Measured Velocity Data Research Articles

PSP and PIV investigations on the VFE-2 configuration in sub- and transonic flow

An overview is given about investigations on a 65° delta wing using the Pressure Sensitive Paint (PSP) and Particle Image Velocimetry (PIV) techniques, carried out in the framework of the International Vortex Flow Experiment 2 (VFE-2). For the delta wing with rounded leading edges and subsonic flow, the occurrence of a flat vortical structure as well as the onset of the primary vortex and the development of a vortex system consisting of an inner and outer vortex is described in dependency of the angle of attack and the Reynolds number. The Q-criterion is applied to the measured velocity data to estimate the circulation strength of individual vortices allowing for a quantitative description of the vortex developments and interactions. Furthermore, a case at transonic flow speeds (M=0.8) is described, showing a sudden occurrence of vortex breakdown above the delta wing, most probably induced by a shock wave.

Boundary layer analysis in turbulent Rayleigh-Bénard convection in air: Experiment versus simulation

We report measurements and numerical simulations of the three-dimensional velocity and temperature fields in turbulent Rayleigh-Bénard convection in air. Highly resolved velocity and temperature measurements inside and outside the boundary layers have been directly compared with equivalent data obtained in direct numerical simulations (DNSs). This comparison comprises a set of two Rayleigh numbers at Ra=3×10(9) and 3×10(10) and a fixed aspect ratio; this is the ratio between the diameter and the height of the Rayleigh-Bénard cell of Γ=1. We find that the measured velocity data are in excellent agreement with the DNS results while the temperature data slightly differ. In particular, the measured mean temperature profile does not show the linear trend as seen in the DNS data, and the measured gradients at the wall are significantly higher than those obtained from the DNS. Both viscous and thermal boundary layer thickness scale with respect to the Rayleigh number as δ(v)~Ra(-0.24) and δ(θ)~Ra(-0.24), respectively.

A practical guide for using proper orthogonal decomposition in engine research

Proper orthogonal decomposition has been utilized for well over a decade to study turbulence and cyclic variation of flow and combustion properties in internal combustion engines. In addition, proper orthogonal decomposition is useful to quantitatively compare multi-cycle in-cylinder measurements with numerical simulations (large-eddy simulations). However, the application can be daunting, and physical interpretation of proper orthogonal decomposition can be ambiguous. In this paper, the mathematical procedure of proper orthogonal decomposition is described conceptually, and a compact MATLAB® code is provided. However, the major purpose is to empirically illustrate the properties of the proper orthogonal decomposition analysis and to propose practical procedures for application to internal combustion engine flows. Two measured velocity data sets from a motored internal combustion engine are employed, one a highly directed flow (each cycle resembles the ensemble average), and the other an undirected flow (no cycle resembles the average). These data are used to illustrate the degree to which proper orthogonal decomposition can quantitatively distinguish between internal combustion engine flows with these two extreme flow properties. In each flow, proper orthogonal decomposition mode 1 is an excellent estimate of ensemble average, and this study illustrates how it is thus possible to unambiguously quantify the cyclic variability of Reynolds-averaged Navier–Stokes ensemble average and turbulence. In addition, this study demonstrates the benefits of comparing two different samples of cycles using a common proper orthogonal decomposition mode set derived by combining the two samples, the effect of spatial resolution, and a method to evaluate the number of snapshots required to achieve convergence.

New 3‐D flow interpolation method on moving ADCP data

A simple but accurate interpolation procedure for obtaining the three‐dimensional distribution of three‐component velocity data, from moving acoustic doppler current profiler (ADCP) observation data, is proposed. For understanding actual flow structure within a river with complex bathymetry, the three‐dimensional mean velocity field provides a basic picture of the flow. For obtaining the three‐dimensional distribution of three‐component velocity data, in this work, anisotropic gridding was introduced in order to remove the random component of measured velocity data caused by the turbulence of the flow and measurement error. A continuity correction based on the pressure equation was used to reduce both random and systematic errors. The accuracy of the developed method was evaluated using three‐dimensional flow simulation data from a detached‐eddy simulation (DES). By using the procedure developed, the complex flow structure surrounding the spur dikes section in the Uji River was successfully visualized and explored. The proposed method shows superiorities in both accuracy and consistency for the interpolated velocity field, as compared to the kriging and inverse‐distance weighted (IDW) methods.

