Showing papers in "Experiments in Fluids in 1994"

Efficient detection of spurious vectors in particle image velocimetry data

Abstract: A statistical model is introduced that describes the occurence of spurious vectors in PIV data. This model is used to investigate the performance of three different post-interrogation procedures: the global-mean, the local-mean and the local-median test. The model is also used to optimize the performance of these procedures. Predicted performances agree very well with those obtained from an artificially generated PIV record. It is demonstrated that the “detectability” as the conventional measure for the reliability of a measured displacement vector is very inefficient, compared to the three tests described here. The local-median test has the highest efficiency.

Refractive index matching methods for liquid flow investigations

Abstract: A difficulty common to most optical diagnostic techniques that are applied to fluid dynamics studies is the refraction of light passing through model and/or test section walls. The method of choice to eliminate refraction problems in liquid flows is to match refractive index. This paper presents techniques for refractive index matching including, (i) arrangement of test section and model, (ii) choice of solid and liquid materials, and (iii) methods for tuning the match. In addition, a new application of refractive index matching to liquid-liquid droplet studies is presented.

Stochastic estimation and proper orthogonal decomposition: complementary techniques for identifying structure

Abstract: The Proper Orthogonal Decomposition (POD) as introduced by Lumley and the Linear Stochastic Estimation (LSE) as introduced by Adrian are used to identify structure in the axisymmetric jet shear layer and the 2-D mixing layer. In this paper we will briefly discuss the application of each method, then focus on a novel technique which employs the strengths of each. This complementary technique consists of projecting the estimated velocity field obtained from application of LSE onto the POD eigenfunctions to obtain estimated random coefficients. These estimated random coefficients are then used in conjunction with the POD eigenfunctions to reconstruct the estimated random velocity field. A qualitative comparison between the first POD mode representation of the estimated random velocity field and that obtained utilizing the original measured field indicates that the two are remarkably similar, in both flows. In order to quantitatively assess the technique, the root mean square (RMS) velocities are computed from the estimated and original velocity fields and comparisons made. In both flows the RMS velocities captured using the first POD mode of the estimated field are very close to those obtained from the first POD mode of the unestimated original field. These results show that the complementary technique, which combines LSE and POD, allows one to obtain time dependent information from the POD while greatly reducing the amount of instantaneous data required. Hence, it may not be necessary to measure the instantaneous velocity field at all points in spacesimultaneously to obtain the phase of the structures, but only at a few select spatial positions. Moreover, this type of an approach can possibly be used to verify or check low dimensional dynamical systems models for the POD coefficients (for the first POD mode) which are currently being developed for both of these flows.

Algorithms for fully automated three-dimensional particle tracking velocimetry

Abstract: A three-dimensional Particle Tracking Velocimetry (3-D PTV) technique has been developed to provide time-resolved, three-dimensional velocity field measurements throughout a finite volume. This technique offers many advantages for fundamental research in turbulence and applied research in areas such as mixing and combustion. The data acquired in 3-D PTV is a time sequence of stereo images of flow tracer particles suspended in the fluid. In this paper, the implementation of the technique is discussed in detail, as well as the results of an extensive statistical investigation of the performance of the algorithms. The technique has been optimized to allow fully automatic processing of long sequences of image pairs in a computationally efficient manner, hereby providing a viable, practical tool for the study of complex flows.

On the mechanism of unsteady shock oscillation in shock wave/turbulent boundary layer interactions

