Surface Stress Sensitive Film Research Articles

Plantar pressure and shear measurement using surface stress-sensitive film

This paper describes the performance of a system for dynamic measurements of pressure and shear forces over the entire plantar surface of the foot. During human locomotion, mechanical stress at the plantar surface is 3D: pressure acting vertically and shear stress acting longitudinally and laterally. The system described here offers gait, foot deformity, diabetic ulceration, and foot-ankle kinetics researchers a means to obtain the full 3D data that is missing in most current research efforts. Surface Stress Sensitive Film™ (S3F) provides the sensing element for this measurement device. S3F is an incompressible elastic film that deforms in response to applied forces. Upon removal of the forces, the film returns to its original shape. The dynamic, 3D deformations are captured with a digital camera. The measured deformations can be analyzed directly, since they are proportional to the applied forces, or used as inputs to a finite element model of the film to reconstruct the pressure and shear force values. System performance is evaluated using both a standard form and human foot contact. The fundamental approach involves comparing the integrated normal and shear loads from the S3F-based sensor to the measurements provided by a force plate. The results showed high linearity (<3% nonlinearity), low uncertainty (<4%), low cross talk (<3%), high spatial resolution (1.6 mm2), no loss of fidelity at walking speeds with a 45 Hz sampling rate, and good repeatability for human foot contact measurements.

Design of an experiment to measure unsteady shear stress and wall pressure transmitted through an elastomer in a turbulent boundary layer

A flat plate that is exposed to a turbulent boundary layer (TBL) experiences unsteady velocity fluctuations which result in fluctuating wall pressures and shear stresses on the surface of the plate. There is an interest in understanding how fluctuating shear stresses and normal pressures generated on the surface of an elastomer layer exposed to a TBL in water are transmitted through the layer onto a rigid backing plate. Analytic models exist which predict these shear stress and normal pressure spectra on the surface of the elastomer as well as those transmitted through the elastomer. The design of a novel experiment is proposed which will utilize Surface Stress Sensitive Films (S3F) to measure the fluctuating shear stress and hydrophones to measure fluctuating normal pressure at the elastomer-plate interface. These experimental measurements would then be compared to models of unsteady shear and unsteady pressure spectra within a TBL for purposes of model validation. This work will present the design of an experiment to measure the unsteady pressure and unsteady shear at the elastomer-plate interface and the methodology for comparing the measured results to the analytic model predictions.

Experimental Comparison of DBD Plasma Actuators for Low Reynolds Number Separation Control

This paper presents experimental work comparing several Dielectric Barrier Discharge (DBD) plasma actuator configurations for low Reynolds number separation control. Actuators studied here are being investigated for use in a closed loop separation control system. The plasma actuators were fabricated in the U.S. Air Force Research Laboratory Propulsion Directorate’s thin film laboratory and applied to a low Reynolds number airfoil that exhibits similar suction surface behavior to those observed on Low Pressure (LP) Turbine blades. In addition to typical asymmetric arrangements producing downstream jets, one electrode configurations was designed to produce an array of off axis jets, and one produced a spanwise array of linear vertical jets in order to generate vorticity and improved boundary layer to freestream mixing. The actuators were installed on an airfoil and their performance compared by flow visualization, surface stress sensitive film (S3F), and drag measurements. The experimental data provides a clear picture of the potential utility of each design. Experiments were carried out at four Reynolds numbers, 1.4 × 105, 1.0 × 105, 6.0 × 104, and 5.0 × 104 at a-1.5 deg angle of attack. Data was taken at the AFRL Propulsion Directorate’s Low Speed Wind Tunnel (LSWT) facility.

Investigation of Vibration Phenomena Induced by Air Flow Over Side View Mirror

This work develops an understanding of the flow mechanisms that induce vibrations on automotive side view mirrors. The unsteady nature of the flow over side view mirrors causes unsteady aerodynamic load distributions and flow-induced vibrations on the mirror assembly. These vibrations generate blurred rear-view images and higher noise levels, affecting the mirror functionality and passenger comfort. Certain geometrical design features of side view mirrors can exacerbate the flow-induced vibration levels of the mirror assembly significantly. This work quantifies the impact of these design features on the vibration amplitude, develops a methodology for testing mirror vibrations in a small, low-speed wind tunnel using only the mirror of interest, and delves into the interactions between the bluff body mirror geometry and its wake. Two similar side view mirror designs were investigated in this work by using laser-based vibrometry, flow visualization, particle image velocimetry, hot film anemometry, and surface stress sensitive film techniques. The magnitude of the vibrations was found to depend on the level of excursion in the dynamic location of flow separation, particularly when characteristic flow frequencies couple with the mirror housing natural frequency.

Measurements of skin friction in water using surface stress sensitive films

The measurement of skin friction on hydrodynamic surfaces is of significant value for the design of advanced naval technology, particularly at high Reynolds numbers. Here we report on the development of a new sensor for measurement of skin friction and pressure that operates in both air and water. This sensor is based on an elastic polymer film that deforms under the action of applied normal and tangential loads. Skin friction and pressure gradients are determined by monitoring these deformations and then solving an inverse problem using a finite element model of the elastic film. This technique is known as surface stress sensitive films. In this paper, we describe the development of a sensor package specifically designed for two-dimensional skin friction measurements at a single point. The package has been developed with the goal of making two-dimensional measurements of skin friction in water. Quantitative measurements of skin friction are performed on a high Reynolds number turbulent boundary layer in the 12 inch water tunnel at Penn State University. These skin friction measurements are verified by comparing them to measurements obtained with a drag plate as well as by performing two-dimensional velocity measurements above the sensor using a laser Doppler velocimetry system. The results indicate that the sensor skin friction measurements are accurate to better than 5% and repeatable to better than 2%. The directional sensitivity of the sensor is demonstrated by positioning the sensor at several orientations to the flow. A final interesting feature of this sensor is that it is sensitive to pressure gradients, not to static pressure changes. This feature should prove useful for monitoring the skin friction on a seafaring vessel as the operating depth is changed.

The development of optical techniques for the measurement of pressure and skin friction

A novel measurement approach is described which has been developed to allow both pressure and skin friction measurements at very low speeds in liquid or gaseous environments. This approach, surface stress-sensitive films, S3F, is based on the deformation of an elastic medium and the transformation of this deformation into surface loads (pressure and shear stresses). Measurements in both air and water flows have been made. A comparative analysis of the S3F and PSP techniques for a low-speed air-flow case is presented.