Film Probe Research Articles

A comparison of shear stress fluctuation statistics between microbubble modified and polymer modified turbulent boundary layers

Shear stress fluctuation statistics, as measured by flush mounted hot film probes, are presented for both microbubble and polymer injection into the same zero pressure gradient turbulent boundary layer. At equivalent values of drag reduction, the moments of the probability distribution are remarkably similar. For drag reductions larger than about 40%, the deviation of the skewness and kurtosis values from their pure water values argue against simple scaling explanations of the drag reduction phenomenon.

SPECTRAL PROPERTIES OF TURBULENT ENERGY IN THE EQUILIBRIUM WAVE NUMBER RANGE

Turbulence measurement is carefully made with some special devices in a gridturbulent flow with small turbulent Reynolds number in order to investigate the detail of viscous wave number subrange. The velocity fluctuations measured by a small hot film probe are analysed to determine the spectral power laws in the equilibrium wave number range. It is established that there exists -7 power spectrum at higher wave numbers in the viscous subrange which is different from the earlier established -13/3 power spectrum for lower wave numbers. Using the energy dissipation evaluated by the transport equation of turbulent energy, it is possible to determine all spectral constants of power laws for the equilibrium range and evaluation of the spectral properties. The spectra predicted with these spectral constants are consistent with many spectra measured by other investigators.

Mean and turbulence characteristics in the near wake of an airfoil.

A experimental investigation of the near wake of an airfoil at the angles of attack of 0° and 8° is reported in this paper. The mean velocity, turbulent energy and Reynolds stress are measured using a split film probe. A four-quadrant analysis is applied to clarify the behavior of the Reynolds stress at the near wake region. The main results obtained in this paper are summarized as follows. The downstream development of the half-width of the wake, the displacement thickness and the momentum thickness with the attack angle is remarkable on the pressure side. Throughout the experimental condition, contribution from the ejection event to the Reynolds stress dominates except at the wake centre regions.

Estimation of velocity eigenfunction and vorticity distributions from the timeline visualization technique

For the case of quasi-periodic flow, it is demonstrated that use of the hydrogen bubble timeline method leads to reasonable estimates of the eigenfunction of the streamwise velocity fluctuation. Both amplitude and phase distributions across an unstable wake flow are well-approximated. It is shown that the vorticity extrema, as well as the degree of concentration of vorticity, are in good agreement with those calculated from linear stability theory. A critical assessment is given of the possible uncertainties associated with this technique: the existence of a finite, but unknown cross-stream velocity component; bubble rise due to buoyancy effects; wake defect created downstream of the bubble wire; and resolution of the digitized image. Furthermore, the uncertainty in the streamwise velocity, arising from existence of a finite cross-stream velocity component, is actually less than that corresponding to a single-element hot film probe over certain regimes of operation.

APPLICATION OF HOT-FILM ANEMOMETRY TO AIR-WATER FLOW IN AERATED STIRRED TANKS

The subject of this paper is the evaluation of the applicability of constant temperature anemometry (CTA) with a modified version of Delhaye's method to aerated stirred tanks. A calibration technique that takes into account the variation in medium temperature was developed and verified experimentally. The directional sensitivity of the conical film probe was investigated in a streamline flow field as well as in the impeller discharge stream in an stirred tank. The reliability of the setup and the technique was verified through the comparison of the vertical and radial velocity profiles of water in a stirred tank with those obtained from the literature. The directional response of the conical probe follows the cosine law for intersection angles smaller than 45° in a streamline flow field, but the directional sensitivity in the impeller discharge stream is rather poor, owing to the rolling characteristics of flow in this region. Due to the inability to detect bubbles smaller than the sensing element of the conical probe, the CTA usually gives lower values of local gas holdup. However, this discrepancy is considered to have no influence on the velocity measurement.

Transition in Pressure-Surface Boundary Layers

Laminar-to-turbulent transition in the presence of Go¨rtler vortices has been investigated experimentally, in the outer wall boundary layer of a curved water channel. Ratios of boundary layer thickness at the start of curvature to wall radius were around 0.05 and core flow turbulence intensities were between 1 and 3 percent. Measurements of intermittency factor were made by hot film probe and of mean and rms velocity by laser anemometer. At Reynolds numbers low enough to allow considerable nonlinear vortex amplification in the laminar region, transition was found to begin sooner and progress faster at a vortex upwash position than at a spanwise-adjacent downwash position. Measured Go¨rtler numbers at transition onset bore little relationship to those often used as transition criteria in two-dimensional boundary layer prediction codes. Little spanwise variation in intermittency occurred at higher Reynolds numbers, where mean velocity profiles at upwash were much less inflected. Toward the end of curvature, favorable pressure gradients estimated to exceed the Launder relaminarization value corresponded with cases of incomplete transition.

