Hot-film Shear Stress Sensor Research Articles

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.

Three-dimensional conditional structure of a high-Reynolds-number turbulent boundary layer

An array of surface hot-film shear-stress sensors together with a traversing hot-wire probe is used to identify the conditional structure associated with a large-scale skin-friction event in a high-Reynolds-number turbulent boundary layer. It is found that the large-scale skin-friction events convect at a velocity that is much faster than the local mean in the near-wall region (the convection velocity for large-scale skin-friction fluctuations is found to be close to the local mean at the midpoint of the logarithmic region). Instantaneous shear-stress data indicate the presence of large-scale structures at the wall that are comparable in scale and arrangement to the superstructure events that have been previously observed to populate the logarithmic regions of turbulent boundary layers. Conditional averages of streamwise velocity computed based on a low skin-friction footprint at the wall offer a wider three-dimensional view of the average superstructure event. These events consist of highly elongated forward-leaning low-speed structures, flanked on either side by high-speed events of similar general form. An analysis of small-scale energy associated with these large-scale events reveals that the small-scale velocity fluctuations are attenuated near the wall and upstream of a low skin-friction event, while downstream and above the low skin-friction event, the fluctuations are significantly amplified. In general, it is observed that the attenuation and amplification of the small-scale energy seems to approximately align with large-scale regions of streamwise acceleration and deceleration, respectively. Further conditional averaging based on streamwise skin-friction gradients confirms this observation. A conditioning scheme to detect the presence of meandering large-scale structures is also proposed. The large-scale meandering events are shown to be a possible source of the strong streamwise velocity gradients, and as such play a significant role in modulating the small-scale motions.

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)

Improved micro thermal shear-stress sensor

Micro hot-film shear-stress sensors have been designed and fabricated by surface micromachining technology which is compatible with IC technology. A polysilicon strip, 2 /spl mu/m/spl times/80 /spl mu/m, is deposited on top of a thin silicon nitride film and functions as the sensor element. By using the sacrificial-layer technique, a cavity (a vacuum chamber of about 300 mtorr), 200/spl times/200/spl times/2 /spl mu/m, is placed between the silicon nitride film and the silicon substrate. This cavity significantly increases the sensitivity of the sensor by reducing the heat loss to the substrate. The frequency response of the sensor, however, is degraded by the cavity. For comparison purposes, a sensor structure without a cavity has also been designed and fabricated on the same chip. When operated in a constant temperature mode, the cutoff frequencies of the sensors with and without a cavity can reach 9 and 130 kHz, respectively. Wind tunnel calibration of the sensor with a cavity shows a sensitivity of about 10 mV/Pa, which is about two orders of magnitude higher than other micromachined shear stress sensors.

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.