Application of empirical and linear methods to VSTOL powered-lift aerodynamics

Available prediction methods applied to problems of aero/propulsion interactions for short takeoff and vertical landing (STOVL) aircraft are critically reviewed and an assessment of their strengths and weaknesses provided. The first two problems deal with aerodynamic performance effects during hover: (1) out-of-ground effect, and (2) in-ground effect. The first can be evaluated for some multijet cases; however, the second problem is very difficult to evaluate for multijets. The ground-environment effects due to wall jets and fountain flows directly affect hover performance. In a related problem: (3) hot-gas ingestion affects the engine operation. Both of these problems as well as jet noise affect the ability of people to work near the aircraft and the ability of the aircraft to operate near the ground. Additional problems are: (4) the power-augmented lift due to jet-flap effects (both in- and out-of-ground effects), and (5) the direct jet-lift effects during short takeoff and landing (STOL) operations. The final problem: (6) is the aerodynamic/propulsion interactions in transition between hover and wing-borne flight. Areas where modern CFD methods can provide improvements to current computational capabilities are identified

Flow Past Bluff Bodies

More than a century ago Kirchhoff solved for the velocity distribution within an elliptical patch of uniform vorticity. That solution became the basis for all further studies of elliptical vortices and has been regarded as the only known exact solution for a steady, elliptical patch of uniform vorticity. In the present paper, an exact solution for a new elliptical patch of uniform vorticity is presented. The vortex is constructed of streamlines of constant eccentricity. By specifying a velocity distribution along either of the principle axes of the vortex, continuity between differentially-spaced streamlines provides the velocity distribution throughout the vortex. Some of the unique features of the vortex are that although the vorticity is uniform throughout the vortex, the angular velocity about the center is non-uniform, unlike the Kirchhoff vortex wherein both are uniform. The point of maximum velocity occurs not at the end of the major axes as in the case of Kirchhoff\u27s vortex, but rather at the end of the minor axes, more nearly approximating the behavior of the twin vortices formed behind bluff bodies. In the present work, a non-orthogonal (non-confocal) elliptical coordinate system is employed to solve for the velocity and pressure distributions within the vortex

