Hydroxyl tagging velocimetry in a supersonic flow over a cavity

Abstract

Hydroxyl tagging velocimetry (HTV) measurements of velocity were made in a Mach 2 (M 2) flow with a wall cavity. In the HTV method, ArF excimer laser (193 nm) beams pass through a humid gas and dissociate H2O into H + OH to form a tagging grid of OH molecules. In this study, a 7 x 7 grid of hydroxyl (OH) molecules is tracked by planar laser-induced fluorescence. The grid motion over a fixed time delay yields about 50 velocity vectors of the two-dimensional flow in the plane of the laser sheets. Velocity precision is limited by the error in finding the crossing location of the OH lines written by the excimer tag laser. With a signal-to-noise ratio of about 10 for the OH lines, the determination of the crossing location is expected to be accurate within +/- 0.1 pixels. Velocity precision within the freestream, where the turbulence is low, is consistent with this error. Instantaneous, single-shot measurements of two-dimensional flow patterns were made in the nonreacting M 2 flow with a wall cavity under low- and high-pressure conditions. The single-shot profiles were analyzed to yield mean and rms velocity profiles in the M 2 nonreacting flow.

Role of diffusion on molecular tagging velocimetry technique for rarefied gas flow analysis

“…The efficiency of MTV has been demonstrated for liquid flows (Koochesfahani and Nocera 2007) at mini (Gendrich et al 1997;Hu and Koochesfahani 2006) and micro (Thompson et al 2005;Elsnab et al 2010) scales. Its application to gaseous flows has been evidenced at millimetric scales in external flow configurations (Koochesfahani 1999;Stier and Koochesfahani 1999;Lempert et al 2003;Pitz et al 2005;ElBaz and Pitz 2012).…”

Vibrationally excited hydroxyl tagging velocimetry

“…In environments in which the tag fluorescence or phosphorescence is rapidly quenched (e.g., oxygen containing gas at atmospheric pressure), MTV methods require a two-step process in which one laser "writes" a tag line (usually via dissociation) and a second laser "reads" the displaced tag line (usually a photo-product from the write phase) [7,8]. For low quenching environments (e.g., low pressure or pure nitrogen at atmospheric pressure), the tag line fluorescence or phosphorescence can persist long enough to record the tag line movement without a second laser [9,10].…”

N2O molecular tagging velocimetry

“…Single-photon photodissociation of H 2 O with a 193 nm ArF laser was used to measure the line-of-sight averaged velocity in a shock tube by recording the displaced line position with UV absorption [17]. Hydroxyl tagging velocimetry (HTV) has been developed where a 193-nm ArF laser photodissociates H 2 O in a single-photon process to produce a large grid (up to 11 × 11 lines) of OH that is imaged by a second laser to determine the velocity in low-and high-temperature reacting flows in a two-dimensional plane [15,[18][19][20][21][22]. The HTV method has been used to measure supersonic flows in wind tunnels [20,21] and rocket exhausts [22].…”

“…Hydroxyl tagging velocimetry (HTV) has been developed where a 193-nm ArF laser photodissociates H 2 O in a single-photon process to produce a large grid (up to 11 × 11 lines) of OH that is imaged by a second laser to determine the velocity in low-and high-temperature reacting flows in a two-dimensional plane [15,[18][19][20][21][22]. The HTV method has been used to measure supersonic flows in wind tunnels [20,21] and rocket exhausts [22]. Although the OH tag can be long-lived at high temperature [15,18], at room temperature the OH lifetime is about 20 µs [19], limiting lowtemperature application to moderate-and high-speed flows.…”