Hot-wire Anemometry Measurements Research Articles

Unsteady Boundary Layers on a Flat Plate Disturbed by Periodic Wakes: Part II—Measurements of Unsteady Boundary Layers and Discussion

As the second part of the study, detailed hot-wire anemometry measurements of wake-affected boundary layers on the flat plate are made. These measurements are organized in order, first, to check the standpoint of the modeling of the wake-induced transition proposed in Part I, and second, to observe wake–boundary layer interaction in detail from a viewpoint of direct and indirect effect of the wake passage upon turbulent spot generation within the boundary layer, as described by Walker (1993). The validity of the presumed state of the wake-affected boundary layer in the distance–time domain, which constitutes the basis of the transition model, is confirmed to great extent. However, it is also found that the criterion for the onset of the wake-induced transition adopted in Part I should be reconsidered. Some successful attempts are therefore made to specify the transition onset.

Experimental testing of diffusion models in a manipulated turbulent boundary layer

An experimental investigation of the performances of one-point closure turbulent diffusion models is performed in an externally manipulated turbulent boundary layer. Detailed hot wire anemometry measurements are performed on a refined two-dimensional grid, allowing the terms of the transport equations of the turbulent stresses to be calculated. The turbulent diffusion terms as well as the convection, production, and viscous diffusion terms are then directly estimated. The dissipation terms of each Reynolds tensor component are estimated by use of local isotropy hypothesis (e ij = 2/3eδ ij ), e being obtained by balancing the turbulent kinetic energy budget. The pressure-strain terms are then obtained by balancing the u' i u' j equations. The results are presented for a streamwise position located near the manipulator trailing edge (x = 2δ 0 ). All of the measurements are performed at relatively large distances from the wall (y + > 120), allowing the minimization of the spatial integration effects of the probes. The balances show the modifications imposed to the classical turbulent boundary-layer equilibria. Knowledge of the different terms allows several experimental applications of the conventional turbulent diffusion models to be made.

Experimental study of the flow in the cavity between rotating disks

An experimental investigation using laser-Doppler anemometry was conducted in the cavity between rotating disks. Measurements of velocity were taken for rotor-stator and counterrotating disk flows. Batchelor-type flow was observed for the rotor-stator flow, and Stewartson-type flow in the counterrotating disk system. The core flow of the counterrotating disks is turbulent for x 2Re Φ as low as 2.16 × 10 4, but the boundary layer flow remains laminar for x 2 Re Φ ≤ 1.1 × 10 5. The velocity measurements show good agreement with previously published hot-wire anemometry measurements and theoretical predictions.

Nonisothermal turbulent boundary-layer adverse pressure gradient large scale thermal structure measurements

Hot-wire anemometry measurements in an incompressible turbulent boundary-layer flow over a heated flat plate under equilibrium adverse-pressure-gradient conditions (beta = 1.8) were made for two different temperature difference cases (10 and 15 C) between the wall and the freestream. Space-time correlations of temperature fluctuations (T`) were obtained with a pair of subminiature temperature fluctuation probes. The mean convection velocities, the mean inclination angles, and coherence characteristics of the T` large-scale structure were determined. The present temperature structures measurements for a nonisothermal boundary layer are compared to the zero-pressure-gradient case with identical temperature differences previously reported, in which the mean convection velocity of the T` structure was a function of position y(sup +) and independent of the limited temperature-difference cases tested. The three major findings of the present study, as compared to the zero-pressure-gradient case, are (1) the mean convection speed of the T` structure under beta = 1.8 pressure-gradient conditions was found to be substantially lower in the logarithmic core region than the zero-pressure-gradient case. Additionally, the mean convection speed is felt by the authors to be a function of pressure-gradient parameter beta; (2) the mean inclination angle of the T` structure to the wall under the adverse-pressure-gradient flow wasmore » 32 deg, which compares favorably to the 30-deg value of the zero-pressure-gradient case; and (3) the limited data suggests that the mean convection velocity of the T` structure is a function of y(sup +) and independent of the limited temperature-difference cases tested. 11 refs.« less

Experimental measurements of large-scale temperature fluctuation structures in a heated incompressible turbulent boundary layer

Hot-wire anemometry measurements over a heated flat plate are made for three different temperature difference cases of 10, 15, and 20°C. Space-time correlations of temperature fluctuations T′ are determined from which mean convection velocities, mean inclination angles, extent in space, and coherence characteristics of T′ large-scale structure are calculated. The T′ mean convection velocity is found as a function of y +. The T′ structure inclination angle is 30 deg. The T′ structure is limited to the viscous defect length in the normal and spanwise directions. However, in the streamwise direction, the structure is mildly dependent on the temperature difference.

