Hot-film Shear Research Articles

Estimation of aerodynamic entrainment in developing wind-blown sand flow.

Understanding aerodynamic entrainment, a critical process in wind-blown sand dynamics, remains challenging due to the difficulty of isolating it from other mechanisms, such as impact entrainment. Aerodynamic entrainment initiates the movement of surface particles, influencing large-scale processes like sediment transport and dune formation. Previous studies focused on average aerodynamic shear stress to estimate entrainment, but the role of impulse events, which cause significant shear stress fluctuations, remains under-explored. We used 12 hot-film shear sensors to measure the spatiotemporal distribution of aerodynamic shear stress during wind-blown sand flow development. We identified impulse events exceeding the entrainment threshold and analyzed their intensity, classifying particle movement as rocking, rolling, or saltation. Results indicate that after a 2-m fetch, sediment mass flux stabilizes, with aerodynamic shear stress decreasing to 78% of the entrainment threshold. We identified key trends, including the stabilization of rocking events beyond x = 4.5 m and a significant decrease in saltation frequency, indicating fully developed wind-blown sand flow. Impulse characteristics stabilize at a greater distance (4.5 m) than sediment transport (2 m) because turbulent airflow evolves more slowly. Our findings show that impulse events significantly influence aerodynamic entrainment. These insights enhance understanding of sediment transport dynamics and improve modeling of sand dune movement.

Application of superstatistical analysis on fluctuant surface shear in particle-laden turbulence boundary layer

We report on an application of superstatistics to particle-laden turbulent flow. Four flush-mounted hot-film wall shear sensors were used to record the fluctuations of the wall shear stress in sand-laden flow. By comparing the scaling exponent in sand-free with that in sand-laden flows, we found that the sand-laden flow is more intermittent. By applying the superstatistics analysis to the friction velocity, we found that the large time scale is smaller when the flow is sand-laden. The probability density of a fluctuating energy dissipation rate measured in sand-laden flow follows a log-normal distribution with higher variances than for sand-free flow. The variance of this dissipation rate is a power law of the corresponding time scale. The prediction based on the superstatistics model is consistent with our structure function exponents zeta _n for sand-free flow. Nevertheless, it overestimates zeta _n for sand-laden flow, especially at higher Reynolds numbers.Graphic abstract

The Effect of Sensor Dimensions on the Performance of Flexible Hot Film Shear Stress Sensors.

This paper presents a study to determine the effect of sensor dimensions (length, width, and thickness) on the performance of flexible hot film shear stress sensors. The sensing component of a hot film sensor is nickel thermistor, and the flexible substrate material is polyimide. Several groups of flexible hot film shear stress sensors with different lengths, widths, and thicknesses were studied. The temperature coefficient of resistance (TCR) was measured. The TCR increased slightly with increasing thickness. The frequency response (time constant) of the flexible hot film shear stress sensor was obtained by the square wave, while the sensitivity was tested in a wind tunnel. The study found that as the sensor length was shortened, the frequency response increased, and the sensitivity decreased.

Experimental Research on Boundary Shear Stress in Typical Meandering Channel

A novel instrument named Micro-Electro-Mechanical System (MEMS) flexible hot-film shear stress sensor was used to study the boundary shear stress distribution in the generalized natural meandering open channel, and the mean sidewall shear stress distribution along the meandering channel, and the lateral boundary shear stress distribution in the typical cross-section of the meandering channel was analysed. Based on the measurement of the boundary shear stress, a semi-empirical semi-theoretical computing approach of the boundary shear stress was derived including the effects of the secondary flow, sidewall roughness factor, eddy viscosity and the additional Reynolds stress, and more importantly, for the first time, it combined the effects of the cross-section central angle and the Reynolds number into the expressions. Afterwards, a comparison between the previous research and this study was developed. Following the result, we found that the semi-empirical semi-theoretical boundary shear stress distribution algorithm can predict the boundary shear stress distribution precisely. Finally, a single factor analysis was conducted on the relationship between the average sidewall shear stress on the convex and concave bank and the flow rate, water depth, slope ratio, or the cross-section central angle of the open channel bend. The functional relationship with each of the above factors was established, and then the distance from the location of the extreme sidewall shear stress to the bottom of the open channel was deduced based on the statistical theory.

