Flow measurement on an oscillating pipe flow near the entrance using the UVP method

 The authors have carried out a study to investigate and clarify the characteristics of purely oscillating pipe flows over the developing region. The main objective of this study is to establish the method of time-dependent velocity profiles obtained by the ultrasonic velocity profile (UVP) measurement method. First, the relationship between the test fluids and the microparticles, as reflectors of ultrasonic pulses, was investigated. In addition, the relationship between the sound speeds of the test fluids and the wall materials was studied. Second, the UVP was used to obtain the instantaneous velocity profiles in oscillating pipe flows, and the developing characteristics of the flows were analyzed. Finally, the “entrance length” (by analogy with a unidirectional pipe flow) required for oscillating pipe flows was analyzed by examining the amplitude of the harmonic spectral components of the oscillating frequency. A fast Fourier transform (FFT) is proposed as the applicable method to estimate the “entrance length”. From the Fourier transform of the velocity on the centerline, nonlinear oscillation of fluid occurs in the “entrance length” of the oscillating flows, and the viscous dissipation of the higher-order velocity harmoncis determines the entrance region. The “entrance length” can be obtained from the dissipation length of the third-order harmonic. These results prove that the UVP method is highly applicable to carry out the flow measurement in the “entrance length” of oscillating pipe flow. Received: 20 March 2000 / Accepted: 10 August 2001     

Measurements of mean and fluctuating wall shear stress beneath spanwise-invariant separation bubbles

 Pulsed-wire measurements of wall shear stress have been made beneath two separation bubbles. In one a cross flow was generated by means of a (25°) swept separation line. Fluctuating stresses in orthogonal “streamwise” and cross-flow directions are very nearly equal and independent of at least moderate cross flow velocity. These fluctuations are largely determined by large-scale motions in the outer flow, whereas the mean shear stresses are not. The pdf of the “streamwise” fluctuations is unchanged by the cross flow. When a cross flow is present the pdf of the cross-flow stresses is similar to the “streamwise” pdf. Dependence on Reynolds number is the same in both flows. Received: 10 April 1998/Accepted: 17 July 1998     

Effects of Periodic Excitation on the Flow Around a D-Shaped Cylinder at Low Reynolds Numbers

The baseline and forced flow around a bluff body with semi-elliptical D-shape was investigated by solving the 2D Navier–Stokes equations at low Reynolds numbers. A D-shape rather than the canonic circular-cylinder was selected due to the fixed separation points in the latter, enabling to study a pure wake rather than boundary-layer control. The correlation between Strouhal and Reynolds numbers, the mean drag, the lift and drag oscillations vs. the Reynolds number and wake structure were investigated and compared to experimental and numerical data. Effects of open-loop forcing, resulting from the influence of zero-mass-flux actuators located at the fixed separation points, were studied at a Reynolds number of 150. Fluidic rather than body motion or volume forcing was selected due to applicability considerations. The motivation for the study was to quantify the changes in the flow field features, as captured by Proper Orthogonal Decomposition (POD) analysis, due to open-loop forcing, inside and outside the “lock-in” regime. This is done in order to evaluate the suitability of low-order-models based on POD modes of this changing flow field, for future feed-back flow control studies. The evolution of the natural and the excited vortices in the Kármán wake were also investigated. The formation and convection regions of the vortex evolution were documented. It was found that the forcing causes an earlier detachment of the vortices from the boundary-layers, but does not affect their circulation or convection speeds. The results of the POD analysis of the near-wake flow show that the influence of the bluff body shape (“D”-shaped versus circular cylinder) on the baseline POD wake modes is small. It was found that the eigenfunctions (mode-shapes) of the POD velocity modes are less sensitive to slot excitation than the vorticity modes. As a result of the open-loop excitation, two types of mode-shape-change were observed: a mode can be exchanged with a lower-energy mode or shifted to a low energy level. In the latter case, the most energetic mode becomes the “actuator” mode. The evolution of one-slot excitation on still fluid (“Synthetic jet”) was studied and compared to published data and to “actuator” modes with external flow present. Based on the current findings, it is hypothesized that the cross-flow velocity POD modes are suitable for feedback control of wake flow using periodic excitation, due to their low sensitivity to the excitation as compared to the streamwise velocity or vorticity modes.     

