This paper deals with time series analysis on the emission of light from methane-air lean premixed flames. Planar flames formed on a flat burner at the equivalence ratio of unity; cellular flames formed at low equivalence ratios, because diffusive-thermal effects have a destabilizing impact in this range. We measured the light emission to get time series of intensity, and obtained the root mean square (RMS) and power spectrum density of the emission intensity normalized by its average value. The RMS increased as the equivalence ratio decreased, which is due to the increase in the instability level. The power spectrum density had a sharp peak at the frequency corresponding to the typical oscillation of premixed flames, and the 1⁄f spectrum appeared in the low frequency range. Moreover, we performed time series analysis on the normalized emission intensity. We obtained the attractor and correlation dimension to investigate the characteristics of the dynamic behavior of flame fronts. The characteristics depended strongly on the equivalence ratio, i.e. on intrinsic instability. Consequently, the obtained results suggest that time series analysis on the emission of light is applicable to diagnostics of the instability of premixed flames.
Flows over two circular cylinders in tandem, side-by-side, and staggered arrangements were analyzed using the overset grid method, which is capable of handling a variety of sizes and arrangements. The Reynolds number was 100 based on the cylinder diameter. The present computation code was validated by comparison with benchmark solutions for flow around a single cylinder. Wind-tunnel experiments were conducted for the side-by-side cylinder flow for comparison with numerical simulations. Calculation showed two critical spacings in the tandem arrangement where the aerodynamic forces and Strouhal number change discontinuously. Three critical spacings and four distinct flow patterns were found numerically in the side-by-side arrangement. Similar critical spacings were found in the staggered arrangement calculation and formed critical lines. Furthermore, a pocket region was found for a staggered arrangement surrounded by the critical line.
Design exploration of a nacelle chine installation was carried out. The nacelle chine improves stall performance when deploying multi-element high-lift devices. This study proposes an efficient design process using a Kriging surrogate model to determine the nacelle chine installation point in wind-tunnel tests. The design exploration was conducted in a wind-tunnel using the JAXA high-lift aircraft model at the JAXA Large-scale Low-speed Wind Tunnel. The objective was to maximize the maximum lift. The chine installation points were designed on the engine nacelle in the axial and chord-wise direction, while the geometry of the chine was fixed. In the design process, efficient global optimization (EGO) which includes Kriging model and genetic algorithm (GA) was employed. This method makes it possible both to improve the accuracy of the response surface and to explore the global optimum efficiently. Detailed observations of flowfields using the Particle Image Velocimetry method confirmed the chine effect and design results.
In transient flight, rotor wakes and tip vortex generated by unsteady blade air-loads and blade motions are fully unsteady and 3-dimensionally-aperiodic, giving rise to significant complicity in accurate analysis compared to steady flight. We propose a hybrid approach by splitting the motions of a maneuvering helicopter into translation and rotation. Translation is simulated using a non-inertial moving (translating) coordinate for which new governing equations are derived, and rotations are simulated by moving each grid in the frame. A flow simulation (CFD) code is constructed by using the hybrid approach, then two simple cases (accelerating/decelerating flight and right-turn flight) for maneuvering helicopter are calculated using the moving overlapped grid method, which is now one of the most advanced techniques for tip-vortex capture. The vortex bundling phenomena, which is a main characteristic of right-turn flight, is well captured by the simulation code. The results of the present study provide better understanding of the characteristics for maneuvering rotorcraft, which can be valuable in full helicopter design.
The performance of a pulsed-heat-source, high-temperature inert gas MHD electrical power generator, which is a candidate space-based laser-to-electrical power converter, is examined by time-dependent, quasi-one dimensional, numerical simulation. In the present MHD generator, the inert gas is assumed to be ideally pulse heated to about 104 K within a short time (∼1 μs) in a stagnant energy input volume. The energy of the high-temperature inert gas is converted to the electricity using the medium of pure inert gas plasma without seeding. The numerical simulation results show that an enthalpy extraction ratio (= electrical output energy/pulsed heat energy) of several tens of percentage can be achieved, which is the same level as the conventional, seeded, nonequilibrium plasma MHD generator. Although there are still many phenomena to clarify and many problems to overcome to build this system, a pulsed-heat-source, high-temperature inert gas MHD generator is worth examining in more detail.
