Showing papers in "Experiments in Fluids in 1987"

Spatial resolution and measurement of turbulence in the viscous sublayer using subminiature hot-wire probes

Abstract: Measurements in the viscous sublayer of a flat-plate turbulent boundary layer in air, using single hot-wire sensors with lengths from 1–60 viscous length scales show that, at a given distance from the surface, the turbulence intensity, flatness factor, and skewness factor of the longitudinal velocity fluctuation are nearly independent of wire length when the latter is less than 20–25 times the viscous length scale (i.e. 20–25 “wall units”), and decrease significantly and abruptly for larger wire lengths. This conclusion is consistent with other workers' probability density functions of streak spacing: the lateral spacing of “streaks” in the viscous sublayer is 80–100 wall units on average with minimum spacing of 20–25 wall units, which implies that signals would be strongly attenuated by wires whose length exceeds 20–25 wall units. To achieve wire lengths of less than 20–25 wall units, subminiature hot wire probes like those described by Ligrani and Bradshaw (1987), having lengths as small as 150 μm, are necessary for sublayer measurements in typical laboratory wind tunnels. As well as the measurements mentioned above, dissipation spectra are presented, to show the effect of spanwise averaging on the high-frequency motions, which is necessarily more severe than the effect on overall intensities.

An experimental investigation of the stability of converging cylindrical shock waves in air

Abstract: An experimental study was made of the stability of converging cylindrical shock waves. The experiments were conducted on annular shock tubes equipped with a double exposure holographic interferometer in the Stoswellenlabor, RWTH Aachen, and in the Institute of High Speed Mechanics, Tohoku University, Sendai, for shock Mach numbers of 1.1 to 2.1 in air. By comparing these two different shock tube experiments, it is found that in the former facility the mode-three instability is predominant at the center of convergence, whereas the mode-four instability appears in the latter setup. The instabilities are denoted in this way because the shock and the flow field behind it reveal a remarkable triangular and quadrangular symmetry, respectively. It is concluded that the converging cylindrical shock wave is always unstable and sensitive to the structure of the annular shock tube. Usefulness of holographic interferometry to this kind of shock wave research is also demonstrated.

Radial viscous fingering patterns in Hele-Shaw cells

Abstract: Viscous fingering experiments were performed by injecting a liquid to radially displace a much more viscous liquid in a Hele-Shaw cell consisting of two parallel closely-spaced glass plates. Both smooth and etched plates were used to study the influence of plate roughness on the fingering mechanism. Effect of flow rate and interfacial tension was also demonstrated. The results show that symmetric dendritic finger patterns can form in the presence of anisotropy provided by an etched square network, for both miscible and immiscible fingers. Chaotic finger patterns can form both in a cell with smooth surfaces and in one having a network of randomly oriented channels etched on one plate. Due to interfacial tension, the immiscible finger patterns are less ramified than their miscible counterparts, are more sensitive to the flow rate and become compact as the flow rate decreases. Possible applications of two-phase displacement studies in Hele-Shaw cells are discussed, which include two-phase flow in porous media and acid fracturing of oil reservoirs.

Effect of laden solid particles on the turbulent flow structure of a round free jet

Abstract: The effect of solid particles on the flow structure of a round air jet in a stagnant surrounding was investigated experimentally. Information on the averaged two-component velocities, the kinetic energies, and the u′ v′-properties were obtained for both phases by means of a monochromatic three beam laser Doppler anemometer. The particle number density was also measured by this system. Glass beads of 64 μm and 132 μm diameter were used for a constant mass loading ratio of 0.3 in a jet with a Reynolds number of 20 000. The lateral mean velocity and number density profiles were expressed by best fitting functions and several invariable coefficients were found. The standard drag force coefficient C D for a single particle was applicable for a dilute particle cloud even in a non-uniform air velocity field.

