Turbulence measurements around a body in a re-circulating water channel by a hot film probe

Measurements were made in the stern boundary layers and near wakes of an elliptic cylinder and a slender ship model. Turbulence intensities, Reynolds stresses, kinematic eddy viscosities and mixing lengths are presented. For the elliptic cylinder, furthermore, auto-correlation and power spectrum are obtained. It is shown that the separation from the cylinder increases the turbulence intensities, and the Kármán vortices enhance the turbulence power at the vortex frequency. All distributions of Reynolds stresses in the thick boundary layer and wake of the ship model show a secondary low peak at about half the thickness.     

Investigation of the stability of a heavy fluid film in a hot heat-conducting gas jet on an inclined phase-transition surface

The stability of the flow of a heavy viscous fluid film flowing along the inclined phase-transition surface is examined. In contrast to [1] wherein it was assumed that a constant temperature is maintained on the free surface, it is assumed here that the fluid film is on the boundary with a gas jet which has finite specific heat and heat conduction. In this connection, the stability criteria differ substantially from [1].Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 4, pp. 10–18, July–August, 1974.     

Effect of ventilation on the flowfield around a sphere

The flowfield around a sphere with and without ventilation was investigated in a wind tunnel over a range of Reynolds numbers in an incompressible flow. At supercritical Re, the pressure drag of a sphere can be nearly nullified by venting only 2% of the frontal area of the sphere to the base through a smooth internal duct. The drag reduction is achieved by increased pressures in the separated flow region close to the base. At high Re, the vent flow breaks through the near wake and brings about symmetry in the global flowfield. When the internal shear is increased by using a rough internal duct, the base pressure is unchanged, but the external flow is accelerated to velocities beyond that achieved by the potential flow around the basic sphere. The findings can be explained by a flow model in which the near wake is aerodynamically streamlined by a pair of counterrotating vortex rings at the base. A roughness element can be made to partially destroy the vortex system at the base and result in a steady asymmetric wake. A 1.2 mm diameter wire placed at 70° was found to overtrip the boundary layer and completely destroy the vortex system. Simultaneously, the turbulent separation is advanced and the drag increased.At subcritical Re, ventilation marginally increases static pressures all over the surface. Since the large pressure differential between the windward and leeward sides is not reduced, the internal flow has a rapid acceleration to a velocity close to that of the free stream. The reverse flow associated with the near wake forces the vent flow to rest within itself and the wake profile is unchanged. The main features of subcritical flow around the basic sphere are retained in spite of ventilation. The upstream effects of ventilation are greater for subcritical flow than for supercritical flow.The work reported was carried out under a study grant from the German Academic Exchange Service (DAAD) in Bonn. The authors wish to thank the Director of DAAD in Bonn for the same. Thanks are due to Dr. F. R. Grosche and colleagues at DLR in Göttingen who assisted in the design, fabrication and wind tunnel testing of the sphere model. Thanks are also due to Prof. D. G. Mabey, visiting Professor, Imperial College, London for useful discussions. The many useful discussions with the research advisors of the first author viz., Dr. P. R. Viswanath of National Aerospace Laboratories and Prof. A. Prabhu of Indian Institute of Science, Bangalore are acknowledged with thanks. The support given by the Head, Experimental Aerodynamics Division, National Aerospace Laboratories is thankfully acknowledged.     

Study of the thermocapillary effect on a wavy falling film using a fiber optical thickness probe

The wave flow of a water film down a vertical plate with a 150×150 mm heater has been experimentally studied. The effect of the heat flux on the film flow leads to the formation of periodically flowing rivulets and thin film between them due to the action of thermocapillary forces in spanwise direction. The local film thickness between rivulets is measured by means of a noncontact fiber optical probe. As the heat flux grows, the average film thickness continuously decreases but upon reaching about 50% of the initial thickness, the film spontaneously breaks down. It is found that the decrease of the wave amplitude between rivulets is caused by the reduction of the local Reynolds number and is in a qualitative agreement with the laws of the hydrodynamics for the isothermal case. That is, no appreciable effect of streamwise thermocapillary forces on the wave amplitudes is detected. The experimental results are in good agreement with recently published data obtained by the capacitance method.     