A technique for investigating road vehicle wakes using a pair of split film probes

A technique for investigating road vehicle wakes using a perpendicular pair of split films is described. Heat transfer laws, temperature effects, binormal cooling and deviation from ideal probe behaviour are all considered. Calibration procedures and experimental technique are described. A typical set of time-mean and fluctuating velocity measurements are presented and discussed.

Measurements of Wall Shear Stress in Axial Flow in a Square Lattice Rectangular Rod Bundle

Hot film probe measurements of the distribution of the wall shear stress were made for axial flow along a rectangular 3 × 6 array of rods with a pitch to diameter ratio, P/D = 4/3, and a wall to diameter ratio, W/D = 2/3. Measurements were performed on rods at several locations and on two duct side walls at a position 62 hydraulic diameters from the entrance. Local shear stress maxima occur near the largest subchannel flow areas with the lowest maximum local shear stress on rods nearest the sidewalls. Maximum to the minimum shear stress ratio on an individual rod is largest for the corner rod. Side wall maximum local shear stress occurs in the first wall subchannel. Overall friction factors calculated from the wall shear stress measurements agree with those calculated from pressure drop data.

Potential core of an axisymmetric laminar jet. (1st report)

This study is intended to clarify the flow pattern in the flow developing region of an axisymmetric laminar water jet issuing into the surrounding calm water. The jet, having initially a potential core region to some extent at the nozzle exit, was studied. The velocity profile in the developing region near the nozzle was measured by using a miniature hot film probe of a cone type. Flow visualization by the hydrogen bubble method was also performed. Experiments were carried out for the jet Reynolds number from 100 to 600. The flow pattern in the developing region was made clear, and the extent of the developing region also discussed. The experimental results were compared with the existing numerical solution.

Experimental studies of turbulence in mercury flows with transverse magnetic fields

Turbulence intensity and energy spectra were investigated experimentally in mercury flows in a 2 × 4.8 cm2 rectangular cross section duct subjected to a transverse magnetic field. Velocity fluctuations were measured by means of a quartz coated hot film probe at Re = 78.5 × 103 and 0 ≦ Ha ≦ 785. The results obtained made possible a number of conclusions related to turbulence suppression and structure changes and to energy transfer mechanisms.

Investigation of the flowfield immediately upstream of a hot film probe

The laminar flowfield in a rectangular channel immediately upstream of a hot film gradient probe with two parallel films was investigated in the range of Reynolds number Repr= 6 to 95, with the Reynolds number based on the probe diameter and the local flow velocity. For this study a photochromic dye flow visualization technique was used. The results show that the smaller the Reynolds number Reprthe larger the influence of the probe is upon the flowfield. No distinct influence of the probe location relative to the channel walls on the flow deceleration process immediately upstream of the probe was observed.

Turbulence Measurements in the Ship-Stern Boundary Layer of a Tanker Model

Turbulence measurements in the boundary layer of a double model are made by a hot film probe. Turbulence intensity, turbulent kinetic energy, Reynolds shear stress, turbulent energy production and kinematic eddy viscosity obtained from three turbulence components are investigated. Measuring positions are selected as followings, which are along a potential flow streamline, across a separation line, on the separation line, just past a bilge and beneath the waterline. It is shown that the turbulence quantities at each position have peculiar distributions throughout the layer since the quantities are influenced by the hull surface curvature, pressure gradient, separation and longitudinal vortex.

Etalonnage des sondes parietales a film chaud en regime permanent

The utilization of hot film probes for the measurement of wall shear stress requires the realization of a calibration channel which permits to study the probes responses, to draw the calibration curves in laminar and turbulent flows and to propose the laws in the following form: Nu = Kτ p ∗ n . The study of the influence of thermal conductivity and relative thickness of the support shows that we obtain a law all the more near of the limit law as the thermal resistance of the support is large. In turbulent flow, we note that in all cases the 1 3 power of reduced wall shear stress is conserved.

Laminar-To-Turbulent Transition on a Body of Revolution With an Extended Favorable Pressure Gradient Forebody

An experimental study of the laminar-to-turbulent transition and resulting hydrodynamic forces on a body of revolution with a long, favorable pressure gradient forebody (i.e., where pressure is dropping and the flow accelerating) is reported. Over a substantial range of body velocity and angle of attack the favorable pressure gradient is shown to postpone transition to the point of laminar separation, and this extended laminar region results in a much lower hydrodynamic drag than is characteristic of an all-turbulent body. The intermittency of the boundary layer and the propagation characteristics of turbulent spots in the extended favorable pressure gradient region are quantified by hot film probes mounted flush with the body surface. The sensitivity of the boundary layer transition to three-dimensional surface roughness elements located in tandem (along a streamline) is also quantified. A number of such elements in tandem causes transition at a lower Reynolds number than would a single element of the same size, this effect becoming more pronounced with increasing number of roughness elements and decreasing space between them.

