Showing papers in "Experiments in Fluids in 1992"

Effect of resolution on the speed and accuracy of particle image velocimetry interrogation

Abstract: Particle image velocimetry incorporates a process by which an image of a flow field, bearing double images of seeding particles, is analyzed in small regions called “interrogation spots.” Each spot is imaged onto a photodetector array whose digitized output is evaluated computationally using the auto-correlation technique. This paper examines the effects of resolving the spot using arrays of various resolutions, motivated primarily by a gain in speed. For this purpose, two specially created test photographs representing (i) uniform flow and (ii) solid body rotation, were interrogated using array sizes ranging from 32 × 32 to 256 × 256. Each reduction in resolution by a factor of two gains a factor of four in interrogation speed, but this benefit is counteracted by a loss in accuracy. The particle image diameter strongly influences accuracy through two distinct error mechanisms. When the particle image is small compared to the pixel size, mean bias error becomes significant due to finite numerical resolution of the correlation function. Conversely, when the particle image is large, random error due to irregularities in the electronic images predominates. The optimum image size, therefore, lies not at either extreme but at an intermediate value such that the particle image is small in an absolute sense, and yet large relative to the pixel size.

Acetone: a tracer for concentration measurements in gaseous flows by planar laser-induced fluorescence

Abstract: This paper explores the use of acetone as a suitable tracer in planar laser-induced fluorescence concentration measurements in gaseous flows. The photophysics and physical properties of acetone relevant to its use as a fluorescent marker are discussed and compared to those of alternative molecular tracers, particularly the biacetyl molecule. Finally, as a direct example, concentration images obtained in a turbulent air jet seeded alternatively with acetone and biacetyl are compared.

Three-dimensional particle imaging with a single camera

Abstract: A new approach to the instantaneous three-dimensional mapping of flow fields is introduced. A single camera system uses defocusing in conjunction with a mask (three pin holes) embedded in the camera lens to decode three-dimensional point sources of light (i.e., illuminated particles) on a single image. The sizes and locations of the particle image patterns on the image plane relate directly to the three-dimensional positions of the individual particles. Using sequential images, particles may be tracked in space and time, yielding whole-field velocity information. Calibration of the system is straightforward, whereas the self-similarity of the particle image patterns can be used in automating the data-extraction process. The described technique was used to obtain particle trajectories in the flow field of a vortex ring impinging on a wall.

Control of low-speed turbulent separated flow using jet vortex generators

Abstract: A parametric study has been performed with jet vortex generators to determine their effectiveness in controlling flow separation associated with low-speed turbulent flow over a two-dimensional rearward-facing ramp. Results indicate that flow-separation control can be accomplished, with the level of control achieved being a function of jet speed, jet orientation (with respect to the free-stream direction), and jet location (distance from the separation region in the free-stream direction). Compared to slot blowing, jet vortex generators can provide an equivalent level of flow control over a larger spanwise region (for constant jet flow area and speed).

Study of vortex breakdown by particle tracking velocimetry (PTV)

Abstract: Using a quasi three-dimensional instantaneous measurement technique, which combines particle tracking velocimetry (PTV) with volume scanning, first quantitative experimental results of the unsteady and asymmetric interior region of vortex breakdown were obtained. The study was carried out in a low speed flow through a cylindrical tube. A vortex was generated by a set of guidevanes and subjected to an adverse pressure gradient causing its breakdown. By scanning a pulsed illuminated planar laser light sheet, a set of meridional and azimuthal “cuts” of the flow was obtained. With PTV the recorded particle paths in the “cuts” were processed in order to obtain the instantaneous two-dimensional velocity field, mean streamlines and vorticity distribution. Moreover, the three-dimensional shape of the appearing breakdown, visualized with fluorescent dye, was reconstructed from the “cuts”. The results revealed that the shape of the bubble nearly equals the streamsurface of the stagnation point. According to the conditions in the water tunnel a single tilted vortex ring at the open rear part of the bubble dominates the interior flow structure of the bubble as first noted by Sarpkaya (1971). The vortical flow is bulged over the bubble, restored and intensified at the lower end. The gathered data lead to the conclusion that the vortex axis remains parallel to the centerline.

Convection velocity measurements in a cylinder wake

Abstract: The convection velocity of vortices in the wake of a circular cylinder has been obtained by two different approaches. The first, implemented in a wind tunnel using an array of X-wires, consists in determining the velocity at the location of maximum spanwise vorticity. Four variants of the second method, which estimates the transit time of vortices tagged by heat or dye, were used in wind and water tunnels over a relatively large Reynolds number range. Results from the two methods are in good agreement with each other. Along the most probable vortex trajectory, there is only a small streamwise increase in the convection velocity for laminar conditions and a more substantial variation when the wake is turbulent. The convection velocity is generally greater than the local mean velocity and does not depend significantly on the Reynolds number.