LDV measurements of a turbulent air-solid two-phase flow in a 90 ° bend

Abstract: Simultaneous measurements of the mean streamwise and radial velocities and the associated Reynolds stresses were made in an air-solid two-phase flow in a square sectioned (10×10 cm) 90° vertical to horizontal bend using laser Doppler velocimetry. The gas phase measurements were performed in the absence of solid particles. The radius ratio of the bend was 1.76. The results are presented for two different Reynolds numbers, 2.2×105 and 3.47×105, corresponding to mass ratios of 1.5×10−4 and 9.5×10−5, respectively. Glass spheres 50 and 100 μm in diameter were employed to represent the solid phase. The measurements of the gas and solid phase were performed separately. The streamwise velocity profiles for the gas and the solids crossed over near the outer wall with the solids having the higher speed near the wall. The solid velocity profiles were quite flat. Higher negative slip velocities are observed for the 100 μm particles than those for the 50 gm particles. At angular displacement θ=0°, the radial velocity is directed towards the inner wall for both the 50 and 100 μm particles. At θ=30° and 45°, particle wall collisions cause a clear change in the radial velocity of the solids in the region close to the outer wall. The 100 μm particle trajectories are very close to being straight lines. Most of the particle wall collisions occur between the θ=30° and 60° stations. The level of turbulence of the solids was higher than that of the air.

Conditional sampling of velocity and scalars in turbulent flames using simultaneous LDV-Raman scattering

Abstract: The laser Doppler velocimeter (LDV) measures the velocity distribution of particles which is often an acceptable representation of the distribution of gas velocities. However, in turbulent two stream mixing flows, the particle velocity distribution will differ from the gas velocity distribution when the particle densities in the two streams are unequal. This bias is explored in a reacting and nonreacting turbulent jet which is surrounded by coflowing air. By adding seed particles to only the coflow air and then to only the jet fluid, the limits of this bias are established. Additional measurements with an LDV triggered laser Raman scattering system demonstrate that the bias in the LDV sampling is propagated to the Raman measurements. An analytical equation is presented which will generate unbiased velocity and scalar distributions from measurements obtained from seeding only one stream at a time.

Measurements of temperature, density, pressure, and their fluctuations in supersonic turbulence using laser-induced fluorescence

Abstract: A laser-induced fluorescence method has been developed that provides simultaneous measurements of temperature, density, and their fluctuations owing to turbulence in unheated compressible flows. Pressure and its fluctuations are also deduced using the equation of state. Fluorescence is induced in nitric oxide that has been seeded into a nitrogen flow in concentrations of 100 ppm. Measurements are obtained from each laser pulse, with a spatial resolution of 1 mm and a temporal resolution of 125 ns. The method was applied to a supersonic, turbulent, boundary-layer flow with a free-stream Mach number of 2. For stream conditions in the range from 150-300 K and 0.3-1 atm, temperature is measured with an uncertainty of approximately 1 percent rms, while density and pressure uncertainties are approximately 2 percent rms.

Two-component laser anemometry measurements of non-reacting and reacting complex flows in a swirl-stabilized model combustor

Abstract: Simultaneous two-component velocity measurements are acquired in a model, complex flow swirl-stabilized combustor using a two-color laser anemometer. A time base computer interface enables the direct measurement of Reynolds stress\((\overline {u'w'} )\) as well as mean and rms axial (u,u′) and azimuthal (w,w′) velocities. The peak value of the normalized Reynolds stress\((\overline {u'w'} /u_{rms} w_{rms} )\) approaches 0.25 which is less than values (∼0.40) obtained by others using indirect, non-simultaneous measurement methods in complex flows, but similar to a direct measurement in a dump combustor without swirl. Isotropy is satisfied except in regions of high unidimensional shear, and both turbulence intensity and normalized Reynolds stress are reduced in the absence of reaction. Relatively small-scale form intermittencies, associated with a fluctuation of the stagnation point and a precessing vortex core, serve to reduce the measured values of the normalized Reynolds stress at the centerline by increasing the apparent turbulence intensity. At an elevated fuel loading, a global-scale form intermittency is invoked and, while likely realistic relative to practical devices, may not be a viable condition for time-averaged calculations.

On the structure of a turbulent spot in a heated laminar boundary layer

Abstract: An artificially generated turbulent spot was investigated experimentally in a heated boundary layer using a rake of mini-thermocouples. Simultaneous temperature traces were used to determine the spot's leading and trailing edge characteristics. The measurements on the centerline of the plate at a constant velocity and variable streamwise positions provided a Rex range of 2.45–12.6 x 105. At one axial station the free stream velocity was varied and off-axis measurements were obtained.

Inlet centerline turbulence effects on reattachment length in axisymmetric sudden-expansion flows

Abstract: The important parameters that affect the reattachment length in an axisymmetric sudden-expansion flow are examined. It is found that inlet centerline turbulence stands out as the most important; namely increased inlet centerline turbulence causes the reattachment length to decrease.

