacement of the larger particle are increased. The reason is maybe larger particles receive larger Saffman lift force. Besides, the normal displacement of ablative particles is much smaller than the horizontal displacement, so the particles are mainly transported horizontally. In order to find the unified law underlying all the regularities, we defined the start-up velocity and found that the normalized particle start-up velocity is a function of the particle and fluid inertia, i.e., the particle horizontal transport velocity is the velocity of fluid or neutral buoyant particle minus the inertia correction term.