Cowl Length Research Articles

Similarity Analysis for Two-Dimensional Hypersonic Inlets Restart

The restart ability of inlets is significant to the robustness of hypersonic airbreathing vehicles. Classic starting theories based on the one-dimensional and inviscid flow assumption cannot predict the restart boundary accurately. To overcome the classic theories, the restart characteristics of two-dimensional contraction ducts in viscous flow are numerically studied. The restart boundaries for different configurations and flow conditions are determined, and the restart similarity laws are discovered. With the proposed dimensionless quantities (the dimensionless cowl length and the effective throat-entrance area ratio), the influences of multiple factors on restart are described simultaneously in a concise self-similar formulation. The restart characteristics can be described with any two variables in cowl angle, dimensionless cowl length, and effective area ratio. The restarts of two-dimensional inlets are classified into three types of the short-cowl, transitional, and long-cowl restarts according to the features of restart boundary lines and the flow structure. The three restart types are dominated by the competition between the cowl shock system and cowl tail expansion waves, the cowl shock system alone, and the complementation between the cowl shock system and throat choke, respectively. The wall pressure criteria for restart are correlated with the dimensionless cowl length and the characteristic separation scale at the restart critical state. It is observed that Kantrowitz limit is not generalizable for restart boundaries.

Computational investigation of the effect of variable cowl length on the flowfield of conical aerospike nozzles

Computational investigation of the effect of variable cowl length on the flowfield of conical aerospike nozzles

Similarity Analysis on Two-Dimensional Hypersonic Inlets Unstart

The prediction of unstart boundaries of hypersonic inlets is significant to the engine control. To correct the previous conclusions based on the one-dimensional and inviscid flow assumption, the present paper numerically investigates the unstart characteristics of two-dimensional contraction ducts in viscous flow that vary with multiple controlling variables. Based on the newly defined dimensionless cowl length and effective internal contraction ratio, the similarity law of unstart is discovered. The universal empirical equation of unstart boundary for different cowl lengths, cowl lip heights, entrance boundary-layer thicknesses, unit Reynolds numbers, and Mach numbers is proposed, which can simplify the design of two-dimensional hypersonic inlets. With varying the dimensionless cowl length, three different types of short-cowl, transitional, and long-cowl unstarts are identified, owing to the different evolution mechanisms of separation region. The unstart displays a separation region growing under the streamwise pressure difference enhanced by the shock system or flow choke. In addition, the wall pressure criteria for unstart are correlated to the characteristic separation scale at unstart critical state and the dimensionless cowl length, which facilitates the unstart detection or early warning. It is observed that the Kantrowitz limit is close to the unstart boundaries for very short cowl and the isentropic limit is close to the unstart boundaries for very long cowl.

Cowl Length Variation on Performance Characteristics of a Single Expansion Ramp Nozzle

Experiments have been carried out to investigate the effect of cowl length variation on performance characteristics of a single expansion ramp nozzle. The performance parameters were estimated for cowl lengths of 0, 25, 50, 75, and 100% with respect to the horizontal length of the ramp. Experiments were conducted for different nozzle pressure ratios ranging between 1.5 and 9. The wall static pressure distribution data were measured from the tests to estimate the various performance parameters, such as axial thrust, normal force, gross thrust, thrust vectoring angle, and coefficient of pitching moment. High-speed schlieren imaging was used to visualize the flow separation and shock patterns and to measure the jet width. The flow was separated from the ramp wall up to a nozzle pressure ratio of 3 for all cowl cases. The shorter cowl length delays the downstream movement of shock-induced boundary separation inside the nozzle as compared to the longer cowl. The cowl trailing-edge flow was more underexpanded than the ramp tip flow. As cowl length increases, the increased restriction results in higher axial thrust and also increases the normal force. The pitching moment and thrust vectoring were dominated by normal force. Overall, as the nozzle pressure ratio increases, the axial force and jet width increase, whereas the normal force and the pitching moment increase up to a certain level and then decrease. As the cowl length increases, the axial thrust, normal thrust, pitching moment, and thrust vector angle increase, while the jet width decreases.

