High Propulsion Research Articles

A Study on the Hydrodynamic Characteristics of a Synthetic Jet Propelled Underwater Vehicle

In this article, the drag characteristics of an underwater vehicle propelled by a synthetic jet (SJ) is numerically studied. The underwater vehicle studied is REMUS Autonomous underwater vehicle (AUV). The cruising speed studied is up to 10 knots. The actuation parameters of the SJ actuator are the same under different cruising speeds. When the SJ actuator works, the drag of the vehicle changes periodically. The SJ-induced additional pressure drag increases with increasing cruising speed. The additional friction force is negligible. The thrust of the actuator decreases with increasing cruising speed. The thrust outputted by the SJ actuator when used for underwater vehicle propulsion is smaller than the thrust of an isolated actuator. The mathematical model of the additional drag, the model of the thrust, and the velocity distribution is given to assist the analysis in this article. 1. Introduction The advance in propulsion technique is the key to the evolution of underwater robots. The most common method for underwater robot propulsion is using screw propellers. Thrust is generated through the high speed rotation of the blades. The fluid is accelerated not only in the axial direction but also in the radial direction; however, only fluid acceleration in the axial direction can generate thrust. A significant part of the energy is wasted in the radial direction without thrust generation. As a result, the efficiency of screw propellers is low. Furthermore, the protruded screw propellers induce significant added resistance. This further reduces the total hydrodynamic efficiency of the vehicle. Nature always inspires the technical revolution in engineering. In nature there exist many marine creatures such as squids adopting jet propulsion. The jet created by squids is called pulsed jet. On the other hand, the jet of screw propellers is called steady jet. Studies about pulsed jet showed that compared with the steady jet, pulsed jet possess higher propulsion efficiency (Krueger 2001; Krueger & Gharib 2003, 2005; Moslemi & Krueger 2010; Lydia et al. 2011; Robert & John 2013). Seikman (1963), Weihs (1977), and recently Krueger and Gharib (2003) have shown that pulsed jets can give rise to a greater average thrust than steady jets of an equivalent mass flow rate. Lydia A. Ruiz, Robert W. Whittlesey, and John O. Dabiri found through experiment that pulsed jet has a propulsion efficiency that is 40% higher than that of steady jet, and a drag-based hydrodynamic efficiency increase more than 70% (Lydia et al. 2011). The mechanism behind this high efficiency is the formation of the vortex. The vortex will induce a radial velocity gradient at the nozzle exit. The velocity distribution becomes more ununiform compared with steady jet. As a result, the pressure at the nozzle exit will increase. This is observed by Krueger through experiment.

Supersonic Dual Bell Axisymmetric Minimum Length Nozzle Conception for High Propulsion Thrust

The aim of this work is to develop a new numerical computing program to design a new form of a dual bell axisymmetric supersonic minimum length nozzle contour, adapted to two different altitudes, usually at sea level and space, giving a supersonic uniform and parallel flow to the exit section. The first bell has a uniform and parallel sonic inlet and a low-altitude adaptation, with a reduced supersonic Mach number, while the second bell has a supersonic inlet and uniform and parallel flow and high altitude adaptation with high supersonic Mach number. The two bells are attached to an inflection point at the exit of the first bell. The passage from low altitude to high altitude is done without any mechanical activation. The purpose of this type of nozzle is the possibility of supersonic flight in two different regimes adapted into two different altitudes, under the assumption of a calorically and thermally perfect gas. This type of nozzle has an inflection point at the attachment point of the second bell to the first bell. The design is done using the method of characteristics. Solving the equations is done numerically by the predictor corrector algorithm. The validation of the results is controlled by the convergence of the critical sections ratio, calculated numerically, to that given by the theory. In this case, all the design parameters automatically converge towards the desired solution.

Optimization, Test and Diagnostics of Miniaturized Hall Thrusters.