Turbulence measurements around a body in a re-circulating water channel by a hot film probe

Measurements were made in the stern boundary layers and near wakes of an elliptic cylinder and a slender ship model. Turbulence intensities, Reynolds stresses, kinematic eddy viscosities and mixing lengths are presented. For the elliptic cylinder, furthermore, auto-correlation and power spectrum are obtained.

Towed-model apparatus for investigating underwater cavity tones

The problem of steady tones generated by flow over a rectangular cavity in a flat plate with a sharp leading edge has been studied previously in air, utilizing a low-noise wind tunnel [S. A. Elder, T. Farabeem, and F. C. DeMetz, J. Acoust. Soc. Am. 72, 532–549 (1982)]. Three tone mechanisms already identified are cavity resonance feedback, edge-force feedback, and cavity turbulence response, the resonance effect being predominant in air. To isolate edge-force effects, new studies are being done underwater where cavity resonance is less pronounced. The towed model system was chosen over the continuous water tunnel, on account of large background noise associated with the latter. Fiberglass model is fabricated into thin fairwater structure designed for good towing stability. Though possessing slight convex curvature, the vertical fairwater approximates a flat plate with 9° angle of attack. Cavity instrumentation includes fixed hydrophone for sound detection and motor-driven hot film probe for velocity profile surveys. Preliminary results are compared with similarity predictions from air data. [Work supported by Naval Sea Systems Command General Hydromechanics Research Program, administered by the David W. Taylor Naval Ship R&D Center, Bethesda, MD.]

Experimental investigations on fast gold-tin metal film second-sound detectors and their application

In order to optimize gold-tin metal film probes as bolometers for measurement of fast second-sound signals in He II, the influence of their composition and of annealing on their electrical properties at low temperatures was investigated systematically. Measurements of the dependence of the voltage-current characteristics on temperature were supplemented by electron microscopy of the film surfaces and by measurement of the resistivity of the composed films at room temperature as a function of the atomic percentage of tin. Fast interdiffusion, which took place after evaporating typically 1000 A of tin onto 200 A of gold, resulted in the development of different phases of the Au-Sn system in the layer. Due to proximity effects between grains of these phases the transition temperature of such a film is reduced to the temperature range of He II. Different types of weak-link behavior which may govern the current-induced breakdown of superconductivity are discussed. As an example of the application of the metal film probes, the circulation of a macroscopic vortex ring was measured using the flow-induced variation of the traveling times of second-sound shock waves. Values of up to several cm2/sec were thus obtained to an accuracy of some 10−2 cm2/sec.

Dynamic In‐Situ Calibration of Hot Film

A device for the calibration of a hot film probe subjected to a simple harmonic motion is described. It works on the basis of constant amplitude and variable periods. Results of calibration with such periodically varying velocities indicate its suitability for dynamic calibration especially for measurements in wave fields.

Turbulence Measurements in the Wake of a Tanker Model on and under the Free Surface

Turbulence measurements in a circulating water channel are made by an X-type hot film probe. From three components of velocity fluctuations turbulence intensities, Reynolds shear stresses and turbulent kinetic energies are obtained. By stern longitudinal vortices formed in the wake the values of turbulence quantities are influenced appreciably. In the surface condition of a model the turbulence intensities of vertical component w and shear stresses -uw are intensely suppressed due to the existence of free water surface. The value of every turbulence quantity takes the maximum value near the vortex center.

Ordered motion of turbulence in a thermally stratified flow under unstable conditions

Measurements of the streamwise and vertical velocity fluctuations and temperature fluctuation in a thermally stratified flow in an open channel under unstable conditions were made with two laser Doppler velocimeters and a film probe operated as a resistance thermometer. The result indicates that the contribution to the total turbulent momentum flux from the sweep motion of large amplitude becomes greatest in the free surface region, while that from the ejection motion remains largest in the wall region. The ejection motion makes the largest contribution to the total turbulent heat transport over the whole measurement range from the wall region to the free surface region for both weakly and strongly unstable conditions. The outward interaction motion makes increasing contribution to the total turbulent momentum flux as well as to the total turbulent heat flux and is closely connected with the ejection motion under the strongly unstable conditions. The mean durations and the mean periods of occurrences of the respective motions decrease with increasing instability.