Thrust Enhancement Research Articles

Swimming freely near the ground leads to flow-mediated equilibrium altitudes

Experiments and computations are presented for a foil pitching about its leading edge near a planar, solid boundary. The foil is examined when it is constrained in space and when it is unconstrained or freely swimming in the cross-stream direction. It was found that the foil has stable equilibrium altitudes: the time-averaged lift is zero at certain altitudes and acts to return the foil to these equilibria. These stable equilibrium altitudes exist for both constrained and freely swimming foils and are independent of the initial conditions of the foil. In all cases, the equilibrium altitudes move farther from the ground when the Strouhal number is increased or the reduced frequency is decreased. Potential flow simulations predict the equilibrium altitudes to within 3 %–11 %, indicating that the equilibrium altitudes are primarily due to inviscid mechanisms. In fact, it is determined that stable equilibrium altitudes arise from an interplay among three time-averaged forces: a negative jet deflection circulatory force, a positive quasistatic circulatory force and a negative added mass force. At equilibrium, the foil exhibits a deflected wake and experiences a thrust enhancement of 4 %–17 % with no penalty in efficiency as compared to a pitching foil far from the ground. These newfound lateral stability characteristics suggest that unsteady ground effect may play a role in the control strategies of near-boundary fish and fish-inspired robots.

Effect of chordwise wing flexibility on flapping flight of a butterfly model using immersed-boundary lattice Boltzmann simulations.

Wing flexibility is one of the important factors not only for lift and thrust generation and enhancement in flapping flight but also for development of micro-air vehicles with flapping wings. In this study, we construct a flexible wing with chordwise flexibility by connecting two rigid plates with a torsion spring, and investigate the effect of chordwise wing flexibility on the flapping flight of a simple butterfly model by using an immersed boundary-lattice Boltzmann method. First, we investigate the effects of the spring stiffness on the aerodynamic performance when the body of the model is fixed. We find that the time-averaged lift and thrust forces and the required power increase with the spring stiffness. In addition, we find an appropriate range of the spring stiffness where the time-averaged lift and thrust forces are larger than those of the rigid wings. The mechanism of the lift and thrust enhancements is as follows: in the downstroke the flexible wings can generate not only the lift force but also the thrust force due to the deformation of wings; in the upstroke the flexible wings can generate not only the thrust force but also the lift force due to the deformation of wings. Second, we simulate free flights when the body of the model can only move translationally. We find that the model with the flexible wings at an appropriate value of the spring stiffness can fly more effectively than the model with the rigid wings, which is consistent with the results when the body of the model is fixed. Finally, we simulate free flights with pitching rotation. We find that the model gets off balance for any value of the spring stiffness. Therefore, the passive control of the pitching motion by the chordwise wing flexibility cannot be expected for the present butterfly model.

Passing-over leading-edge vortex: The thrust booster in heaving airfoil

The thrust generated by heaving airfoil in incompressible flow is studied both numerically and theoretically. It is found that the behaviors of leading-edge vortex (LEV) under different airfoil’s transverse velocities have an important influence on the thrust. When this velocity is small, the LEV is advected downstream regularly; however, as transverse velocity increases, the LEV can be pushed back to the leading edge again and pass over it to the opposite side of the airfoil. In cases with the LEV passing over the leading edge, the maximum transient thrust can be enhanced by several times compared with that in the single-stroke motion. The reason and critical condition for the occurrence of passing-over leading-edge vortex (PO-LEV) are found. Based on the source of the pressure Poisson equation, a near-field force theory is developed and used together with the boundary vorticity flux (BVF) theory to clarify the thrust enhancement mechanisms.

