New developments and applications of skin-friction measuring techniques
TL;DR: In this article, a survey of skin-friction measurement techniques (oil-film interferometry, wall hot wire, surface fence and wall pulsed wire) is presented.
Abstract: This survey covers recent developments and applications of four skin-friction measurement techniques (oil-film interferometry, wall hot wire, surface fence and wall pulsed wire). Comparisons of the techniques with each other and with other methods are presented. Applications in attached and separated fully turbulent boundary layers and in highly accelerated laminar-like flows will be shown to demonstrate the application range and the limits of the various techniques.
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TL;DR: In this paper, the authors distill the salient advances of recent origin, particularly those that challenge textbook orthodoxy, and highlight some of the outstanding questions, such as the extent of the logarithmic overlap layer, the universality or otherwise of the principal model parameters, and the scaling of mean flow and Reynolds stresses.
Abstract: Wall-bounded turbulent flows at high Reynolds numbers have become an increasingly active area of research in recent years. Many challenges remain in theory, scaling, physical understanding, experimental techniques, and numerical simulations. In this paper we distill the salient advances of recent origin, particularly those that challenge textbook orthodoxy. Some of the outstanding questions, such as the extent of the logarithmic overlap layer, the universality or otherwise of the principal model parameters such as the von Karman “constant,” the parametrization of roughness effects, and the scaling of mean flow and Reynolds stresses, are highlighted. Research avenues that may provide answers to these questions, notably the improvement of measuring techniques and the construction of new facilities, are identified. We also highlight aspects where differences of opinion persist, with the expectation that this discussion might mark the beginning of their resolution.
716 citations
TL;DR: In this article, three relatively modern categories of skin-friction measurement techniques are broadly classified as microelectromechanical systems (MEMS)-based sensors, oil-film interferometry, and liquid crystal coatings.
384 citations
TL;DR: In this article, two independent experimental investigations of the behavior of turbulent boundary layers with increasing Reynolds number were recently completed, and the results are summarized here, utilizing the profiles of the mean velocity, for Reynolds numbers based on the momentum thickness ranging from 2500 to 27,000.
Abstract: Two independent experimental investigations of the behavior of turbulent boundary layers with increasing Reynolds number were recently completed. The experiments were performed in two facilities, the Minimum Turbulence Level (MTL) wind tunnel at Royal Institute of Technology (KTH) and the National Diagnostic Facility (NDF) wind tunnel at Illinois Institute of Technology (IIT). Both experiments utilized oil-film interferometry to obtain an independent measure of the wall-shear stress. A collaborative study by the principals of the two experiments, aimed at understanding the characteristics of the overlap region between the inner and outer parts of the boundary layer, has just been completed. The results are summarized here, utilizing the profiles of the mean velocity, for Reynolds numbers based on the momentum thickness ranging from 2500 to 27 000. Contrary to the conclusions of some earlier publications, careful analysis of the data reveals no significant Reynolds number dependence for the parameters desc...
332 citations
Cites methods from "New developments and applications o..."
...The overlap region in turbulent boundary layers 31 The skin-friction was measured independently of the velocity measurements using oil-film interferometry in a set-up similar to Fernholz et al. (1996), see Fig....
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01 Jan 1999
TL;DR: In this paper, the authors investigated the high Reynolds number zero pressure gradient turbulent boundary layers in an incompressible flow without any effects of heat-transfer and found that the inner limit of overlap region was found to scale on the viscous length scale (ν/uτ) and was estimated to be y = 200.
