Bulletin of the American Physical Society
77th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 24–26, 2024; Salt Lake City, Utah
Session L33: Drag Reduction II: General |
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Chair: Jae Sung Park, University of Nebraska - Lincoln Room: 255 E |
Monday, November 25, 2024 8:00AM - 8:13AM |
L33.00001: Transpiration and spanwise wall oscillations in laminar channel flow: A path to energy savings Leo Mangalath, Daniel Floryan Judicious use of wall transpiration or spanwise wall oscillations can significantly reduce drag in pressure-driven flow. The net energy expenditure, however, is greater than for the uncontrolled laminar flow. Recent work has shown that it is theoretically possible to achieve sub-laminar net energy expenditure by combining transpiration with spanwise wall oscillations. Here, we examine this theory via direct numerical simulations of channel flow in the laminar regime. We consider streamwise-traveling waves of transpiration and spanwise wall oscillations, finding that waves whose size is on the order of the channel height can reduce drag when traveling upstream. We present a detailed analysis of the energy budget, including a scaling analysis of the dissipation induced by the control. The balance of the dissipation and flux of spanwise kinetic energy at the wall determines the net energy expenditure relative to the uncontrolled flow, and the scaling analysis suggests what levels of net energy expenditure are possible. |
Monday, November 25, 2024 8:13AM - 8:26AM |
L33.00002: Control for Turbulent Drag Reduction by Wall-normal Blowing and Suction Ching-Te Lin, Vinod Ramakrishnan, Andres Goza, Kathryn H Matlack, Jane Bae This study investigates the impact of wall-normal blowing and suction on turbulent drag in a turbulent channel flow. The research explores the effect of various parameters, e.g., actuation frequency, wavenumber, and waveform types (such as standing and streamwise-traveling waves) on the turbulent kinetic energy (TKE). Numerical analysis indicates a decrease and increase of TKE, respectively, in the vicinity of the blowing and suction regions of the channel wall. Further examination of two-point velocity correlations reveal enhanced streamwise rolls coinciding with reduced drag. Subsequently, the study incorporates closed-loop control, simulating surface velocity akin to subsurface structures, e.g., metamaterials responding to fluid forces at the channel wall. This will provide a foundation for modeling and identifying surface and subsurface feedback control methods that could potentially mitigate turbulent drag. |
Monday, November 25, 2024 8:26AM - 8:39AM |
L33.00003: Importance of relative phase between wall pressure and transpiration for drag reduction Simon S Toedtli, Anthony Leonard, Beverley J McKeon Wall-based active and passive control strategies for drag reduction in low Reynolds number turbulent flows cause at least two commonly observed modifications of the flow dynamics: attenuation or amplification of the near-wall cycle, and generation of spanwise rollers. The present study aims to clarify the underlying flow physics and considers a closed-loop wall transpiration, which is represented as a superposition of Fourier modes. The aforementioned modifications of the flow dynamics are shown to be associated with transpiration at distinct spatial scales. Whether control amplifies or attenuates the scales depends on the phase of the transpiration relative to the background flow, which can be parametrized by the wall pressure. Attenuation of the near-wall cycle and drag reduction occur when transpiration at large streamwise scales is in-phase with the wall pressure, while spanwise rollers and one route to drag increase are associated with out-of-phase transpiration at large spanwise scales. The coupled dynamics of the transpiration and wall pressure are discussed along with implications for control and modeling of wall-bounded flows. |
Monday, November 25, 2024 8:39AM - 8:52AM |
L33.00004: Surrogate modeling for optimization of actuation parameters for active drag reduction in turbulent boundary layer flows Fabian Hübenthal, Matthias Meinke, Wolfgang Schröder As environmental conditions and rising energy costs pose technological and economic challenges to air transportation, aerodynamic improvements are needed to reduce energy demand, cost, and environmental impact. A promising technique to actively reduce the aerodynamic viscous drag are spanwise traveling transversal surface waves to manipulate the near-wall turbulent boundary layer. Given the flow conditions the goal is to choose the actuation parameters, such that the drag reduction and the net power savings are optimized, while other essential aerodynamic properties, such as the lift-to-drag ratio for airfoils, are neutrally or positively affected. |
Monday, November 25, 2024 8:52AM - 9:05AM |