Miniaturized spacecraft and satellites require smart, highly efficient and durable low-thrust thrusters, capable of extended, reliable operation without attendance and adjustment. Thermochemical thrusters which utilize thermodynamic properties of gases as a means of acceleration have physical limitations on their exhaust gas velocity, resulting in low efficiency. Moreover, these engines demonstrate extremely low efficiency at small thrusts and may be unsuitable for continuously operating systems which provide real-time adaptive control of the spacecraft orientation, velocity and position. In contrast, electric propulsion systems which use electromagnetic fields to accelerate ionized gases (i.e., plasmas) do not have any physical limitation in terms of exhaust velocity, allowing virtually any mass efficiency and specific impulse. Low-thrust Hall thrusters have a lifetime of several thousand hours. Their discharge voltage ranges between 100 and 300 V, operating at a nominal power of <1 kW. They vary from 20 to 100 mm in size. Large Hall thrusters can provide fractions of millinewton of thrust. Over the past few decades, there has been an increasing interest in small mass, low power, and high efficiency propulsion systems to drive satellites of 50-200 kg. In this work, we will demonstrate how to build, test, and optimize a small (30 mm) Hall thruster capable of propelling a small satellite weighing about 50 kg. We will show the thruster operating in a large space environment simulator, and describe how thrust is measured and electric parameters, including plasma characteristics, are collected and processed to assess key thruster parameters. We will also demonstrate how the thruster is optimized to make it one of the most efficient small thrusters ever built. We will also address challenges and opportunities presented by new thruster materials.

Fundamental Trade-offs in Communication and Trajectory Design for UAV-Enabled Wireless Network

The use of unmanned aerial vehicles (UAVs) as aerial communication platforms is of high practical value to future wireless systems such as 5G, especially for swift and on-demand deployment in temporary events and emergency situations. Compared to the static terrestrial base stations (BSs) in cellular networks, UAV-mounted aerial BSs possess stronger line-of-sight links with the ground users due to their high altitude as well as high and flexible mobility in 3D space, which can be exploited to enhance the communication performance. On the other hand, unlike terrestrial BSs that have reliable power supply, aerial BSs in practice have limited onboard energy, but require high propulsion power to stay airborne and support high mobility. Motivated by the above new considerations, this article aims to revisit some fundamental trade-offs in UAV-enabled communication and trajectory design. Specifically, it is shown that the communication throughput, delay, and (propulsion) energy consumption can be traded off among each other by adopting different UAV trajectory designs, which sheds new light on their existing trade-offs in terrestrial communication. The main design challenges and promising directions for future research are also discussed.

Оптимізація швидких міжпланетних траекторій з використанням ядерного дворежимного двигуна

Interplanetary missions require fast and fuel-efficient transfers. Combining small times transfers of high-thrust and efficiency of low-thrust propulsion can provide a good compromise. Saving an amount of fuel from the initial high-thrust burn and using it to correct the trajectory could lead to an economy of fuel. We investigated the optimal way to take advantages of both high and low-thrust propulsion benefits in order to maximize the payload mass of the mission. Using a simple model of ideal engine of limited power and the transporting trajectory method, we determined the analytical expression of final payload mass. The solution of the optimization problem gave us the optimal repartition of fuel between high and low-thrust maneuvers for a given thrust of thermal propulsion and electrical power of low-thrust propulsion system. As the mass of the low-thrust propulsion system depends on the electrical power, we took it into account to determine the optimal electrical power for a sprint trajectory in a given time. As a result, we could obtain the interval of transfer time for which the combination of high and low thrust becomes optimal.

Precooler-design & engine-performance conjugated optimization for fuel direct precooled airbreathing propulsion

Design study of the precooling heat exchanger specified for high speed airbreathing propulsion was carried out from the perspective of overall engine performance. The precooler proposed for the SABRE engine was adopted as the representative configuration for evaluation. Design procedure of the precooler was developed and incorporated into a cycle analysis model such that the variation in engine performance can be assessed as the design inputs is altered. By means of the differential-evolution algorithm, the design characteristics of the precooler were clarified through the idea of Pareto-Optimality. Precooler total mass and axial length, together with engine specific impulse, was included as optimization objectives on account of the results of parametric analysis. Pareto-optimal-fronts were obtained with the influence of some key design parameters such as the precooling temperature and the intake efficiency was analyzed. The results show that precooler design is a tradeoff between the optimization objectives considered. Selection of a larger precooling temperature helps to increase engine specific impulse and reduce the mass of precooler, although engine specific thrust is decreased slightly, whereas larger intake efficiency is extremely preferred for precooled cycles from not only the performance side of the engine, but more importantly, the geometry side of the precooler.

