Single-camera Doppler global velocimetry based on frequency modulation techniques

The main advantage of the described Doppler global velocimeter (DGV) systems based on frequency modulation (FM) or frequency shift keying (FSK) is that no reference detector is required. The frequency variation of the laser light during one modulation period additionally allows an on-line calibration of the complete DGV system. Thus, the new method has the potential to reduce the uncertainty of conventional DGV velocity measurements since time resolved velocity field measurements on a spinning disc have shown standard deviations down to 0.02 m/s. On investigating flow fields, velocity components notably less than 0.5 m/s were resolved.     

Heterodyne Doppler global velocimetry

Doppler Global Velocimetry (DGV) is an imaging flow measurement technique which allows the measurement of the velocity distribution in a plane. In DGV the frequency shift of scattered light from moving particles within the flow is used to determine the local flow velocity. Heterodyne Doppler Global Velocimetry (HDGV) is a new approach which combines the imaging and geometrical characteristics of DGV with the measurement principles of reference beam laser Doppler anemometry. The frequency shifted scattered light from the flow tracers is heterodyned with a reference beam from the same light source. Due to interference the result of this superposition is a harmonic intensity modulated signal. This signal is detected using a smart pixel detector array to obtain the velocity distribution. Two different experiments are presented. The first experiment compares the measured velocity distribution of a rotating disk with its actual velocity. The second experiment demonstrates the capability of the technique to measure a real flow.     

Fractal-Generated Turbulence in Opposed Jet Flows

The opposed jet configuration presents a canonical geometry suitable for the evaluation of calculation methods seeking to reproduce the impact of strain and re-distribution on turbulent transport in reacting and non-reacting flows. The geometry has the advantage of good optical access and, in principle, an absence of complex boundary conditions. Disadvantages include low frequency flow motion at high nozzle separations and comparatively low turbulence levels causing bulk strain to exceed the turbulent contribution at small nozzle separations. In the current work, fractal generated turbulence has been used to increase the turbulent strain and velocity measurements for isothermal flows are reported with an emphasis on the axis, stagnation plane and the distribution of mean and instantaneous strain rates. Energy spectra were also determined. The instrumentation comprised hot-wire anemometry and particle image velocimetry with the flows to both nozzles seeded with 1  $\upmu$ m silicon oil droplets providing a relaxation time of ? 3 $\upmu$ s. It is shown that fractal grids increase the turbulent Reynolds number range from 48–125 to 109–220 for bulk velocities from 4 to 8 m/s as compared to conventional perforated plate turbulence generators. Low frequency motion of the order 10 Hz could not be completely eliminated and probability density functions were determined for the location of the stagnation plane. Results show that the fluctuation in the position of the stagnation plane is of the order of the integral length scale, which was determined to be 3.1±0.1 mm at the nozzle exits through the use of hot-wire anemometry. Flow statistics close to the fractal plate located upstream of the nozzle exit were also determined using a transparent glass nozzle.     

Pulsed, high-power LED illumination for tomographic particle image velocimetry

This paper investigates the use of high-power light-emitting diode (LED) illumination for tomographic particle image velocimetry (PIV) as an alternative to traditional laser-based illumination. Modern solid-state LED devices can provide averaged radiant power in excess of 10 W and by operating the LED with short high current pulses theoretical pulse energies up to several tens of mJ can be achieved. In the present work, a custom-built drive circuit is used to drive a Luminus PT-120 high-power LED at pulsed currents of up to 150 A and 1 μs duration. Volumetric illumination is achieved by directly projecting the LED into the flow to produce a measurement volume of ≈3–4 times the size of the LED die. The feasibility of the volumetric LED illumination is assessed by performing tomographic PIV of homogenous, grid-generated turbulence. Two types of LEDs are investigated, and the results are compared with measurements of the same flow using pulsed Nd:YAG laser illumination and DNS data of homogeneous isotropic turbulence. The quality of the results is similar for both investigated LEDs with no significant difference between the LED and Nd:YAG illumination. Compared with the DNS, some differences are observed in the power spectra and the probability distributions of the fluctuating velocity and velocity gradients. These differences are attributed to the limited spatial resolution of the experiments and noise introduced during the tomographic reconstruction (i.e. ghost particles). The uncertainty in the velocity measurements associated with the LED illumination is estimated to approximately 0.2–0.3 pixel for both LEDs, which compares favourably with similar tomographic PIV measurements of turbulent flows. In conclusion, the proposed high-power, pulsed LED volume illumination provides accurate and reliable tomographic PIV measurements in water and presents a promising technique for flow diagnostics and velocimetry.     