Measurement of locally heated liquid film thickness by a double-fiber optical probe

An experimental investigation of thermocapillary deformations in a film of 10% ethyl alcohol solution in water, flowing down a plate with a heater of length 6.7 mm and width 68 mm, is performed. Heating of the film results in the formation of a horizontal liquid bump at the top edge of the heater. On the heater the flow divides into vertical rivulets with a thin film between them. Film deformations in the bump and the thin film between the rivulets are investigated. Local film thickness is measured by means of a double-fiber optical probe. The method is based on the dependence of the intensity of reflected light on the distance between the probe and the reflecting surface. The measurement results are compared to those previously obtained using the schlieren method. The experiment is controlled by three parameters. They are, with their respective values, the plate inclination angle (4–90°), the Reynolds number (0.15–62) and the heat flux density (0–4.5 W/cm²).     

Experimental evaluation of pneumatic atomizers by hot film

The spray characteristics of pneumatic atomizers are examined using hot film anemometry. Measurements of drop size distribution, concentration profile and drop frequency are carried out over wide ranges of relative position between air peripheral jet exit and the concentric water nozzle exit. The objective is to clarify the role of turbulence in producing a wide spectrum of particle sizes and numbers in the spray. The output signal from hot film contained multispikes with different amplitudes and thickness which has been correlated to the drop number density and the droplet size through the use of probability density function. Generally it is found that an increase of the guide relative position enhances the mixing process and produces finer spray. This is attributed to the utilization of high wave number eddies in shortening the liquid jet break up length.

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Versuchsauswertung eines pneumatischen Pulverisators durch einen Heißfilm

Zusammenfassung Die Spraykennzeichen eines pneumatischen Pulverisators sind mit der Heißfilmanemometrie untersucht worden. Es wurden Messungen über die Tropfengrößenverteilung, Konzentrationsprofile und Tropffrequenz über weite Bereiche der relativen Position von äußerem Luftstromausgang und dem konzentrischen Wasserdüsenausgang durchgeführt. Das Ziel ist die Funktionsklärung der Turbulenzen in der Produktion von einem weiten Spektrum der Teilchengrößen und Teilchenzahlen im Spray. Das Ausgangssignal vom Heißfilm beinhaltet mehrere Impulsspitzen mit verschiedenen Amplituden und Breiten. Diese wurden für die Tropfenzahldichte und Tröpfchengröße mit der Wahrscheinlichkeitsdichte-Funktion berechnet. Im allgemeinen wurde herausgefunden, daß ein Erhöhen der relativen Position des Führungsrohres den Mischprozeß verbessert und einen feineren Spray ergibt. Dies ist darauf zurückzuführen, daß hochfrequente Wirbel die Abbruchlänge des Flüssigkeitsstroms verkürzen.

     

Downstream boundary effects on the spectral characteristics of a swirling flowfield

The spectral characteristics and the structural response of a swirling flowfield are investigated subject to a non-axisymmetric disturbance and a contraction imposed downstream. Two natural frequencies are noted in different regions of the undisturbed swirling flowfield, one is due to a precessing vortex core and the other to the most amplified downstream azimuthal instability. The downstream contraction usually causes compression of the central recirculation zone into two side-lobes, increases the dominant frequencies and forms a straight central vortex core with a high axial velocity. The dominant downstream instability frequency depends linearly on the inlet Reynolds number and on the mode of the breakdown. For the downstream non-axisymmetric disturbance, such as the passing of the turbine blades, the fundamental frequency is not altered by the disturbance and the oscillation strength of the downstream instability is greatly reduced as the excitation frequency remains unmatched with the dominant downstream natural frequency. Downstream azimuthal instability promotes the breakdown recirculation.A version of this paper was presented at the 26th AIAA Aerospace Sciences Meeting, Reno, Nevada, 11–14, Jan. 1988     

A comparison between spray cooling and film flow cooling during the rewetting of a hot surface

The paper is dealing with a research carried out at the Institute of Thermal-Fluid Dynamics to investigate the rewetting of a hot surface. The rewetting of the hot surface by spray cooling has been analyzed in previous works. After the droplet impingement, the liquid film falls along the surface, and rewetting by falling film takes place. The experiment was characterized by a 1-dimensional liquid spray, i.e., drops having a uniform, constant diameter, impinging on the heated surface. The cooling rate of the hot surface has been detected as a function of wall temperature, drop diameter and velocity, and impact point of the spray. The working feature of the spray is based on the varicose rupture of the liquid jet: imposing a periodic (symmetrical) perturbation with appropriate amplitude and frequency on the jet surface, the flow is “constrained” to break soon after leaving the nozzle, eventually obtaining constant diameter drops, depending on the nozzle diameter and liquid velocity. In this paper, previous results with spray cooling are compared with experimental runs in which the spray injection is replaced with a falling film all along the test section. The rewetting velocity has been calculated from the response of the thermocouples placed on the heated wall and using a digital image system based on the video image registered during the runs.     

