The launching of a waterjet rocket has been a very popular idea in recent years. Its basic propulsion principle makes use of the high-pressurized air inside the rocket’s main body to swiftly expel the water out of the nozzle and thus generate thrust. The waterjet rocket is characterized with nature, interest, combustionlessness, environmental friendliness, simplicity, and minimal cost. Moreover, it is a very good science model for propulsion analysis, design, experiment, and education because of an abundance of easily adjustable key parameters. This model also features separately stored energy and mass of the propellant, in contrast to a conventional rocket. However, related literature shows that no in-depth theoretical analysis of the waterjet rocket has been attempted for various reasons. In this research, the propulsion dynamics of a waterjet rocket is analyzed by simultaneously solving the momentum and the newly derived generalized power equations to predict its flight histogram, computationally, and convolutionally. This integrated energy approach synthesizes the internal and external dynamics analyses together and ingeniously takes full advantage of the clear power supply of pressurized air in a waterjet rocket. The analysis results are generally agreeable with the experimental flight data. While the new power equation herein gives a complete spectrum of physical parameters to be manipulated, there will be wider room in quest of better rocket propulsion performance, especially through the heuristic research of this versatile but affordable waterjet rocket.
The dynamic instability of a slender front-fixed-aft-free elastic strip in a uniform airflow is studied analytically. The analysis is performed by using the unsteady slender body aerodynamic theory and the beam theory. The analysis reveals that flutter may occur as a result of coupling between the first and second bending oscillation modes as a result of aerodynamic force when the flow speed overcomes the critical speed. One approximate method is derived, and the flutter speed, mode, and frequency are examined numerically. These approximate results are compared with other approximate ones derived by the two-term Galerkin method. And when the virtual-air-mass to strip mass ratio is sufficiently small, the flutter speed by the present method coincides with that by the two-term Galerkin method.
The Semi-Lagrangian (SL) scheme developed for incompressible Navier-Stokes equations written in generalized coordinates has been explored to accurately solve the high Reynolds number flows. The instability related to the advection term of the Navier-Stokes equations is classified into linear instability and nonlinear instability. The former is controlled by the CFL condition, and the latter is due to the aliasing error. The linear instability is naturally eliminated by employment of the SL scheme. The Kawamura scheme is creatively applied to approximate the first derivatives appearing in the Hermite interpolation function to remove the nonlinear instability. The resulting numerical scheme is unconditionally stable for incompressible flows at all Reynolds numbers. Accurate numerical solutions of the unsteady flow around a 2D circular cylinder at Reynolds numbers below 100,000 have been carried out. It was found that this flow has a wide range of wave number modes. Numerous grid numbers and a small time step length must be used to guarantee the accuracy. Furthermore, a very long time average should be conducted to compare the data with the experimental measurement.
The method of complex characteristics is used to describe four kinds of localized disturbances in the boundary layer on a flat plate. The disturbances are those introduced by vibrating ribbon, two-dimensional pulse through a slit parallel to the leading edge, continuous excitement through a small hole on the plate, and an instantaneous jet from the same hole. The corresponding four equation systems are numerically solved to show fundamental properties of these disturbances. It is also intended to estimate quantitative effects of the leading-edge sweep angle and of the boundary-layer nonparallelism on the development of a three-dimensional wave packet.
A three-dimensional grid adaptation method using the concept of anisotropic Cartesian grid has been developed to improve the efficiency of an existing Cartesian grid adaptation method by reducing the total number of cells needed to compute a flow field. The present grid adaptation is carried out by performing both grid coarsening and refinement in such a way that the cell aspect ratio can take an arbitrary value, keeping the grid smoothness. This flexibility necessitates the concept of unstructured approach. Two test cases: a cylinder in supersonic flow and an ONERA M6 wing in transonic flow show that the present method can well capture 2D and 3D flow features. In the cylinder case, the number of cells after the final adaptation cycle is one or two orders of magnitude less than that of the corresponding isotropic grid; in the ONERA M6 case it becomes about half as many. This remarkable saving in the 2D case is achieved because the spanwise domain is covered by only one cell in the present method, unlike conventional isotropic approaches. As a result of the decrease in the number of cells, the anisotropic grid requires less time to solve the flow and less amount of total memory, though the amount of memory per cell actually increases because of its unstructured property.
This paper studies the control problem of spacecraft with mobile appendages. In such a system, the motions of a spacecraft’s main body and the appendages act as disturbances to each other through nonlinear dynamic coupling. To this problem we apply the robust gain-scheduled control. From the viewpoints of synthesis and implementation, three types of approaches, i.e., simultaneous, sequential, and local design are proposed. Their capabilities are investigated through some numerical studies and compared with one another.
Fully-developed pilot-induced oscillation (PIO) is an important issue to be solved in the development of modern fly-by-wire flight control systems. A new method is presented to analyze the limit cycle phenomenon of the PIO, including the effects of actuator rate limiting and feedback control. By using describing function method, the frequency and the amplitude of the PIO limit cycle are obtained analytically. It is shown that the predictions obtained with this present method closely match results of the simulation.
The plume flow field of an arcjet thruster with hydrazine decomposed gas as the propellant has been numerically analyzed to reveal a backflow field, using Direct Simulation Monte Carlo (DSMC). Hydrazine decomposed gas is approximated by a mixture of gas composed of N2, H2, and H. Four different nozzle lip shapes have been prepared to investigate their effects on the backflow, and species existing in the backflow have also been examined. The present results of mass flux at various angles measured from the plume axis have been compared with experiments, and a qualitatively fairly good agreement has been obtained.