The natural-laminar-flow (NLF) design for the wing of the national experimental supersonic transport (NEXST) aims to suppress crossflow (CF) instability near the leading edge. We computationally investigate the growth of various stationary CF disturbances on the outer wing of the NEXST-1 model using the prediction system of boundary-layer transition. According to the N factors obtained, the growth of those disturbances is completely suppressed around the designed angle of attack, which shows that the NLF design for the NEXST-1 wing is valid.
A scaling law on the wear of three-grid optics is proposed for the lifetime estimation of ion engines. The charge exchange ions erode the accel grid holes; therefore, the reduced retarding potential results in electron backstreaming, which is the end of life of the three-grid optics. The arithmetic formulas are induced by use of the space-charge-limited current density law and a newly introduced parameter called the modified effective acceleration length. The model is based on the following scenario of grid life: (1) Charge exchange ion erosion enlarges the accel grid hole diameter, (2) which shortens the modified effective acceleration length between the screen and accel grids and (3) reduces the retarding potential to keep the beam current constant under the space-charge limited flow. The characteristic of the proposed formulas is that the lifetime of the three-grid optics strongly depends on the beam current and the accel grid voltage. The correlation between the arithmetic formulas and the numerical simulations showed good qualitative agreement with one another in all regions of the normalized perveance per hole and achieved quantitative consistency in its high value.
This work presents a comparison of matching asymptotic solutions for the limiting case of the restricted three-body problem by the use of perturbation methods. The problem is of a singular-perturbation type. We investigate two alternative methods to deal with it: the classical method of matched asymptotic expansions and the improved method of matched asymptotic expansions. Two expansions, outer and inner, are involved. The outer expansion breaks down in the inner region where sharp changes occur, and the inner expansion becomes nonuniformly valid in the outer region. To obtain a uniformly valid composite solution, we need a matching procedure to relate these two expansions. Instead of straightforward matching of the outer and inner expansions to higher-order terms, in the improved technique the higher-order solutions are derived by generating perturbations between the lower-order composite solutions and the exact solutions. The perturbation equations are then integrated in the outer and inner regions, respectively, for a higher-order matching. Improved asymptotic solutions of second order are obtained for the limiting case of the restricted three-body problem. Compared to the solutions obtained by the classical method of matched asymptotic expansions and the pure numerical integration for various values of a small parameter μ, the improved asymptotic solutions are very accurate. Moreover, the asymptotic solutions obtained by use of the improved method give better accuracy than those using the classical method over wide ranges of the small parameter.
Compressible flows over cavities with a series value of length-to-depth ratio (L⁄D) were investigated experimentally, with the objective being to elucidate the mechanism of the transition of types of cavity flows as their L⁄D increases or decreases. For open-type cavity flows, the freedom of backflow inside the cavity is found to be crucial in smoothing out adverse pressure gradient. The spreading of the shear layer and its gradual approach toward the cavity floor as L⁄D increases tend to suppress the freedom of backflow, causing the cavity flow to change from the open-type to the transitional-type. For closed-type cavity flows, the finite thickness of the upstream boundary layer leads to the presence of three kinds of characteristic lengths that correspond to the recirculation region, the deflection of the main flow and the recovery or the abrupt rise of the pressure, respectively. The compression fans at the foot of the impingement and the exit shock waves will approach each other well in advance of the possible merge of the vertices of the conventionally defined separation and recompression wakes. As the L⁄D of a closed cavity decreases, the reattached boundary layer on the mid-portion of the cavity floor will have less developing distance, thus it will be more susceptible to adverse pressure gradient and prone to separation. For the present two-dimensional cavity models, the critical values of L⁄D were found to be about 10 and 14 for the transition of cavity flows from the open- to the transitional-type and from the transitional- to the closed-type, respectively. The sum of the pressure lengths at the front and rear wakes agrees remarkably well with the second critical L⁄D.
In this paper, two recently introduced parameter identification (PID) methods are applied to the real-time estimation of aerodynamic coefficients from the flight data of the NASA F/A-18 HARV aircraft. The study specifically addresses the computational effort for each PID technique, which can be a decisive factor for on-line real-time application purposes. The results are also compared with off-line parameter identification results obtained through the well-known Maximum Likelihood method as well as wind tunnel data. Following a coding for the two on-line methods organized to minimize the computations, the required on-line computational effort associated with the frequency domain PID method is shown to be lower than that with the time domain PID method by almost one order of magnitude. The overall results show that two on-line PID methods exhibit consistent performance. The frequency domain-based method seems to provide estimates closer to the Maximum Likelihood and wind tunnel results for both longitudinal and lateral/directional dynamics.
The effect of the test gas on the flow field around a 120° apex angle blunt cone has been investigated in a shock tunnel at a nominal Mach number of 5.75. The shock standoff distance around the blunt cone was measured by an electrical discharge technique using both carbon dioxide and air as test gases. The forebody laminar convective heat transfer to the blunt cone was measured with platinum thin-film sensors in both air and carbon dioxide environments. An increase of 10 to 15% in the measured heat transfer values was observed with carbon dioxide as the test gas in comparison to air. The measured thickness of the shock layer along the stagnation streamline was 3.57±0.17 mm in air and 3.29±0.26 mm in carbon dioxide. The computed thickness of the shock layer for air and carbon dioxide were 3.98 mm and 3.02 mm, respectively. The observed increase in the measured heat transfer rates in carbon dioxide compared to air was due to the higher density ratio across the bow shock wave and the reduced shock layer thickness.
Small disturbances superimposed on the growing boundary-layer flow along a long swept wing are governed by partial differential equations with respect to the local chordwise Reynolds number and the nondimensional vertical coordinate. For a simple and widely applicable method of stability estimation, however, it is desirable to reduce the exact disturbance equations to an eigenvalue problem of the corresponding ordinary differential equations, as in the stability analysis of two-dimensional parallel flows. This paper proposes such a simple model of the ordinary differential system that includes the most important terms of boundary-layer nonparallelism and wall curvature. Numerical computations show that the eigensolutions can properly describe multi-instability characteristics of the three-dimensional boundary layers near the attachment line of a long swept wing.
Multi-instability characteristics of the three-dimensional boundary layer around a yawed circular cylinder are investigated by an eN method involving the effects of wall curvature and nonparallelism. Velocity profiles of the boundary layer are approximated by members of Falkner-Skan-Cooke similarity solutions and the local dispersion relation of each member is determined from the nonparallel eigenvalue problem proposed in part 1 of this study. A complex ray theory is adopted in the integration procedure for N factor, and a numerical estimation of N is made to describe wedge-shaped disturbances originating from a point source. The analysis using these methods shows that the influence of wall curvature and nonparallelism stabilizes and destabilizes the flow, respectively, though their quantitative effects on the N factor depend on kinds of instability, range of frequency, and values of flow parameters. It is also found that the destabilizing effect of nonparallelism increases with the increase of sweep angle.