<p>Ground vehicles are subjected to crosswind from various origins such as weather, topography of the ambient environment (land, forest, tunnels, high bushes...) or surrounding traffic. The trend of lowering the weight of vehicles imposes a stronger need for understanding the coupling between crosswind stability, the vehicle external shape and the dynamic properties. Means for reducing fuel consumption of ground vehicles can also conflict with the handling and dynamic characteristics of the vehicle. Streamlined design of vehicle shapes to lower the drag can be a good example of this dilemma. If care is not taken, the streamlined shape can lead to an increase in yaw moment under crosswind conditions which results in a poor handling.</p><p>The development of numerical methods provides efficient tools to investigate these complex phenomena that are difficult to reproduce experimentally. Time accurate and scale resolving methods, like Detached-Eddy Simulations (DES), are particularly of interest, since they allow a better description of unsteady flows than standard Reynolds Average Navier-Stokes (RANS) models. Moreover, due to the constant increase in computational resources, this type of simulations complies more and more with industrial interests and design cycles.</p><p>In this thesis, the possibilities offered by DES to simulate unsteady crosswind aerodynamics of simple vehicle models in an industrial framework are explored. A large part of the work is devoted to the grid design, which is especially crucial for truthful results from DES. Additional concerns in simulations of unsteady crosswind aerodynamics are highlighted, especially for the resolution of the wind-gust boundary layer profiles. Finally, the transient behaviour of the aerodynamic loads and the flow structures are analyzed for several types of vehicles. The results simulated with DES are promising and the overall agreement with the experimental data available is good, which illustrates a certain reliability in the simulations. In addition, the simulations show that the force coefficients exhibit highly transient behaviour under gusty conditions.</p> / ECO2 Crosswind Stability and Unsteady Aerodynamics for Ground Vehicles

No description available.

Approved for public release, distribution unlimited / Over the past few decades, advances in aeronautic and control technologies have established a new vision for future air transportation systems. NASA has initiated the motion with several programs supporting the "highway of the sky," a system of launch pads and air pathways enabling smaller and more easily piloted aircraft to travel the open space above instead of busy freeways and crowded city streets. Previous investigations into crossflow fan technology as a propulsion source have identified its potential for use in personal aircraft and vertical takeoff and landing applications. To further development, performance characteristics must be determined for the possible configurations and under variable conditions to understand factors critical to design. This experiment studied flow characteristics of a crossflow fan incorporating 30 blades of six inch length in a six inch diameter rotor. Comparison was made against the performance of a fan of similar design but one-fourth the length span previously tested. Results were plotted for various parameters along constant speed lines of operation and general trends were determined. These results were used to quantitatively deduce scaling relationships for this device. / US Navy (USN) author.

No description available.

<div>Many important measurements of low-amplitude instabilities related to hypersonic laminar-turbulent boundary-layer transition have been successfully performed with 1-MHz PCB132 pressure sensors. However, there is large uncertainty in measurements made with PCB132 sensors due to their poorly understood response at high frequency. The current work continues efforts to better characterize the PCB132 sensor with a low-pressure shock tube, using the pressure change across the incident shock as an approximate step input. </div><div> </div><div> New vacuum-control valves provide precise control of pre-run pressures in the shock tube, generally to within 1\% of the desired pressure. Measurements of the static-pressure step across the shock made with Kulite sensors showed high consistency for similar pre-run pressures. Skewing of the incident shock was measured by PCB132 sensors, and was found to be negligible across a range of pressure ratios and static-pressure steps. Incident-shock speed decreases along the shock tube, as expected. Vibrational effects on the PCB132 sensor response are significantly lower in the final section of the driven tube.</div><div> </div><div> Approximate frequency responses were computed from pitot-mode responses. The frequency-response amplitude varied by a factor of 5 between 200--1000 kHz due to significant resonance peaks. Measurements with blinded PCB132 sensors indicate that the resonances in the frequency response are not due to vibration. </div><div> </div><div> Using the approximate frequency response measured with the shock tube to correct the spectra of wind-tunnel data produced inconclusive results. Correcting pitot-mode PCB132 wind-tunnel data removed a possible resonance peak near 700 kHz, but did not agree with the spectrum of a reference sensor in the range of 11--100 kHz. </div>

Ph.D. thesis, Faculty of Engineering (Mechanical Engineering), University of the Witwatersrand, 1993 (2 v.)

