Simultaneous Measurement of Velocity and Fluctuating Pressure in a Turbulent Wing-tip Vortex Using Triple Hot-film Sensor and Miniature Total Pressure Probe

We have developed a probe-system for simultaneous measurement of three velocity components and pressure in turbulent flows. A miniature total pressure probe is placed adjacent to the sensors of a triple hot-film probe in order to achieve the spatial resolution which is equivalent to that of the triple hot-film probe itself. The instantaneous static pressure is calculated from measured velocity and total pressure by means of a newly developed processing method based on the Bernoulli equation for unsteady flows. The measurements were undertaken in a turbulent wing-tip vortex flow. The look-up table method is employed for the calibration of the hot-film probe so accurate velocity data could be obtained over a wide range of the flow-attack angles. It is also demonstrated that the present probe-system is capable of measuring fluctuations in both velocity and pressure in the 20?C650 Hz frequency range. The distribution of the fluctuating pressure obtained by this indirect method is in good agreement with the results from direct measurements of static pressure, demonstrating the promising performance of the present method. Furthermore, an improvement in the ability to make measurements of the velocity?Cpressure correlation across the wing-tip vortex is achieved. This improvement is possible because the effects of lateral velocity components are properly taken into account in the present formulation. The investigation regarding the transport equation budget for turbulent kinetic energy shows an anomalous structure of turbulence in this flow, mainly due to the meandering of the vortex, and the measurement of pressure diffusion is found to play an important role in the characterization of this kind of flow.     

Multiple hot-film sensor array calibration and skin friction measurement

A method for the calibration of closely spaced multiple hot-film sensor (MHFS) arrays of nickel-on-polyimide-substrate type was presented in this paper. The hot-film sensors were calibrated individually and in situ against the values of the wall-shear stresses determined from boundary-layer velocity profiles (developed on the front portion of a circular cylinder) through a specially designed wall-mounted hot-wire probe. The accuracy and feasibility of the present method was examined through the subsequent measurements of the skin-friction distributions on a circular cylinder (including the removal and bonding of the calibrated MHFS from a 10-cm diameter calibration cylinder and onto the surface of a 8-cm diameter test cylinder) and comparison of their consistency with the published results. The present MHFS calibration method is important in the non-intrusive multipoint measurements of the mean skin friction along the surfaces of two-dimensional aerodynamic objects of interest.     

Some expériences and considerations on measuring turbulence in water with hot films

Results are presented of a four year experience with conical hot-film sensors for measurements of the longitudinal component of turbulence in water. Data are presented on the reduced sensitivity by fouling. Special consideration has been given to the amplitude response of these sensors. For scientific purposes careful calibration in the lower frequency range is required; RMS values may be obtained that are accurate to within 5%. With “technical clean” water, as has been used in rigs for industrial research, this accuracy may be within 10%. If dynamic calibration is not possible, existing theories may be used as a first good approximation if the geometry of the sensor is specified in detail.     

Measurement of unsteady boundary layer developed on an oscillating airfoil using multiple hot-film sensors

 The spatial-temporal progressions of the leading-edge stagnation, separation and reattachment points, and the state of the unsteady boundary layer developed on the upper surface of a 6 in. chord NACA 0012 airfoil model, oscillated sinusoidally within and beyond the static-stall angle, were measured using 140 closely-spaced, multiple hot-film sensors (MHFS). The MHFS measurements show that (i) the laminar separation point and transition were delayed with increasing α and the reattachment and relaminarization were promoted with decreasing α, relative to the static case, (ii) the pitchup motion helped to keep the boundary layer attached to higher angles of attack over that could be obtained statically, (iii) the dynamic stall process was initiated by the turbulent flow separation in the leading-edge region as well as by the onset of flow reversal in the trailing-edge region, and (iv) the dynamic stall process was found not to originate with the bursting of a laminar separation bubble, but with a breakdown of the turbulent boundary layer. The MHFS measurements also show that the flow unsteadiness caused by airfoil motion as well as by the flow disturbances can be detected simultaneously and nonintrusively. The MHFS characterizations of the unsteady boundary layers are useful in the study of unsteady separated flowfields generated by rapidly maneuvering aircraft, helicopter rotor blades, and wing energy machines. Received: 17 June 1997 / Accepted: 10 December 1997     

