High-speed Flow Measurements Research Articles

Multi-point FLEET velocimetry in a Mach 4 Ludwieg tube using a diffractive optical element.

A diffractive optical element was paired with femtosecond laser electronic excitation tagging (FLEET) velocimetry and used to probe multiple locations in a high-speed wind tunnel. Two configurations were explored, one that uses the traditional method of viewing from a perspective orthogonal to the beam axis and another that uses a perspective parallel to the beam axis. In the latter, the FLEET emissions are viewed as points that can allow for FLEET measurements in a wall normal fashion without the laser needing to impinge upon the surface. The configurations are demonstrated in a Mach 4 Ludwieg tube, highlighting their utility in high-speed flow measurements.

VUV to IR Emission Spectroscopy and Interferometry Diagnostics for the European Shock Tube for High-Enthalpy Research

The European Shock Tube for High-Enthalpy Research is a new state-of-the-art facility, tailored for the reproduction of spacecraft planetary entries in support of future European exploration missions, developed by an international consortium led by Instituto de Plasmas e Fusão Nuclear and funded by the European Space Agency. Deployed state-of-the-art diagnostics include vacuum-ultraviolet to ultraviolet, visible, and mid-infrared optical spectroscopy setups, and a microwave interferometry setup. This work examines the specifications and requirements for high-speed flow measurements, and discusses the design choices for the main diagnostics. The spectroscopy setup covers a spectral window between 120 and 5000 nm, and the microwave interferometer can measure electron densities up to 1.5 × 1020 electrons/m3. The main design drivers and technological choices derived from the requirements are discussed in detail herein.

TCC-III Engine Benchmark for Large-Eddy Simulation of IC Engine Flows

A collaborative effort is described to benchmark the TCC-III engine, and to illustrate the application of this data for the evaluation of sub-grid scale models and valve simulation details on the fidelity of Large-Eddy Simulations (LES). The TCC-III is a spark ignition 4-stroke 2-valve engine with a flat head and piston and is equipped with a full quartz liner for maximum optical access that allows high-speed flow measurements with Particle Image Velocimetry (PIV); the TCC-III has new valve seats and a modified intake-system compared to previous configurations. This work is an extension of a previous study at an engine speed of 800 RPM and an intake manifold pressure (MAP) of 95 kPa, where a one-equation eddy viscosity LES model yielded accurate qualitative and quantitative predictions of ensemble averaged mean and RMS velocities during the intake and compression stroke. Here, experimental data were acquired with parametric variation of engine speed and intake manifold absolute pressure to assess the capability of LES models over a range of operating conditions of practical relevance. This paper focuses on the repeatability and accuracy of the measured PIV data, acquired at 1 300 RPM, at two different MAP (95 kPa and 40 kPa), and imaged at multiple data planes and crank angles. Two examples are provided, illustrating the application of this data to LES model development. In one example, the experimental data are used to distinguish between the efficacies of a one-equation eddy viscosity model versus a dynamic structure one-equation model for the sub-grid stresses. The second example addresses the effects of numerical intake-valve opening strategy and local mesh refinement in the valve curtain.

Velocity and density field measurements by digital speckle method

Velocity and density field measurements based on image processing of laser speckle or pseudo-speckle pattern have been developed. Laser speckle velocimetry (LSV) or white-light speckle velocimetry (WSV), which corresponds to a high-image-density PIV, gives a local velocity vector map or whole field velocity contour map of a two-dimensional flow field seeded densely with fine particles. This technique has an advantage in high-speed flow measurement without limit of frame rate except for directional ambiguity. New techniques of laser speckle photography and laser speckle interferometry by means of digital image processing have been developed recently for density field measurements. In laser speckle photography, a local density gradient vector map is reconstructed by cross-correlation evaluation between the reference and the object speckle patterns. In laser speckle interferometry, an equi-density contour map is reconstructed by image subtraction between the reference and the object interferometric speckle patterns.