Wave flume experiments on two-dimensional oscillating water column wave energy devices

Abstract: Experiments were performed in a wave flume with regular waves of very small amplitude-to-wavelength ratio (less than 0.01). Their purpose was mainly the validation of the oscillating surface pressure theory of Sarmento and Falcao (1985) applied to wave energy absorption by oscillating water column (OWC) devices. Experimental and theoretical curves for the efficiency and for the reflection and transmission coefficients were obtained and compared. The test also included the validation of the two-wave-gauge experimental procedure used to decouple direct and reflected wave trains, as well as the effects of increasing the depth of immersion of the OWC overhang on the efficiency-wave period curves.

Three-dimensional boundary-layer transition on a cone at Mach 3.5

Abstract: A boundary-layer transition study on a sharp, 5° half-angle cone at various angles of attack was conducted at Mach 3.5. Transition data were obtained with and without significantly reduced freestream acoustic disturbance levels. A progressive downstream and upstream motion of the transition front on the windward and leeward rays, respectively, of the cone with angle of attack was observed for the high noise level data in agreement with data trends obtained in conventional (“noisy”) wind tunnels. However, the downstream movement was not observed to the same degree for the low noise level data in the present study. Transition believed to be crossflow dominated was found to be less receptive to freestream acoustic disturbances than first-mode (Tollmien-Schlichting) dominated transition. The previously-developed crossflow transition Reynolds number criterion, χtr,max≈200, was found to be inadequate for the current case. An improved criterion is offered, which includes compressibility and flow-geometry effects.

A reliable method for determining the measurement volume size and particle mass fluxes using phase-Doppler anemometry

Abstract: A reliable method for the calibration of the measurement volume cross-section has been developed, in order to correct the particle size distribution measured by a phase-Doppler anemometer (PDA) with respect to the counting bias in favour of the large particles. Furthermore, this method allows the measurement of particle concentration or mass flux with high accuracy in two-dimensional two-phase flows. A comparison of the mass flow rate obtained by the integration of the mass flux measured in a water spray by applying this method with the global mass balance showed a difference of about 5%. The basis of the present method is the detection of the amplitude of the filtered Doppler signal in connection with the particle size measured by the PDA. The detection process is performed using an electronic circuit which validates the Doppler burst and has additionally the advantage that the highest amplitude portion of the burst can be selected for digitizing, acquisition and subsequent processing. Therefore, this method has the great advantage that the processed part of the burst has the highest signal-to-noise ratio which results in high accuracies for frequency and phase estimation.

Visualization study of the flow in and around a Savonius rotor

Abstract: Flow in and around a Savonius rotor has been studied by flow visualization experiments, and the rotation effect is discussed in comparison with the measured pressure distributions on the blade surfaces. It is observed that the flow separating regions on the blade surfaces are fairly reduced by the rotation effect and the flow through the overlap is weakened by the appearance of resisting flow. The former contributes to the torque production of the rotating rotor while the latter acts as a resistance. These phenomena together with the stagnation effect on the front side of the rotor contribute to the power producing mechanism of the Savonius rotor.

Experimental investigation of viscous effects upon a breakup of droplets in high-speed air flow

Abstract: The deformation and disintegration of water and silicone oil droplets were investigated experimentally in a shock tube. Optical visualization was performed by means of the shadowgraph method. Droplets with diameters in the range of 200 to 500 μm were generated by an oscillating capillary. The smallest Weber number in the present experiments is close to the critical value of the breakup. The droplets disintegrated in the stamen or bag mode for moderate values of the Weber number. The effect of the viscosity of the liquid on the breakup mode and the breakup time is discussed.

