Bulletin of the American Physical Society
72nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 64, Number 13
Saturday–Tuesday, November 23–26, 2019; Seattle, Washington
Session C26: Drag Reduction II |
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Chair: Barton Smith, Utah State University Room: 608 |
Sunday, November 24, 2019 8:00AM - 8:13AM |
C26.00001: Drag reduction of three-dimensional riblets on a flat plate turbulent boundary layer Gioacchino Cafiero, Gaetano Iuso We performed an experimental investigation of the turbulent flow on a flat plate presenting micro roughness. In addition to the typical longitudinal micro-grooves, commonly referred to as riblets, we also investigate the performance of “three-dimensional” riblets, i.e. presenting a sinusoidal pattern. Further to the typical cross-section parameters that characterize longitudinal riblets, namely depth ($h$) and spacing ($s$) of the micro grooves, the sinusoidal riblets add two more parameters: the wavelength ($\lambda$) and the amplitude ($A$). In our study, we consider a parabolic profile ($s/h$=0.7) for the cross-section of the micro-grooves and we study two different sinusoidal riblets varying the amplitude ($A$=0.6mm and $A$=0.15mm), for a fixed value of the wavelength $\lambda/s$=64mm. Our load cell measurements show a consistent effect of the amplitude of the sinusoidal riblets on the friction drag reduction. In particular, while the longitudinal riblets feature drag reductions of the order of 7.7\% at $s^+$=13 (in good agreement with Bechert et al. 1997), the sinusoidal riblets can achieve values as large as 10\% for similar values of $s^+$. Stereoscopic-PIV measurements show the different near wall structure of the flow, when the three-dimensional riblets are employed. [Preview Abstract] |
Sunday, November 24, 2019 8:13AM - 8:26AM |
C26.00002: Streamwise evolution of drag reduced turbulent boundary layer with dilute polymer solutions Yash Shah, Serhiy Yarusevych The effect of dilute polymer solutions on the evolution of flat-plate turbulent boundary layer has been investigated experimentally. Three different injection concentrations of 100, 500 and 1000 ppm of polyethylene oxide (PEO) were injected through an inclined slot on the plate and particle image velocimetry measurements were performed to characterise boundary layer development and compare the results to the baseline cases without injection and with water injection. Drag reduction (DR) was found to decrease approximately linearly with injection distance for all injection concentrations. The results show that polymer injection has a significant effect on the flow statistics, including the extent of viscous sublayer and the characteristics of the log layer. The polymer injection also attenuates peak velocity fluctuations and moves them away from the wall with increasing polymer concentration. It is also shown that the ejection and sweep motions are weakened notably by polymer injection. [Preview Abstract] |
Sunday, November 24, 2019 8:26AM - 8:39AM |
C26.00003: Passive Actuation of Scales Modeled after Shark Scales to Delay Separation in a Steady Turbulent Boundary Layer Chase Parsons, Amy Lang, Leonardo Santos, Andrew Bonacci, Sarah Foley Delaying the onset of flow separation is of great interest in the field of fluid mechanics to improve the overall aerodynamic efficiency of aircraft. This project seeks to investigate passive flow control using shortfin mako shark inspired manufactured scales in turbulent boundary layer separation. Previous studies have demonstrated the effectiveness of similar devices placed inside the separation bubble. In this study, the scales are placed in front of the separation point to investigate the effectiveness to delay separation. Reversing flow is the primary mechanism causing the actuation of the shark scales, so under these test conditions, it is hypothesized that reversing flow low speed streaks can actuate and be controlled by the scales, thus delaying the onset of separation. To generate a controlled adverse pressure gradient, a rotating cylinder induces separation at a chosen location within a flat plate turbulent boundary layer ranging from Re$=$495,000 to Re$=$710,000. With this thick boundary layer, DPIV is used to measure the flow characteristics. The goal is to better understand the fundamental mechanisms by which shark scales can induce passive flow control with the aim of fabricating surfaces suitable for real aircraft applications. [Preview Abstract] |
Sunday, November 24, 2019 8:39AM - 8:52AM |
