Taking into account the fact that the state of turbulence approaches a two-dimensional state close to the wall, a new Reynolds-stress turbulence model is proposed. The rotational properties of two-dimensional turbulence differ significantly from that of three-dimensional turbulence. Under the system rotating about the axis perpendicular to the plane, the turbulence correlations between fluctuating velocities remain invariant, while those combined with fluctuating pressure vary. Based on these mathematical properties, a proposal for the pressure-strain correlation is made. Trial computations for turbulent channel flow that the present Reynolds-stress model can work well to reproduce strong anisotropic near-wall behaviors of Reynolds stresses, which cannot be predicted by previous turbulence models.
Two-dimensional characteristics of the boiling two-phase flow of liquid nitrogen in a duct flow are numerically investigated to contribute to the further development of new high-performance cryogenic engineering applications. First, the governing equations of the boiling two-phase flow of liquid nitrogen based on the unsteady drift-flux model are presented and several flow characteristics are numerically calculated taking account the effect of cryogenic flow states. Based on the numerical results, a two-dimensional structure of the boiling two-phase flow of liquid nitrogen is shown in detail, and it is found that the phase change of liquid nitrogen occurs in quite a short time interval compared with that of two-phase pressurized water at high temperature. Next, it is clarified that the distributions of pressure and the void fraction in a two-phase flow show a tendency different from those of fluids at room temperature because of the decrease in sound velocity due to large compressibility and the rapid phase change velocity in a cryogenic two-phase mixture flow. According to these numerical results, the fundamental characteristics of the cryogenic two-phase flow are predicted. The numerical results obtained will contribute to advanced cryogenic industrial applications.
To clarify influences induced by non-uniform ground surface on I.G.E. hover performance of a rotor, a numerical prediction method is developed by combining a free-wake method with a panel method, where the most important feature is the ability to determine blade flapping motions to be consistent with the deformed wake geometry. The ground surface beneath the rotor is substituted for quadratic panels with unknown ground vortex strength which are determined by virtue of the non-penetration conditions at the collocation points. The rotor blades are meodeled by the lifting lines with a constant circulation which results in a wake structure to compose of deformed helical line vortices trailing from the blade tips. The numerical procedure is programmed as an interactive two-stage process, where the spatial arrangements of tip vortices are calculated at first by moving their nodal points with updated local velocities induced by the ground and trailing vortices and then proceed to the second stage where the equation of blade flapping motion is solved using averaged induced velocity distribution on the rotor disc. Iterations are executed until both the wake geometry and blade flapping motions are converged simultaneously. In this paper, we introduce typical numerical results obtained for a rotor hovering in close proximity above a uniformly inclined flat surface and discuss influences of the ground inclination angle and rotor height on the wake geometry, flowfields around the rotor and the rotor-induced torque. The ground interaction effect on the amplitude and phase angle of the blade flapping motion are also investigated, and their unique dependencies on the operating circumstances are clarified.
Motion tracking sliding-mode control theory is utilized to design a homing-missile guidance law, which induces missile-target line-of-sight angular rate to track an arbitrarily given signal, such that observability for the target tracking with bearings-only measurements is enhanced. An adaptive two-step filter is used to estimate missile-target relative range, relative velocity, and target acceleration involved in the guidance law during the whole guidance period. Simulation studies indicate that the presented guidance law provides good performance for the filter, robustness to estimation errors, and good terminal effectiveness.
In order to investigate the oscillation mode of an underexpanded impinging jet on a flat plate and the behavior of the plate shock waves formed in the vicinity of the plate, the authors visualize the flow field and measure the pressure on the plate. Visualization is made using Mach-Zehnder interferometry and density contours are drawn by analyzing fringe shifts obtained from interferograms under the assumption of axisymmetric flow. At a nozzle pressure ratio of 3.0, the shape of the plate shock wave changes repeatedly from mormal to curved, from curved to normal etc., with nozzle-plate spacings. The shape of the plate shock depends on the location of the plate and strength of the shock wave. When the spacing exceeds a certain value, the mode of oscillation changes markedly. Pressure correlations show that the jet structure under oscillation has four different types of flow patterns: quasi-steady, axisymmetric, whirling and random.
Numerical simulation of combustion around a strut in supersonic airflow at Mach 1.5 was conducted. In previous papers, experimental results on flame-holding characteristics have been shown for the strut divided into two parts, indicating the effectiveness of the flame-holding characteristics of this strut. Stable flame-holding is due to a comparatively long residence time in the subsonic flow region between the two parts of the strut. The present study is analytical evidence of the stable flame-holding of this strut. The Stahl and Warnatz’s detailed chemistry of hydrogen/oxygen reactions and the Baldwin Lomax turbulence algebraic model were employed to simulate the chemical reaction and turbulent flow, respectively. Flame structures such as distributions of chemical species and temperature were obtained. For example, the predicted density distributions explicitly showed an attached shock wave, expansion fans and shear layers, and had good agreement with the shadowgraph of the experiment. The overall equivalence ratio in the space between two strut parts was calculated to evaluate the reaction time in the space between the struts and a particle trace analysis was performed to evaluate the residence time in the space. By obtaining the Damköhler number from two characteristic times, two flame-holding limits, namely the chemical kinetic limit at small interval between two struts and the dynamic limit at large interval, were discussed. The numerical results were qualitatively consistent with the previous experimental results.