High-Reynolds-number flat-plate turbulent boundary layer measurements (original) (raw)
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Turbulence profiles from a smooth flat-plate turbulent boundary layer at high Reynolds number
Experimental Thermal and Fluid Science, 2012
Much is known about smooth-flat-plate turbulent boundary layers (TBLs) at laboratory-scale Reynolds numbers because of a wealth of experimental data. However, smooth-flat-plate TBL data are much less common at the high Reynolds numbers typical of aerodynamic and hydrodynamic applications (Re x $ 10 8-10 10), and at the even higher Reynolds numbers of many geophysical flows. This paper presents new LDV-measured profiles of the stream-wise velocity variance, the wall-normal velocity variance, and the Reynolds shear stress from the TBL that formed on a smooth flat plate at Karman numbers from 15,000 to 60,000 (Re x from 75 million to 220 million). The experiments were conducted in the William B. Morgan Large Cavitation Channel on a polished (k + < 0.2) flat-plate test model 12.9 m long and 3.05 m wide at water flow speeds up to 20 m s À1. The TBL on the model developed in a mild favorable pressure gradient having an acceleration parameter K $ 10 À10. When plotted with the usual inner and outer scalings, the stream-wise velocity variance profiles display a Reynolds number dependence that is consistent with prior lower Reynolds-number zero-pressure-gradient TBL measurements. However, using the same normalizations, the profiles of wall-normal velocity variance and Reynolds shear stress are found to be Reynolds number independent, or nearly so, when experimental uncertainties are considered.
Physics of Fluids, 2010
Wall-pressure fluctuations were investigated within a high-Reynolds-number turbulent boundary layer (TBL) modified by the addition of dilute friction-drag-reducing polymer solutions. The experiment was conducted at the U.S. Navy's Large Cavitation Channel on a 12.9 m long flat-plate test model with the surface hydraulically smooth (k+<0.2) and achieving downstream-distance-based Reynolds numbers to 220×106. The polymer (polyethylene oxide) solution was injected into the TBL through a slot in the surface. The primary flow diagnostics were skin-friction drag balances and an array of flush-mounted dynamic pressure transducers 9.8 m from the model leading edge. Parameters varied included the free-stream speed (6.7, 13.4, and 20.2 m s-1) and the injection condition (polymer molecular weight, injection concentration, and volumetric injection flux). The behavior of the pressure spectra, convection velocity, and coherence, regardless of the injection condition, were determined primarily based on the level of drag reduction. Results were divided into two regimes dependent on the level of polymer drag reduction (PDR), nominally separated at a PDR of 40%. The low-PDR regime is characterized by decreasing mean-square pressure fluctuations and increasing convection velocity with increasing drag reduction. This shows that the decrease in the pressure spectra with increasing drag reduction is due in part to the moving of the turbulent structures from the wall. Conversely, with further increases in drag reduction, the high-PDR regime has negligible variation in the mean-squared pressure fluctuations and convection velocity. The convection velocity remains constant at approximately 10% above the baseline-flow convection velocity, which suggests that the turbulent structures no longer move farther from the wall with increasing drag reduction. In light of recent numerical work, the coherence results indicate that in the low-PDR regime, the turbulent structures are being elongated in the streamwise direction and occurring at decreasing frequency. In the high-PDR regime, the rate of occurrence continues to decrease until large-scale coherent turbulent structures are potentially no longer present.
