Showing papers in "Experiments in Fluids in 1990"
TL;DR: In this article, an experimental investigation was made of the deformation and breakup of water droplets at nearly critical Weber numbers, where experiments were conducted in a small horizontal wind tunnel.
Abstract: Experimentally determined values of the critical Weber number available from the literature are scattered over a very wide range ofWec from 2.2 to 99.6. To study one possible source of these discrepancies an experimental investigation was made of the deformation and breakup of water droplets at nearly critical Weber numbers. Experiments were conducted in a small horizontal wind tunnel. A continuous stream of uniform water droplets was allowed to fall perpendicularly to the continuous stream of air. The time histories of water droplets were recorded by using a high speed camera. Five different basic behaviours of water droplets were recorded in the range ofWe = 11 to 14. It was found that an increase in the Weber number in this region resulted in an increased percentage of droplets with regular bag type breakup.
233 citations
TL;DR: In this paper, a small airfoil is operated in combined harmonic plunging and pitching motions to generate thrust in a still air environment, by full utilization of dynamic stall vortices large thrust coefficients were attained.
Abstract: A small airfoil is operated in combined harmonic plunging and pitching motions to generate thrust in a still air environment. By full utilization of dynamic stall vortices large thrust coefficients were attained. The vortical signature of thrust is a simple vortex street with the character of a jet stream.
151 citations
TL;DR: In this paper, the authors provide a framework for determining the experimental resolution in PLIF experiments and give error estimates for concentration measurements in turbulent jet mixing experiments using biacetyl as the molecular tracer.
Abstract: The planar laser-induced fluorescence (PLIF) technique is attractive for instantaneous and non-intrusive imaging of species concentration in gaseous flows. This paper provides a framework for determining the experimental resolution in PLIF experiments and gives error estimates for concentration measurements in turbulent jet mixing experiments using biacetyl as the molecular tracer. The procedures to correct for experimental artifacts in the PLIF images are outlined. Images of the instantaneous, average, rms, and dissipation of concentration in a turbulent jet are presented.
137 citations
TL;DR: In this article, an experimental investigation was conducted to examine the dynamic stall characteristics of a NACA 23012 aerofoil section at a Reynolds number of 1.5 million and the static stall mechanism was determined to be via abrupt upstream movement of trailing edge separation.
Abstract: An experimental investigation was conducted to examine the dynamic stall characteristics of a NACA 23012 aerofoil section at a Reynolds number of 1.5 million. Time-dependent data were obtained from thirty miniature pressure transducers and three hot film gauges situated at the mid-span of the wing. The static stall mechanism of the NACA 23012 was determined to be via abrupt upstream movement of trailing edge separation. Under dynamic conditions, stall was found to occur via leading edge separation, followed by a strong suction wave that moved across the aerofoil. This suction wave is characteristic of a strong moving vortex disturbance. Evidence of strong secondary vortex shedding was also found to occur, and this appears symptomatic of dynamic stall only at low Mach numbers. Some evidence of flow reversals over the trailing edge of the aerofoil were indicated prior to the development of leading edge separation and dynamic stall.
123 citations
TL;DR: In this article, a circular cylinder placed close to a horizontal plane wall was investigated experimentally and the fluctuating fluid forces acting on the cylinder sharply increased, and the regular vortex shedding, i.e. Karman vortex streets, started to form beyond a critical gap height.
Abstract: The flow around a circular cylinder placed close to a horizontal plane wall was investigated experimentally. Fluctuations of lift and drag of the cylinder and wall interference effects were studied in terms of the gap height between the cylinder and wall and the thickness of the turbulent wall boundary layer. The fluctuating fluid forces acting on the cylinder sharply increased, and the regular vortex shedding, i.e. Karman vortex streets, started to form beyond a critical gap height. The formation of Karman vortex streets was abruptly interrupted when the bottom of the cylinder came in contact with the outer layer of the boundary layer developed on the wall. This critical gap height correlated well with the thickness of the boundary layer.
122 citations
TL;DR: In this article, the velocity field of a circular water jet impinging onto a flat plate has been measured using particle image velocimetry, or PIV, producing thousands of simultaneous two-dimensional velocity measurements for each realization.