C26.00004: Flow mechanism at the interface layer of the bio-inspired coated surface in a turbulent channel flow Venkatesh Pulletikurthi, Carlo Scalo, Luciano Castillo We have investigated the turbulent channel flow coated with bio-inspired surface via fully unstructured Pseudo-spectral method. The present study is inspired by the denticles present on the skin of the mako shark. Previous experimental studies by Bocanegra et. al (2018)\footnote{Evans, H. B., Hamed, A. M., Gorumlu, S., Doosttalab, A., Aksak, B., Chamorro, L. P., \& Castillo, L. (2018). Engineered bio-inspired coating for passive flow control. Proceedings of the National Academy of Sciences, 115(6), 1210-1214} revealed that the denticle like micro structures ($85$ $\mu m$) delayed the separation in an adverse pressure gradient flow. However, in spite of several studies to understand the flow near the wall, the interaction at the interface layer and inside the microstructures is not fully understood. Here, we use high resolution adaptive mesh refinement based DNS simulations to resolve the flow in the interface layer and inside the microstructures to shed light on the mechanism between the interface layer which is responsible for the drag reduction and/or delay in separation. \%This study also will help to model the interface layer boundary conditions for numerical modeling. [Preview Abstract] |
Sunday, November 24, 2019 8:52AM - 9:05AM |
C26.00005: Deviations in Polymer Drag Reduction Performance with Mechanical Degradation Zeeshan Saeed, Yasaman Farsiani, Dr. Brian Elbing Polymer drag reduction studies, although show great promise in flow control applications, are significantly limited by a problem; mechanical degradation of polymers in shear flows. Insights into this problem were made by comparing the drag reduction performance (slope increments on Prandtl-Karman (P-K) plots) of Polyethylene oxide (PEO) samples with and without degradation. The molecular weights of PEO samples-a measure of the extent of their degradation-were determined by matching the onset of drag reduction i.e. the intersection of the polymeric curve with the Prandtl-von Karman law on the P-K plots. Range of mean molecular weights (0.6 -- 8 million g/mol) of PEO samples were included in the test matrix. Higher molecular weight samples were mechanically degraded to lower mean molecular weights that matched the molecular weights of available non-degraded samples (e.g. 4 million g/mol was degraded to 0.6 million g/mol). Comparisons of resulting slope increments determined from P-K plots of the degraded and non-degraded samples were then scaled with a function based on their mean molecular weights to show how polydispersity is central in determining the flow characteristics. This presentation reports the findings from the experiments and analysis mentioned above [Preview Abstract] |
Sunday, November 24, 2019 9:05AM - 9:18AM |
C26.00006: Drag-reduction curves for anisotropic permeable substrates Garazi Gomez-De-Segura, Ricardo Garcia-Mayoral We present DNSs of channel flows bounded by modelled streamwise-preferential permeable substrates. The resulting drag curves are similar to those of riblets. For small permeabilities, the curves exhibit a linear regime, where drag reduction is proportional to the difference between the streamwise and spanwise permeabilities. This breaks down for a critical value of the wall-normal permeability, beyond which spanwise-coherent, Kelvin-Helmholtz-like structures develop, and the performance begins to degrade. We present simple linearised models to predict both the linear regime and its breakdown, which yield accurate a priori estimates for the substrates' performance. The largest drag reduction observed in our DNSs is $\approx 20-25\%$ at a friction Reynolds number $Re_{\tau} = 180$, at least twice that obtained for riblets. [Preview Abstract] |
Sunday, November 24, 2019 9:18AM - 9:31AM |
C26.00007: Role of wall-attached structures in frictional drag reduction by streamwise Min Yoon, Hyung Jin Sung The role of wall-attached structures in the frictional drag reduction by the Navier slip is explored. The wall-attached structures are extracted from the clusters of streamwise velocity fluctuations in a turbulent channel flow (\textit{Re}$_{\tau }=$ 470). The skin friction coefficient ($C_{f})$ is decreased by 35{\%}. A dataset of the no-slip condition (\textit{Re}$_{\tau }=$ 577) is also included for comparison. The wall-attached structures extend toward the upstream in the vicinity of the wall by the slip. The convection velocity of the wall-attached structures increases near the wall, leading to the wide influence on the inner region via roll-cell motions. The vortical structures circumscribing the wall-attached structures are attenuated, since the mean shear of the structures is decreased by the slip. The contribution of the wall-attached structures to $C_{f}$ is quantified through the skin friction decomposition, which can be derived from the mean vorticity equations. The advective vorticity transport and vortex stretching terms around the wall-attached structures are found to dominate the contributions to the frictional drag. The wall-attached structures are responsible for 53.2{\%} of the total reduction of $C_{f}$. [Preview Abstract] |
