Showing papers in "Experiments in Fluids in 1986"

Power spectra of fluid velocities measured by laser Doppler velocimetry

Abstract: The power spectrum and the correlation of the laser Doppler velocimeter velocity signal obtained by sampling and holding the velocity at each new Doppler burst are studied Theory valid for low fluctuation intensity flows shows that the measured spectrum is filtered at the mean sample rate and that it contains a filtered white noise spectrum caused by the steps in the sample and hold signal In the limit of high data density, the step noise vanishes and the sample and hold signal is statistically unbiased for any turbulence intensity

Turbulent thermal convection in wide horizontal fluid layers

Abstract: Turbulence in thermal convection is investigated for flows in which the production of turbulence energy is due solely to buoyancy, and the statistics of the flow are homogeneous in horizontal planes. New experimental results for high Rayleigh number unsteady turbulent convection in a horizontal layer heated from below and insulated from above are presented and compared to turbulent Rayleigh convection, convection in the planetary boundary layer, and laboratory penetrative convection. Mean temperature fields are correlated in terms of wall layer scales and convection scales. Joint statistics of turbulent temperature and horizontal velocity and vertical velocity through fourth order are presented for the core region of the convection layer.

On the spread of rectangular jets

Abstract: Results are presented of some characteristic features of turbulent free jets issuing from rectangular exits having different geometries and small aspect ratios. Emphasis is placed on the measurement in the flow field near the exit. The measured quantities include mean velocity, mean temperature and stream-wise turbulence intensity. It is found that the growth and decay of the spanwise saddle-backed profile for temperature is different from that for velocity, and that the streamwise turbulence intensity distribution shows a remarkable change according to aspect ratio, exit shape and exit velocity.

Turbulent boundary layers on rough surfaces

Abstract: The influence of three-dimensional, stochastic roughness on the properties of turbulent boundary layers has been investigated experimentally in order to obtain an improved understanding and description of such effects on the boundary-layer flow over turbomachinery blading. Four rough surfaces were examined: a surface representative of newly-finished turbo-machinery blading, a variant of this surface with different roughness parameters, a sand-cast surface and a mesh surface with a deterministic roughness geometry. A skin-friction correlation has been obtained, and trends in this correlation and the roughness function caused by changes in roughness parameters have been identified.

Turbulence generation by electric body forces.

Abstract: The effect of an electric body force on vorticity production and turbulence generation in a gas is investigated by examination of the governing electrohydrodynamic equations. The theoretical concepts are illustrated by hot-film anemometer measurements of the electrically induced turbulence in a large scale electrostatic precipitator. The results indicate dramatic increases in turbulence and diffusivity due to the corona discharge and suggest that turbulence control in this application requires a modification in electrode geometry.

A study of a bluff-body combustor using laser sheet lighting

Abstract: Laser sheet lighting is used to study reacting flows with and without heat release in an axisymmetric, unducted and vertically mounted bluff-body combustor. The fuel, which is seeded with titanium tetrachloride vapor, is ejected from a jet located in the center of the bluff-body. The TiCl4 in the dry fuel reacts spontaneously with the water in the annulus air to form titanium dioxide particles. High speed movies and visual observations of vertically and horizontally located sheets of laser light provided remarkably detailed visualization (via Mie scattering) of the vortex dynamics in the near-wake region of the bluff-body.

On the measurement of wall shear stress

Abstract: The differential pressure reading from a static hole pair is utilized for determination of the local wall shear stress Both the hole diameter and forward-facing angle are varied to test the sensitivity of the device The static hole pair in tested in a two-dimensional zero pressure gradient turbulent boundary layer on a smooth surface The calibrating values for the local wall shear is determined from the universal scaling laws for the mean velocity profile in the inner part of the turbulent boundary layer The static hole pair is found to be sensitive to imperfections in the manufacturing process, and needs an individual calibration in order to make accurate measurements of the local skin friction possible

The flow regime in the turbulent near wake of a hemisphere

Abstract: The work presented is a wind tunnel study of the near wake region behind a hemisphere immersed in three different turbulent boundary layers. In particular, the effect of different boundary layer profiles on the generation and distribution of near wake vorticity and on the mean recirculation region is examined. Visualization of the flow around a hemisphere has been undertaken, using models in a water channel, in order to obtain qualitative information concerning the wake structure.

