Optics: measuring and testing – By light interference – For dimensional measurement
Reexamination Certificate
1999-09-27
2002-07-30
Front, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
For dimensional measurement
C356S477000, C356S505000
Reexamination Certificate
active
06426796
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to wall shear stress sensors. In particular, it relates to a fiber optic wall shear stress sensor.
BACKGROUND OF THE INVENTION
A significant fraction of the total resistance to motion on airplanes and ships is due to surface friction, or skin friction. Measuring and determining this friction is complex and further complicated by the limited availability of skin friction gages. This is in contrast to the availability of gages for pressure, temperature and strain. Early work in this area involved obtaining data in pipe flows, where the wall shear can be directly related to the pressure drop, which is relatively simple to measure accurately. By measuring the small frictional force exerted on a movable element of the surface, one is able to obtain direct measurements of wall shear stress. These types of measurements work well for laminar, transitional and turbulent flows without prior knowledge of the state of the flow.
A great number of techniques for the measurement of wall shear stress have been devised over the years, ranging from inferring the skin friction from measuring the boundary layer profile or using some correlation or analogy to the direct measurement of the force on a surface. Although all of these techniques can be shown to work for some flow regime, indirect methods, such as Stanton tubes, Preston tubes, and surface hot-wire techniques to name a few, have not been shown to be reliable for complex flows such as 3D and/or unsteady cases with rough wall, curved walls, flows with injection or suction, or high-speed flows, especially those with high enthalpies, combustion and/or impinging shocks. Alternatively, direct measurements do not require any foreknowledge of the flow or its properties and can provide accurate results all the regimes mentioned above.
Direct measurements, refers to techniques that separate a small element, referred to as a floating head, from the wall and measures the tangential force that the flow imparts on it. Direct measurements are the most believable of all the techniques. The sensor is measuring the actual shear on the surface, without respect to the fluid, the state of the boundary layer, or chemical reactions. Since the floating head is level with the wall, the measurement is non-intrusive to the flow. The forces are very small, sometimes requiring large floating heads and expensive instrumentation to obtain accurate results. The direct measurement technique generally falls into two categories, nulling and non-nulling.
In the nulling design, called this because the floating head is returned to its null (or original) position, the floating head is acted upon by shear, but the sensor provides a restoring force to the head to keep it in place. The magnitude of that restoring force is monitored, which is equal to the shear force acting on the floating head. In one example, the floating head was supported by a beam which pivoted around a spring near the base. The movement of the head was sensed by a pair of capacitance plates, with an electromagnet used to supply the restoring force. Other nulling designs have required mechanical linkages and motors to return the head to the null position. Although the nulling design does remove some of the possible error introduced by the movement of the head, the sensors are very complex, mechanically unreliable, and have a slow time response, on the order of seconds.
In a non-nulling design, the floating head is allowed to deflect under the shearing load. This type of design is much simpler than a nulling design, allowing the removal of the restoring force mechanism and thus making much smaller time responses possible. However, this arrangement does allow the structure of the sensor to flex and this variation may introduce spurious results.
Another type of non-nulling gage is a design where the floating head is supported by tethers around its periphery. This design is sensitive to normal pressure changes because the tethers are more sensitive to motion normal to the wall than parallel to it. In macro-designs of the tether sensor, where the displacement was measured by strain gages mounted on the tethers, the sensor was not only sensitive to normal pressures, but also to temperature changes as the strain gages were very near the flow. In supersonic flows, wall temperature can change significantly during a test. However, the heat flux boundary conditions are significantly different at the floating head than the rest of the wall, leading to significant temperature differences and, therefore, significant errors in the shear measurement. In micromachined designs, the strain gages have been replaced by capacitance or by an external laser/photoiode system. However, the drawbacks of the table-top design remain, limiting the use of the sensor to simple flows without large temperature or pressure variation.
Another non-nulling design is a cantilever beam concept. In this design, the floating head is attached to a single cantilever beam which flexes with the application of shear to the head. The displacement is measured by the strain caused by the displacement at the base of the beam. If the displacements are kept to a minimum, the issue of head protrusion into the flow is negligible. This design offers high stiffness for normal forces, while being relatively weak for tangential forces, providing a sensor that is insensitive to normal pressure variations. The concept is very simple, rugged and has a small time response. If the sensing head is prepared from similar materials as the surrounding wall, errors from temperature mismatches will be minimized. Also, the concept can be easily extended to measure in two directions, removing the directional ambiguity of other designs.
Winter (“An Outline of the Techniques Available for the Measurement of Skin Friction in Turbulent Boundary Layers,”
Prog. Aerospace Sci
., 1977, Vol. 18, pp. 1-57) was able to use resistance strain gages in a large balance, which allowed sensitive measurements in a static environment. Schetz and Nerney (“Turbulent Boundary Layer with Injection and Surface Roughness,”
ATAA Journal
, 1977, Vol. 15, No. 9, pp. 1288-1294) made the next step to semi-conductor strain gages, improving the output 100 fold. A series of skin friction sensors based on strain gages mounted on the cantilever beam in a non-nulling arrangement have also been produced that are useful for measuring subsonic or cool supersonic flows. However, as the Mach number range increases, hot flows are encountered. In addition, the study of supersonic flows with injection of combustible gases such as hydrogen also necessitated a new design of skin friction gages. High enthalpy flows are simulated using an object. During this simulation, the skin friction gage encounters both high temperatures and high electromagnetic field environments.
Acharya et al. (“Development of a Floating Element for the Measurement of Surface Shear Stress,”
AIAA Journal
, March 1985, Vol. 23, No. 1, pp. 410-415) describe a floating-element instrument having a central component which is a tension galvanometer which supports the element itself. The position of a target on the back of the element is determined using a fiber-optic scanner. A reflector is lined-up perfectly with the fiber. The reflector is only partially reflective, and therefore, only part of the light is returned through the fiber. The returning light is shined on a photodiode and transformed to voltage. The nulling part of the circuit moves the head and the reflector back to the same voltage, which is the same location on the reflector. This design is extremely complicated requiring a feedback mechanism and an actuator to rezero the head. The alignment of the fiber and the reflector must be precise, making construction of the device difficult. Use of the partial reflection reflector limits the temperature that the sensor can be used. Since all of the electronics are internal, the sensor is large, complex and temperature sensitive. Moreover, this design can only be used for nulling applications.
By
Jones Mark E.
Murphy Kent A.
Pulliam Wade J.
Schetz Joseph A.
Bryant Joy L.
Front Frank G.
Lee Andrew H.
Luna Innovations Inc.
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