Separated flow past an airfoil is characterized by multiple natural frequencies associated with various instabilities.
- Traditional separation control forces at a single frequency and often neglects the influence of actuator dynamics (resonance).
Questions & Objectives
- Is the effectiveness of a control strategy related to the natural flow instabilities and/or interactions?
Objective 1: Identify and analyze the various global dynamics in a separated flow configuration
- Can we detect and leverage natural instabilities to understand, model, and control separated flow in a more efficient and systematic manner?
Objective 2: Investigate the effectiveness and efficiency of targeting the instabilities in open-loop experiments
Objective 3: Apply observations to design and implement effective closed-loop control strategies
Challenges & Approach
1. Traditional experimental methods for detection of flow frequency content are limited
- Localized dynamic sensors (location sensitive)
- High-rate global measurements ($$$)
Approach: Use modal analysis methods amenable to experimental techniques to obtain low-order, global estimates
2. Characteristics of 2D flow separation on airfoils is highly dependent on angle of attack and surface curvature
Approach: A canonical flat plate model retains the essential separation characteristics, eliminates curvature effects, and is amenable to both simulations and experiments
1. Estimate global flow dynamics (time-resolved velocity fields)
2. Identify characteristic frequencies/instabilities
3. Perform open-loop control
Target characteristic frequencies by modulating actuator resonance. The extent and height of the time-averaged separation bubble is reduced during control.
Forcing at or near the natural shear layer frequency "feeds" the characteristic roll-up of the shear layer, which increases mixing between the high-momentum freestream and the low-momentum separated flow. This ultimately reattaches the flow further upstream with little control effort, or cost, described by the coefficient of momentum Cμ.
DMD extracts modal structures of the characteristic shear layer and wake components and results reveal interaction between the wake and separation bubble dynamics
Most efficient OL control scheme targets multiple flow instabilities rather than a single instability
- CL control reattaches the flow (time average) by reducing the controlled shear layer oscillations