Session A25: Flow Control: General

Comparative Study on the Aerodynamic Effects of Various Jet Deflectors on a Mach 0.1 Steady Jet

In this study, we analyze the effect of three distinct jet deflector designs - solid plate, porous, and super ellipse - on a steady jet at a Mach number of 0.10 exiting from a 2-inch nozzle. The Reynolds numbers based on the nozzle exit diameter and the jet exit velocity is 103,000. The jet deflector prototypes were created using 3D printing to elucidate their distinct aerodynamic effects. All the deflectors are placed at a distance of 3D downstream from the nozzle exit, where d is the nozzle exit diameter. The centerline of the nozzle is aligned with the centerline of the jet deflectors with the deflectors placed perpendicular to the nozzle exit plane. Through a comparative evaluation of these models, our goal is to delineate their discrete flow control mechanisms and their effectiveness in jet deflection, turbulence modulation, and flow control. Z-type Schlieren imaging is used for visualizing flow phenomena to observe the changes in flow structures and patterns induced by each deflector. Particle Image Velocimetry (PIV) will be used to procure high-fidelity, two-dimensional velocity field data. The combined use of these techniques allows for a comprehensive understanding of the complex flow dynamics associated with each deflector design. Our preliminary findings indicate diverse flow characteristics unique to each design. The porous deflector exhibits potential for enhanced jet control, while the super ellipse model suggests a potential for noise mitigation. This research will contribute to the fundamental comprehension of jet deflection phenomena and could potentially inspire the development of more effective jet deflector designs in diverse applications, such as gas turbine engines and aerospace propulsion systems.   

Stabilizing the flow past twin cylinders using rotation

For modest Reynolds numbers, the steady-state flow past twin (side-by-side) cylinders (separated by a diameter) is unstable. This flow can be stabilized for Reynolds numbers up to 66 by using cylinder rotation. We investigate the quality of the controlled flow when using linear, quadratic, or cubic feedback that are computed using the quadratic-quadratic regulator formulation and the accompanying QQR software. The cubic feedback law is able to more effectively stabilize the flow while also requiring less control energy to do so. For Reynolds numbers 67 and above, an additional asymmetric unstable mode appears and the actuation mechanisms are not sufficient. Similarly, for larger spacing between the cylinders, the antiphase synchronized vortex shedding can also not be stabilized with the rotational actuation mechanism. This agrees with the recent study of these flows by Carini et al. (JFM, 2014). Thus an additional actuation mechanism, such as inflow perturbation or mass injection is required for these cases.   

Active Tracers for Hydraulic Control of Cooled Short Circuits: Bench-scale Demonstration and Forward Modeling

Commercially successful geothermal systems require balance between thermal and hydraulic
performance. An injector-producer well pair with exceptional hydraulic performance, for instance, may
have inadequate thermal performance if the effective heat transfer surface area is insufficient. In such a
circumstance the current state-of-the-art is to abandon such well pairs once production well
temperatures fall below design/operating criteria. Here, an "active" tracer is introduced as a novel
solution that enables cooled "short circuits" to be sealed off and circulating fluids to be redirected to
hotter flow paths. This treatment increases the effective heat transfer area and subsequently improves
thermal performance by increasing production well temperatures. Bench-scale laboratory experiments
are presented and the anticipated improvement to thermal performance is determined from a
hypothetical case.   

Enhanced Modelling of 3D Synthetic Jet Actuators In Crossflow

This paper examines three different approaches used in the literature for simulating a 3D synthetic jet actuator (SJA). Using a dynamic mesh, the first method models the whole SJA, including the vibrating diaphragm. The second models only the neck of the SJA and applies the Womersley solution to the bottom of the neck. The third method eliminates the SJA and applies the Womersley solution directly to the jet slot exit. All three methods were modelled in a crossflow with Reynolds number based on the boundary layer thickness of Re_δ=7,000 using unsteady Reynolds-Averaged Navier-Stokes. The results show that the flow characteristics are similar for the first two methods, but the third produces unrealistic backflow and high vorticity during ingestion. The computational cost of the second approach was 70% lower than the dynamic mesh approach. A parametric study was conducted to further investigate the source of deviation by the third method. The volume ratio between the ingested flow and the neck volume was found to be an important parameter for simulating the SJA using the Womersley solution. This dimensionless parameter must be less than unity for accurate simulations of the SJA.   