A trapezoidal wing equivalent to a Janatella leucodesma’s wing in terms of aerodynamic performance in the flapping flight of a butterfly model

Wing planform is one of the most important factors for lift and thrust generation and enhancement in flapping flight. In a previous study based on a simple numerical model of a butterfly, we found that the wing planform of an actual butterfly (Janatella leucodesma) is more efficient than any rectangular or trapezoidal wing planform. In the present study, we make a hypothesis that the efficient aerodynamic performance of a butterfly’s wings can be reproduced by the following four geometrical parameters of wing planform: aspect ratio, taper ratio, position of the rotational axis for the geometric angle of attack, and sweepback angle. In order to test this hypothesis, we explore a trapezoidal wing planform equivalent to an actual butterfly’s wing planform in terms of aerodynamic performance in a parameter space consisting of these four parameters. We use a simple butterfly model composed of two rigid thin wings and a rod-shaped body and calculate the aerodynamic performance of the model by an immersed boundary–lattice Boltzmann method to find such a trapezoidal wing planform. As a result, we find a trapezoidal wing planform which gives almost the same lift, thrust, pitching moment, power, and power-loading coefficients as an actual butterfly’s wing planform. Furthermore, in the free flight of the butterfly model with pitching motion control, the flight behavior of the model with the resulting trapezoidal wing planform is almost the same as that with an actual butterfly’s wing planform.

Flow interactions of two- and three-dimensional networked bio-inspired control elements in an in-line arrangement

We present experiments that examine the modes of interaction, the collective performance and the role of three-dimensionality in two pitching propulsors in an in-line arrangement. Both two-dimensional foils and three-dimensional rectangular wings of AR = 2 are examined. In contrast to previous work, two interaction modes distinguished as the coherent and branched wake modes are not observed to be directly linked to the propulsive efficiency, although they are linked to peak thrust performance and minimum power consumption as previously described (Boschitsch et al 2014 Phys. Fluids 26 051901). In fact, in closely-spaced propulsors peak propulsive efficiency of the follower occurs near its minimum power and this condition reveals a branched wake mode. Alternatively, for propulsors spaced far apart peak propulsive efficiency of the follower occurs near its peak thrust and this condition reveals a coherent wake mode. By examining the collective performance, it is discovered that there is an optimal spacing between the propulsors to maximize the collective efficiency. For two-dimensional foils the optimal spacing of X* = 0.75 and the synchrony of ϕ = 2π / 3 leads to a collective efficiency and thrust enhancement of 42% and 38%, respectively, as compared to two isolated foils. In comparison, for AR = 2 wings the optimal spacing of X* = 0.25 and the synchrony of ϕ = 7 π / 6 leads to a collective efficiency and thrust enhancement of 25% and 15%, respectively. In addition, at the optimal conditions the collective lateral force coefficients in both the two- and three-dimensional cases are negligible, while operating off these conditions can lead to non-negligible lateral forces. Finally, the peak efficiency of the collective and the follower are shown to have opposite trends with increasing spacing in two- and three-dimensional flows. This is correlated to the breakdown of the impinging vortex on the follower wing in three-dimensions. These results can aid in the design of networked bio-inspired control elements that through integrated sensing can synchronize to three-dimensional flow interactions.

Thrust Augmentation by Airframe-Integrated Linear-Spike Nozzle Concept for High-Speed Aircraft

The airframe-integrated linear-spike nozzle concept applied to an external nozzle for high-speed aircraft was evaluated with regard to the thrust augmentation capability and the trim balance. The main focus was on the vehicle aftbody. The baseline airframe geometry was first premised to be a hypersonic waverider design. The baseline aftbody case had an external nozzle comprised of a simple divergent nozzle and was hypothetically replaced with linear-spike external nozzle configurations. Performance evaluation was mainly conducted by considering the nozzle thrust generated by the pressure distribution on the external nozzle surface at the aftbody portion calculated by computer simulation at a given cruise condition with zero angle of attack. The thrust performance showed that the proposed linear-spike external nozzle concept was beneficial in thrust enhancement compared to the baseline geometry because the design of the proposed concept had a compression wall for the exhaust flow, which resulted in increasing the wall pressure. The configuration with the boattail and the angled inner nozzle exhibited further improvement in thrust performance. The trim balance evaluation showed that the aerodynamic center location appeared as acceptable. Thus, benefits were obtained by employing the airframe-integrated linear-spike external nozzle concept.

