scholarly journals Investigation of the Flow behind the Roughness Element on the UAV Surface at a Favorable Pressure Gradient

2020 ◽  
Vol 15 (2) ◽  
pp. 61-72
Author(s):  
Pavlenko ◽  
Kaprilevskaya ◽  
Kozlov ◽  
Katasonov
Keyword(s):  
Boundary Layer ◽  
Experimental Study ◽  
Liquid Crystal ◽  
Wind Tunnel ◽  
Smooth Surface ◽  
Roughness Element ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Liquid Crystal Thermography ◽  
Favorable Pressure Gradient

In a wind tunnel of low subsonic speeds, an experimental study was conducted of the windward flow of a trapezoidal model of a flying wing (UAV) with a locally installed perturbation generator in the region of maximum susceptibility on its surface. The generator was a three-dimensional roughness element whose height was comparable to the thickness of the boundary layer. The uniqueness of the work was that the experiments were carried out in a wind tunnel at real flight Reynolds numbers on a UAV model at a scale of 1:1. The results of visualization of the flow near a smooth surface and behind roughness were obtained using the method of liquid crystal thermography. The internal structure and processes of development of the longitudinal perturbation behind the roughness downstream were studied in detail using the thermoanemometry method.

Roughness Element Geometry Required for Wind Tunnel Simulations of the Atmospheric Wind

1977 ◽  
Vol 99 (3) ◽  
pp. 480-485 ◽  
Author(s):  
I. S. Gartshore ◽  
K. A. De Croos
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Wind Tunnel ◽  
Roughness Element ◽  
Three Dimensional ◽  
Wall Stress ◽  
Wide Range ◽  
Drag Plate ◽  
Rough Boundaries ◽  
Single Figure

Using a data correlation for the wall stress associated with very rough boundaries and a semi-empirical calculation method, the shape of boundary layers in exact equilibrium with the roughness beneath them is calculated. A wide range of roughness geometries (two- and three-dimensional elements) is included by the use of equivalent surfaces of equal drag per unit area. Results can be summarized in a single figure which relates the shape factor of the boundary layer (its exponent if it has a power law velocity profile) to the height of the roughness elements and their spacing. New data for one turbulent boundary layer developing over a long fetch of uniform roughness is presented. Wall shear stress, measured directly from a drag plate is combined with boundary layer integral properties to show that the shear stress correlation adopted is reasonably accurate and that the boundary layer is close to equilibrium after passing over a streamwise roughness fetch equal to about 350 times the roughness element height. An example is given of the way in which roughness geometry may be chosen from calculated equilibrium results, for one particular boundary layer thickness and a shape useful for simulating strong atmospheric winds in a wind tunnel.

The Role of Two-Dimensional Roughness Element in the Laminar-Turbulent Process in the Favorable Pressure Gradient of the Swept Wing

2014 ◽  
Vol 9 (4) ◽  
pp. 65-73
Author(s):  
Stepan Tolkachev ◽  
Valeria Kaprilevskaya ◽  
Viktor Kozlov
Keyword(s):  
Liquid Crystal ◽  
Roughness Element ◽  
Frequency Interval ◽  
Two Dimensional ◽  
Swept Wing ◽  
Liquid Crystal Thermography ◽  
Roughness Elements ◽  
Favorable Pressure Gradient ◽  
Turbulent Process

In the article using a liquid crystal thermography investigated the development of stationary and secondary disturbances, which were excited by cylindrical and two-dimensional roughness elements. It was shown, that two-dimensional roughness element has a destabilizing effect on disturbances, induced by cylindrical roughness element. Also the twodimensional roughness element is able to excite the stationary structures, and then the secondary disturbances the frequency interval of which is lower than in the case of stationary vortices excitation by cylindrical roughness element

scholarly journals Transient growth in the flow past a three-dimensional smooth roughness element

2013 ◽  
Vol 724 ◽  
pp. 642-670 ◽  
Author(s):  
S. Cherubini ◽  
M. D. De Tullio ◽  
P. De Palma ◽  
G. Pascazio
Keyword(s):  
Boundary Layer ◽  
Boundary Layer Flow ◽  
Roughness Element ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Base Flow ◽  
Energy Gain ◽  
Roughness Elements ◽  
Layer Flow ◽  
Optimal Disturbances

AbstractThis work provides a global optimization analysis, looking for perturbations inducing the largest energy growth at a finite time in a boundary-layer flow in the presence of smooth three-dimensional roughness elements. Amplification mechanisms are described which can bypass the asymptotical growth of Tollmien–Schlichting waves. Smooth axisymmetric roughness elements of different height have been studied, at different Reynolds numbers. The results show that even very small roughness elements, inducing only a weak deformation of the base flow, can localize the optimal disturbance characterizing the Blasius boundary-layer flow. Moreover, for large enough bump heights and Reynolds numbers, a strong amplification mechanism has been recovered, inducing an increase of several orders of magnitude of the energy gain with respect to the Blasius case. In particular, the highest value of the energy gain is obtained for an initial varicose perturbation, differently to what found for a streaky parallel flow. Optimal varicose perturbations grow very rapidly by transporting the strong wall-normal shear of the base flow, which is localized in the wake of the bump. Such optimal disturbances are found to lead to transition for initial energies and amplitudes considerably smaller than sinuous optimal ones, inducing hairpin vortices downstream of the roughness element.

