Propulsion Aerodynamics for a Novel High-Speed Exhaust System

Abstract

A key requirement to achieve sustainable high-speed flight and efficiency improvements in space access lies in the advanced performance of future propulsive architectures. Such concepts often feature high-speed nozzles, similar to rocket engines, but employ different configurations tailored to their mission. Additionally, they exhibit complex interaction phenomena between high-speed and separated flow regions at the base, which are not yet well understood. This paper presents a numerical investigation on the aerodynamic performance of a representative, novel exhaust system, which employs a high-speed nozzle and a complex-shaped cavity region at the base. Reynolds-Averaged Navier–Stokes computations are performed for a number of nozzle pressure ratios (NPRs) and freestream Mach numbers in the range of 2.7 < NPR < 24 and 0.7 < M∞ < 1.2, respectively. The corresponding Reynolds number lies within the range of 1.06 × 106 < Red < 1.28 × 106 based on the maximum diameter of the configuration. The impact of the cavity is revealed by direct comparison to an identical noncavity configuration. Results show a consistent trend of increasing base drag with increasing NPR for both configurations, owing to the jet entrainment effect. Cavity is found to have no impact on the incipient separation location of the nozzle flow. At conditions of M∞ = 1.2 and high NPRs, the cavity has a significant effect on the aerodynamic performance, transitioning nozzle operation to underexpanded conditions. This results in approximately 12% higher drag coefficient compared to the noncavity case and shifts the minimum NPR required for positive gross propulsive force to higher values.

Wind Tunnel Installation Effects on the Base Flow for a High-Speed Exhaust System

“…Analyses on exhaust systems, where base-embedded cavities are employed as an integrated design aspect of the system, necessary for successful operation, have not been previously reported in the public domain. However, the signiőcance of such cavities has been demonstrated in previous work by the authors [27], where the existence of the cavity alone, transitioned the nozzle operation into the under-expanded condition at 𝑀 ∞ = 1.2. This led to a notable increase of approximately 12% in the drag coefficient with respect to an identical, non-cavity conőguration, as a result of higher base drag.…”

“…Additionally, as discussed previously, such installation effects due to high blockage can be quite common in academic facilities, owing to the combination of small wind tunnels [29] and the requirement for a relatively large model. However, in the context of base ŕows, it is usually the local ŕow characteristics at the surface of the base that are of critical importance [14,23,27]. Consequently, it is vital and of high practical value, to investigate whether a tunnel-constrained ŕow could feature local ŕow similarity at the base with respect to equivalent, unbounded ŕow conditions, even if the global ŕow topology differs notably.…”

“…The domain is discretized using a multi-block, fully structured grid approach [50]. The computational domain, boundary conditions as well as meshing approach, are adopted from previous work [27], involving an identical conőguration under free-stream conditions, featuring a slightly modiőed cavity shape. Figure 13 shows the computational mesh employed for this study (left) along with a close-up view on the cavity and nozzle regions (right).…”

“…The GCIs [42] corresponding to the medium mesh for the drag and nozzle standard velocity coefficients (𝐶 𝑑 and 𝐶 𝑉 ,𝑠 ), are 0.26% and 0.75%, respectively. Consequently, the medium grid of approximately 1.2 million cells is employed [27].…”