Hardware-in-the-loop simulation of high-speed maglev transportation five-segment propulsion system based on dSPACE

Aim: For exploring and testing the key technology of high-speed maglev transportation propulsion control system, this paper designs and establishes a hardware-in-the-loop (HIL) real-time simulation system of the high-speed maglev transportation five-segment propulsion system.&#x0D; Materials and methods of the studies: According to the route conditions and propulsion segment division of Shanghai maglev demonstration and operation line, the real-time simulation platform based on dSPACE multiprocessor systems is implemented. The simulation system can achieve the functional simulation of all the high-power related equipment in the 5-segment area, including 8 sets of high-power converter units, 2 sets of medium-power converter units, 2 sets of low-power converter units, five-segment trackside switch stations and long-stator linear synchronous motors. The mathematical models of linear motors and converters are built in MATLAB/Simulink and System Generator, after compiling, they can be downloaded and executed in Field Programmable Logic Array (FPGA). All the interfaces connecting the simulation system to the propulsion control system physical equipment use real physical components as in the field, such as analog I/O, digital I/O, optical signals and Profibus.&#x0D; Results: By using CPU+FPGA hardware configuration, the simulation steps are greatly shortened and the response speed and accuracy of real-time simulation system are improved. The simulation system can simulate multiple operating modes such as multi-segment, multi-vehicle, double-track, double-feeding, step-by-step stator section changeover, and so on. The simulation results show that the maximum speed of the simulation system can reach 500 km/h.&#x0D; Conclusion: This HIL system can provide detailed real-time on-line test and verification of high speed maglev propulsion control system.

Micromotor Pills as a Dynamic Oral Delivery Platform.

Tremendous progress has been made during the past decade toward the design of nano/micromotors with high biocompatibility, multifunctionality, and efficient propulsion in biological fluids, which collectively have led to the initial investigation of in vivo biomedical applications of these synthetic motors. Despite these recent advances in micromotor designs and mechanistic research, significant effort is needed to develop appropriate formulations of micromotors to facilitate their in vivo administration and thus to better test their in vivo applicability. Herein, we present a micromotor pill and demonstrate its attractive use as a platform for in vivo oral delivery of active micromotors. The micromotor pill is comprised of active Mg-based micromotors dispersed uniformly in the pill matrix, containing inactive (lactose/maltose) excipients and other disintegration-aiding (cellulose/starch) additives. Our in vivo studies using a mouse model show that the micromotor pill platform effectively protects and carries the active micromotors to the stomach, enabling their release in a concentrated manner. The micromotor encapsulation and the inactive excipient materials have no effects on the motion of the released micromotors. The released cargo-loaded micromotors propel in gastric fluid, retaining the high-performance characteristics of in vitro micromotors while providing higher cargo retention onto the stomach lining compared to orally administrated free micromotors and passive microparticles. Furthermore, the micromotor pills and the loaded micromotors retain the same characteristics and propulsion behavior after extended storage in harsh conditions. These results illustrate that combining the advantages of traditional pills with the efficient movement of micromotors offer an appealing route for administrating micromotors for potential in vivo biomedical applications.

Experimental and theoretical study of ultraviolet-induced structural/optical instability in nano silicon-based luminescence

Nano silicon is emerging as an active element for UV applications due to quantum confinement-induced widening of the Si bandgap, amenability to integration on Si, and less sensitivity to temperature. NanoSi-based UV applications include deep space exploration, high temperature propulsion, solar photovoltaics, and particle detection in high energy accelerators. However, the viability of the technology is limited by a complex nanoSi optical quenching instability. Here, we examined the time dynamics of UV-induced luminescence of sub 3-nm nanoSi. The results show that luminescence initially quenches, but it develops a stability at ∼50% level with a time characteristic of minutes. Upon isolation, partial luminescence recovery/reversibility occurs with a time characteristics of hours. To discern the origin of the instability, we perform first principles atomistic calculations of the molecular/electronic structure in 1-nm Si particles as a function of Si structural bond expansion, using time dependent density functional theory, with structural relaxation applied in both ground and excited states. For certain bond expansion/relaxation, the results show that the low-lying triplet state dips below the singlet ground state, providing a plausible long-lasting optical trap that may account for luminescence quenching as well as bond cleavage and irreversibility. Time dynamics of device-operation that accommodates the quenching/recovery time dynamics is suggested as a means to alleviate the instability and allow control of recovery, which promises to make it an effective alternative to UV-enhanced Si or metal-based wide-bandgap sensing technology.

Development of Swimming Abilities in Squid Paralarvae: Behavioral and Ecological Implications for Dispersal.

