Wall Shear Stress Measurements Research Articles

Temperature dependence of constant current hot-film sensors: Investigation with application to temperature correction for wall-shear stress measurements

Hot-film sensors measure wall-shear stress based on the forced convective heat transfer. Variations of ambient temperatures lead to significant measurement errors for such thermal sensors and must be corrected for sometimes. Although methods for correcting the temperature shifts of hot-film sensors driven in constant temperature or constant voltage mode have been suggested over the years, the temperature dependence and relevant correction schemes of constant current mode of operation are still open questions. In this study, temperature dependence characteristics of constant current hot-film sensors are investigated within relatively large temperature changes in air and water channel flows (28 °C and 16 °C, respectively). The specific property of constant overheat ratios under varying temperatures for constant current driving mode is found and a quite simple temperature correction function is derived on that basis to eliminate the temperature dependence without any assumed heat transfer correlation. With the correction function, the data at different ambient temperatures are collapsed to single curves with high R2 factors over 0.99, typically. Furthermore, after correction, the relative errors of the measured wall-shear stress are reduced to within ±6 % of the true values. It will then enable the constant current hot-film sensors to calibrate at only one ambient temperature instead of a multiple range of temperatures as with many other schemes, which is more practical and convenient.

Calorimetric wall-shear-stress microsensors for low-speed aerodynamics

This article describes the design, calibration, and testing of new calorimetric microsensors for the measurement of wall shear stress in low-speed aerodynamic flows. The sensors are made of three beams of platinum-plated silicon nitride suspended over a small cavity. Their range of operation and their bandwidth are of the order of ±10\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\pm 10$$\\end{document} Pa and 1 kHz, respectively. Results from experimental campaigns in a laminar separation bubble, a turbulent separation bubble, and on a NACA 0015 airfoil at low Reynolds number indicate a high sensitivity and an inherent capacity to measure instantaneous backflow. This demonstrates the capability of the new sensors to accurately determine the mean and fluctuating wall shear stress in laminar, transitional, and turbulent separating and reattaching flows.

Wall shear stress measurement using a zero-displacement floating-element balance

The floating-element (FE) principle, introduced nearly a century ago, remains one of the most versatile direct wall shear stress measurement methods. Yet, its intrinsic sources of systematic error, associated with the flow-exposed gap, off-axis load sensitivity, and calibration, have thus far limited its widespread application. In combination with the lack of standard designs and testing procedures, measurement reliability still hinges heavily on individual judgement and expertise. This paper presents a framework to curb these limitations, whereby the design and operation of a FE balance are leveraged by an analytical model that attempts to capture the behaviour and predict the relative contribution of the systematic sources of error. The design is based on a parallel-shift linkage and a zero-displacement force-feedback system. The FE has a surface area of 200×200\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$200\ imes 200\\,$$\\end{document} mm, and measurement sensitivity is adjustable depending on the surface condition and the Reynolds number. It is thus suitable for application in a wide range of low-speed, boundary-layer wind tunnels, small or large scale. Measurements of the skin friction coefficient over a smooth wall show a remarkable agreement with oil-film interferometry, especially for Reθ>1.3×104\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$${\\rm Re}_{\ heta } > 1.3 \ imes 10^4$$\\end{document}. The discrepancy relative to the empirical Coles–Fernholz relation (κ=0.39\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\kappa = 0.39$$\\end{document} and C=4.352\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$C = 4.352$$\\end{document}) is within 0.5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$0.5\\,$$\\end{document}%, and the level of uncertainty is below 1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1\\,$$\\end{document}% for a confidence interval of 95\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$95\\,$$\\end{document}%.