Video systems for PIV recording

CCD video cameras have been used extensively in recent years for image capture in fluid flow measurements with the particle image velocimetry (PIV) technique. Recent advances in CCD imaging technology have improved and extended the PIV technique to new applications, ranging from rotating machinery to 3D measurements. The progressive scan interline transfer CCD allows the time interval between two consecutive frames to be very small. Combined with frame straddling technique and the cross correlation analysis method, video cameras with such CCD design can be used for high-speed flow measurements, without the need for image shifting or image tagging methods to resolve directional ambiguity. Investigation of the pixel fill factor in CCD designs has indicated that the particle image diameter of two pixels or more is required to minimize the error in particle image location, which is important for accurate PIV measurements. Results of the investigation on a commercial video camera with the progressive scan interline transfer CCD design for its capability in high-speed flow measurements are presented. It is shown that this camera can achieve images for a time interval as small as s and for velocities of up to 250 m in the experiment.

High-speed flow measurements in a two-stroke engine by photon-correlation laser velocimetry

Three-dimensional mapping of the spatial structure and the temporal evolution of the air flow field in a commercially available two-stroke engine requires the ability to resolve a wide range of flow velocities (±250 m/s) at measurement locations near the cylinder wall and the piston crown. Laser-Doppler velocimetry that uses photon-correlation signal processing and Fourier-transform analysis permits ensemble-averaged velocity-distribution functions to be extracted from scattered-light signals that are sampled at nearly the Nyquist limit and whose strength is typically less than 1 photon per Doppler cycle. Selected results from a systematic study illustrate the impulsive jet-like flow through the transfer ports into the engine cylinder, the complexity of the resultant in-cylinder flow field, and the likelihood of large-scale flow-field variations from one engine cycle to another.

Modern experimental techniques for high-speed flow measurements

ITHIN the last few years there has been a marked increase in interest in supersonic and hypersonic flow regimes. This has been partly as a result of the establishment of a transatmpspheric vehicle and the National Aerospace Plane (NASP) program. Addressing the technology problems for these future systems will provide a significant challenge for the aerospace community. Supersonic and hypersonic airbreathing vehicles are highly integrated systems involving strongly coupled technologies. The conceptual design process for various components requires analytical and experimental tools. The flowfield regimes include the internal combustion of propulsion systems, the external flow of the vehicle, and the overall performance of the system. The rebirth of interest in high-speed flows follows a period of inactivity. During the 1960's, considerable research was conducted on supersonic and hypersonic flows. Much of the research was directed at understanding the problem of re-entry. Test facilities such as supersonic/hypersonic wind tunnels, shock tunnels, and shock tubes were established. The diagnostic instrumentation in these facilities provided limited data. The instrumentation included pressure transducers, heattransfer gages, schlieren and inter ferometry, and gas-sampling probes. The data were sufficient to help understand the flow phenomena. With the advent of computer and laser technologies, the activities in the area of computational fluid dynamics (CFD) and sophisticated instrumentation were accelerated. The design and analysis of re-entry-type vehicles have become increasingly dependent upon numerical schemes for predicting the flowfield.1 The adequacy of CFD as the sole design tool for aerodynamic vehicles in the foreseeable future is doubtful. 2 CFD produces more detailed aerodynamic information faster than previously possible, particularly on simple configurations such as an airfoil section, a fuselage, or even a complete wing. However, reliable modeling of complex complete configurations including vortex interactions, boundary-layer transition, and chemistry are not yet available. Even the detailed physics of the flow near the leading or trailing edge of a wing is very difficult, if not impossible,, to compute within the required accuracy. Experimental aerodynamics, therefore, remain an important component in the design of future transport aircraft. This is even more true in terms of development of the propulsion system. To examine the applicability of various diagnostic techniques, it is necessary to establish the measurement requirements in terms of the parameters and the measurement environments. This in turn requires the examination of the available or proposed design facilities. Figure 1 shows the performance envelope of some existing ground test facilities.3 Also shown is the performance profile for a proposed entry research vehicle (ERV). The research vehicle is proposed for investigation of basic flowfield phenomena for high-altitude/ hypervelocity flights. Nonintrusive measurement techniques are also needed in support of candidate flight experiments. Examination of the performance envelope of existing flow facilities (e.g., Fig. 1) will help determine the required dynamic range of the instrumentation employed in these facilities.4 It is also generally agreed that the environments associated with supersonic and hypersonic test facilities are harsh both inside and outside the test facility. Inside, the measurement environment is characterized by high temperatures, high velocities, and high noise levels. Outside, it is characterized by high noise and vibration levels and by varying temperature and pressure fields, which can adversely affect the alignment and overall performance. For application of optical techniques, this requires that all optical components be mounted independent of