Feedback control for form-drag reduction on a bluff body with a blunt trailing edge

Abstract

The objective of this numerical study is to increase the base pressure on a backwardfacing step via linear feedback control, to be ultimately translated to a drag reduction on a blunt-based bluff body. Two backward-facing step cases are simulated: a laminar two-dimensional (2D) flow at a Reynolds number of Re θ = 280, and a turbulent three-dimensional (3D) flow at Re θ = 1500 using large-eddy simulation. The control is effected by a full-span slot jet with zero-net-mass-flux, and two jet locations are examined. Linear system identification is performed to characterize the flow response to actuation, used to synthesize a control law. The control strategy is based on the premise that an attenuation of the instantaneous pressure fluctuations on the base of the step should lead to an increase in the time-averaged base pressure. Open-loop harmonic forcing is examined within a broad frequency range for both the 2D and 3D flows, which are found to respond differently to actuation. The controllers based on disturbance attenuation lead to sensible increases in base pressure (up to 70 % in 2D and 20 % in 3D) with higher efficiency than the best results achieved in open-loop. The results support the conjecture about the link between the base pressure fluctuations and mean, although it is shown that such a black-box model approach is not suitable for optimization without further physical insight.

Reducing the pressure drag of a D-shaped bluff body using linear feedback control

“…The aim is to use linear feedback control to achieve an increase in mean pressure on the back face or base of the geometry, so as to achieve a pressure drag reduction. This approach has previously been successful for BFS flows [15]; the present study reveals some theory underpinning the link between mean drag and fluctuations, and extends application to bluff bodies exhibiting interacting shear layers. A notable feature of the present control strategy is that it should be implementable in real experiments outside of the wind tunnel, even though we are using computational flow simulations as a test-bed.…”

“…Numerical simulations of such flows have been used as a test-bed in several active control studies, in which simplification of the Navier-Stokes equations was either used as a basis for feedback controller design [9,10], or input-output models were used as a basis for feedforward controller design [11,12]. For a BFS whose separating boundary layer has 3-D flow fluctuations at sufficiently high Reynolds number, the wake flow exhibits global instability and is an "oscillator" flow; most previous studies agree on the existence of a shedding/step mode and a lower frequency flapping or bubble pumping mode [13][14][15]. Previous numerical [15,16] and experimental [8,[17][18][19] feedback control studies have been performed; with the exception of [15], the sensor signal has been either based on wake velocity measurements or a measure of the reattachment length.…”

High-frequency forcing of a turbulent axisymmetric wake

“…A recent review of the subject by Choi et al (2008) includes techniques that are either passive (for example, Heenan & Morrison 1996, 1998Strykowski & Sreenivasan 1990;Parezanovic & Cadot 2012) or active (Zaman & Hussain 1981;Gaster et al 1985;Kim & Choi 2005;Vukasinovic et al 2010), the latter with or without feedback (Kang & Choi 2002;Dahan et al 2012). Effective direct-wake control (as opposed to separation delay), whether in open or closed loop, requires an understanding of the large-scale organised structures and their interaction.…”

Modelling for robust feedback control of fluid flows

“…Such loop-shaping controllers therefore provide a convenient framework for dealing with the parametric, dynamic and disturbance uncertainties encountered when attempting to control flows (1.1) from controllers designed upon simpler models (1.2). This simplicity of designing robust controllers has thus meant that H ∞ loop-shaping controllers have found use in a variety of applications, ranging from the flight control of vertical take-off aircraft (Hyde et al 1995), control of combustion oscillations (Chu et al 2003), bluff body form-drag reduction (Dahan et al 2012) and wind-turbine active blade-pitch control (Lu et al 2014).…”