Design of a double parabolic supersonic nozzle and performance evaluation by experimental and numerical methods

Purpose The purpose of this paper is to design a double parabolic nozzle and to compare the performance with conventional nozzle designs. Design/methodology/approach The throat diameter and divergent length for Conical, Bell and Double Parabolic nozzles were kept same for the sake of comparison. The double parabolic nozzle has been designed in such a way that the maximum slope of the divergent curve is taken as one-third of the Prandtl Meyer (PM) angle. The studies were carried out at Nozzle Pressure Ratio (NPR) of 5 and also at design conditions (NPR = 3.7). Experimental measurements were carried out for all the three nozzle configurations and the performance parameters compared. Numerical simulations were also carried out in a two-dimensional computational domain incorporating density-based solver with RANS equations and SST k-ω turbulence model. Findings The numerical predictions were found to be in reasonable agreement with the measured experimental values. An enhancement in thrust was observed for double parabolic nozzle when compared with that of conical and bell nozzles. Research limitations/implications Even though the present numerical simulations were capable of predicting shock cell parameters reasonably well, shock oscillations were not captured. Practical implications The double parabolic nozzle design has enormous practical importance as a small increase in thrust can result in a significant gain in pay load. Social implications The thrust developed by the double parabolic nozzle is seen to be on the higher side than that of conventional nozzles with better fuel economy. Originality/value The overall performance of the double parabolic nozzle is better than conical and bell nozzles for the same throat diameter and length.

A mechanism of thrust enhancement on a heaving plate due to flexibility at moderately low Reynolds numbers

A numerical study is conducted to investigate the force mechanisms for a 3D heaving flexible plate from the perspective of a diagnostic force element analysis. The problem is relevant to a flapping fish-tail simplified as a flat plate with the leading edge held heaving in the space. The flow is assumed to be laminar with the Reynolds numbers fixed at Re=200 or 500, and the Strouhal number St ranging from 0.1 to 0.6, and the flexure amplitude of the plate a0 from 0.1 to 0.25 (dimensionless). As the finite plate is set into unsteady motion, complicated vortex patterns around the plate are generated. It is shown that heaving, whilst increasing thrust generation, also reduces the frictional drag, yet the flexibility promotes thrust generation at the expense of accruing more frictional drag. In the literature, the force (thrust) exerted on the tail-mimicking plate is largely credited to the vortices (vorticity) in the wake. However, this study performs a regional force analysis to show that the vorticity in the wake region supplies approximately 20–30% of the total thrust, especially in the cases of strong thrust generation. Comparable contributions come also from the regions direct above and below the heaving plate (mainly including the attached vortices) as well as from the two side regions (mainly including the tip vortices) next to the flexible plate. In addition, the potential motion associated with the unsteady flapping and the contribution from the surface vorticity are non-negligible constituent force components. The relative importance of the various force contributions was analyzed in detail, and the results may shed light on how a flapping tail generates propulsive efficiency above the critical Strouhal number St=0.2.

Thrust Enhancement of a Convergent-Divergent Nozzle by Using CFD

Thrust Enhancement of a Convergent-Divergent Nozzle by Using CFD

The swimming patterns and energy-saving mechanism revealed from three fish in a school

Fish in nature are believed to favor their swimming performance by swimming in a school. We investigated numerically the thrust and power consumption of a school of fish with an arrangement of one upstream and two downstream. The upstream fish enhances the thrust with an area of high pressure at the heads of the two downstream fish, but the region of low pressure created between the two downstream fish generates a strong suction and is detrimental to the swimming of the upstream fish. The thrust forces increase but the power consumptions of the downstream fish decrease. The downstream fish in this arrangement can avoid the jet flow of the upstream fish and benefit from the counter flow at the verge of the upstream wake. The power-saving mechanism is obvious and sensitive to the longitudinal distance when the fish are in a school within a small lateral distance. The maximum average power consumption in a school is 20% less than not in a school. The mechanisms of thrust enhancement and energy saving in a school of fish interact in manifold ways, and provide insight into the bio-inspired design for arrays of underwater vehicles.