Blood flow effects in a patient with a thoracic aortic endovascular prosthesis

This work analyzes hemodynamic phenomena within the aorta of two elderly patients and their impact on blood flow behavior, particularly affected by an endovascular prosthesis in one of them (Patient II). Computational Fluid Dynamics (CFD) was utilized for this study, involving measurements of velocity, pressure, and wall shear stress (WSS) at various time points during the third cardiac cycle, at specific positions within two cross sections of the thoracic aorta. The first cross-section (Cross-Section 1, CS1) is located before the initial fluid bifurcation, just before the right subclavian artery. The second cross-section (Cross-Section 2, CS2) is situated immediately after the left subclavian artery. The results reveal that, under regular aortic geometries, velocity and pressure magnitudes follow the principles of fluid dynamics, displaying variations. However, in Patient II, an endoprosthesis near the CS2 and the proximal border of the endoprosthesis significantly disrupts fluid behavior owing to the pulsatile flow. The cross-sectional areas of Patient I are smaller than those of Patient II, leading to higher flow magnitudes. Although in CS1 of Patient I, there is considerable variability in velocity magnitudes, they exhibit a more uniform and predictable transition. In contrast, CS2 of Patient II, where magnitude variation is also high, displays irregular fluid behavior due to the endoprosthesis presence. This cross-section coincides with the border of the fluid bifurcation. Additionally, the irregular geometry caused by endovascular aneurysm repair contributes to flow disruption as the endoprosthesis adjusts to the endothelium, reshaping itself to conform with the vessel wall. In this context, significant alterations in velocity values, pressure differentials fluctuating by up to 10%, and low wall shear stress indicate the pronounced influence of the endovascular prosthesis on blood flow behavior. These flow disturbances, when compounded by the heart rate, can potentially lead to changes in vascular anatomy and displacement, resulting in a disruption of the prosthesis-endothelium continuity and thereby causing clinical complications in the patient.

Wall shear stress measurement of turbulent bubbly flows using laser Doppler displacement sensor

A novel wall shear stress meter is developed to promote studies of frictional drag reduction by injecting air bubbles into a turbulent boundary layer. Such studies require a tool having a time resolution sufficiently high to capture the unsteady effect of turbulent characteristics due to various bubble passage patterns. The present study adopts the principle of the heterodyne interferometer to capture of time variation of wall shear stress in turbulent flow up to 500 Hz in sampling frequency. We demonstrate the performance of the tool by applying it to bubbly flows beneath a flat-bottom model ship running at 3 m/s in a towing water reservoir, where bubbles experience wavy passage around a few Hz in frequency. The data obtained by the proposed shear stress meter agree well with Dean's formula for single-phase turbulent channel flow. The scattering of the data across three trials is sufficiently small, at approximately 3 %. This high reproducibility is a great advantage over conventional direct measurements, which have low reproducibility and a large scatter of the data of approximately 15 %. The obtained results represent reasonable drag reductions by injecting bubbles with different void fractions. We also evaluate histogram of the wall shear stress fluctuation to characterize the effect of drag reduction by unsteady passage of bubbles.

Experimental verification of wall shear stress measurement with MEMS sensors array for underwater applications with flat plate benchmark tests

Underwater wall shear stress measurement based on a newly developed MEMS sensors array is experimentally verified utilizing a set of flat plate benchmark tests conducted in a precision water flume. The array is firstly calibrated in a well-defined 2D flow and then flush embedded on the plate surface to measure WSS directly. The local freestream velocities ranging from 0.1 m/s to 0.7 m/s are adopted for the test cases of naturally developing flow and deliberately stimulated turbulence. The boundary layer profiles of interests are correspondingly detected with LDV, and mean WSS values are extrapolated from suitable fits to known profile shapes for flat plate with a zero pressure gradient. The results from the MEMS sensors match well with those from the LDV velocity profile fits and are in good agreement with empirical estimations with an overall bias of less than 5%. Also, the direct simultaneous measurements from multiple sensors on the MEMS array favorably exhibit the streamwise variation of the wall-bounded flow. Generally, the MEMS sensors array-based underwater wall shear stress measurement is characterized by desirable accuracy and spatial resolution for further practical applications.