Abstract: An experimental investigation into the mechanism of shock wave oscillation in compression ramp-generated shock wave/turbulent boundary layer interactions is presented. Particular emphasis is focused upon documenting the respective roles played by both burst-sweep events in the turbulent boundary layer immediately upstream of the interaction and the downstream separated shear layer upon unsteady shock front motion. Unlike the majority of compression ramp experiments which involve bulk separation and large-scale shock motion, consideration is given here to comparatively “weak” interactions in which the streamwise spatial excursion of the shock front is always less than one boundary layer thickness. In this manner any shock motion due to upstream burst-sweep events should be more apparent in relation to that oscillation associated with the separated region. A discrete Hilbert transform-based conditional sampling technique is used to obtain wall pressure measurements conditioned to burst-sweep events. The conditional sampling technique forms the basis by which the instantaneous shock motion is conditioned to the occurrence of upstream bursting. The relationship between the separation bubble and shock motion is also explored in detail. The results of the experiments indicate that the separation bubble represents a first-order effect on shock oscillation. Although it is demonstrated theoretically that the burst-sweep cycle can also give rise to unsteady shock motion of much lower amplitude, the experiments clearly demonstrate that there is no discernible statistical relationship between burst events and spanwise coherent shock front motion.

Turbulence: the chief outstanding difficulty of our subject

Abstract: A review of interesting current topics in turbulence research is decorated with examples of popular fallacies about the behaviour of turbulence. Topics include the status of the Law of the Wall, especially in compressible flow; analogies between the effects of Reynolds number, pressure gradient, unsteadiness and roughness change; the status of Kolmogorov's universal equilibrium theory and local isotropy of the small eddies; turbulence modelling, with reference to universality, pressure-strain modelling and the dissipation equation; and chaos. Fallacies include the mixing-length concept; the effect of pressure gradient on Reynolds shear stress; the separability of time and space derivatives; models of the dissipation equation; and chaos.

Measuring the two-dimensional structure of a wavy water surface optically: A surface gradient detector

Abstract: A new method of measuring the slopes of a water surface covered with short waves is developed. A camera is placed far above the water surface looking downward so that it receives only approximately vertical rays of light emerging from the water surface from a source below. A large lens is positioned horizontally underwater. A plane light source in the form of a translucent colored screen is placed horizontally in the focal plane below this lens. Corresponding to each value of water surface slope, regardless of observer position, there is one and only one point of origin on the color screen from which light rays can enter the camera. When the color screen has a suitable two-dimensional color pattern, we are able to detect the gradient of the surface elevation throughout the field of view of the camera. This refraction slope detector has been used to find statistical properties of short wind waves in a wind-wave channel where a broad angular beam width of capillary ripples and short gravity waves contribute to the surface slopes. In these experiments waves were generated by winds ranging from 5 m/s to 10 m/s at a fetch of 24 m. The wavenumber spectra of short wave slopes have two distinguishing features: a dip at the capillary-gravity transition and steep slopes in the capillary range. Surface shapes resembling the shape of solitary capillary-gravity waves have been found from profiles of wave elevation deduced by integration of the elevation gradient.

Leading edge shape for flat plate boundary layer studies

Abstract: In experimental boundary layer studies, a flat plate with some shaped nose piece is generally used; this is often prone to flow separation at the junction. By analysing the development of a laminar boundary layer on a two-parameter family of nose shapes, it is found that a cubic super-ellipse of axis ratio 6 or higher is a reasonable optimum shape for avoiding separation on or due to such nose-pieces.

Characterization of the PVC phenomena in the exhaust of a cyclone dust separator

Abstract: A laboratory scale cyclone dust separator with swirl numbers varying from 3.043 to 1.790 was used to examine the effects of different downstream pipework configurations, flowrates and swirl numbers upon the size, shape, and position of the precessing vortex core. Also examined was the effect the precessing vortex core had on the reverse flow zone, and the relationship between the two.

Control of three-dimensional phase dynamics in a cylinder wake

Abstract: Recently there has been a surge of new interest in three-dimensional wake patterns In the present work, we have devised a method to control the spanwise end conditions and wake patterns using “end suction”, which is both continuously-variable and admits transient control Classical steady-state patterns, such as parallel or oblique shedding or the “chevron” patterns are simply induced The wake, at a given Reynolds number, is receptive to a continuous range of oblique shedding angles (θ), rather than to discrete angles, and there is excellent agreement with the “cos θ” formula for oblique-shedding frequencies We show that the laminar shedding regime exists up to Reynolds numbers (Re) of 205, and that the immense disparity among reported critical Re for wake transition (Re = 140–190) can be explained in terms of spanwise end contamination Our transient experiments have resulted in the discovery of new phenomena such as “phase shocks” and “phase expansions”, which can be explained in terms of a simple model assuming constant normal wavelength of the wake pattern Peter Monkewitz (Lausanne) also predicts such transient phenomena from a Guinzburg-Landau model for the wake