The Berlin oil channel for drag reduction research

Abstract: For drag reduction research an oil channel has been designed and built. It is also well suited for investigations on turbulent flow and in particular on the dynamics of the viscous sublayer near the wall. The thickness of the viscous sublayer (y+= 5) can be varied between 1 and 4 mm. Surfaces with longitudinal ribs (“riblets”), which are known to reduce drag, can have fairly large dimensions. The lateral spacing of the ribs can lie between 3 and 10 mm, as compared to about 0.5 mm spacing for conventional wind tunnels. It has been proved by appropriate tests that the oil channel data are completely equivalent to data from other facilities and with other mean flow geometries. However, the shear stress data from the new oil channel are much more accurate than previous data due to a novel differential shear force balance with an accuracy of ±0.2%. In addition to shear stress measurements, velocity fluctuation measurements can be carried out with hot wire or hot film probes. In order to calibrate these probes, a moving sled permits to emulate the flow velocities with the fluid in the channel at rest. A number of additional innovations contribute to the improvement of the measurements, such as, e.g., (i) novel adjustable turbulators to maintain equilibrium turbulence in the channel, (ii) a “bubble trap” to avoid bubbles in the channel at high flow velocities, (iii) a simple method for the precision calibration of manometers, and (iv) the elimination of (Coulomb) friction in ball bearings. This latter fairly general invention is used for the wheels of the calibration unit of the balance. The channel has a cross section of 25 × 85 cm and is 11 m long. It is filled with about 4.5 metric tons of baby oil (white paraffine oil), which is transparent and odorless like water. The kinematic viscosity of the oil is v = 1.2×10−5 m2/s, and the highest (average) velocity is 1.29 m/s. Thus, the Reynolds number range (calculated with the channel width, 0.25 m) lies between 5,000 and 26,800 for fully established turbulent flow.

Measurement of two-dimensional velocity fields in porous media by particle image displacement velocimetry

Abstract: Local measurements of the phase function and of two components of the velocity can be performed in transparent porous media by means of particle image displacement velocimetry (P.I.D.V.). Some preliminary results are presented and discussed.

Silicon based flow sensors used for mean velocity and turbulence measurements

Abstract: Small and directional sensitive silicon based sensors for velocity measurements have been designed and fabricated using microelectronic technology. Single-chip as well as double-chip sensors for the determination of mean velocity and turbulent stresses have been developed. To determine the performance of these silicon sensors, comparisons with conventional hot-wire sensors were done in a well-defined two-dimensional turbulent flat plate boundary layer at a constant Reynolds number of 4.2 · 106. All the silicon sensors were found to have a spatial and frequency resolution that makes them suitable for turbulence measurements. In the studied flow field the measured profiles of mean velocities and Reynolds stresses of all silicon sensors show the same accuracy as corresponding hot-wire measurements. The silicon sensors are also shown to operate with good resolution even when the temperature of the heated part of the chip is reduced considerably.

A comparison of the drag-reducing benefits of riblets in internal and external flows

Abstract: Equivalent drag-reducing performance is observed with 3 M riblets in fully developed internal (pipe) and developing external (flat plate) flows. Drag reduction begins around h+ = 3, peaks between 6 and 9% at about h+ = 12, and becomes zero for a value of h+ between 20 and 30. In laminar pipe flow no significant change in drag is observed with the 3 M riblets present. At high Reynolds numbers, after exhibiting fully rough behaviour, friction factors for the 3 M riblets are observed to monotonically decrease with increasing Reynolds number.

Flow and temperature measurement of natural convection in a Hele-Shaw cell using a thermo-sensitive liquid-crystal tracer

Abstract: The temperature and flow field of natural convection in a Hele-Shaw cell is visualized by using a liquid-crystal tracer. The tracer photographs obtained by this method are compared with the interferograms of previous experiments using the same experimental setup, and the applicability of the present methods is validated. Quantitative data of the temperature and velocity were obtained by applying a colour-image-processing technique to the visualized images.

Signal-processing techniques for low signal-to-noise ratio laser Doppler velocimetry signals

Abstract: A variety of methods have been developed to obtain acurate frequency estimates from laser Doppler velocimetry (LDV) signals. Rapid scanning and fiber optic LDV systems require robust methods for extracting accurate frequency estimates with computational efficiency from data with poor signal-to-noise ratios. These methods typically fall into two general categories, time domain parametric techniques and frequency domain techniques. The frequency domain approach is initiated by transforming the Doppler bursts into the frequency domain using the fast Fourier transform (FFT). From this basic transformation a variety of interpolation procedures (parabolic, Gaussian, and centroid fits) have been developed to optimize the frequency estimation accuracy. The time domain approaches are derived from the parametric form of a sinusoid. The estimation of constants in this relationship is performed to satisfy specific constraints, typically a minimization of a variance expression. A comparison of these techniques is presented using simulated signals and additive Gaussian and Poisson white noise. The statistical bias and random errors for each method are presented from 200 signal simulations at each condition. Frequency estimation via the FFT with zero-padding and a Gaussian interpolation scheme was found to produce the lowest bias and random errors.