Ideal incompressible flow around a wedge tip

A planar analog of conical flows is considered: an inviscid incompressible fluid flow around a wedge tip. A class of conical flows is found where vorticity is transported along streamlines by the potential component of velocity. Problems of a wave “locked” in the corner region and of a flow accelerating along the rib of a dihedral angle are considered. By analogy with an axisymmetric quasi-conical flow, a planar quasi-conical flow of the fluid is determined, namely, the flow inside and outside the region bounded by tangent curves described by a power law. Conditions are found where vorticity and swirl produce a significant effect. An approximate solution of the problem of the fluid flow inside a “zero” angle is obtained. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 48, No. 2, pp. 57–65, March–April, 2007.     

Shock wave structures ahead of nonuniform fan cascades

We study shock wave structures (SWS), consisting of shock waves and expansion waves between them, that occur in supersonic flow past nonuniform fan cascades when the velocity component normal to their front (“axial” component) is subsonic. The cascade nonuniformity is due to the scatter in the setting angles of identical blades, either sharp or blunt. A result of the uniformity is the generation of combined noise, whose frequencies are much smaller than the fundamental frequency of the uniform cascade, and slower nonlinear SWS attenuation. The accurate and fast “simple wave method” and “nonlinear acoustics approximation”, together with numerical algorithms for integrating Euler equations on overlapping grids (in calculating flow past blunt edges) and on SWS-adapted grids, are applied to determine the “guiding” action of nonuniform cascades and to describe the SWS evolution. The application of the Fourier analysis gives the sound field spectrum. The use of blades with rectilinear initial regions of the “backs” for reducing supersonic fan blade noise is efficient only at small (less than 0.25°) scatter in the setting angles. The shock wave structures attenuate more rapidly ahead of nonuniform cascades composed of blunt blades than ahead of those with sharp blades. For uniform cascades the blade bluntness effect is not large.     

Numerical simulation of 3-D turbulent flows over dredged trenches

A 3-D free surface flow in open channels based on the Reynolds equations with thek-ε turbulence closure model is presented in this paper. Insted of the “rigid lid” approximation, the solution of the free surface equation is implemented in the velocity—pressure iterative procedure on the basis of the conventional SIMPLE method. This model was used to compute the flow in rectangular channels with trenches dredged across the bottom. The velocity, eddy viscosity coefficient, turbulent shear stress, turbulent kinetic energy and elevation of the free surface can be obtained. The computed results are in good agreement with previous experimental data.     

Gas-dynamic signs of explosive eruptions of volcanoes. 2. Model of homogeneous-heterogeneous nucleation. Specific features of destruction of the cavitating magma

The dynamics of state of the crystallite-containing magma is studied within the framework of the gas-dynamic model of bubble cavitation. The effect of crystallites on flow evolution is considered for two cases: where the crystallites are cavitation nuclei (homogeneous-heterogeneous nucleation model) and where large clusters of crystallites are formed in the magma in the period between eruptions. In the first case, decompression jumps are demonstrated to arise as early as in the wave precursor; the intensity of these jumps turns out to be sufficient to form a series of discrete zones of nucleation ahead of the front of the main decompression wave. Results of experimental modeling of an explosive eruption with ejection of crystallite clusters (magmatic “bombs”) suggest that a cocurrent flow of the cavitating magma with dynamically varying properties (mean density and viscosity) transforms to an independent unsteady flow whose velocity is greater than the magma flow velocity. Experimental results on modeling the flow structure during the eruption show that coalescence of bubbles in the flow leads to the formation of spatial “slugs” consisting of the gas and particles. This process is analyzed within a combined nucleation model including the two-phase Iordansky-Kogarko-van Wijngaarden model and the model of the “frozen” field of mass velocities in the cavitation zone. __________ Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 50, No. 2, pp. 167–177, March–April, 2009.     

Modeling of jet flows of a viscous fluid by the discrete vortex method

Results obtained previously by the discrete vortex method with a “viscous” correction are generalized. The boundaries of applicability of this method are determined. Previous results obtained for a flow past a flat plate are supplemented with solution convergence estimates. Exhaustion of a plane jet of a viscous incompressible fluid into the ambient space is modeled. The geometric parameters of the jet (its half-width, shapes of the streamwise velocity profiles, and intensity of oscillations) are analyzed. The calculated results are found to agree well with experimental data and with results calculated by other methods.     