Experimental Study of Flow Fields around a Perforated Breakwater

This study investigates flow fields and energy dissipation due to regular wave interaction with a perforated vertical breakwater, through velocity data measurement in a two-dimensional wave tank. As the waves propagate through the perforated breakwater, the incoming wave energy is reflected back to the ocean, dissipated due to very turbulent flows near the perforations and inside the chamber, and transmitted through the perforations of the breakwater. This transmitted energy is further reduced due to the presence of the perforated back wall. Hence most of the energy is either reflected or dissipated in the vicinity of the structure, and only a small amount of the incoming wave energy is transmitted through the structure. In this study, particle image velocimetry (PIV) technique was employed to measure two-dimensional instantaneous velocity fields in the vicinity of the structure. Measured velocity data was treated statistically, and used to calculate mean flow fields, turbulence intensity and turbulent kinetic energy. For investigation of the flow pattern, time-averaged mean velocity fields were examined, and discussed using the cross-sections through slot and wall for comparison. Flow fields were obtained and compared for various cases with different regular wave conditions. In addition, turbulent kinetic energy was estimated as an approach to understand energy dissipation near the perforated breakwater. The turbulent kinetic energy was distributed against wave height and wave period to see the dependence on wave conditions.

High spatial resolution mapping of surface velocities and depths for shallow overland flow

ABSTRACTPoint measurements of flow rate, depth or velocity are not sufficient to validate overland flow models, particularly when the interaction of the water with the soil surface creates a complex flow geometry. In this study, we present the coupling of two techniques obtaining spatial data of flow depths and surface velocity measurements for water depths as low as 1 mm. Overland flow experiments were performed in the laboratory at various flow rates and slopes on two surfaces. The first surface was 120 cm by 120 cm showing three undulations of sinusoidal shape with an amplitude of 1 cm and a wavelength of 20 cm, while the second was a 60 cm by 60 cm moulded reproduction of a seedbed with aggregates up to 2 cm in size. Large scale particle image velocimetry (LSPIV) was used for velocity measurements with a sub‐centimetre spatial resolution. An instantaneous‐profile laser scanner was used to map flow depths with a sub‐millimetre spatial resolution. A sensitivity analysis of the image processing of the LSPIV showed good robustness of the method. Comparison with measurements performed with hot film anemometer and salt velocity gauge showed that LSPIV surface velocities were representative of the flow. Water depths measured with the laser scanner were also in good agreement with single‐point measurements performed with a dial indicator. Spatially‐distributed flow rates could be computed by combining both presented techniques with a mean relative error less than 20%. Copyright © 2012 John Wiley & Sons, Ltd.

Modelling the failure modes in geobag revetments

In recent years, sand filled geotextile bags (geobags) have been used as a means of long-term riverbank revetment stabilization. However, despite their deployment in a significant number of locations, the failure modes of such structures are not well understood. Three interactions influence the geobag performance, i.e. geobag-geobag, geobag-water flow and geobag-water flow-river bank. The aim of the research reported here is to develop a detailed understanding of the failure mechanisms in a geobag revetment using a discrete element model (DEM) validated by laboratory data. The laboratory measured velocity data were used for preparing a mapped velocity field for a coupled DEM simulation of geobag revetment failure. The validated DEM model could identify well the critical bag location in varying water depths. Toe scour, one of the major instability factors in revetments, and its influence on the bottom-most layer of the bags were also reasonably represented in this DEM model. It is envisaged that the use of a DEM model will provide more details on geobag revetment performance in riverbanks.

