Search results

This study aims to numerically study the three-dimensional (3D) flow field characteristics in a conical convergent divergent (CD) nozzle with an internal strut system to describe the effect of struts on producing a side force for thrust vectoring applications.

Struts are solid bodies. When inserted into the supersonic region of the axisymmetric CD nozzle, it induces a shock wave that causes an asymmetric pressure distribution predominantly over the internal surface of the diverging wall of the C-D nozzle, creating a net side force similar to the secondary injection thrust vectoring control method. Numerical simulations were performed by solving Unsteady Reynolds Averaged Navier–Stokes equations with re-normalized group k–ϵ turbulence model. Cylindrical struts of various heights positioned at different locations in the divergent section of the nozzle were investigated at a nozzle pressure of 6.61.

Thrust vectoring angle of approximately 3.8 degrees was obtained using a single cylindrical strut with a dimensionless thrust (%) and total pressure loss of less than 2.36% and 2.67, respectively. It was shown that the thrust deflection direction could also be changed by changing the strut insertion location. A strut located at half of the diverging length produced a higher deflection per unit total pressure loss.

Using a lightweight and high-temperature resistant material, such as a strut, strut insertion-based thrust vectoring control might provide an alternative thrust vectoring method in applications where a longer period of control is needed with a reduced overall system weight.

This study describes the 3D flow field characteristics which result in side force generation by a supersonic nozzle with an internal strut.

The purpose of this paper is to investigate the aerodynamics characteristics (especially the side force/moment and rolling characteristics), to analyze the impacts generated by different parameters of wrap-around fins (WAFs) and to find the corresponding mechanism.

The paper has adopted three different types of WAFs for the rocket configurations and the sub-regions divided technology to investigate the lateral and rolling characteristics of WAFs, including the fins with variations in span to chord ratio, thickness, leading-edge sweep, curvature radius, fin numbers, setting angles and rotated angles. Simulations have been performed at Mach numbers from 3 to 4 through an angle-of-attack range of about 0° to 10° and at model rolling angles of 45° to 90°.

The paper shows that the WAF configurations can greatly improve the longitudinal stability and enhance the longitudinal aerodynamic characteristics for the whole rocket. The total drag of the whole rocket is mainly stemmed from the body, while the drag generated by the WAF account for only about 7.42 per cent. The extra side forces and rolling moments are due largely to the unequal pressure distributions on both sides of the fin (windward or leeward). Maintaining a certain negative setting angle (d) can effectively avoid the coning movement and improve the flight stability at high angles of attack. The size of the span and chord are two main factors in controlling the longitudinal characteristics. For the side force/moment and rolling characteristics, different geometric parameters of the WAFs have played different roles.

The paper provides the qualitative and quantitative analysis for different WAFs configurations by investigating the curves of different parameters and contouring of static pressure distributions. Findings can provide some suggestions for the designers for avoiding some significant dynamic problems, such as Magnus instability and roll rate variations during flight.

One of the crucial questions confronting strategy and entrepreneurship scholars continues to be: where do new industry sectors come from? Extant literature suffers from a supply-side “skew” that focuses unduly on the role of heroic figures and celebrity CEOs, at the expense of demand-side considerations. In response, the purpose of this paper is to examine societal demand for entrepreneurial innovations. Employing historical data spanning nearly a century, the author assess more completely the role of latent demand-side signaling in driving the quantity and diversity of entrepreneurial innovation.

Applying the methods of historical econometrics, this study employs historical artifacts and cliometric models to analyze textual data in drawn from three distinctive sources: Popular Science Monthly magazine, from its founding in 1872 to 1969; periodicals, newsletters, club minutes, films and radio transcripts from the Science Society, from 1921 to 1969; and programs and news accounts from the US National High School Science Fair, from 1950 to 1969. In total, 2,084 documents containing 33,720 articles and advertisements were coded for content related to pure science, applied science and commercialized science.

Three key findings are revealed: vast opportunity spaces often exist prior to being occupied by individuals and firms; societal preferences play a vital role in determining the quantity and diversity of entrepreneurial activity; and entrepreneurs who are responsive to latent demand-side signals are likely to experience greater commercial success.

This study intentionally draws data from three markedly different textual sources. The painstaking process of triangulation reveals heretofore unobserved latencies that invite fresh perspectives on innovation discovery and diffusion.

