Serpentine Nozzle Research Articles

Comprehensive optimization design of aerodynamic and electromagnetic scattering characteristics of serpentine nozzle

Comprehensive optimization design of serpentine nozzle with trapezoidal outlet was studied to improve its aerodynamic and electromagnetic scattering performance. Serpentine nozzles with different center offsets and different ratios of the bases of the trapezoidal outlet were generated based on curvature control regulation. Computational Fluid Dynamics (CFD) simulations have been conducted to obtain the flow field in the nozzle, and Forward-Backward Iterative Physical Optics (FBIPO) method was applied to study the electromagnetic scattering characteristics of the nozzle. Guarantee Convergence Particle Swarm Optimization (GCPSO) algorithm based on Radial Basis Function (RBF) neural network was used to optimize the geometry of the nozzle in consideration of its aerodynamic and electromagnetic scattering characteristics. The results show that the GCPSO method based on RBF can be used to optimize the aerodynamic characteristics of the internal flow and the scattering characteristics of the cavity of the serpentine nozzle with irregular outlet. The optimized model has a higher center offset and a lower ratio of the bases of the trapezoidal outlet after optimization compared to the original model. The optimized model leads to a slight change in aerodynamic performance, with a total pressure recovery coefficient increase of 0.31% and a discharge coefficient increase of 0.41%. In addition, the Radar Cross Section (RCS) decreases also by around 83.33% and the overall performance is significantly improved, with a decrease of the optimized objective function by around 38.74%.

Concurrent multi-scale optimization of hybrid composite plates and shells for vibration

In this paper, a concurrent multi-scale optimization framework is established for hybrid composite plates and shells, where fiber volume, fiber orientation and stacking sequence can be optimized simultaneously. Firstly, a finite element model of shell structure that contains patches is established. Then, the candidate material set of hybrid composites is calculated and assembled by different combinations of fiber volume, fiber orientation and stacking sequence at the material and laminate scales. Furthermore, the Discrete Material Optimization (DMO) method is employed to perform the concurrent multi-scale optimization. Two illustrative examples are used to verify the effectiveness of the proposed optimization framework, including a simple example of a hybrid composite plate and a complex engineering example of a double serpentine nozzle. In comparison to optimal results by the traditional constant-stiffness design method, the optimal results of the proposed framework achieve significant 19.8% and 14.0% improvements in the fundamental frequency under the constraint of material cost, respectively. It can be concluded that the proposed concurrent multi-scale optimization framework has huge potential in adaptive stiffness tailoring for hybrid composite plates and shells with complex multi-scale design variables, which can make full use of hybrid composite materials to improve the structural performance against vibration while maintaining the low material cost.

Infrared signature of serpentine nozzle with engine swirl

Serpentine nozzle is used to improve the stealth performance of military aircraft due to its ability to suppress the infrared signature. Investigation of the effect of engine swirl on the infrared signature of the serpentine nozzle is the primary objective of this study. The results showed that the engine swirl could effectively suppress the infrared signature, while inducing a small performance loss. Moreover, the engine swirl mainly contributed to the infrared signature suppression from the gas. The hot streak induced by the engine swirl affected the wall infrared signature level; however, exhibited limited effect on the total infrared signature level of the serpentine nozzle. The swirl angle significantly affected the infrared signature level of the serpentine nozzle. Compared to the no-swirl case, the average reduction of the total infrared signature level increased from 2.29% to 13.84% in the horizontal plane, and from 5.83% to 31.21% in the vertical plane with the increase in the swirl angle from 5° to 20°.

Effect of Nozzle Geometry on Flowfield for High Subsonic Jets

The present Paper aims to present a comparative study of the external and internal flow characteristics through an axisymmetric circular convergent nozzle, a nonserpentine convergent nozzle with an elliptic exit, and a shallow single serpentine convergent nozzle with an elliptic exit under the fully expanded condition at a nozzle pressure ratio of 1.6. The static wall pressure measurements reveal that, due to the axisymmetric shape, the conventional circular nozzle shows symmetric jet development. However, the flow through the serpentine nozzle follows its curvature, making the flow through it asymmetric. The upper and the lower walls have different flow expansion due to the secondary flow created by the wall curvature and transition. This asymmetric nature of the flow through the serpentine nozzle also vectors the external jet toward the lower wall. The saddle-shaped velocity profile is observed along the major axis plane of serpentine nozzle. The nozzles with elliptic exits show higher mass entrainment and faster velocity decay as compared to the conventional circular nozzle. The nonserpentine elliptic nozzle shows the maximum velocity decay with the fastest decay rate among the three nozzles considered. This nozzle also shows the maximum mass entrainment across its major as well as minor axes and thus reduces the length of the external jet more effectively.