Effect of curvature on the turbulence of a two-dimensional jet

Abstract: An experimental study is presented which details the turbulence characteristics of a two dimensional offset jet. A single offset ratio (height of the nozzle above the solid parallel wall by nozzle width) of seven is used with a nozzle Reynolds number of 15,000. Measurements are presented in the preattachment jet region made with a dual frequency shifted laser Doppler anemometer. The relative strain rates resulting from curvature on the upper and lower sides of the jet are determined and their effects on the r. m. s. fluctuating velocity components are presented.

Near-field turbulence properties of single- and two-stream plane mixing layers

Abstract: Detailed mean flow and turbulence measurements have been made in the near-field of two plane mixing layers in air with a maximum velocity of 21 m/s. The experimental rig enabled mixing layers of velocity ratios 0 and 0.46 to be generated simultaneously. Cases with both tripped and untripped initial boundary layers were studied. In all cases, it was found that the two-stream layer developed to the self-preserving state in a distance much shorter than the single-stream layer, which followed accepted criteria for the development distance. The asymptotic levels of the turbulence quantities in the two-stream layer and the development of the single-stream layer showed agreement with existing data. The results suggest that the two-stream mixing layer should provide a better test case for the development of turbulence models and calculation methods than the single-stream mixing layer.

Measurements of single spherical particle trajectories with lateral migration in a slit with one porous wall under laminar flow conditions

Abstract: Lateral migration of spherical, neutrally buoyant particles moving in Poiseuille flow was measured in a slit with a porous membrane at one wall. Neutrally buoyant particles with diameters of 226 μm were injected into 22% glycerol-water solution flowing vertically in a slit channel (1.6 × 50 × 152 mm). The particles were illuminated with a strobe at 25 flashes/sec and photographed with a 4 × 5 camera under dark field conditions. Measured particle trajectories are compared with theoretically predicted trajectories based on Altena and Belfort's (1984) model. The theoretical trajectories are within the limits of error in the measured trajectories. By confirming the Altena and Belfort (1984) model within the range of experimental conditions tested here, inertial interactions should be included when modeling membrane fouling of dilute suspensions (Belfort et al. 1982).

Wall pressure and its relation to turbulent structure of a boundary layer

Abstract: Wall pressure fluctuations were measured in relation to the structure of coherent motions of a turbulent boundary layer. Their high and low frequency parts were found to be related with the large scale motions of the outer layer and the bursts in the wall region respectively. Based on the experimental results, a structure model of the coherent motions is presented.

Methods of measuring vorticity in turbulent flows

Abstract: Experimental methods which have been developed over the last three decades to measure one or more components of the vorticity vector are described and assessed. These include methods which estimate the velocity gradients comprising the various vorticity components, methods which sense fluid rotation directly, and methods which sense other fluid properties which can be related to vorticity.

The role of photographic parameters in laser speckle or particle image displacement velocimetry

Abstract: The parameters involved in obtaining the multiple exposure photographs in the laser speckle velocimetry method (to record the light scattering by the seeding particles) were optimized. The effects of the type, concentration, and dimensions of the tracer, the exposure conditions (time between exposures, exposure time, and number of exposures), and the sensitivity and resolution of the film on the quality of the final results were investigated, photographing an experimental flow behind an impulsively started circular cylinder. The velocity data were acquired by digital processing of Young's fringes, produced by point-by-point scanning of a photographic negative. Using the optimal photographing conditions, the errors involved in the estimation of the fringe angle and spacing were of the order of 1 percent for the spacing and +/1 deg for the fringe orientation. The resulting accuracy in the velocity was of the order of 2-3 percent of the maximum velocity in the field.