It has been widely anticipated that thrust vectoring could be an effective method of providing sufficient levels of stability and control for highly manoeuvrable and flexible Unmanned Combat Air Vehicles (UCAVs). The present project aims to understand the interactions of delta wing vortical flows and thrust vectoring, with an emphasis on unsteady aspects. Food-colouring dye flow visualization, Laser-induced fluorescent flow visualization, Particle Image Velocimetry (PIV) and force measurements were conducted in the water and wind tunnels over a range of dimensionless frequencies and jet momentum coefficients. Both slender and nonslender wings were tested with the purpose of understanding the effect of sweep angle on the aerodynamics-propulsion interaction. The interaction of statically pitched trailing-edge jets with leading-edge vortices over stationary delta wings was studied. It was found that under-vortex blowing with rectangular nozzle at stall and post-stall regimes could yield the maximum effectiveness of trailing-edge blowing, due to the promotion of earlier reattachment and delay of vortex breakdown. The effect of nozzle geometry can be important, because the entrainment effect of the jet depends on it. Studies of the flow field reveal strong jet-vortex interactions, distortion of jet vortices, and merging of wing and jet vortices. The dynamic responses of wing vortical flows to dynamic trailing-edge blowing exhibit hysteresis and phase lag, which increases with the increasing dimensionless frequency of jet momentum. Time delay for the decelerating jet is significantly larger than that for the accelerating jet. Sweep angle has no significant influence on the effect of unsteady trailing-edge blowing. From a design aspect, hysteresis and time delay need to be considered for the flight control systems.

