Bulletin of the American Physical Society
74th Annual Meeting of the APS Division of Fluid Dynamics
Volume 66, Number 17
Sunday–Tuesday, November 21–23, 2021; Phoenix Convention Center, Phoenix, Arizona
Session M23: Flow Control IV |
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Chair: Ronald Hanson, York University Room: North 224 A |
Monday, November 22, 2021 1:10PM - 1:23PM |
M23.00001: Drag control on a fully rough oscillating flat plate Edgardo J Garcia, Ateeb Ahmad, Fazle Hussain The dynamics of a turbulent channel flow over spanwise square bars on the wall with sinusoidal spanwise oscillations are studied by direct numerical simulations. For smooth wall channels, spanwise wall oscillations have been shown to reduce skin friction drag by as much as 30%. In a practical flow, wall roughness needs to be considered where form drag is non-negligible. Here, we consider a configuration where 93% of the drag comes from form drag; the ratio of pitch (λ) to the height of bars (k) is λ/k=7.5, where the height of the bars in wall units is k^{+}=50, which is considered a “fully rough” wall. A 25% form drag reduction is obtained with the wall oscillations considered. Inspection of the mean streamlines shows that the secondary recirculation region downstream from the bars is completely suppressed while the recirculation upstream slightly increases in size. Surprisingly, the optimal amplitude and frequency for drag reduction in a smooth wall used here as a starting point also give significant form drag reduction, despite the fact that skin-friction drag is negligible in this scenario. |
Monday, November 22, 2021 1:23PM - 1:36PM |
M23.00002: Effects of external body forcing on the flow physics in turbulent channel flow Cesar A Leos, Jae Sung Park Direct numerical simulations are carried out to investigate the controlled effects of external body forcing in a turbulent channel flow. The transitional Reynolds number studied is Re_{τ} = 85, and the analysis is performed by distinguishing new asymmetric states using a 20% threshold from the standard deviation of their symmetric counterparts. The numerical results reveal a maximum drag reduction of 25% in the total wall shear stress of the controlled channel with a penetration depth of 0.03 and an oscillation period of 10. From statistical probability, it is determined that not only does the shear stress decrease but becomes narrower, which indicates improved accuracy. During the symmetric states of the uncontrolled case, the wall shear is found to be close to the mean value, while the controlled channel exhibits shear stress lower than the mean. In addition, results show that the average bulk velocity for the controlled channel is higher near the uncontrolled wall, rather than the wall where force is applied. The total number of symmetric states for the uncontrolled and controlled channel was found to be 10% and 3%, respectively. These essential observations enhance a fundamental understanding of the drag reduction process, and the dependence of Reynolds number is further discussed. |
Monday, November 22, 2021 1:36PM - 1:49PM |
M23.00003: Control of large-scale motions in boundary layers Alexandros Tsolovikos, Akshit Jariwala, Pranav Sridhar, Saikishan Suryanarayanan, Efstathios Bakolas, David Goldstein Turbulent flows are dominated by large-scale motions with temporal and spatial coherence. In particular, LSMs in boundary layers, which can span several times the boundary layer thickness in the streamwise direction, contain a large fraction of the turbulent kinetic energy of the streamwise velocity component, contribute significantly to the average Reynolds shear stresses, and transport momentum within the turbulent boundary layer. Given their importance, we pose the question of whether we can manipulate the movement of these LSMs and bring them closer to the wall in order to energize a boundary layer. To answer that question, we introduce a set of synthetic LSMs in a direct numerical simulation of a laminar boundary layer by generating a series of aligned hairpin vortices via force fields. We then target these structures via an actuator modeled as a body force field with a pitch angle towards the wall that generates a region of downwash that traps and pushes the synthetic LSMs toward the wall. The body force required to move the oncoming LSMs is computed via a model predictive control framework (Tsolovikos et al., AIAA J. 2021). The effect of moving these coherent structures toward the wall on the vorticity and momentum transport in the boundary layer is studied. |
Monday, November 22, 2021 1:49PM - 2:02PM |
