This paper presents our work of an experimental examination of the flow over a NACA 0012 Airfoil at low Reynolds numbers and large angle of attack using particle imaging velocimetry. The Reynolds numbers examined were 5,000, 30,000, and 60,000, while the angles of attack ranged from 8 to 12 degrees in 2 degree increments. This work was motivated by reports that lift and drag measurements for airfoils operating at Reynolds numbers less than roughly 40,000 could not be made due to flow unsteadiness. This is puzzling in that the flow should be laminar at these Reynolds numbers, which are an order of magnitude lower than the flat-plate transition Reynolds number of 500,000. To this end we examined a sequence of flow field measurements of the instantaneous velocity field. We observed mean streamline patterns that were very representative of those we would find for a strictly steady flow, however a random pattern of significant fluctuations in the velocity and vorticity were observed. The intensity of these fluctuations increased with Reynolds number and angle of attack.
The numerical search for the optimum shape of an aerofoil is of great interest for aircraft and turbomachine designers. Unfortunately, this process is very computationally intense and can require a large number of individual flow field simulations resulting in very long CPU run times. One of the core issues that the designer must deal with is how to describe the shape of the airfoil. Clearly, we can not treat the profile on a point by point basis as the problem would have an infinite number of degrees of freedom. Hence the typical practice is to resort to using a series of curves, such as polynomials and Bezier curves, to describe the profile. This typically reduces the number of degrees of freedom to a much smaller, manageable number.The influence of the selection of the parameterization on the optimization has received relatively little consideration to date. We can anticipate that some parameterizations will be less suitable for describing the profile shape and may result in slower convergence times.Our paper will discuss a new airfoil parameterization, Bezier-PARSEC, that was developed to extend and improve the typical Bezier parameterization found in use. This parameterization was found to fit the known shape of a wide range of existing airfoil profiles as well as resulting in accelerated convergence for aerodynamic optimization using Differential Evolution. Our presentation will present the development and details of the Bezier-PARSEC parameterization and provide evidence that the parameterization is suitable and accelerates convergence.
A comprehensive experimental examination is presented of the effect of hot-wire length for a single-wire probe. Specially constructed hot wires, with 2.5 μm and 5.0 μm nominal diameters and lengths from 0.3 mm to 2.8 mm, were used to measure the one-dimensional spectrum of the streamwise fluctuating velocity component u as well as the skewness and flatness of both u and du/dt. All measurements were made in air at Y+=150 (corresponding to Y/δ=0.1) in a boundary layer growing inside a pipe. The Reynolds number was 37 000 based on the velocity at the centerline and on the boundary-layer thickness. The longest wires exhibited some attenuation at the highest wavenumbers. This attenuation was found to roughly confirm the correction to the spectrum due to Wyngaard. A correction for the microscale λ, and thus dissipation, was examined and found to be correct to a fair degree (within 8%). This correction is based on the early work of Skramstad and of Frankiel, but with the integral scale of R11 (0, 0, r) assumed to equal the λ scale. The measurements of skewness and flatness did not indicate an experimentally discernible effect of the length of the hot wire.
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