在這篇論文中，我們考察亞音速流體流進流出一給定的有限長管道的問題，目的在於找到管道入口和出口處內蘊的（物理上可接受的）邊界條件。我們首先刻劃一組物理邊界條件，借此可以確定長方形管道中亞音速無旋流的存在性和唯一性。在給定管道入口處的流量及水平來流方向和出口處的適當壓力條件下，存在兩個正的常數m₀和m₁(m₀ < m₁)，使得當流量m ∈ [m₀,m₁)，在長方形管道中存在唯一的亞音速無旋流。流體的水平速度是恒正的，並且當流量m趨於m₁時，流體的最大速度也將趨於音速。問題的困難來自于由流量蘊含的非局部項和出口處的壓力條件。我們首先引入伯努利常數作參數的附屬問題来處理非局部项，然後建立流量和伯努利常數之間的單調關係。為處理壓力條件引起的斜導數缺失問題，我們利用角速度和壓力來重新描述這一問題。我們利用Moser迭代來得到角速度的最大模估計，以確保流體的水平速度恒為正。 / 我們接下來考察彎曲管道中一般來流方向和管道壁的幾何結構對亞音速無旋流的影響。我們發現來流方向和管道壁的傾斜角度和出口壓力起著相似的作用。管道壁的曲率也起著很重要的作用。我們的結果也可以推廣到二維亞音速歐拉流和三維軸對稱亞音速歐拉流的情形。 / 接下來我們考慮三維有限長管道中的亞音速歐拉流的情形，這是最有趣和最困難的情形，也是論文的核心部份。我們在二維管道中給定的邊界條件在三維有一個自然的推廣。這些重要的提示對我們尋求歐拉方程組新的分解及借此理解其中的雙曲與橢圓耦合的結構是至關重要的。我們的新的分解的關鍵想法在於利用伯努利定律來約化速度場。具體的做法是通過定義新的變量 [附圖] 及通過 [附圖] ，利用伯努利函數B 來代替u₁。這樣我們可以更深入地挖掘伯努利定律的作用，期望借此可以稍微簡化一下複雜的歐拉方程組。對伯努利函數為常數的流體，我們找到了一個新的守恆量，這跟二維的約化的旋度的情形相似。讓人驚奇的是，我們還可以找到一組新的守恆律，這一情況即使在二維也從未被人注意到。我們利用這一分解來證明長方體管道中靠近某些特殊亞音速流并滿足給定的入口處的來流方向及伯努利函數和出口處的壓力條件的亞音速歐拉流的存在性和唯一性。同樣的想法可以用於不可壓歐拉方程組、自相似歐拉方程組、帶阻尼項的歐拉方程組、定常歐拉泊松方程組和定常的歐拉麥克斯韋方程組。 / 最後我們考慮歐拉泊松方程組某些定常亞音速解的結構穩定性。如果帶亞音速背景電荷的背景解的馬赫數和電場都比較小的話，那麼背景解關於背景電荷、來流方向、伯努利函數、出口壓力的小撓動是結構穩定的。在我們的數學分析中新的元素在於求解帶斜導數邊界條件和Dirichlet邊界條件的混合型的二階強耦合的橢圓型方程組。 / In this thesis, we investigate an inflow-outflow problem for subsonic gas flows in a nozzle with finite length, aiming at finding intrinsic (physically acceptable) boundary conditions on upstream and downstream. We first characterize a set of physical boundary conditions that ensure the existence and uniqueness of a subsonic irrotational flow in a rectangle. Our results show that suppose we prescribe the horizontal incoming flow angle at the inlet and an appropriate pressure at the exit, there exists two positive constants m₀ and m₁ with m₀ < m₁, such that a global subsonic irrotational flow exists uniquely in the nozzle, provided that the incoming mass flux m ∈ [m₀,m₁). The maximum speed will approach the sonic speed as the mass flux m tends to m₁. The new difficulties arise from the nonlocal term involved in the mass flux and the pressure condition at the exit. We first introduce an auxiliary problem with the Bernoulli’s constant as a parameter to localize the nonlocal term and then establish a monotonic relation between the mass flux and the Bernoulli’s constant to recover the original problem. To deal with the loss of obliqueness induced by the pressure condition at the exit, we employ the formulation in terms of the angular velocity and the density. A Moser iteration is applied to obtain the L∞ estimate of the angular velocity, which guarantees that the flow possesses a positive horizontal velocity in the whole nozzle. / As a continuation, we investigate the influence of the incoming flow angle and the geometry structure of the nozzle walls on subsonic flows in a finitely long curved nozzle. It turns out to be interesting that the incoming flow angle and the angles of inclination of nozzle walls play the same role as the end pressure. The curvatures of the nozzle walls play an important role. We also extend our results to subsonic Euler flows in the 2-D and 3-D asymmetric cases. / Then it comes to the most interesting and difficult casethe 3-D subsonic Euler flow in a bounded nozzle, which is also the essential part of this thesis. The boundary conditions we have imposed in the 2-D case have a natural extension in the 3-D case. These important clues help us a lot to develop a new formulation to get some insights on the coupling structure between hyperbolic and elliptic modes in the Euler equations. The key idea in our new formulation is to use the Bernoulli’s law to reduce the dimension of the velocity field by defining new variables [with formula] and replacing u₁ by the Bernoulli’s function B through [with formula] In this way, we can explore the role of the Bernoulli’s law in greater depth and hope that may simplify the Euler equations a little bit. We find a new conserved quantity for flows with a constant Bernoulli’s function, which behaves like the scaled vorticity in the 2-D case. More surprisingly, a system of new conservation laws can be derived, which is never been observed before, even in the two dimensional case. We employ this formulation to construct a smooth subsonic Euler flow in a rectangular cylinder by assigning the incoming flow angles and the Bernoulli’s function at the inlet and the end pressure at the exit, which is also required to be adjacent to some special subsonic states. The same idea can be applied to obtain similar information for the incompressible Euler equations, the self-similar Euler equations, the steady Euler equations with damping, the steady Euler-Poisson equations and the steady Euler-Maxwell equations. / Last, we are concerned with the structural stability of some steady subsonic solutions for the Euler-Poisson system. A steady subsonic solution with subsonic background charge is proven to be structurally stable with respect to small perturbations of the background charge, the incoming flow angles and the end pressure, provided the background solution has a low Mach number and a small electric field. The new ingredient in our mathematical analysis is the solvability of a new second order elliptic system supplemented with oblique derivative conditions at the inlet and Dirichlet boundary conditions at the exit of the nozzle. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Weng, Shangkun. / Thesis (Ph.D.)--Chinese University of Hong Kong, 2012. / Includes bibliographical references (leaves 176-187). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Abstract also in Chinese. / Chapter 1 --- Introduction --- p.11 / Chapter 2 --- Subsonic irrotational flows in a rectangular nozzle --- p.26 / Chapter 2.1 --- Introduction --- p.27 / Chapter 2.2 --- Reduction of the problem and main results --- p.33 / Chapter 2.2.1 --- An auxiliary problem --- p.34 / Chapter 2.2.2 --- Restrictions on the end pressure --- p.34 / Chapter 2.2.3 --- Main results --- p.36 / Chapter 2.3 --- Unique solvability of the auxiliary problem --- p.38 / Chapter 2.3.1 --- Reformulation of the auxiliary problem --- p.38 / Chapter 2.3.2 --- Proof of Theorem 2.2.2 --- p.40 / Chapter 2.4 --- The relationship between the mass flux m and the Bernoulli’s constant B --- p.56 / Chapter 3 --- Subsonic irrotational flows in a 2-D finitely long curved nozzle --- p.61 / Chapter 3.1 --- Introduction --- p.62 / Chapter 3.2 --- Reduction of the problem and main results --- p.65 / Chapter 3.2.1 --- An auxiliary problem --- p.65 / Chapter 3.2.2 --- Main results --- p.66 / Chapter 3.3 --- Unique solvability of the auxiliary problem --- p.68 / Chapter 3.3.1 --- Reformulation of the auxiliary problem --- p.68 / Chapter 3.3.2 --- Proof of Theorem 3.2.1 --- p.69 / Chapter 4 --- Subsonic Euler flows in a divergent nozzle --- p.85 / Chapter 4.1 --- Introduction --- p.86 / Chapter 4.2 --- Subsonic Euler flows in a 2-D divergent nozzle --- p.87 / Chapter 4.2.1 --- Formulation of the problem and main results . --- p.87 / Chapter 4.2.2 --- Proof of Theorem 4.2.2 --- p.92 / Chapter 4.3 --- Subsonic Euler flows in a three-dimensional divergent conic nozzle with an asymmetric end pressure --- p.97 / Chapter 4.3.1 --- Formulation of the problem and main results . --- p.97 / Chapter 4.3.2 --- Proof of Theorem 4.3.2 --- p.102 / Chapter 5 --- A new formulation for the 3-D compressible Euler equations --- p.108 / Chapter 5.1 --- Introduction --- p.109 / Chapter 5.2 --- A new formulation for the 3-D compressible Euler equations --- p.114 / Chapter 5.3 --- The 3-D compressible Euler equations with a constant Bernoulli’s function --- p.117 / Chapter 5.3.1 --- A new conserved quantity --- p.117 / Chapter 5.3.2 --- A system of new conservation laws --- p.122 / Chapter 5.4 --- Subsonic Euler flows in a rectangular cylinder --- p.128 / Chapter 5.4.1 --- Extension to the domain Ωe = [0, 1] × T² --- p.130 / Chapter 5.4.2 --- Main results --- p.130 / Chapter 5.4.3 --- Proof of Theorem 5.4.1 --- p.132 / Chapter 5.5 --- A new formulation for the 3-D incompressible Euler equations --- p.139 / Chapter 5.5.1 --- A new formulation for the 3-D incompressible Euler equations --- p.139 / Chapter 5.5.2 --- The 3-D incompressible Euler equations with a constant Bernoulli’s function --- p.142 / Chapter 5.6 --- Appendix: The verification of (5.3.4)-(5.3.6) --- p.145 / Chapter 6 --- On steady subsonic flows for the Euler-Poisson models --- p.148 / Chapter 6.1 --- Introduction --- p.149 / Chapter 6.2 --- Preliminary --- p.152 / Chapter 6.2.1 --- A new formulation for the Euler-Poisson equations --- p.152 / Chapter 6.2.2 --- Background solutions --- p.153 / Chapter 6.3 --- Structural stability of background solutions --- p.156 / Chapter 6.3.1 --- Extension to the domain Ωe = [0, 1] × T² --- p.158 / Chapter 6.3.2 --- Main results --- p.159 / Chapter 6.3.3 --- Proof of Theorem 6.3.1 --- p.162 / Chapter 7 --- Discussions and Future works --- p.170 / Chapter 7.1 --- Subsonic flows in a finitely long nozzle --- p.170 / Chapter 7.2 --- The transonic shock problem in a 3-D divergent nozzle --- p.172 / Chapter 7.3 --- Dynamical stability of a transonic shock in nozzles --- p.174 / Bibliography --- p.175

