The present thesis aims at shedding some light for future applications of &amp / #956 / AVs by investigating the hovering mode of flight by flapping motion. In this study, a detailed numerical investigation is performed to investigate the effect of some geometrical parameters, such as the airfoil profile shapes, thickness and camber distributions and as well as the flapping motion kinematics on the aerodynamic force coefficients and vortex formation mechanisms at low Reynolds number. The numerical analysis tool is a DNS code using the moving grid option. Laminar Navier-Stokes computations are done for flapping motion using the prescribed kinematics in the Reynolds number range of 101-103. The flow field for flapping hover flight is investigated for elliptic profiles having thicknesses of 12%, 9% and 1% of their chord lengths and compared with those of NACA 0009, NACA 0012 and SD 7003 airfoil profiles all having chord lengths of 0.01m for numerical computations. Computed aerodynamic force coefficients are compared for these profiles having different centers of rotation and angles of attack. NACA profiles have slightly higher lift coefficients than the ellipses of the same t/c ratio. And one of the most important conclusions is that the use of elliptic and NACA profiles with 9% and 12% thicknesses do not differ much as far as the aerodynamic force coefficients is concerned for this Re number regime. Also, two different sinusoidal flapping motions are analyzed. Force coefficients and vorticity contours obtained from the experiments in the literature and present study are compared. The validation of the present computational results with the experimental results available in the literature encourages us to conclude that present numerical method can be a reliable alternative to experimental techniques.

The present thesis is an experimental study of some aspects of unsteady two dimensional boundary layers subject to adverse pressure gradient. An adverse pressure gradient usually leads to boundary layer separation or an instability which may result in transition to turbulence. Unsteady boundary layer separation is not yet fully understood and there is no specific criterion proposed in literature for its occurrence. The details of separation depend on the Reynolds number, the geometry of the body (streamlined or bluff) and the type of imposed unsteady motion (impulsive, oscillatory etc.). Similarly there are many unknowns with respect to instability and transition in unsteady boundary layers, especially those having a streamwise variation. For unsteady flows it is useful to break up the pressure gradient term in the unsteady boundary layer equation into two components:(Formula) is the velocity at the edge of the boundary layer. The first term of the right hand side of this equation may be called the temporal component (Πt) which signifies acceleration or deceleration in time of the free stream and the second term is the spatial component (Πx) which represents the spatial or convective acceleration of the free stream. Many of the studies on instability in unsteady flows found in literature are carried out in straight tubes or channels, where the Πx term is absent. However, in many cases, especially in biological systems both terms are present. An example is the unsteady flow over the moving body of a fish. To study the effects of Πt and Πx on unsteady separation and instability we have built an unsteady water tunnel where the two components can be systematically varied. The flow is created by a controlled motion of a piston. By a suitable combination of the geometry of the model and the piston motion, different types of separation bubbles may be generated. In our studies the piston motion follows a trapezoidal variation: constant acceleration from rest, followed by constant velocity and then deceleration to zero velocity. We have chosen two geometries. One is a bluff body and thus has a high value of Πx and other is a small angle diffuser with a divergence angle 6.2° and thus having a small value of Πx. Upstream and downstream of the diffuser are long lengths of constant cross section. We have performed experiments with the above mentioned geometries placed in the tunnel test section. Flow is visualized using the laser induced fluorescence technique by injecting a thin layer of fluorescein dye on the test wall. Numerical simulations have been done using the software FLUENT. Boundary layer parameters like boundary layer, displacement and momentum thicknesses are calculated from the simulations and used to analyze the experimental results. For the flow in the diffuser, quasi-steady stability analysis of the instantaneous velocity profiles gives a general idea of stability behavior of the flow. Two types of experiments have been done with the bluff body. One is the unsteady boundary layer separation and the formation of the initial vortex for a flow that is uniformly accelerated from rest. We have found some scalings for the formation time (tv) of the separation vortex. The second type of experiment was to study the vortex shedding from the separating shear layer after the boundary layer has fully separated. At high enough Reynolds number shear layer vortices are seen to shed from the separation bubble. The Strouhal number based on the momentum thickness and the velocity at the edge of the boundary layer just upstream of the separation point is found to vary between 0.004 and 0.008. This value is close to the Strouhal number value of 0.0068 found in laminar separation bubbles on a flat plate. The second part of the study concerns with the evolution of the flow in the small angle diffuser with a mild variation of the spatial component of the pressure gradient. From the experimental visualizations we have found that the ratio of Πx and Πt at the start of the deceleration phase of the piston motion is an important parameter that determines the type of instability. This value of Πx/Πt is controlled by controlling the piston deceleration: a large deceleration gives a low Πx/Πt value and a low deceleration gives a large Πx/Πt value. Three types of instabilities have been observed in our experiments. In Type I, the first vortex forms at the maximum pressure gradient point (MPGP) and which grows disproportionately with time. However, instability vortices are seen later at other locations around the MPGP. In type II an array of vortices over a certain length are observed; the vortices grow with time. In Type III, which we observe for low decelerations, we observe initial vortices only in the diffuser section in the deceleration phase of the piston motion. Type III instability is similar to the one observed in dynamic stall experiments. In all cases the instability is very localized - it occurs only over some length of the boundary layer. Transition to turbulence, which is also localized, is observed at higher Reynolds numbers. The non-dimensional time for vortex formation is not very different from that found in straight channel experiments. Quasi-steady linear stability analyses for the boundary layer at the MPGP both for the top and the bottom walls show that the flow is absolutely unstable for some cases. In summary, the thesis looks at in a unified way the separation and instability of unsteady boundary layers with reverse flow. It is hoped that the results will be useful in predicting and understanding onset of separation and instability in practically occurring unsteady flows.

