A kilohertz frame rate cinemagraphic PIV system for laboratory-scale turbulent and unsteady flows

A kilohertz frame rate cinemagraphic particle image velocimetry (PIV) system has been developed for acquiring time-resolved image sequences of laboratory-scale gas and liquid-phase turbulent flows. Up to 8000 instantaneous PIV images per second are obtained, with sequence lengths exceeding 4000 images. The two-frame cross-correlation method employed precludes directional ambiguity and has a higher signal-to-noise ratio than single-frame autocorrelation or cross-correlation methods, facilitating acquisition of long uninterrupted sequences of valid PIV images. Low and high velocities can be measured simultaneously with similar accuracy by adaptively cross-correlating images with the appropriate time delay. Seed particle illumination is provided by two frequency-doubled Nd:YAG lasers producing Q-switched pulses at the camera frame rate. PIV images are acquired using a 16 mm high-speed rotating prism camera. Frame-to-frame registration is accomplished by imaging two pairs of crossed lines onto each frame and aligning the digitized image sequence to these markers using image processing algorithms. No flow disturbance is created by the markers because only their image is projected to the PIV imaging plane, with the physical projection device residing outside the flow field. The frame-to-frame alignment uncertainty contributes 2% to the overall velocity measurement uncertainty, which is otherwise comparable to similar film-based PIV methods. Received: 11 July 2000 / Accepted: 21 June 2001 Published online: 29 November 2001     

Fiber-optic probe measurements of void fraction and bubble size distributions beneath breaking waves

Experiments were performed to determine the accuracy of single-tip fiber-optic probes for making simultaneous measurements of the void fraction and bubble size distributions under breaking waves. Tests in a vertical bubble column showed that the normalized RMS error in the void fraction measurements was ∼10%. The relationship between bubble rise time and bubble velocity was investigated in a unidirectional flow cell. Similar to previous studies the rise time and bubble velocity were found to be related by a power law equation. The probes can provide mean bubble velocities accurate to ±10% when a minimum of ∼15 individual bubble velocities are averaged. The fiber-optic probes were deployed beneath a plunging breaking wave in a laboratory wave channel. The slope and shape of the bubble cord length size distribution measured with the probes was found to agree closely with the size distribution measured from digital video recordings. The probes were then positioned in the splash-up zone of a plunging breaker and the resulting cord length distribution had a shape and slope that was in agreement with previous measurements. These results demonstrate that single-tip fiber optic probes can provide accurate simultaneous measurements of the void fraction and bubble sizes inside the dense bubble clouds entrained by breaking waves.     

Surfactant-induced effects on turbulent swirling flows

 We employ digital particle imaging velocimetry (DPIV) to investigate the influence of a drag reducing cationic surfactant additive, cetyltrimethyl-ammonium chloride (CTAC), on turbulent swirling flows generated in a cylindrical vessel either by a rotating disk or a rotating disk fitted with vertical flat blades. The largest concentration of CTAC used in this study (0.05 ≤ C ≤ 0.5 mmol/l) is an order of magnitude smaller than those used in experimental investigations of surfactant induced drag reduction in turbulent pipe/channel flows. Even for such dilute systems, a number of dramatic and intriguing effects are observed. In the case of disk-driven flow, it is shown that the surfactant has a non-monotonic influence on turbulence intensity: both radial and axial root mean square velocity fluctuations first increase with increasing surfactant concentration C, reach a maximum and decrease upon further increase in C. Moreover, the maximum intensity is attained at a concentration that is practically independent of the angular frequency Ω of the disk. For the flow driven by bladed impeller, the introduction of the surfactant leads to flow reversal at the impeller plane for low concentrations. Enhancement in the radial and azimuthal mean velocities is also observed. For relatively larger concentrations (=0.5 mmol/l), a mean flow field that consists of multiple transient mixing pockets emerges as Ω exceeds a critical value. Plausible mechanisms are proposed to explain these observations. Received: 11 September 2000 Accepted: 10 April 2001     