Digital interferometry for flow visualization

Abstract: Digital holographic interferometry is a hybrid optical-digital technique for determining the phase of an interferogram. This technique improves the accuracy of interferometric measurement of fluid properties and enhances the utility of interferometric flow visualization. Displays of the interferometric phase produce excellent images of weakly refracting two-dimensional flows and can be used to produce integral projection images of three dimensional flows which differ from and complement schlieren and shadowgraph images. The technique is explained herein and examples of its use in both continuous wave and pulsed interferometry are presented.

Generation and manipulation of uniform shear with the use of screens

Abstract: A procedure for the generation of uniformly sheared turbulent flow in the wind-tunnel using separate channels with adjustable blockage is described in detail. Measurements of the reduction of mean shear in a flow crossing transverse grids and gauzes with uniform solidities are also reported: for small flow disturbances, i.e. small relative screen element thickness, d/h, the shear reduction factor, A, was found comparable to predictions, A p; however, for d/h > 0.01, the ratio A/A p decreased rapidly with increasing d/h. Results on the effect of screen insertion on the upstream shear and the combined effects of multiple screens are briefly discussed.

A pulsed light generator for high speed photography

Abstract: A pulsed light generator for stroboscopic photography which makes use of LED's has been developed. The maximum light power which may be extracted from a single LED is approximately 1 Watt. The pulse repetition rate ranges up to 1 MHz and the length of the pulses is adjustable between 0.05 and 1 μs. The light emitted from the diodes is especially suited for the observation of phase objects by interferometric methods due to its pronounced coherence. Results of the electrical and optical performance of the generator are presented.

The effects of inlet conditions and expansion ratio on the onset of flow reversal in swirling flow in a sudden expansion

Abstract: Experiments are reported in which the minimum swirl intensity required to produce a central recirculation zone in a swirling sudden expansion flow is determined as a function of expansion ratio and inlet conditions. Using a swirl generator which allows for independent variation of velocity profile shape and swirl number, it is shown that an inlet tangential velocity distribution with a large solid body vortex core or an axial velocity profile with a maximum on the axis will lead to a higher critical swirl.

Secondary flow in horizontal channels heated from below

Abstract: Experiments have been performed to investigate onset and development of the buoyancy driven secondary flow in a horizontal parallel plate channel with uniform bottom heating. Flow visualization in water (Pr ≈ 7) was performed by injecting a continuous sheet of dye into the bottom boundary layer just up-stream of the heated surface, and operating conditions in the ranges 125 < Re < 1,000 and4.7 x 104 < Gr * < 8.0 x 106 were considered. Top, side, and end views revealed onset of the secondary flow as thermal plumes, which rise from the heated surface and form pairs of counter-rotating vortices. Subsequent development of the flow is characterized by a breakdown in the regular plume structure and transition to buoyancy driven turbulence. Onset of the secondary flow is advanced by increasing the heat flux and/or decreasing the flow rate, and results may be correlated in terms of a critical Grashof number and a dimensionless longitudinal distance. Liquid crystal sheets applied to the heated surface reveal significant spanwise temperature variations due to the secondary flow. The unsteadiness of the flow is discussed and comparisons are made to previous experimental and numerical work.

Techniques for high-speed digital imaging of gas concentrations in turbulent flows

Abstract: Techniques have been developed to measure the concentration of nozzle gas quantitatively in transitional and turbulent flow regimes at high framing rates in two dimensions. Elastically scattered light from a seeded jet, illuminated by a sheet of laser light, was recorded in three ways. The first involved the use of a rotating mirror to sequentially place images onto different portions of a relatively slow vidicon-based imaging system to achieve a high effective framing rate (2–10 kHz) for up to four frames. The second used a portion of a monolithic 128×128 photodiode array connected directly to a high-speed analog to digital converter to achieve a framing rate of 1.136 kHz. The third used a fast video camera and recorder (6 kHz) to store images on magnetic tape in analog form, which were later digitized upon playback. A comparison of these techniques is presented along with a discussion of factors important in the use of high-speed digital imaging systems. It is shown that the temporal development of large-scale structures can be digitally recorded in two dimensions, with the advantage of being both quantitative and visually interpretable.