The visualization of the flowfield about a delta wing with spanwise blowing

This paper describes a nonintrusive method for the visualization of the flow about a delta wing with spanwise blowing jets, based on the schlieren technique. The effects of the jet/leading-edge vortex interference are visualized by using both air and helium for the jets. The visualization of the leading-edge vortex trajectories and their breakdown, as well as the influence of the jets on them is achieved by spanwise blowing of air. The visualization of the jets' paths and the effects of the leading-edge vortices on these paths is achieved by spanwise blowing of helium.     

Theoretical study of the flowfield inside a hydrocyclone for higher Reynolds numbers

The axi-symmetric turbulent boundary layers formed on the conical surface and under the roof of different hydrocyclones have been studied for higher Reynolds numbers. Calculating the total flux through the boundary layers it has been found that the mathematical model for the flowfield, presented in a previous paper [2] for the 1/7th power law velocity distribution, holds even for higher Reynolds numbers and for different cyclones with vortex finder diameters greater than that of apex opening. Theoretical results agree well with previous experimental findings.A part of this work was performed in the Mining Engineering Department of Delft University of Technology, The Netherlands.     

Investigation on a coupled Navier–Stokes–Direct Simulation Monte Carlo method for the simulation of plume flowfield of a conical nozzle

To assess the plume effects of space thrusters, the accurate plume flowfield is indispensable. The plume flow of thrusters involves both continuum and rarefied flow regimes. Coupled Navier–Stokes–Direct Simulation Monte Carlo (NS–DSMC) method is a major approach to the simulation of continuum‐rarefied flows. An axisymmetric coupled NS–DSMC solver, possessing adaptive‐interface and two‐way coupling features, is investigated in this paper for the simulation of the nozzle and plume flows of thrusters. The state‐based coupling scheme is adopted, and the gradient local Knudsen number is used to indicate the breakdown of continuum solver. The nitrogen flows in a conical nozzle and its plume are chosen as the reference case to test the coupled solver. The threshold value of the continuum breakdown parameter is studied based on both theoretical kinetic velocity sampling and coupled numerical tests. Succeeding comparisons between coupled and full DSMC results demonstrate their conformities, meanwhile, the former saves 58.8% computational time. The pitot pressure evaluated from the coupled simulation result is compared with the experimental data proposed in literatures, revealing that the coupled method makes precise predictions on the experimental pitot pressure. Copyright © 2014 John Wiley & Sons, Ltd.     

Investigation of temperature distribution in a piecewise-homogeneous seam with pressed-in hot fluid

Temperature distribution in a piecewise-homogeneous finiteseam exposed to hot fluid and the effect of inhomogeneity in the permeability and thermal properties of separate zones of a seam on the redistribution of temperature fields for flat parallel or axial flow of a pressed-in hot liquid are studied. The differential equations which describe the process of temperature distribution in accordance with [1] are solved for various initial and boundary conditions. Exact analytic formulas are obtained which are useful in numerical computations. The problem under consideration is related to important engineering problems in hydrology, geothermy, as well as in the development of oil or gas fields [2–4, 5].     

Investigation of the effect of the Coriolis force on a thin fluid film on a rotating disk

The effect of the Coriolis force on the evolution of a thin film of Newtonian fluid on a rotating disk is investigated. The thin-film approximation is made in which inertia terms in the Navier–Stokes equation are neglected. This requires that the thickness of the thin film be less than the thickness of the Ekman boundary layer in a rotating fluid of the same kinematic viscosity. A new first-order quasi-linear partial differential equation for the thickness of the thin film, which describes viscous, centrifugal and Coriolis-force effects, is derived. It extends an equation due to Emslie et al. [J. Appl. Phys. 29, 858 (1958)] which was obtained neglecting the Coriolis force. The problem is formulated as a Cauchy initial-value problem. As time increases the surface profile flattens and, if the initial profile is sufficiently negative, it develops a breaking wave. Numerical solutions of the new equation, obtained by integrating along its characteristic curves, are compared with analytical solutions of the equation of Emslie et al. to determine the effect of the Coriolis force on the surface flattening, the wave breaking and the streamlines when inertia terms are neglected.     

