Analytical and experimental investigations of dual-plane PIV

In its 'classical' form particle image velocimetry (PIV) extracts two components of the flow velocity vector by measuring the displacement of tracer particles within a double-pulsed laser light sheet. The method described in this paper is based on the additional recording of a third exposure of the tracer particles in a parallel light sheet, which is slightly displaced with respect to the first one. The particle images resulting from these three exposures are stored on separate frames. The locations of the correlation peaks, as obtained by cross-correlation methods, are used to determine the projections of the velocity vectors onto the plane between both light sheets. In the manner described below, the amplitudes of these peaks are used to obtain information about the velocity component perpendicular to the light sheet planes. The mathematical background of this method is described in the paper. Numerical simulations show the influence of the main parameters (e.g. light sheet thickness, light sheet displacement and out-of-plane component) on the resolution and reliability of the new method. Two different recording procedures and their results will be shown to demonstrate the ease of operation when applying this technique to liquid flows

PIV Measurements of unsteady transonic flow fields above a NACA 0012 Airfoil

Particle Image Velocimetry (PIV) is increasingly used to investigate unsteady velocity field instantaneously. At DLR an experimental set-up for PIV has been developed which can be operatred under the rough environmental conditions of a large wind tunnel. This system has successfully been applied as well to low speed as to high speed flows (U &#61 10. 500 m/s). The evaluation and post processing of the PIV recordings runs fully automatical on a work station. The paper will describe investigations of instantaneous transonic flow fields above a NACA 0012 airfoil at different angles of attack. Some improvements of the PIV technique had to be carried out in order to be able to utilize the PIV technique in a high speed wind tunnel with a measuring time of less than 20 seconds. Among these improvements are a fas focusing device for the recording camera, on line observation of seeding with tracer particles, master control unit for automatic control of the whole measuring sequence and the data aquisition of pressure, temperature, anlge of attack etc. A high speed image shifting system is required in order to be able to measure even in reverse flows or in order to optimize the dynamic range of the evaluation. For this purpose a high speed rotating mirror system was developed. The frequency of rotation ranges from 1 Hz to 50 Hz, thus covering a range of shift velocities from 10 m/s to 500 m/s (at a ratio of reduction of 1:7 between oberservation plane in the flow to the recording plane)

Application of PIV technique to transonic flows

Particle Image Velocimetry (PIV) is increasingly used to investigate unsteady velocity fields instantaneously. At DLR an experimental set-up for PIV has been developed which can be operated under the rough environmental conditions (noise, vibrations) of a large wind tunnel. This system has successfully been applied as well to low speed as to high speed flows (U &#61 10. 500 m/s). The evaluation and post processing of the PIV recordings runs filly automatical on a work station

Application of PIV technique to transonic flows in a blow-down wind tunnel

Particle Image Velocimetry (PIV) is increasingly used to investigate unsteady velocity fields instantaneously. At DLR an experimental setup for PIV has been developed which can be operated in transonic flows under the rough environmental conditions (noise, vibrations) of a large high speed wind tunnel. This PIV-system has been successfullyapplied to flow field measurements in the velocity range from U = 10 to 500 m/s. The evaluation and post processing of the PIV recordings runs fully automatical on a workstation. Two different aerodynamic investigations of instantaneous transonic flow fields around a bluff cylinder and around a NACA 0012 airfoil have been performed in a blow-down wind tunnel. The experience gained during these experiments and some technical improvements of the PIV technique which are necessary to enable its application in transonic flows are described

Application of Particle Image Velocimetry in Highly Three-Dimensional Flows

The image shifting technique, originally developed to resolve the directional ambiguity, also provides the opportunity to influence other parameters relevant to the PIV technique and thus, to enable its application of pulse separation and image shift is required to obtain high quality data of velocity vector fields even in strongly three dimensional flows. It has already been reported in the literature that errors will result from the fact thatthe projection of the 3D velocity vector of the real flow field into the 2D plane of recording is not a parallel projection, as it is mostly assumed for simplification, but a perspective projection

Application of the PIV measurement technique for aerodynamic research in EC projects