First observations on break-up of particle agglomerates in shock waves

Abstract: The destruction of solid agglomerates suspended in argon by means of shock waves (1

The water towing tank as an experimental facility

Abstract: Wind tunnels and flumes have become standard laboratory tools for modeling a variety of aerodynamic and hydrodynamic flow problems. Less available, although by no means less useful, are facilities in which a model can be towed (or propelled) through air or water. This article emphasizes the use of the water towing tank as an experimental tool for aerodynamic and hydrodynamic studies. Its advantages and disadvantages over other flow rigs are discussed, and its usefulness is illustrated through many examples of research results obtained over the past few years in a typical towing tank facility.

The effect of trip wire roughness on the performance of the Wortmann FX 63-137 airfoil at low Reynolds numbers

Abstract: An experimental investigation was conducted on the performance and boundary layer characteristics of the Wortmann FX 63-137 airfoil with and without trip wire roughness. Data were obtained through use of a three-component strain gage force balance and static pressure measurement equipment at a test Reynolds number of R c = 100,000. Emphasis was placed on determining the effect of trip wire placement and size on such performance parameters as (C l /C d )max and (C 3/2 /C d )max. Prediction of transition location by the criterion due to Tani and Gibbings was found to have limited application. Most trip wire locations resulted in degraded performance, but for some locations, minimum drag was reduced, maximum lift to drag ratio increased, and hysteresis averted.

Boundary layer two-dimensionality in wind tunnels

Abstract: Dimotakis, P. E.; Maike-Lye, R. C.; Papantoniou, D. A. 1983: Structure and dynamics of round turbulent jets. Phys. Fluids 26, 3185-3192 Gad-el-Hak, M.; Blackwelder, R. F.; Riley, J. J. 1981: On the growth of turbulent regions in laminar boundary layers. J. Fluid Mech. 110, 73-95 Liu, H.-T.; Lin, J.-T.; Delisi, D. P.; Robben, F. A. 1977: Proceed. of the Symp. on Flow in Open Channels and Closed Conduits held at NBS, NBS Special Publication 484, Gaithersburg, MD, USA

Average wavelength of organised structures in the turbulent far wake of a cylinder

Abstract: The average wavelength of organised structures in the far wake of a circular cylinder is inferred from several different estimates based primarily on wind tunnel measurements. Spectra of the lateral velocity fluctuation and cross-spectra between this fluctuation and the temperature fluctuation, at either the same point or at a different point in space, provide relatively unambiguous estimates of the average wavelength of the structures. Dye photographs in a water tunnel provide a less accurate estimate of the average wavelength of the structures. However, all estimates indicate that the average wavelength increases with streamwise distance, at a rate consistent with the self-preserving growth of the wake.

On principal noise of the laser Doppler velocimeter

Abstract: The principal noise of the Laser Doppler Velocimeter is shown to be due to transit broadening rather than the most commonly suggested signal shot; the phase ambiguity noise is also shown as less serious for the dual-beam Laser Doppler Velocimeter. On the basis of optical geometry and Fourier analysis of electric fields interacting with moving particles, the transit broadening noise has been calculated; some supporting experimental evidence is also discussed along with means for reducing this principal noise.

Effective particle size range in laser-Doppler anemometry

Abstract: The present paper points out that all existing laser-Doppler anemometer systems do not only operate within a finite range of Doppler frequencies but also work within a relatively narrow range of signal amplitudes. It is shown that this corresponds to a finite, and usually to an extremely small, range of particle diameters which contributes to the final LDA measurements. Because of this, the particle size distribution has to be matched to the LDA-system used for measuring particle velocities. If this is not taken into account in particle seeding, low data rates will result in spite of very high particle passage rates through the measuring control volume. This is shown experimentally and is supported by theoretical considerations.

Characteristics of a trailing flap flow with small separation

Abstract: Detailed measurements of pressure and velocity characteristics are reported for the flow on and downstream of a trailing flap, whose 16 degree angle of incidence resulted in boundary layer separation, a small region of recirculating flow, and a curved downstream wake. Emphasis is placed on the region of recirculating flow and on the downstream wake. The characteristics were quantified by the use of pressure, hot-wire, and flying hot-wire probes. The flow characteristics are compared to those of a similar flow but with a larger region of recirculation reported by Thompson and Whitelaw (1985). The relative importance of the terms in the transport equations for mean momentum and turbulence energy are quantified and the implications of the results for viscous-inviscid interaction methods and turbulence modeling are discussed.