Physics of Fluids, 2013
The current study explores the influence of polymer drag reduction on the nearwall velocity distribution in a turbulent boundary layer (TBL) and its dependence on Reynolds number. Recent moderate Reynolds number direct numerical simulation and experimental studies presented in White et al. [Phys. Fluids 24, 021701 (2012)] have challenged the classical representation of the logarithmic dependence of the velocity profile for drag-reduced flows, especially at drag reduction levels above 40%. In the present study, high Reynolds number data from a drag reduced TBL is presented and compared to the observations of White et al. [Phys. Fluids 24, 021701 (2012)]. Data presented here were acquired in the TBL flow on a 12.9-m-long flat plate at speeds to 20.3 m s −1 , achieving momentum thickness based Reynolds number to 1.5 × 10 5 , which is an order of magnitude greater than that available in the literature. Polyethylene oxide solutions with an average molecular weight of 3.9 × 10 6 g mol −1 were injected into the flow at various concentrations and volumetric fluxes to achieve a particular level of drag reduction. The resulting mean near-wall velocity profiles show distinctly different behavior depending on whether they fall in the low drag reduction (LDR) or the high drag reduction (HDR) regimes, which are nominally divided at 40% drag reduction. In the LDR regime, the classical view that the logarithmic slope remains constant at the Newtonian value and the intercept constant increases with increasing drag reduction appears to be valid. However, in the HDR regime the behavior is no longer universal. The intercept constant continues to increase linearly in proportion to the drag reduction level until a Reynolds-number-dependent threshold is achieved, at which point the intercept constant rapidly decreases to that predicted by the ultimate profile. The rapid decrease in the intercept constant is due to the corresponding increase in the profile slope in the HDR regime. There was significant scatter in the observed slope in the HDR regime, but the scatter did not appear to be Reynolds number dependent. Finally, the ultimate profiles for flows at maximum drag reduction were examined and did not exhibit a logarithmic functional relationship, which is the classical empirical relationship suggested by Virk [J. Am. Inst. Chem. Eng. 21, 625-656 (1975)]. C 2013 AIP Publishing LLC. [http://dx.
Journal of Fluid Mechanics, 2011
Polymer drag reduction, diffusion and degradation in a high-Reynolds-number turbulent boundary layer (TBL) flow were investigated. The TBL developed on a flat plate at free-stream speeds up to 20m s(-1). Measurements were acquired up to 10.7 m downstream of the leading edge, yielding downstream-distance-based Reynolds numbers up to 220 million. The test model surface was hydraulically smooth or fully rough. Flow diagnostics included local skin friction, near-wall polymer concentration, boundary layer sampling and rheological analysis of polymer solution samples. Skin-friction data revealed that the presence of surface roughness can produce a local increase in drag reduction near the injection location (compared with the flow over a smooth surface) because of enhanced mixing. However, the roughness ultimately led to a significant decrease in drag reduction with increasing speed and downstream distance. At the highest speed tested (20m s(-1)) no drag reduction was discernible at the first measurement location (0.56 m downstream of injection), even at the highest polymer injection flux (10 times the flux of fluid in the near-wall region). Increased polymer degradation rates and polymer mixing were shown to be the contributing factors to the loss of drag reduction. Rheological analysis of liquid drawn from the TBL revealed that flow-induced polymer degradation by chain scission was often substantial. The inferred polymer molecular weight was successfully scaled with the local wall shear rate and residence time in the TBL. This scaling revealed an exponential decay that asymptotes to a finite (steady-state) molecular weight. The importance of the residence time to the scaling indicates that while individual polymer chains are stretched and ruptured on a relatively short time scale (similar to 10-3 s), because of the low percentage of individual chains stretched at any instant in time, a relatively long time period (similar to 0.1 s) is required to observe changes in the mean molecular weight. This scaling also indicates that most previous TBL studies would have observed minimal influence from degradation due to insufficient residence times.
High Reynolds number experimentation in the US Navy's William B Morgan Large Cavitation Channel
Measurement Science and Technology, 2005
The William B Morgan Large Cavitation Channel (LCC) is a large variable-pressure closed-loop water tunnel that has been operated by the US Navy in Memphis, TN, USA, since 1991. This facility is well designed for a wide variety of hydrodynamic and hydroacoustic tests. Its overall size and capabilities allow test-model Reynolds numbers to approach, or even achieve, those of full-scale air-or water-borne transportation systems. This paper describes the facility along with some novel implementations of measurement techniques that have been successfully utilized there. In addition, highlights are presented from past test programmes involving (i) cavitation, (ii) near-zero pressure-gradient turbulent boundary layers, (iii) the near-wake flow characteristics of a two-dimensional hydrofoil and (iv) a full-scale research torpedo.