Experimental investigation of the effect of extended cowl on the flow field of planar plug nozzles

Plug and ramp nozzles offer many advantages over the conventional converging-diverging counterpart and have received much attention in the recent decades. Variants of plug nozzles have been investigated for a wide range of operating conditions. The performance of these nozzles is dependent on the flow development on the plug or ramp surface, which in turn is greatly influenced by the cowl geometry. Experiments are conducted to study the planar plug nozzle flowfield for Mach 1.8 and 2.2 using half nozzle geometry. The work primarily focuses on the influence of the cowl length on the flow evolution on the plug surface at different nozzle pressure ratios (NPRs). The cowl of the outer nozzle is extended to 10, 30, 50 and 100% of the full plug length. This allows the supersonic flow to partially expand internally ahead of the throat section. Schlieren images are used to visualize the wave structure at different pressure ratios for different cowl lengths. For low NPRs, the nozzle with extended cowl behaves more like a conventional planar nozzle with strong shock waves. It is observed that the cowl length influences the pressure distribution on the plug surface only for low NPRs. The effect of side walls on the flow field of planar plug nozzles is also studied.

Aerodynamic Characteristics of an Airfoil with Boundary Layer Ingestion in Subsonic Flow

The influence of boundary layer ingestion (BLI) on the aerodynamics of airfoil RAE2822 in subsonic is investigated by numerical method. Based on the calculation, it is found that the mass flow ratio (MFR), intake height, cowl length and intake position in the chord can affect on the lift, drag and pitching moment remarkable. Considering a maximum lift-over-drag, intake position in the chord should be rearward as possible with a certain high MFR at a small angle of attack and a considerable intake height. There is a proper MFR which makes drag lowest.

COMPUTATIONAL STUDY OF THE EFFECT OF GEOMETRIC PARAMETERS ON THE PERFORMANCE OF SINGLE EXPANSION RAMP NOZZLE FLOWS

The present work deals with the computational study of the single expansion ramp nozzle (SERN) flows for the supersonic regime. Theoretical calculations are carried out for different geometries with certain assumptions. Based on the initial calculation and with the help of Method of Characteristics, nozzle configuration with similar axisymmetric convergent portion and distinct non-axisymmetric divergent portions are drafted. Commercial CFD software ANSYS 12 with Navier Stokes code is used for the computational studies. The study begins with a simple upper flat ramp at different angles with a very small lower cowl at fixed angles. The divergent section of a conventional three-dimensional nozzle is used as the upper ramp of the SERN. The study includes the variation of the ramp at different angles ranging from 16 deg. to 24 deg. with 2 deg. intervals, the cowl angle ranging from 0 deg. to 6 deg. with 2 deg. intervals and the length of the cowl varying from 0.2, 0.4, 0.6, 0.8 and 1.0 times the length of the ramp. The performance parameters like Thrust, Normal force and Mach Variation for these different contours are analyzed and compared. The various parameters used are nozzle length, cowl length, cowl angle, ramp-angle, and ramp length. The shape of the SERN is optimized for the design parameters. The grid sensitivity computational analysis is also performed before finalizing the grid for study.

Numerical simulations of the flow through the inlet and isolator of a Mach 4 dual mode scramjet

AbstractResults from numerical simulations of the three dimensional flow in the intake-isolator of a dual mode scramjet are presented. The FANS calculations have utilised the SST k -ω turbulence model. The effect of cowl length and cowl convergence angle on the inlet mass capture ratio, flow distortion, shock strength and pressure rise are studied in detail. Three cowl lengths and four or five cowl convergence angles for each cowl length are considered. The predicted values of the dimensionless wall static pressure and inlet mass capture ratio are compared with experimental data reported in the literature. The numerical predictions are shown to agree well with the experimental data. In addition, details of the flow field such as shocks, expansion fans and shock boundary interaction are also captured accurately. Inlet unstart is also demonstrated for one case.

