scholarly journals Sustained drag reduction in a turbulent flow using a low-temperature Leidenfrost surface

2016 ◽  
Vol 2 (10) ◽  
pp. e1600686 ◽  
Author(s):  
Dhananjai Saranadhi ◽  
Dayong Chen ◽  
Justin A. Kleingartner ◽  
Siddarth Srinivasan ◽  
Robert E. Cohen ◽  
...  
Keyword(s):  
Drag Reduction ◽  
Skin Friction ◽  
Superhydrophobic Surface ◽  
Pressure Fluctuations ◽  
Slip Boundary Condition ◽  
Superhydrophobic Surfaces ◽  
Friction Drag ◽  
Slip Boundary ◽  
Large Water ◽  
High Reynolds

Skin friction drag contributes a major portion of the total drag for small and large water vehicles at high Reynolds number (Re). One emerging approach to reducing drag is to use superhydrophobic surfaces to promote slip boundary conditions. However, the air layer or “plastron” trapped on submerged superhydrophobic surfaces often diminishes quickly under hydrostatic pressure and/or turbulent pressure fluctuations. We use active heating on a superhydrophobic surface to establish a stable vapor layer or “Leidenfrost” state at a relatively low superheat temperature. The continuous film of water vapor lubricates the interface, and the resulting slip boundary condition leads to skin friction drag reduction on the inner rotor of a custom Taylor-Couette apparatus. We find that skin friction can be reduced by 80 to 90% relative to an unheated superhydrophobic surface for Re in the range 26,100 ≤ Re ≤ 52,000. We derive a boundary layer and slip theory to describe the hydrodynamics in the system and show that the plastron thickness is h = 44 ± 11 μm, in agreement with expectations for a Leidenfrost surface.

Effects of the air layer of an idealized superhydrophobic surface on the slip length and skin-friction drag

2016 ◽  
Vol 790 ◽  
Author(s):  
Taeyong Jung ◽  
Haecheon Choi ◽  
John Kim
Keyword(s):  
Drag Reduction ◽  
Skin Friction ◽  
Superhydrophobic Surface ◽  
Slip Length ◽  
Joint Probability ◽  
Turbulent Channel Flow ◽  
Water Interface ◽  
Friction Drag ◽  
Recirculating Flow ◽  
Air Water Interface

The anisotropy of the slip length and its effect on the skin-friction drag are numerically investigated for a turbulent channel flow with an idealized superhydrophobic surface having an air layer, where the idealized air–water interface is flat and does not contain the surface-tension effect. Inside the air layer, both the shear-driven flow and recirculating flow with zero net mass flow rate are considered. With increasing air-layer thickness, the slip length, slip velocity and percentage of drag reduction increase. It is shown that the slip length is independent of the water flow and depends only on the air-layer geometry. The amount of drag reduction obtained is in between those by the empirical formulae from the streamwise slip only and isotropic slip, indicating that the present air–water interface generates an anisotropic slip, and the streamwise slip length ($b_{x}$) is larger than the spanwise one ($b_{z}$). From the joint probability density function of the slip velocities and velocity gradients at the interface, we confirm the anisotropy of the slip lengths and obtain their relative magnitude ($b_{x}/b_{z}=4$) for the present idealized superhydrophobic surface. It is also shown that the Navier slip model is valid only in the mean sense, and it is generally not applicable to fluctuating quantities.

scholarly journals Traces of surfactants can severely limit the drag reduction of superhydrophobic surfaces

2017 ◽  
Vol 114 (28) ◽  
pp. 7254-7259 ◽  
Author(s):  
François J. Peaudecerf ◽  
Julien R. Landel ◽  
Raymond E. Goldstein ◽  
Paolo Luzzatto-Fegiz
Keyword(s):  
Drag Reduction ◽  
Slip Boundary Condition ◽  
External Flow ◽  
Superhydrophobic Surfaces ◽  
Stagnation Points ◽  
Slip Boundary ◽  
Low Concentrations ◽  
Air Water Interface ◽  
Surfactant Effects ◽  
The Impact

Superhydrophobic surfaces (SHSs) have the potential to achieve large drag reduction for internal and external flow applications. However, experiments have shown inconsistent results, with many studies reporting significantly reduced performance. Recently, it has been proposed that surfactants, ubiquitous in flow applications, could be responsible by creating adverse Marangoni stresses. However, testing this hypothesis is challenging. Careful experiments with purified water already show large interfacial stresses and, paradoxically, adding surfactants yields barely measurable drag increases. To test the surfactant hypothesis while controlling surfactant concentrations with precision higher than can be achieved experimentally, we perform simulations inclusive of surfactant kinetics. These reveal that surfactant-induced stresses are significant at extremely low concentrations, potentially yielding a no-slip boundary condition on the air–water interface (the “plastron”) for surfactant concentrations below typical environmental values. These stresses decrease as the stream-wise distance between plastron stagnation points increases. We perform microchannel experiments with SHSs consisting of stream-wise parallel gratings, which confirm this numerical prediction, while showing near-plastron velocities significantly slower than standard surfactant-free predictions. In addition, we introduce an unsteady test of surfactant effects. When we rapidly remove the driving pressure following a loading phase, a backflow develops at the plastron, which can only be explained by surfactant gradients formed in the loading phase. This demonstrates the significance of surfactants in deteriorating drag reduction and thus the importance of including surfactant stresses in SHS models. Our time-dependent protocol can assess the impact of surfactants in SHS testing and guide future mitigating designs.