Abstract: The velocity field of a circular water jet impinging onto a flat plate has been measured using particle image velocimetry, or PIV. The velocity field has been recorded at several instants in time, producing thousands of simultaneous two-dimensional velocity measurements for each realization. The instantaneous velocity, vorticity and rate-of-strain fields reveal the interaction of vortices near the impinging wall within the radial wall jet downstream from the stagnation point. An ensemble average of the instantaneous fields produces a mean velocity field of the jet flow, which reveals many of the processes leading to boundary layer separation and vortex breakaway within the wall jet. The PIV system extracts the velocity measurements using a two-dimensional autocorrelation method, and can obtain thousands of highly accurate velocity measurements within a few minutes. The structure found in these experiments may be similar to the ground level structure of atmospheric microburst phenomena.
115 citations
TL;DR: In this paper, the normal and streamwise components of the velocity fields of turbulent flow in a channel at low Reynolds numbers have been measured with laser-Doppler techniques, and good agreement is found for single-point moments through fourth order.
Abstract: Normal and streamwise components of the velocity fields of turbulent flow in a channel at low Reynolds numbers have been measured with laser-Doppler techniques. The experiments duplicate the conditions used in current direct numerical simulations of channel flow, and good, but not exact, agreement is found for single-point moments through fourth order. In order to eliminate LDV velocity bias and to measure velocity spectra, the mean time interval between LDV signals was adjusted to be much smaller than the smallest turbulence time scale. Spectra of the streamwise and normal components of velocity at locations spanning the channel are presented.
108 citations
TL;DR: In this paper, a split film probe of hot wire anemometer was used to investigate the flow field generated by two identical jets of air issuing from plane parallel nozzles in a common end wall and mixing with the ambient room air.
Abstract: Measurements of mean velocity components, mean flow direction, turbulent intensities and Reynolds shear stress were made with a split film probe of hot wire anemometer to investigate the flow field generated by two identical jets of air issuing from plane parallel nozzles in a common end wall and mixing with the ambient room air. Due to the sensitivity of the split film probe to the flow direction, the reverse flow in the converging region was detected by the split film probe and observed by flow visualization. The mean velocity approaches self-preservation in both the converging and the combined regions, while the turbulent intensities and Reynolds shear stress approach self-preservation in the combined region only. The trajectory of the maximum velocity is almost unchanged by variance of nozzle spacing in the converging region. The distance of the merging point from the nozzle exit increases linearly with nozzle spacing. The spread of the converging jet increases more rapidly than that of the combined jet.
89 citations
TL;DR: In this paper, a speckle photographic method is applied for analyzing a compressible turbulent air flow with density fluctuations, where the flow is passed through a turbulence grid. Butterworth et al. determined spatial correlation coefficients, turbulent length scales and energy spectra under the assumption of homogeneous isotropic turbulence.
Abstract: A speckle photographic method, which is sensitive to changes of gradients in fluid density, is applied for analyzing a compressible turbulent air flow with density fluctuations. Spatial correlation coefficients, turbulent length scales, and energy spectra are determined under the assumption of homogeneous isotropic turbulence. The experiments are performed in a shock tube where the flow is passed through a turbulence grid. Measurements are taken before and after the turbulent regime interacts with the normal shock wave reflected from the tube's end wall. Amplification of the turbulence intensity by the shock interaction process is verified quantitatively and is shown to be restricted to the lower wave numbers in the spectrum.
81 citations
TL;DR: In this article, an experiment was performed to evaluate the characteristics of various fluorescent dyes used as tracers for concentration measurements in water flows, by laser induced fluorescence, and the results showed that, in terms of the stability of the solution, fluorescein is inferior to either Rhodamine B or Rhodamine 6G and that for concentrations of rhodamine B less than 0.08 mg/1 the response of fluorescent to the incident light is linear.
Abstract: An experiment was performed to evaluate the characteristics of various fluorescent dyes used as tracers for concentration measurements in water flows, by laser induced fluorescence. Three common fluorescent dyes (fluorescein, rhodamine B and rhodamine 6G) were used, to select the most suitable fluorescent dye and identify its range of linear response. The results showed that, in terms of the stability of the solution, fluorescein is inferior to either rhodamine B or rhodamine 6G and that for concentrations of rhodamine B less than 0.08 mg/1 the response of fluorescent to the incident light is linear.
