Extent Of Laminar Flow Research Articles

Weakly nonlinear stages of boundary-layer transition initiated by modulated Tollmien–Schlichting waves

AbstractWeakly nonlinear interactions involving amplitude-modulated Tollmien–Schlichting waves in an incompressible, two-dimensional aerofoil boundary layer are investigated experimentally. Selected resonant regimes are examined with emphasis on the regimes where more than one fundamental Tollmien–Schlichting (TS) wave is present in the flow. The experiments were performed on an NLF-type aerofoil section for glider applications. Disturbances with controlled frequency-spanwise-wavenumber spectra were excited in the boundary layer and studied by phase-locked hot-wire measurements. The results show that nonlinear mechanisms connected with the steepening of the primary TS wave modulation do not play any significant role in the transition scenarios studied. It is also shown that modulations of the two-dimensional fundamental waves tend to generate additional modes at modulation frequency. These low-frequency disturbances are found to be produced by a non-resonant quadratic combination of spectral components of the primary, modulated TS wave. The investigations show that the efficiency of the process is higher for three-dimensional low-frequency modes in comparison with two-dimensional modes. Thus, the emergence of three-dimensionality for the low-frequency waves does not require any resonant interactions. In a subsequent nonlinear stage, the self-generated detuned subharmonics are found to be strongly amplified due to resonant interactions with the primary TS waves. The sequence of weakly nonlinear mechanisms found and investigated here seems to be the most likely route to the laminar–turbulent transition, at least for two-dimensional boundary layers of aerofoils with a long extent of laminar flow and in a ‘natural’ disturbance environment.

Aerodynamic Considerations in the Design of Truss-Braced-Wing Aircraft

The paper describes a study of the effects of several key aerodynamic considerations on the conceptual design of minimum fuel / emissions, long range transport, truss-braced wing aircraft configurations. This unconventional configuration has a large benefit over conventional cantilever wing configurations. The truss system enables an increased aspect ratio with lower sweep, thickness ratio, and chords, thus exploiting natural laminar flow. The design problem is solved by an MDO process, which takes into account both aerodynamic and structural considerations. The paper contains several studies, each of which investigates the dependency of the design space on a specific aerodynamic parameter such as the extent of laminar flow on the wing, cruise Mach number, maximum cruise twodimensional lift coefficient, the supercritical characteristics of the airfoil, winglet influence, and intersection fairing design. In addition, various fuselage drag reduction technologies are investigated: fuselage relaminarization, surface riblets, tailless arrangements and Goldschmied apparatus. All of these studies illustrate large potential of the truss-braced wing along with additional drag reduction technologies, which may substantially decrease the fuel weight and vehicle emissions. The paper emphasizes the importance of appropriate wing section airfoils, which can satisfy various contradicting criteria of natural laminar flow, supercritical characteristics, high lift coefficient, and low drag coefficient. Finally, these studies illustrate the importance of fuselage drag reduction for a low induced drag, NLF aircraft like a truss-braced wing configuration.

A Practical and Robust Method to Experimentally Determine the Location of the Region of Transition from Laminar to Turbulent Flow on a Towed Body

When performing model-scale resistance tests on bodies towed in water it is normal practice to deliberately stimulate turbulence close to the leading edge to permit appropriate scaling of resistance measurements. Alternatively, tests on towed bodies can address the flow characteristics over the body in the absence of deliberately tripped turbulent flow. In the second case an appreciation of the extent of laminar and turbulent flow over the body is of particular interest. While sophisticated and expensive methods are available to experimentally detect the location of transition on bodies in fluid flow, these are still particularly challenging when applied on bodies being towed through water. In this paper a straightforward alternative experimental method to detect transition on towed bodies is provided. This transition detection method simply uses conventional experimental measurements of drag on a towed body, subject to deliberate turbulence stimulation, to deduce the extent of laminar flow. The transition detection method is theoretically outlined and an experimental flow visualisation procedure demonstrates the required effectiveness of the turbulence stimulators used. An experimental demonstration of the proposed method is provided using two alternative turbulence stimulation devices. The conclusions indicate that the proposed method provides a practical and robust method to experimentally detect transition on a towed body with reasonable accuracy.

Turbulence Effects on the Design and Performance of Low-Drag Keels

Achievement of laminar boundary-layer flow over the keel of sailing yachts offers great potential for increased boat speed resulting from the reduction in viscous drag. The intensity and the spectral energy distribution of small-scale turbulence in the upper layer of the ocean can have a large effect on the extent of laminar flow and, hence, the drag reduction. A detailed summary and interpretation is provided of measured turbulent intensities and length scales in the upper ocean. An analysis is given of the effect of small-scale turbulence on boundary-layer transition and profile drag for two laminar-flow foils at zero and moderate leeway angles. Using the modified en-transition criterion it is shown that the level of small-scale oceanic turbulence is an important parameter in the design of a laminar-flow foil. At high turbulence levels, the foil with a more forward location of the minimum-pressure point generates less drag than the foil with a more aft location of the minimum-pressure point at the same flow conditions.

Flow at exit of an electrically-heated water nozzle discharging in air

The extent of laminar flow at the exit of a nozzle due to favorable pressure gradient, and the influence of heating on the amount of laminar flow present at the nozzle exit, was visualized using a high-resolution, image-motion compensating camera which provides high-speed macrophotography of the exit jet. Photographs are presented which indicate the effect of electrical heating on the extent of laminar flow in the shear layer of two nozzles, 1/10-inch (2.5 mm) and 1/4-inch (6.4 mm) diameter respectively. Other data give the effect of upstream pressure and electrical power input on the visually observed properties of the nozzle exit flow.

Surface Flow Patterns as Visualised by Dust Deposits on the Blades of a Fan

Investigations by one of the authors in connection with the design of a fan for a blower type of wind tunnel showed that regular and repeatable dust patterns occurred on the blades of a one-quarter scale model fan of 18 inches diameter. Dust was deposited on the fan blades along the leading-edge and on the suction surface over an area thought to be the turbulent region of the boundary layer. The introduction of isolated protuberances on the dust free area of a blade gave rise to turbulence wedges in which dust was also deposited and this was interpreted as confirmation of the coincidence of the dust deposits with regions of turbulent boundary-layer flow. These deposits showed the existence of a considerable extent of laminar flow on the suction surface of each blade close to the root, a region where high lift coefficients would be expected with associated adverse pressure gradients. Two-dimensional wind tunnel experiments were made to confirm the interpretation of the observed dust patterns by comparison with the smoke filament and volatile liquid methods of flow visualisation and these are reported in Reference 2.