Investigation of Infrared Signature of Serpentine Nozzle for Turbofan

Infrared stealth technology is one of the essential abilities for advanced stealth aircraft. The exhaust system is the main infrared radiation source of aircraft. The serpentine nozzle is a potential configuration that could efficiently suppress the infrared signature of the exhaust system, but there are few papers focused on the criteria for designing a serpentine nozzle from the view of infrared suppression. Investigating the effect of the shield ratio on the infrared signature of the serpentine nozzle is the primary purpose of this paper. For the serpentine nozzle, the gas infrared signature level in the vertical plane is greater than that in the horizontal plane, and the wall infrared signature level at the negative azimuth angle in the vertical plane is greater than that at the positive azimuth angle. The shield ratio has an important impact on the infrared signature of the serpentine nozzle, and the analysis indicates that the serpentine nozzle with has a better stealth performance; additionally, the visible area ratio is suggested to be under 0.041.

Flow Characteristics of Double Serpentine Convergent Nozzle With Different Inlet Configuration

Serpentine nozzles have been used in stealth fighters to increase their survivability. For real turbofan aero-engines, the existence of the double ducts (bypass and core flow), the tail cone, the struts, the lobed mixers, and the swirl flows from the engine turbine, could lead to complex flow features of serpentine nozzle. The aim of this paper is to ascertain the effect of different inlet configurations on the flow characteristics of a double serpentine convergent nozzle. The detailed flow features of the double serpentine convergent nozzle including/excluding the tail cone and the struts are investigated. The effects of inlet swirl angles and strut setting angles on the flow field and performance of the serpentine nozzle are also computed. The results show that the vortices, which inherently exist at the corners, are not affected by the existence of the bypass, the tail cone, and the struts. The existence of the tail cone and the struts leads to differences in the high-vorticity regions of the core flow. The static temperature contours are dependent on the distributions of the x-streamwise vorticity around the core flow. The high static temperature region is decreased with the increase of the inlet swirl angle and the setting angle of the struts. The performance loss of the serpentine nozzle is mostly caused by its inherent losses such as the friction loss and the shock loss. The performance of the serpentine nozzle is decreased as the inlet swirl angle and the setting angle of the struts increase.

Internal flow and external jet characteristics of double serpentine nozzle with different aspect ratio

In order to increase the survivability of the fighter aircraft, the serpentine nozzle has been applied in series of stealth bombers and unmanned aerial vehicles due to its excellent potentiality of evidently suppressing the infrared radiation signatures and radar cross section emitted by engine exhausts. Among the geometric parameters of the serpentine nozzle, the aspect ratio (AR) at the nozzle exit is one of the most critical parameters for the nozzle design as the infrared suppression effect could be greatly enhanced with the increment of AR by strengthening the mixing between the exhaust plume and atmosphere; the aim of this paper is to study the influence of the AR on the flow characteristics of the double serpentine nozzle. The flow features of six double serpentine convergent nozzles, i.e. AR = 3, 5, 7, 9, 11, 15 respectively, were numerically simulated with the shear stress transport κ–ω turbulent model adopted, which had been validated by the experimental data. The characteristics of internal flow and external jet, and the aerodynamic performances of these six nozzles were compared. Results show that the Ma contours at the symmetric plane are different due to the distinct flow accelerations caused by the change of the curvature and the duct height for diverse AR, and the surface pressure and the shock wave features are different correspondingly. The lateral divergence and the lateral convergence characteristics of the nozzle configuration lead to opposite lateral flow under diverse AR, and the change of lateral width induced different lateral pressure gradient, then lead to various lateral vortex distributions. The length of potential core is the contribution of the comprehensive effects of geometry parameters, and it is decreased with the increase of AR due to the dominated effect of the increased mixing area; however, the declining rate is slowed down. The AR of 5 should be chosen for the best aerodynamic performance of the double serpentine nozzle under the qualifications to completely shield the high-temperature turbine.

Influences of shield ratio on the infrared signature of serpentine nozzle

The criteria for designing the serpentine nozzle on the aspect of infrared suppression is undocumented currently. The aim of this paper is to investigate the influences of the shield ratio on the infrared signature of the serpentine nozzle. Firstly, a validation study was conducted to get the reliable computational method of the infrared signature. Then, the infrared signatures of the serpentine nozzles with variable shield ratios were investigated numerically, and the comparison between the serpentine nozzle and the circular nozzle was also conducted. Results show that 28.9% of the infrared radiation intensity, on average, can be reduced by the single serpentine nozzle, as compare with the circular nozzle. The shield ratio has little effect on the infrared signature of gas outside of the serpentine nozzle exit. And the completely shielded serpentine nozzle cannot bring the greatest benefit to the infrared stealth. The single serpentine nozzle with shield ratio of 0.75 and the double serpentine nozzle with shield ratio of 0.5 have better infrared stealth performance than the others. The visible area ratio can be used as an indicator to evaluate the infrared signature of the serpentine nozzle. In order to provide an effective infrared suppression, the visible area ratio of the single serpentine nozzle and the double serpentine nozzle should be restricted under 0.15 and 0.35, respectively.