Improvement of extraction method of the Fizeau fringes spacing in the oil film interferometry and measurement of wall shear stress on an airfoil surface

Improvement of extraction method of the Fizeau fringes spacing in the oil film interferometry and measurement of wall shear stress on an airfoil surface

Assessment of aerodynamic roughness parameters of turbulent boundary layers over barnacle-covered surfaces

Full-scale drag penalty predictions of flows over rough walls require surface roughness characterisation from laboratory experiments or numerical simulations. In either approach, it is necessary to determine the so-called equivalent sand-grain roughness height (k_s). There are several steps involved in determining this aerodynamic roughness lengthscale, but its procedure typically includes a combination of measurement of wall-shear stress (tau _w) using direct or indirect methods as well as analysis of velocity profiles. Indirect methods usually rely on assumptions made about flow and its scaling including the validity of universal outer-layer similarity. However, the implications of the underlying assumptions involved in full-scale drag prediction are unclear. In this work, we carry out wind tunnel measurements over a realistic rough surface (from a fouled ship-hull) to evaluate the impact of different methods with an emphasis on using the outer-layer similarity hypothesis for full-scale drag predictions. Wall-shear stress is measured using an in-house floating-element drag balance (DB), and velocity profiles are obtained using particle image velocimetry (PIV), allowing the evaluation of k_s, and the associated wake parameters through several methods. The aerodynamic roughness parameters hence obtained are used for full-scale drag penalty calculations. It is observed that the predicted drag penalty can vary by over 15% among the different methods highlighting the care that should be taken when employing such methods.

Effect of Particle Migration on the Stress Field in Microfluidic Flows of Blood Analog Fluids at High Reynolds Numbers.

In the present paper, we investigate how the reductions in shear stresses and pressure losses in microfluidic gaps are directly linked to the local characteristics of cell-free layers (CFLs) at channel Reynolds numbers relevant to ventricular assist device (VAD) applications. For this, detailed studies of local particle distributions of a particulate blood analog fluid are combined with wall shear stress and pressure loss measurements in two complementary set-ups with identical flow geometry, bulk Reynolds numbers and particle Reynolds numbers. For all investigated particle volume fractions of up to 5%, reductions in the stress and pressure loss were measured in comparison to a flow of an equivalent homogeneous fluid (without particles). We could explain this due to the formation of a CFL ranging from 10 to 20 μm. Variations in the channel Reynolds number between Re = 50 and 150 did not lead to measurable changes in CFL heights or stress reductions for all investigated particle volume fractions. These measurements were used to describe the complete chain of how CFL formation leads to a stress reduction, which reduces the apparent viscosity of the suspension and results in the Fåhræus-Lindqvist effect. This chain of causes was investigated for the first time for flows with high Reynolds numbers (Re∼100), representing a flow regime which can be found in the narrow gaps of a VAD.

Development and Evaluation of a Multifrequency Ultrafast Doppler Spectral Analysis (MFUDSA) Algorithm for Wall Shear Stress Measurement: A Simulation and In Vitro Study.

Cardiovascular pathology is the leading cause of death and disability in the Western world, and current diagnostic testing usually evaluates the anatomy of the vessel to determine if the vessel contains blockages and plaques. However, there is a growing school of thought that other measures, such as wall shear stress, provide more useful information for earlier diagnosis and prediction of atherosclerotic related disease compared to pulsed-wave Doppler ultrasound, magnetic resonance angiography, or computed tomography angiography. A novel algorithm for quantifying wall shear stress (WSS) in atherosclerotic plaque using diagnostic ultrasound imaging, called Multifrequency ultrafast Doppler spectral analysis (MFUDSA), is presented. The development of this algorithm is presented, in addition to its optimisation using simulation studies and in-vitro experiments with flow phantoms approximating the early stages of cardiovascular disease. The presented algorithm is compared with commonly used WSS assessment methods, such as standard PW Doppler, Ultrafast Doppler, and Parabolic Doppler, as well as plane-wave Doppler. Compared to an equivalent processing architecture with one-dimensional Fourier analysis, the MFUDSA algorithm provided an increase in signal-to-noise ratio (SNR) by a factor of 4-8 and an increase in velocity resolution by a factor of 1.10-1.35. The results indicated that MFUDSA outperformed the others, with significant differences detected between the typical WSS values of moderate disease progression (p = 0.003) and severe disease progression (p = 0.001). The algorithm demonstrated an improved performance for the assessment of WSS and has potential to provide an earlier diagnosis of cardiovascular disease than current techniques allow.