Comparative Analysis of Velocity Decomposition Methods for Internal Combustion Engines

Different signal processing technique performances are compared to each other with regard to separating the mean and fluctuating velocity components of a simulated one-dimensional unsteady velocity signal comparable to signals observed in internal combustion engines. A simulation signal with known mean and fluctuating components was generated using experimental data and generic turbulence spectral information. The simulation signal was generated based on observations on the measured velocity data obtained using LDV in a motored Briggs-and-Stratton engine at about 600 RPM. Experimental data was used as a guide to shape the simulated signal mean velocity variation; fluctuating velocity variations with specified spectrum and standard deviation was used to mimic the turbulence. Cyclic variations were added both to the mean and the fluctuating velocity signals to simulate prescribed cyclic variations. The simulated signal was introduced as input to the following algorithms: ensemble averaging; high-pass filtering; Proper-Orthogonal Decomposition (POD); Wavelet Decomposition (WD) and Wavelet Decomposition/Principal Component Analysis (WD/PCA). The results were analyzed to determine the best method in correctly separating the mean and the fluctuating velocity information, indicating that the WD/PCA performs better compared to other techniques.

Distribution of turbulence statistics in open-channel flow

Turbulence is ubiquitous in most environmental flows. Therefore, highly resolved measurement of turbulent quantities is one of the challenges in hydraulics. As turbulence is a multi-scale physical phenomenon, the measurement resolution should be sufficient to capture all these scales. Therefore, research on turbulent flow gained momentum after the introduction of high-resolution measurement devices such as Laser Doppler Velocimeter (LDV) and Acoustic Doppler Velocimeter (ADV). In this study, a set of experiments were conducted by using ADV to measure the flow turbulence in a trapezoidal open channel on rough bed which is commonly observed in a typical riverine environment. In the experiments, vertical distribution of the turbulent quantities was investigated. The experiments are conducted by using a four component ADV with maximum sampling frequency of 200 Hz. Measured velocity data are filtered to eliminate the artifacts due to spikes, low correlation and SNR velocity values. The filtered data are then analyzed to obtain the statistical parameters that characterize the flow turbulence in the water column ranging from the smallest scale to integral length scale. Key words: Turbulence, open channel, spectral analysis.

Near-Transducer Errors in ADCP Measurements: Experimental Findings

Acoustic Doppler current profilers (ADCPs) are not able to accurately determine velocity near their transducers and near the bed. These limitations have restricted the use of ADCPs to flow depths that are large enough to allow acquisition of few directly measured velocity data that can be subsequently used to accurately estimate vertical velocity profiles and flow discharge in cross sections. While the causes that make ADCPs unable to collect data in the near-bed region are relatively well documented, the causes of near-transducer errors have not yet been fully understood and are only partly documented. We present results from an experimental study aimed at characterizing the systematic errors due to the combined effect of acoustic interference and instrument-induced flow disturbance near a Janus-configured ADCP. The study comprises: (1) concurrent measurements with an ADCP and an acoustic Doppler velocimeter (ADV) under the ADCP; (2) measurements of the flow disturbance produced by the ADCP in the vertical and horizontal planes; and (3) ADV measurements along the path of the acoustic beams ensonified by the ADCP during a measurement. Results suggest that ADCPs bias low the velocity profiles with respect to the undisturbed velocity profiles, mostly because of the flow disturbance induced by the ADCP, with acoustic effects playing a secondary role. For the range of flows we studied, both undisturbed and disturbed profiles exhibit similar shapes when plotted in dimensionless form, with the bulk flow velocity and the ADCP diameter (D) as characteristic scales. The differences between the undisturbed and the ADCP-disturbed profiles extend up to a distance of about 1.5D from the ADCP, except for the profiles measured at locations where the flow depth is close to D for which the boundary layer induced by the ADCP interacts with the one induced by the flume bed.

