AISCOS a New Approach to Active Flow Control of Separation

2018

An innovative solution dedicated for an effective control of strong separation of air flow has been proposed. The solution is based on a system of coupled pairs of nozzles located on the surface, where the strong flow separation is forecasted. When such phenomenon appears, in each pair of coupled nozzles a flow is activated, in such a way, that one of the nozzles starts blowing, while the other starts sucking the air from the main flow region. A matrix of such fluidic devices is able to significantly alleviate and even completely eliminate the strong separation of the flow on a given surface. The discussed solution has been investigated and partially optimized based on a computational-simulation approach. Additionally, an innovative idea of a fluidic actuator, feeding the nozzles with both the overpressure and underpressure has been presented.

Universal Journal of Mechanical Engineering

Flow separation is expected to have the effect of increasing aerodynamic drag due to decreased pressure distribution at the rear of the vehicle. The faster the flow separation occurs, the lower the pressure distribution is in the area, thereby reducing vehicle performance. Therefore, flow modification is needed with expected effects on the separation delay and the reduction in wake and vortex formation. This modification can be done through the application of suction active control in the separation area. The research is intended to analyze the effect of suction active control on flow characteristics, pressure distribution and aerodynamic drag on vehicle models with suction velocity variations. The test model used is an Ahmed model modified by changing the orientation of the flow. The study used a numerical computational approach with a standard k-epsilon turbulence model at 19.4 m/s upstream velocity. Results revealed that the use of flow active control was able to reduce wake and vortex formation through separation delay and to increase the minimum pressure coefficient by 73% on the model with Usc 2 suction velocity of 0.5 m/s, gaining the highest drag coefficient reduction of 10.897% in the same model.

AIP Advances, 2022

This study deals with the wall resolved Unsteady Reynolds-Averaged Navier-Stokes (URANS) simulation of boundary layer flow separation over a circular hump model and its active control. An array of Synthetic Jet Actuators (SJAs) is implemented in the hump model to introduce a train of vortex rings into the boundary layer to control flow separation. The OpenFOAM solver is used to numerically simulate and analyze the fluid flow using the k-ω shear stress transport model. Hot wire anemometry and particle image velocimetry measurements are carried out to evaluate the accuracy of the URANS technique as well as the effectiveness of SJAs by comparing numerical predictions to experimental data. The time-averaged results are in a good agreement with experimental results and demonstrate a successful application of SJAs to delay the flow separation by the interactions of vortical structures with the separated shear flow. The three-dimensional simulation also reveals that near wall coherent flow structures (streamwise and spanwise vortices) are responsible for the wall shear stress components. The results can be used to better understand the performance of SJAs and to further improve future actuator configurations.

AIAA Journal, 2010

Compressible large-eddy simulations of turbulent flow over a wall-mounted hump with active flow control are performed and compared with previous experiments. The flow is characterized by the unsteady separation before the steep trailing edge, which naturally reattaches downstream of the hump to form an unsteady turbulent separation bubble. The low Mach number large-eddy simulation demonstrated a good prediction of surface pressure coefficient, separation-bubble length, and velocity profiles compared with experiments. The effect of compressibility on the baseline flow is documented and analyzed and is found to increase the separation-bubble size, due to a reduced growth rate. Control is applied just before the natural separation point via steady suction and zero-net-mass-flux oscillatory forcing, and steady suction is shown to be more effective in decreasing the size of the separation bubble and pressure drag for the control parameters investigated. Controlled flow at a compressible subsonic Mach number is applied, and found to be slightly less effective than the same control parameters at low Mach numbers.

1st Flow Control Conference, 2002

Active flow control in the form of periodic zeromass-flux excitation was applied at the slat shoulder of a simplified high-lift airfoil to delay flow separation. The NASA Energy Efficient Transport (EET) supercritical airfoil was equipped with a 15% chord simply hinged leading edge slat and a 25% chord simply hinged trailing edge flap. The cruise configuration data was successfully reproduced, repeating previous experiments. The effects of flap and slat deflection angles on the performance of the airfoil integral parameters were quantified. Detailed flow features were measured as well, in an attempt to identify optimal actuator placement. The measurements included, steady and unsteady model and tunnel wall pressures, wake surveys, arrays of surface hot-films, flow visualization and Particle Image Velocimetry (PIV). High frequency periodic excitation was applied to delay the occurrence of slat stall and improve the maximum lift by 10 to 15%. Low frequency amplitude modulation was used to reduce the oscillatory momentum coefficient by roughly 50% with similar aerodynamic performance. Nomenclature AFC Active Flow Control AM Amplitude Modulation α Angle of Attack µ c steady blowing momentum coefficient , cq J ≡ µ c oscillatory blowing momentum U ρ R c chord Reynolds number, ν / c U ∞ ≡ T temperature U, u average and fluctuating streamwise velocity x/c normalized streamwise location X sp distance from baseline separation to reattachment z spanwise location ν kinematic viscosity ρ density Abbreviations LE leading edge TE trailing edge < > phase locked values Subscripts b baseline flow conditions c cavity d de-rectified hot-wire data j conditions at blowing slot N Normalized according to text R reattachment S separation ∞ free-stream conditions 2D two-dimensional 3D three-dimensional Superscripts ′ root mean square of fluctuating value

