Parametric Study on Influence of a Discontinuous Step in a Nozzle Contour on Side-load

Analysis of the acoustic signature produced by truncated ideal contour and thrust-optimized parabolic nozzles is conducted during both fixed and transient (startup) operations. The truncated ideal contour nozzle experiences free-shock separation flow, whereas the thrust-optimized parabolic nozzle experiences both free-shock separation and restricted-shock separation flow states during startup. This study provides a direct comparison of the acoustic signature produced during free-shock separation and restricted-shock separation flow states while operating under identical nozzle pressure ratios. During a transient episode, the continuous wavelet transform is used to compare the acoustic signatures produced by the nozzles. The truncated ideal contour nozzle demonstrates a gradual increase in broadband frequency energy with increasing nozzle pressure ratio and with broadband shock noise appearing at higher nozzle pressure ratios. The thrust-optimized parabolic nozzle, however, displays a much larger sensitivity to the nozzle pressure ratio. In particular, the free-shock separation to restricted-shock separation transition, which occurs around nozzle pressure ratio 24.4, is weakly revealed in the acoustic signature along sideline angles to the nozzle. At nozzle pressure ratio 13, the acoustic signal observed at shallow angles to the nozzle decreases abruptly across a broad range of frequencies. The latter phenomenon is attributed to the formation of an open-ended subsonic core surrounded by a supersonic annular flow in the thrust-optimized parabolic nozzle during free-shock separation operations of the nozzle, which does not occur in the truncated ideal contour nozzle.

Restricted shock separation (RSS) occurs in new bell type rocket nozzles such as the compressed truncated perfect nozzle and the thrust optimized nozzle. RSS may cause two serious problems. The first is the large side load on the nozzle and the second is the damage on the regenerative cooling tubes. In the present paper, the method to suppress RSS is introduced. Both numerical and experimental flow analyses or the shutdown transient are performed. The results show that a small step in the nozzle surface can change the location of the separation point at higher nozzle pressure ratio (NPR) than the RSS occurrence and RSS can be supressed. Additionally, the results of side load measurement show that the side load caused by the step is smaller than that caused by RSS.

In high-speed flight regimes, the drag plays a significant role in determining the overall performance of the aerospace vehicle. One proposed solution is to integrate a single expansion ramp nozzle (SERN) at the aft of the vehicle to mitigate the drag effects. This work investigates the effects of nozzle geometry on flow and performance characteristics of SERN with a thrust-optimized parabolic and minimum-length nozzle ramp contours, which have not been explored with regard to these aspects. A computational study is carried out on these two ramp contours using compressible Reynolds Averaged Navier–Stokes equations over a range of nozzle pressure ratio (NPR). In the tested range of NPR, the restricted shock separation is found to be the chief mode of flow separation in SERN, along with the change in the size of recirculation zone and separation bubble. At NPR 3, the thrust-optimized parabolic ramp contour shows an increment of thrust coefficient by 29.2% over the minimum-length nozzle; whereas at NPR 3.5, the minimum-length nozzle demonstrates an increment of thrust coefficient by 56.3% over the thrust-optimized parabolic at 0-deg M cowl angle. Further, in the tested range of NPR, the minimum-length nozzle shows an improved lift coefficient at 5 deg cowl angle.

A method of designing rocket nozzle wall contours to yield optimum thrust is presented. The rocket exhaust gases are treated under the assumption of isentropic, adiabatic and frictionless flow. Nozzle length and ambient pressure appear as governing restrictions under which the thrust is maximized. The solution of this extremal problem yields flow properties required along a suitably chosen control surface, and the nozzle contour is constructed by the method of characteristics to yield this required flow field. A typical numerical example is given. For the propellant combination used in the example, it is inferred from available experimental data that the expanding exhaust gases are in chemical equilibrium. The varying gas properties associated with chemical equilibrium are accounted for in the thrust optimization method. The low altitude performance of such a contoured nozzle is discussed.

This study has been carried out to assess the efficacy of the flow regulations in the form of tiny jets to regulate the pressure in the base region of an abruptly expanded duct. Four tiny jets of 1mm diameter placed at 90° intervals at 6.5 mm distance from the main jet in the wake region of the base were employed as flow management mechanism. The experiments were conducted at the inertia level of M = 2.5 & 3.0. The jets from the nozzles were expanded abruptly into a circular duct with four cross-sectional areas of 2.56, 3.24, 4.84 and 6.25. The L/D ratio of the enlarged duct considered was from 10 to 1 and experiments were conducted for Nozzle Pressure Ratio (NPR) from 3 to 11. Since the jets Mach numbers are high and the highest NPR tested was 11 which imply that the flow remains over expanded, even though, with increase in the NPR, the level of over expansion will decrease. It is well known that for over expanded nozzles an oblique shock will be formed at the nozzle lip, which in turn will result in the increase of the base pressure once it passes through the shock wave. From the results it is observed that for the NPRs 3 and 5 there is no appreciable gain in the base pressure, and hence, control employed as tiny jets are not effective, however, at NPR 7, 9, and 11 there is remarkable change in the base pressure values. This clearly indicates that NPR plays a significant role to decide on the magnitude of the base pressure and the control efficacy of the flow regulation mechanism as the tiny jets. It is found that the present method of flow regulation mechanism can be used as effective regulator of the base flows in an abruptly expanded duct. The control does not alter the nature of the flow in the enlarge duct.