The electrodiffusional theory for the wall shear stress measurement by two-strip probe

This article deals with the derivation of the fundamental theory describing the mass transport on the active surface of a two-strip mass transfer probe for an arbitrary direction of fluid flow. The existence of such a general theory is a critical point for the application of the electrodiffusion method using a two-strip probe. Considering the different possible sizes of the probe segments, the analytical formulas for the average mass transfer coefficients are derived and presented in dimensionless forms for both segments. The correctness of the derived analytical expressions is verified by numerical solution of the convection-diffusion transport equation. A methodology for possible experimental data treatment is also proposed. It is based on the evaluation of two current signals collected from the segments of a two-strip probe. From the derived equations, it is possible to determine the magnitude and direction of the wall shear rate vector for both frontal and reverse flow regimes. From the analysis of the current ratio predictions for different probe geometries, an optimal probe configuration is found with respect to the sensitivity of the flow direction measurement.

Preliminary study on modeling blood flow velocity profile for ultrasonic measurement of wall shear stress

The wall shear stress (WSS) contains useful information for the diagnosis of vascular diseases. To improve the accuracy of WSS measurements by ultrasound, some studies have attempted to correct the underestimation of the WSS by approximating a blood flow velocity profile measured by ultrasound to a velocity distribution model such as laminar flow. In this study, a blood flow velocity profile measured by ultrasound was modeled by the Womersley flow model, in order to correct underestimation of WSS in cases of measurements of vessels with complex geometries such as stenotic vessels. The simulation results showed that the rms error of the estimated WSSs was 45.8% from the stenotic vessel with a stenotic ratio of 40% when the spatial gradient was estimated by fitting a linear model to the measured flow profile. Such an error was improved from 45.8% to 25.3% by fitting a Womersley flow model to the measured blood velocity profile.

Measurement of Skin Friction in the University of Texas at Arlington Arcjet

Arcjet flows are known to have significant uncertainties associated with the fundamental flow quantities. A detailed characterization is critical to augment the current understanding of these complex flows as well as developing, screening, and qualifying thermal protection systems. Classically, estimations of skin friction have been made indirectly using heat flux analogies. These techniques, however, are marred by high levels of measurement uncertainty and require a priori assumptions about the flowfield. Directly measuring the wall shear stress removes the demand for these assumptions. A direct measurement of shear stress in supersonic high-enthalpy flow at the Office of Naval Research/University of Texas at Arlington arcjet facility has been conducted. The measured wall shear stress was (average combined calibration uncertainty), which matches well with estimations for laminar flow. Two shear stress sensors separated by a distance of 1.905 cm (0.75 in.) were integrated into a 20 deg half-angle shear-wedge platform. The two sensors tracked together within 10% of each other, implying a uniform flowfield over the region studied. Approximately 1.5 s into the test window, a steady increase in skin friction was observed. The cause of this nonphysically accurate increase in the shear stress measurement is discussed.

Downstream persistence of frictional drag reduction with repetitive bubble injection

Frictional drag reduction using repetitive bubble injection is investigated in a turbulent boundary layer beneath a 36-m-long flat-bottom model ship towed at 8.0 m/s. Bubbles are injected with air flow rates periodically fluctuating at a repetition frequency of 0.5–2.0 Hz. The drag-reduction ratio reaches 24%, which is a 5% improvement relative to the conventional method of continuous bubble injection. Measurement of the local wall shear stress acting on the bottom plate and optical visualization of bubbly flow reveal that the transition of the status of bubble dispersion causes the streamwise decay of local drag reduction. The downstream persistence of the drag-reduction effect can be improved by a factor of 15 through the use of repetitive bubble injection. The estimation of the full-scale performance indicates that repetitive bubble injection provides approximately 7 times the frictional drag reduction provided by continuous bubble injection for long ship hulls exceeding 130 m.