74 citations
TL;DR: In this article, a helium/air binary gas jet with a jet Reynolds number of 4,300 and a jet-to-ambient fluid density ratio of 0.64 was measured using a laser/hotwire technique.
Abstract: Simultaneous measurements of species volume concentration and velocities in a helium/air binary gas jet with a jet Reynolds number of 4,300 and a jet-to-ambient fluid density ratio of 0.64 were carried out using a laser/hot-wire technique. From the measurements, the turbulent axial and radial mass fluxes were evaluated together with the means, variances and spatial gradients of the mixture density and velocity. In the jet near field (up to ten diameters downstream of the jet exit), detailed measurements of ϱ′ u′/ϱ
0
U
0, ϱ′ v′/ϱ
0 U0, ϱ u′ v′/ϱ
0
U
0
2
, ϱ′u′
2
/ϱ
0
U
0
2
and ϱ′ v′
2
/ϱ
0
U
0
2
reveal that the first three terms are of the same order of magnitude, while the last two are at least one order of magnitude smaller than the first three. Therefore, the binary gas jet in the near field cannot be approximated by a set of Reynolds-averaged boundary-layer equations. Both the mean and turbulent velocity and density fields achieve self-preservation around 24 diameters. Jet growth and centerline decay measurements are consistent with existing data on binary gas jets and the growth rate of the velocity field is slightly slower than that of the scalar field. Finally, the turbulent axial mass flux is found to follow gradient diffusion relation near the center of the jet, but the relation is not valid in other regions where the flow is intermittent.
TL;DR: In this article, a computerized infrared (IR) scanning radiometer is employed to characterize the boundary layer development over a model wing, having a Gottingen 797 cross-section, by measuring the temperature distribution over its heated surface.
Abstract: A computerized infrared (IR) scanning radiometer is employed to characterize the boundary layer development over a model wing, having a Gottingen 797 cross-section, by measuring the temperature distribution over its heated surface. The Reynolds analogy is used to relate heat transfer measurements to skin friction. The results show that IR thermography is capable of rapidly detecting location and extent of transition and separation regions of the boundary layer over the whole surface of the tested model wing. Thus, the IR technique appears to be a suitable and effective diagnostic tool for aerodynamic research in wind tunnels.
TL;DR: The accuracy of the Particle Image Velocimetry technique was investigated using synthetic images having known characteristics as mentioned in this paper, and it was found that as long as the images have a good contrast, particle locations can be determined with sub-pixel accuracy and particle velocities can be reliably determined within a few percent.
Abstract: The accuracy of the Particle Image Velocimetry technique was investigated using synthetic images having known characteristics. Algorithms were developed to process images automatically without operator assistance. This allowed to parametrically investigate the influence of the various parameters (image contrast, image noise, particle density, distribution of sizes of particles and particle displacement between frames) on the accuracy of the technique. It was found that as long as the images have a good contrast, particle locations can be determined with sub-pixel accuracy and particle velocities can be determined within a few percent.The accuracy of the Particle Image Velocimetry technique was investigated using synthetic images having known characteristics. Algorithms were developed to process images automatically without operator assistance. This allowed to parametrically investigate the influence of the various parameters (image contrast, image noise, particle density, distribution of sizes of particles and particle displacement between frames) on the accuracy of the technique. It was found that as long as the images have a good contrast, particle locations can be determined with sub-pixel accuracy and particle velocities can be determined within a few percent.
TL;DR: A review of the use of iodine in experimental fluid mechanics, including basic formulas for saturated and non-saturated fluorescence (excited with narrowband or broadband lasers), line shape, line strength and quenching behavior is given in this paper.