L33.00005: The effects of near-wall/outer regions and domain size on turbulent drag reduction via external body forces Timothy A Alo, Jae Sung Park Turbulent flows are well-known to exhibit a wide range of flow motions. Different flow control methods aim to target different motions for drag reduction. In this study, direct numerical simulations are performed up to a friction Reynolds number of 1000 to investigate the effects of different flow regions and domain size on drag reduction via external spanwise body forces. We first show the overall effect of the body force, where the effectiveness of drag reduction diminishes with increasing the Reynolds number (Re), especially around a friction Re of 500. To understand the drag contribution from different flow regions, the Fukagata-Iwamoto-Kasagi (FIK) identity is used. For the no-control case, the contribution of the outer region increases with Re, while the contribution of the near-wall region gradually decreases. However, the body force reduces drag by controlling the outer region, while barely affecting the near-wall region. Lastly, focusing on a friction Re of 500, we attempt to examine the impacts of the domain size on drag reduction. The premultiplied energy spectra are computed, indicating that as the domain size increases, the decrease in the maximum achievable drag reduction is mainly due to the energization of large-scale structures near the channel centerline. This study reemphasizes the need to control the outer region and large-scale motions when designing drag reduction strategies for higher Reynolds number and larger domain flows. |
Monday, November 25, 2024 9:05AM - 9:18AM |
L33.00006: ABSTRACT WITHDRAWN
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Monday, November 25, 2024 9:18AM - 9:31AM |
L33.00007: Sub-laminar drag over slip surfaces in turbulent channel flow Alexander J Rogge, Alexia Martinez Ibarra, Simon Song, Jae Sung Park Turbulent flow control is of importance due to its potential benefits, particularly regarding drag reduction for energy savings. It was once conjectured that the minimum drag of a constant mass-flux turbulent channel flow is its corresponding laminar drag. However, several studies have shown that the sub-laminar drag can be sustainably achieved, for example, by blowing/suction, surface grooves, or curved flows. In this study, we will investigate the sub-laminar drag due to slip or hydrophobic surface control and explore its underlying mechanisms in turbulent channel flows. Direct numerical simulations are performed with the inclusion of the slip surface up to a friction Reynold number of 125. As the slip length increases, sub-laminar drag instants start to emerge, and their occurrence and duration are dramatically increased when more than 30% of drag reduction is achieved. As a result, the total fraction of time spent in the sub-laminar drag is substantially increased. A key underlying mechanism for the sub-laminar drag is the creation of negative Reynolds shear stress in the channel-center region, not in the wall region. The dependence of the Reynolds number on the sub-laminar drag will also be discussed. |
Monday, November 25, 2024 9:31AM - 9:44AM |
L33.00008: Adaptive flow control over a sphere using smart morphable skin Anchal Sareen, Rodrigo Vilumbrales-Garcia, Putu Brahmanda Sudarsana Dimples on a sphere's surface are known to significantly reduce drag, but the optimal dimple depth varies with the Reynolds number. In this study, we devised an adaptive surface morphing strategy that adjusts dimple depth in response to changing flow velocity, minimizing drag over a sphere across a wide range of Reynolds numbers. We conducted systematic experiments for a Reynolds number range of Re = 6 x 104 - 1.3 x 105 and dimple depth ratios of k/d = 0 - 2 x 10-2 using simultaneous force and particle image velocimetry measurements in a subsonic wind tunnel. Our results indicate the existence of a critical optimal dimple depth ratio for a fixed Reynolds number. As the depth ratio increases, drag decreases monotonically until reaching a critical point, after which drag begins to increase. This behavior correlates with the upstream movement of the flow separation location. From these comprehensive experiments, we developed a model that relates the optimal dimple depth to the Reynolds number for minimizing drag. Implementing this model in our surface morphing strategy demonstrated real-time drag reduction of up to 50%, with the dimples automatically adjusting their size based on input flow velocity to achieve minimum drag. |
Monday, November 25, 2024 9:44AM - 9:57AM |
