A Flexible Hot-Film Sensor Array for Underwater Shear Stress and Transition Measurement

Abstract

A flexible hot-film sensor array for wall shear stress, flow separation, and transition measurement has been fabricated and implemented in experiments. Parylene C waterproof layer is vapor phase deposited to encapsulate the sensor. Experimental studies of shear stress and flow transition on a flat plate have been undertaken in a water tunnel with the sensor array. Compared with the shear stress derived from velocity profile and empirical formulas, the measuring errors of the hot-film sensors are less than 5%. In addition, boundary layer transition of the flat plate has also been detected successfully. Ensemble-averaged mean, normalized root mean square, and power spectra of the sensor output voltage indicate that the Reynolds number when transition begins at where the sensor array located is 1.82 × 105, 50% intermittency transition is 2.52 × 105, and transition finishes is 3.96 × 105. These results have a good agreement with the transition Reynolds numbers, as measured by the Laser Doppler Velocimetry (LDV) system.

High sensitive flexible hot-film sensor for measurement of unsteady boundary layer flow

“…If we ignore the spanwise thermal diffusion, assume that the thermal boundary layer is contained within the velocity boundary layer and effects of natural convection are much smaller than forced convection. The relationship between the sensor input power P and shear stress τ w can be expressed as [16,22]:…”

“…The onset and end, as well as 50% intermittent of transition locations can be resolved from the hot-film sensor output [16]. Characterized by the alternating current (AC) output of hot-film sensor, the AC output voltage is stable with very little fluctuation in the laminar boundary layer.…”

“…The hot-wire anemometry [13,14] is an intrusive measurement method, it can only provide pointwise flow velocity and hard to obtain the flow-field information close to the surface of the experimental model. The flexible hot-film sensors [15][16][17][18] based on the MEMS technology are well fit on curved surfaces for in situ multipoint measurements. Meanwhile, the sensors possess high temporal/spatial resolution and neglectable interference to the flow due to its very small thickness of micrometer order.…”

Boundary layer state detection using piezoelectric sensors

“…The intrinsic differences between the boundary layer states include the shear stress at the skin of the airfoil, the heat transfer between the air and the skin, and the increased high frequency pressure fluctuations [1]. Given these differences, various measurement techniques to be used in wind tunnel testing have been developed to determine the local state of the flow around the airfoil, such as pressure or sound transducers [9][10][11], hot film sensors [12][13][14][15], infrared radiation (IR) thermography [16][17][18], and shear stress measurements [19][20][21][22]. Pressure or sound transducers either measure the absolute pressure occurring at discrete locations on an airfoil, or rely on dynamic pressure fluctuations.…”

Fabrication and static calibration of double-layer thermal film sensor for fluid wall shear stress measurement

“…The hot-film sensors fabricate on flexible polymer substrate are well fit on curved surfaces with high temporal/spatial resolution and less interference to the flow for in-situ multipoint measurements. Hence, the flexible thermal sensor is a convenient and effective method to determine the shear stress in the boundary layer, both simultaneously and non-intrusively [5].…”