Flow Characterization for a Shallow Single Serpentine Nozzle with Aft Deck

An experimental investigation is carried out to characterize the flow through a shallow single serpentine curved convergent nozzle with an aft deck under fully expanded and underexpanded conditions with nozzle pressure ratios ranging from 1.1 to 2.2 for cold flows by wall static pressure measurements and flow visualization of the exit jet. The study revealed initial deceleration of the flow due to the turning of the streamlines caused by the nozzle curvature. Asymmetric coefficient of pressure variation in the cross-stream direction across various nozzle pressure ratios was observed, which is caused by secondary flow created by wall curvature and transition. The acceleration of the core flow due to geometric transition, as well as the additional nozzle length, causes early transition to sonic and supersonic flows, resulting in the formation of shock cells at relatively lower Mach number in the absence of the aft deck, as can be seen from the schlieren images. However, the presence of the constant area extension section reduced the flow asymmetry. Furthermore, the presence of the aft deck prevented the formation of barrel shocks and is expected to prevent the formation Mach disk(s) at higher nozzle pressure ratios. Thus, the curvature, transition, and aft deck complemented each other.

Influences of Design Parameters on a Double Serpentine Convergent Nozzle

Serpentine nozzles are supplied in stealth bombers and unmanned aerial vehicles (UAVs) to evidently suppress the infrared radiation signatures (IRSs) emitted by engine exhausts. It is commonly known that excessive geometric parameters are included in the double serpentine nozzle design and, as a result, the aim of this paper is to study the influences of the design parameters on the performance of double serpentine nozzle. To this end, the design method of the serpentine nozzle was concisely introduced, and the qualifications to completely shield turbine were given. Simulations using six different turbulence models were conducted and compared to the experimental data in order to determine the suitable turbulence model for serpentine duct simulations. Then, the effects of geometric design parameters at the first serpentine paragraph exit (the dimensionless width of W1/D, area of A1/Ain, and offset distance of ΔY1/L1) on the flowfield, and the performance of double serpentine nozzle was investigated numerically. The validation study shows that the simulations with shear-stress transport (SST) κ–ω turbulence model adopted can accurately predict the flux rate, the axial thrust, and the static pressure of the experimental nozzle, and therefore, SST κ–ω turbulence model is the most suitable turbulence model in the selected six turbulence models to be used for the simulation of the double serpentine nozzles. The numerical results show that friction loss increases with the increment of W1/D due to the increased wetted perimeter, but small value of W1/D would lead to large secondary flow loss; even the shock loss appears because of the steep curvature of the second turning. Small area of the first serpentine duct A1/Ain induces high flow velocity in the first duct, which corresponds to large friction loss. Steep offset distance of the first serpentine duct ΔY1/L1 induces high local losses. As the geometric design parameters of the double serpentine nozzle interact mutually with the qualifications to completely shield the turbine, the range of design parameters should be synthetically chosen during the design progress. Thus, the width of the first serpentine duct W1/D is recommended to be from 1.0 to 1.3. The area of the first serpentine duct A1/Ain should be as large as possible, and the offset distance of the first serpentine ΔY1/L1 should be small in the permission range of the design parameters.

Experimental and computational investigation of double serpentine nozzle

Infrared radiation signatures of gas turbine engine exhaust are suppressed markedly when equipped with a serpentine nozzle compared to an axisymmetric nozzle. The aim of this paper is to research more detailed flow characteristics of the serpentine nozzle, and to this end a double serpentine nozzle cold fluid test was conducted in this paper, static pressures on the nozzle walls surface were measured, and schlieren flow visualizations downstream of the nozzle exit were observed. Then numerical simulations of the experimental model were carried out using CFD software with k-ɛ turbulence model adopted. And the effects of geometric design parameters (the length ratio of first S length to second S length and the centerline distributions) on serpentine nozzle performance were investigated numerically. Detailed flow characteristics were presented including the distributions of static pressure, Ma number (streamlines), wall shear stress (limited streamlines), and the total pressure. Results show good agreement between the experimental data and computation. Static pressure distributions on the upper and down walls surface of double serpentine nozzle are completely different compared to the traditional axisymmetric nozzle. The rapid turning and steep passage slope of the serpentine nozzle would result in high friction loss and strong secondary flow loss, hence the value of the length ratio of first S passage to second S passage is recommended to be chosen from 2:5 to 2:3. The centerline distributions are crucial to the nozzle design for its influence on air acceleration inside the nozzle. The centerlines with a rapid turning at the exit would result in a high Ma number, which brings on high friction loss and secondary loss at the turnings. For maximum efficiency of centerline distributions, it is recommended that curves with a gentle turning at each serpentine passage exit should be chosen.

Effect of Aspect Ratio on Flow Through Serpentine Nozzles

4, providing a Dean number of 8400 (based upon radius of curvature at the centerline of the nozzle). The experimental portion of the study was conducted by three-component laser Doppler velocimetry using a five-beam probe. A computational study was performed on the interior of the nozzle using a commercial code that employed a Reynolds stress model. With noted exceptions, good correlation was observed between the computational and experimental solutions. Secondary-flow velocities at the nozzle exit revealed evidence of a complex pattern of streamwise vortices generated by the two bends. It was found that despite the common Dean number, the difference in aspect ratio led to substantially different velocity fields. The flow was dominated by a single pair of counterrotating streamwise vortices in one case, whereas its counterpart yielded a complex pattern of multiple smaller vortices.