For the past few years particle image velocimetry (PIV) has been increasingly used for aerodynamic research and de-velopment. This is mainly due to the unique feature of the PIV technique, which allows the recording of a complete velocity field in a plane of the flow within a few microseconds. Thus, PIV provides information about unsteady flow fields, which is difficult to obtain with other ex-perimental techniques. The short acquisition time and fast availability of data reduce the operational time, and hence cost, in large scale test facilities. Technical progress made in the last years allowed the PIV teams of the European research establishments on aeronautics to develop reliable, modular PIV systems for use in industrial wind tunnels. Development, and application of PIV as well as information exchange about PIV with industrial end-users have been considerably supported by EC. This paper will summarize the state-of-the-art of PIV by describing the basic features of DLR's PIV system and will present some results of recent PIV applications within EC funded projects such as EUROPIV 1 and 2, PivNet, APIAN, WAVENC, EUROWAKE, C-Wake, EUROLIFT, and HELIFLOW. Most of these results have been obtained by joint teams of different EREA members, European aeronautical industry and wind tunnels

Theoretical and experimental aspects of PIV recording utilizing photographic film and mechanical image shifting

The current level of technology allows photographic recording to achieve a higher spatial resolution than can be obtained with current methods of digital video recording, especially if using large formats. Though CCD sensors with increasing resolutions are continuously under development and brought into the market, it will be some time until video recording techniques can match the spatial resolution of present day film material. Another problem associated with video PIV is the requirement to store the huge amount of data, which may be captured during a measuring campaign. Thus, for time being, the photographic technique is the method of choice for PIV applications requiring high velocity and spatial resolution. One major disadvantage of the photographic technique is that - at present - it is difficult to record the images of the tracer particles onto two different photographic plates, especially in the case of aerodynamic investigations where pulse separations on the order of a few microseconds are required. That indicates that the problem of directional ambiguity removal has to be solved for photographic PIV in a reliable and flexible manner using a technique such as image shifting. Additionally, the application of an image shift to the PIV recording allows PIV measurements in strongly three-dimensional flows as well as in flows with strong velocity gradients. By 'negative' image shifting the resolution of the velocity measurement can be increased with a slight reduction to the spatial resolution. A rotating mirror system, which is able to provide very high and easily adjustable shift velocities, will be describled in the following. Problems such as the angular control of the rotating mirror and its synchronization with the laser pulses have been solved. A detailed theoretical model of the imaging of tracer particles by means of a rotating mirror system has been developed in order to be able to correct certain errors appearing at imaging via a rotating mirro

The Importance of Image Shifting to the Applicability of the PIV Technique for Aerodynamic Investigations

The great interest in PIV measurements in many different fields of research requires also a flexible image shifting system which can be applied to a variety of experimental situations. Image shifting is primarily known as a method for eliminating the ambiguity of the direction of the velocity verctor within PIV recordings. Further capabilities of the image shifting technique which increase the general applicability of the PIV-method will be described. The advantages of the proposed proceeding is demonstrated by referring to a variety of wind tunnel-experiments carried out by us

Theoretical and experimental aspects of image shifting by means of a rotating mirror system at particle image velocimetry

One possibility of remove the ambiguity of the direction (i.e. sign) of the velocity vectors at the utilization of Particle Image Velocimetry is to employ the technique of image shifting when recording the images of the tracer particles. The increasing number of applications of the PIV technique expected in near future requires a flexible image shifting system, which can be easily adapted to most different problems and experimental set-ups. Such a system, which must able to provide very high and easily adjustable shift velocities, can be attained by utilizing a rotating mirror for image shifting. In contrast to other image shifting methods no additional demands have to made on the light source and the scattering behaviour of the tracer particles. Problems as e.g. generating and controlling the rotation of the mirror and synchronizing the laser pulses with the angular position of the mirror have been solved successfully. A detailed theoretical model of the imaging of tracer particles by means of a rotating mirror system, which is required to obtain information about the accuracy of the method, is presented here for the first time. Further capabilities of the image shifting technique, which - in general - increase the applicability of the PIV-method for measuring complex flow fields are also described. The efficiency of the rotating mirror system is demonstrated by referring to the results of different wind tunnel measurements