The incompressible flow over a normal step in a channel was investigated by making measurements of the static wall pressures upstream of the step, on the step face, and downstream of the step. Several parameters were varied in order to determine their influence on the pressure field. These parameters included the free stream velocity, step height, channel height, and the boundary layer thickness. Tests were made using water as the working medium. The results indicate that the pressure field in the vicinity of the step base is highly dependent on the ratio of step height to boundary layer thickness, and that the far field is influenced strictly by the ratio of channel height to step height. This distinction becomes more clear when the step is at least two times the boundary layer thickness.

The aerospike engine was first devised in the early 1960s where it provided new means of reaching orbit in a single stage. The paper aimes to demonstrate the viability of the technology by showcasing the increased nozzle thrust efficiency over the conventional bell nozzle. Various truncations were applied to the nozzle and each was subjected to two conditions, an over-expansion and near optimum condition. The nozzle contour was developed using the simple approximation method and was chosen to replicate that of the XRS-2200. This anchored the data, thereby validating the computational fluid dynamics (CFD) simulation. Simulations were completed for at nozzle pressure ratios (NPR) of 58 and 15. Velocity vector plots and contours were generated in which the recirculation region can be clearly identified. This region is a result of the negative thrust contribution of the base and grows increasingly negative when the truncation applied increases in addition to when the exhaust flow is over-expanded. The results demonstrate the performance gain of the full-length aerospike nozzle, , over all other truncations. At NPRs of 58 and 15 it showed 1.5% and 10.3% gain respectively in the nozzle thrust efficiency compared to . There are, many impracticalities related to the full-length aerospike including cooling on the nozzle. Therefore, provide a realistic nozzle truncation that would be implemented. Although radical design changes to the rocket will be required for the adaptation of the aerospike engines, the changes will be beneficial in the long term. By increasing the nozzle thrust efficiency compared to bell nozzles, less fuel will be required per launch. Furthermore, removing the multi-stage method currently used, overall, the rocket will have an increased reliability due to its reduced complexity.

Numerical study was conducted on the LOX-RP1 engine bell nozzle to replace the dual bell nozzle for determining the optimum expansion ratio and greater thrust at sea level. Here base nozzle is a conventional Rao’s TIC nozzle, where throat radius, exit radius, inflection angle and exit angles are 138mm, 826mm, 33° and 8° respectively. The total length of dual bell nozzle and existing bell nozzle are kept same. Numerical analyses were carried out in ANSYS FLUENT software on different dual bell nozzle geometry to evaluate the thrust and expansion ratio. Numerical analysis is performed by two dimensional, axi-symmetric, steady state, pressure based solver with SST k-ω turbulence model at different ambient pressures to recreate the patterns of flow in the nozzle at different nozzle pressure ratio (NPR) and expansion ratio. The profile has made by commercial software SOLID WORKSTM. Numerical simulations and flow separation locations are validated with the experimental data published earlier. It is observed that with area ratio 150 and inflection angle 15° the thrust has increased by 5.46% at sea level and around 10.5% in vacuum, where the increase in exit radius and mass is only 96 mm and 6 kg respectively.

The CFD numerical simulations were carried out to investigate the base drag characteristics of a projectile with base bleed unit with a central jet. Different base bleed grain types with different combustion temperatures were used. The goal was to find a way to effectively control the base flow for base drag reduction and optimisate the latter using an adequate CFD software. Axisymmetric, compressible, mass-averaged Navier-Stokes equations are solved using the k-? SST, transition k-kl-?, and RSM turbulence models. The various base flow characteristics are obtained by the change in the non-dimensionalized injection impulse. The results obtained through the present study show that there is an optimum bleed condition for all base bleed grains tested. That optimum is dependent on the temperature of the grain combustion products. The optimum reduces the total drag for 6,9% in the case of air injection at temperature of 300 K and reaches up to 28% in the case of propellant combustion products injection at almost 2500 K. Besides, the increasing of molecular weight has a role no less important than temperature of the combustion products in terms of base drag reduction.