Wall Shear Stress Measurement in Carotid Artery Phantoms with Variation in Degree of Stenosis Using Plane Wave Vector Doppler

Wall shear stress (WSS) plays an important role in the formation, growth, and rupture of atherosclerotic plaques in arteries. This study measured WSS in diseased carotid artery phantoms with degrees of stenosis varying from 0 to 60% with both steady and pulsatile flow. Experiments were performed using in silico and real flow phantoms. Blood velocities were estimated using plane wave (PW) vector Doppler. Wall shear stress was then estimated from the velocity gradient near the wall multiplied by the viscosity of a blood-mimicking fluid. The estimated WSS using the in silico phantom agreed within 10% of the ground-truth values (root-mean-square error). The phantom experiment showed that the mean WSS and maximum WSS increased with the increasing degree of stenosis. The simulation and experiment results provide the necessary validation data to give confidence in WSS measurements in patients using the PW vector Doppler technique.

Measurement of wall shear stress on an airfoil surface by using the oil film interferometry with PIV analysis applied to Fizeau fringes

In this study, oil film interferometry (OFI) was applied to the flow around the suction surface of a two-dimensional airfoil under high lift conditions to measure the wall shear stress. However, existing OFI methods have difficulty measuring the wall shear stress in images affected by reverse and secondary flows. Therefore, the particle image velocimetry (PIV) method was applied to the Fizeau fringe images to determine the direction of progress from the calculated velocity vector. An airfoil with a wing section of NACA0012 was used, the Reynolds number was set to 8 × 104, and angle of attack was set at 11°. A direct spatial domain correlation was used for the PIV analysis method. At x/c = 0.75 to 0.83, the measured local skin friction coefficient was good agreeing with the large eddy simulation calculated results reported by Miyazawa et al. (Transaction of the JSME, Series B, Vol.72, No.721 (2006)). However, a large difference between the measured and calculated local skin friction coefficients occurred at x/c = 0.17 near the re-attachment point. The wall shear stress is determined by calculating the dominant frequencies using fast Fourier transform (FFT) analysis from the matrix data obtained by mean of the pixel intensities in the analysis region in the span direction. If the Fizeau fringe image is uniform in the span direction, the program can calculate the periodic waves. When the Fizeau fringes are tilted to the analysis range, the FFT analysis of the obtained matrix data results in an error because the dominant frequency cannot be calculated. Therefore, velocity vectors were detected near the re-attachment point by adapting PIV to the Fizeau fringe images. The local skin friction coefficients were calculated by OFI measurements with the Fizeau fringe images that were rotated by the angle of the velocity vector determined by PIV.

Neuroendocrine pancreatic tumors: about 13 cases

This article intends to revisit the electro-diffusional theory for the wall shear stress measurement from mass transfer probes of rectangular shape by considering the existence of two components of the wall shear rate (i.e., axial and transversal). General analytical formulas for the effective transfer length and the dimensionless mass transport coefficient were derived as a function of two parameters: a dimensionless angle of the flow direction, relative to the leading edge of the probe, and the aspect ratio between the width and the length of the strip probe. The correctness of the analytical relations for arbitrary flow direction and the aspect ratio was confirmed by numerical solutions of the transport equation in the convective-diffusive regime. It has also been proved that the differences between the Lévêque solution and the general analytical formula exhibit a significant deviation for a specific range of parameters. In the case of the three-dimensional boundary layers, in addition to the magnitude of the wall shear stress, the direction of the fluid flow in the vicinity of the probe’s surface is of paramount importance. Accordingly, a measurement methodology is proposed using two strip probes with different aspect ratios. The resulting equations required to quantify the magnitude of the wall shear rate vector and the dimensionless angle are also derived.

Utility of vector flow mapping technology in quantitative assessment of carotid wall shear stress in hypertensive patients: A preliminary study.