Ionic strontium fluorescence as a method for flow tagging velocimetry

A flow tagging technique based upon ionic fluorescence in strontium is investigated for applications to velocity measurements in gas flows. The method is based upon a combination of two laser based spectroscopic techniques, i.e. resonantly-enhanced ionisation and laser-induced ionic fluorescence. Strontium is first ionised and then planar laser-induced fluorescence is utilised to give 2D `bright images' of the ionised region of the flow at a given time delay. The results show that this method can be used for velocity measurements. The velocities were measured in two types of air–acetylene flames – a slot burner and a circular burner yielding velocities of 5.1 ± 0.1 m/s and 9.3 ± 0.2 m/s, respectively. The feasibility of the method for the determination of velocities in faster flows than those investigated here is discussed. Received: 5 November 1998/Accepted: 19 January 2000     

Spray characteristics of a gasoline direct injector injector with short durations of injection

The sprays into atmosphere from a GDI injector were visualised and the velocity and droplet characteristics measured at an injection pressure of 50 bar and at different injection durations, with emphasis on short injection periods. The images show that the initial and closing delay times were 0.225 and 0.2 ms, respectively, and that the cone angle increased with injection duration to a constant value of 62° at 0.5 ms. They also revealed large droplets ahead of the main spray with its smaller droplets. An injection duration of 0.15 ms led to fuel leaving the injector with little atomisation, but a 30% increase led to the formation of the cone, which was present for times greater than 0.5 ms. The poor atomisation associated with short injection durations and the initial phase of longer injections, was due to low swirl velocities. The droplet velocities were higher in the initial phase of injection than in the main phase, with values up to 50 m/s. The Sauter mean diameters of the initial and main-spray droplets were approximately 55 and 35 μm respectively and with a tendency to decrease with time from the start of injection. Received: 20 October 2000/Accepted: 30 March 2001     

Comparison of Gaussian particle center estimators and the achievable measurement density for particle tracking velocimetry

A series of numerical simulations were conducted to investigate the performance of two particle center estimation algorithms for Particle Tracking Velocimetry: a simple three-point Gaussian estimator and a least-square Gaussian. The smallest position error for images with reasonable noise levels was found to be approximately 0.03 pixels for both estimators using particles with diameters of 4 pixels. As both estimators performed equally well, use of the simple three-point Gaussian algorithm is recommended because it executes 100 times faster than the least-square algorithm. The maximum achievable measurement density and accuracy for the three-point Gaussian estimator were determined with a numerical simulation of an Oseen vortex. Uncertainty measures have been introduced to filter out unreliable displacement measurements. It was found that 4 to 5 velocity vectors could be obtained within a 32 × 32 pixel area with an average displacement error of 0.1 pixels. This doubles the spatial resolution of conventional cross-correlation based Particle Image Velocimetry at comparable accuracy. Received: 26 June 1998/Accepted: 12 October 1999     

Comparative evaluation of three heat transfer enhancement strategies in a grooved channel

 Results of a comparative evaluation of three heat transfer enhancement strategies for forced convection cooling of a parallel plate channel populated with heated blocks, representing electronic components mounted on printed circuit boards, are reported. Heat transfer in the reference geometry, the asymmetrically heated parallel plate channel, is compared with that for the basic grooved channel, and the same geometry enhanced by cylinders and vanes placed above the downstream edge of each heated block. In addition to conventional heat transfer and pressure drop measurements, holographic interferometry combined with high-speed cinematography was used to visualize the unsteady temperature fields in the self-sustained oscillatory flow. The locations of increased heat transfer within one channel periodicity depend on the enhancement technique applied, and were identified by analyzing the unsteady temperature distributions visualized by holographic interferometry. This approach allowed gaining insight into the mechanisms responsible for heat transfer enhancement. Experiments were conducted at moderate flow velocities in the laminar, transitional and turbulent flow regimes. Reynolds numbers were varied in the range Re = 200–6500, corresponding to flow velocities from 0.076 to 2.36 m/s. Flow oscillations were first observed between Re = 1050 and 1320 for the basic grooved channel, and around Re = 350 and 450 for the grooved channels equipped with cylinders and vanes, respectively. At Reynolds numbers above the onset of oscillations and in the transitional flow regime, heat transfer rates in the investigated grooved channels exceeded the performance of the reference geometry, the asymmetrically heated parallel plate channel. Heat transfer in the grooved channels enhanced with cylinders and vanes showed an increase by a factor of 1.2–1.8 and 1.5–3.5, respectively, when compared to data obtained for the basic grooved channel; however, the accompanying pressure drop penalties also increased significantly. Received on 5 April 2001     