Further report on the propagation of spark-produced N waves through turbulence

A laboratory model experiment to study the effect of turbulence on waveform and rise time of sonic booms has previously been reported [B. Lipkens and D. T. Blackstock, J. Acoust. Soc. Am. 90, 2307(A) (1991)]. Electric sparks produce the N waves, which then propagate through turbulence generated by a plane jet. Downscaling of the model turbulence from atmospheric turbulence was designed to match downscaling of the model N wave (in terms of duration and rise time) from the sonic boom. Reported here are hot wire anemometry measurements of the jet, which confirm that the turbulence characteristics are indeed those of a classical plane jet. The measured values of the integral length scale, Taylor microscale, and Kolmogorov microscale show that the desired downscaling from atmospheric turbulence was achieved. Also reported are measurements that show how rise time and wave shape distortion vary with propagation distance through the jet, turbulence intensity, and peak amplitude of the N wave. [Work supported by NASA.]

Measurements of turbulent boundary layer Prandtl numbers and space-time temperature correlations

Hot-wire anemometry measurements in an incompressible turbulent boundary-layer flow over a heated flat plate at zero pressure gradient were made using an x probe and a temperature fluctuation probe. The measurements were made for three different temperature difference cases between the wall and the freestream 10, 15, and 20 o C. Simultaneous measurements of the two components of the velocity and the temperature were made at the same point using an x probe and a temperature fluctuation probe

Starting Flow and Heat Transfer Downstream of a Backward-Facing Step

Experiments are performed to study the starting process of heat transfer downstream of a backward-facing step. A Ludwieg tube wind tunnel is employed to produce the incompressible flow, which accelerates from a zero velocity to a steady state value with an accelerating period of 7 ms and a steady-state period of 12 ms. Hot-wire anemometry and heat flux gages are used to measure the flow and heat transfer history, respectively. The onset of transition in the free shear layer shows that the disturbance originates from the top corner of the step, then propagating to the free stream. The velocity and turbulence profiles in the free shear layer reach steady-state values after the leading edge disturbance traverses to the measurement locations. In regions upstream and far downstream of the step, heat flux history data suggest the transformation of the flow from laminar to transitional and finally to turbulent flow. Hot-wire anemometry measurements indicate high-frequency turbulence with a short characteristic time. In the recirculating region, however, a longer characteristic time is observed because of the existence of large-scale eddies. The dimensionless reattachment length (xr/H) is shown to increase with time from the bottom corner (xr/H = 0) in the laminar regime to a maximum value of 13.6 in the transitional regime, and decreases to a constant values of 7.6 in the turbulent regime. The steady-state flow field and heat transfer compare favorably with existing data obtained using steady-state techniques.

Acoustic waves over a transpiring surface

An investigation of the effect on the acoustic waves due to wall mass injection in a piston-driven closed-end test chamber has been conducted. Dry ice is used as a source of mass injection. Acoustic pressure is measured with a pressure transducer mounted at the closed end. A comparison with the results with no side wall mass addition shows a significant increase in the acoustic pressure amplitude due to the interaction of the acoustic wave and side wall mass injection near resonance. A shift in the lowest resonant frequency between the two cases has also been detected. High-speed flow visualization and hot wire anemometry measurements all suggest that turbulence may occur in the acoustic boundary layer above the transpiring surface. This, in turn, feeds more energy into the acoustic wave. This increase in amplitude has never been reported before to the authors' best knowledge. [Work supported by U.S. Air Force Astronautics Laboratory and Morton Thiokol, Inc.]

Role of computational fluid mechanics in the analysis of prosthetic heart valve flow.