Measurements of two- and three-dimensional waves in a channel, including the vicinity of cut-off frequencies

Abstract: Measurements of water wave profiles were performed in a rectangular flume equipped with a modular wavemaker. This particular wavemaker could generate both two- and three-dimensional waves. A method is proposed to evaluate quantitatively the deviations of a spacial flow field from the two-dimensional one. Plane propagating waves, as well as pure sloshing waves with their crests parallel to the walls, were generated in the flume. In all cases the measured amplitudes were compared against linear theory predictions.

Rayleigh scattering for density measurements in premixed flames

Abstract: Rayleigh scattering measurements for molecular number density in turbulent, premixed CH4-air flames are discussed, and data for both flamelet passage time distributions and power spectral density functions are reported and compared to the recent predictions of Bray, Libby and Moss (1984). Measurement problems associated with variations in mixture-averaged Rayleigh scattering cross section, index of refraction fluctuations, finite spatial and temporal resolution and with scattering from particles are discussed. It is concluded that these effects are relatively minor in the reported experiments. Correction procedures are suggested for the effects of cross section variation and of finite resolution.

An experimental study of the lateral migration of a droplet in a creeping flow

Abstract: The distribution of droplets in a plane Hagen-Poiseuille flow of dilute suspensions has been measured by a special LDA technique. This method assumes a well defined relation between the velocity of the droplets and their lateral position in the channel. The measurements have shown that the droplet distribution is non-uniform and depends on the viscosity ratio between the droplets and the carrier liquid. The results have been compared with a theory by Chan and Leal describing the lateral migration of suspended droplets.

An LDV system for turbulence length scale measurements

Abstract: An LDV system for making spatial correlation measurements of velocity fluctuations in turbulent nonreacting and reacting flows is presented. The LDV system is the dual beam type and consists of an elongated probe volume and a two-point optical fiber detector. Results are presented of the integral length scale measured in a nonreacting grid generated turbulent flow.

Streamwise pseudo-vortical structures and associated vorticity in the near-wall region of a wall-bounded turbulent shear flow

Abstract: Streamwise pseudo-vortical motions near the wall in a fully-developed two-dimensional turbulent channel flow are clearly visualized in the plane perpendicular to the flow direction by a sophisticated hydrogen-bubble technique. This technique utilizes partially insulated fine wires, which generate hydrogen-bubble clusters at several distances from the wall. These flow visualizations also supply quantitative data on two instantaneous velocity components, ν and w, as well as the streamwise vorticity, ω x . The vorticity field thus obtained shows quasi-periodicity in the spanwise direction and also a double-layer structure near the wall, both of which are qualitatively in good agreement with a pseudo-vortical motion model of the viscous wall-region.

Visualization of immiscible displacement in a three-dimensional transparent porous medium

Abstract: Visualisation du deplacement d'huile dans une solution aqueuse dans un assemblage transparent de particules de quartz. La structure et la distribution des trois phases est vue par lasers differentes couleurs de fluorescence en lumiere laser

Concentration distributions in a model combustor

Abstract: A hot-wire concentration probe with a spatial resolution of 0.13 mm is used to measure concentration in a model cylindrical combustor. The flow inside the combustor is simulated by injecting a helium jet into a cylindrical confinement with or without swirling air flow present. Mean concentrations are essentially zero outside of the jet region, indicating complete confinement of the scalar field by the swirling flow. Consequently, concentration fluctuations are found to be relatively weak compared to velocity fluctuations, and are maximum off-axis at a point which corresponds to the interface between helium and air flows. However, in the absence of a swirling air flow, the helium diffuses quickly to fill the combustor. The resulting helium concentrations are constant and do not resemble the jet-like behavior of the velocity field.

Aerodynamic force measurements over missile configurations in IISc shock tunnel at M ∞ = 5.5

Abstract: On utilise un systeme d'equilibrage de force a accelerometre pour mesurer la portance, la trainee et les coefficients de quantite de mouvement. Comparaison avec des resultats theoriques

Space and space-time longitudinal velocity correlations in the turbulent far wake of a flat plate in incompressible flow

Abstract: This paper presents longitudinal space and space-time conventional velocity correlations measured in the wake of a two-dimensional flat-plate at zero incidence, with fully-turbulent boundary layers at its sharp trailing edge. Iso-correlation contour plots are given in the plane of symmetry and normal to this plane in the assumed asymptotic (in the mean velocity sense) part. Comparisons with initially laminar 2-D wake results show that the classical Double-Roller-Eddy model developed for initially laminar wakes is not adapted to initially turbulent wakes; on the other hand, comparison with supersonic wake of the same nature seems to confirm this conclusion. The lack of detailed experimental data in this field is outlined, together with the need of a refined theoretical approach to the coherent structures existing in such flows.

