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Title:Wake bursting: a computational and experimental investigation for application to high-lift multielement airfoil design
Author(s):Pomeroy, Brent William
Director of Research:Selig, Michael S
Doctoral Committee Chair(s):Selig, Michael S
Doctoral Committee Member(s):Elliott, Gregory S; Vassberg, John C; Chamorro, Leonardo P
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
high lift
airfoil design
wake bursting
split film
wind tunnel
Reynolds-averaged Navier Stokes (RANS)
Computational fluid dynamics (CFD)
Abstract:High-lift aerodynamic flowfields are complex, and the potentially-adverse wake development associated with these high-lift systems is not fully understood. Thus, an exhaustive investigation including both experimental and computational efforts is needed to gain an increased understanding of the flowfield. Previous work indicates the strong off-the-surface adverse pressure gradients created by flaps may cause the main-element wake to "separate" in an aerodynamic phenomena known as wake bursting. Previous experimental research efforts to study wake bursting over a multielement airfoil are lacking a detailed study of the burst wakes in a wide range of spatial coordinates. In addition, no thorough comparison between the experimentally-captured data and computational simulations of a high-lift multielement airfoil has been performed. A variety of different experimental and computational tools can be used to study the burst-wake flowfield. These experimental techniques include the standard aerodynamic-performance and flow-visualization techniques in addition to complex wake survey methods. These wake surveys can be executed with one of a variety of probes to capture unsteady or steady data such as pressures or velocities. Because all desired flowfield parameters cannot be captured by one probe, results from different probes must be carefully analyzed and compared to other data such that a full understanding of the flowfield can be gained. Computational methods to study the burst-wake flowfield must adequately solve both the inviscid and viscous regions of the flowfield. Computations can be performed with low-order coupled viscous/inviscid program in addition to more-robust Navier-Stokes solvers, such as Reynolds-averaged Navier Stokes (RANS) programs. It is necessary to carefully compare the experimental and computational results such that the flowfield can be understood in greater detail. These comparisons will also yield insight into the effects of experimental testing environments and the weaknesses of computational solvers. Results for a three-element airfoil, consisting of a main element and a double-slotted flap, were determined using various experimental methods. Experimental results included aerodynamic polars, flow visualization, and wake surveys with both split-film and 7-hole probes. The split-film probe yielded two-dimensional unsteady velocity measurements while the 7-hole probe was use to capture time-averaged velocity vector, static pressure, and total pressure. The burst-wake region consisted of increased turbulence intensities and extremely-high turbulence production when compared to the flow outside of the wake. An increase in wake thickness with increasing downstream distance was captured from each probe, and the relationship between the wake thickness and freestream conditions was established. Low Reynolds numbers and increased angle of attack yielded the thickest wakes of all tested freestream conditions. In addition, flaps with extremely small gap sizes also yielded increased wake bursting than the large-gap airfoils. In general, minimal differences in the burst-wake flowfield were observed for small- or large-overhang simulations, and the results are found in the experiment. Two-dimensional computational results were captured for low-order and higher-order methods including both a panel code and a RANS program. In general, the RANS solver indicated larger and thicker wakes than the wakes predicted by the panel code or captured in the wake surveys. This result of larger and thicker wakes was found to be independent of the selected turbulence model. Further investigation suggested a large vortex resulted from the junction between the airfoil and the wind tunnel wall at both ends of the airfoil. These vortices introduced non-constant spanwise lift distributions for the multielement airfoil. When the effect of the vortices was considered, computational results matched the experimental data better than without the vortices, but differences still remained. These differences are attributed to difficulties in turbulence model development for computational simulations as well as limitations of the experimental probes (mainly resulting from finite probe size). Numerous different airfoils were designed such that the presence of the burst wakes was mitigated when compared to a baseline airfoil at a given value of lift for specific freestream conditions. These three airfoils were designed using different geometry constraints and different freestream conditions. Wake bursting was reduced by moving the transition point far downstream and applying a very weak pressure gradient on the forward portion of the suction surface. Increased aft loading, increased thickness in the upstream part of the airfoil, and careful control of the boundary layer yielded increased aerodynamic performance, including reduced drag and increased lift-to-drag ratio, due in large part to decreased wake bursting.
Issue Date:2016-04-21
Rights Information:Copyright 2016 Brent Pomeroy
Date Available in IDEALS:2016-07-07
Date Deposited:2016-05

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