Experimental Investigation of Crossflow in the Presence of a Laminar Separation Bubble in Low-Speed Flow

Abstract

An experimental investigation has been conducted to explore the behavior of crossflow in the presence of a laminar separation bubble in low-speed flow. Crossflow and laminar separation bubbles have been studied separately in experiment and simulation. However, there is little work with both flow features existing concurrently. In this work, a modified NACA 643-618 airfoil is tested at a variety of Γ, α, and ReC to define a parameter space for the interaction between crossflow and laminar separation bubbles. Discrete roughness elements were necessary to promote the most unstable stationary crossflow modes and expand the parameter space to conditions relevant to stationary crossflow vortices and laminar separation bubbles. Characteristics of the laminar separation bubbles on the airfoil were obtained through time-averaged pressure measurements and infrared thermography. Infrared thermography was also used to detect the formation of stationary crossflow vortices. Time-resolved hotwire measurements provided information about primary travelling crossflow waves and the Kelvin-Helmholtz instability near the transition region of the laminar separation bubble. The first portion of this investigation showed that Γ has a negligible impact on laminar separation bubble characteristics when ReC = 200, 000. In the unforced case (no roughness elements), for [Γ = 45◦ , α ≤ −5 ◦ , ReC = 600, 000] stationary crossflow vortices were present, but little evidence exists for a laminar separation bubble at these conditions. Passive forcing with roughness elements created observable stationary crossflow vortices at [Γ = 45◦ , α = 0◦ , ReC = 600, 000]: a case where surface pressure measurements showed evidence of a laminar separation bubble. In the absence of a laminar separation bubble, the frequency content around the transition location for [Γ = 45◦ , α = −8 ◦ , ReC = 600, 000] (with roughness elements added, a case with crossflow dominated transition) exhibits primary crossflow instability modes in line with predictions from parabolized stability equations. In the presence of crossflow and a laminar separation bubble, the frequency content around the transition location for [Γ = 45◦ , α = 0◦ , ReC = 600, 000] aligns well with predictions from parabolized stability equations for primary crossflow instability frequencies and displays higher frequencies associated with the Kelvin-Helmholtz instability- as suggested by linear stability theory. The formation of distinct peaks in this higher frequency range calls for further investigation to better understand the underlying instability mechanisms and their potential interaction.