Thrust Enhancement on a Two Dimensional Elliptic Airfoil in a Forward Flight

Thrust Enhancement on a Two Dimensional Elliptic Airfoil in a Forward Flight

Hydrodynamics of swimming in stingrays: numerical simulations and the role of the leading-edge vortex

Stingrays, in contrast with many other aquatic animals, have flattened disk-shaped bodies with expanded pectoral ‘wings’, which are used for locomotion in water. To discover the key features of stingray locomotion, large-eddy simulations of a self-propelled stingray, modelled closely after the freshwater stingray,Potamotrygon orbignyi, are performed. The stingray’s body motion was prescribed based on three-dimensional experimental measurement of wing and body kinematics in live stingrays at two different swimming speeds of 1.5 and$2.5L~\text{s}^{-1}$($L$is the disk length of the stingray). The swimming speeds predicted by the self-propelled simulations were within 12 % of the nominal swimming speeds in the experiments. It was found that the fast-swimming stingray (Reynolds number$Re=23\,000$and Strouhal number$St=0.27$) is approximately 12 % more efficient than the slow-swimming one ($Re=13\,500$,$St=0.34$). This is related to the wake of the fast- and slow-swimming stingrays, which was visualized along with the pressure on the stingray’s body. A horseshoe vortex was discovered to be present at the anterior margin of the stingray, creating a low-pressure region that enhances thrust for both fast and slow swimming speeds. Furthermore, it was found that a leading-edge vortex (LEV) on the pectoral disk of swimming stingrays generates a low-pressure region in the fast-swimming stingray, whereas the low- and high-pressure regions in the slow-swimming one are in the back half of the wing and not close to any vortical structures. The undulatory motion creates thrust by accelerating the adjacent fluid (the added-mass mechanism), which is maximized in the back of the wing because of higher undulations and velocities in the back. However, the thrust enhancement by the LEV occurs in the front portion of the wing. By computing the forces on the front half and the back half of the wing, it was found that the contribution of the back half of the wing to thrust in a slow-swimming stingray is several-fold higher than in the fast-swimming one. This indicates that the LEV enhances thrust in fast-swimming stingrays and improves the efficiency of swimming.

Dual Drive Booster for a Two-Spool Turbofan: Performance Effects and Mechanical Feasibility

A novel two-spool turbofan engine configuration is described which uses a booster powered by both the low an high pressure spools. Design and off-design performance analysis shows the operating characteristics of the configuration, and a mechanical feasibility study of the gearbox is presented. The trends toward ever higher engine overall pressure ratio and bypass ratio have resulted in a combination of higher pressure ratio and lower blade speed in the booster compressor of conventional two-spool turbofans. This combination gives rise to many stages in the booster and/or lower booster efficiency and also a higher degree of off-design mismatch between the core compressors. The current paper describes an engine architecture which aims to alleviate both these issues by powering the booster compressor from both low and high pressure spools through an epicyclic gear system. We have called this engine architecture the dual drive booster. The concept gives the engine designer greater flexibility to optimize component performance and work split, resulting in the potential for lower cruise specific fuel consumption and higher hot-day takeoff thrust capability than current engine configurations. The gear system is described along with the mathematical derivation of the booster rotational speed in terms of LP- and HP-spool speeds. Both the design point and off-design performance modeling have been conducted and comparison is made between a conventional turbofan and a turbofan fitted with the dual drive booster. The results show a significant enhancement in takeoff thrust due to the better speed match of the booster. The paper also describes the results of a preliminary study into the design and mechanical feasibility of the engine architecture and gear system. The presented concept is an alternative to the conventional turbofan and should be considered during the conceptual design of future aircraft engines.

