Doppler Global Velocimeter Research Articles

High-speed light field camera and frequency division multiplexing for fast multi-plane velocity measurements.

Non-intrusive fast 3d measurements of volumetric velocity fields are necessary for understanding complex flows. Using high-speed cameras and spectroscopic measurement principles, where the Doppler frequency of scattered light is evaluated within the illuminated plane, each pixel allows one measurement and, thus, planar measurements with high data rates are possible. While scanning is one standard technique to add the third dimension, the volumetric data is not acquired simultaneously. In order to overcome this drawback, a high-speed light field camera is proposed for obtaining volumetric data with each single frame. The high-speed light field camera approach is applied to a Doppler global velocimeter with sinusoidal laser frequency modulation. As a result, a frequency multiplexing technique is required in addition to the plenoptic refocusing for eliminating the crosstalk between the measurement planes. However, the plenoptic refocusing is still necessary in order to achieve a large refocusing range for a high numerical aperture that minimizes the measurement uncertainty. Finally, two spatially separated measurement planes with 25×25 pixels each are simultaneously acquired with a measurement rate of 0.5 kHz with a single high-speed camera.

Planar near-nozzle velocity measurements during a single high-pressure fuel injection

In order to reduce the fuel consumption and exhaust emissions of modern Diesel engines, the high-pressure fuel injections have to be optimized. This requires continuous, time-resolved measurements of the fuel velocity distribution during multiple complete injection cycles, which can provide a deeper understanding of the injection process. However, fuel velocity measurements at high-pressure injection nozzles are a challenging task due to the high velocities of up to 300 m/s, the short injection durations in the $$\upmu\text{s}$$ range and the high fuel droplet density especially near the nozzle exit. In order to solve these challenges, a fast imaging Doppler global velocimeter with laser frequency modulation (2D-FM-DGV) incorporating a high-speed camera is presented. As a result, continuous planar velocity field measurements are performed with a measurement rate of 200 kHz in the near-nozzle region of a high-pressure Diesel injection. The injection system is operated under atmospheric surrounding conditions with injection pressures up to 1400 bar thereby reaching fuel velocities up to 380 m/s. The measurements over multiple entire injection cycles resolved the spatio-temporal fluctuations of the fuel velocity, which occur especially for low injection pressures. Furthermore, a sudden setback of the velocity at the beginning of the injection is identified for various injection pressures. In conclusion, the fast measurement system enables the investigation of the complete temporal behavior of single injection cycles or a series of it. Since this eliminates the necessity of phase-locked measurements, the proposed measurement approach provides new insights for the analysis of high-pressure injections regarding unsteady phenomena.

Speckle noise influence on measuring turbulence spectra using time-resolved Doppler global velocimetry with laser frequency modulation

A novel Doppler global velocimeter (DGV) with high temporal resolution is presented as a tool for measuring spatially resolved flow turbulence spectra for three components in order to characterize complex flows, e.g. in turbomachines. The proposed DGV technique is based on a sinusoidal laser frequency modulation. Its maximum available measurement rate equals the modulation frequency and amounts currently to 100 kHz. The harmonic analysis of the detector array signals reduces errors due to detector offset drifts, detector sensitivity changes, ambient light, camera misalignment and beam splitting errors in comparison with conventional DGV systems. The achievable statistical errors are considered by theoretical investigations and by experiments regarding detector noise as well as temporal and spatial scattered light fluctuations, e.g. due to speckles. An error propagation finally provides the determination of the noise power spectral density occurring as virtual turbulence in the measured turbulence spectra. It amounts to about 1.2 × 10−4 (m2 s−2) Hz−2 for mean flow velocities up to 40 m s−1 and 1 nW mean scattered light power per detector element. It rises for higher flow velocities in dependence on the flow turbulence. For the example of a nozzle flow with a mean velocity of 85 m s−1, which is disturbed by a cylinder, the final uncertainty is demonstrated to result in an effective bandwidth of the acquired turbulence spectra of 10 kHz and is thus sufficiently high for flow turbulence analysis. The measured velocity spectra agree well with comparison measurements using a hot-wire anemometer.

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.

Dopplerglobal velocimetry data in circular jets

A two-component Doppler global velocimeter (DGV)system has been improved through the use of vapour-limitediodine cells that have temperature-independent responses, alongwith nonpolarizing beam splitters and lower f-number lenses.Two-component DGV velocity measurements have been obtained fora 1 inch diameter uniform circular jet flow at a nominal exit velocity of 60 m s-1, as well as for an annular jet and aswirling jet. These data generally agree with earlier pointDoppler velocimeter and hot wire anemometer results towithin about 2-4 m s-1, and display a total variabilityfrom a smooth curve of ±2-3 m s-1. This level ofaccuracy has been obtained for a system that uses a cw argonion laser and eight-bit CCD cameras and digitizers. Exceptions tothis level of accuracy are noted in regions of significantsecondary scattering, due to scattered laser light that isreflected off the lip of the jet nozzle, as well as in regionsof low smoke seeding levels, resulting in low signal-to-noiseratios. A significant amount of the variability of the datafrom a smooth curve is due to the flat field correction.

Uncertainty estimates in DGV systems due to pixel location andvelocity gradients

The accuracy of Doppler global velocimeter systems is dependent uponmany parameters. The ability to obtain accurate values of the light intensityat specified locations in space can easily be the largest contributor touncertainty in the resultant velocity measurement. The effects of lightintensity spatial gradients and the accuracy of spatial locations of theindividual measurements are analysed to provide estimates of the magnitude ofthese uncertainties and to determine how these uncertainties can bereduced.

Results for a Two-Component Doppler Global Velocimeter

A two-component Doppler global velocimeter (DGV) system is described, and velocity measurements obtained to quantify its accuracy are presented. Molecular iodine vapor cells are used as frequency discriminating e lters to determine the Doppler shift of laser light that is scattered off of seed particles in a e ow, from which velocity is determined. Results are presented for velocity distributions over the surface of a rotating wheel, a fully developed pipe e ow, and a freejet. For the rotating wheel DGV results, rms noise levels display standard deviations of § 1 m/s, whereas total velocity range errors are between § 1 and 2 m/s out of 59 m/s. The rms error is dominated by residual errors in the e at-e eld correction, and the 8-bit camera resolution, whereas velocity range errors are largely ine uenced by iodine cell calibration accuracy. Measurements for fully developed turbulent pipe e ow and axisymmetric jet e ow show good agreement with pitot-static probe data. A random velocity offset error due to iodine cell side arm temperature variations of about 4 ‐5 m/s has been found to be the dominant error source in the present DGV system. A reference tab has been used to record the zero velocity signals in the pipe and jet e ows.

Doppler global velocimetry: problems and pitfalls

A 1D Doppler global velocimeter (DGV) system was designed and constructed. During the evaluation of this system, inaccuracies were observed which could be attributed to the performance of many individual components. Items requiring evaluation included the performance of video cameras, the transfer lens, the beam splitter, the ALF cell construction, the absorption line filter (ALF) cell charging, and video recording computer boards. Descriptions of the problems and corrections are detailed.