Altitude adaptability plays a significant role in designing a single stage to orbit vehicles. The aerospike nozzles outperform traditional bell nozzles in the case of altitude adaptability application as on the external spike surface, the pressure flow is unrestricted. With the variation in altitude, pressure on the spike surface adjusts to the shift in ambient pressure. The diagnostic investigation of fluid flow characteristics of aerospike nozzles during the streamtube compression at various nozzle pressure ratios (NPRs) is of topical interest. V.R.S.Kumar et al., (Physics of Fluids, 2022) reported conclusively the phenomenon of streamtube compression and sonic-fluid-throat effect in both internal and external fluid flow systems. Investigation on the actual physics of streamtube compression in external flow field of aerospike nozzles is an interesting research topic because it is well-known that the external streamline is unconstrained in a typical aerospike nozzle. A literature review reveals that cowl geometry is one of the dominant factors affecting the performance of aero spike nozzle. In this paper, three parameters have been considered for the diagnostic investigation, viz., the nozzle pressure ratio (NPR), truncation levels of spike length and the length of the cowl. In the research analysis, the obtained streamtube compression region followed by flow acceleration is seen from the perspective of variation in basic fluid properties such as temperature, viscosity, Mach number, and turbulence. Streamtube compression leads to Sanal flow choking and normal shock wave generation causing an enhancement of entropy. An increase in entropy increases the gas temperature and viscosity. An increase in viscosity leads to the subsequent streamtube compression due to the flow stickiness. This cascade effect leads to the formation of convergent-divergent shaped streamtubes in the supersonic flow regime of aero spike nozzles creating series of normal/oblique shock waves until the flow reaches subsonic flow condition. We have comprehended that the sonic-fluid-throat effect and undesirable shock wave can be negated by injecting suitable fluid to the Sanal flow choking region with high heat capacity ratio. This study is a pointer towards the design optimization of aero spike nozzles with an optimum cowl length for achieving the altitude compensation lucratively for various aerospace applications.

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The effects of an exhaust plume and nozzle length on the flow organization of axisymmetric base flows have been studied experimentally at Mach numbers of 0.76 and 2.2 using particle image velocimetry. From the measured velocity data, the mean pressure field was computed. The application of different nozzle lengths resulted in flow cases in which the shear layer impinges on the model, on the flow downstream of the model, or intermittently on the model and on the flow. The results showed that, for intermediate nozzle lengths, the overall pressure level downstream of the base decreased in the transonic flow cases and increased in the supersonic flow cases, indicating an entrainment effect in transonic flow and a displacement effect in supersonic flow. An increase in nozzle length was found to lead to a higher local pressure at the nozzle exit, which seemed to result in more overexpanded plumes in the transonic flow cases and less underexpanded plumes in the supersonic flow cases.

During the ascent phase of a launcher, base buffeting may occur for transonic conditions which may result in large side loads on the nozzle. This effect is well known and has been subject of extensive studies in literature. The present work is conducted in the framework of the ESA TRP “Launcher Base Flows and Shock Interactions Regions Improved Load Characterisation”, where high speed PIV measurements and unsteady pressure measurements performed in the DNW-HST on a 1:60 scale Ariane 5 are compared to detached eddy simulations for the same configuration and flow conditions. The main goal of the investigation is to identify how well the computations are capable of predicting the salient features of the transonic buffeting phenomenon..

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.

In suddenly expanded flows, due to the abrupt expansion of the flow from a nozzle into an enlarged duct, the pressure reduces in the base region of the enlarged duct which increases the base drag. The techniques used to increase the base pressure are namely passive control technique and active control technique. In passive control technique the geometrical modifications are employed by providing splitter plates, ribs, cavities etc. while in active control technique the secondary control jets are provided in the base region of an enlarged duct to increase base pressure up to atmospheric pressure. The air blowing pressure from secondary control jets should be optimum. This paper presents the computational fluid dynamic (CFD) analysis to optimize blowing pressure ratio i.e. the ratio of inlet pressure of control jets to the atmospheric pressure, to increase base pressure up to atmospheric pressure in the base region of an enlarged duct. In the present study, CFD analysis was carried out for different air blowing pressure ratios to optimize it. Flow and geometry parameters considered for the analysis are Mach number, area ratio, nozzle pressure ratio and blowing pressure ratio. Mach numbers considered for analysis are 1.5, 2.0 and 2.5. Area ratios and nozzle pressure ratios considered for analysis are 2, 5 and 8. The CFD analysis is done for different combinations of Mach numbers, area ratios, and the nozzle pressure ratios by varying blowing pressure ratio from 2 to 8 in step of 1. Based on analysis results anyone can select optimum value of blowing pressure ratio at a given Mach number, area ratio and nozzle pressure ratio to increase base pressure nearly up to atmospheric pressure.