YIP Understanding Dynamics and Control of Unsteady Separation via Texture-Induced 3D Boundary Layers
Abstract
The overarching goal of the proposed research is to develop micro-texture-based passive control strategies to control boundary layer separation over aerodynamic surfaces. Specifically, we use textures to methodologically inducing weak secondary flows within the boundary layer that result in a disproportionate effect on the separation characteristics. We hypothesize that surface characteristics, such as skin-friction anisotropy, inhomogeneity and texture 3-dimensionality, can be engineered to target specific turbulent mechanisms responsible for generation of these secondary flows. Such passive control of separation characteristics using surface-texturing provides a reliable and cost-effective pathway to achieve superior aerodynamic performance. This is critical for developing next generation flight vehicles with improved stability and control in unsteady environments (in-ground effects, IGEs), improved noise characteristics and superior maneuverability.To this end, we will develop a versatile class of 3-dimensional micro-textures based on converging-diverging (`herringbone ) riblets with a parametric control of (a) surface characteristics (anisotropy, inhomogeneity and 3-dimensionality), and (b) constituent turbulent bottom-up mechanisms that disproportionately alter the separation behavior. We do this with a specific goal of achieving separation localization and delay (Objectives - 1&3) and improved separation resilience in steady and unsteady free-stream conditions (Objectives - 2&4). These goals are achieved in four objectives using state-of-the-art volumetric measurements of turbulence in a high Reynolds number boundary layer over a flat-plate. In Objective-1, we understand the turbulent mechanisms that result in amplification of weak texture-induced 3-dimensionality and the interactions between the texture-dominant sub-layer turbulence (around 1-5% of the boundary layer thickness) and outer-layer large-scale turbulence. Using 2D-riblet textures (without any texture 3-dimensionality), we parameterize the role of texture parameters in establishing these mechanisms that resulted in the highly three-dimensional separation observed in our preliminary experiments. These serve as the baseline behaviors and framework for Objectives-2 through 4. In Objective-2, we explore the role of texture 3-dimensionality, where we superpose a controlled low-order model atop the textured surface using a bespoke #Dynamic Surface# capability. We focus on controlling the spanwise and wall-normal transport terms by inducing a local, wall-normal component to the minimum skin-friction direction. Based on our preliminary experiments, we argue that this provides sufficient mechanistic control on the turbulent processes to enable us engineer the secondary flows and consequently achieve the desired improvements to mean separation behavior. In Objective-3, we understand the texture-induced changes to separation transients and non-equilibrium effects in the vicinity of separation. We focus on the incipient separation behavior, and the transient spanwise growth of separated flow through an inhomogeneous environment. These will be the most resolved measurements of separation over non-canonical surfaces till date, and will enable us understand the precise mechanisms by which the separation initiation occurs, and how texture-induced changes can mitigate the mean separation. In Objective-4, we build on this knowledge to develop optimal texturing 3-dimensionalities to enhance separation resilience in unsteady environments. We focus onfree-stream unsteadiness of two basic kinds -- (a) an advecting pressure perturbation that translates over the surface, and (b) a case of unsteady global change in free-stream pressure distribution. Aerodynamically, these are representative of the pressure fluctuations experienced in in-ground effects (IGEs) and aircraft pitching, respectively.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jan 13, 2025
- Source ID
- N000142512082
Entities
People
- Gokul Pathikonda
Organizations
- Arizona State University
- Office of Naval Research
- United States Navy