Diogo Barros

On the need for a nonlinear subscale turbulence term in POD models as exemplified for a high-Reynolds-number flow over an Ahmed body

We investigate a hierarchy of eddy-viscosity terms in proper orthogonal decomposition (POD) Galerkin models to account for a large fraction of unresolved fluctuation energy. These Galerkin methods are applied to large eddy simulation (LES) data for a flow around a vehicle-like bluff body called an Ahmed body. This flow has three challenges for any reduced-order model: a high Reynolds number, coherent structures with broadband frequency dynamics, and meta-stable asymmetric base flow states. The Galerkin models are found to be most accurate with modal eddy viscosities as proposed by Rempfer & Fasel (J. Fluid Mech., vol. 260, 1994a, pp. 351–375; J. Fluid Mech. vol. 275, 1994b, pp. 257–283). Robustness of the model solution with respect to initial conditions, eddy-viscosity values and model order is achieved only for state-dependent eddy viscosities as proposed by Noack, Morzynski & Tadmor (Reduced-Order Modelling for Flow Control, CISM Courses and Lectures, vol. 528, 2011). Only the POD system with state-dependent modal eddy viscosities can address all challenges of the flow characteristics. All parameters are analytically derived from the Navier–Stokes-based balance equations with the available data. We arrive at simple general guidelines for robust and accurate POD models which can be expected to hold for a large class of turbulent flows.

Bluff body drag manipulation using pulsed jets and Coanda effect

The impact of fluidic actuation on the wake and drag of a three-dimensional blunt body is investigated experimentally. Jets blowing tangentially to the main flow allow to force the wake with variable frequency and amplitude. Depending on the forcing conditions, two flow regimes can be distinguished. First, in case of broadband actuation with frequencies comprising the natural wake time scale, the convection of the jet structures enhances wake entrainment, shortens the length of the recirculating flow and increases drag. Secondly, at higher actuation frequencies, shear-layer deviation leads to fluidic boat-tailing of the wake. It additionally lowers its turbulent kinetic energy thus reducing the entrainment of momentum towards the recirculating flow. The combination of both mechanisms produces a raise of the base pressure and reduces the drag of the model. Both actuation regimes are characterized by complementary velocity, pressure and drag measurements at several upstream conditions and control parameters. By adding curved surfaces to deviate the jets by the Coanda effect, periodic actuation is reinforced and drag reductions of about 20% are achieved. The unsteady Coanda blowing not only intensifies the flow deviation and the base pressure recovery but also preserves the unsteady high-frequency forcing effect on the turbulent field. The present results encourage further development of fluidic control to improve the aerodynamics of road vehicles and provide a complementary insight into the relation between wake dynamics and drag.

Control of a three-dimensional blunt body wake using low and high frequency pulsed jets

We present experimental evidence of pressure changes in the wake of a three-dimensional blunt body by the use of periodic pulsed jets. The jets are pulsed tangentially to the shear layers separated from the trailing-edges of a classical square-back Ahmed body. The Reynolds number based on the models height is ReH = 3.105. Significant decrease (respectively increase) of the rear pressure are achieved considering low and high frequency pulsing. Low frequency actuation (St = Hf/Uo = 0.4, where f is the frequency and Uo the upstream velocity) is shown to enhance the global wake mode and to increase drag. On the contrary, dynamical effects associated to the particular flow control strategy provides a significant drag decrease for the higher frequency forcing (St = 11.5). Time-averaged pressure on the back surface of the model and velocity measurements on the wake illustrates the main effects of such actuation and introduce new strategies for drag control of three-dimensional geometries.

Resonances in the forced turbulent wake past a 3D blunt body

We study the resonances of a forced turbulent wake past a flat-based bluff body using symmetric and antisymmetric actuation modes. The natural, unforced wake flow exhibits broadband dynamics superimposed on oscillatory motions linked to the reminiscent laminar Benard-von Karman instability in the turbulent flow. Harmonic and subharmonic resonances can be controlled by the phase relationship of periodic forcing and are linked to the symmetry properties of vortex shedding. Symmetric forcing leads to a strong subharmonic amplification of vortex shedding in the wake, but no harmonic excitation. The robustness of the subharmonic resonance is confirmed at different Reynolds numbers. Antisymmetric actuation, however, promotes a harmonic resonance with very similar wake and drag features.

Effects of Unsteady Coanda Blowing on the Wake and Drag of a Simplified Blunt Vehicle

The impact of periodic forcing on the wake past a square back bluff body is experimentally studied. By the use of pulsed jets in combination with a Coanda effect, shear layer forcing allows to recover over 30 % of the model’s base pressure. The actuation frequency is an order of magnitude higher than the natural flow instabilities. Velocity measurements indicate that this direct wake control modifies the vorticity development along the shear layers and shrinks the width of the recirculating flow region down. At the same time, the velocity fluctuations in the near wake decrease, without measurable impact on the oscillatory vortex shedding. With this control strategy, both the flow deviation and the base pressure recovery are dependent on the upstream Reynolds number. Particle image velocimetry data and pressure measurements are used to discuss the origin of these observations.