An experimental study of a water droplet impinging on a liquid surface

An experimental study is presented for water droplet impingement on a liquid surface. The impaction process was recorded using a high-speed digital camera at 1,000 frames/s. The initial droplet diameter was fixed at 3.1 mm ± 0.1 mm, and all experiments were performed in atmospheric air. The impact velocity was varied from 0.36 m/s to 2.2 m/s thus varying the impact Weber number from 5.5 to 206. The impacted liquid surface consisted of two fluids, namely water and methoxy-nonafluorobutane, C₄F₉OCH₃ (HFE7100). The depth of the water and HFE7100 pool was varied from 2 mm to 25 mm. The collision dynamics of water in the HFE7100 pool was observed to be drastically different from that observed for the water droplet impingement on a water pool. The critical impact Weber number for jet breakup was found to be independent of liquid depth. Water–HFE7100 impact resulted in no jet breakup over the range of velocities studied. Therefore, no critical impact Weber number can be defined for water–HFE7100 impact. Received: 27 June 2001/Accepted: 29 November 2001     

Characteristics of air and water velocity fields during natural convection

Air and water velocity fields have been simulated during natural convection, using a two-dimensional volume of fluid (VOF) model. The results have shown that during unstable thermal stratification, the root-mean-square (RMS) airside velocities are an order of magnitude higher than the RMS waterside velocities, whereas, during the stable thermal stratification, the velocity magnitudes are comparable for air and water sides. Furthermore, the magnitude of the air velocity changed more rapidly with the change in the bulk air–water temperature difference than the water velocity, indicating that the air velocities are more sensitive to the bulk air and water temperature difference than the water velocities. A physical model of the heat and mass transfer across the air–water interface is defined. According to this model, the vortices on the air and water sides play an important role in enhancing the heat and mass transfer. Due to the significance of the flow velocities in the transport process, it has been proposed that the correlations for the heat and mass transfer during natural convection should be improved by incorporating the flow velocity as a parameter.     

On the accuracy of a MJPEG-based digital image compression PIV-system

 In this communication, the Digital Image Compression (DIC) – PIV system is introduced. The present system allows the measurement of mean and RMS velocities in turbulent flow fields, using JPEG digital image compression technique for on-line recording of thousands of images. The decompression and subsequent analysis of the images, performed by means of digital cross-correlation technique, is carried out off-line. Errors incurred by the application of the compression method are assessed and discussed. The effect of the compression is firstly analysed by linearly traversing (synthetic) computer-generated PIV-images at constant velocity. Secondly, accurate LDA measurements and data from direct numerical simulation (DNS) are used as a basis for the analysis in a low Reynolds number open water channel flow. The results show that excellent agreement between LDA and DIC–PIV measurements for mean and RMS velocities can be achieved using a compression factor up to 12. Received: 27 August 1996 / Accepted: 15 December 1998     

Bentonite slurries—zeta potential,yield stress,adsorbed additive and time-dependent behaviour

Bentonite clay is a vital ingredient of drilling mud. The time-dependent and high shear thinning yield stress behaviour of drilling mud is essential for maintaining wellbore stability and to remove cuttings, cool and clean the drill bit of debris. As-prepared 3, 5 and 7 wt.% bentonite clay slurries displayed time-dependent behaviour where the yield stress (measured after quick stirring) decreased with time of rest. An equilibrium value is reached after 24 h. Despite the low solids concentration, the yield stress is already relatively high and is displayed at all pH level. The yield stress is maximum at pH 2 and minimum at pH ∼ 7. This yield stress is due to the formation of gel structure by the swelling clay particles. However the addition of phosphate additives such as (PO₃)^19 −, (P₃O₁₀)^5 − and (P₂O₇)^4 − completely dispersed the clay slurries at pH above 6. At pH below 6, yield stress is still present but is 3-folds smaller than that of the pure bentonite slurry. With phosphate additives, the magnitude of the critical zeta potential at the complete dispersion pH is ca 48 mV. However for the pure bentonite, the slurry remained flocculated at zeta potential of >50 mV in magnitude. Interestingly, (P₂O₇)^4 − anions is more effective than the other two phosphate additives in reducing the yield stress at low pH, ∼ 2.0.     

