Browsing by Author "Tsentis, Spyros"

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    Propulsion aerodynamics for a novel high-speed exhaust system
    (American Society of Mechanical Engineers (ASME), 2023-09-28) Tsentis, Spyros; Goulos, Ioannis; Prince, Simon; Pachidis, Vassilios; Zmijanovic, Vladeta
    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 yet not well understood, but are critical in terms of pressure and viscous forces. This paper presents a numerical investigation on the aerodynamic performance of a representative novel exhaust system, which employs a high-speed, truncated, ideal contoured 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 free stream 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. A decomposition of the drag domain forces exposes the major trends between the constituent elements. The impact of the cavity on the aerodynamic characteristics of the apparatus is revealed by direct comparison to an identical non-cavity configuration. Results show a consistent trend of increasing base drag with increasing NPR for all examined M∞ for both configurations. This is attributed to the jet entrainment effect and to the lower base pressure imposed by the higher jet flow expansion. The cavity region is found to have almost no impact on the incipient separation location of the nozzle flow. At low supersonic speeds of M∞ = 1.2 and high NPRs, the cavity has a significant effect on the aerodynamic performance, transitioning nozzle operation to under-expanded conditions. This results in approximately 12% higher drag coefficient compared to the non-cavity case and shifts the minimum NPR for which the system produces positive gross propulsive force to higher values.
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    ItemOpen Access
    Propulsion aerodynamics for a novel high-speed exhaust system
    (American Society of Mechanical Engineers, 2023-09-13) Tsentis, Spyros; Goulos, Ioannis; Prince, Simon; Pachidis, Vassilios; Zmijanovic, Vladeta
    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. This paper presents a numerical investigation on the aerodynamic performance of a representative novel exhaust system, which employs a high-speed, truncated, ideal contoured 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 free stream 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 on the aerodynamic characteristics is revealed by direct comparison to an identical non-cavity configuration. Results show a consistent trend of increasing base drag with increasing NPR for all examined M∞ 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 under-expanded conditions. This results in approximately 12% higher drag coefficient compared to the non-cavity case and shifts the minimum NPR for which the system produces positive gross propulsive force to higher values.
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    Wind tunnel installation effects on the base flow for a high-speed exhaust system
    (AIAA, 2024-01-04) Tsentis, Spyros; Goulos, Ioannis; Prince, Simon; Pachidis, Vassilios; Zmijanovic, Vladeta; Saavedra, Josè
    It is envisaged that future propulsion concepts will enable high-speed flight and improve space access. However, their aerodynamic behavior is not yet well understood, especially at the base where severe flow separation occurs, requiring further analyses using both numerical and experimental techniques. This paper presents a numerical investigation of the wind tunnel installation effects on a representative, sub-scale, high-speed exhaust system. The analysis facilitates an ongoing design of experiments and de-risking activity. The apparatus features a truncated, ideal-contoured nozzle and an axially symmetric cavity region embedded at the base. The viable design space owing to high blockage is identified in terms of maximum approach Mach number. A systematic jet vectoring effect is observed in all cases examined. The origins of this effect are investigated and attributed solely to the pressure distribution asymmetry caused by the existence of the wing-pylon. Additionally, local flow similarity at the base of the tunnel-installed model with respect to unconstrained flow is investigated and presented, along with a proposed methodology to establish comparability. This analysis is of increased practical importance, due to the size range of most closed transonic tunnels found in academic research facilities. Results show that the pressure distribution at pre-choking tunnel conditions agrees within less than 1.5% and 0.1% for the base and cavity wall surfaces, respectively. At post-choking tunnel operation, the base pressure distribution of the model exhibits increased deviations in the azimuthal direction of up to 7.5%. The base pressure distribution in the corresponding unconstrained flow case falls within the observed pressure range of the tunnel-installed model, while the pressure distribution along the cavity wall agrees within less 1%. The findings of this study suggest that a jet vectoring effect could potentially manifest to wingtip mounted nacelles, usually incorporated in future, high-speed vehicles. Finally, it is demonstrated, that local flow similarity exists at the base with respect to unbounded flow, even for post-choking tunnel conditions, which is critical in base flows and base drag reduction analyses.