Examination of the yaw-angle bias in three-dimensional boundary layers

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The Internatinonal Conference on Materials Science Mechanical and Automation Engineerings and Technology in ÇEŞME/İZMİR (IMSMATEC’18), April 10-12 2018

Examination of the Yaw-Angle Bias in Three-Dimensional Boundary

Layers

Musa Özkana_{, Volkan R. Akkaya}b_{and Peter J. Thomas}c

a _{Bilecik Şeyh Edebali Üniversitesi, Bilecik, Turkey, E-mail: musa.ozkan@bilecik.edu.tr} b _{Muğla Sıtkı Koçman Üniversitesi, Muğla, Turkey, E-mail: volkan.akkaya@mu.edu.tr}

c _{The University of Warwick, Coventry, UK, E-mail: P.J.Thomas@warwick.ac.uk}

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2nd International Conference on Material Science and Technology in Cappadocia (IMSTEC’17), October 11-13, 2017, Nevsehir, Turkey

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2nd International Conference on Material Science and Technology in Cappadocia (IMSTEC’17), October 11-13, 2017, Nevsehir, Turkey

Abstract

The investigation of three-dimensional boundary layers has recently been highly significant since this type of flows can occur, for instance, over highly swept wings of an aircraft. Therefore, researchers have been examining the aspects of these boundary layers by means of both theoretical and experimental studies. A rotating-disk flow, for instance, is a simple and easily repeatable example of a three-dimensional boundary-layer flow and thus related experiments are generally conducted over the surface of these disks.

Hot-wire anemometers are mostly used for the investigation of these boundary layers because some of them are specially desgined to investiage boundary layer flows and also they have an ability to capture rapid fluctuations in turbulent boundary layers. As a result of its nature, the rotating-disk boundary layer has three velocity components in azimuthal, radial, and axial directions. However, due to their design, hot-wire probes can only measure the flow in one direction that is perpendicular to the wire. Therefore, the probe needs to be positioned accordingly to capture three velocity components individually. This approach results in several problems.

One of the main difficulties of conducting hot-wire mesurements in three-dimensional boundary layers is called yaw-angle bias and the purpose of our study is to address this issue experimentally in detail. There are two strong components that have cooling effect on the hot-wire probe close to the disk surface. When the probe is aligned to measure the azimuthal velocity component, for instance, it encounters a cooling effect caused by the radial component of the velocity along the sensor or vice versa.

In the current study, the hot-wire probe was positioned appropriately to measure the voltages that correspond to the normal and the tangential velocities. Subsequently, the yaw coefficient, ky, was calculated by

means of the below equation for various velocities ranging from 0 to 1 m/s. Ve2 = UN2 + ky2 UT2

Here, UN is the normal velocity component and UT is the tangential velocity component. The tangential

component, here, is the one that has the along wire cooling effect on the hot-wire probe and results in the yaw-angle bias. An average value for the yaw coefficient was calculated using various data and eventually the appropriate correction factor and formula was obtained which can be employed to any hot-wire measurements carried out in three-dimensional boundary layers where similar bias effects are observed.

Keywords: Hot-wire anemometer, Rotating-disk flow, Three-dimensional boundary layers, Yaw-angle bias