A model of average velocity in oscillating turbulent boundary layers

A model to calculate the vertical distribution of the average velocity in the oscillating turbulent boundary layer forced by the overlaying oscillating free flow is discussed. The model is founded on the Rotationally Anisotropic Turbulence theory. The oscillating boundary layer is treated within the approximation of average velocity depending on the time and vertical coordinate only. The description results in an analytical expression for the flow velocity analogous to the expression for the flow velocity in the boundary layer of oscillating pipe flow. The resulting expressions for the vertical velocity profile are compared with respective laboratory data obtained in two tests of an oscillatory water tunnel experiment. It is shown that similar to the description of oscillating flow in tubes, the outcome of the suggested model with the appropriate values of the model parameters matches the measured velocity data reasonably well.

Direct bed shear stress measurements in bore-driven swash

Direct measurements of bed shear in the swash zone are presented. The data were obtained using a shear plate in medium and large-scale laboratory bore-driven swash and cover a wide range of bed roughness. Data were obtained across the full width of the swash zone and are contrasted with data from the inner surf zone. Estimates of the flow velocities through the full swash cycle were obtained through numerical modelling and calibrated against measured velocity data. The measured stresses and calculated flow velocities were subsequently used to back-calculate instantaneous local skin friction coefficients using the quadratic drag law. The data show rapid temporal variation of the bed shear stress through the leading edge of the uprush, which is typically two–four times greater than the backwash shear stresses at corresponding flow velocity. The measurements indicate strong temporal variation in the skin friction coefficient, particularly in the backwash. The general behaviour of the skin friction coefficient with Reynolds number is consistent with classical theory for certain stages of the swash cycle. A spatial variation in skin friction coefficient is also identified, which is greatest across the surf-swash boundary and likely related to variations in local turbulent intensities. Skin friction coefficients during the uprush are approximately twice those in the backwash at corresponding Reynolds number and cross-shore location. It is suggested that this is a result of the no-slip condition at the tip leading to a continually developing leading edge and boundary layer, into which high velocity fluid and momentum are constantly injected from the flow behind and above the tip region. Finally, the measured stress data are used to determine the asymmetry and cross-shore variation in potential sediment transport predicted by three forms of sediment transport formulae.

Elasticity of stishovite and acoustic mode softening under high pressure by Brillouin scattering

Brillouin scattering measurements on single-crystal stishovite, a high-pressure polymorph of SiO 2, were carried out to 22 GPa. Acoustic velocities in three 30-μm thick crystal platelets of synthetic stishovite were measured in a forward symmetric scattering geometry, and the full set of elastic constants were retrieved to 12 GPa. The measured velocity data were fit to Christoffel’s equation, yielding ambient-pressure elastic constants of C 11 = 455(1) GPa, C 33 = 762(2) GPa, C 12 = 199(2) GPa, C 13 = 192(2) GPa, C 44 = 258(1) GPa, and C 66 = 321(1) GPa. The elastic modulus ( C 11 − C 12)/2 was observed to decrease with pressure, indicating acoustic mode softening, consistent with theoretical predications for the behavior of stishovite as it approaches the transition to the CaCl 2-type phase. The bounds on the aggregate adiabatic bulk and shear moduli are K S0 = 315(1) GPa, G 0 = 240(1) GPa for the Voigt bound, K S0 = 301(1) GPa, G 0 = 216(1) GPa for the Reuss bound. Pressure derivatives of aggregate bulk and shear moduli were constrained to be ( ∂K S/ ∂P) T0 = 4.34(16) and ( ∂G/ ∂P) 0 = 0.7(1) for the Reuss bound, and ( ∂K S/ ∂P) T0 = 4.0(1) and ( ∂G/ ∂P) 0 = 1.1(1) for the Voigt–Reuss–Hill (VRH) average, respectively, by fitting the data to Eulerian finite strain equations. The volume compression curve obtained from our Brillouin measurement is in very good agreement with previous compression studies up to 50 GPa.