Sunday, November 24, 2019 9:31AM - 9:44AM |
C26.00008: The Baseball Seam: Clever and Capable Passive Flow Control Barton Smith, Andrew Smith It is obvious to any observer of baseball that the aerodynamics of the ball are important, both for pitched and batted balls. Much has been written about the well-known Magnus Effect, or the force on a moving ball due to its rotation. Less is known about forces due to the wake of the ball. Baseball seams make baseballs very interesting when compared to other sports balls. They play two roles: As many have speculated, when located in the favorable pressure gradient on the front of the ball, they can cause laminar flow to become turbulent, which subsequently modifies the wake of the ball. More surprisingly, when located in the adverse gradient on the back of the ball, they can also modify the location of boundary layer separation and can make the wake (and thus the force on the ball) asymmetric, leading to movement. In this talk, we will discuss these effects and the possibility of the existence of the ``laminar express'' 2-seam fastball that moves due an asymmetric wake rather than by Magnus effect. [Preview Abstract] |
Sunday, November 24, 2019 9:44AM - 9:57AM |
C26.00009: Passive control of vortex-induced vibration of a sphere. Anchal Sareen, John Sheridan, Kerry Hourigan, Mark Thompson Although passive methods for controlling vortex-induced vibrations (VIV) are extensively studied for a circular cylinder, such methods remain unexplored for a basic three-dimensional bluff body, a sphere. In this study, we use a surface trip wire as a passive method to control sphere VIV. The effect of a surface trip is experimentally investigated for varying diameter (1.25 x 10$^{\mathrm{-2}} \quad \le \quad k/d \le $ 6.63 x10$^{\mathrm{-2}})$ and stream wise location ({\o} $=$ 20$^{\mathrm{o}}$ -70$^{\mathrm{o\thinspace }}$from the stagnation point) of the trip wire for a wide range of reduced velocities (3 $\le $U$^{\mathrm{\ast }}\le $20). It was found that the vibration amplitude decreases progressively with the increase in the stream wise location angle ({\o}) of the trip wire. The control was highly effective in mode II and mode III of the VIV response with maximum reduction of up to 97.8{\%} for {\o} $=$60$^{\mathrm{o.\thinspace }}$Interestingly, thicker trip wires (\textit{k/d \textgreater }1.25 x 10$^{\mathrm{-2}})^{\mathrm{\thinspace }}$were more effective in mode I, but showed a galloping response for higher reduced velocities. $^{\mathrm{\thinspace \thinspace }}$ [Preview Abstract] |
Sunday, November 24, 2019 9:57AM - 10:10AM |
C26.00010: Underlying Mechanisms of Drag Reduction in Turbulent Flows Alex Rogge, Jae Sung Park Turbulent flow control is of great importance in fundamentals and applications due to the potential benefits associated with it, particularly regarding drag reduction for energy savings. In this study, we will investigate three strategies to better understand their underlying mechanisms for drag reduction in turbulent channel flows. These strategies include using a spanwise body force, adding a small concentration of long-chain polymers into the fluid, and using a superhydrophobic surface on the channel walls. Direct numerical simulations were performed to elucidate the mechanisms at play. Analysis is based on the lifetime of turbulent phases represented by the active and hibernating phases of minimal channel turbulence (Xi and Graham PRL 2010). Given similar drag reduction percentages, the polymer and slip cases show similar mechanisms, while the body force case shows a different mechanism. The polymers and slip surfaces cause hibernating phases to happen more frequently, while the phase duration remains almost constant. The body forces prolong the duration of hibernating phases, while these phases become less frequent. Lastly, these drag reduction mechanisms and their various behaviors with respect to control parameters will be further discussed and analyzed. [Preview Abstract] |
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