Experiments on the effect of acceleration on the drag of tapwater bubbles

Three-dimensional (3D) trajectories of spherical air bubbles passing through a converging part of a rectangular channel have been measured. Bubble diameters, d _b, were less than 1 mm and the Reynolds numbers, Re _b, for stagnant tapwater and for mean liquid velocity vˉ _L=0.25 m/s were in about same range. Received: 15 January 2001 / Accepted: 12 June 2001     

Turbulent flow in a channel at a low Reynolds number

A two-component laser Doppler velocimeter with high spatial and temporal resolution was used to obtain measurements for fully developed turbulent flow of water through a channel with an aspect ratio of 12 : 1 at Re=5700 (based on the centerline velocity and the half-height of the channel). Statistical quantities that were determined are the mean streamwise velocity, the root-mean-square of the fluctuations of the streamwise and the normal velocities, the Reynolds shear stress and higher order moments. Turbulence production is calculated from these quantities. Turbulence statistics obtained from experiments are compared with results from a direct numerical simulation at the same Reynolds number. The good agreement validates a recent DNS, at Re=5700, which is approximately twice as large as used in most previous studies. Received: 12 May 1997 / Accepted: 8 April 1998     

Rheological study of concentrated suspensions in pressure-driven shear flow using a novel in-line ultrasound Doppler method

 In this work a novel in-line non-invasive rheological measuring technique is developed and tested in pilot plant and industrial-scale applications. The method is based on a combination of the ultrasonic pulsed echo Doppler technique (UVP) and pressure difference method (PD). The rheological flow properties are derived from simultaneous recording and on-line analysis of the velocity profiles across the tube channel and related radial shear stress profiles calculated from the pressure loss along the flow channel. It is shown that the in-line UVP-PD technique allows for the non-invasive rheological flow behaviour characterization of non-transparent and highly concentrated suspensions. Received: 8 May 2000 / Accepted: 22 June 2001 Published online: 29 November 2001     

Droplet evaporation on porous and non-porous ceramic solids heated from top

 This paper presents the results of an experimental investigation, of the effect of radiation heat, on the evaporation of five droplet sizes of pure water, softly deposited on porous and non-porous ceramic solids, at temperature ranging from 75 to 250 °C. Both solids were instrumented with several surface and in-depth thermocouples, and had the same thermal properties. Results show that, the droplet evaporation time, and the surface recovery time for the porous solid were shorter than that of non-porous solid for the same droplet size under identical conditions. Also, smaller droplets were more efficient for cooling both solids. The results were compared with data for the evaporation of water droplets on similar ceramic solids heated from bottom (Abu-Zaid M; Atreya A (1994) J Heat Transfer 116: 694–701). The comparison shows that, the heat radiation has a significant effect of reducing evaporation time, recovery time, and droplet volume of influence for both solids, at the same initial surface temperature. Received on 6 December 1999 / Published online: 29 November 2001     

Effects of coincidence window and measuring volume size on laser Doppler velocimetry measurement of turbulence

The effects of coincidence window and measuring volume size on two-component laser velocimeter measurement of turbulence in an isothermal liquid flow through a concentric annular channel were studied. Three different coincidence windows (100–500 μs) and three different measuring volume sizes (diameter, 5–9 wall units; spanwise length, 24–91 wall units) were used in a flow of Reynolds number 31,500 and data density spanning the high end of intermediate to the low end of high (3–6). While no significant effects of the coincidence window and measuring volume size were found on the time-mean velocity and turbulence intensities, the streamwise Reynolds shear stress measured near a wall was found to be markedly affected by both. The smallest feasible measuring volume along with an appropriate coincidence window provides good measurement of the shear stress. Received: 8 September 1999/Accepted: 11 July 2000     

