Separation Pattern Transition Research Articles

The Influence of External Flow Field on the Flow Separation of Overexpanded Single-Expansion Ramp Nozzle

Flow separation and transitions of separation patterns are common phenomena of nozzles working with a wide Mach range. The maximum thrust method is applied to design the single-expansion ramp nozzle (SERN) for specific operating conditions. The nozzle is used to numerically simulate the transition processes of separation patterns under the linear change in the external flow Mach number and the actual trajectory take-off condition of a rocket-based combined cycle (RBCC), to investigate the mechanism through which the external flow field influences the separation pattern transition during acceleration. The computational fluid dynamics (CFD) method is briefly introduced, followed by experimental validation. Then, the design procedure of SERN is described in detail. The simulation results indicate that as the external Mach number increases, the flow field in the nozzle undergoes transitions from RSS (ramp) to FSS, and finally exhibits a no-flow separation pattern. The rate at which the external Mach number varies has little effect on the transition principle of the nozzle flow separation patterns, but it has a significant effect on the critical Mach number of the transition points. The external flow field of the nozzle has an airflow accumulation effect during acceleration, which can delay the transition of the flow separation pattern.

Comparative Study of OpenFOAM Solvers on Separation Pattern and Separation Pattern Transition in Overexpanded Single Expansion Ramp Nozzle

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.

Numerical Investigation on Mechanism of External Flow Field–Induced Separation Pattern Transition

The single-expansion ramp nozzle (SERN) has a compact structure and adaptability under off-design conditions, which is one of the nozzle configurations that the combined cycle engine gives priority to. Flow separation is likely to occur when the nozzle operates at low-pressure ratios. Subsonic regions, such as separation bubbles, recirculation zone, and boundary layer, also exist in the overexpanded flow field. At this point, the varying external flow field can affect the flow field inside the nozzle, which may lead to the transition of the flow separation pattern. The majority of relevant studies are conducted in a static environment because of experimental conditions limitations. This paper numerically simulates the transitions of separation patterns during the acceleration and deceleration processes of a short flap SERN. The separation transitions mechanism is analyzed, as well as the performance impact of the transitions. Current results show that the separation pattern transition happens during both the acceleration and deceleration processes, and free shock separation is a relatively stable state. The separation pattern transition occurs as a function of the external flow field, recirculation zone, boundary layer, and separation shock wave coupling. Compared with previous studies on the long flap SERN, the coupling mechanisms of the internal and external flow fields of the two nozzles were found to be different. The shock wave structure of the short flap SERN’s flow field is stable during the transition processes, and the nozzle performance does not exhibit a step change. The hysteresis of the short flap SERN is also weaker.

Performance evaluation of a second throat exhaust diffuser with a thrust optimized parabolic nozzle

Abstract In the present study, the experimental analysis of a second-throat exhaust diffuser (STED) performance has been conducted for the high altitude test of a parabolic bell-type nozzle. The diffuser starting performance has been explored considering two different approaches, namely the gradual and instantaneous increase of the nozzle chamber pressure. Also, the influence of pre-evacuating the internal regions of the system on the diffuser starting performance has been studied. Numerical simulations have been carried out to have more physical insight into some test results. It is demonstrated that the non-dimensional hysteresis range in STED starting performance with a bell-type nozzle is up to 4 times larger than that with a conical nozzle. The pre-evacuation of the internal area does not affect the diffuser minimum starting pressure. However, starting the diffuser with pre-evacuation takes 50 to 70 percent less time than without pre-evacuation. Also, it is illustrated that in spite of large differences in minimum starting pressure of STED with conical and bell-type nozzles, the STED breakdown pressure (minimum operating pressure) is independent of nozzle profile. Moreover, it is shown with experimental evidences and numerical analyses that the creation of restricted shock separation (RSS) inside the bell nozzle leads to some unexpected behaviors in STED starting and breakdown behaviors. Finally, it is demonstrated that the instantaneous increase in the nozzle chamber pressure with steeper slope and pre-evacuation of the internal regions can eliminate separation pattern transition from common free shock separation to unwanted RSS.

Phase separation pattern transition of three-domain vesicles

Based on the Helfrich elastic curvature energy model, the stable shapes for the two patterns of three-domain phase separation are studied in detail for the experimental parameters with direct minimization method in order to explain the interesting experimental results by Yanagisawa et al. (2010 Phys. Rev. E 82 051928). According to their experimental results, there are two transition processes. In the first process, the three-domain vesicles are formed, which are metastable. After several tens of minutes, the three-domain vesicles begin to bud, which is the second process. In the first process, the three-domain vesicles are formed with two patterns. The pattern with the liquid-ordered (Lo) phase in the middle with roughly cylindrical shape and two cap-shape liquid disordered (Ld) domains on each side of the Lo domain is termed pattern I in our paper, and the pattern with Ld domain in the middle with roughly cylindrical shape and two cap-shape Lo domains on each side is referred to as pattern Ⅱ. In the same paper of M. Yanagisawa et al., an approximate calculation is made with the vesicle shapes of the two patterns approximately represented by spheroids. Their calculation shows that the transition point of the two patterns is at o* 0.27 in the case of = 0.02 (or v = 0.942) and = 50, in contrast with the experimental result of o* 0.5. Here o is the area fraction of Lo phase, and is the excess area (which is usually represented by reduced volume v in the previous literatures), is the reduced line tension at the boundary of two adjacent domains. Thus the problem comes down to whether the transition point of the two patterns conforming with the experimental result can be obtained by the Helfrich elastic curvature energy theory if one performs a more precise calculation. Our calculation is performed with the direct minimization method, with the two boundaries of domains constrained in two parallel planes, this is an effective method to guarantee the smoothness of the boundary. To allow the vesicle to have a sufficient freedom to evolve, only constraints of fixed reduced volume and area fraction are imposed (The usual implementation method of constraints with the enclosed volume and the area of each phase fixed is not appropriate in this case. It does not allow the vesicle to have enough freedom to evolve, since the two boundaries are constrained in two preassigned planes). For the experimental parameters of = 50 and = 0.02, the transition point for the two patterns is obtained to be o* = 0.49, which is quite close to the experimental result of o* = 0.5. In order to understand the budding process in the second process, a detailed study is also made with the direct minimization method. It is found that the budding process can occur only for high enough value ( qslant 7.0) and permeable membrane (in other words, no constraint of reduced volume is exerted). One possible mechanism of the permeation is the temporary passage caused by the defect in the bilayer membrane due to large reduced line tension, which needs to be further checked experimentally. The three-domain vesicles found in the experiment have rotational symmetry in the case of small (or large v). What is more, they have a reflective symmetric plane perpendicular to the rotational symmetric axis, thus only vesicles with Dh symmetry are considered in this paper.