Thesis (M.S.)--Mississippi State University. Department of Aerospace Engineering. / Title from title screen. Includes bibliographical references.

Thesis (Ph. D.)--Brigham Young University. Dept. of Mechanical Engineering, 2003. / Includes bibliographical references (p. 133-136).

No description available.

In unsteady flows, dune dimensions may vary considerably from fully-developed dimensions produced from a flat bed under a steady and uniform flow. Specifically, dune height and length are observed to lag discharge when the flow is non-steady, resulting in dimensions that are out of phase with the prevailing flow. This research attempts to provide some insight into the behaviour of dunes when the flow is suddenly changed, as well as the time-scale of the related dune changes. Nineteen experimental runs were carried out in the 21-m long, 0.76-m wide Sediment Transport Flume at Queen’s University. In ten of these runs the flow depth, h, was suddenly increased (h2/h1 > 1) and in nine runs the flow depth was suddenly decreased (h2/h1 < 1). In all runs, the slope of free surface was kept equal to the slope of the bed such that the change in flow rate is represented by the change in the flow depth. Seven ratios of the two flow depths, h2/h1 (varying between 0.49 and 2.29), were performed for three slopes (S = 1/792, 1/534, 1/341). The bed material was a coarse, poorly-graded silica sand (D50 = 1.0 mm). Longitudinal bed profiles were taken along the centreline of the flume approximately every 6–8 minutes to determine the transient dune dimensions and the time at which the dunes acquired their new equilibrium dimensions. This time is termed the duration of dune development, Td. Through dimensional and physical considerations, Bielenberg (2006) established that the dimensionless counterpart of Td should be a function of the material number, relative flow depth, relative flow intensity, and ratio of the flow depths h2/h1. The present experiments were carried out to investigate the influence of the relative flow intensity and h2/h1 on the duration of dune development. Results indicate that equilibrium dune dimensions do not depend on the initial shape of the bed. After the period Td, the dunes tend to be similar to those produced from a flat bed. It is found that Td is strongly dependent on h2/h1, and weakly dependent on the flow intensity. All other determining variables remaining the same, Td decreases with increasing values of flow intensity. Semi-empirical relations for the dimensionless duration of dune development are presented. / Thesis (Master, Civil Engineering) -- Queen's University, 2007-09-24 09:45:13.624

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