High-Reynolds-Number Turbulent-Boundary-Layer Surface Pressure Fluctuations With Bubble or Polymer Additives

2005 ◽  
Author(s):  
E. S. Winkel ◽  
B. R. Elbing ◽  
D. R. Dowling ◽  
S. L. Ceccio ◽  
M. Perlin
Keyword(s):  
Drag Reduction ◽  
High Reynolds Number ◽  
Dynamic Pressure ◽  
Pressure Fluctuations ◽  
Leading Edge ◽  
Friction Drag ◽  
Flow Rates ◽  
Reduction Experiment ◽  
High Reynolds ◽  
Friction Drag Reduction

This paper reports multi-point dynamic pressure fluctuation measurements made beneath a high-Reynolds-number turbulent boundary layer (TBL) with wall-injection of air or polymer additives for the purpose of skin-friction drag reduction. Two independent experiments were conducted in the U.S. Navy’s Large Cavitation Channel (LCC) on a 12.9 m long, 3.05 m wide hydro-dynamically smooth (k+ < 1) flat plate at free-stream speeds from 6.5 to 20.0 m/s. The first, a bubble drag reduction experiment (BDR), involved injecting gas at flow rates ranging from 100 to 800 CFM (17.8 to 142.5 liter/s per meter of injector span) from one of two injectors located 1.32 and 9.78 m from the model leading edge. The second, a polymer drag reduction experiment (PDR), involved injecting polymer from a single slot injector, 1.32 m from the leading edge, at flow-rates ranging from 6 to 30 GPM (0.14 to 0.71 liter/s per meter of injector span). Dynamic pressure measurements were made with 16 flush-mounted transducers in “L”-shaped arrays located 10.7 and 9.8 m (70 × 106 < ReX < 210 × 106) from the leading edge for the BDR and PDR experiments, respectively. Measurements show modifications in the spectra, stream-wise coherence, and convection velocity of the pressure fluctuations due to the presence of gas or polymer in the near-wall region of the TBL. At the dynamic pressure measurement locations the maximum skin-friction drag reduction approached 100% for the BDR experiment and 63% for the PDR experiment.

scholarly journals Noise Reduction Effect of Superhydrophobic Surfaces with Streamwise Strip of Channel Flow

2021 ◽  
Vol 11 (9) ◽  
pp. 3869
Author(s):  
Chen Niu ◽  
Yongwei Liu ◽  
Dejiang Shang ◽  
Chao Zhang
Keyword(s):  
Reynolds Number ◽  
Drag Reduction ◽  
Noise Reduction ◽  
Channel Flow ◽  
Superhydrophobic Surface ◽  
Sound Field ◽  
Flow Noise ◽  
Slip Boundary ◽  
Eddy Simulation ◽  
Reduction Effect

Superhydrophobic surface is a promising technology, but the effect of superhydrophobic surface on flow noise is still unclear. Therefore, we used alternating free-slip and no-slip boundary conditions to study the flow noise of superhydrophobic channel flows with streamwise strips. The numerical calculations of the flow and the sound field have been carried out by the methods of large eddy simulation (LES) and Lighthill analogy, respectively. Under a constant pressure gradient (CPG) condition, the average Reynolds number and the friction Reynolds number are approximately set to 4200 and 180, respectively. The influence on noise of different gas fractions (GF) and strip number in a spanwise period on channel flow have been studied. Our results show that the superhydrophobic surface has noise reduction effect in some cases. Under CPG conditions, the increase in GF increases the bulk velocity and weakens the noise reduction effect. Otherwise, the increase in strip number enhances the lateral energy exchange of the superhydrophobic surface, and results in more transverse vortices and attenuates the noise reduction effect. In our results, the best noise reduction effect is obtained as 10.7 dB under the scenario of the strip number is 4 and GF is 0.5. The best drag reduction effect is 32%, and the result is obtained under the scenario of GF is 0.8 and strip number is 1. In summary, the choice of GF and the number of strips is comprehensively considered to guarantee the performance of drag reduction and noise reduction in this work.

scholarly journals Base flow modulations for skin-friction drag reduction

2011 ◽  
Vol 318 (3) ◽  
pp. 032008 ◽  
Author(s):  
Jens H M Fransson ◽  
Alessandro Talamelli
Keyword(s):  
Drag Reduction ◽  
Skin Friction ◽  
Base Flow ◽  
Friction Drag ◽  
Friction Drag Reduction
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