Abstract: Laser-induced fluorescence of iodine seed molecules can be used for both flow visualization and accurate measurements of gasdynamic properties. This paper gives an introductory review of the use of iodine in experimental fluid mechanics, including basic formulas for saturated and non-saturated fluorescence (excited with narrowband or broadband lasers), line shape, line strength and quenching behavior. Techniques for the seeding of the molecules into the gas flow and the safe handling of the gas are discussed. Finally, a simple numerical example is given for the calculation of absorption and fluorescence signals with discussion of the measurement of gasdynamic properties.
TL;DR: The behavior of supersonic mixing layers under three conditions has been examined by schlieren photography and laser Doppler velocimetry as mentioned in this paper, and it was found that higher levels of secondary freestream turbulence did not increase the peak turbulence intensity observed within the mixing layer, but slightly increased the growth rate.
Abstract: The behavior of supersonic mixing layers under three conditions has been examined by schlieren photography and laser Doppler velocimetry. In the schlieren photographs, some large-scale, repetitive patterns were observed within the mixing layer; however, these structures do not appear to dominate the mixing layer character under the present flow conditions. It was found that higher levels of secondary freestream turbulence did not increase the peak turbulence intensity observed within the mixing layer, but slightly increased the growth rate. Higher levels of freestream turbulence also reduced the axial distance required for development of the mean velocity. At higher convective Mach numbers, the mixing layer growth rate was found to be smaller than that of an incompressible mixing layer at the same velocity and freestream density ratio. The increase in convective Mach number also caused a decrease in the turbulence intensity (σu/ΔU).
TL;DR: In this paper, high-resolution digital imaging, using planar laser-induced light scattering is developed for analysis of gaseous flowfields, implying both high spatial resolution and wide signal dynamic range, can be readily processed to yield twodimensional distributions of species concentrations and, in turn, accurate two-dimensional images of concentration gradient and turbulence scales.
Abstract: High resolution digital imaging, using planar laser-induced light scattering is being developed for analysis of gaseous flowfields. High resolution image data, implying both high spatial resolution and wide signal dynamic range, can be readily processed to yield two-dimensional distributions of species concentrations and, in turn, accurate two-dimensional images of concentration gradients and turbulence scales. Critical aspects of the technique are discussed; details of the design and the performance of the imaging system are presented; and results for laminar, transitional, turbulent and birfurcating nitrogen jets, using planar-laser-induced fluorescence of biacetyl, are reported. Initial results from imaging processing and the potential implications for flowfield analysis are described
TL;DR: In this article, the mixing of two feed streams in a reactor, one with a fluorescent tracer, the other without, results in a fluctuating concentration field, due to the turbulent flow.
Abstract: The mixing of two feed streams in a reactor, one with a fluorescent tracer, the other without, results in a fluctuating concentration field, due to the turbulent flow. Fluorescence spectroscopy allows the characterization of the fluctuations at small scale and high frequencies. Measurements have been made with a spatial resolution of about 30 μm and up to a frequency of 5,000 Hz. Methods have been developed to determine the variance (intensity of segregation) and the power spectra. The spectra can be used to calculate the integral scale of the fluctuations, and in some cases the microscale and dissipation rate. Two optical setups are presented, one based on a nonfocused and the other on a focused laser beam. It is shown that only the focused system has sufficiently high laser flux density and sufficiently small measurement volume to give useful results at the desired characteristic size and frequency. As a demonstration of the method, the turbulent mixing in a continuous stirred tank reactor has been studied. Experiments were carried out in a 225 cm3 baffled reactor, stirred by a six-bladed Rushton disk turbine. The effects of stirring speed and position on the mixing were investigated.
TL;DR: In this article, the compressible spreading rate of two supersonic coaxial jets were studied experimentally, and the authors determined the experimental compression rate using Schlieren photography and total-pressure measurements.
Abstract: The compressible spreading rates of two supersonic coaxial jets were studied experimentally. The center jet had a fully-expanded Mach number of 3, and the outer jet of M = 1.8. The geometries of the center jet were circular and rectangular with two configurations, both with a 3∶1 aspect-ratio. The convective Mach numbers were varied in the range between 0.25 < M
c< 2.25. This was accomplished by varying the density ratio between the center jet and the coaxial jet, as well as the velocity ratio. The experimental compressible spreading rate was determined using Schlieren photography and total-pressure measurements. The spreading rate of the center circular jet decreased with increasing convective Mach number until it reached a constant value of 0.2 to 0.3 of the incompressible spreading rate for M
c>1.4. The rectangular jets exhibited a similar drop, at the same range of M
c, but their spreading rate was higher relative to the circular jet in the entire convective Mach number range.
