Abstract
The paper is focused on quantitative evaluation of the value of the wall shear stress in liquids with low viscosity by means of the method of the hot film anemometry in a laminar and turbulent flow. Two systems for calibration of probes are described in the paper. The first of these uses an innovative method of probe calibration using a known flow in a cylindrical gap between two concentric cylinders where the inner cylinder is rotated and a known velocity profile and shear rate, or shear stress profile, is calculated from the Navier-Stokes equations. This method is usable for lower values of the wall shear stress, particularly in the areas of laminar flow. The second method is based on direct calibration of the probes using a floating element. This element, with a size of 120x80 mm, is part of a rectangular channel. This method of calibration enables the probe calibration at higher shear rates and is applicable also to turbulent flow. Values obtained from both calibration methods are also compared with results of measurements of the wall shear stress in a straight smooth channel for a certain range of Reynolds numbers and compared with analytical calculations. The accuracy of the method and the influence of various parasitic phenomena on the accuracy of the measured results were discussed. The paper describes in particular the influence of geometric purity of the probe location, the impact of various transfer phenomena, requirements for the measured liquid and layout of the experiment.
Highlights
Wall shear stress (WSS) plays a very important role in the research of fluid flow and transfer of momentum and energy; its determination has been currently discussed quite frequently
The paper is focused on quantitative evaluation of the value of the wall shear stress in liquids with low viscosity by means of the method of the hot film anemometry in a laminar and turbulent flow
This paper focuses on a method that makes use of connection between the intensity of cooling the heating element in body of the probe flowing in the fluid and the magnitude of the shear rate, or alternatively the shear stress in the area of the probe (Reynolds analogy [12])
Summary
Wall shear stress (WSS) plays a very important role in the research of fluid flow and transfer of momentum and energy; its determination has been currently discussed quite frequently. The calibration with the help of two concentric cylinders designed in this paper is advantageous due to the possibility of precise control of the liquid temperature in the device, the possibility of continuous measurement and the need for only a small sample of the liquid measured As it turned out, the disadvantage of this method is a relatively narrow application area, for values of the wall shear stress approximately to 01 Pa. the value is sufficient for many applications, especially in the field of hemodynamics, for other applications it was necessary to develop a second calibration device working on the principle of direct measurement of WSS. The second method has an advantage of direct probe calibration with a known level of force effect on the channel wall and possibility of using both for laminar and for turbulent area of the flow and for non-Newtonian fluids
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