Investigation of nonstationary heat transfer when a hot turbulent jet interacts with a shield

An experimental and theoretical investigation has been made of nonsteady heat transfer between a shield and a hot turbulent jet impinging normally on it. A system of dimensionless parameters is found for simulating the nonsteady interaction of high-temperature jets with shields. A system of thermocouples was used to measure the spatial and temporal distributions of the temperature in a gas above a shield, and also on the surface and within the shield. A numerical investigation was made of the nonsteady heat transfer in a shield with boundary condition specified in the form of the experimental temperature distribution of the front surface of the shield and with allowance for convective and radiative loss of heat from the other surfaces of the shield. The obtained solution is used to find the characteristics of nonsteady heat transfer (the coefficient of heat transfer and the Stanton number) on the front surface of the shield. These characteristics are used to solve the problem of periodic exposure of a shield to a heat flow produced by the switching on of a high-temperature jet.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 2, pp. 130–138, March–April, 1979.     

Investigation of a dense,chemically reacting plasma by the high-frequency probe potential modulation method

A method is proposed for investigating a chemically reacting plasma by means of an electric probe whose potential is modulated by a high-frequency sinusoidal voltage. The method is based on the use of a numerical solution of the problem of an electric probe introduced under negative potential into a steady-state low-temperature plasma formed in a mixture of chemically reacting molecular gases. The conditions under which the charged particle concentration in the region undisturbed by the probe is constant as a result of equilibrium between the ionization and recombination rates are examined.Translated from Izvestiya Akademii Nauk SSSR, Mekhanika Zhidkosti i Gaza, No. 6, pp. 114–119, November–December, 1987.     

Investigation of high-speed combustible gas ignited by a hot gas jetproduced in the shock tube

The high-speed combustible gas ignited by a hot gas jet, which is induced by shock focusing, was experimentally investigated. By use of the separation mode of shock tube, the test section of a single shock tube is split into two parts, which provide the high-speed flow of combustible gas and pilot flame of hot gas jet, respectively. In the interface of two parts of test sections the flame of jet was formed and spread to the high-speed combustible gas. Two kinds of the ignitions, 3-D “line-flame ignition” and 2-D “plane-flame ignition”, were investigated. In the condition of 3-D “line-flame ignition” of combustion, thicker hot gas jet than pure air jet, was observed in schlieren photos. In the condition of 2-D “plane-flame ignition” of combustion, the delay time of ignition and the angle of flame front in schlieren photos were measured, from which the velocity of flame propagation in the high-speed combustible gas is estimated in the range of 30–90m/s and the delay time of ignition is estimated in the range of 0.12–0.29ms. PACS 47.40.Nm; 82.40.FpPart of this paper was presented at the 5th International Workshop on Shock/Vortex Interaction, Kaohsiung, October 27–31, 2003.     

Numerical analysis of a flowfield produced by a pair of rectilinear vortices approaching a circular cylinder

Two kinds of discrete vortex methods and a direct method to solve the Navier-Stokes equations were carried out for the flow-field induced by a pair of rectilinear vortices approaching a circular cylinder. The major features of the flowfield observed by smoke visualization, i.e., the large scale secondary vortices and the subsequent rebounding trajectories of the primary vortices were well simulated by both numerical methods.     

Flow development upstream of a fence

The effect of Reynolds number on the flow development upstream of a rigid, non-porous, static fence is investigated experimentally. The flow field is measured using time-resolved, two-component particle image velocimetry at Reynolds numbers based on fence height of 18000, 36000, and 54000. The results show that a laminar separation bubble forms upstream of the junction vortex at the base of the fence. The mean extent of the bubble decreases with increasing Reynolds number, with mean separation moving downstream and mean reattachment moving upstream. In the aft portion of the bubble, shear layer vortices form and are shed at scaled frequencies and wavelengths that are comparable to laminar separation bubble shedding in low Reynolds number airfoils and flat plates with an imposed adverse pressure gradient. The strong periodicity of the associated coherent structures and the proximity of shear layer roll-up relative to the fence should be taken into consideration in the relevant designs due to potential implications to structural loading. A simple flow separation prediction model combining inviscid fence flow solution with Thwaites’ method is introduced and shows good agreement with the experimental results for the Reynolds number range considered.