Forced nonlinear oscillator with nonsymmetric dry friction

In this paper we discuss an approximately steady motion of an oscillator as a single whole with a constant “on the average” velocity. For that purpose we analyze the position and stability of some special points of the phase portrait. In the presence of internal excitation and nonsymmetric Coulomb dry friction, a motion of the oscillator with a constant “on the average” velocity is possible. The algebraic equation for this constant velocity is found. For different parameters of the model there exist at most three regimes of motion with a constant velocity, but only one or two of them are stable. The theoretical results obtained can be used for the design of worm-like moving robots.     

Motion of two interconnected mass points under action of non-symmetric viscous friction

This paper deals with the analysis of a one-dimensional motion of two mass points in a resistive medium. The force of resistance is described by small non-symmetric viscous friction acting on each mass point. The magnitude of this force depends on the direction of motion. The mass points are interconnected with a kinematic constraint or with an elastic element. Using the averaging method the expressions for the stationary “on the average” velocity of the systems’s motion as a single whole is found. In case of a small degree of non-symmetry an explicit expression for the stationary “on the average” velocity of the system is derived. For the other case we obtained algebraic equations for the corresponding stationary velocity.     

Studies on nonlinear stability of three-dimensional H-type disturbance

The three-dimensional H-type nonlinear evolution process for the problem of boundary layer stability is studied by using a newly developed method called parabolic stability equations (PSE). The key initial conditions for sub-harmonic disturbances are obtained by means of the secondaryinstability theory. The initial solutions of two-dimensional harmonic waves are expressed in Landau expansions. The numerical techniques developed in this paper, including the higher order spectrum method and the more effective algebraic mapping for dealing with the problem of an infinite region, increase the numerical accuracy and the rate of convergence greatly. With the predictor-corrector approach in the marching procedure, the normalization, which is very important for PSE method, is satisfied and the stability of the numerical calculation can be assured. The effects of different pressure gradients, including the favorable and adverse pressure gradients of the basic flow, on the “H-type“ evolution are studied in detail. The results of the three-dimensional nonlinear “H-type“ evolution are given accurately and show good agreement with the data of the experiment and the results of the DNS from the curves of the amplitude variation, disturbance velocity profile and the evolution of velocity.     

New insights into particle image velocimetry data using fuzzy-logic-based correlation/particle tracking processing

Digital particle image velocimetry (DPIV) data processing has been developed to the point where DPIV image data are processed via auto- or cross-correlation techniques in near real time and the results are displayed on screen as they are processed. Correlation techniques are highly desirable, since they provide velocity measurements on a regular grid, which are readily comparable to CFD predictions of the flow field. In high-speed flows, particle lag effects are always of concern; however, the correlation operation does not provide any means for minimization or elimination of systematic errors in the recorded particle image data. In this paper, we present a combined correlation processing/particle tracking technique providing “super-resolution” velocity measurements. Fuzzy-logic principles are employed to maximize the information recovery in the correlation operation and to determine the correct particle pairings in the tracking operation. The combined correlation/particle tracking technique is applied to DPIV data obtained in the diffuser region of a high-speed centrifugal compressor producing velocity vector maps with an average density of 6 vectors/mm². Inspection of the particle tracking results revealed large particles that were not following the flow. Using preknowledge of the flow field, the biased velocity estimates arising from large particles in the flow were removed, thereby improving the accuracy of the measurements. Received: 21 October 1999/Accepted: 19 August 2000     

Molecular Tagging Velocimetry (MTV) measurements in gas phase flows

 Recent developments in Molecular Tagging Velocimetry (MTV) using the phosphorescence of biacetyl are described for gas-phase flows. With improvements in tagging, detection, and processing schemes, whole-field measurements of two components of the velocity vector are obtained simultaneously, typically at more than 300 points over a plane. Application of this measurement approach is demonstrated in mapping the velocity and vorticity fields of the intake flow into a “steady flow rig” model of an internal combustion engine. Received: 29 August 1997/Accepted: 16 September 1998     

Development of an X-ray computed tomography (CT) system with sparse sources: application to three-phase pipe flow visualization

This paper describes the application of a two-beam X-ray computed tomography (CT) system to multiphase (gas–oil–water) flow measurement. Two high-voltage (160 keV) X-ray sources are used to penetrate a 4-in. (101.6 mm ID) pipeline. A rotating filter wheel mechanism is employed to alternately “harden” and “soften” the X-ray spectra to provide discrimination between the three phases. Because this system offers only two projections, conventional back-projection algorithms are ineffective and thus a new reconstruction technique has been developed. A matrix equation is formed, to which additional “smoothing equations” are added to compensate for the lack of projection data. The tomographic result is obtained by computing an inverse matrix. This is a one-off computation and the inverse is stored for repeated use; reconstructed images from synthesized data demonstrate the effectiveness of this technique. Three-phase tomographic images of a horizontal slug flow are presented, which clearly show the mixing of oil and water layers within the slug body. The relevance of this work to the offshore oil and gas industry is summarized.     