L33.00009: Shark Skin-Inspired Surface Features to Enhance the Hydrodynamic Performance of Flapping Fin Propulsion Marshall Graybill, Nicole W Xu Power constraints are a major limiting factor in uncrewed underwater vehicle (UUV) performance. Increases in UUV efficiency and endurance can benefit efforts in remote environmental monitoring, ocean exploration, and underwater surveillance. We designed and tested a robotic tandem fin propulsion system by varying the fins’ spacing, geometry, and phase offset, combined with shark skin-inspired surface structures for enhanced hydrodynamic benefits. Surface structures were modeled from the interlocking, tooth-like denticles of shark skin, which are known to alter vortex interactions within the boundary layer to reduce drag and increase lift. We constructed a robotic experimental setup that can create flapping motions inspired by finned aquatic animals to test the combined effects of denticles on the drag, lift, and thrust of single and tandem 3D flapping fins with various motion profiles. By measuring the forces and torques on fins with and without denticles, we determined how various denticle configurations (e.g., denticle size, placement, and quantity) alter the lift and thrust production of both leading and trailing fins. By drawing inspiration from multiple model organisms, such as sharks, polypteruses, and flying fish, we demonstrate how influences from multiple model organisms can be combined to create technologies for more efficient and maneuverable UUVs. |
Monday, November 25, 2024 9:57AM - 10:10AM |
L33.00010: The Effect of Sliding Surfaces on Flow around Solid Bodies Xuanhong An, Burak A Tuna, Rajan Kumar, James H. Buchholz Incorporating a sliding surface on a solid body to align with the incoming flow is an effective approach for drag reduction. One practical implementation of this concept is to replace the surface with a passive roller-belt system. However, the impact of such a system on flow characteristics remains largely unexplored. This study investigates a solid body with part of its surface replaced by a roller-belt system using both computational and experimental methods. A comprehensive flow physics investigation is conducted to analyze the effects of the sliding surface on flow features such as boundary layer thickness, vortex shedding, and flow separation. Additionally, parametric studies are performed to examine the influence of various design parameters on the drag reduction and lift generation performance of these systems. From the preliminary investigation, a 20\% friction drag reduction is observed in the numerical simulation. |
Monday, November 25, 2024 10:10AM - 10:23AM |
L33.00011: Wake Dynamics of Heavy Road Vehicles: Characterization and Control with Rear Flaps Xianyang Jiang, Jacky Zhang, Max Weissenbacher, Isabella Fumarola, Georgios Rigas In the context of climate change, heavy road vehicles are a major contributor to global pollution. Therefore aerodynamic devices are paramount in reducing their drag and improving their energy efficiency. The main source of drag comes from pressure drag due to the flow separating at the rear part of the vehicle. A successful solution to control the flow is installing flaps on the top and the sides of the rear of the vehicle. Here we present a state-of-the-art experiment on the wake dynamics of a realistic scaled model of a truck in a wind tunnel, with a rolling road to simulate the ground effect. The experiment is carried out at different yaw angles. For each case, a parametric study to determine the optimal shape and angle configuration of the side and top flaps is carried out. A time-resolved three-dimensional particle tracking method (Shake-the-Box) and planar particle image velocimetry (PIV) are used to reconstruct the development of the wake. Additionally, the pressure at the rear of the truck is measured to enhance the understanding of wake dynamics and a load cell is used to measure the forces and moments exerted on the vehicle. Our setup enables for the first time an accurate spatio-temporal measurement of the coherent structures linked to the generation of drag. The results give an optimised flap configuration to reduce drag, offering valuable insights for practical applications in the heavy road vehicle industry. |
Monday, November 25, 2024 10:23AM - 10:36AM |
L33.00012: Reinforcement learning for turbulent drag reduction of realistic road vehicles with dynamic flaps Junjie Zhang, Isabella Fumarola, Max Weissenbacher, Xianyang Jiang, Georgios Rigas The complex wake dynamics behind road vehicles is a dominant contributor to aerodynamic drag. Using Reinforcement Learning (RL) and digital environments (Direct Numerical Simulations), we demonstrate that dynamic rear flaps can fully stabilize the vortex shedding instability in laminar regimes. Next, we extend our study to heavy road vehicle models using RL in a wind tunnel environment. A real-time, time-critical control loop was established to enable online interactions between RL controllers and the flow environment. In this setup, the RL-trained controller receives instant pressure feedback from the truck's surface and outputs control signals as motor angles. The rear pitching flap, motorized by servo motors, acts as the control surface. The primary challenges in this experiment are controlling the highly turbulent wake of the vehicle and addressing partial observability due to the limited surface-mounted pressure sensors and signal delays. To overcome these challenges, we incorporate memory-based neural networks and efficient RL algorithms, demonstrating a significant reduction in instability and showcasing the effectiveness of our approach. |
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