Abstract: This thesis deals with the problem of high Reynolds number zero pressuregradient turbulent boundary layers in an incompressible flow without any effects of heat-transfer. The zero-pressure gradient turbulent boundary layer is one of the canonical shear flows important in many applications and of large theoretical interest. The investigation was carried out through an experimental study in the MTL wind-tunnel at KTH, where the fluctuating velocity components and the fluctuating wall-shear stress in a turbulent boundary layer were measured using hot-wire and hot-film anemometry. Attempts were made to answer some basic and “classical” questions concerning turbulent boundary boundary layers. The classical two layer theory was confirmed and constant values of the slope of the logarithmic overlap region (i.e. the von Karman constant) and the additive constants were found and estimated to κ = 0.38, B = 4.1 and B1 = 3.6 (δ = δ95). The inner limit of overlap region was found to scale on the viscous length scale (ν/uτ) and was estimated to be y = 200, i.e. considerably further out compared to previous knowledge. The outer limit of the overlap region was found to scale on the outer length scale and was estimated to be y/δ = 0.15. This also means that a universal overlap region only can exist for Reynolds numbers of at least Reθ ≈ 6000. The values of the newly determined limits explain the Reynolds number variation found in some earlier experiments. Measurements of the fluctuating wall-shear stress using the hot-wire-onthe-wall technique and a MEMS hot-film sensor show that the turbulence intensity τr.m.s./τw is close to 0.41 at Reθ ≈ 9800. A numerical and experimental investigation of the behavior of double wire probes were carried out and showed that the Peclet number based on wire separation should be larger than about 50 to ensure an acceptably low level of thermal interaction. Results are presented for two-point correlations between the wall-shear stress and the streamwise velocity component for separations in both the wallnormal-streamwise plane and the wall-normal-spanwise plane. Turbulence producing events are further investigated using conditional averaging of isolated shear-layer events. Comparisons are made with results from other experiments and numerical simulations. Descriptors: Fluid mechanics, turbulence, boundary layers, high Reynolds number, zero-pressure gradient, hot-wire, hot-film anemometry, oil-film interferometry, structures, streak spacing, micro-electro-mechanical-systems.
322 citations
Cites background or methods from "New developments and applications o..."
...The accuracy of the mean skin friction value τw measured by this oil-film method is better than ±4% (Fernholz et al. 1996)....
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...The skin-friction was measured independently of the velocity measurements using oil-film interferometry in a set-up similar to Fernholz et al. (1996), see Fig....
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...Evaluation of the recent high Reynolds number experiment in the German-Dutch wind-tunnel by Fernholz et al. (1995) also yields a significantly lower value of the von Kármán constant....
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TL;DR: In this article, the mean velocity distributions of two-dimensional fully developed turbulent plane-channel flows were analyzed for different Reynolds numbers up to Reτ≈5×103, where Reτ is based on the wall friction velocity and the channel half-height.
Abstract: This article is concerned with the mean velocity distributions of two-dimensional fully developed turbulent plane-channel flows. To yield reliable information, the authors performed detailed hot-wire measurements for more than 12 Reynolds numbers. The experimental investigations covered a wide range of the Reynolds numbers up to Reτ≈5×103, where Reτ is based on the wall friction velocity and the channel half-height. From the distribution of the mean velocity gradient (dU+/dy+)=f(y+) the entire flow field was analyzed, resulting in a logarithmic region for the mean velocity profile in the inertial sublayer, extending almost up to the center of the channel at higher Reynolds numbers. The analysis of the experimental results yield a value of the von Karman constant, κ, close to 0.37(≈1/e) independent of the Reynolds number and the additive constant B=3.70, which is close to 10/e, i.e., U+=e ln y++10/e=(1/0.37)ln y++3.70.
200 citations
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TL;DR: In this paper, it is shown that in sufficiently strong favorable and adverse pressure gradients the inner-law velocity distribution breaks down completely, and it is suggested that this breakdown is associated with reversion to laminar flow.