Experimental study on the bed shear stress under breaking waves

The object of present study is to investigate the bed shear stress on a slope under regular breaking waves by a novel instrument named Micro-Electro-Mechanical System (MEMS) flexible hot-film shear stress sensor. The sensors were calibrated before application, and then a wave flume experiment was conducted to study the bed shear stress for the case of regular waves spilling and plunging on a 1:15 smooth PVC slope. The experiment shows that the sensor is feasible for the measurement of the bed shear stress under breaking waves. For regular incident waves, the bed shear stress is mainly periodic in both outside and inside the breaking point. The fluctuations of the bed shear stress increase significantly after waves breaking due to the turbulence and vortexes generated by breaking waves. For plunging breaker, the extreme value of the mean maximum bed shear stress appears after the plunging point, and the more violent the wave breaks, the more dramatic increase of the maximum bed shear stress will occur. For spilling breaker, the increase of the maximum bed shear stress along the slope is gradual compared with the plunging breaker. At last, an empirical equation about the relationship between the maximum bed shear stress and the surf similarity parameter is given, which can be used to estimate the maximum bed shear stress under breaking waves in practice.

220 MEMSを用いた熱式壁面せん断応力センサの開発および壁乱流構造に関する研究(OS3-2 乱流現象の実験とシミュレーション(2),OS3 乱流現象の実験とシミュレーション)

220 MEMSを用いた熱式壁面せん断応力センサの開発および壁乱流構造に関する研究(OS3-2 乱流現象の実験とシミュレーション(2),OS3 乱流現象の実験とシミュレーション)

Control of a decelerating boundary layer. Part 3: Optimization of round jets vortex generators

In the previous two parts of this study [G. Godard, M. Stanislas, Control of a decelerating boundary layer. Part 1: Optimization of passive vortex generators, Aerospace Sci. Technol. 10 (3) (2006) 181–191; G. Godard, J.M. Foucaut, M. Stanislas, Control of a decelerating boundary layer. Part 2: Optimization of slotted jets vortex generators, Aerospace Sci. Technol. 10 (5) (2006) 394–400], two different types of vortex generators were characterized and optimized in an adverse pressure gradient boundary layer. The model used was a bump in a boundary layer wind tunnel, which mimics the adverse pressure gradient on the suction side of an airfoil at the verge of separation. The present contribution describes the results of a test campaign performed in the same facility to optimize round jets devices with continuous blowing. The optimization was done as previously with hot film shear stress probes. The results show that the optimized jet devices give performances comparable to standard passive vortex generators in terms of skin friction. They also allow a quantitative comparison between three different types of vortex generators: passive devices, slotted and round jets. This comparison is performed in both co- and counter-rotating configurations.

Control of a decelerating boundary layer. Part 2: Optimization of slotted jets vortex generators

In the first part of this contribution [G. Godard, M. Stanislas, Control of a decelerating boundary layer. Part 1: Optimization of passive vortex generators, Aerospace Sci. Technol. 10 (3) (2006) 181–191], an optimization of passive vortex generators was performed in an adverse pressure gradient boundary layer. The model used was a bump in a boundary layer wind tunnel, which mimics the adverse pressure gradient on the suction side of an airfoil at the verge of separation. The present contribution describes the next step of the study: a test campaign was performed in the same facility to optimize slotted jets devices with both continuous and pulsed blowing. The optimization was done using hot film shear stress probes. The results show quantitatively the improvement brought by the slotted jets devices in terms of skin friction increase. They also show that the tested devices are less effective than equivalent passive devices.

Control of a decelerating boundary layer. Part 1: Optimization of passive vortex generators

The control of boundary layer separation on the suction side of an airfoil at high angle of attack has been renewed by the possibilities of active control. Nevertheless, such an active control needs a deep understanding of the flow to manipulate and of the actuating flow, both being 3D and unsteady. For that purpose, a model experiment has been designed in the frame of a coordinated European project called AEROMEMS, with a simpler (2D) geometry and with a dilatation of the scales in order to be able to characterize the actuation flow. This model is a bump in a boundary layer wind tunnel, which mimics the adverse pressure gradient on the suction side of an airfoil at the verge of separation. The present contribution describes preliminary tests done to optimize standard passive devices before testing active systems. The optimization was done with hot film shear stress probes, the characterization with hot wire anemometry and PIV. The results show quantitatively the improvement brought by the passive devices in terms of skin friction. They also show the mechanism which is at the origin of this improvement. The next step of the project is to replace passive devices by synthetic jets.

