Experimental measurement of turbulence statistics and transverse integral scale of Mach 4 nozzle flow of the ONR-UTA arc jet wind tunnel

This study reports the first converged, non-intrusive measurement of a transverse integral scale of turbulence in an arc-jet flow, specifically a Mach 4 nitrogen freestream flow within the ONR–UTA (Office of Naval Research-University of Texas at Arlington) arc-heated plasma wind tunnel, Leste, using Femtosecond Laser Electronic Excitation Tagging (FLEET) velocimetry. A total of 4722 FLEET images capturing tagged fluid parcels were collected. To isolate the FLEET signal from the intense arc-jet background radiation, a 725 nm long-pass filter and a background subtraction algorithm were applied. The spatio-temporal displacement of the laser-induced tags was analyzed to extract axial velocity and its fluctuations along the transverse profile of the FLEET tag. Velocity determination employed a sliding signal box with 50 % overlap under a one-dimensional flow assumption, validated by comparing the FLEET tag length with the nozzle exit diameter and half-angle of the conical arc-jet nozzle. Of the dataset, 563 images were suitable for velocity measurements, and two-point correlation was used to determine the transverse integral scale. Previous work reported the first FLEET velocity measurements at the Mach 6 nozzle centerline, demonstrating the feasibility of quantitative velocimetry in arc-jet flows despite challenging background emissions. Subsequent studies extended this approach to Mach 4 flow for preliminary integral scale estimates; however, convergence was limited by the small number of usable images. The present study completes this progression by providing the first converged measurement of an integral scale of turbulence in an arc-jet flow using FLEET, which, to the authors’ knowledge, has not been previously reported in the literature. The correlation curve exhibits an almost monotonic decay to a minimum of 0.12, which, together with the low uncertainty of approximately 1.6 %, provides strong confidence in the measurement. Under the selected conditions, the mean axial velocity varied between 2330 and 2370 m/s, the turbulence intensity ranged from 3.7 to 4.5 %, and the transverse integral scale was 12.9 mm.