Precise flow rate measurements of natural gas under high pressure with a laser Doppler velocity profile sensor

This paper reports about the first application of a laser Doppler velocity profile sensor for precise flow rate measurements of natural gas under high pressure. The profile sensor overcomes the limitations of conventional laser Doppler anemometry (LDA) namely the effect of spatial averaging and the effect of fringe spacing variation (virtual turbulence). It uses two superposed, fan-like interference fringe systems to determine the axial position of a tracer particle inside the LDA’s measurement volume. Consequently, a spatial resolution of about 1 μm can be achieved and the effect of virtual turbulence is nearly eliminated. These features predestine the profile sensor for flow rate measurements with high precision. Velocity profile measurements were performed at the German national standard for natural gas, one of the world′s leading test facilities for precision flow rate measurements. As a result, the velocity profile of the nozzle flow could be resolved more precisely than with a conventional LDA. Moreover, the measured turbulence intensity of the core flow was of 0.14% mean value and 0.07% minimum value, which is significantly lower than reference measurements with a conventional LDA. The paper describes the performed measurements, gives a discussion and shows possibilities for improvements. As the main result, the goal of 0.1% flow rate uncertainty seems possible by an application of the profile sensor.     

Near-wall measurements of turbulence statistics in a fully developed channel flow with a novel laser Doppler velocity profile sensor

This paper reports on the measurements of the near-wall turbulence statistics in a fully developed channel flow. The flow measurements were carried out with a novel laser Doppler velocity profile sensor with a high spatial resolution. The sensor provides both the information of velocity and position of individual tracer particles inside the measurement volume. Hence, it yields the velocity profile inside the measurement volume, in principle, without the sensor being mechanically traversed. Two sensor systems were realized with different techniques. Typically the sensor has a relative accuracy of velocity measurement of 10⁻³ and the spatial resolution of a few micrometers inside the measurement volume of about 500 μm long. The streamwise velocity was measured with two independent sensor systems at three different Reynolds number conditions. The resulting turbulence statistics show a good agreement with available data of direct numerical simulations up to fourth order moment. This demonstrates the velocity profile sensor to be one of the promising techniques for turbulent flow research with the advantage of a spatial resolution more than one magnitude higher than a conventional laser Doppler technique.     

Turbulent decay in the near field of multi-scale and conventional grids

Wind tunnel turbulence generated by a conventional and two multi-scale grids has been investigate. The grids were all designed to produce turbulence with the same integral scale, so that a direct comparison could be made between the flows, both in physical and scaled space. It has been suggested in the literature (e.g. Hurst and Vassilicos, 2007) that for a particular class of multi fractal grids, the turbulence decay depends exponentially on the distance from the grid. After a short distance where the flow is highly dependent of the geometry, it was found that the exponential decay is not unique to a particular geometry, but may be found over the same streamwise distances also behind the multi-scale grids, as well as for the conventional grid.By comparing the probability density functions measured using laser Doppler and hot wire anemometry it is shown that hot wire measurements may contain severe errors if taken too close to the grid. It is shown that negative streamwise velocity components may occasionally be found as far as 10 times the mesh widths downstream of the grid. Since hot wire anemometry is not able to measure the sign of the velocity vector, this leads to a folding of the data which artificially increases the derived mean velocity and, more seriously, reduces the width of the probability distribution. Hence the interpreted turbulent stress is reduced.     

Investigation of turbulence modification in a non-reactive two-phase flow

In a two-phase flow the influence of a dispersed phase on the turbulence properties of a continuous phase, known as turbulence modification, is investigated. An experimental approach is discussed that is suitable for studying the decay of grid-generated turbulence in a vertically orientated wind tunnel with a cross-section large enough to avoid influences from walls. Phase Doppler anemometry is used to characterize both single and two-phase flow by measuring mean axial and radial velocity components, velocity fluctuations, turbulent kinetic energy, and integral time scales. By direct comparison of results from single- and two-phase flows, the feedback of the dispersed phase on the continuous phase can be isolated. The data is used to deduce a source term for particle-induced turbulence production appropriate for a numerical simulation of the flow, based on the Reynolds-averaged Navier-Stokes equations. Although of special importance for a detailed understanding of turbulent two-phase combustion, additional complexity introduced by evaporation and chemical reactions is avoided by using glass beads as dispersed phase.     