Initial Conditions and Near-Field Dynamics in Turbulent Liquid Sheets

High-speed liquid “curtains” have been proposed to protect solid structures in fusion energy applications. Minimizing free-surface waves and spray formation in such flows is important for effective protection in this application. In this work, free-surface waves and turbulent breakup were studied experimentally in jets of water issuing from a rectangular nozzle into ambient air at a Reynolds number of 1.2 × 10⁵. Laser-Doppler anemometry was used to characterize the streamwise and transverse velocity components in the nozzle for two different flow calming section designs. Planar laser-induced fluorescence was used to measure the free-stream position in the near-field of the sheet. The results suggest that transverse velocity fluctuations in the nozzle are the primary factor in determining the amplitudes of free-surface waves. Removing a small amount of low-speed fluid immediately downstream of the nozzle exit (“boundary-layer cutting”) is shown to both significantly reduce free-surface waves and the amount of spray due to turbulent breakup. Overall, boundary-layer cutting appears to have the greatest benefit when used on a “well-conditioned” turbulent liquid sheet.     

Numerical simulation of aerosol sampling from an air flow to an input tube

The processes of sampling (aspiration) to an input tube of an aspiration probe from an ambient air flow are studied numerically. The air flow is simulated on the basis of three-dimensional Navier-Stokes equations for an incompressible fluid. The method proposed allows calculation of the aspiration efficiency in the case of rather complicated shapes of the limiting trajectories of the particles. Dependences of the aspiration efficiency on the mean velocity of suction of air into the tube and the size of particles for a given free-stream velocity are obtained. Research Institute of Aerobiology, State Research Center of Virusology and Biotechnology “Vector,” Novosibirsk Region, Kol’tsovo 633159. Institute of Computing Technologies, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 5, pp. 113–122, September–October, 1999.     

Imaging spectroscopy of the nonequilibrium shock front radiation in air

The nonequilibrium radiation of shock fronts in air is experimentally investigated by means of the imaging spectroscopy technique. Shock velocity ranges from 9.7 to 11.6 km/s and initial pressure from 13.3 to 41.6 Pa. The spectral diagnostic system consists of an imaging spectrograph, a streak camera, a gated image-intensified CCD camera and a personal computer for data acquisition/processing. This spectral diagnostic system is capable of simultaneous wavelength-, intensity- and time-resolved spectroscopic measurements in the nanosecond order. The image processing of the streak images includes a combined smoothing/deconvolution process in the time direction to diminish experimental noise effects and the temporal broadening due to the streak camera entrance slit. Wavelength range is chosen to investigate the first negative band of . “Large” and “slim” streak image types are observed. In the “large” streak images greater contribution from (1-)(1,0) behind the radiation peak is observed. Experimental data are compared with a streak image numerically simulated. The numerical simulation agrees better with the “slim” streak image. Received 7 July 1995 / Accepted 10 January 1996     

Vortex stretching and filaments

This paper presents a new technique to produce controlled stretched vortices. Intense elliptical vortices are created by stretching of an initial vorticity sheet. The initial vorticity comes from a laminar boundary layer flow and the stretching is parallel to the vorticity vectors. This low velocity flow enables direct observation of the formation and destabilization of vortices. Visualizations are combined with quasi-instantaneous measurements of a full velocity profile. The velocity profile is obtained with an ultrasonic pulsed Doppler velocimeter. The evolution of the central diameter of the vortices is related to the stretching. It is observed that destabilization occurs by pairing of two vortices, by hairpin deformation, and by breakdown of vortices into a “coil shape”.     

Nucleation and growth of a gas bubble in magma

The dynamics of a “collective” gas bubble in the magma melt during its decompression was numerically studied on the basis of a complete mathematical models of an explosive volcanic eruption. It is shown that the bubble size distribution obtained for the nucleation process has one peak, which allows considering a “collective” bubble. The main stages of bubble growth due to gas diffusion and changes in the viscosity of the medium are determined. It is shown that the high viscosity of the melt makes possible the transition from the Rayleigh equation to a simpler relation for the radial velocity of the bubble.