On the scaling of air layer drag reduction
Journal of Fluid Mechanics, 2013
Air-induced drag reduction was investigated on a 12.9 m long flat plate test model at a free stream speed of 6:3 m/s. Measurements of the local skin friction, phase velocity profiles (liquid and gas) and void fraction profiles were acquired at downstream distances to 11.5 m, which yielded downstream-distance-based Reynolds numbers above 80 million. Air was injected within the boundary layer behind a 13 mm backward facing step (BFS) while the incoming boundary layer was perturbed with vortex generators in various configurations immediately upstream of the BFS. Measurements confirmed that air layer drag reduction (ALDR) is sensitive to upstream disturbances, but a clean boundary layer separation line (i.e. the BFS) reduces such sensitivity. Empirical scaling of the experimental data was investigated for: (a) the critical air flux required to establish ALDR; (b) void fraction profiles; and (c) the interfacial velocity profiles. A scaling of the critical air flux for ALDR was developed from balancing shear-induced lift forces and buoyancy forces on a single bubble within a shear flow. The resulting scaling successfully collapses ALDR results from the current and past studies over a range of flow conditions and test model configurations. The interfacial velocity and void fraction profiles were acquired and scaled within the bubble drag reduction (BDR), ALDR and transitional ALDR regimes. The BDR interfacial velocity profile revealed that there was slip between phases. The ALDR results showed that the air layer thickness was nominally three-quarters of the total volumetric flux (per unit span) of air injected divided by the free stream speed. Furthermore, the air layer had an average void fraction of 0.75 and a velocity of approximately 0.2 times the free stream speed. Beyond the air layer was a bubbly mixture that scaled in a similar fashion to the BDR results. Transitional ALDR results indicate that this regime was comprised of intermittent generation and subsequent fragmentation of an air layer, with the resulting drag reduction determined by the fraction of time that an air layer was present.
Diffusion of drag-reducing polymer solutions within a rough-walled turbulent boundary layer
Physics of Fluids, 2010
The influence of surface roughness on diffusion of wall-injected, drag-reducing polymer solutions within a turbulent boundary layer was studied with a 0.94 m long flat-plate test model at speeds of up to 10.6 m s-1 and Reynolds numbers of up to 9×106. The surface was hydraulically smooth, transitionally rough, or fully rough. Mean concentration profiles were acquired with planar laser induced fluorescence, which was the primary flow diagnostic. Polymer concentration profiles with high injection concentrations (>=1000 wppm) had the peak concentration shifted away from the wall, which was partially attributed to a lifting phenomenon. The diffusion process was divided into three zones-initial, intermediate, and final. Studies of polymer injection into a polymer ocean at concentrations sufficient for maximum drag reduction indicated that the maximum initial zone length is of the order of 100 boundary layer thicknesses. The intermediate zone results indicate that friction velocity and roughness height are important scaling parameters in addition to flow and injection conditions. Lastly, the current results were combined with those in Petrie et al. [``Polymer drag reduction with surface roughness in flat-plate turbulent boundary layer flow,'' Exp. Fluids 35, 8 (2003)] to demonstrate that the influence of polymer degradation increases with increased surface roughness.
In order to lower down the fuel consumption and to achieve higher speed of ship, reduction in ship resistance has been the major topic of research for a long time. The most important factor in ship resistance is skin friction resistance. Micro-bubble has been shown to be able to reduce skin friction. This micro-bubble method gives the possibility to lower the friction without any change in the present hull form of a ship. The application of the micro-bubble technique reduces the surface friction by a variation of the viscosity of the fluid around the ship and makes a modification in the structure of the turbulent boundary layer. However, not much is known about the correct size, quantity, area of coverage which can effectively form a skin friction reducing mechanism. There are many established methods, such as Venturi tube type generator, tangential water-jet and forming of dissolve air and also a chemical process, such as electrolysis, may result in bubble production [1]. The use o...