A new laser vorticity probe — LAVOR: Its development and validation in a turbulent boundary layer

Vorticity measurements, which are scarce at the present time, can provide valuable dynamical information, particularly in unsteady and separated flows. Advances in laser Doppler anemometry and optical techniques have furnished the opportunity for the development of a non-intrusive vorticity probe with very fine spatial and temporal resolutions. The laser vorticity probe (LAVOR), which makes use of minimal laser beams and optical components, is capable of measuring velocity gradients with a separation distances as small as 0.3 mm. Velocity gradients are measured using two points on the same probe volume. However, unlike other techniques, the LAVOR also provides the instantaneous velocity at each point in the probe volume, and so provides additional dynamical information. The LAVOR probe was used in a fully turbulent two-dimensional boundary layer, and the data obtained are compared with the existing hot-wire vorticity data obtained in the same wind tunnel facility and with data obtained in other facilities. The spatial resolution is of the order of three Kolmogorov microscale units. Published online: 19 October 2002     

Simultaneous Measurement of Velocity and Fluctuating Pressure in a Turbulent Wing-tip Vortex Using Triple Hot-film Sensor and Miniature Total Pressure Probe

We have developed a probe-system for simultaneous measurement of three velocity components and pressure in turbulent flows. A miniature total pressure probe is placed adjacent to the sensors of a triple hot-film probe in order to achieve the spatial resolution which is equivalent to that of the triple hot-film probe itself. The instantaneous static pressure is calculated from measured velocity and total pressure by means of a newly developed processing method based on the Bernoulli equation for unsteady flows. The measurements were undertaken in a turbulent wing-tip vortex flow. The look-up table method is employed for the calibration of the hot-film probe so accurate velocity data could be obtained over a wide range of the flow-attack angles. It is also demonstrated that the present probe-system is capable of measuring fluctuations in both velocity and pressure in the 20?C650 Hz frequency range. The distribution of the fluctuating pressure obtained by this indirect method is in good agreement with the results from direct measurements of static pressure, demonstrating the promising performance of the present method. Furthermore, an improvement in the ability to make measurements of the velocity?Cpressure correlation across the wing-tip vortex is achieved. This improvement is possible because the effects of lateral velocity components are properly taken into account in the present formulation. The investigation regarding the transport equation budget for turbulent kinetic energy shows an anomalous structure of turbulence in this flow, mainly due to the meandering of the vortex, and the measurement of pressure diffusion is found to play an important role in the characterization of this kind of flow.     

The measurement of velocity gradients in fluid flow by laser light scattering

The motion of the speckle pattern in the far field limit is uniquely determined by the velocity gradient of the light scattering medium. This is the basic physical effect of a non-invasive method that allows the measurement of velocity gradients with high spatial and temporal resolution down to 75 m and 1 ms, respectively. We present three different measurement schemes for the analysis of the scattered light. The experimental results refer to mean gradients obtained from measurements on turbulent channel flows.     

Measurements of a supersonic turbulent boundary layer by focusing schlieren deflectometry

 Some novel, non-intrusive, high-frequency, localized optical measurements of turbulence in compressible flows are described. The technique is based upon focusing schlieren optics coupled with high-speed quantitative measurement of light intensity fluctuations in the schlieren image. Measurements of density gradient fluctuations confined to a thin slice of the flowfield are thus obtained. The new instrument was used to investigate the structure of a two-dimensional, adiabatic, wind tunnel wall boundary layer at a Mach number of 3. The measurements were compared to data obtained using hot-wire anemometry and good agreement was found between the two. Distributions of broadband convection velocity of large-scale structures through the boundary later were also measured. In marked contrast to earlier results, it is shown here that the convection velocity is essentially identical to the local mean velocity. Further, results obtained using the VITA conditional sampling technique shed new light on the turbulent boundary layer structure. Overall, the data presented herein serve to validate the new measurement technique. Received: 12 February 1997/Accepted: 31 January 1998     

Dissipation rate estimation from PIV in zero-mean isotropic turbulence

Measuring the turbulent kinetic energy dissipation rate in an enclosed turbulence chamber that produces zero-mean flow is an experimental challenge. Traditional single-point dissipation rate measurement techniques are not applicable to flows with zero-mean velocity. Particle image velocimetry (PIV) affords calculation of the spatial derivative as well as the use of multi-point statistics to determine the dissipation rate. However, there is no consensus in the literature as to the best method to obtain dissipation rates from PIV measurements in such flows. We apply PIV in an enclosed zero-mean turbulent flow chamber and investigate five methods for dissipation rate estimation. We examine the influence of the PIV interrogation cell size on the performance of different dissipation rate estimation methods and evaluate correction factors that account for errors related to measurement uncertainty, finite spatial resolution, and low Reynolds number effects. We find the Re _λ corrected, second-order, longitudinal velocity structure function method to be the most robust method to estimate the dissipation rate in our zero-mean, gaseous flow system.     

Hot-wire spatial resolution effects in measurements of grid-generated turbulence

We use grid-generated turbulence as a benchmark flow to test the effects of spatial resolution on turbulence measurements with hot wires. To quantify the spatial filtering, measurements of the turbulence statistics and spectra downstream of the grid were made using hot wires of varying length and compared to the results from a new nanoscale thermal anemometry probe, which has a sensing length of the order of, or smaller than, the Kolmogorov scale. In order to separate the effects of temporal and spatial filtering, a study was performed to ensure that the data were free of the artifacts of temporal filtering so that differences in the measurements could be wholly attributed to spatial filtering. An empirical correlation for the attenuation of the streamwise Reynolds stress due to spatial filtering is constructed, and it is shown that these grid turbulence results relate directly to the near-wall region of wall-bounded flows, where the effects of spatial filtering are most acutely felt. The effect of spatial filtering on the streamwise spectrum function is observed to extend to almost all wavenumbers, even those significantly lower than the length of the hot wire itself. It is also shown that estimates of the Kolmogorov scale are affected by spatial filtering when wires longer than the Kolmogorov length are used.     