Measurement of multiple gas-bubble velocities in gas–liquid flows using hot-film anemometry

A dual-probe hot-film anemometry technique has been developed to measure multiple gas-bubble velocities corresponding to different gas-bubble size groups in air–water flows. A data reduction scheme using wavelet analysis combined with a phase-detection technique is used to discriminate the hot-film anemometer output signals into signals corresponding to different bubble size groups. The phase and bubble size discrimination is based on the magnitude and time derivative of the signal, and the streamwise length of the gas bubbles. A cross-correlation between the discriminated signals from the two probes yields an average time difference of arrival of the gas bubbles at the two sensor locations. The velocities are estimated from the distance between the sensors and the time difference of arrival. The mean bubble size is estimated from the chord length distribution. Measurements performed in vertical-up air–water slug flow show the technique to be a viable method for obtaining bubble velocity and size information. The velocity measurements from the hot-film anemometry are corroborated using a high-speed quantitative flow visualization system. Received: 22 December 1999/Accepted: 8 May 2001     

The response of a turbulent boundary layer to artificial disturbances

With periodic fluid injection through small slots, a turbulent boundary layer is artificially disturbed on scales that are of the order of those of the natural quasi-periodic events. The periodic phase-average of the streamwise fluid velocity is determined from hot-film measurements, and used to find the coherent velocity component as defined by the triple decomposition. It appears that, when a disturbance is active, the generated flow pattern is very similar to the one caused by the interaction of a crossflow and a jet. However, when it is terminated, the turbulent boundary layer returns to its undisturbed state.     

Simultaneous measurements of reactive scalar and velocity in a planar liquid jet with a second-order chemical reaction

This paper presents a new experimental approach for simultaneous measurements of velocity and concentration in a turbulent liquid flow with a chemical reaction. For the simultaneous measurements, we developed a combined probe consisting of an I-type hot-film probe and an optical fiber probe based on the light absorption spectrometric method. In a turbulent planar liquid jet with a second-order chemical reaction $(\mathrm{A}+\mathrm{B}\rightarrow \mathrm{R})$ , streamwise velocity and concentrations of all reactive species are measured by the combined probe. The turbulent mass fluxes of the reactive species are estimated from the simultaneous measurements. The results show that the influence of the chemical reaction on the turbulent mass flux of the reactant species near the jet exit is different from its influence in other regions, and the turbulent mass flux of the product species has a negative value near the jet exit and a positive value in other regions.     

Simultaneous DPTV/PLIF measurements of a turbulent jet

Simultaneous measurements of instantaneous velocity and concentration fields were performed using digital particle tracking velocimetry and planar laser- induced fluorescence for a turbulent jet at a Reynolds number of 3000. The measurements of mean velocity, turbulent stresses, mean concentration, concentration variance, and turbulent flux of tracer all collapse onto self-similar profiles in the far field of the jet. The measurements showed excellent agreement with previous point velocity and concentration measurements. It is concluded that the system is an effective means of measuring the velocity and concentration distributions and turbulent characteristics. Received: 7 July 1999/Accepted: 9 February 2000     

In-situ swinging arm calibration for hot-film anemometers

A method of calibration for hot-film anemometers is presented. A swinging arm that moves under the influence of gravity serves as both a calibration mechanism and a probe support. The velocity of the probe is found by differentiating the angular position history of the arm and multiplying it with the arm length. Limitations on the quality of calibration data while the arm is accelerating are discussed. The hot film voltage output is then matched to the velocity to find the two constants in King's law. The calibration was tested by taking velocity profile measurements in a laminar boundary layer. The results of these compared well to the Blasius profile.     

