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基準形状に正弦波状，あるいは局在する波状の変形を加えることで，超音速自然層流機首を設計することを試みた．変形のパラメータだけでなく，一様流に関するパラメータも変化させ，最も自然層流効果の大きくなる形状を調査した．その結果，変形によって周方向と軸方向の圧力勾配比が減少すると，側面の境界層遷移を支配する横流れ不安定を抑制し自然層流効果をもたらすことがわかった．また，この圧力勾配比を小さくするには，圧力分布のピーク位置，すなわち圧力係数が負圧側で極大となる位置をトップラインとボトムラインで適切に配置することが重要であることがわかった．そして，そのような自然層流効果をもたらす変形形状の設計指針を獲得し，自然層流機首となる形状例が得られた．正弦波変形における自然層流効果は，実験的にも検証できた．一方，全抵抗の増加を抑制するには，局在波変形の方が有効であることが示された．, The transition characteristics were examined numerically on candidate shapes in order to design a natural laminar flow nose for a supersonic transport. Two types of deformation from a baseline shape were examined. One is sinusoidal type, and another is localized type. Not only parameters of geometry which define the deformation, but also the conditions of uniform flow were varied to obtain the largest effect of natural laminar flow. In results, the crossflow instability, which dominates the boundary-layer transition at the side area, was found to be suppressed from the small ratio of pressure gradients in azimuthal and axial directions at the side area. Moreover it was found that the relative location of bump in pressure distribution along leeward and windward rays is a key to yield small ratio of pressure gradient in azimuthal direction. The effect of natural laminar flow for sinusoidal deformation was confirmed experimentally. On the other hand, the localized deformation was found to be more effective in order to suppress increasing of total drag than the sinusoidal deformation., 形態: カラー図版あり, Physical characteristics: Original contains color illustrations

The effects of aerodynamic asymmetries on hypersonic vehicle controllability and performance were investigated for a wide range of geometries. Asymmetric conditions were introduced by an isolated surface roughness that forces boundary-layer transition resulting in a turbulence wedge downstream of the disturbance. The disturbance simulates the effects of physical deformations that may exist on a vehicle surface or leading edge, such as protruding edges of thermal protection system tiles or non-uniform surface roughness. Both multi-objective and robust multi-objective optimization studies were performed. Traditional multi-objective optimization methods were used to identify vehicle designs that are best suited to withstand spanwise asymmetric boundary-layer transition while retaining its performance and payload requirements. Trade-offs between vehicle controllability and performance were analyzed. A novel multi-objective based robust optimization method to solve single-objective optimization problems with environmental parameter uncertainty was proposed and tested. Unlike commonly used robust optimization methods, the multi-objective method formulates an optimization problem such that post-optimality data handling techniques can identify multiple robust designs from a single solution set. This allows for comparisons to be made between different types of robust designs, thus providing more information about the design space. Comparisons were made between the robust multi-objective optimization formulation and conventional robust regularization- and aggregation-based methods. The results, performance, and philosophies of each method are discussed. Design trends were identified for classifying the optimum and robust optimum designs of hypersonic vehicle shapes under boundary-layer transition uncertainties. Traditional multi-objective optimization results show that two types of vehicle shapes bound the set of Pareto-optimal solutions: wedge-like and cone-like. The L

Reducing skin friction is important in nature and in many technological applications. This reduction may be achieved by reducing stresses in turbulent boundary layers, for instance tailoring biomimetic rough skins. Here we take a second approach consisting of keeping the boundary layer laminar as long as possible by forcing small optimal perturbations. Because of the highly non-normal nature of the underlying linearized operator, these perturbations are highly amplified and able to modify the mean velocity profiles at leading order. We report results of wind-tunnel experiments in which we implement this concept by using suitably designed roughness elements placed on the skin to enforce nearly optimal perturbations. We show that by using this passive control technique it is possible to sensibly delay transition to turbulence., QC 20100525

Challenges in discerning the intricacies of boundary layers buffeted by free-stream turbulence (FST) over a flat-plate have a renewed interest in the recent years. The FST transition scenario is dominated by unsteady streamwise streaks with spanwise scales that are dependent on the initial conditions at the LE. Many old transition studies show that the transition Reynolds number is simply a function the turbulence intensity (Tu). The current parameter variation study is the largest, performed in a low turbulence wind tunnel by varying the FST conditions namely, Tu in between [1%-7%] and the FST integral length scale in the range of [4.3-35.4] mm as well as the free-stream velocity in [6-15] m/s. Here, we present experimental results from 83 unique FST cases. The results confirm more recent studies that the transitional Reynolds number is not only a function of Tu but also the integral Reynolds number based on the integral length scale and the free-stream speed and that the transitional Reynolds number shows a twofold effect with respect to integral length scale. There is a threshold Tu where an increase in integral length scale goes from advancing to postponing transition, which is in agreement with recent experimental findings. Furthermore, our results also agree with previous studies that the FST condition at the LE have a strong influence on the spanwise wavelength of the streaks. We show that the averaged streak spacing, normalized by the FST integral length scale, follows a simple empirical power-law built on the FST Reynolds number., QC 20220531