This paper constitutes the most panoramic investigation to-date of the influence wielded by latent demand-side forces in the discovery and commercialization of innovation.

The fluid flow in a rotating channel is obviously different from that in a stationary channel due to the existence of Coriolis force, which, in turn, enhances the heat transfer on the trailing side and reduces the heat transfer on the leading side. The purpose of this paper is to study various rib configurations combined with channel orientation on heat transfer and frictional loss in a rotating channel.

In the present study, the k-ω SST model was used as the turbulence model. The fluid flow direction in the channel is radially outward. The angle between the rotation axis and leading side is 45°. The channel aspect ratio (W/H) is 2, the blockage ratio (e/D_n ) is 0.1 and the pitch ratio (P/e) is 10. The Reynolds number is fixed at 10,000 and the rotation number varies from 0 to 0.7. Angled ribs, reversed angled ribs, standard V-shaped ribs and outer-leaning V-shaped ribs, are examined.

It is found that the reversed angled rib configuration and the outer-leaning V-shaped rib configuration display better heat transfer performance than the V-shaped ribs in rotating condition, which is in contrast to stationary condition. At the leading side, the reversed angled rib and the outer-leaning V-shaped rib show better performance in recovering the heat transfer recession due to the negative effects of the Coriolis force.

In the present study, the fluid is incompressible with constant thermophysical properties and the flow is steady.

The results of this study will be helpful in design of ribbed channels internal cooling for turbine blade.

The results imply that the rib configuration combined with channel orientation significantly impacts the heat transfer performance in a rotating channel. The reversed angled rib and the outer-leaning V-shaped rib show better heat transfer performance than standard V-shaped ribs, especially at high Rotating numbers, which is in contrast to stationary condition. The outer-leaning V-shaped rib has a relatively good heat transfer uniformity along the widthwise direction.

Friction stir welding (FSW) is a novel method for joining materials without using consumables and without melting the materials. The purpose of this paper is to present the state of the art in robotic FSW and outline important steps for its implementation in industry and specifically the automotive industry.

This study focuses on the robot deflections during FSW, by relating process forces to the deviations from the programmed robot path and to the strength of the obtained joint. A robot adapted for the FSW process has been used in the experimental study. Two sensor‐based methods are implemented to determine path deviations during test runs and the resulting welds were examined with respect to tensile strength and path deviation.

It can be concluded that deflections must be compensated for in high strengths alloys. Several strategies can be applied including online sensing or compensation of the deflection in the robot program. The welding process was proven to be insensitive for small deviations and the presented path compensation methods are sufficient to obtain a strong and defect‐free welding joint.

This paper demonstrates the effect of FSW process forces on the robot, which is not found in literature. This is expected to contribute to the use of robots for FSW. The experiments were performed in a demonstrator facility which clearly showed the possibility of applying robotic FSW as a flexible industrial manufacturing process.

This paper aims to investigate the mechanisms lying behind the cycloidal rotor under hovering status.

Experiments were conducted to validate the numerical simulation results. The simulations were based on unsteady Reynolds-averaged Navier–Stokes (URANS) equations solver and the sliding mesh technique was used to model the blade motion. 2D and 2.5D simulations were made to investigate the 3D effects of turbulence. The effects of pressure and viscosity were compared to study the significance of the blade motion on force generation.

The 2.5D numerical simulation cannot produce more accurate results than the 2D counterpart. The pitching motion of the blade results in dynamic stall. The dynamic stall vortices induce parallel blade vortex interaction (BVI) upon downstream blades. The interactions between the blades delay the stall of the blade which is beneficial to the thrust generation. The blade pitching motion is the dominant contributor to the force generation and the turbulence is the secondary. Strong downwash in the rotor cage varied the inflow velocity as well as the effective angle of attack (AOA) of the blade.

Cycloidal rotor is a propulsion device that can provide omni-directional vectored thrust with high efficiency and low noise. To understand the mechanisms lying behind the cycloidal rotor helps the authors to design efficient cycloidal rotors for aircraft.

The authors discovered that the blade pitching motion plays primary role in force generation. The effects of the dynamic stall and BVI were studied. The reason why cycloidal rotor can be more efficient was discussed.

The purpose of this paper is to examine the ability of the harmonic balance method for predicting the aerodynamic characteristics of rigid finned spinning vehicle.