Evaluation of three techniques for wall-shear measurements in three-dimensional flows

Recent improvements in three techniques for measuring skin friction in two- and three-dimensional turbulent wall-bounded shear flows are presented. The techniques are: oil-film interferometry, hot wires mounted near the wall, and surface hot-film sensors based on MEMS technology. First, we demonstrate that the oil-film interferometry technique can be used to measure the skin-friction magnitude and its direction in two- and three-dimensional wall-bounded shear flows. Second, a simple method is outlined to measure the skin friction with a wall wire located outside of the viscous sublayer. Finally, a systematic study of the parameters influencing wall-friction measurements with MEMS sensors is presented. The results demonstrate that accurate measurements of the mean skin friction with MEMS sensors are possible in two- and three-dimensional wall flows. Measurements by the three techniques are compared to each other and to past measurements in the same facility.     

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. Published online: 19 December 2002     

Evaluation of flush mounted hot-film sensors for skin friction reduction measurements in viscoelastic polymer solutions

The purpose of this investigation was to evaluate the performance of flush mounted hot-film sensors for mean wall shear stress measurement in turbulent flows of dilute drag reducing polymer solution. A series of pipe flow expriments were conducted over a range of Reynolds numbers and polymer solution concentrations to compare the level of skin friction drag reduction measured by hot-film sensors with values calculated from pipe pressure drop. It is shown that water calibrated hot-film sensors consistently underestimate the wall shear stress suggesting that Reynolds analogy is not valid in dilute polymer solutions. The Newtonian form of the relationship between the wall shear stress and the heat transfer remains valid in dilute polymer solutions. However, multiplicative and additive factors in the relationship are shown to increase linearly with the logarithm of the polymer concentration.     

Compressible flow hot-wire calibration

 A resistance thermal anemometer operated in compressible flow is sensitive to density, velocity, and total temperature fluctuations. To address these effects, a relatively inexpensive compressible flow calibration facility has been developed and tested. Characteristics of a hot-film anemometer in compressible flow are determined and a correction technique to account for differences in mean density between the calibration facility and the flow under study is proposed. Received: 19 October 1995 / Accepted: 18 October 1996     

Experimental study of transient forced turbulent separation and reattachment on a bevelled trailing edge

The purpose of this experimental study is to analyze the transient processes of the separation and reattachment of a turbulent boundary layer at the trailing edge of a splitter plate. The separation is driven by a steady pneumatic injection, considered here as an actuator controlling the mixing layer. Particle image velocimetry and hot-film measurements are performed at various stages of the processes. The results highlight the behavior of each transient. Analysis of the time scale of the processes is realized by means of bi-orthogonal decomposition. This gives essential information (e.g., time scale, qualitative features) for future applications using duty cycle mode to excite instabilities of the mixing layer.     

Vapor core turbulence in annular two-phase flow

 This paper reports a new technique to measure vapor turbulence in two-phase flows using hot-film anemometry. Continuous vapor turbulence measurements along with local void fraction, droplet frequency, droplet velocity and droplet diameter were measured in a thin, vertical duct. By first eliminating the portion of the output voltage signal resulting from the interaction of dispersed liquid droplets with the HFA sensor, the discrete voltage samples associated with the vapor phase were separately analyzed. The data revealed that, over the range of liquid droplet sizes and concentrations encountered, the presence of the droplet field acts to enhance vapor turbulence. In addition, there is evidence that vapor turbulence is significantly influenced by the wall-bounded liquid film. The present results are qualitatively consistent with the limited data available in the open literature. Received: 17 August 1998/Accepted: 12 April 1999     

Experimental study of the boundary layer over an airfoil in plunging motion

This is an experimental study on the boundary layer over an airfoil under steady and unsteady conditions.It specifically deals with the effect of plunging oscillation on the laminar/turbulent characteristics of the boundary layer.The wind tunnel measurements involved surfacemounted hot-film sensors and boundary-layer rake.The experiments were conducted at Reynolds numbers of 0.42×10 6 to 0.84 × 10 6 and the reduced frequency was varied from 0.01 to 0.11.The results of the quasi-wall-shear stress as well as the boundary layer velocity profiles provided important information about the state of the boundary layer over the suction surface of the airfoil in both static and dynamic cases.For the static tests,boundary layer transition occurred through a laminar separation bubble.By increasing the angle of attack,disturbances and the transition location moved toward the leading edge.For the dynamic tests,earlier transition occurred with increasing rather than decreasing effective angle of attack.The mean angle of attack and the oscillating parameters significantly affected the state of the boundary layer.By increasing the reduced frequency,the boundary layer transition was promoted to the upstroke portion of the equivalent angle of attack,but the quasi skin friction coefficient was decreased.     