Flow shaping and thrust enhancement of sidewall bounded oscillating cantilevers

An oscillating cantilever is employed in a vast number of applications ranging from electronics cooling to propulsion. The motion can be driven at resonance by piezoelectrics which make it an energy efficient source of flow generation from a robust solid state device. Commonly known as piezoelectric fans, they have been the topic of numerous studies, and although many applications ultimately require mounting the cantilever within an enclosure of some form, much of the literature only considers idealized conditions, with walls far removed from the beam. Although it is commonly understood that, in general, sidewalls will help direct the flow in a desired direction, there is little knowledge into what impact this has on key performance characteristics such as power consumption, thrust, or convection enhancement. In this paper, in order to develop a strategic design approach for the enclosure, the thrust produced by a cantilever operating at resonance is quantified with two sidewalls present for a range of beam to wall spacings. Additionally, the sensitivity of the thrust on the relative location of the downstream edge of the sidewalls to the free end of the cantilever (fan tip) is experimentally investigated. It is found that the sidewall gap has little effect on thrust enhancement, except for very small gaps, and that the tip location plays a very large and interesting role in power consumption. In effect, there are cantilever tip locations where one can obtain substantial thrust enhancement with little or no extra power consumption, suggesting that flow shaping has the potential to positively impact the performance. The findings in the paper provide not only a relevant basis for further study, but also a very meaningful indication of the most effective orientation when considering the enclosure design around the oscillating cantilever.

Thrust Enhancement of a Microwave Ion Thruster

Two optical fiber measurement techniques are used in this paper to reveal the physical mechanism of the enhancement of the thrust force of the electron cyclotron resonance ion thruster. The beam current of the thruster was increased in previous studies by changing the propellant injection method. In this study, to observe the difference in plasma distributions, optical fiber probes were inserted into the thruster under beam acceleration. The first measurement was laser absorption spectroscopy. By traversing the optical fiber, the number densities of were obtained along the center axis. The second measurement was an electric-optic element probe measurement conducted to measure the intensities of the microwave electric field. Both measurements suggest that there is plasma in the waveguide in the conventional model of the thruster. This phenomenon is possibly caused by the leakage of electrons from the electron cyclotron resonance region to the waveguide. As a result, this paper concludes that the suppression of plasma in the waveguide is a very important measure to improve the performance of microwave thrusters.

Thrust Enhancement Through Bubble Injection Into an Expanding-Contracting Nozzle With a Throat

This paper addresses the concept of thrust augmentation through bubble injection into an expanding-contracting nozzle with a throat. The presence of a throat in an expanding-contracting nozzle can result in flow transition from the subsonic regime to the supersonic regime (choked conditions) for a bubbly mixture flow, which may result in a substantial increase in jet thrust. This increase would primarily arise from the fact that the injected gas bubbles expand drastically in the supersonic region of the flow. In the current work, an analytical 1D model is developed to capture choked bubbly flow in an expanding-contracting nozzle with a throat. The study provides analytical and numerical support to analytical observations and serves as a design tool for nozzle geometries that can achieve efficient choked bubbly flows through nozzles. Starting from the 1D mixture continuity and momentum equations, along with an equation of state for the bubbly mixture, expressions for mixture velocity and gas volume fraction were derived. Starting with a fixed geometry and an imposed upstream pressure for a choked flow in the nozzle, the derived expressions were iteratively solved to obtain the exit pressures and velocities for different injected gas volume fractions. The variation of thrust enhancement with the injected gas volume fraction was also studied. Additionally, the geometric parameters were varied (area of the exit, area of the throat) to understand the influence of the nozzle geometry on the thrust enhancement and on the flow conditions at the inlet. This parametric study provides a performance map that can be used to design a bubble augmented waterjet propulsor, which can achieve and exploit supersonic flow. It was found that the optimum geometry for choked flows, unlike the optimum geometry under purely subsonic flows, had a dependence on the injected gas volume fraction. Furthermore, for the same injected gas volume fraction the optimum geometry for choked flows resulted in greater thrust enhancement compared to the optimum geometry for purely subsonic flows.