Spline-Based Cardiac Motion Tracking Using Velocity-Encoded Magnetic Resonance Imaging

This paper deals with the problem of tracking cardiac motion and deformation using velocity-encoded magnetic resonance imaging. We expand upon an earlier described method and fit a spatiotemporal motion model to measured velocity data. We investigate several different spatial elements both qualitatively and quantitatively using phantom measurements and data from human subjects. In addition, we also use optical flow estimation by the Horn-Schunk method as complementary data in regions where the velocity measurements are noisy. Our results show that it is possible to obtain good motion tracking accuracy in phantoms with relatively few spatial elements, if the type of element is properly chosen. The use of optical flow can correct some measurement artifacts but may give an underestimation of the magnitude of the deformation. In human subjects the different spatial elements perform quantitatively in a similar way but qualitative differences exists, as shown by a semiquantitative visual scoring of the different methods.

Evolutionary optimised nonlinear function for linearisation of constant temperature anemometer

A simple software method to derive linearised output for constant temperature anemometer (CTA) is presented. The method uses a nonlinear ratiometric–logarithmic function, which consists of two parameters whose optimal values are determined by minimising the objective function (mean square error) to improve the linearity of CTA signal. Covariance matrix adopted evolutionary strategy algorithm, which generates optimal values consistently, is employed to determine the optimal values of linearisation parameters. The proposed linearisation algorithm was implemented using LabVIEW 7.1 Professional Development System in a personal computer that provides the facility to interface with the National Instruments data acquisition module PCMCIA-NI DAQCard-6024E. Experimental studies have been carried out using practical air-flow velocity measurement data obtained form Dantec Dynamics practical guide. The performance measures such as full-scale error and root mean square error are considered to compare the performance of the proposed method with the methods reported for linearisation of transducers. Experimental results reveal that the proposed evolutionary optimised nonlinear function-based software lineariser works well for CTA, and it can be suitable for computer-based flow measurement/control systems.

Measurement of Longshore Sediment Transport Rates in the Surf Zone on Galveston Island, Texas

Longshore sediment transport in the surf zone on Galveston Island, Texas, was studied to develop a new technique involving optical instruments rapidly calibrated in situ and to compare measured transport rates with those predicted by the well-known Coastal Engineering Research Center (CERC) formula. This method used an instrumented sled equipped with a LISST-100 for particle size distribution determination, four optical backscatter sensors (OBSs) for turbidity measurements, and two velocity sensors for longshore current measurements. The sled was pulled across the surf zone, occupying 10 to 15 stations spaced about 10 m apart, for approximately 3 minutes each.The OBS data were calibrated with the LISST-100 particle size distribution data, thereby overcoming the difficulties associated with the use of these sensors in the presence of a mix of sand and fine particles. Subsequently, these data were fit to a logarithmic profile to determine the average vertical distribution of suspended sand concentration, assuming that the fine particles were vertically well mixed at each station. A logarithmic profile of average longshore current was also computed based on the measured velocity data. The longshore sediment transport rate was calculated as the spatial integral of the product of suspended sand concentration and velocity and related to the wave conditions at the point of breaking. Measured rates ranged from 86,000 to 231,000 m3/y, and transport was found to be greatest in the vicinity of the sand bars. The popular CERC formula gave sediment transport rates significantly greater than the observed values, with the difference between the two on the order of 100%. The average CERC transport coefficient, K1, computed from our measurements was determined to be 0.19 ± 0.12.

A method for estimating wall friction in turbulent wall-bounded flows

We describe a simple method for estimating turbulent boundary layer wall friction using the fit of measured velocity data to a boundary layer model profile that extends the logarithmic profile all the way to the wall. Two models for the boundary layer profile are examined, the power-series interpolation scheme of Spalding and the Musker profile which is based on the eddy viscosity concept. The performance of the method is quantified using recent experimental data in zero pressure gradient flat-plate turbulent boundary layers, and favorable pressure gradient turbulent boundary layers in a pipe, for which independent measurements of wall shear are also available. Between the two model profiles tested, the Musker profile performs much better than the Spalding profile. Results show that the new procedure can provide highly accurate estimates of wall shear with a mean error of about 0.5% in friction velocity, or 1% in shear stress, an accuracy that is comparable to that from independent direct measurements of wall shear stress. An important advantage of the method is its ability to provide accurate estimates of wall shear not only based on many data points in a velocity profile but also very sparse data points in the velocity profile, including only a single data point such as that originating from a near-wall probe.

