Skin-Friction-Based Identification of the Critical Lines in a Transonic, High Reynolds Number Flow via Temperature-Sensitive Paint.

In this contribution, three methodologies based on temperature-sensitive paint (TSP) data were further developed and applied for the optical determination of the critical locations of flow separation and reattachment in compressible, high Reynolds number flows. The methodologies rely on skin-friction extraction approaches developed for low-speed flows, which were adapted in this work to study flow separation and reattachment in the presence of shock-wave/boundary-layer interaction. In a first approach, skin-friction topological maps were obtained from time-averaged surface temperature distributions, thus enabling the identification of the critical lines as converging and diverging skin-friction lines. In the other two approaches, the critical lines were identified from the maps of the propagation celerity of temperature perturbations, which were determined from time-resolved TSP data. The experiments were conducted at a freestream Mach number of 0.72 and a chord Reynolds number of 9.7 million in the Transonic Wind Tunnel Göttingen on a VA-2 supercritical airfoil model, which was equipped with two exchangeable TSP modules specifically designed for transonic, high Reynolds number tests. The separation and reattachment lines identified via the three different TSP-based approaches were shown to be in mutual agreement, and were also found to be in agreement with reference experimental and numerical data.

Free boundary problems in shock reflection/diffraction and related transonic flow problems.

Shock waves are steep wavefronts that are fundamental in nature, especially in high-speed fluid flows. When a shock hits an obstacle, or a flying body meets a shock, shock reflection/diffraction phenomena occur. In this paper, we show how several long-standing shock reflection/diffraction problems can be formulated as free boundary problems, discuss some recent progress in developing mathematical ideas, approaches and techniques for solving these problems, and present some further open problems in this direction. In particular, these shock problems include von Neumann's problem for shock reflection-diffraction by two-dimensional wedges with concave corner, Lighthill's problem for shock diffraction by two-dimensional wedges with convex corner, and Prandtl-Meyer's problem for supersonic flow impinging onto solid wedges, which are also fundamental in the mathematical theory of multidimensional conservation laws.

Numerical Investigation of Transonic Flow-Induced Spontaneous Condensation in Micro-Ejector Nozzles.

Micro-cooling systems are compact refrigeration systems widely applicable in microchemical analysis, biomedicine, and microelectromechanical systems (MEMS). These systems rely on the use of micro-ejectors to achieve precise, fast, and reliable flow and temperature control. However, the efficiency of micro-cooling systems is hindered by spontaneous condensation occurring downstream of the nozzle throat and within the nozzle itself, impacting the performance of the micro-ejector. A micro-scale ejector mathematical model describing wet steam flow was simulated to investigate the steam condensation phenomenon and its influence on flow, incorporating equations for liquid phase mass fraction and droplet number density transfer. The simulation results of wet vapor flow and ideal gas flow were compared and analyzed. The findings revealed that the pressure at the micro-nozzle outlet exceeded predictions based on the ideal gas assumption, while the velocity fell below it. These discrepancies indicated that condensation of the working fluid reduces the pumping capacity and the efficiency of the micro-cooling system. Furthermore, simulations explored the impact of inlet pressure and temperature conditions on spontaneous condensation within the nozzle. The results demonstrated that the properties of the working fluid directly influence transonic flow condensation, underscoring the importance of selecting appropriate working fluid parameters for nozzle design to ensure nozzle stability and optimal micro-ejector operation.

Three-Dimensional Analysis of Biomimetic Aerofoil in Transonic Flow.

Since the invention of the aircraft, there has been a need for better surface design to enhance performance. This thirst has driven many aerodynamicists to develop various types of aerofoils. Most researchers have strongly assumed that smooth surfaces would be more suitable for air transport vehicles. This ideology was shattered into pieces when biomimetics was introduced. Biomimetics emphasized the roughness of a surface instead of smoothness in a fluid flow regime. In this research, the most popular 0012 aerofoils of the National Advisory Committee for Aeronautics (NACA) are considered to improve them, with the help of a surface pattern derived from the biological environment. Original and biomimetic aerofoils were designed in three dimensions with the help of Solidworks software and analyzed in the computational flow domain using the commercial code ANSYS Fluent. The implemented biomimetic rough surface pattern upgraded the NACA 0012 aerofoil design in the transonic flow regime. Lift and viscous forces of the aerofoil improved up to 5.41% and 9.98%, respectively. This research has proved that a surface with a little roughness is better than a smooth surface.