An approximate amplitude attenuation correction for hot-film shear stress sensors

A correction method, based on experimental results, has been developed to remedy the amplitude attenuation that occurs when statically calibrated hot-film shear stress sensors are used in air. The correction method is necessary in applications where typically two-dimensional arrays of measurement points are needed and other sensors, such as hot wires, cannot be employed. The method was developed with a primary aim of obtaining the correct power spectral density of an ensemble-averaged signature from an array of hot-film shear stress sensors. The hot-film sensors are corrected by comparing their individual power spectral densities to a reference spectrum obtained with a single hot wire, slightly elevated but within the viscous sublayer of the turbulent boundary layer. The method is verified by comparing the corrected hot film’s turbulence statistics, power spectral density, and correlation coefficients with the corresponding results from the hot wire.

Unsteady Bed Shear Stresses Induced by Navigation: Laboratory Observations

Time-dependent bed shear stresses induced by the passage of a barge tow have been measured with hot film shear stress sensors in a 1:25 scale model. Conditions typical of those observed for Upper Mississippi River navigation traffic were simulated in the experimental facility. Two sets of experiments were carried out: the first set consisted of simultaneous shear stress measurements at different locations for a variety of flow depths and boat operating conditions, providing space-time distributions of ensemble averaged wall shear stresses. The second set included a large number of realizations gathered for one particular flow condition at a single position, allowing analysis of the time evolution of the turbulence characteristics (i.e., standard deviation) of the bed shear stresses. The results of the first set of experiments show that for all the experimental conditions the basic patterns of the shear stress are similar, with two regions of high shear stress associated with the passage of the bow and the...

A219 三次元数値シミュレーションによる熱膜せん断応力センサ周りの流れと熱伝導解析(オーガナイズドセッション4 : 自然対流)

A219 三次元数値シミュレーションによる熱膜せん断応力センサ周りの流れと熱伝導解析(オーガナイズドセッション4 : 自然対流)

An in vitro study on the effect of branch points on the stability of coronary artery flow

Empirical correlations for the onset of turbulence in pulsatile flow through a straight tube and a 45° T-bifurcation are presented. We pumped three different test fluids of kinematic viscosity 0.008–0.035 cm 2 s −1 through four straight tubes 0.4–3.0 cm in diameter and three 45 degree T-bifurcations 0.45–2.2 cm in diameter. A Scotch yoke mechanism provided an oscillatory sine wave flow component of known stroke volume and frequency. To determine transition to turbulence, we adjusted the mean flow independently until we detected signal instabilities from hot film or electrochemical wall shear stress probes. The critical peak Reynolds number was found to correlate with two independent dimensionless groups: the Womersley parameter and the Strouhal number. We derived power law functions of these groups to provide an accurate and convenient method of predicting transition in both straight and bifurcating tubes. When compared to pulsatile flow through the straight tube, the presence of flow separation within the 45° T-bifurcation induced flow instabilities at lower values of the peak Reynolds number. The correlation for the 45° T-bifurcation is also a suitable model for predicting transition at coronary branch points, which we previously studied in an in vitro pulse duplicator. Flow instabilities at coronary branch points may play an important role in atherogenesis.

Numerical and experimental studies of annular flume flow

Annular flume studies of cohesive sediment beds are routinely used to investigate the erosion characteristics of the bed and the transport of suspended sediment. In order to interpret the results of such studies, an accurate description of the flow field and bed stress distribution is required. In this paper the flow field and bed stress distribution are calculated for a miniature annular flume using the computational fluid dynamics (CFD) package Harwell-FLOW3D. Particular attention is paid to the effect of singularities at the edges of the annular lid. The predictions are compared with data obtained from a miniature annular flume mounted on a Carri-med rheometer. By modifying the package, calculations are also made for inelastic non-Newtonian fluids, and the results are compared with data obtained with a Weissenberg rheogoniometer and flush mounted hot-film shear stress probes.