Cavity and Wake Flows

The phenomenon of wake formation behind a body moving through a fluid, and the associated resistance of fluids, must have been one of the oldest experiences of man. From an analytical point of view, it is also one of the most difficult problems in fluid mechanics. Rayleigh, in his 1876 paper, observed that "there is no part of hydrodynamics more perplexing to the student than that which treats of the resistance of fluids." This insight of Rayleigh is so penetrating that the march of time has virtually left no mark on its validity even today, and likely still for some time to come. The first major step concerning the resistance of fluids was made over a century ago when Kirchhoff (1869) introduced an idealized inviscid-flow model with free streamlines (or surfaces of discontinuity) and employed (for steady, plane flows) the ingenious conformal-mapping technique that had been invented a short time earlier by Helmholtz (1868) for treating two-dimensional jets formed by free streamlines. This pioneering work offered an alternative to the classical paradox of D’Alembert (or the absence of resistance) and laid the foundation of the free-streamline theory. We appreciate the profound insight of these celebrated works even more when we consider that their basic idea about wakes and jets, based on a construction with surfaces of discontinuity, was formed decades before laminar and turbulent flows were distinguished by Reynolds (1883), and long before the fundamental concepts of boundary-layer theory and flow separation were established by Prandtl (1904a). However, there have been some questions raised in the past, and still today, about the validity of the Kirchhoff flow for the approximate calculation of resistance. Historically there is little doubt that in constructing the flow model Kirchhoff was thinking of the wake in a single-phase fluid, and not at all of the vapor-gas cavity in a liquid; hence the arguments, both for and against the Kirchhoff flow, should be viewed in this light. On this basis, an important observation was made by Sir William Thomson, later Lord Kelvin (see Rayleigh 1876) "that motions involving a surface of separation are unstable" (we infer that instability here includes the viscous effect). Regarding this comment Rayleigh asked "whether the calculations of resistance are materially affected by this circumstance as the pressures experienced must be nearly independent of what happens at some distance in the rear of the obstacle, where the instability would first begin to manifest itself." This discussion undoubtedly widened the original scope, brought the wake analysis closer to reality, and hence should influence the course of further developments. An expanded discussion essentially along these lines was given by Levi-Civita (1907) and was included in the survey by Goldstein (1969). Another point of fundamental importance is whether the Kirchhoff flow is the only correct Euler (or outer) limit of the Navier-Stokes solution to steady flow at high Reynolds numbers. If so, then a second difficulty arises, a consequence of the following argument: We know that the width of the Kirchhoff wake grows parabolically with the downstream distance x, at a rate independent of the (kinematic) viscosity u. If Prandtl’s boundary-layer theory is then applied to smooth out the discontinuity (i.e. the vortex sheet) between the wake and the potential flow, one obtains a laminar shear layer whose thickness grows like (ux/U)^-1/2 in a free stream of velocity U. Hence, for sufficiently small u/U the shear layers do not meet, so that the wake bubble remains infinitely long at a finite Reynolds number, a result not supported by experience. (For more details see Lagerstrom 1964, before p. 106, 131; Kaplun 1967, Part II.) The weaknesses in the above argument appear to lie in the two primary suppositions that, first, the free shear layer enveloping the wake would remain stable indefinitely, and second (perhaps a less serious one), the boundary-layer approximation would be valid along the infinitely long wake boundary. Reattachment of two turbulent shear layers, for instance, is possible since their thickness grows linearly with x. By and large, various criticisms, of the Kirchhoff flow model have led to constructive refinements of the free-streamline theory rather than to a weakening of the foundation of the theory as a valuable idealization. The major development in this direction has been based on the observation that the wake bubble is finite in size at high Reynolds numbers. (The wake bubble, or the near-wake, means, in the ordinary physical sense, the region of closed streamlines behind the body as characterized by a constant or nearly constant pressure.) To facilitate the mathematical analysis of flows with a finite wake bubble, a number of potential-flow models have been introduced to give the near-wake a definite configuration as an approximation to the inviscid outer flow. These theoretical models will be discussed explicitly later. It suffices to note here that all these models, even though artificial to various degrees, are aimed at admitting the near-wake pressure coefficient as a single free parameter of the flow, thus providing a satisfactory solution to the state of motion in the near part of the wake attached to the body. On the whole, their utility is established by their capability of bringing the results of potential theory of inviscid flows into better agreement with experimental measurements in fluids of small viscosity. The cavity flow also has a long, active history. Already in 1754, Euler, in connection with his study of turbines, realized that vapor cavitation may likely occur in a water stream at high speeds. In investigating the cause of the racing of a ship propeller, Reynolds (1873) observed the phenomenon of cavitation at the propeller blades. After the turn of this century, numerous investigations of cavitation and cavity flows were stimulated by studies of ship propellers, turbomachinery, hydrofoils, and other engineering developments. Important concepts in this subject began to appear about fifty years ago. In an extensive study of the cavitation of water turbines, Thoma (1926) introduced the cavitation number (the underpressure coefficient of the vapor phase) as the principal similarity parameter, which has ever since played a central role in small-bubble cavitation as well as in well-developed cavity flows. Applications of free-streamline theory to finite-cavity flows have attracted much mathematical interest and also provided valuable information for engineering purposes. Although the wake interpretation of the flow models used to be standard, experimental verifications generally indicate that the theoretical predictions by these finite-wake models are satisfactory to the same degree for both wake and cavity flows. This fact, however, has not been widely recognized and some confusion still exists. As a possible explanation, it is quite plausible that even for the wake in a single-phase flow, the kinetic energy of the viscous flow within the wake bubble is small, thus keeping the pressure almost unchanged throughout. Although this review gives more emphasis to cavity flows, several basic aspects of cavity and wake flows can be effectively discussed together since they are found to have many important features in common, or in close analogy. This is in spite of relatively minor differences that arise from new physical effects, such as gravity, surface tension, thermodynamics of phase transition, density ratio and viscosity ratio of the two phases, etc., that are intrinsic only to cavity flows. Based on this approach, attempts will be made to give a brief survey of the physical background, a general discussion of the free-streamline theory, some comments on the problems and issues of current interest, and to point out some basic problems yet to be resolved. In view of the vast scope of this subject and the voluminous literature, efforts will not be aimed at completeness, but rather on selective interests. Extensive review of the literature up to the 1960s may be found in recent expositions by Birkhoff & Zarantonello (1957), Gilbarg (1960), Gurevich (1961), Wehausen (1965), Sedov (1966), Wu (1968), Robertson & Wislicenus (1969), and (1961)