Optimization of the injection process for drag-reducing additives

Abstract: The primary objective of this experimental study was to determine the optimum combination of additive concentration, additive flowrate, injector angle and injector width for injecting a drag-reducing additive into a channel flow of water. The experiments were designed to keep an effective concentration of the additive in the buffer region where previous experiments have shown the additives directly affect the turbulent structures. Flow visualization of the wall-layer structures was conducted for the optimum combination of injection variables at the streamwise locations where the drag reduction peaked and where the additive became fully mixed with the channel flow of water.

Near-wall velocity distributions within a straight conical diffuser

Abstract: The measured mean velocity profiles at the various stations along a conical diffuser (8° total divergence angle) were found to consist of log regions, half-power law regions and linear regions. The describing coefficients for the inner half-power law region (which followed a rather narrow log region) differed from the standard values due to the axi-symmetric geometry and lack of moving equilibrium of the flow as it attempted to adjust to a varying adverse pressure gradient. However, these coefficients (like those for the linear region) correlated with the local wall shear stress and the kinematic pressure gradient.

Drag reducing outer-layer devices in rough wall turbulent boundary layers

Abstract: The ability of outer-layer devices to reduce wall shear stress over a substantial streamwise distance in rough-wall turbulent boundary layers has been studied experimentally. The devices examined are a pair of thin flat ribbons placed in tandem as well as those having symmetric airfoil sections. The wall conditions examined are smooth, d- and k-type transverse-groove and sandgrain roughnesses. The wall drag is found to be reduced from the respective normal levels in all rough walls. All k-type rough walls exhibit a similar level of relative wall drag reduction which is also smaller than that in a smooth-wall. The d-type rough walls exhibit a transitional behaviour — the relative wall drag reduction drops from the smooth wall level to that of the k-type roughness with increasing roughness Reynolds number. However, the absolute reductions in the local wall shear stress are similar in both the rough and smooth walls. On the other hand, the relative reductions are lower in the rough walls because of a higher reference drag which is caused by the unique presence of a pressure component on which the devices are not as effective.

Experimental studies of turbulence in mercury flows with transverse magnetic fields

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

Verification of LDA and seed generator performance

Abstract: An essential step in the application of laser-Doppler anemometry (LDA) to turbulent flow research is to verify the capability of the measurement system to respond adequately to the anticipated velocity fluctuations. Problems of seed particle velocity fidelity can arise due to inadequate detectability of micron-size seed particles by the LDA system or improper seeding techniques. As a result of the present investigation, we caution that reported LDA measurements in piston engines could be in error. A convenient method to check for the occurrence of these problems is described.

Thermofluiddynamic experiments with a heated and rotating circular cylinder in crossflow: Part 2.1: Boundary layer profiles and location of separation points

Abstract: Two-dimensional boundary layer profiles of heated and rotating circular cylinders in crossflow were investigated for subcritical freestream-Reynolds-numbersRe = 3.4 · 104 andRe = 4.8 · 104. The peripheral speed of the cylinder surface corresponds to velocity ratios 0 ≦ ξ ≦ 2(α, circular/freestream-velocity). Special attention was focused on the location of separation points, which was determined as a function of the velocity ratio a whereby a strong influence of wall temperature was revealed.

Velocity measurements in the field of an internal gravity wave by means of speckle photography

Abstract: Speckle velocimetry with forward scattering has been applied to measure and visualize the two-dimensional velocity field in an internal gravity wave. The wave was produced by towing a cylinder in vertical direction, normal to its axis, through stratified salt water. Neutrally buoyant tracer particles whose density was matched with the density distribution of the stratification were uniformly distributed in the test fluid. The experimental results verify the results of a linear theory in the far field of the wave.