Experimental and numerical studies on the interaction between combustion of a hydrogen jet and an incident shock wave were performed. When the incident shock wave was introduced upstream of the injection slot, the boundary-layer separation region was expanded extensively. The penetration height, defined as the height of the Mach disk, with the incident shock wave was less than that without the incident shock wave. Combustion was confirmed when the incident shock wave was introduced downstream of the fuel injection slot. However, combustion was not confirmed with the incident shock wave upstream of fuel injection slot. The mechanism of these phenomena was examined based on the numerical simulation results in terms of the characteristic residence time in the separation region near the fuel injection slot.
This paper investigates the interplanetary trajectories associated with the impulsive deflection of a potentially hazardous asteroid (PHA) considering the uncertainty of the velocity increment that a spacecraft gives to the PHA at the collision. The velocity increment is assumed to have uncertainties of magnitude and direction due to estimation errors of asteroid shape and mass distributions. The uncertainty is modeled using a convex model assuming that magnitude and direction vary independently. The effect of uncertainty is assessed by evaluating the worst (i.e. minimum) value for the closest approach distance between the PHA and Earth. The worst value of the closest approach distance can be determined analytically without searching the whole convex hull. The optimal spacecraft trajectory is designed by maximizing the worst value of the closest approach distance in terms of the Earth departure date and the asteroid arrival date of the spacecraft under C3 (Earth departure energy) constraint. Using a numerical example with a fictitious asteroid, the importance of considering the velocity increment uncertainty is demonstrated by comparing the optimal trajectory with the deterministic optimal trajectory. The uncertainty of the velocity increment direction is shown to have a significant effect on the deflection of the PHA.
This paper deals with the State-Dependent Riccati Equation (SDRE) method for designing a rotorcraft flight controller. It focuses on the design of the SDRE controller when a highly complex rotorcraft mathematical model is used. The requirements of the rotorcraft model are investigated to design the SDRE controller and to validate the final designs. Since the SDRE method can be applied to a deterministic system, adequate fidelity in the rotorcraft mathematical model is crucial to guarantee controller performance. However, a complex mathematical model generally prevents us from analytically deriving the State Dependent Coefficient (SDC) form of the system equations, which conforms to the basic structure of the SDRE method. This paper proposes a pure numerical procedure for SDC factorization of the motion equation. The numerical methods available to solve the algebraic Riccati equation are selected to cope with the inherent system instability and are applied to the trajectory tracking problems. The overall feature of the present approach is highlighted through analysis of a bob-up and turn maneuver. The results can be utilized as a guide for appropriate selection of rotorcraft mathematical models and numerical methods in designing a robust SDRE controller.
Predictions of helicopter BVI noise using three-dimensional Euler code with a single blade grid are conducted under three different conditions: BVI noise caused by (1) interaction between rotating blades and vortex shed from fixed wing vortex generator, (2) interaction between rotating blades and tip vortices shed from preceding blades, and (3) interaction between rotating blades with elastic deformation and shed tip vortices. In the CFD calculation, the Field Velocity Approach (FVA) and Scully’s vortex model are used to import the wake information into the calculation grid and to determine the induced velocity made by tip vortices, respectively (cases 1–3). Beddoes generalized wake model is used to prescribe the tip vortices position in the wake (cases 2 and 3). Information about blade elastic deformation is imported from HART II project experimental data into the calculation (case 3). Acoustic analyses based on Ffowcs-Williams and Hawkings (FW-H) equation are conducted subsequently in each case. The results from the calculations show good agreement with experiments in all three cases, indicating that application of FVA, Scully’s model, and Beddoes generalized wake model is effective for BVI noise prediction in this study, which is intended for low calculation cost using a single blade grid. Also, use of blade elastic deformation data in the calculation shows marked improvement in calculation precision. Consequently, the method used in this study can predict BVI noise under various conditions of wake or blade deformation with acceptable precision and low calculation cost.