Three-dimensional Turbulent Boundary Layer in a Shrouded Rotating System

A thre-dimensional direct numerical simulation is combined with a laboratory study to describe the turbulent flow in an enclosed annular rotor-stator cavity characterized by a large aspect ratio G = (b − a)/h = 18.32 and a small radius ratio a/b = 0.152, where a and b are the inner and outer radii of the rotating disk and h is the interdisk spacing. The rotation rate Ω considered is equivalent to the rotational Reynolds number Re = Ωb ²/ν= 9 .5 × 10⁴ (ν the kinematic viscosity of water). This corresponds to a value at which experiment has revealed that the stator boundary layer is turbulent, whereas the rotor boundary layer is still laminar. Comparisons of the computed solution with velocity measurements have given good agreement for the mean and turbulent fields. The results enhance evidence of weak turbulence by comparing the turbulence properties with available data in the literature (Lygren and Andersson, J Fluid Mech 426:297–326, 2001). An approximately self-similar boundary layer behavior is observed along the stator. The wall-normal variations of the structural parameter and of characteristic angles confirm that this boundary layer is three-dimensional. A quadrant analysis (Kang et al., Phys Fluids 10:2315–2322, 1998) of conditionally averaged velocities shows that the asymmetries obtained are dominated by Reynolds stress-producing events in the stator boundary layer. Moreover, Case 1 vortices (with a positive wall induced velocity) are found to be the major source of generation of special strong events, in agreement with the conclusions of Lygren and Andersson (J Fluid Mech 426:297–326, 2001).     

Measurement of multiple gas-bubble velocities in gas–liquid flows using hot-film anemometry

A dual-probe hot-film anemometry technique has been developed to measure multiple gas-bubble velocities corresponding to different gas-bubble size groups in air–water flows. A data reduction scheme using wavelet analysis combined with a phase-detection technique is used to discriminate the hot-film anemometer output signals into signals corresponding to different bubble size groups. The phase and bubble size discrimination is based on the magnitude and time derivative of the signal, and the streamwise length of the gas bubbles. A cross-correlation between the discriminated signals from the two probes yields an average time difference of arrival of the gas bubbles at the two sensor locations. The velocities are estimated from the distance between the sensors and the time difference of arrival. The mean bubble size is estimated from the chord length distribution. Measurements performed in vertical-up air–water slug flow show the technique to be a viable method for obtaining bubble velocity and size information. The velocity measurements from the hot-film anemometry are corroborated using a high-speed quantitative flow visualization system. Received: 22 December 1999/Accepted: 8 May 2001     

Optical probe visualization of air–water flow structure through a sudden area contraction

 Single-fiber optical probes were used to investigate the time-averaged structure of gas–liquid horizontal flow through a sharp-edged sudden area contraction. The probes allowed to measure the local void fraction distribution over several cross sections of a pipe having an inner diameter of 0.08 m upstream and 0.06 m downstream of the sudden contraction. The water mass flow rate was 3 kg/s, while the gas fraction of the volume flow ranged from 0.2 to 0.8. The local void fraction was plotted as a function of its two spatial coordinates, so that a representation of the time-averaged gas distribution over the cross section could be obtained. The contraction was shown to considerably alter the distribution of the phases, so that the correlations for straight pipes appear no longer suitable. Received: 27 August 2001 / Accepted: 19 November 2001     

Gas jets in fluidized beds: The influence of particle size,shape and density on gas and solids entrainment

Gas has been injected in two-dimensional fluidized beds of solids different in size, density and shape. The ranges of solids sizes and bed heights were such as to produce relatively steady permanent jets.The mechanics of dispersion of these jets has been studied measuring jet angles, jet gas and solids velocity profiles, and particle entrainment velocities. The proportions of total mass and momentum flowrates pertaining to gas and solids have been calculated from these data.