Refraction through cylindrical tubes

Abstract: When viewed from the outside, objects placed inside containers with cylindrical walls (i.e. test tubes, beakers, pipes) appear distorted because of the curvature of the interfaces and the differing refractive indices of the media. We have developed a method for measuring the coordinates of a particle located inside a straight cylindrical tube by viewing it from two directions using a camera. A set of equations is derived which maps all points inside the tube to the camera image plane; ray tracing diagrams are shown for several important cases, indicating variable distortion, hidden regions, multiple images, and critical reflections. An experimental test was performed to check the calculations; excellent verification was obtained.

The self-preservation of homogeneous shear flow turbulence

Abstract: An analysis of the equations governing homogeneous shear flow shows the possibility of solutions which are self-preserving at all scales of motion, and that these solutions are dependent on the initial conditions. The appropriate velocity scale is the one obtained from the turbulence kinetic energy, q 2/2, while the length scale is the Taylor microscale, λ. Two cases of self-preserving flow are identified: one corresponding to constant mean shear, the other to a mean shear which is inversely proportional to time. For the first case (the only one considered in detail) the principal results of the postulated similarity are that λ is constant, while q 2 varies exponentially with time. The ratio of the turbulence energy production rate to its dissipation rate remains constant. It is also shown that the energy spectra scale over all wavenumbers with q 2 and λ, and that they have shapes determined by the initial conditions. The experimental evidence is generally consistent with the theory.

The influence of kinematic waves on jet break down

Abstract: The breakdown of a liquid jet is commonly considered as an effect of the surface tension. However, essential aspects of the boundary wave amplification and the final disintegration of velocity modulated jets can be explained by non-linear travelling time effects, which are of purely kinematic nature. The experiments are carried out on jets with radii between 1.2 to 2.5 mm and Weber numbers between 17 to 200. The experimental results are compared with a theoretical solution which neglects the surface tension.

A concentration probe for the study of mixing in supersonic shear flows

Abstract: An aspirating hot-film probe is developed to measure local mean gas composition in supersonic flows. The probe consists of a constant temperature hot-film sensor operating in a channel with a choked exit. Thus, the flow over the hot film is influenced only by total temperature, total pressure, and gas concentration. The use of the probe requires a separate measurement of the total temperature in the gas flow. The probe has a spatial resolution of 0.011 in. and shows acceptable sensitivity to flow angularity. The probe is used in the study of an unheated supersonic air/helium mixing layer in a 23 cm × 23 cm supersonic wind tunnel. Data are presented in raw form and after reduction to concentration and mean flow quantities.

Frequency response and instantaneous temperature profile of cold-wire sensors for fluid temperature fluctuation measurements

Abstract: The information of the wire response is necessary for the estimation of corrections and uncertainty of temperature measurements. This paper describes the theoretical response of cold-wire sensors to temperature fluctuations in a fluid flow. Existing transfer functions of cold wires are approximate and implicit functions of frequency. We present the exact solutions of heat conduction equations for a cold wire and stubs taking account of the prong effect. Because the solutions have simple forms of elementary functions, we can easily calculate the frequency response of cold wires. Sample calculations are given under several typical conditions. Also, the instantaneous temperature profiles of a cold wire are obtained for the first time.

External and internal flow fields of plunging breakers

Abstract: This first stage of a laboratory study of the turbulent flow-field induced by wave-breaking involves the application of a flow-visualization technique from which external appearance and internal flow-structures of the surf zone have been examined. Plunging breakers were generated in a wave flume, whilst suitable light sources were deployed. To trace the water particle motion, fluorescent coatings in combination with suitable illuminating light sources, such as an Argon-Ion laser and ultraviolet lamps, were used. Whilst a particular light source was suitable for studying the water body in the surf zone, it might not be for studying particle motion in the air-bubble zone having a high concentration of air entrainment. It was found that the motion in the surf zone was highly three-dimensional with a dividing region formed beneath the impinging surface rollers, and that the breaking phenomenon was cyclic and deterministic in character.

An experimental study of two-phase turbulent coaxial jets

Abstract: The effect of solid particles on the flow structure of axisymmetric turbulent coaxial jets has been studied A laser-Doppler anemometer was used to measure the mean and fluctuation velocities of both phases, and a Malvern laser diffraction instrument was applied to measure particle size and concentration A series of velocity ratios and particle loading ratios were investigated, and the results were analysed for the effects of these ratios on the mixing characteristic and the similarity behavior of the jet The effects of particle diameter and its distribution were also studied as well as their influence on the coaxial jet behavior

Flow visualization of interactions among large coherent structures in an axisymmetric jet

Abstract: Interactions between large coherent structures are visualized with both schlieren photography in two air jets and dye photography in a water jet. The density difference needed for the schlieren technique is provided by an electrically heated wire ring surrounding the jet. External forcing with either single axisymmetric, single non-symmetric, combined axisymmetric or combined non-symmetric modes was applied. It was found that forcing the jet with a pair of different spinning modes leads to azimuthal distortions of the mean flow. This observation confirms and explains existing hotwire data. Simultaneous excitation with two axisymmetric modes may produce structures of higher modes or even cause structurally undistinguishable development. Streamwise structures are observed both in the unforced jet and in the axisymmetrically forced jet. They do not seem to be caused by a Gortier instability from the concave curvature of the conventional nozzle, since they were also found in a jet flow from a specially designed nozzle with only convex contraction surface.

