Bulletin of the American Physical Society
66th Annual Meeting of the APS Division of Fluid Dynamics
Volume 58, Number 18
Sunday–Tuesday, November 24–26, 2013; Pittsburgh, Pennsylvania
Session E25: Flow Control III - Drag Reduction |
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Chair: Wade Huebsch, West Virginia University Room: 320 |
Sunday, November 24, 2013 4:45PM - 4:58PM |
E25.00001: Reduction of turbulent skin-friction drag by oscillating discs Daniel Wise, Pierre Ricco A new drag-reduction method, based on the active technique proposed by Ricco \& Hahn (2013), i.e. steadily rotating flush-mounted discs, is studied by DNS. The effect of sinusoidally oscillating discs on the turbulent channel-flow drag is investigated at $Re_{\tau}=180$, based on the friction velocity of the stationary-wall case and the half channel height. A parametric investigation on the disc diameter, tip velocity and oscillation period yielded a maximum drag reduction of 18.5\%. Regions of net power saved, calculated by considering the power spent to enforce the disc motion against the viscous resistance of the fluid, are found to reach up to 6.5\% for low disc tip velocities. Significantly, the characteristic time-scale for the oscillating disc forcing is double that for the steadily rotating discs, representing a further step towards industrial implementation. The oscillating disc forcing, similar to the steadily rotating disc forcing, creates streamwise-elongated structures between the discs. These structures - largely unaffected by the periodic wall forcing and persisting throughout the entire period of the oscillation - are the main contributor to the additional Reynolds stresses term created by the disc forcing, and are important for the drag reduction mechanism. [Preview Abstract] |
Sunday, November 24, 2013 4:58PM - 5:11PM |
E25.00002: Designing smart duct geometries for low frictional losses Gertraud Daschiel, Veronika Krieger, Jovan Jovanovic, Bettina Frohnapfel In turbulent flows through triangular ducts the friction factor is significantly reduced compared to the well-proven Blasius correlation. The passages of reduced friction are detected close to the duct corners in which the flow also shows a strong tendency in the turbulent fluctuations towards the statistical axisymmetric state. Within the present investigation direct numerical simulations of turbulent flows through non-circular ducts are carried out. The duct shapes are designed with the goal to reduce frictional losses in the turbulent state by forcing turbulent fluctuations towards statistical axisymmetry in a wide part of the flow domain. In this respect, the influence of the corners' opening angle and the surface curvature are investigated. Interestingly, the state of statistical axisymmetry is also reported to lead to a stabilization of disturbances in laminar flows and consequently delay the breakdown to turbulence. From this finding it might be expected that duct geometries leading to this particular statistical properties of the turbulent fluctuations also can have beneficial effects in the delay of the laminar to turbulent transition process. First numerical experiments that tackle this point will be presented. [Preview Abstract] |
Sunday, November 24, 2013 5:11PM - 5:24PM |
E25.00003: Model-based analysis of the effect of spanwise wall oscillations on drag reduction at high Reynolds numbers Armin Zare, Rashad Moarref, Mihailo Jovanovic Experiments and numerical simulations have shown that drag-reducing ability of spanwise wall oscillations in turbulent channels deteriorates as the Reynolds number increases. Recent work by Moarref and Jovanovic (J. Fluid Mech., vol. 707, 2012) has demonstrated the predictive power of a model-based approach for controlling turbulent flows. In the present study, we use a linearized stochastically-forced model to reveal the Reynolds number independent effects of wall oscillations on drag reduction. This allows us to extend the predictive capability of our simulation-free approach to high Reynolds numbers. We show that the influence of wall oscillations at low Reynolds numbers is confined to the streamwise and spanwise wavelengths that correspond to the universal inner-scaled eddies in wall turbulence. Since wall oscillations do not suppress large scale eddies, which are responsible for increased drag in the uncontrolled flow, we conclude that wall oscillations have weaker influence on drag reduction at higher Reynolds numbers. In addition, our observations enable predictions of drag reduction trends at high Reynolds numbers. [Preview Abstract] |
Sunday, November 24, 2013 5:24PM - 5:37PM |
E25.00004: Drag-Reduction Effectiveness of Riblet Films in Adverse Pressure Gradients Aaron Boomsma, Fotis Sotiropoulos Riblet films are micro-grooved structures that are widely known to passively reduce skin friction. Past studies have almost solely focused on riblet performance in channel-flows. However, possible applications of riblets include wind turbine blades, gas turbine blades, and other complex bodies that are exposed to non-zero pressure gradient flows---specifically adverse pressure gradients. We use high-resolution large eddy simulations of turbulent flow over three-dimensional riblets under an adverse pressure gradient. We analyze the computed results to quantify drag reduction effectiveness for different riblet shapes and to examine pertinent turbulent structures to gain a fundamental understanding of riblet performance. [Preview Abstract] |
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