Wavelength Impact on Flow Control through Backward Traveling Wave Oscillations: A Numerical Study

This study investigates the role of wavelength on flow control using surface morphing oscillations. These oscillations are characterized by low amplitude, backward (opposite to the airfoil's forward motion) traveling waves induced on the suction side of the airfoil at 15⁰ angle of attack (AOA), and Reynolds Number (Re) of 50,000. Large eddy simulations (LES) were conducted using the sharp interface curvilinear immersed boundary (CURVIB) method. The wavelength is varied from ƛ* = 0.05 – 0.9 (ƛ* = ƛ/L; ƛ: wavelength, L: airfoil chord length) to simulate different boundary layer thickness and wave generation conditions. A baseline simulation is performed at AOA = 15⁰ and Re = 50,000. Next, the role of wavelength (ƛ* = 0.05 – 0.9) on flow control is explored via traveling waves with an amplitude of a* = 0.001 (a* = a/L; a: amplitude), frequency of f* = 8.0 (f * = fL/U; f: frequency, U: freestream velocity). While our previous results showed that traveling waves of ƛ* =0.44 suppressed stall, the role of wavelength on traveling wave flow control is yet to be understood. This work is supported by National Science Foundation (NSF) grant CBET 1905355, and the computational resources are provided by High Performance Research Computing (HPRC) group at Texas A&M University.   

Harnessing rotational symmetry in reinforcement-learning-based control of the flow around a four-roll mill

The four-roll mill is designed to study the morphology of a drop at the centre of an extensional flow formed by four rotating cylinders with the same speed but alternate signs. However, the drop is prone to escaping due to the unstable nature. Thus it is necessary to implement flow control to correct the drop's trajectory. Traditional model-based controllers face challenges when attempting this task, as an accurate flow model is hard to obtain in real-world conditions. To address this issue, we apply a model-free deep-reinforcement-learning-based (DRL) control scheme to adjust the speeds of the four cylinders in real-time, aiming to drive the drop back to the centre when it drifts away. The drop is modeled as a rigid particle transported by the flow and the flow itself is initially modeled by a linear superposition of four rotlets and then extended to the realistic 2-D direct numerical simulations. In both cases, it is found that with a proper choice of state representations and reward functions, the learnt policy is effective in driving the drop back to the centre from any random initial position. One key to the training lies in embedding the rotational symmetry inherent in the geometry into the DRL framework. This way, the control policy learnt in one quadrant can be directly applied to control drops located in the other three quadrants. Compared to the naive training, the symmetry-aware agent has a much smaller state space, resulting in higher sampling efficiency and faster learning speed.   

Performance Improvements in Wall-normal Momentum Injection with Axisymmetric Dielectric Barrier Discharge Plasma Actuators

Dielectric barrier discharge (DBD) plasma actuators have seen a strong interest in active flow control over the last two decades. Plasma synthetic-jet actuators (PSJAs) are DBD jets capable of producing zero-net mass flux momentum injection in atmospheric conditions with near instantaneous response and no moving parts. Due to the edge-normal momentum injection characteristics of PSJAs, most studies focused on surface wall jet configurations with spanwise geometries (see straight-edge, serrated, fingered electrodes). Opposing and annular geometries have been briefly explored for the purpose of wall-normal thrust by producing opposing flows, albeit with relatively low velocity and forcing.

We further examine the electro-mechanical characteristics of axisymmetric plasma synthetic jet actuators (APSJA’s) with improved dielectrics and larger diameters than previous studies, resulting in substantial improvements in velocity and momentum injection with comparable power consumption. We investigate entrained and injected flow behavior through experimental and numerical approaches Combined, this provides extended voltage-frequency-diameter-thrust relationships for APSJAs as well as finer intricacies including entrainment patterns, self-similarity, and jet dissipation. Combined with electrical current analysis, the APSJA is found to be an effective method of producing wall-normal momentum-injection.