This study investigates the development of swimming abilities and its relationship with morphology, growth, and nourishment of reared Doryteuthis opalescens paralarvae from hatching to 60 days of age. Paralarvae (2.5–11 mm mantle length – ML) were videotaped, and their behavior quantified throughout development using computerized motion analysis. Hatchlings swim dispersed maintaining large nearest neighbor distances (NND, 8.7 ML), with swimming speeds (SS) of 3–8 mm s-1 and paths with long horizontal displacements, resulting in high net to gross displacement ratios (NGDR). For 15-day-old paralarvae, swimming paths are more consistent between jets, growth of fins, length, and mass increases. The swimming pattern of 18-day-old paralarvae starved for 72 h exhibited a significant reduction in mean SS and inability to perform escape jets. A key morphological, behavioral, and ecological transition occurs at about 6 mm ML (>35-day old), when there is a clear change in body shape, swimming performance, and behavior, paths are more regularly repeated and directional swimming is evident, suggesting that morphological changes incur in swimming performance. These squid are able to perform sustained swimming and hover against a current at significantly closer NND (2.0 ML), as path displacement is reduced and maneuverability increases. As paralarvae reach 6–7 mm ML, they are able to attain speeds up to 562 mm s-1 and to form schools. Social feeding interactions (kleptoparasitism) are often observed prior to the formation of schools. Schools are always formed within areas of high flow gradient in the tanks and are dependent on squid size and current speed. Fin development is a requisite for synchronized and maneuverable swimming of schooling early juveniles. Although average speeds of paralarvae are within intermediate Reynolds numbers (Re < 100), they make the transition to the inertia-dominated realm during escape jets of high propulsion (Re > 3200), transitioning from plankton to nekton after their first month of life. The progressive development of swimming capabilities and social interactions enable juvenile squid to school, while also accelerates learning, orientation and cognition. These observations indicate that modeling of the lifecycle should include competency to exert influence over small currents and dispersal patterns after the first month of life.

Alignment of Thermally Conducting Nanotubes Making High-Performance Light-Driving Motors

Light-actuating devices that can produce selective motions at small scales are highly desired for on-demand manipulation. For conventional photothermal motors that mostly encounter the homogenous light-induced heat diffusion at the liquid/air interface, it is challenging to effectively control the actuating direction and enhance the actuating speed. To this end, here, we explore aligned thermally conducting one-dimensional nanomaterials to make light-driving motors where the light-induced heat can be transmitted to the water surface along the length direction of the aligned one-dimensional nanomaterials to generate a localized surface tension gradient for high spatial resolution propulsion. When multiwalled carbon nanotubes were studied as a demonstration, the aligned active layer generated sufficient propulsion to drive a centimeter-sized motor that was 10 000 times higher in mass of the actuating layer on water. In addition, the actuating direction had been accurately controlled by varying the illuminated region of the active aligned nanotube layer. The resulting light-driving motors can move as fast as 4.19 cm/s (or 5.2 body length per second), which exceeded the previous motors based on the light activation.

Investigation of the hydrodynamic performance of crablike robot swimming leg

The existing amphibious robots cannot usually enjoy a superior adaptability in the underwater environment by replacing the actuators. Based on the bionic prototype of the Portunus trituberculatus, a new leg-paddle coupling crablike robot with a composite propulsion of walking legs and swimming legs is developed, with both the abilities of walking and swimming under water. By simulation and experiment, the effects of the phase difference, the flapping amplitude and the angular bias of the coupling movement, as well as the Strouhal number on the hydrodynamic performance of the swimming legs are studied, and the time dependent tail vortex shedding structure in a cycle is obtained. Both experimental and numerical results indicate that the thrust force with a high propulsion efficiency can be generated by a flapping swimming leg. This work can further be used for analysis of the stability and the maneuverability of the swimming leg actuated underwater vehicles.

Propulsion Principles of Water Striders in Sculling Forward through Shadow Method

Semi-aquatic arthropods skate on water surfaces with synergetic actions of their legs. The sculling forward locomotion of water striders was observed and analyzed in situ to understand and reproduce the abovementioned feature. The bright–edged elliptical shadows of the six legs of a water strider were recorded to derive the supporting force distributions on legs. The propulsion principles of water striders were quantitatively disclosed. A typical sculling forward process was accomplished within approximately 0.15 s. Water striders lifted their heads slightly and supported their weight mainly by the two driving legs to increase the propulsion force and reduce the water resistance during the process. The normalized thrust–area ratio (defined as the ratio of the propulsion force to the projected area) was usually lower than 0.4 after sculling for approximately 0.08 s. The entire normal supporting force remained nearly constant during a stroke to reduce the mass center fluctuation in the normal direction. In addition, water striders could easily control the locomotion direction and speed through the light swinging of the two hind legs as rudders. These sculling principles might inspire sophisticated biomimetic water-walking robots with high propulsion efficiency in the future.