Studies on Unsteady Shock Interactions Near a Generic Scramjet Inlet

An experimental and numerical study is presented to show the effect of cowl length and angle on the ramp/cowl shock interaction phenomena fora two-dimensional planar scramjet inlet model. Experiments areconducted in a hypersonic shock tunnel, at Mach 8, at four lengths of owl and three cowl angles. Investigations include schlieren flow Visualization near the cowl region and static pressure and heat transfer rate measurement inside the inlet chamber. Various ramp/cowl shock interaction processes resulted for different cowl configurations have been visualized using a high-speed camera. Edney type-II interference pattern is observed for 131 and 141-mm cowl lengths,whereas it is an Edney type-I interference pattern in case of a 151 mm cowl with all their typical features resulting because of the ramp/cowl shock interaction. Experiments with a cowl configuration other than 0deg show the flow to he established through the inlet because or the reduced contraction ratio. Heat transfer peaks can be observed for the10 and 20-deg cowl cases where flow through the inlet is found to be established. These may serve as the possible locations of fuel injection.

Performance Characteristics of an Annular ConicalAerospike Nozzle with Freestream Effect

An experimental investigation has been carried out to study the performance and base pressure characteristics of a Mach 2.0 annular conical aerospike nozzle with and without freestream flow. The effect of cowl length, plug length, and plug contour variation on the nozzle performance and base pressure characteristics is studied. It is observed that the overexpansion shock from the internal nozzle, overexpansion shock on the spike surface, and the expansion fan from the cowl lip of the internal nozzle dominate the overall flowfield development. The presence of freestream flow reduces the nozzle performance by approximately 4% relative to static conditions. Base pressure characteristics are observed to be strongly influenced by the movement of these shocks on the plug surface, and their subsequent interaction with the inner shear layer controls the base-wake closure. Relative to the conical plug conflguration, the contoured plug shows considerably enhanced base pressure characteristics. Real-time pressure measurements on the spike reveal highly unsteady flow in the intermittent region of separation.

Numerical flow visualization of a Single Expansion Ramp Nozzle with hypersonic external flow

Numerical simulation of scramjet asymmetric nozzle flow is carried out to visualize and investigate the effects of interaction between engine exhaust and hypersonic external flow. The Single Expansion Ramp Nozzle (SERN) configuration studied here consists of flat ramp and a cowl with different combinations of ramp angle and cowl geometry. Using PARAS 3D, simulations are performed for a free stream Mach number of 6.5 that constitutes the external flow around the vehicle. Appropriate specific heats ratio has been simulated for the jet and free stream flow. External shock wave due to jet plume interaction with free stream flow, the internal barrel shock wave and the shear layer emanating from the cowl trailing edge and sidewalls are well captured. Wall static pressure distribution on the nozzle ramp for different nozzle expansion angles has been computed for both with and without side fence. Axial thrust and normal force have been evaluated by integrating the wall static pressure. Effect of cowl length variation and side fence on the SERN performance has also been studied and found to be quite significant. Based on this study, an optimum ramp angle at which the SERN generates maximum axial thrust is obtained. SERN angle of 20o was found to be optimum when the flight axis coincides with nozzle axis.

Design of an advanced nacelle for a very high bypass ratio engine

AbstractThe next challenge for future engines is to increase the bypass ratio in order to improve the propulsive efficiency. These very high bypass ratio engines improve the fuel consumption but create new technical challenges. These challenges include the fact that larger engines require bigger nacelles, hence the external drag and weight increase; furthermore the installation of these engines on civil aircraft is more challenging. Therefore important work has been carried out to optimise the design of these new nacelles and their installation on the airframe.The present work is based on a new engine, with a very high bypass ratio. For this engine Snecma has designed an advanced nacelle with a low diameter-low length fan cowl and a core cowl mounted thrust reverser. A very short secondary flow nozzle is required to improve accessibility to the core engine and its accessories. The fan cowl length is dramatically reduced.Different solutions were investigated to design the inlet and the fan cowl. Based on 2D potential inverse codes and 3D Euler codes, CFD calculations were made to optimise the aerodynamic lines for cruise conditions while keeping good off-design performance. A highly challenging exhaust nozzle was designed to maintain good efficiency with very tough geometric constraints. For this Snecma used 2D Navier-Stokes CFD codes. New reverse systems were studied for this type of nacelle, for example core cowl mounted thrust reversers. The optimisation of these highly 3D geometries was performed using 2D and 3D Navier-Stokes calculations.