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A directional event detector for conditional laser imaging

Conditional sampling of laser-image data in bi-directional nonperiodic unsteady flows requires an event detector which can resolve the characteristic direction of the flow. To avoid storing data which does not contain events of interest, event detection must be performed in real time. This paper describes a mixed analogue-TTL circuit which determines the characteristic flow direction from signals at three analogue inputs. When the analogue inputs indicate an event arriving from the preferred direction, the detector allows trigger signals to reach the imaging camera. The event detector has been used for conditionally-sampled PIV of the internal flow in an oscillating-jet nozzle. In this particular example, streamlines of the conditionally-averaged flow have a spiral focus and two half saddles which are not resolved by conventional time averaging. Introduction Conditional sampling is a well-established method for investigating the structure of unsteady flows. Van Atta [9] provides an early history, and Antonia [1] describes a number of variations on the technique. Before the development of laser imaging for experimental research, conditional sampling was most often used in conjuction with hot-wire anemometry or cold-wire thermometry. The essence of conditional averaging is to detect an event in the flow and to use the time or location of the event as an origin or reference point for ensemble averaging. The conditionalaveraging process therefore has two components, (a) ensemble averaging, and (b) event-detection. For the measured parameter u, a simple form of ensemble average can be written as u x t 1 N N ∑ i 1 u x t τi (1) where x is the space domain, t is the time domain, and events are detected at N points in time, τi : 1 i N . The average may include a weighting function. For quasi-periodic flows such as oscillating jets, time between adjacent events may mapped onto the interval 0 2π so that the conditionally-averaged parameter u x t becomes periodic [11]. Event detection is less easy to summarise because the available choice of detection criterion is virtually unlimited, especially if the data is recorded continuously and the detection algorithm is applied to the recorded data. Some of the simpler schemes only compare the instantanteous event signal with a fixed discriminator level. Readers interested in a more detailed discussion of event-detection may wish to consult reviews and other articles on conditional sampling [1, 2, 6, 8, 9, 10]. In years before 1990, conditional sampling was used with hotwire and other point-measurement sensors. The usual procedure was to store a continuous data record first; then a conditionalsampling filter would selectively extract data from the record and deliver it to the averaging algorithm. For PIV, the volume of data from the apparatus is so high that data not containing events of interest cannot be stored, and event detection must be performed in real time. Of published investigations making use of conditionallysampled PIV, the largest proportion ( 53%) synchronise the PIV sampling with the phase of mechanically or acoustically driven unsteadiness in the flow (for example, Li et al. [5]). Another 37% are investigations of flow produced by turbines, rotary pumps or helicopter rotors where PIV is phase-locked to the rotation of a shaft. At the time of writing, the authors know of only five examples (i.e. the remaining 10% of articles) of conditionallysampled PIV where there was no apparent external excitation of the unsteady flow and the event-detection signal was obtained from the flow itself. Of these only one oscillating flow, the precessing-jet flow of Wong et al. [12] was bi-directional. Wong used an Atmel 8-bit microprocessor to resolve the direction of oscillation. In this paper, we present an alternative method of sensing direction — that is, with a TTL circuit. Design Requirements The specific requirement is for a device which controls the conditional sampling of PIV in a naturally oscillating-jet flow. The oscillating jet is produced by flow through an orifice into a short but larger diameter chamber (Figure 1). Flow from the inlet orifice reattaches asymmetrically and precesses around the circumference of the chamber [4]. For the tests described in this paper the event detector receives signals from an array of three pressure sensors which are distributed over part of the circumference and are turned to face the inlet orifice. It is important not to bias the conditional average by using a detection algorithm which does not account for asymmetry of the unsteady flow. If, for example, precession in either direction were equally probable, a detection scheme which lacks sensitivity to direction would produce false symmetry in the conditional averages [3]. We therefore require that sampling occur only when precession is in a specified direction (i.e. clockwise or counter-clockwise). With three sensors, the event detector can also perform a crude form of spatial correlation so that smallscale features in the flow are less likely to be accepted as events.