Numerical and Experimental Analysis of Transient Flow in Roots Blower

The performance of rotary positive displacement machines highly depends on the operational clearances. It is widely believed that computational fluid dynamics (CFD) can help understanding internal leakage flows. Developments of grid generating tools for analysis of leakage flows by CFD in rotary positive displacement machines have not yet been fully validated. Roots blower is a good representative of positive displacement machines and as such is convenient for optical access in order to analyse internal flows. The experimental investigation of flow in optical roots blower by phase-locked PIV (Particle Image Velocimetry) performed in the Centre for Compressor Technology at City, University of London provided the velocity field suitable for validation of the simulation model. This paper shows the results of the three-dimensional CFD transient simulation model of a Roots blower with the dynamic numerical grids generated by SCORG and flow solution solved in ANSYS CFX flow solver to obtain internal flow patterns. The velocity fields obtained by simulation agree qualitatively with the experimental results and show the correct main flow features in the working chamber. There are some differences in the velocity magnitude and vortex distribution. The flow field in roots blower is highly turbulent and three-dimensional. The axial clearances should be included, and the axial grids should be refined in the simulation method. The paper outlines some directions for future simulation and experimental work. The work described in this paper is a part of the large project set to evaluate characteristics of the internal flow in rotary positive displacement machines and to characterize leakage flow

Sinusoidal-gust generation with a pitching and plunging airfoil

The generation of uniform, periodic gust disturbances in an experimental context is demonstrated using a single oscillating airfoil. A pitching and heaving symmetric airfoil is suggested as a simpler alternative to existing gust-generation methods. The Theodorsen theory of unsteady aerodynamics is used as an analytical tool to dictate the kinematics necessary to produce well-defined sinusoidal gusts downstream of the airfoil. These analytic predictions improve the symmetry of fluctuations in the vertical velocity induced by the airfoil, as well as minimize the influence of vorticity shed by the oscillating airfoil. The apparatus is shown to produce smooth, repeatable gusts with high amplitudes and reduced frequencies compared to other gust-generation mechanisms in the literature. Furthermore, the control of downstream flow properties by airfoil motion kinematics has applications in experimental aerodynamics, the design of rotorcraft and light aerial vehicles, and biological propulsion.Comment: Under revie

Two staggered finite circular cylinders in cross-flow

Circular cylinders in cross-flow have been extensively studied in the last century. However, there are still many unsolved problems in this area, one of which is the flow structure around two staggered finite circular cylinders. This thesis mainly focuses on an experimental investigation of the vortex shedding characteristics of two staggered finite circular cylinders of equal diameter in cross-flow. Wind tunnel experiments were conducted to measure the vortex shedding frequency at the mid-height of the two cylinders and along the height of the two cylinders. Two identical circular cylinders of aspect ratio AR = 9 were partially immersed in a flat-plate turbulent boundary layer, where the boundary layer thickness to cylinder height ratio at the location of the cylinders was δ/H = 0.4. The Reynolds number based on the cylinder diameter was ReD = 2.4z x ~104. Centre-to-centre pitch ratios of P/D = 1.125, 1.25, 1.5, 2, 2.5, 3, 4 and 5 were examined and the incidence angle was incremented in small steps from á = 0° to 180°. For each configuration of the cylinders, the vortex shedding frequency, represented in dimensionless form as the Strouhal number, St, was measured with a single-component hot-wire anemometer. Also, a seven-hole pressure probe was used to measure the time-averaged wake velocity field behind the cylinders at selected configurations in order to get a better understanding of the wake structure.The vortex shedding frequencies measured at the mid-height of the cylinders clearly showed the similarities and differences of vortex shedding between two staggered finite and infinite circular cylinders. The Strouhal number behavior of the two finite circular cylinders is generally similar to that of two infinite circular cylinders, but the values of St for the two finite cylinders were found for most cases to be smaller than the case of the infinite cylinders.The measurements of vortex shedding frequency along the heights of each finite cylinder revealed that, for most incidence angles, the value of the Strouhal number remains constant along the height of the cylinder, but a notable variation in the shape and strength of the vortex shedding peak along the heights of the cylinders is observed. Sharp and strong peaks in the power spectra are measured around the mid-height of the cylinder. Broader and weaker peaks are found both at the base of the cylinder and near the free end. At several particular configurations, the vortex shedding frequency changes along the height of the cylinder, caused by the varying flow pattern in the vertical direction.Wake measurements showed the velocity field behind the two finite circular cylinders arranged in tandem configurations of P/D = 1.125, 2 and 5. The experimental data revealed that the flow structure behind two finite circular cylinders arranged in a tandem configuration is much more complicated than that of the single finite circular cylinder. The downwash flow from the tip of the downstream cylinder is weaker due to the flow interaction between the free ends of two cylinders, and this downwash flow becomes stronger with increasing P/D. A similar trend happens to the vorticity of the tip vortex structures. However, the upwash flow behind the downstream cylinder is not strongly affected by the existence of the upstream cylinder