Particle image velocimetry in a variable density flow: application to a dynamically evolving microburst

Abstract: A fondamental difficulty in the experimental study of gravity-driven flows using particle image velocimetry (PIV) and other optical diagnostic techniques is the problem associated with variations in the refractive index within the fluid. This paper discusses a method by which the refractive indices of two fluids are matched while maintaining density differences of up to 4%. Aqueous solutions of glycerol and potassium phosphate are used to achieve precise index matching in the presence of mixed and unmixed constituents. The effectiveness of the method is verified in a PIV study of a laboratory-scale model of an atmospheric microburst where planes of two-dimensional velocity vectors are obtained in the evolving flow field.

A combined OH/acetone planar laser-induced fluorescence imaging technique for visualizing combusting flows

Abstract: A combined OH/acetone planar laser-induced fluorescence (PLIF) imaging technique that provides simultaneous visualizations of regions of unburned fuel and of combustion in a reacting flow is described. OH marks the location of chemical reaction and of combustion products, and acetone vapor, which is seeded into the fuel stream, marks unburned fuel. A single pulse from an ultraviolet laser is used to simultaneously excite both the OH and acetone, and the fluorescence from each is detected on separate cameras. Acetone spectroscopy and chemistry are reviewed to provide a basis for interpreting acetone fluorescence signals in high-temperature combusting environments. The imaging technique is applied to two nonpremixed turbulent reacting flows to assess the utility of the technique for visualizing the instantaneous flow structure and to illustrate the dependence of the interpretation of the acetone PLIF images on the flow conditions.

A molecular filter based velocimetry technique for high speed flows

Abstract: A molecular absorption filter-based diagnostic technique has been developed to obtain instantaneous, planar velocity measurements in high speed flows. This paper presents the details of the technique and its application in high Reynolds number compressible mixing layers with convective Mach numbers of 0.51 and 0.86. Pressure broadening was used to “tune” the absorption profile of the filter to a given flow field/optical arrangement. It is shown that two filters can be used when background scattering is significant: one as the velocity-discriminating filter, the other to eliminate unwanted background light from the reference camera. Collected images show instantaneous variations in velocity within both the highly organized roller type structures of the M c = 0.51 mixing layer and the unorganized structures found in both mixing layers. An uncertainty analysis showed that the measurement uncertainty was ±8.6% for the current measurements, but improvements to reduce the uncertainty to ±3% are suggested.

Azimuthal velocity in supercritical circular Couette flow

Abstract: Although the stability of supercritical circular Couette flow has been studied extensively, results for the velocity field of the flow are limited. The azimuthal velocity profiles for the Taylor vortex, wavy vortex, and turbulent Taylor vortex flow in the annulus between a rotating inner cylinder and a fixed outer cylinder with fixed end conditions were measured using laser Doppler velocimetry. The azimuthal velocity was measured at about 300 points per vortex pair, distributed in both the radial and axial directions. This measurement procedure was repeated for several Reynolds numbers within each flow regime to study both the spatial dependence and the Reynolds number dependence of the azimuthal velocity. The experimental results for the Taylor vortex flow regime compare well with the Davey perturbation expansion of the Navier-Stokes equations about the circular Couette flow solution [J. Fluid Mech. 14, 336 (1962)]. The measured azimuthal velocity fields also indicate two predominant effects with increasing Reynolds number: the magnitude of the radial gradient of azimuthal velocity near both cylinders increases and the radial outflow region between pairs of vortices becomes increasingly jet-like.