Abstract: Preston's method of measuring skin friction in the turbulent boundary layer makes use of a circular Pitot tube resting on the wall. On the assumption of a velocity distribution in the wall region common to boundary layer and pipe flows the calibration curve for the Pitot tube can be obtained in fully developed pipe flow. Earlier experiments suggested that Preston's original calibration was in error, and a revised calibration curve has been obtained and is presented here.From experiments in strong favourable and adverse pressure gradients, limits are assigned to the pressure-gradient conditions within which the calibration can be used with prescribed accuracy. It is shown that in sufficiently strong favourable gradients the ‘inner-law’ velocity distribution breaks down completely, and it is suggested that this breakdown is associated with reversion to laminar flow.As an incidental result, values have been obtained for the constants occurring in the logarithmic expression for the inner-law velocity distribution.
730 citations
TL;DR: In this article, the authors discuss force-measurement balances, the use of the velocity profile, pressure measurements by surface pitot tubes or about obstacles, and the analogies of heat transfer, mass transfer or surface oil flow.
337 citations
TL;DR: In this paper, the fluctuating wall-shear stress was measured with various types of hotwire and hot-film sensors in turbulent boundary layer and channel flows, and the rms level of the streamwise wall shear stress fluctuations was found to be 40% of the mean value, which was substantiated by measurements of streamwise velocity fluctuations in the viscous sublayer.
Abstract: The fluctuating wall‐shear stress was measured with various types of hot‐wire and hot‐film sensors in turbulent boundary‐layer and channel flows. The rms level of the streamwise wall‐shear stress fluctuations was found to be 40% of the mean value, which was substantiated by measurements of the streamwise velocity fluctuations in the viscous sublayer. Heat transfer to the fluid via the probe substrate was found to give significant differences between the static and dynamic response for standard flush‐mounted hot‐film probes with air or oil as the flow medium, whereas measurements in water were shown to be essentially unaffected by this problem.
333 citations
TL;DR: In this paper, a simple relation is obtained between the film thickness variation and the skin friction distribution, and the results confirm the theory and show that the method gives reasonably accurate measurements of skin friction distributions in low speed flows.
Abstract: An oil film on a solid surface moves under the action of gravity or of the pressure gradient and skin friction resulting from the flow of air over the oil surface. Such oil flows are studied experimentally and theoretically. If the film is thin enough, the dominant force is the skin friction, and a simple relation is obtained between the film thickness variation and the skin friction distribution. The other forces give a perturbation which may be estimated and which decreases with time. The appropriate film thickness is of the order of 10 mu m and so is conveniently measured by interferometry. Experimental results confirm the theory and show that the method gives reasonably accurate measurements of skin friction distribution in low speed flows.
246 citations
TL;DR: In this paper, the authors measured mean velocity, Reynolds-shear-stress and Reynolds-normal-stress distributions of highly turbulent and disturbed flow over a bluff plate with a long splitter plate in its plane of symmetry.
Abstract: Experiments were performed in the highly turbulent and disturbed flow over a bluff plate with a long splitter plate in its plane of symmetry. The flow separates at the sharp bevelled edge of the bluff plate, forms a free shear layer on top of the reverse-flow region which is bounded on its other side by the splitter plate, and reattaches on the splitter plate over a narrow region curved in spanwise direction. Downstream of reattachment the shear flow adjusts slowly to the wall boundary conditions. Measurements of mean velocity, Reynolds-shear-stress and Reynolds-normal-stress distributions were carried out by hot-wire and pulsed-wire anemometry. The latter technique was used in those regions of the flow where reverse flow occurred or where the flow was highly turbulent. Spectra and integral lengthscales were measured to investigate the state and structure of the flow. The large-eddy structure in the inner region of the flow had lengthscales in the two cross-stream directions which were approximately equal, indicating a fast break-up of spanwise structures just downstream from separation. Mean and fluctuating quantities showed a self-similar behaviour in a short region upstream of reattachment and ‘profile similarity’ in the separated shear layer and along the splitter plate downstream from reattachment. Probability-density distributions of skin friction were measured and used to calculate mean and fluctuating values. No flapping of the reattaching shear layer could be observed. Pulsed-wire measurements revealed that the logarithmic law of the wall does not hold either in the reverse-flow region or in a region about half the length of the bubble downstream from reattachment.
224 citations