ISIS: An Instrument for Measuring Erosion Shear Stress In Situ

An instrument for measuring shear stress for erosion in situ(ISIS) has been developed to measure the erosion shear stress of muddy sediments on intertidal mud flats. Erosion shear stress is defined in this paper as the minimum applied bed shear stress required to initiate erosion and remove sediment from the bed surface. An applied shear stress is generated by the flow through and around a specially shaped bell head, which draws water radially across the bed into the centre of the bell head. The applied shear stress is a function of the distance from the bell head to the bed surface and the discharge through the system. The design of ISIS was assisted by the use of a computational numerical flow modelling package. The operating conditions giving the most even shear stress across the whole test section were discharges of 0·01–0·6 ls −1, and bell-to-bed distance of 4–8 mm giving a shear stress of 0·02–5 Nm −2. The ISIS system was calibrated using hot film shear stress probes. The calibration data gave a 92% fit to the calibration function for shear stress. Laboratory measurements with ISIS of the erosion shear stress of mud beds consolidated for c. 1·5 days, showed surface shear stresses of 0·11–0·24 Nm −2. These were very similar to values of surface erosion shear stress measured for the same mud in an annular flume. The ISIS system was used to measure surface erosion shear stresses on the mud flats at Portishead and Blue Anchor Bay in the Severn Estuary, U.K. Surface erosion shear stresses at Portishead were generally in the range 0·2–0·5 Nm −2. The surface erosion shear stresses measured at Blue Anchor Bay, which included mud and sand, ranged between 0·1–1·9 Nm −2.

Bed shear stress measurements for vims Sea Carousel

The results of bed shear stress measurements for vims Sea Carousel under clear-water and flat-bed conditions are presented. The measurements were made using a flush mounted hot film shear stress sensor mounted on the bottom of a laboratory version of the vims Sea Carousel. The experimental results confirm the relationship between the spatial-average bed shear stress, τ (Pascal), and the ring rotation speed, Ω (rpm), from a previous numerical study: τ = 0.0114 Ω 1.69 . The measurements show a maximum of 15% difference with that given by the equation. We also found the side wall effects extend outward less than 2.5 cm for all ring speeds and the bed shear stress distributions are reasonably uniform.

Application of the flush mounted hot film gauge for measuring blast wave surface shear

Use is made of the fast response capability of the hot film shear gauge to perform unique shock tube and field test blast wave surface shear measurements. Mean flow turbulent calibration data are presented from three different test facilities: a low speed wind tunnel, a high pressure pipe facility, and a laboratory scale shock tube. Favorable agreement in calibration data between the three facilities is demonstrated. The good correlation in calibration results validates the ‘‘established’’ formulation procedure and provides a unique verification of available shock tube shear data. Measurements of ground shear under clean wall, i.e., nondusty, flow conditions are reported for blast waves created by high explosive detonations under large scale field test conditions. Satisfactory scaling of results in terms of event yield and overpressure location is demonstrated.

A dual film anemometer probe for measuring arterial shear reversal

Wall shear rate was measured in the canine thoracic aortal in vivo by means of a hot-film anemometer and flush-mounted shear probe. Simultaneous measurement of blood temperature at the probe tip was accomplished by means of a thermistor film located close to the hot-film shear sensor on the probe tip. Blood temperature measurements were utilised to monitor the reversal of the thermal boundary layer produced by the heated shear sensor during blood flow reversal. Temperature data were utilised in an attempt to correct wall shear rates by adjusting the overheat ratio in response to the varying near wall temperature. Wall shear rate data and blood viscosity data were utilised to compute wall shear stress values. The average change in peak shear stress resulting from temperature corrections was 16%.

Hot-film wall shear probe for measurements at flexible walls

A new wall shear probe for measurement at flexible walls, such as membranes or arteries is presented. The design is based on commercially available hot-film wall shear probes. Rapid fixation at the membrane and sealing of the probe hole is guaranteed by a cap screw. The membrane is pressed between the screw and a flange. The probe is calibrated in stationary Couette flow by means of a calibration device which was described in an earlier paper (Tillmann and Schlieper, 1979). Application of the probe in an artificial blood pump chamber is demonstrated and first results for the wall shear stress distribution at different sites of the chamber are presented.

Position sensitivity of hot-film shear probes

The position sensitivity of hot-film shear probes was studied with the use of pulse-echo ultrasound. The ultrasonic probe was placed opposite the hot-film probe across a rigid tube. The accuracy of measuring hot-film probe position by ultrasonic methods was studied with the aid of a mechanical micrometer. A similar study was conducted in vivo on the thoracic aorta of dogs to maintain flush conditions between the hot-film probe and arterial wall. The sensitivity of hot-film probes to position with respect to a rigid wall was studied. Probe position was determined by ultrasonic methods. The results showed a 20% deviation in hot-film anemometer bridge voltage with a change in probe position of 0.3 mm from the wall. This could result in a 50% error in computed wall shear stress. Resolution of the method of ultrasonic positioning of the hot-film probe was determined to be 0.1 mm.