Flow visualization and LDV measurement of fully developed laminar flow in helically coiled tubes

Abstract: Secondary flow structure in helically coiled tubes is characterized by laser light-sheet flow visualization photographs and laser Doppler velocimetry measured velocity vector field. The torsion-to-curvature ratio and Reynolds number, based on the tube diameter and bulk average axial velocity, were varied from 0.06 to 5.55 and from 35 to 330, respectively, to study their effects on the secondary flow patterns. Good agreement was found between the results obtained in the present work and those predicted previously. That the dominance of the torsion effect on the secondary flow is not limited to low Reynolds numbers as reported previously is pointed out. Moreover, the transformation of two recirculating cells into one cell is documented in detail.

The influence of the type of gas on the reduction of skin friction drag by microbubble injection

Abstract: The influence of the type of gas on the performance of microbubble skin friction reduction was investigated on an axisymmetric body. Gases were selected which covered a wide range of densities and solubilities. Integrated skin friction measurements, which span a range of velocities (U∞) from roughly 10 to 20 m/s and tunnel pressures from 1 to 2.6 atm, are presented as a function of gas flow rate. All gases show qualitatively similar behavior. The gas volume flowrate, referenced to injector ambient conditions (tunnel temperature and pressure), is shown to correlate the drag reducing behavior of all the gases at one velocity, independent of pressure. A normalization based on the volume flowrate through the turbulent boundary layer is shown to nearly collapse all the results independent of velocity or pressure. The results indicate that high ambient pressures may degrade the drag reducing capabilities of highly soluble gases.

A quantitative study of the flow around an impulsively started circular cylinder

Abstract: The detailed flow structure behind an impulsively started circular cylinder has been investigated experimentally. The Reynolds number based on the steady state velocity and the diameter of the cylinder was 500 to 3,000. This work is unique in that unsteady spatial velocities were measured simultaneously by a quantitative visualization technique — Laser Induced Photochemical Anemometry (LIPA). The surface vorticity at g/q = π/2 and vorticity distribution behind the cylinder in the Lagrangian coordinates (i.e. coordinates fixed on the cylinder) were calculated from the measured velocities. The surface vorticity shows in the early stage of flow development a close agreement with the previous results obtained by analytical and numerical approaches. The large-field velocity and vorticity information provides an insight into the formation process of the vortices downstream of the cylinder. In addition to the quantitative information, the results of visualized flow pattern obtained by LIPA technique are also presented.

Experimental criteria for the determination of fractal parameters of premixed turbulent flames

Abstract: The influence of spatial resolution, digitization noise, the number of records used for averaging, and the method of analysis on the determination of the fractal parameters of a high Damkohler number, methane/air, premixed, turbulent stagnation-point flame are investigated in this paper. The flow exit velocity was 5 m/s and the turbulent Reynolds number was 70 based on a integral scale of 3 mm and a turbulent intensity of 7%. The light source was a copper vapor laser which delivered 20 nsecs, 5 mJ pulses at 4 kHz and the tomographic cross-sections of the flame were recorded by a high speed movie camera. The spatial resolution of the images is 155 × 121 μm/pixel with a field of view of 50 × 65 mm. The stepping caliper technique for obtaining the fractal parameters is found to give the clearest indication of the cutoffs and the effects of noise. It is necessary to ensemble average the results from more than 25 statistically independent images to reduce sufficiently the scatter in the fractal parameters. The effects of reduced spatial resolution on fractal plots are estimated by artificial degradation of the resolution of the digitized flame boundaries. The effect of pixel resolution, an apparent increase in flame length below the inner scale rolloff, appears in the fractal plots when the measurent scale is less than approximately twice the pixel resolution. Although a clearer determination of fractal parameters is obtained by local averaging of the flame boundaries which removes digitization noise, at low spatial resolution this technique can reduce the fractal dimension. The degree of fractal isotropy of the flame surface can have a significant effect on the estimation of the flame surface area and hence burning rate from two-dimensional images. To estimate this isotropy a determination of the outer cutoff is required and three-dimensional measurements are probably also necessary.