TL;DR: A technique is described, using a minimum of experimental complexity, which overcomes the directional ambiguity problem in Particle Image Velocimetry.
Abstract: A technique is described, using a minimum of experimental complexity, which overcomes the directional ambiguity problem in Particle Image Velocimetry. Examples of the application of the technique to three different flow regimes are described and typical images presented. A description is given of the algorithms used to interpret the images and sample vector fields presented.
TL;DR: In this paper, the effect of increasing Reynolds number on the evolution of steady three-dimensional flow between the rollers is reported, showing that an increase in Reynolds number beyond this critical value results in a loss of symmetry in the flow pattern.
Abstract: Flow visualization has been used to study the motion of a Newtonian fluid in a four-roll mill. Observations of the effect of increasing Reynolds number on the evolution of steady three-dimensional flow between the rollers are reported. The flow visualization experiments have been conducted for two values of the aspect ratio (defined as the ratio of roller length to gap width between rollers). The rollers have been constrained to rotate in a manner which generates an approximately two-dimensional pure extensional flow near the center of the apparatus at low speeds. Steady, symmetrically positioned vortices grow near the top and bottom walls of the container as the Reynolds number is increased up to a critcal value. An increase in Reynolds number beyond this critical value results in a loss of symmetry in the flow pattern. The nature of this transition is strongly dependent upon the aspect ratio.
TL;DR: In this paper, the mean velocities and velocity fluctuations are reported for both phases in two downward flowing, low-Reynolds-number, particle-laden jets for spherical particles having a number-average diameter of 163 μm.
Abstract: Mean velocities and velocity fluctuations are reported for both phases in two downward flowing, low-Reynolds-number, particle-laden jets. The Reynolds number based on the initial bulk air flow is 780. Initial solid-air volume flux ratios of 0.0027 and 0.0225 are studied for spherical particles having a number-average diameter of 163 μm. Both cases may be considerd as gravity driven flows, and in most regions of these jets there is a measurable slip velocity between phases. Gas-phase velocities are obtained using a combination of amplitude discrimination and velocity filtering. The velocity filtering is done on the computer as a postprocessing step. Comparison of the two jets shows that high particle loading decreases the slip velocity, decreases the velocity fluctuations of both phases, and decreases the spreading rate of the particles.
TL;DR: In this paper, a set of Plexiglass containers with rounded trapezoidal cross sections were designed and built to test the validity of Concus and Finn's existence theorem for a bounded free liquid surface at zero gravity.
Abstract: Plexiglass containers with rounded trapezoidal cross sections were designed and built to test the validity of Concus and Finn's existence theorem (1974, 1983) for a bounded free liquid surface at zero gravity. Experiments were carried out at the NASA Lewis two-second drop tower. Dyed ethanol-water solutions and three immiscible liquid pairs, with one liquid dyed, were tested. High-speed movies were used to record the liquid motion. Liquid rose to the top of the smaller end of the containers when the contact angle was small enough, in agreement with the theory. Liquid interface motion demonstrated a strong dependence on physical properties, including surface roughness and contamination.
TL;DR: In this article, the authors used particle image-velocimetry for flow visualization and analysis, which offers the advantage of measuring the entire flow field (velocity field, streamlines etc) in a selected plane within the fluid at a given instant of time.
Abstract: Thermocapillary convection is studied experimentally using particle-image-velocimetry for flow visualization and analysis This method offers the advantage of measuring the entire flow field (velocity field, streamlines etc) in a selected plane within the fluid 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 for different Marangoni numbers
TL;DR: In this paper, three methods of data analysis, namely ensemble averaging, high pass filtering and cycle-by-cycle smoothing, are applied to velocity data obtained by laser Doppler velocimetry in a model axisymmetric and a realistic engine configuration.