Transport equations for the mean energy and temperature dissipation rates in grid turbulence

 Simultaneous measurements of velocity and temperature fluctuations were made in the turbulent flow downstream of a grid-heated screen combination. The magnitudes of n and m, the power-law exponents for the decay of turbulent energy and temperature variances, are nearly equal. This approximate equality is in conformity with the locations of the peaks in the three-dimensional turbulent energy and temperature spectra. The values of n and m are consistent with the isotropic forms of the transport equations for the mean turbulent energy and temperature variance. They are also consistent, allowing for the measurement accuracy, with the isotropic forms of the equations for the mean turbulent energy and temperature dissipation rates. Received: 20 October 1998 / Accepted: 26 March 1999     

A miniature triple hot-wire probe for wall bounded flows

 Four orthogonal and one non-orthogonal miniature triple hot-wire probes have been developed and tested in a two-dimensional turbulent boundary layer. The influence of the different wire configurations on measurements of the Reynolds stresses has been studied. A directional calibration with an analytical solution for the wire response equations was used for the measurements of the non-orthogonal probe and was tested for the orthogonal probes in order to correct their possible geometrical imperfections. It is shown that a directional calibration does not significantly improve the quality of the measurements for precisely manufactured orthogonal probes and that measuring errors are related rather to the measuring volume, the size of the domain of unique solutions for the instantaneous velocity vector and interference effects, i.e. the wire configuration itself. Received: 7 February 1997/Accepted: 18 November 1997     

Stereoscopic particle image velocimetry measurements of the flow around a Rushton turbine

 The principles of stereoscopic particle image velocimetry (PIV), including distortion compensation, were applied to the turbulent flow in a vessel stirred by a Rushton turbine. An angular offset configuration was used and tilt-axis lens mounts were incorporated in order to satisfy the Scheimpflug condition, significantly reducing the ordinarily large depth of field requirements of such configurations. A distortion compensation procedure, or in situ calibration, was utilized in place of the ray tracing, or mechanical registration, used in previous studies. The calibration procedure was validated using two tests, one a rigid translation of a speckle target, the other the viscous flow between two concentric cylinders. The results of the tests suggest the success with which the distortion compensation procedure may be applied to real fluid flows. Phase-locked instantaneous data were ensemble averaged and interpolated in order to obtain mean 3-D velocity fields on a cylindrical shell enclosing the turbine blade. From these fields, the tip vortex pairs and the radial jet documented in previous studies of mixer flows were easily identified. Received: 5 February 1999/Accepted: 1 December 1999     

An experimental study into the effects of streamwise and spanwise acceleration in a turbulent boundary layer

We compare two turbulent boundary layers produced in a low-speed water channel experiment. Both are subjected to an identical streamwise pressure gradient generated via a lateral contraction of the channel, and an additional spanwise pressure gradient is imposed on one of the layers by curving the contraction walls. Despite a relatively high streamwise acceleration, hot-film probe measurements of the mean-velocity distributions show that the Reynolds number increases whilst the coefficient of friction decreases downstream. Visualization of the viscous layers using hydrogen bubbles reveal an increase in the non-dimensional streak spacing in response to the acceleration. Changes in statistical moments of the streamwise velocity near the wall suggest an increased dominance of high-velocity fluctuations. The near-wall streaks and velocity statistics have little sensitivity to the boundary layer three-dimensionality induced by the spanwise pressure gradient, with the boundary-layer crossflow velocity reaching 11 % that of the local freestream velocity.