Pressure distributions behind a rearward facing segmented step

Abstract: Effects of 3-D disturbances on a separating flow past a rearward facing step have been studied with emphasis on pressure distributions in the separation and reattachment regions. The results show that the 3-D disturbances to a separating shear layer diminish rapidly and the flow downstream of reattachment appears to have a 2-D global shearlayer structure.

Laser-induced fluorescence modulation techniques for velocity measurements in gas flows

Abstract: Broadband fluorescence of iodine, excited at 514.5 nm by a single-mode argon-ion laser tuned to the quasi-linear part of an absorption line, was used to detect the Doppler shift and hence the velocity of iodine molecules seeded in a nitrogen jet flow. The slope of the absorption line profile was measured directly using a frequency shift introduced by acoustooptic modulators (AOMs). A velocity of 36 m/s was measured in a jet of N2 at 60 Torr in 2 ms with an accuracy of 11%. To reduce experimental noise, the laser beams were switched at 125 KHz and signal-tuned amplification was used.

Experimental study of thermally stratified unsteady flow by nmr-ct.

Abstract: Thermally stratified unsteady flow caused by two-dimensional surface discharge of warm water into a rectangular reservoir is investigated. Experimental study is focused on the rapidly developing thermal diffusion at small Richardson number. The basic objectives are to develop a measurement system for the unsteady flow phenomena and to study the interfacial mixing between a flowing layer of warm water and the underlying body of cold water. Mean velocity field measurement is carried out by using NMR-CT (Nuclear Magnetic Resonance — Computerized Tomography). It detects a quantitative flow image of any desired section in any direction of flow. Transient mean temperature profiles are obtained by fine thermocouple arrays and a microcomputer-based data acquisition system. Results show that the warm layer penetrates more rapidly into the cold layer at smaller Richardson number because of decrease instability. This is clearly verified by flow visualization using thymol blue solution. It is found that the transport of heat across the interface is more vigorous than that of momentum.

Errors in particle sizing by LDA due to turbidity in the incident laser beams

Abstract: Measurements have been made of the effect of flow turbidity on the visibility and pedestal amplitude of an anemometer signal when incident laser beams are interrupted by particulate flow. The purpose is to assess the likely accuracy of particle sizing and the reliability of discrimination between continuous and particulate phase velocities. Optical depths of field were varied between 2.5 × 10−2 and 14 × 10−2 mm the diameter of the interrupting particles ranged between 14 and 800 μm in six discrete ranges and the corresponding void fractions lay between 0.003% and 0.378%. The incident beam diameter was approximately 400 μm. The measured size is subject to both systematic and random errors when inferred from measurements of pedestal amplitude: the random error increases as the ratio of the incident beam diameter to that of the particulate phase decreases. Systematic errors corresponding to a 10% underestimation of diameter occur at void fractions of 0.003%, 0.01% and 0.018% for particles below 40 μm 75 μm and 105 μm respectively over a 5 cm depth of field. The r.m.s. error is smaller than 7% for particles below 40 μm for all conditions studied but increases with increasing diameter and exceeds 10% at void fractions greater than 0.1% for particles above about 100 μm. The random error in measured diameter derived from measurements of visibility is influenced mostly by the flow turbidity over the 5 cm of the incident beams closest to the measuring volume. For interrupting particles smaller than about 100 μm the r.m.s. error is similar to that for measurements based on the pedestal amplitude. Discrimination of the velocity signal between the particulate and dispersed phase, based on the separation of pedestal amplitudes, is likely to be unreliable if the particle diameter is comparable to the diameter of the incident beams and if the probability of two particles simultaneously present in each beam is not negligible. A method for estimating the level of turbidity at which discrimination is no longer possible is described.

Development of a portable laser sheet

Abstract: A portable laser sheet illumination source for flow visualization experiments has been developed utilizing fiber optic cable and standard optical bench components. This technique improves the convenience and safety aspects of using laser light by containing the concentrated laser energy inside a flexible and shielded fiber cable. Transmission efficiencies up to seventy-five percent have been obtained with a twenty meter long fiber cable and careful alignment of optical components. The current optical configuration can transmit a two-watt Argon-ion laser beam and produce a sheet one-half by one meter in size and approximately two to three millimeters in thickness.