M23.00004: On the Use of Thermal Energy Deposition for Active Control of a Turbulent Mixing Layer Ashish Singh, Jesse C Little Nanosecond dielectric barrier discharge (ns-DBD) plasma actuators are known to control flows through thermal energy deposition. However, unlike momentum addition type actuators (e.g. ac-DBDs), the relationship between control authority and the amplitude of energy deposition in the flow is not well understood. This investigation quantifies the density/temperature gradients introduced as a function of actuation parameters using background oriented schlieren (BOS) in a low-speed turbulent mixing layer. The effect of changes to the mean properties of the flow are modelled by linear stability theory (LST) using a 2-D inviscid model of the mixing layer with variable mean temperature & density profiles that are obtained from BOS. At increased forcing amplitudes, a drop in the growth rate (-α_{i}) of the peak unstable mode is noted, as well as a shift in the neutral point to lower frequencies. These results suggest that excessive changes to mean temperature/density (due to high amplitude forcing) can have detrimental effects on control authority.This implies that there is some optimal level of energy deposition for controlling a given flow |
Monday, November 22, 2021 2:02PM - 2:15PM |
M23.00005: Undulated Splitter Plate Trailing Edge Effects on a Supersonic Nozzle Emma D Gist, Rishov Chatterjee, Mark Glauser Spanwise perturbations on a splitter plate trailing edge are assesed as a means to mitigate a high frequency signal at 34kHz in a complex nozzle. The flowfield is comprised of two streams, mach 1.6 and mach 1 which merge behind a splitter plate, enter into a rectangular single expansion ramp nozzle (SERN) and exit over an aft-deck. The wavenumber implemented was guided by simulations as it showed a diminishment of the dominant tone by inducing streamwise vortices. Particle Image Velocimetry and pressure measurements are used to realize differences in the controlled flow from that of the nominal for various aft-deck lengths. The nominal and no deck configurations show an upwards vectoring of the mean jet plume whereas the half deck shows no change from the uncontrolled flow. The 34kHz tone ceased in the wavy splitter plate cases however other peaks were amplified. Spectral Analysis Modal Methods (SAMMs) will be leveraged to gain insight into the flow physics responsible for these amplifications. |
Monday, November 22, 2021 2:15PM - 2:28PM |
M23.00006: Effect of Angle of Attack on Flow Control using Traveling Waves UCHENNA E OGUNKA, Amir Akbarzadeh, Iman Borazjani This study numerically investigates the effect of the angle of attack (AOA) on the aerodynamic performance (lift coefficient, C_{L}, and drag coefficient, C_{D}) of an airfoil under active flow control using low amplitude surface actuation in the form of backward (opposite to the airfoil's forward motion) traveling waves on the suction side of the airfoil. A NACA0018 airfoil with a low Reynolds number (Re = 50,000) is simulated using large eddy simulations (LES) curvilinear immersed boundary (CURVIB) method at different angles of attack (AOA = 0^{0} to 20^{0}). While our previous simulations indicated the effectiveness of backward traveling waves at pre-stall (AOA = 10^{0}) and near stall angle of attack (AOA = 15^{0}), the effectiveness of these waves at post-stall AOA (AOA = 20^{0}) is not understood. The simulation of the baseline case at AOA = 20^{0} is performed first. Afterward, the role of the backward traveling waves with a low amplitude range of a* = 0.001 - 0.002 (a* = a/L; a: amplitude, L: chord length of the airfoil) and a reduced frequency range of f* = 4 - 16 (f* = fL/U; f: frequency, U: the freestream velocity) will be investigated at the post-stall angle of attack (AOA = 20^{0}). This work is supported by National Science Foundation (NSF) grant CBET 1905355, and the computational resources are provided by High Performance Computing (HPRC) group at Texas A&M University. |
Monday, November 22, 2021 2:28PM - 2:41PM Not Participating |
M23.00007: Unsteady plasma vortex generators for flow separation control of an A-pillar shape bluff body Patricia Sujar Garrido, Ramis Örlü The aerodynamic drag accounts for more than 20% of the total energy loss of heavy-duty vehicles and around 20% is induced by the tractor at zero degrees yaw angle. One of the contributions comes from the flow separated at the A-pillars (front corners) of the tractor. The aerodynamic design of these corners has improved during the last decades, however, the side winds have a significant detrimental effect on the drag and a real heavy-duty vehicle will be mainly subjected to varying yaw angles. To investigate this point, a bluff body with a square-back and round corners on the leading edges has been chosen as a generic geometry. Our geometry has the flexibility to study separation on different front and back designs and yaw effects (side winds) to demonstrate the potential of unsteady plasma vortex generators (VGs). The effect of the excitation amplitude and duty-cycle parameters on the evolution of the starting vortex by means of Dielectric Barrier Discharge (DBD) VGs has previously been studied; however, there is still a lack of studies exploring its advantage in complex flow-control configurations such as the A-pillar. Particle Image Velocity measurements will detail the dynamics of the unsteady actuation for separation control at different configurations. |
