جستجوی مقالات مرتبط با کلیدواژه « single expansion ramp nozzle » در نشریات گروه « مکانیک »

Flow separation in overexpanded single expansion ramp nozzles (SERN) involves complex phenomena, such as shock waves, expansion waves, turbulent boundary layers, and shear layers. Computational fluid dynamics plays a crucial role in studying unsteady flow behaviour in supersonic nozzles, allowing for an investigation into the dynamic flow field characteristics. However, the application of OpenFOAM as a numerical tool for studying SERN in the field of compressible flows, particularly in the overexpansion state where the flow field characteristics are more complex, has received relatively less attention. In this study, the flow field characteristics of an overexpanded SERN under different turbulence models are investigated through a combination of experiments and numerical calculations. The qualitative and quantitative predictive performance of two compressible flow solvers in OpenFOAM, namely, rhoCentralFOAM and sonicFOAM, are compared in terms of flow separation pattern and separation pattern transitions within the overexpanded SERN. The ability of rhoCentralFOAM and sonicFOAM to accurately predict complex flow states is evaluated. Results indicate that the numerical simulations conducted using rhoCentralFOAM and sonicFOAM successfully capture flow separation, separated shock waves, separated bubbles and shear layers for two types of restricted shock separation patterns at the same nozzle pressure ratio (NPR), demonstrating agreement with experimental results. However, sonicFOAM initiates the transition in the separation pattern 0.0773 NPR earlier than rhoCentralFOAM during the whole separation pattern transition process of the SERN. The transition process in sonicFOAM lasts longer and exhibits a greater variation in NPR. SonicFOAM fails to accurately predict certain aspects, such as the pressure rise after the separation bubble, the reattachment shock wave, and tends to overestimat the length of the separation shock length. Consequently, sonicFOAM cannot be recommended as a suitable solver for accurately capturing the separation pattern of an overexpanded nozzle.

Single Expansion Ramp Nozzle (SERN) with large expansion ratio is normally adopted for the hypersonic aircraft in consideration of integrated aircraft / propulsion system / nozzle design. Under low Mach number and low Nozzle Pressure Ratio (NPR, the ratio of inlet total pressure to outlet static pressure) conditions, the flow in the SERN is a state of severe over-expansion, the internal resistance increased obviously, the flow quality and performance of the SERN sharply deteriorated. How to effectively improve the SERN performance under over-expanded condition has become an important issue in the integrated design of hypersonic propulsion system. The passive cavity flow control technique was introduced on the upper expansion ramp of the SERN in this paper, the three-dimensional flowfield in the passive cavity SERN was investigated numerically, suitable NPR range for passive cavity flow control and impacts of passive cavity parameters were discussed. Results show the SERN performance is effectively improved when the passive cavity is applied from NPR of 5 to 10. Compared with the baseline SERN, 3.13 % improvement can be achieved for the thrust coefficient of the passive cavity SERN when NPR is 5. The passive cavity has the capacity of regulating the induced shock intensity or restraining flow separation, the reason for the change in its function is decided by the relative position between the induced shock and the passive cavity position on the upper expansion ramp of the SERN. As for each function of the passive cavity, an optimum position for the passive cavity structure exists on the upper expansion ramp. Among the primary geometric parameters of the passive cavity structure, percent of porosity is a crucial factor to affect the SERN performance by adjusting separation zone size and its separation starting position, and the improvement effectiveness of the axial thrust coefficient drops with the decrease of percent of porosity. The cavity depth and the aperture size are not sensitive to the performance of passive cavity SERN as compared to the effect of the percent of porosity.