14-Transition on BOLT forebody
Experimental hypersonic transition investigation over the BOLT forebody
Authors: Loïc Sombaert1,2,†, François Nicolas1, Mathieu Lugrin1, Nicolas Severac1, Jean-Marc Luyssen1, Sylvain Morilhat1, Jean-Pierre Tobeli1, Sébastien Esquieu2, Reynald Bur1
1 DAAA, ONERA, Université Paris Saclay, F-92190 Meudon – France
2 CEA/CESTA, 33116 Le Barp cedex – France
† Corresponding author
Email: † loic.sombaert@onera.fr / francois.nicolas@onera.fr / mathieu.lugrin@onera.fr / nicolas.severac@onera.fr / jean-marc.luyssen@onera.fr / sylvain.morilhat@onera.fr / jean-pierre.tobeli@onera.fr / sebastien.esquieu@cea.fr / reynald.bur@onera.fr
Keywords: experimental hypersonic, 3D boundary-layer transition, complex geometry
Abstract:
Building on the recent successful transition flight experiments, such as HIFiRE-1 or HIFiRE-5b, the Boundary-Layer Transition (BOLT) experiment has been designed by the Air Force Office of Scientific Research (USAF/AFOSR) in 2017 [1]. It aims at leveraging our abilities in understanding the physics behind boundary-layer transition in hypersonic flight by gathering an international community of researchers to prepare for the future BOLT-1b flight.
With highly swept leading edges and low-curvature concave surfaces, it is a complex geometry specifically designed to assess prediction tools without knowing, a priori, the dominant transition mechanisms.
An experimental investigation has been carried out in the R2Ch hypersonic blow-down facility of ONERA Meudon over a 1:3 subscale BOLT model. By setting different reservoir conditions (pressure, temperature) for each run, a large Reynolds number range (from 5.0 x 105 to 1.44 x 107) is reached at Mach numbers of 6 and 7, going from a fully laminar to a highly turbulent flow over the main surfaces of the model. With the simultaneous use of infrared thermography and high-frequency wall pressure measurements (PCB® and Kulite®), the campaign aims at assessing the development of transition and the occurrence of related instabilities.
The infrared measurements highlight a sudden rise of wall heat-flux linked to laminar-to-turbulent transition (up to 7 times higher when the flow becomes turbulent along the centerline at Mach 7). The Reynolds number sweep, associated with heat-flux data, helps in defining the flow topology: at low Re, it shows the development of two counter-rotating vortices in the central region, induced by the bow shock. Then, while continuously increasing Re, two turbulent lobes appear on the upper side of the body (Fig.1, left). The transition over the centerline comes out later, as a result of contamination from these turbulent lobes.
Analysis of power spectral densities of wall pressure from PCB® sensors draws attention to the growth and breakdown of instabilities. For now, three regions display an amplification of instabilities at different frequency ranges: one located in the front part of the model (f : [60,250] kHz), two supported by the centerline region ( f : [20,150] kHz, f : [100,200] kHz), and a last one inside the two side lobes where transition first appears ( f : [90,200] kHz) (Fig.1, right).
A cross-validation between ONERA-R2Ch and DLR-H2K measurements has been performed for three Reynolds numbers, showing good agreement in the frequency range for the latter instability at Mach 7 [2]. The mechanisms leading to these instabilities are under investigation and further physical insights will be provided by the time of the symposium.
Figure 1: Stanton number highlighting transition lobes (left), growth and breakdown of an instability over BOLT upper side given by PCB11 (right)
References
[1] B. M. Wheaton, D. C. Berridge, T. D. Wolf, R. T. Stevens, and B. E. McGrath, Boundary Layer Transition (BOLT) Flight Experiment Overview, AIAA Aviation Forum, Fluid Dynamics Conference, 2018.
[2] T. Thiele, S. Willems, and A. Gülhan, DLR AS-HYP Transition Experiments on BOLT-Model in H2K, DLR Technical Report, 2021.
