Direct measurement of wall shear stress in a reattaching flow with a photonic sensor

Abstract

Wall shear stress measurements are carried out in a planar backward-facing step flow using a micro-optical sensor. The sensor is essentially a floating element system and measures the shear stress directly. The transduction method to measure the floating element deflection is based on the whispering gallery optical mode (WGM) shifts of a dielectric microsphere. This method is capable of measuring floating element displacements of the order of a nanometer. The floating element surface is circular with a diameter of ∼960 μm, which is part of a beam that is in contact with the dielectric microsphere. The sensor is calibrated for shear stress as well as pressure sensitivity yielding 7.3 pm Pa −1 and 0.0236 pm Pa −1 for shear stress and pressure sensitivity, respectively. Hence, the contribution by the wall pressure is less than two orders of magnitude smaller than that of shear stress. Measurements are made for a Reynolds number range of 2000-5000 extending to 18 step heights from the step face. The results are in good agreement with those of earlier reports. An analysis is also carried out to evaluate the performance of the WGM sensor including measurement sensitivity and bandwidth.

Monitoring of pressurized pipelines with whispering gallery mode-based sensors

“…The sensitivity, expressed in pm/Pa is calculated assuming the use of a tunable distributed feedback laser diode with a nominal central wavelength of 1.312 µm 27 and was found to range between 0.0113 pm/Pa for a resonator diameter of 1 mm and 0.0149pm/Pa for a resonator diameter of 6 mm. The sensor resolution, which is the smallest possible change that a sensor can perceive, is determined, for a WGM-based sensor, by the minimum resolvable shift 19 , which is, in turn, dictated by the optical quality factor according to the following expression:…”

“…A change of the pressure applied to the internal wall of the pipe causes a mechanical strain on the perimeter of the resonator and, as a consequence, a shift of optical modes occurs as explained in detail in section 2 and section 3. Due to their high resolution, high sensitivity, and small size, WGM sensors have been developed to detect many different physical quantities including electric fields [13][14][15][16] , magnetic fields 15,17,18 , wall shear stress [19][20][21] , temperature [22][23][24] , applied loads 25,26 , displacement [27][28][29][30][31] , and radiant energy 32 among others. Section 2 of this paper explains how the WGM phenomenon is utilized in sensing applications.…”

Mathematical Model for Electric Field Sensor Based on Whispering Gallery Modes Using Navier’s Equation for Linear Elasticity

“…These applications include wall shear stress calculation [3,4]; temperature measurement [5][6][7]; biosensing [8][9][10][11][12][13]; hemozoin nanocrystals detection in aquatic environment [14]; nanoparticle detection [15]; and acceleration measurement [16][17][18]. In this paper, WGM based optical cavities are used to detect and measure the applied electric field.…”

Behavior of Current Flow on Lead Zirconate Titanate Surfaces during Mechanical Impact

“…Sensors, which allow in-situ detection of mechanical stress, have found wide interest in several industrial branches [1][2][3]. As result of this industrial interest, new sensor designs and physical measurement concepts for impact and stress measurements were developed in the last years [4][5][6][7].…”