Expansion ratio effects on the separated shear layer and reattachment downstream of a backward-facing step

Experiments were carried out to study the behavior of the incompressible turbulent separated shear layer and subsequent reattachment, downstream of a backward-facing step in a channel. The main objective of the study was to determine the effect of the expansion ratio on the development of the mean velocity and turbulence intensity in the shear layer and on the evolution of wall static pressure downstream of the step. The step height-to-upstream channel height ratio was varied between 0.5 and 2.13 while all inlet conditions were kept constant. Both hot-wire anemometry and frequency shifted laser Doppler anemometry were used for the velocity measurements. The Reynolds number based on free stream velocity and channel height upstream of the step was 16,600. The expansion ratio was found to have a particularly strong influence in the development of the turbulent, separated shear layer. Larger step height-to-inlet channel height ratios lead to higher turbulence intensities and faster growth of the unstable shear layer. As a result of this, shorter normalized reattachment lengths occurred with lager expansion ratios. For all the expansion ratios studied, the mean reattachment lenght was uniform along the spanwise direction except very near the side walls.     

The effects of flow split ratio and flow rate in manifolds

This paper reports a combined experimental and numerical investigation of three-dimensional steady turbulent flows in inlet manifolds of square cross-section. Predictions and measurements of the flows were carried out using computational fluid dynamics and laser Doppler anemometry techniques respectively. The flow structure was characterized in detail and the effects of flow split ratio and inlet flow rate were studied. These were found to cause significant variations in the size and shape of recirculation regions in the branches, and in the turbulence levels. It was then found that there is a significant difference between the flow rates through different branches. The performance of the code was assessed through a comparison between predictions and measurements. The comparison demonstrates that all important features of the flow are well represented by the predictions.     

Low Reynolds number axisymmetric turbulent boundary layer on a cylinder

In the present study, an axisymmetric turbulent boundary layer growing on a cylinder is investigated experimentally using hot wire anemometry. The combined effects of transverse curvature as well as low Reynolds number on the mean and turbulent flow quantities are studied. The measurements include the mean velocity, turbulence intensity, skewness and flatness factors in addition to wall shear stress. The results are presented separately for the near wall region and the outer region using dimensionless parameters suitable for each case. They are also compared with the results available in the open literature.The present investigation revealed that the mean velocity in near wall region is similar to other simple turbulent flows (flat plate boundary layer, pipe and channel flows); but it differs in the logarithmic and outer regions. Further, for dimensionless moments of higher orders, such as skewness and flatness factors, the main effects of the low Reynolds number and the transverse curvature are present in the near wall region as well as the outer region.     

Comparison of turbulent channel and pipe flows with varying Reynolds number

Single normal hot-wire measurements of the streamwise component of velocity were taken in fully developed turbulent channel and pipe flows for matched friction Reynolds numbers ranging from 1,000 ≤ Re _τ ≤ 3,000. A total of 27 velocity profile measurements were taken with a systematic variation in the inner-scaled hot-wire sensor length l ⁺ and the hot-wire length-to-diameter ratio (l/d). It was observed that for constant l ⁺ = 22 and l/d >~200l/d \gtrsim 200, the near-wall peak in turbulence intensity rises with Reynolds number in both channels and pipes. This is in contrast to Hultmark et al. in J Fluid Mech 649:103–113, (2010), who report no growth in the near-wall peak turbulence intensity for pipe flow with l ⁺ = 20. Further, it was found that channel and pipe flows have very similar streamwise velocity statistics and energy spectra over this range of Reynolds numbers, with the only difference observed in the outer region of the mean velocity profile. Measurements where l ⁺ and l/d were systematically varied reveal that l ⁺ effects are akin to spatial filtering and that increasing sensor size will lead to attenuation of an increasingly large range of small scales. In contrast, when l/d was insufficient, the measured energy is attenuated over a very broad range of scales. These findings are in agreement with similar studies in boundary layer flows and highlight the need to carefully consider sensor and anemometry parameters when comparing flows across different geometries and when drawing conclusions regarding the Reynolds number dependency of measured turbulence statistics. With an emphasis on accuracy, measurement resolution and wall proximity, these measurements are taken at comparable Reynolds numbers to currently available DNS data sets of turbulent channel/pipe flows and are intended to serve as a database for comparison between physical and numerical experiments.