The aerodynamic characteristics of a rigid four-finned spinning vehicle at Mach number 2.5 and angle of attack of 20 degrees are simulated using the harmonic balance method and the unsteady time-accurate approach based on the dual-time method. The numerical results are analyzed, and the computed aerodynamic coefficients of the harmonic balance method are compared with those of the dual-time method. The influence of the number of harmonics is presented. The computed Magnus force and moment coefficients are compared with the experimental data. The flow fields at different roll angles are presented. The computational efficiency of harmonic balance method is analyzed.

The results show that the aerodynamic coefficients of spinning vehicle could be predicted by the harmonic balance method with reasonable accuracy compared with the dual-time method. For the harmonic balance method, the accuracy of the computed leeward side flow is relatively poor compared with that of the computed windward side flow. Meanwhile, the computational efficiency is influenced by initial guess and the intensity of unsteady effect.

The harmonic balance method could be used for the aerodynamic prediction of spinning vehicle, which may improve the efficiency of vehicle design.

This paper presents the results of the harmonic balance method for simulating the aerodynamic characteristics of finned spinning vehicle. The accuracy and efficiency of the method are analyzed.

IN the following notes the phenomenon of ground looping (which may be defined as an uncontrollable fast turn on the ground) is briefly analysed by semi‐technical methods. The analysis has been restricted to a condition which in the opinion of experienced pilots causes trouble most frequently: namely, a landing with dead stick, when there is no slipstream available to make the rudder more effective. The equations of lateral motion for an aeroplane in contact with the ground have been written clown as an introduction to more thorough study. Finally, certain tentative conclusions have been drawn for application in aeroplane design with a view to eliminating or at least reducing ground loop tendencies.

The purpose of this paper is to develop a directional and roll control system for unmanned combat air vehicle (UCAV) automatic takeoff roll, with the objective of keeping the UCAV along the runway centerline and keeping the wings level, especially when there is a crosswind.

The nonlinear model of the UCAV during takeoff roll is established. The model is linearized about the lateral‐directional equilibrium point at different forward speeds. The approximate directional model and roll model are extracted using time‐scale decomposition technique. Then the directional control law and roll control law are developed using gain scheduling approach. Nose wheel steering, differential brake and rudder are used as the primary directional control device at low, medium and high speeds, respectively, according to both the qualitative and quantitative analysis of their control effectiveness at different speeds. A priority matrix is developed to determine the secondary control device which is used if the primary control device fails, thus the directional control system can have a certain degree of fault tolerance.

This work developed the directional control law and roll control law by using gain scheduling approach. Experimental results verified that the developed directional and roll control system has high robustness and satisfactory fault tolerance: it can guarantee a safe takeoff under a 50 ft/sec crosswind, even if one directional control device fails, which satisfies the relevant criteria in MIL‐HDBK‐1797.

The directional and roll control system developed can be easily applied to practice and can steer the UCAV during takeoff roll safely, which will considerably increase the autonomy of the UCAV.

The paper shows how time‐scale decomposition technique is employed to extract the approximate directional model and roll model, which simplifies model analysis and control law design. A fault‐tolerant directional control system is designed to improve safety during takeoff.

This paper aims to present a deflection model to improve positional accuracy of industrial robots. Earlier studies have demonstrated the lack of accuracy of heavy-duty robots when exposed to high external forces. One application where the robot is pushed to its limits in terms of forces is friction stir welding (FSW). This process requires the robot to deliver forces of several kilonewtons causing deflections in the robot joints. Especially for robots with serial kinematics, these deflections will result in significant tool deviations, leading to inferior weld quality.

This paper presents a kinematic deflection model, assuming a rigid link and flexible joint serial kinematics robot. As robotic FSW is a process which involves high external loads and a constant welding speed of usually below 50 mm/s, many of the dynamic effects are negligible. The model uses force feedback from a force sensor, embedded on the robot, and predicts the tool deviation, based on the measured external forces. The deviation is fed back to the robot controller and used for online path compensation.

The model is verified by subjecting an FSW tool to an external load and moving it along a path, with and without deviation compensation. The measured tool deviation with compensation was within the allowable tolerance for FSW.

The model can be applied to other robots with a force sensor.

The presented deflection model is based on force feedback and can predict and compensate tool deviations online.