Visualization of scientific arts and some examples of applications

In this paper, implementation and visualization of scientific arts are described using some examples of application in subject research areas, such as sculpture, archeology, fine arts and information aesthetics, which have been discussed through the Scientific Art Session at FLUCOME9, Tallahassee, Florida, 2007-9. In the application to sculpture, stereo visualization techniques, such as anaglyph stereo visualization and integral imaging technique, are introduced to realize the three-dimensional geometry of sculpture to enhance visual impact on the art. The second application is the flow visualization technique for archeology, where the vortices behind the river stones are studied to understand the origin of patterns on Jomon pottery. Interestingly, such vortex patterns also appear in the paintings of fine arts. The third example is the visualization of information aesthetics, where the Web information, such as public media and stock market, are visualized through scientific techniques. These example of visualization of scientific arts provide the present state of the art in interdisciplinary visualization

Application of an Upwind High Resolution Finite-Differencing Scheme and Multigrid Method in Steady-State Incompressible Flow Simulations

The analysis and design of a submarine propulsor requires the ability to predict the characteristics of both laminar and turbulent flows to a higher degree of accuracy. This report presents results of certain benchmark computations based on an upwind, high-resolution, finite-differencing Navier-Stokes solver. The purpose of the computations is to evaluate the ability, the accuracy and the performance of the solver in the simulation of detailed features of viscous flows. Features of interest include flow separation and reattachment, surface pressure and skin friction distributions. Those features are particularly relevant to the propulsor analysis. Test cases with a wide range of Reynolds numbers are selected; therefore, the effects of the convective and the diffusive terms of the solver can be evaluated separately. Test cases include flows over bluff bodies, such as circular cylinders and spheres, at various low Reynolds numbers, flows over a flat plate with and without turbulence effects, and turbulent flows over axisymmetric bodies with and without propulsor effects. Finally, to enhance the iterative solution procedure, a full approximation scheme V-cycle multigrid method is implemented. Preliminary results indicate that the method significantly reduces the computational effort

Active and Adaptive Flow Control of Twin-Tail Buffet and Applications

Modern fighter aircraft with dual vertical tails are operated at high angles of attack. The vortex generated by leading edge extension (LEX) breaks down before reaching the two vertical tails. The wake of highly unsteady, turbulent flow causes unbalanced broadband aerodynamic loading on the tails and may produce severe buffet on the tails and lead to tail fatigue failure. Flow suction along the vortex cores (FSVC) is investigated as an active control method for tail-buffet alleviation. Suction tubes have been tilted at different angles to study the control effectiveness of suction tubes orientation. Flow field response, aerodynamic loading and aeroelastic results are compared with the no-control case. These flow modifications produce lower tip bending and rotation angle deflections and accelerations. Moreover, the root bending and twisting moments are reduced in comparison with the no-control case. However, there was no shift in the frequencies at which the peaks of the power spectral density (PSD) responses occurred. The primary effect of the FSVC methods is the amplitude reduction of the aeroelastic responses up to 30%. A parametric investigation is conducted and the best control effectiveness is obtained with the suction tubes tilted at −10°. Next, the twin-tail buffet alleviation is addressed by using adaptive flow control, and an adaptive active control method is developed. Control ports, whose locations are determined according to the locations of a range of high-pressure difference, are placed within a small area on the tail surfaces. Flow suction and blowing are applied through these control ports in order to equalize the pressures on the two surfaces of the tail. Mass flow rate through each port is proportional to the pressure difference across the tail at the location of this port. Comparing the flow field and aeroelastic response with the no-control case, the normal-force and twisting-moment distributions are substantially decreased along with the damping of their amplitudes of variation. The bending-deflection and rotation-angle responses have not changed their sign. The PSD of the root bending moment and root twisting moment have shown substantial decreases of more than 70%. The tail tip acceleration responses have shown similar decreases too. Next, a parallel high-order compact-scheme code (PHCC) is developed to investigate flow control more accurately and more efficiently. The validation cases are presented and compared with theoretical results, experimental results and other computational results. The PHCC results show good accuracy and high efficiency. Flow computational simulations of Jet and Vortex Actuator (JaVA) or synthetic jet have been investigated. The computational results show good agreement with the experimental data and other computational results. Simplified 2D models, which include an airfoil under the effect of JaVAs and synthetic jet actuators, are developed and investigated for control effectiveness. Simulation results show: with properly selected parameters, the oscillating amplitude of pressure difference and normal force acting on airfoil can be reduced, the peak of the normal force PSD can be reduced and the frequencies at which the peaks of the pressure difference PSD responses occurred can be shifted to higher frequency levels. Too low or too high exciting frequencies have no effect or adverse effect. Low exciting velocity may not produce enough disturbances to suppress the pressure oscillation