2013

Slope of pressure with respect to chord dimension Re Reynolds number based on chord length U ∞ Freestream velocity V lif t Lift-based performance function w i Trapezoidal summation weights Z lif t Lift-approximating function

2000

Design and implementation of a digital feedback controller for a flow control experiment was performed. The experiment was conducted in a cryogenic pressurized wind tunnel on a generic separated configuration at a chord Reynolds number of 16 million and a Mach number of $0.25$. The model simulates the upper surface of a $2 airfoil at zero angle-of-attack. A moderate favorable pressure gradient, up to $5 pressure gradient which is relaxed towards the trailing edge. The turbulent separation bubble, behind the adverse pressure gradient, is then reduced by introducing oscillatory flow excitation just upstream of the point of flow separation. The degree of reduction in the separation region can be controlled by the amplitude of the oscillatory excitation. A feedback controller was designed to track a given trajectory for the desired degree of flow reattachment and to improve the transient behavior of the flow system. Closed-loop experiments demonstrated that the feedback controller was a...

Direct simulations are carried out in a flow configuration devised for investigating zero-net-mass-flux (or synthetic) jet based active separation control. The numerical configuration consists of an airfoil section at zero incidence in a free-stream. A separation bubble of prescribed size is created on the top surface of the airfoil at the aft-chord location by applying blowing and suction on the top boundary of the computational domain. Such separated flows are generally characterized by three distinct time scales corresponding to the shear layer, the separation zone, and the vortex shedding in the wake; therefore the resulting flowfield can be considered as a canonical separated airfoil flow. Simulations of this flow over two different airfoil sections at a chord Reynolds number of 60, 000 subject to zero-net-mass-flux (ZNMF) perturbation of the boundary layer at different characteristic time scales are presented. Simulations of the flow over an elliptic airfoil indicate that ZNMF forcing at a frequency corresponding to the separation zone or the shear layer draws a better response as compared to excitation at the wake vortex-shedding frequency. Results also show that locating the ZNMF device at the separation point leads to a more effective separation control, whereas ZNMF forcing inside the separation bubble does not significantly alter the baseline separated flow. Results from the second set of simulations over a flat plate with elliptic leading edge and blunt trailing edge indicate that the entire system comprised of the shear layer, the separation zone, and the wake is locked on to a single frequency.

AIAA Conf paper, 2020

An experimental study on the application of active flow control (AFC) to a 1:8.4 scale model of a swept wing in a landing configuration was conducted. The wing is fitted with an Ultra High Bypass Ratio (UHBR) engine nacelle. The highly efficient UHBR engines characterized by a large diameter that interferes with the flow around the wing, degrading its performance. An innovative active flow control device, creating steady suction and pulsed blowing (PB), was installed in the leading-edge region of the wing, above the nacelle, and its performance was experimentally evaluated. The effects of the suction and PB mechanisms were examined individually and simultaneously, using relevant normalized parameters to pave the way to a full-scale wind tunnel test. It was shown that the AFC devices increase the lift by up to 3%, redirected the flow to the desired downstream direction and reduced the size of the separation zone created due to the implementation of the UHBR nacelle. The next step is validating the small-scale results of this study in full-scale wind tunnel tests that hopefully make the technology flight test ready. I. Nomenclature Ar = Model reference area, As = Area of suction holes b = Span length c = Chord length CD = Drag coefficient CL = Lift coefficient Cp = Pressure coefficient, − ∞ 1 2 ∞ ∞ 2 Cq = Mass flow coefficient, ∞ ∞ Cµ = Momentum coefficient, 2 2 ∞ ∞ 2 = 2∞ ∞ 2 F + = Strouhal number, / ∞ ṁ = Pulsed blowing inlet mass flow Pin = Pulsed blowing inlet pressure Psuc = Suction pressure Re = Reynolds number, ∞ / U∞ = Free-stream velocity ρ = Air density

AIAA Journal, 2006