There are different mechanisms which lead to laminar-turbulent transition on a wing. One of the many reasons transition occurs is due to impingement of insects, surface roughness, external sound or other disturbance sources, in particular arising from freestream turbulence. The image depicts a scenario where the laminar flow over a wing undergoes transition to turbulence due to the presence of a turbulator.
Reference: Simon, Bernhard, et al. “IR thermography for dynamic detection of laminar-turbulent transition.” Experiments in Fluids 57.5 (2016): 93.
Flow visualisation images show transition to turbulent flow due to the flow separating from the surface in the presence of an adverse pressure gradient- the flow is from left to right.
Reference: Jagadeesh, Chetan, and Hermann Fasel. “Experimental investigation of the structure and dynamics of laminar separation bubbles.” 50th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition. 2013.
Infrared thermography conducted on a swept wing displaying the transition process from laminar flow to a turbulent one due to surface imperfections.
Reference: Davidson, Todd SC, et al. “Lights, Camera, Data: Optical Technique Development at ARA.”
A Temperature Sensitive Paint image for the upper surface of a left wing at Re 7M – flow is from top to bottom in the image. The lighter regions signify laminar flow and darker regions, turbulent flow. The downstream limit of laminar flow on the outboard wing is the result of the shock. The triangular wedges are the result of some form of surface contamination near the leading edge of the wing. The study of the impact of different surface imperfections on transition at flight Reynolds numbers is an ongoing research study.
Reference: Crouch, Jeffrey, et al. “Assessment of the National Transonic Facility for Natural Laminar Flow Testing.” 48th AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition. 2010.
A high-speed digital Schlieren system is used to visualise the flow field in the vicinity of the seal.
Reference: Jagadeesh, C. City University
These imaging techniques help to understand the influence of the seal position on the flow.
In the video the flow is from left to right, over a bump. The free-stream Mach number is 0.75. The flow accelerates over the 2-dimensional bump, reaching sonic conditions, resulting in a shockwave. Upstream of the shockwave a small surface excrescence results in the disruption of flow, thereby influencing the position of the so-called lambda shockwave.
The last two images demonstrate the use of Schlieren based flow imaging to identify and study the complex flow phenomenon, especially related to the development of boundary layers and its interaction with shock waves (at Mach 0.8 and at Mach 1.0).
Reference: Jagadeesh, C. City University