Micro-PIV measurements of blood flow in extraembryonic blood vessels of chicken embryos

The hemodynamic characteristics of blood flow are important in the diagnosis of circulatory diseases, since such diseases are related to wall shear stress of cardiovascular vessels. In chicken embryos at early stages of development, it is possible to directly visualize blood flow inside blood vessels. We therefore employed a micro-PIV technique to assess blood flow in extraembryonic venous and arterial blood vessels of chicken embryos, using red blood cells (RBCs) as tracers and obtaining flow images of RBCs using a high-speed CMOS camera. The mean velocity field showed non-Newtonian flow characteristics. The blood flow in two venous vessels merged smoothly into the Y-shaped downstream vein without any flow separation or secondary flow. Vorticity was high in the inner regions, where the radius of curvature varied greatly. A periodic variation of temporally resolved velocity signals, due to beating of the heart, was observed in arterial blood vessels. The pulsating frequency was obtained by fast Fourier transform analysis using the measured velocity data. The measurement technique used here was useful in analyzing the hemodynamic characteristics of in vivo blood flow in chicken embryos.

Quantification of Volumetric In-Cylinder Flow of SI Engine Using 3-D Laser Doppler Velocimetry ( II )

Simultaneous 3-D LDV measurements of the in-cylinder flows of three different engine setups were summarized for the quantification of the flow characteristics in each vertical or horizontal plane, and in entire cylinder volume. The ensemble averaged-velocity, tumble and swirl motions, and turbulent kinetic energy during the intake and compression strokes were examined from the measured velocity data (approximately 2,000 points for each engine setup). The better spatial resolution of the 3-D LDV allows measurements of the instantaneous flow structures, yielding more valuable information about the smaller flow structures and the cycle-to-cycle variation of these flow patterns. Tumble and swirl ratios, and turbulent kinetic energy were quantified as planar and volumetric quantities. The measurements and calculation results were animated for the visualization of the flow, and hence ease to analysis.

Optically sliced measurement of velocity and pH distribution in microchannel

A simultaneous measurement technique for the velocity and pH distribution was developed by using a confocal microscope and a 3CCD color camera for investigations of a chemical reacting flow field in a microchannel. Micron-resolution particle image velocimetry and laser induced fluorescence were utilized for the velocity and pH measurement, respectively. The present study employed fluorescent particles with 1 μm diameter and Fluorescein sodium salt whose fluorescent intensity increases with an increase in pH value over the range of pH 5.0–9.0. The advantages of the present system are to separate the fluorescence of particles from that of dye by using the 3CCD color camera and to provide the depth resolution of 5.0 μm by the confocal microscope. The measurement uncertainties of the velocity and pH measurements were estimated to be 5.5 μm/s and pH 0.23, respectively. Two aqueous solutions at different pH values were introduced into a T-shaped microchannel. The mixing process in the junction area was investigated by the present technique, and the effect of the chemical reaction on the pH gradient was discussed by a comparison between the proton concentration profiles obtained from the experimental pH distribution and those calculated from the measured velocity data. For the chemical reacting flow with the buffering action, the profiles from the numerical simulation showed smaller gradients compared with those from the experiments, because the production or extinction of protons was yielded by the chemical reaction. Furthermore, the convection of protons was evaluated from the velocity and pH distribution and compared with the diffusion. It is found that the ratio between the diffusion and convection is an important factor to investigate the mixing process in the microfluidic device with chemical reactions.