Coaxial atomization of a round liquid jet in a high speed gas stream: A phenomenological study

Abstract: Coaxial injectors have proven to be advantageous for the injection, atomization and mixing of propellants in cryogenic H2/O2 rocket engines. Thereby, a round liquid oxygen jet is atomized by a fast, coaxial gaseous hydrogen jet. This article summarizes phenomenological studies of coaxial spray generation under a broad variation of influencing parameters including injector design, inflow, and fluid conditions. The experimental investigations, performed using spark light photography and high speed cinematography in a shadow graph setup as main diagnostic means, illuminate the most important processes leading to atomization. These are identified as turbulence in the liquid jet, surface instability, surface wave growth and droplet detachment. Numerical simulations including free surface flow phenomena are a further diagnostic tool to elucidate some atomization particulars. The results of the study are of general importance in the field of liquid atomization.

Mode spectra of natural disturbances in a circular jet and the effect of acoustic forcing

Abstract: A modal spectrum technique was used to study coherent instability modes (both axisymmetric and azimuthal) triggered by naturally occurring disturbances in a circular jet. This technique was applied to a high Reynolds number (400,000) jet for both untripped (transitional) and tripped (turbulent) nozzle exit boundary layers, with both cases having a core turbulence level of 0.15%. The region up to the end of the potential core was dominated by the axisymmetric mode, with the azimuthal modes dominating further downstream. The growth of the azimuthal modes was observed closer to the nozzle exit for the jet with a transitional boundary layer. Whether for locally parallel flow or slowly diverging flow, even at low levels of acoustic forcing, the inviscid linear theory is seen to be inadequate for predicting the amplitude of the forced mode. In contrast, the energy integral approach reasonably predicts the evolution of the forced mode.

Full three-dimensional measurements of the cross-flow separation region of a 6:1 prolate spheroid

Abstract: The flow in the cross-flow separation region of a 1.37 m long, 6:1 prolate spheroid at 10° angle of attack was investigated with a novel 3-D fiber-optic Laser Doppler Velocimeter (LDV). The probe was used to measure three simultaneous, orthogonal velocity components from within the model. The design and operation of this LDV probe is described and velocity, Reynolds stress, and velocity triple-product measurements are presented from the inner boundary layer through the boundary-layer edge.

An analogue study for flame flickering

Abstract: An analogue experiment is proposed to simulate flame flickering comprising a free ascending column fed on its side with a light gas (helium) emerging from a vertical slot in ambient air. The convective motion of the helium jet is considered to represent the motion of burnt gases of buoyant jet flames. The helium jet is accelerated by buoyancy effects and the flow field is similar to that of burnt gases observed for real buoyant flames. The vertical velocity profile of the steady helium jet is measured at different vertical distances. The unsteady helium jet is also studied by measuring the instability frequency as a function of ambient pressure at different injection flow rates, and by analyzing the tomography images of the helium jet. The instability morphology is the same as that observed on real buoyant flames. We conclude that this type of instability can be approximately characterized by the maximum vertical velocityu max, and the distance δ betweenu max in the helium ascending column andu = o in the ambient air. For this type of instability the local vorticity is proportional to $$\frac{{u_{\max } }}{\delta }$$ which can be influenced by gravity and ambient pressure. Theoretical prediction of the instability frequency as a function of gravity and ambient pressure has been obtained, and is in good agreement with the experimental results.

CCD Recording method for cross-correlation PIV development in unstationary high speed flow

Abstract: Up to now, the use of CCD cameras, for cross-correlation development in Particle Image Velocimetry (PIV), is reduced to relatively slow flows. An original storage method of two images on the two half frames of a video camera permits now to decrease the interval between exposures (10 μs). Therefore, it is possible to study high speed flows. Applications are shown of a jet and a turbulent flame propagation.