Monday, November 22, 2021 2:41PM - 2:54PM |
M23.00008: Development and decay of streamwise vorticity in the laminar boundary layer due to a step input by an array of plasma actuators Hossein Khanjari, Ronald E Hanson, Philippe Lavoie The temporal development of the streamwise vorticity and resulting velocity streaks generated by a spanwise array of plasma actuators that are operated in a burst mode within a Blasius boundary layer is studied. The effect of the actuators is modeled by a momentum source using a commercial CFD code. The source is calibrated with an experimental dataset for steady actuation. Using a burst mode, the Blasius boundary layer is given a step input to the body force for 100 boundary layer turnover and then permitted to relax to the baseline flow. Near the wall an inverse response in the streamwise velocity is detected. The streamwise vorticity data are used to explain the corresponding inverse shear stress observed during the simulated activation and deactivation of the plasma actuators. It is shown that secondary streamwise vortices occurring below the primary titled flow structures lead to the inverse flow response during unsteady forcing. |
Monday, November 22, 2021 2:54PM - 3:07PM |
M23.00009: Delay of boundary layer transition by means of miniature vortex generators André Weingärtner, Santhosh Babu Mamidala, Jens H Fransson The efficacy of spanwise velocity gradients (SVGs) to damp the growth of Tollmien-Schlichting waves and thus delay transition has been shown numerous times. One efficient means to generate SVGs in the mean flow is by employing miniature vortex generators (MVGs), featuring a height of less than the boundary layer thickness, that create streamwise vortices which then cause stable streamwise streaks inside the boundary layer. By combining multiple MVGs to a spanwise array, periodic streaks in the base flow can be generated. Previous investigations have mainly focussed on blade-type MVGs, that are efficient in generating streaks and delaying transition, but sensitive in practical applications. In this study, a new geometry -- a triangular wedge -- is employed and the characteristics of this type of MVG are examined. Furthermore, it has been shown that the transition delay is effective only while the SVGs are present in the flow. Naturally, they will decay with the downstream distance and need to be augmented at some point, which can be done using a second array of MVGs. However, the exact properties (e.g. location, height) of the second array remain to be determined and will be assessed in this study. |
Monday, November 22, 2021 3:07PM - 3:20PM |
M23.00010: Drag reduction in near-critical turbulent open-channel flow over undular bottoms Dominik Murschenhofer, Wilhelm Schneider Reduced drag is often desirable in hydraulic engineering, e.g. to increase the mass flow in an open channel. Steady turbulent open-channel flow with very large Reynolds numbers is considered. The Froude number in the reference state is assumed to be close to the critical value 1, i.e., Fr = 1 + (3/2)ε with ε « 1. By allowing for a slightly uneven bottom with bottom elevations of the order O(ε^{5/2}), a steady-state version of an extended Korteweg–de Vries (KdV) equation, describing the non-dimensional free-surface elevation, is derived by an asymptotic analysis. The amplitudes of the free-surface elevation are of O(ε). Choosing the bottom elevation of a particular undular shape, i.e., a superposition of a linear and a periodic part, leads to stationary periodic cnoidal waves as solutions of the extended KdV equation, as if the flow were inviscid. The analysis of the wall friction distribution over a wave period in comparison to a plane bottom shows that bottom elevations as small as O(ε^{5/2}) lead to drag changes of order O(ε). It turns out that the vast majority of feasible parameter configurations results in drag reduction. Thus, the present analysis shows a suitable way for passively lowering the effective friction coefficient in near-critical turbulent open-channel flows. |
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