No description available.

The low pressure (LP) exhaust system presents a promising avenue for improving the performance of large steam turbines. For this reason, LP exhaust systems have attracted the attention of the research community for decades. Nevertheless, we still lack understanding of the flow physics and loss mechanisms in the exhaust system, especially at part-load conditions. It is also unclear how the exhaust system should be designed when its required operating range widens. This thesis provides solutions to these aerodynamic issues through experimental and numerical investigations, and provides tools that could contribute to better designs of LP exhaust systems. Firstly, the Computational Fluid Dynamics (CFD) solver ANSYS CFX was validated against experiments performed on a scaled test rig under representative part-load flow conditions. This validation exposed the weakness of Reynolds-averaged Navier-Stokes (RANS) CFD when there is a highly swirling flow and large separation regions in the exhaust diffuser. To facilitate the numerical studies, a series of tools were also developed. A design suite, ExhaustGen, was used to automate the pre- and post-processing of CFD calculations. The exhaust diffuser was parametrised using "Minimum Energy Curves", which reduce the dimension of parameter space. Further, a suitable stage-hood interface treatment (Multiple Mixing Planes) was chosen to predict the circumferentially non-uniform flow in the exhaust hood at low computational cost. Numerical investigation of the baseline geometry provided insights into the key flow features and loss mechanisms in the exhaust system, over a wide range of operating conditions. In particular, the bearing cone separation was identified as a key source of loss at part-load conditions. The effect of stage-hood interaction on the performance and design of the exhaust system was studied by varying the rotor blade design, which can positively influence system performance. Finally, a global sensitivity study was performed to identify the most influential design parameters of the exhaust hood. These findings allow, for the first time, LP exhaust hood performance maps to be constructed, so that the benefits of choosing a suitable hood geometry and blade design can be revealed. The thesis also offers contribution towards formulating LP exhaust system design guidance for a wide operating range.