Abstract: Cyclic variations and turbulence characterisation are considered in the context of in-cylinder flows. Three methods of data analysis, namely ensemble averaging, high pass filtering and cycle-by-cycle smoothing, are applied to velocity data obtained by laser Doppler velocimetry in a model axisymmetric and a realistic engine configuration. The interpretation of the results is supported by similar analysis of a steady-state simulation of in-cylinder flow with emphasis on the discrimination between random and deterministic flow variations. The results suggest that it is not always possible to identify cyclic variations as deterministic processes and that turbulence estimates based on filtering techniques may be misleading. The differences in the estimates of mean values are, however, small.
TL;DR: Active control of fundamental two-and three-dimensional amplified modes in an axisymmetric jet is presented by introducing localized acoustic disturbances produced by an azimuthal array of miniature speakers placed in the close proximity of the jet lip on the exit face as discussed by the authors.
Abstract: Active control of fundamental two- and three-dimensional amplified modes in an axisymmetric jet is presented. This is done by introducing localized acoustic disturbances produced by an azimuthal array of miniature speakers placed in the close proximity of the jet lip on the exit face. The independent control of each speaker output allows different azimuthal amplitude and phase distributions of periodic input pressure disturbances. Coupled with this was the development and use of a circular smoke-wire for visualizing shear layer modes around the complete jet circumference.
TL;DR: A review of the digital techniques used in the analysis of these images is carried out and both twodimensional and onedimensional techniques are analyzed and the problems involved in their use are discussed.
Abstract: In Particle Image Velocimetry the analysis of the multi-exposed photographic plate can be carried out either by means of the Young's fringe analysis or by a direct technique. With this second approach the image of each interrogation area, enlarged by a microscope, is recorded by means of a digital image acquisition system. After some preliminary filtering operations the digitized image can be analyzed with numerical tools. In this paper a review of the digital techniques used in the analysis of these images is carried out. Both twodimensional and onedimensional techniques are analyzed and the problems involved in their use are discussed.
TL;DR: In this article, an experimental system using integral interferometric imaging and computer tomography for visualizing the structure of a turbulent, vertical helium jet is described, where integral images and tomographic integral data were obtained using a pulsed phase-shifted interferometer.
Abstract: An experimental system using integral interferometric imaging and computer tomography for visualizing the structure of a turbulent, vertical helium jet is described. Integral images and tomographic integral data were obtained using a pulsed phase-shifted interferometer. The integral images revealed a sinuous overall jet structure and large-scale buckling motions in the far-field. Tomographic reconstruction of jet cross-sections at numerous axial locations were made for three turbulent jets at two different Reynols numbers, 2,800 and 4,300. Tomographic images revealed unmixed ambient fluid far inside the jet boundary and a bimodal concentration distribution. Image interpretation and experimental errors are discussed.
TL;DR: In this paper, the optical field of a laser diode is described satisfactorily as a product of a Gaussian and a truncated Lorentzian distribution, and the influence of various design parameters is examined by means of an extensive computational study as well as experimental evaluation involving precise scanning of the measuring volume.
Abstract: A laser Doppler anemometer with a laser diode as the light source, has several advantages: i.e., low power consumption, compactness, and low cost. In order to be fully benefitted by these favorable characteristics, the measurement uncertainty, associated with wavefront distortion in the measuring volume, should be minimized. Furthermore, proper attention should be given to system misalignment caused by external perturbations, such as thermal expansion of the diode-collimator assembly. These considerations lead to a computational procedure for optimizing the layout of the semiconductor LDA system. Calculations are based on a generalized relation for fringe non-uniformity combined with a simulation model for the anemometry system. For this purpose, the optical field of a laser diode is described satisfactorily as a product of a Gaussian and a truncated Lorentzian distribution. The influence of various design parameters is examined by means of an extensive computational study as well as experimental evaluation involving precise scanning of the measuring volume. The performance is improved by employing a small focal length collimator and a large focal length front lens. For measurement of turbulence intensities smaller than 1%, it may become necessary to collect the signals in the side scatter and to use a frequency-domain signal processor. For such an application, temperature control may also be necessary, but it should be applied to the entire diode-collimator assembly and not just to the laser diode as suggested in previous publications.