Hypersonic N₂ boundary layer flow velocity profile measurements using FLEET

Abstract

Femtosecond laser electronic excitation tagging (FLEET) velocimetry was used in the boundary layer of an ogive-cylinder model in a Mach-6 Ludwieg tube. One-dimensional velocity profiles were extracted from the FLEET signal in laminar boundary layers from pure N 2 flows at unit Reynolds numbers ranging from 3.4 × 10 6 / m to 3.9 × 10 6 / m . The effects of model tip bluntness and the unit Reynolds number on the velocity profiles were investigated. The challenges and strategies of applying FLEET for direct boundary layer velocity measurement are discussed. The potential of utilizing FLEET velocimetry for understanding the dynamics of laminar and turbulent boundary layers in hypersonic flows is demonstrated.

Hypersonic FLEET velocimetry and uncertainty characterization in a tripped boundary layer

“…The excellent SNR, as well as relatively short exposure duration compared to the emission decay time constant, minimized the impact of random and systematic errors in the identification of this centroid. The subscript m in the variables µ 1,m and µ 2,m indicates that the these are 'measured' value that in general contain significant contributions from random and systematic error terms as later explicitly defined in equation (12). The displaced centroid had a reduced SNR and a relatively long gate duration, and was not fit as a line to retain spanwise resolution.…”

“…For this reason the random and systematic error in µ 1 is neglected in future calculations. The measured centroid of the displaced gate was modeled as the summation of the real tagged molecule location (µ i,2 ), systematic error (µ i,2,err (τ )), and random error (µ ′ i,2,err (SNR)) as shown in equation (12). This definition of the centroid was plugged into equation ( 2) to produce equation ( 13), using the Reynolds decomposition to separate mean and fluctuating velocity…”

“…A single femtosecond beam was split into a grid pattern and imaged in a bore sight configuration to track two-dimensional velocity and vorticity [11]. FLEET has also been applied in the wall-normal imaging orientation with the femtosecond beam terminating on the surface of the model or in a beam port in the model [12,13]. Work by Limbach showed heating of the gas caused by traditional FLEET diagnostics [7,14], motivating the development of Selective two-photon absorptive resonance femtosecond laser electronic excitation tagging (STARFLEET).…”

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

“…An excellent review of the theory and previous uses of FLEET is provided by Danehy et al [15]. The FLEET technique has had great success in high-speed wind tunnel measurements, including in the Texas A&M ACE tunnel [16], the Purdue Mach 6 Quiet Ludwieg Tube [17], the AFRL Mach 6 Ludwieg Tube [18], and AEDC Tunnel 9 at Mach 18 [19]. FLEET utilizes a focused femtosecond laser pulse to dissociate nitrogen molecules in air which then recombine and slowly decays back down to the ground state, causing a microseconds-long relatively broadband emission in the visible spectrum [10].…”