Two generic concepts for space propulsion based on thermal nuclear fusion

In the present work, two different concepts for fusion based space propulsion are compared. While the first concept is based solely on propulsion by hypothetic ejection of fusion products and hence may be called ash drive, the second one uses an additional coolant for thrust enhancement. Since this coolant was initially assumed to be gaseous and since it is doing most of the propulsion work, the name of “working gas drive” has been proposed. Propulsive characteristics for both types are evaluated for four fusion reactant couples (D–T; D–3He; 3He–3He; 11B–p). In working gas drives, only hydrogen is considered as coolant due to its exceptionally good caloric and propulsive properties.The results of comparative studies show that while ash drives excel working gas drives in terms of specific impulse the latter yield considerably more thrust than ash drives. Another major drawback of the ash drives is relatively small thrust efficiencies. The plasma power has to be disposed of nearly entirely as waste heat leading to prohibitive radiator masses.

Propulsive performance of unsteady tandem hydrofoils in a side-by-side configuration

Experimental and analytical results are presented on two identical bio-inspired hydrofoils oscillating in a side-by-side configuration. The time-averaged thrust production and power input to the fluid are found to depend on both the oscillation phase differential and the transverse spacing between the foils. For in-phase oscillations, the foils exhibit an enhanced propulsive efficiency at the cost of a reduction in thrust. For out-of-phase oscillations, the foils exhibit enhanced thrust with no observable change in the propulsive efficiency. For oscillations at intermediate phase differentials, one of the foils experiences a thrust and efficiency enhancement while the other experiences a reduction in thrust and efficiency. Flow visualizations reveal how the wake interactions lead to the variations in propulsive performance. Vortices shed into the wake from the tandem foils form vortex pairs rather than vortex streets. For in-phase oscillation, the vortex pairs induce a momentum jet that angles towards the centerplane between the foils, while out-of-phase oscillations produce vortex pairs that angle away from the centerplane between the foils.

Biomimetic and Live Medusae Reveal the Mechanistic Advantages of a Flexible Bell Margin

Flexible bell margins are characteristic components of rowing medusan morphologies and are expected to contribute towards their high propulsive efficiency. However, the mechanistic basis of thrust augmentation by flexible propulsors remained unresolved, so the impact of bell margin flexibility on medusan swimming has also remained unresolved. We used biomimetic robotic jellyfish vehicles to elucidate that propulsive thrust enhancement by flexible medusan bell margins relies upon fluid dynamic interactions between entrained flows at the inflexion point of the exumbrella and flows expelled from under the bell. Coalescence of flows from these two regions resulted in enhanced fluid circulation and, therefore, thrust augmentation for flexible margins of both medusan vehicles and living medusae. Using particle image velocimetry (PIV) data we estimated pressure fields to demonstrate a mechanistic basis of enhanced flows associated with the flexible bell margin. Performance of vehicles with flexible margins was further enhanced by vortex interactions that occur during bell expansion. Hydrodynamic and performance similarities between robotic vehicles and live animals demonstrated that the propulsive advantages of flexible margins found in nature can be emulated by human-engineered propulsors. Although medusae are simple animal models for description of this process, these results may contribute towards understanding the performance of flexible margins among other animal lineages.

Fuzzy Thrust Staging Kinematics for the NERVA Strap-On Boosters

NERVA small space launcher under development in Romania by Politehnica University is based on the solution of hard take-off with three strep-on SRM boosters. Due to the very high thrust enhancement a lift-off loading of more than 40 g-s appears, which is quite challenging. The main challenge is connected with the possible uneven thrust and burn duration of the three SRE-s of the first stage, which we call fuzzy thrust. Experimental data show variations of up to 0.5 seconds in the duration of the powered phase of SRE-s, from a total of 3.0 seconds of a mean burn. Yet undetermined variations in the total ignition delay between individual motors are a stronger concern. While for the unique booster of the standard SA-2 vehicle the ignition delay has no impact, when those engines are forced to work in parallel the individual differences become catastrophic. Severe imbalance between the two lateral engines may occur, ending in a potentially severe damage of the launching rail, possible loss of flight stability along the boost phase and damage of the second stage structure. The problems regarding the efficiency of the propulsion system that accompany this project are considered and solutions are proposed for the high thrust augmentation of the booster stage. Cryogenic vs storable propellants are studied for thrust augmentation of the boosters.