Constant-temperature anemometry in hypersonic flow: critical issues and sample results

Abstract: The critical issues are examined in the application of constant-temperature hot-wire anemometry to hypersonic boundary layers While continuous turbulence measurements are more challenging to make in hypersonic flows, it is shown here that the difficulties can be overcome for a wide range of flow conditions An extensive review of the literature reveals that many of the heat transfer complexities associated with hypersonic anemometry have already been resolved Frequency-response tests, calibration results and boundary-layer measurements in hypersonic flow are also presented A hot-wire frequency response of about 500 kHz was obtained in a Mach 11 flow, and the resulting boundary-layer spectra are smooth and repeatable

Concentration flux measurements of a scalar quantity in turbulent flows

Abstract: A method for determination of velocity-concentration fluxes is presented that combines two conventional imaging techniques, particle image velocimetry (P.I.V.) and planar laser-induced fluorescence (P.L.I.F.). The passive concentration jet was a perfect mixture of fluorescein dye and solid particles submerged in an isotropic homogeneous turbulent channel. The light intensity fluoresced by the dye and the light intensity scattered by the particles were recorded separately on two synchronized cameras by using appropriate high and low-pass filters. Two different sets of images were thus obtained simultaneously. Once digitized and numerically processed, they provide the space and time evolution of velocity and concentration instantaneous fields. Thus, the velocity-concentration correlations can easily be determined. The statistical results for velocity and concentration are compared with classical results in order to validate the technique. We finally report some results giving velocity-concentration fluxes.

Generating high freestream turbulence levels

Abstract: This paper presents a method of generating a highly turbulent freestream flow, up to levels of 20% with a relatively uniform mean velocity field. This method was developed as a result of a combined water channel and wind tunnel study. The method for generating these high turbulence levels includes using high-velocity jets issuing into a mainstream cross-flow. A range of turbulence levels can be generated, using this same flow geometry, by adjusting the jet-to-mainstream velocity ratio or the Reynolds number of the flow.

LDA measurements in a turbulent boundary layer over a d-type rough wall

Abstract: Two-component LDA measurements in smooth wall and d-type rough wall boundary layers indicate that there are important differences in the turbulence intensities over a significant distance above the two surfaces. All the Reynolds stresses—especially the shear stress—are larger over the d-type rough wall, reflecting the relatively strong momentum exchange which occurs over the roughness cavities. Flow visualisations indicate that this exchange is associated with strong ejections of cavity fluid into the outer flow.

Instantaneous concentration profiles of oxygen accompanying absorption in a stratified flow

Abstract: Absorption of oxygen from air to water is studied for a stratified flow. Instantaneous concentration profiles in the liquid, close to the interface, are measured by utilizing oxygen quenching of the fluorescence from pyrenebutyric acid. When waves are present very large temporal variations of the concentration profile and of the mass transfer rate are observed. These results seem to rule out the notion that turbulence in the liquid is controlling mass transfer.

Double row vortical structures in the near field region of a plane wall jet

Abstract: The qualitative and quantitative behaviour of double row vortical structures in the near field region of a plane wall jet are studied experimentally by flow visualization and hot-wire measurements Ensemble averaging is employed to investigate the interaction of vortices with the wall In the flow visualization study, a double row vortical structure, which includes a primary vortex formed in the outer layer region and a secondary vortex induced in the inner layer region, and the vortex lift-off phenomenon are clearly observed during the development of the wall jet The phase averaged results of the velocity measurements show that the instability leading to induction of the secondary vortex is stimulated by the primary vortex In the early stage of wall jet transition, the inflection point of the inner layer velocity profile moves transversely from the wall surface to the inner layer region due to passage of the well-organized primary vortex in the outer layer region The inner layer instability is thus induced and the instability wave rolls up to form the secondary vortex Furthermore, the secondary vortex will convect downstream faster than the primary vortex, and this difference in convective speed will lead to the subsequent phenomenon of vortex lift-off from the wall surface

Neutrally buoyant bubbles used as flow tracers in air

Abstract: Research has been performed to determine the accuracy of neutrally buoyant and near-neutrally-buoyant bubbles used as flow tracers in an incompressible potential flowfield. Experimental and computational results are presented to evaluate the quantitative accuracy of neutrally buoyant bubbles using a commercially available helium bubble generation system. A two-dimensional experiment was conducted to determine actual bubble trajectories in the stagnation region of a NACA 0012 airfoil at 0° angle of attack. A computational scheme evaluating the equation of motion for a single bubble was also used to determine the factors which affect a bubble's trajectory. The theoretical and computational analysis have shown that neutrally buoyant bubbles will trace complex flow patterns faithfully in the flowfield of interest. Experimental analysis revealed that the use of bubbles generated by the commercially available system to trace flow patterns should be limited to qualitative measurements unless care is taken to ensure neutral buoyancy.