Coupling of RF antennas to large volume helicon plasma

Large volume helicon plasma sources are of particular interest for large scale semiconductor processing, high power plasma propulsion and recently plasma-material interaction under fusion conditions. This work is devoted to studying the coupling of four typical RF antennas to helicon plasma with infinite length and diameter of 0.5 m, and exploring its frequency dependence in the range of 13.56-70 MHz for coupling optimization. It is found that loop antenna is more efficient than half helix, Boswell and Nagoya III antennas for power absorption; radially parabolic density profile overwhelms Gaussian density profile in terms of antenna coupling for low-density plasma, but the superiority reverses for high-density plasma. Increasing the driving frequency results in power absorption more near plasma edge, but the overall power absorption increases with frequency. Perpendicular stream plots of wave magnetic field, wave electric field and perturbed current are also presented. This work can serve as an important reference for the experimental design of large volume helicon plasma source with high RF power.

ACOUSTİC SİGNATURE OF FOUR CUTTERS EV BURNER USİNG PİLOT İNJECTOR FUEL SHİFT

ABSTRACTThe basic mechanisms leading to combustion oscillations is in the presence of pressure waves, the flow features large scale motions which drive the instability. The dynamics of the flame is then dominated by processes of hydrodynamic instability, vortex roll-up, vortex interactions, front and reacting stream pulsations, periodic extinctions and resignations, self-acceleration.Combustion instabilities are a major problem in the design of high performance propulsion systems (rocket motors, jet engine afterburners, ramjets). Instabilities are also observed in power plants and in various industrial processes. They are characterized by large oscillations of the flow parameters, which have many undesirable effects. Low frequency oscillations induce large mechanical vibrations in the system, including the combustion chamber, the feeding lines and the connected rotating machinery. Unstable operation enhances, the heat transfer rates at the combustor walls and in extreme cases this may lead to serious damage and even a total loss of the system.The present work investigates the changes induced by pilot injections in the acoustic signature of a newly designed 4-slot EV burner using pilot injector at reacting and non-reacting conditions. These changes are affecting the flame stabilization especially in the lean premixed combustion due to the large structures resulting from vortex breakdown and the swirling shear-layers. Pilot injector offered good results in flame stability. The injector position influences the flow field and in turns the flame stability. Different pilot injector fuel shift inside the burner are tested.All the results are to be compared with those obtained by thepreviously developed 2-slot and 4-slot configuration without pilot injector. This allows the identification of the merits and/or drawbacks of both designs.

Overall performance design of paralleled heat release and compression system for hypersonic aeroengine

The hydrogen powered hypersonic precooled combined cycle engine is an ideal candidate for next generation high speed clean civil aviation propulsion. It utilizes the energy inside the high speed air flow to power the air compression, in order to improve engine performance. Paralleled heat release and compression system, with several separated channels, is proved to be a crucial component to guarantee the favourable performance of the hypersonic precooled combined cycle engine. The system can sharply reduce the fuel consumption and cycle compressor pressure ratio (over 34% reduction in fuel consumption and over 45% reduction in cycle compressor pressure ratio, compared with ordinary single-route flow path design). The system is theoretically and quantitatively analysed. Operation mechanism, parameters design, feasibility and installing effects of the system are discussed. A performance simulation model is built for the paralleled system. The equivalent component method is proposed for measuring its effect on the engine overall performance. Inside the system, higher compression efficiency, lower pressure loss, more branches, higher heat transfer capacity and heat exchanger effectiveness can enhance the performance, while the feasible region, weight and size effects, and technical risks cannot be ignored either. For the engine, the installation of the paralleled system remarkably improves the engine performance and simplifies the compressor design. The paralleled system, with its advantage of coolant flow and compression work reduction, also can be beneficial to other applications in energy and power field.

High pressure waterjet propulsion with thrust vector control system applied on underwater vehicles

Maneuverability is one of the most important performances of underwater vehicles. To improve the maneuverability of underwater vehicles, a high-pressure waterjet propulsion system combined with a Thrust Vector Control System (TVCS) was presented, which controlled the sailing trajectory through adjusting the waterjet deflection angle and thrust vector instead of rudders and sterns. The TVCS was established in the form of 3RPS Parallel Manipulator and the position feedback was provided by measuring the displacements of three actuator cylinders. The relationship between the thrust direction and the displacements of the actuator cylinders was established through the procedure of solving an inverse kinematics problem. Simultaneously, the motion transformation function of the system was derived, and the Fuzzy-PID strategy was adopted. Accordingly, the Simulink model was built, and the simulation indicated that the system had good performance. In addition, the sailing trajectory was obtained, and the results showed that the turning diameter was 78 m, and it was independent of thrust and sailing speed. The underwater vehicle utilizing high-pressure water jet propulsion with TVCS had good maneuverability at either high or low speed conditions.

