CBET-EPSRC: Transition and Turbulence in Compressible Boundary Layers Subjected to Concave Surface Curvature

Ohio State University, The, Columbus OH

Investigators

Abstract

This research was funded under the NSF Engineering - UKRI Engineering and Physical Sciences Research Council opportunity NSF 1-067. Any solid body that moves through a fluid, for example air or water, experiences a resistive drag force that slows its motion down. Cars, trucks, motorcycles, airplanes, ships, and submarines all require a lot of fuel to overcome this drag force and move through air or water. A large part of this resistance is caused by friction effects between the surface and the fluid. These effects are confined in a very thin region near the surface, called the boundary layer. The boundary layer causes no real harm when its motion is very regular and ordered (laminar motion) because the resistive force is very small. However, on large bodies, like airplanes and trucks, the boundary layer is often chaotic because there are a lot of vortices and whirls (turbulent motion), which give a huge resistive force. The change of a boundary layer from laminar to turbulent is called laminar-turbulent transition. The knowledge of this change is essential information for the design of transport vehicles because it is critical to predict how much drag force is generated. This project seeks to study laminar-turbulent transition in air flowing over very fast airplanes. The focus is on the high-speed boundary layers over concave parts of the airplanes, like the bottom of the wing, because these curved surfaces are much more common than flat surfaces. These very fast flows also occur inside the engines, over turbine blades in particular, that compress gases to make the airplanes fly. A systematic theoretical and numerical study will be conducted in collaboration with researchers at the University of Sheffield to provide a complete understanding of when, where, and how a high-speed boundary layer flowing over concave solid surfaces transitions to turbulence. Integrated with the research effort, a variety of educational activities will be undertaken to showcase and instill the excitement of cyberphysics discovery in students at all levels, as well as to prepare a highly-trained workforce in high-speed flows and advanced cyberinfrastructure. The overall objectives of this CBET-EPSRC proposal are to advance the fundamental understanding of all the stages of laminar-turbulent transition in compressible boundary layers flowing over concave solid surfaces, starting from the entrainment of the free-stream disturbances into a laminar boundary layer (i.e., the receptivity stage) through transition up to a fully turbulent flow. The inclusion of the receptivity stage is a fundamental ingredient for transition prediction that has not been previously explored for subsonic and supersonic boundary-layer flows subject to streamwise concave curvature. The proposed approach includes receptivity analysis based on advanced nonlinear asymptotic theory, boundary-layer instability computations, and direct numerical simulations (DNS). A systematic theoretical and numerical study will be conducted to clarify the role of Gortler vortices, streamwise-elongated, counter-rotating structures caused by concave curvature, the dynamics of which is extremely difficult to predict and control. The detailed study of boundary-layer physics will also be combined with large-eddy simulations (LES) and Reynolds-averaged Navier-Stokes (RANS) to assess the performance of subgrid-scale (SGS) and RANS models for transitional flows dominated by Gortler vortices. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.