Unsteady transitions of separation patterns in single expansion ramp nozzle

The single expansion ramp nozzle is one of the optimal configurations for a planar rocket-based combined cycle engine because of its good integration and self-adaptability at off-design operation. The single expansion ramp nozzle is seriously overexpanded when the vehicle is at low speed, resulting in complex flow separation phenomena. Several separation patterns have been found in the single expansion ramp nozzle. Numerical simulations have shown that the transition between these separation patterns occurs in the nozzle startup and shutdown processes. However, only a few relevant experimental studies have been reported. This study reproduces the nozzle startup and shutdown processes using wind tunnel experiments. Two restricted shock separation patterns are observed in the experiment, namely, a separation bubble either forms on the ramp or the flap. The detailed flow fields in the transition processes are captured using a high-speed camera. The shock wave structures in the two separation patterns, influences of the nozzle pressure ratio (NPR) on the separation patterns and changes of the shock waves in the transition processes are discussed in detail. Shock wave instabilities accompany the separation transition, which usually takes less than 5 ms. The nozzle pressure ratios corresponding to the separation pattern transition are different in the startup and shutdown processes, which leads to a hysteresis effect.

Numerical investigation of separation pattern and separation pattern transition in overexpanded single expansion ramp nozzle

AbstractFlow separation results in many problems to single expansion ramp nozzle (SERN) and hypersonic vehicle. However, little research has been conducted on the separation patterns and their effects on SERN’s performance. In the present paper, the numerical simulation is adopted to get the intuitive results and help to analyse the separation phenomena in SERN thoroughly. The main separation pattern is the restricted shock separation (RSS) in SERN, and the free shock separation (FSS) only appears in a small range of the nozzle pressure ratio (NPR), which is much different from the axisymmetric rocket nozzle. Further CFD results show that the separation pattern transition makes great effects on the performance of SERN, especially the lift. Moreover, the performance of SERN has an extreme in the separation pattern transition because of the main jet impinging on the expansion ramp. The transitions occur in both the startup and shutdown processes but the critical nozzle pressure ratios of the separation pattern transitions are different, which leads to a hysteresis loop of SERN performance.

Side-Load Phenomena in Highly Overexpanded Rocket Nozzles

The operation of rocket engines in the overexpanded mode, that is, with the ambient pressure considerably higher than the nozzle exit wall pressure, can result in dangerous lateral loads acting on the nozzle. These loads occur as the boundary layer separates from the nozzle wall and the pressure distribution deviates from its usual axisymmetric shape. Different aerodynamic or even coupled aerodynamic/structural mechanic reasons can cause an asymmetric pressure distribution. A number of subscale tests have been performed, and three potential origins of side loads were observed and investigated, namely, the pressure fluctuations in the separation and recirculation zone due to the unsteadiness of the separation location, the transition of separation pattern between free-shock separation and restricted-shock separation, and aeroelastic coupling, which indeed cannot cause but do amply existing side loads to significant levels. All three mechanisms are described in detail, and methods are presented to calculate their magnitude and pressure ratio at which they occur.

A Study of Side Load in 2-D Nozzles during Shutdown

In the present paper, the experimental and numerical investigations are performed to clarify the mechanism of the side load in rocket nozzles using two types of nozzles-modified ideal nozzle and modified compressed truncated perfect (CTP) nozzle. In the experiment, the flow field is visualized by the Schlieren method and asymmetric flow patterns are observed. In the modified CTP nozzle, two types of the separation patterns are observed. One is "free shock separation" and the other is "restricted shock separation." The numerical simulations predict that the transition of separation pattern from the FSS to the RSS causes a large impulsive side load. It is also shown that the flow in a two-dimensional nozzle can be three dimensional due to flow separation on the side wall.

Appearance of Restricted Shock Separation in Rocket Nozzles

Uncontrolled flow separation in nozzles of rocket engines is not desired because it can lead to dangerous lateral forces. Different origins for side loads were identified in the past. Meanwhile, it is proven that in thrust-optimized or parabolic nozzles, a major side load occurs as a result of the transition of separation pattern from free shock separation to restricted shock separation and vice versa. Reasons for the transition between the separation patterns are discussed, and the cap-shock pattern, which is identified to be the cause of this transition, is closely analyzed. It turns out that this pattern can be interpreted as an inverse Mach reflection of the internal shock at the nozzle axis. To prove the transition effect as main side-load driver, a subscale test campaign has been performed. Two different nozzle contours, a thrust-optimized and a truncated ideal nozzle with equal performance data, were tested. Highest side loads were measured in the thrust-optimized nozzle, when the separation pattern changes from free to restricted shock separation. Side loads measured in the truncated ideal nozzle were only about one-third as high as in the thrust-optimized nozzle.