A Stiffness Adjustment Mechanism Based on Negative Work for High-efficient Propulsion of Robotic Fish

The applications of robotic fish require high propulsive efficiency mechanism to prolong the mission time. Though many methods were applied, robotic fish still suffers from low efficiency. To improve the efficiency of robotic fish, this paper proposes a variable stiffness mechanism which is based on the negative work. The live fish adjusts its body stiffness to save energy when the muscles do negative work. Inspired by the live fish, a control mechanism based on negative work is proposed to change the stiffness of the robotic fish for higher efficiency. Changing the stiffness of the robotic fish is to change the joint-stiffness. A fuzzy controller is introduced to mimic the variable stiffness mechanism of the fish and depicts the relationship between the stiffness and the negative work. To evaluate the performance of this controller, a two-joint robotic fish model is established based on its kinematic model and hydrodynamic model. The evaluation results show that the robotic fish reduces the energy consumption and improves the propulsion efficiency when introducing the variable stiffness mechanism. Different environments with the control mechanism impact differently on propulsive efficiency. This mechanism may provide a high efficient propulsion control method for the robotic fish.

Analytical approach to spacecraft formation-flying with low-thrust relative spiral trajectories

This work addresses the growing need for an intuitive, systematic approach to low-thrust spacecraft formation flying by extending shape-based continuous thrust trajectory design methods to the relative motion of two spacecraft. There is widespread interest in distributed space systems for their low costs, broad capabilities, and high redundancy. This trend introduces a new challenge for trajectory design when combined with the increasing prevalence of low-thrust, high specific impulse electric propulsion systems. That challenge is met herein with a geometrically intuitive, semi-analytical solution to the low-thrust problem. Beginning with the equations of relative motion of two spacecraft, an unperturbed chief and a continuously-thrusting deputy, a thrust profile is constructed which transforms the equations into a form that is solved analytically. The resulting relative trajectories are the family of sinusoidal spirals, which provide diversity for design and optimization based upon a single thrust parameter. Closed-form expressions are derived for the trajectory shape and time-of-flight for two prescribed relative velocity behaviors, and used to develop a novel patched-spirals trajectory design and optimization method. The example problem of a servicer spacecraft establishing and reconfiguring a formation around a target in geostationary earth orbit is used to demonstrate the application of the patched spirals technique as well as the advantages of the relative spiral trajectories over impulsive maneuvers. The sensitivity of the trajectory solutions to deviations from the underlying assumptions, uncertainties in the state, and errors in thrust are studied through high-fidelity simulation.

Computational and experimental study on dynamics behavior of a bionic underwater robot with multi-flexible caudal fins

PurposeThis paper aims to develop a novel type of bionic underwater robot (BUR) with multi-flexible caudal fins. With the coordinate movement of multi-caudal fins, BUR will combine the undulation propulsion mode of carangiform fish and jet propulsion mode of jellyfish together organically. The use of Computational Fluid Dynamics (CFD) and experimental method helps to reveal the effect of caudal fin stiffness and motion parameters on its hydrodynamic forces.Design/methodology/approachFirst, the prototype of BUR was given by mimicking the shape and propulsion mechanism of both carangiform fish and jellyfish. Besides, the kinematics models in both undulation and jet propulsion modes were established. Then, the effects of caudal fin stiffness on its hydrodynamic forces were investigated based on the CFD method. Finally, an experimental set-up was developed to test and verify the effects of the caudal fin stiffness on its hydrodynamic forces under different caudal fin actuation frequency and amplitude.FindingsThe results of this paper demonstrate that BUR with multi-flexible caudal fins combines the hydrodynamic characteristics of undulation and jet propulsion modes. In addition, the caudal fin with medium stiffness can generate larger thrust force and reduce the reactive power.Practical implicationsThis paper implies that robotic fish can be equipped with both undulation and jet propulsion modes to optimize the swimming performance in the future.Originality/valueThis paper provides a BUR with multi-propulsion modes, which has the merits of high propulsion efficiency, high acceleration performance and overcome the head shaken problem effectively.