Velocity, vorticity, and mach number

The boundary layers developing on a single and two tandem circular cylinders were examined using multiple hot-film sensor arrays for Re=2.4–5.1×104. Hot-wire and surface pressure measurements, and smoke-wire flow visualization were also made to better understand the flow pattern and the evolution of the vortex street. The results show that, by use of the sensor arrays in conjunction with a bank of constant-temperature anemometers, (i) the effects of the upstream cylinder on the boundary layer developing on the downstream cylinder, (ii) the frequency of the vortex shedding, and (iii) the locations of flow separation and reattachment can be determined non-intrusively and simultaneously. These measurement capabilities will provide a practical means for the characterization and manipulation of unsteady flow phenomena.

Nonintrusive measurements of the boundary layer developing on a single and two circular cylinders

The present study continues the work described in Part I of this paper in utilizing a specifically designed apparatus for generating a known near-wall fluctuating flow field for the purpose of quantifying the dynamic response of a flush-mounted hot-element wall shear stress gauge. Two different types of wall shear stress gauges were tested: a flush-mounted hot wire in contact with the wall substrate and commercially available quartz substrate hot-film gauges with different thicknesses of quartz coating. It is found that the flush-mounted hot-wire gauge has a much higher dynamic response than the quartz substrate gauges, although it is still lower than the marginally elevated hot-wire configuration. This may suggest the possible use of a marginally elevated hot wire as a wall shear stress gauge to ensure sufficient responsiveness for time-resolved wall shear stress measurements.

http://iopscience.iop.org/article/10.1088/0957-0233/9/5/006/pdf

Dynamic response of a hot-wire anemometer. Part II: A flush-mounted hot-wire and hot-film probes for wall shear stress measurements

A review of heat transfer and skin friction measurement methods is presented with particular emphasis on techniques that yield details of time-resolved properties A description of the calibration methods necessary to insure accurate measurements is included Examples of recent unsteady heat transfer and skin friction measurements with interpretations of the meaning and importance of the results are given

Time-Resolved Heat Transfer and Skin Friction Measurements in Unsteady Flow

A floating-element instrument has been built based upon a precision galvanometer as the essential balance mechanism. The coil current necessary to maintain the element in its null position is directly proportional to the imposed force and is controlled by a feedback loop in which a key component is a fiber-optic scanner. A series of experiments performed in smooth-wall turbulent boundary layers confirms that the principal uncertainty in determining surface shear stress using the instrument arises from the buoyancy force due to the pressure variation within the gap surrounding the element. It is shown that, for a range of element geometries tested, the flow in the gap does not influence the boundary layer and the buoyancy force is adequately modeled by the expression/f(dp/djc)7rD/4. The value of/for elements with t/g greater than about 40 is found to be 0.43, close to the value 0.5 for a linear variation of gap pressure. For smaller values of t/g, the value of/is also smaller and less well defined. With this model for the buoyancy force, measurement accuracy is better than 5% for surface shear forces greater than about 300 /*N.

Development of a floating element for the measurement of surface shear stress

An experimental investigation of the time-dependent response of hot element wall shear stress gages in unsteady periodic air flows is reported. The study has focused on wall shear stress in laminar oscillating flows produced on a flat plate by a free stream velocity composed of a mean component and a superposed sinusoidal variation. Two types of hot element gages, platinum film and flush wire, were tested for values of reduced frequency ranging from 0.14 to 2.36. Values of the phase angle of the wall shear stress variation relative to the free stream velocity, as indicated by the hot element gages, are compared with numerical prediction. The comparisons show that the gages indicate a wall shear stress variation that lags the true variation, and that the gages will also not indicate the correct wall shear stress variation in periodic turbulent flows.

Timothy A. Giddings

Papers

Response of hot-element wall shear-stress gages in laminar oscillating flows