The paper aims to present the strengths and weaknesses of computational fluid mechanics (CFM) compared with existing in vitro haemodynamic techniques such as LDA, hot-wire anemometry and static pressure measurement, and to discuss the application of CFM to the analysis of prosthetic heart valve flow. It presents a brief summary of existing methods used in analysing in vitro heart valve flow (along with CFM), the special problems inherent in each method and the quantities each is capable of giving. The analysis then goes on to present examples of information yielded by CFM. It also contains a brief discussion of the data requirements for the analysis of heart valve performance and the clinical relevance of fluid dynamic occurrences. The conclusion drawn is that CFM has a major role to play in the analysis of prosthetic heart valve flow and design because of the improved availability of data afforded and the increased correlation between CFM and clinical flow regimes, due to the ability to consider realistic models of blood.

Induction System Effects on Small-Scale Turbulence in a High-Speed Diesel Engine

The influence of the induction system on small-scale turbulence in a high-speed, automotive diesel engine was investigated under variable swirl conditions. The induction system was made up of two equiverse swirl tangential ducts, and valves of the same size and lift. Variable swirl conditions were obtained by keeping one of the inlet valves either closed or functioning, and by changing engine speed. The investigation was carried out for two induction system configurations: with both ducts operating and with only one of them operating. Two different engine speeds were considered, one relatively low (1600 rpm) and the other quite high (3000 rpm), the latter being the highest speed at which engine turbulence has been measured up to now. Cycle-resolved hot-wire anemometry measurements of air velocity were performed throughout the induction and compression strokes, under motored conditions, along a radial direction at an axial level that was virtually in the middle of the combustion chamber at top dead center. The velocity data were analyzed using the nonstationary time-averaging procedure previously developed by the authors. Correlation and spectral analysis of the small-scale turbulence so determined was also performed. The turbulence intensity and its degree of nonhomogeneity and anisotropy were sensibly influenced by the variable swirl conditions, depending on both the intake system configuration and engine speed; they generally showed an increase with increasing swirl intensity, at the end of the compression stroke. A similar trend was observed in the cyclic fluctuation of both the mean velocity and turbulence intensity. The micro time scale of turbulence was found to be almost uniform during induction and compression, showing a slight dependence on the measurement point and on the intake system configuration, but a more sensible dependence on the engine speed. No effect of the cylinder wall on turbulence was apparent.

Endwall Boundary Layer Flows and Losses in an Axial Turbine Stage

In order to gain an insight into boundary layer phenomena in axial flow turbines, experimental studies were carried out on a large-scale, low speed, single stage machine. Low and high frequency instrumentation supplemented by flow visualizations were used to determine the details of the flows and losses in the stator and rotor blade rows. Measurements of the turbine stator flows, which were obtained at two different inlet wall boundary layer thicknesses, were generally consistent with typical linear cascade observations. A major difference concerned the presence of appreciable radially directed flow. Hot-wire anemometry measurements at the turbine rotor exit, revealed strong secondary flows at the high turning hub section. The flow pattern was seen to be influenced by the spacing between the two rows of blades and by the rotor’s instantaneous position with respect to the periodic flow field arising from the upstream stator row.

Hot-wire and hot-film anemometry and conditional measurements: A report on Euromech 132

The European Mechanics Colloquium, Euromech 132, was held at the Ecole Centrale de Lyon from 2 to 4 July 1980. Specific areas of hot-wire or hot-film anemometry were presented and discussed, more especially the effect of the finite time constant of the wire supports, the use of yawed hot wires in supersonic flows, the possible improvement of vorticity meters, and multi-point measurements of wall-shear-stress fluctuations. Other subjects described during the meeting included a new technique for concentration measurements in flames, developments and new uses of digitization and conditional sampling, pattern recognition analysis of fluid flow from multi-point, multi-time velocity measurements, and new turbulence measurements in complex flows and in fluid-flow machinery.An exhibition of hot-wire and hot-film anemometers and associated equipment was held during the colloquium.

Radial Pressure Gradient in Turbulent Pipe Flow

Measurements made with a Prandtl static pressure probe have demonstrated that a radial pressure gradient does exist in turbulent pipe flow with approximately the magnitude predicted by Sandborn from hot-wire anemometry measurements.