Buoyancy and thermocapillary flow analysis by the combined use of liquid crystals and PIV

Abstract: Thermocapillary and buoyancy convection is studied experimentally using particle-image-velocimetry with liquid crystal tracers for flow visualization and analysis. This method offers the advantage of measuring the entire flow field (velocity field, temperature distribution etc.) in a selected plane within the fluid simultaneously at a given instant of time in contrast to point by point methods like laser-Doppler-velocimetry (LDV). The paper describes the method and presents quantitative results of both, a thermocapillary and a buoyancy flow experiment. Data of the latter are compared with LDV-results and theoretical predictions, respectively.

Mixing in plasma and low density jets

Abstract: This study was undertaken to examine the mechanisms which produce the large entrainment measured near the exit of thermal plasma torches. A research facility was constructed to examine low density jet behavior under similar dimensionless conditions as those produced by thermal plasma spray torches; the Reynolds number based on jet diameter and average properties was 1000, and the ratio of jet to ambient density was 0.07. This very low density jet produced organized vortex structures which were partially responsible for the rapid entrainment of external air. The formation of these organized structures could be disrupted by introducing turbulence, but the rapid entrainment process was not significantly affected. The structure of the jet produced by a commercial plasma torch was examined and compared to the low density research jet. At low gas flow rates the plasma jet also displayed the formation of coherent vortex structures, the passage frequency of which compared favorably with that measured in the low density research jet. At higher gas flow rates the shear layer of the plasma jet rapidly broke down producing relatively small scale turbulence. Visualizations of the hot plasma core were compared against measurements of the torch voltage fluctuations caused by arc instabilities. At low flow rates the arc voltage fluctuations were quite low and the plume was very steady. At higher flow rates the arc voltage fluctuations increased and produced “surging” and “whipping” in the hot potential core. It is believed that this low frequency unsteadiness is partially responsible for the rapid entrainment measured in plasma torches.

Circular Couette flow with pressure-driven axial flow and a porous inner cylinder

Abstract: In a rotating filter separator a suspension is introduced at one end of the annulus between a rotating porous inner cylinder and a fixed impermeable outer cylinder. The filtrate is removed through the inner cylinder and the concentrate is removed from the opposite end of the annulus from which the suspension entered. The flow in a rotating filter separator is circular Couette flow with a pressure-driven axial flow and a suction boundary condition at the inner cylinder. Flow visualization was used to determine the effect of the Taylor number, axial Reynolds number, and radial Reynolds number on the types of flows present in the annulus. A rich variety of secondary vortical flows appear, depending upon the flow parameters. The radial inflow at the inner cylinder delays the appearance of supercritical circular Couette flow and prevents the appearance of certain flow regimes that have a helical vortex structure. Nevertheless, the average azimuthal velocity measured using laser Doppler velocimetry indicates that the velocity profile is nearly the same for all supercritical flow regimes.

Velocity measurements of wake-induced unsteady flow in a linear turbine cascade

Abstract: Extensive velocity measurements have been taken in a linear turbine cascade with unsteady oncoming wakes. The unsteady wakes were generated by moving cylinders on a squirrel cage device. The Reynolds number was 1.1 × 105, and the Strouhal number varied from o to 7.36. The blade-to-blade flow and the boundary layers on the suction side were measured with a hot-wire anemometer. The results were obtained in ensemble-averaged form so that periodic unsteady processes can be studied. Of particular interest was the transition of the boundary layer. The boundary layer remained laminar in the case without wakes. The passing wakes caused transition, and the beginning of transition moves forward as the wake-passing frequency increases. Unlike in the flat plate study of Liu and Rodi (1991a) the boundary layer state hardly changed with time, although the turbulence level in the boundary layer showed clear periodic response to the passing wakes.