Blood flowing in the arterial lumen acts on the surface of the vessel wall to form wall shear stress (WSS). To date, there has been limited research on the utility of non-invasive technology in the accurate quantification of carotid WSS in patients with hypertension (HP). The present study aimed to explore the usage of vascular vector flow mapping (VFM) in the quantitative assessment of carotid WSS in hypertensive patients at an early stage and to validate its clinical utility. A total of 50 individuals confirmed without carotid plaques were grouped into a HP group (n = 25) and a control (CON) group (n = 25) according to blood pressure. An ALOKA LISENDO 880 Color Doppler Ultrasound with a L441 3-15 MHZ probe was used to obtain a longitudinal section scan to determine the regions of interests (ROIs) of the common carotid artery. VFM-based WSS measurements were obtained by selecting the ROI with optimal image quality from three full cardiac cycles. WSS-derived measurements, including WSSmax, WSSmin, and WSSmean, were analyzed and compared between the HP and CON groups. In addition, the correlations between WSS-derived measurements and the carotid artery intima-media thickness (IMT) were also analyzed. There were significant statistical differences in WSSmax and WSSmean between patients in the HP and CON groups. Specifically, the HP group had significantly decreased WSSmax and WSSmean compared to the CON group (WSSmax: 1.781 ± 0.305 Pa vs. 2.286 ± 0.257 Pa; WSSmean: 1.276 ± 0.333 Pa vs. 1.599 ± 0.293 Pa, both p < 0.001). However, there was no statistical difference in WSSmin between the groups (0.79 ± 0.36 vs. 0.99 ± 0.42, p = 0.080). Additionally, Spearman's correlation analysis indicated that the WSS-derived parameters were negatively correlated with the IMT (p < 0.001). Vascular VFM technology shows promising results in the quantitative assessment of difference in hemodynamics of the vascular flow field between patients with HP and normal controls. Difference in WSS may serve as a potential predictor for the development of arteriosclerosis risks.

Label-free measurement of wall shear stress in the brain venule and arteriole using dual-wavelength third-harmonic-generation line-scanning imaging.

Wall shear stress (WSS) is of fundamental physiological and pathological significance. Current measurement technologies suffer from poor spatial resolution or cannot measure instantaneous values in a label-free manner. Here we demonstrate dual-wavelength third-harmonic-generation (THG) line-scanning imaging, for instantaneous wall shear rate and WSS measurement in vivo. We used the soliton self-frequency shift to generate dual-wavelength femtosecond pulses. Simultaneous acquisition of dual-wavelength THG line-scanning signals extract blood flow velocities at adjacent radial positions for instantaneous wall shear rate and WSS measurement. Our results show the oscillating behavior of WSS in brain venules and arterioles at micron spatial resolution in a label-free manner.

Toward quantitative wall shear stress measurements: a comparative study on the impact of RGB-to-hue conversion algorithms on liquid crystal diagnostics

Liquid crystal diagnostics is a capable tool for determining quantitative wall shear stress distributions with high spatial resolution, which can be applied to almost any surface shape. A standard consumer camera is typically used to record the scattered light of the liquid crystals as red, green, and blue RGB data. This RGB data has to be converted to a hue-based color space in order to perform a state-of-the-art calibration procedure. Algorithms for this purpose are numerous in the literature. However, a considerable number of them show a wide range of resulting hue values due to different trigonometric relations. This renders some conversion algorithms unsuitable for calculating physical wall shear stresses, as their magnitude and distribution depend on the conversion algorithm used. For this reason, the choice of an inappropriate conversion algorithm may compromise the measurement accuracy and subsequent comparability significantly. The main objective of this paper is to give recommendations for the use of appropriate algorithms to determine physical wall shear stresses. In a first step, synthetic liquid crystal data is converted using algorithms described in the literature. The preselected algorithms are then applied to liquid crystal data from a flat plate wind tunnel experiment to illustrate their influence on the determined uncalibrated wall shear stress distribution. The final discussion serves as guidelines for the post-processing of liquid crystal data and their subsequent comparability.