The aeroacoustics of finite wall-mounted square cylinders

2017 ◽  
Vol 832 ◽  
pp. 287-328 ◽  
Author(s):  
Ric Porteous ◽  
Danielle J. Moreau ◽  
Con J. Doolan
Keyword(s):  
Boundary Layer ◽  
Experimental Study ◽  
Reynolds Number ◽  
Wind Tunnel ◽  
Three Dimensional ◽  
Flow Structures ◽  
Hot Wire ◽  
Radiated Noise ◽  
Aspect Ratios ◽  
Square Cylinders

This paper presents the results of an experimental study that relates the flow structures in the wake of a square finite wall-mounted cylinder with the radiated noise. Acoustic and hot-wire measurements were taken in an anechoic wind tunnel. The cylinder was immersed in a near-zero-pressure gradient boundary layer whose thickness was 130 % of the cylinder width, $W$. Aspect ratios were in the range $0.29\leqslant L/W\leqslant 22.9$ (where $L$ is the cylinder span), and the Reynolds number, based on width, was $1.4\times 10^{4}$. Four shedding regimes were identified, namely R0 ($L/W<2$), RI ($2<L/W<10$), RII ($10<L/W<18$) and RIII ($L/W>18$), with each shedding regime displaying an additional acoustic tone as the aspect ratio was increased. At low aspect ratios (R0 and RI), downwash dominated the wake, creating a highly three-dimensional shedding environment with maximum downwash at $L/W\approx 7$. Looping vortex structures were visualised using a phase eduction technique. The principal core of the loops generated the most noise perpendicular to the cylinder. For higher aspect ratios in RII and RIII, the main noise producing structures consisted of a series of inclined vortex filaments, where the angle of inclination varied between vortex cells.

scholarly journals Study on Oil-Film Interferometry Measurement Technique of Hypersonic Boundary Layer Transition

2018 ◽  
Vol 36 (6) ◽  
pp. 1156-1161
Author(s):  
Hao Dong ◽  
Shicheng Liu ◽  
Xi Geng ◽  
Keming Cheng
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Skin Friction ◽  
High Speed ◽  
Smooth Surface ◽  
Roughness Element ◽  
Boundary Layer Transition ◽  
Layer Transition ◽  
Oil Film ◽  
Hypersonic Wind Tunnel

Prediction of boundary layer transition is important for the design of hypersonic aircrafts. The study of boundary layer transition of hypersonic flow around a flat plate using oil-film interferometry was investigated at Φ500mm traditional hypersonic wind tunnel. In order to measure the skin friction fast and precisely on the hypersonic wind tunnel, the traditional oil-film interferometry technique is improved. A high-speed camera is used to capture the images of fringes and the viscosity of the silicon oil is modified according to the wall temperature measured by thermocouples during the test. The skin frictions of smooth surface and the surface with single square roughness element were measured. For the smooth surface, the boundary layer is laminar. However, the boundary layer transition is promoted by wake vortices induced by the roughness element. Both the results of skin friction with and without the roughness element are in good agreement with the simulation results correspondingly, indicating high accuracy of the oil film interferometry technique.

Streamwise absolute instability of a three-dimensional boundary layer at high Reynolds numbers

1998 ◽  
Vol 373 ◽  
pp. 111-153 ◽  
Author(s):  
OLEG S. RYZHOV ◽  
EUGENE D. TERENT'EV
Keyword(s):  
Boundary Layer ◽  
Wind Tunnel ◽  
Wave Packets ◽  
Three Dimensional ◽  
Reynolds Numbers ◽  
Absolute Instability ◽  
Streamwise Direction ◽  
High Reynolds Numbers ◽  
Dimensional Boundary ◽  
High Reynolds

The simplest receptivity problem of linear disturbances artificially excited in a three-dimensional boundary layer adjacent to a solid surface is studied in the framework of the generalized triple-deck theory. In order to provide a mathematical model to be compared with experimental data from wind-tunnel tests we consider the base flow over a swept flat plate. Then crossflow in the near-wall region originates owing to an almost constant pressure gradient induced from outside with a displacement body on top. A pulsed or vibrating ribbon installed on the solid surface serves as an external agency provoking initially weak pulsations. A periodic dependence of the ribbon shape on a coordinate normal to the streamwise direction makes the receptivity problem effectively two-dimensional, thereby allowing a rigorous analysis to be carried out without additional assumptions.The most striking result from the asymptotic theory is the discovery of streamwise absolute instability intrinsic to a three-dimensional boundary layer at high Reynolds numbers. However, due to limitations imposed on the receptivity problem no definite conclusions can be made with regard to possible continued convection of disturbances in the crossflow direction. An investigation of the dispersion-relation roots points to the fact that wave packets of different kinds can be generated by an external source operating in the pulse mode. Rapidly growing wave packets sweep downstream, weaker wave packets move against the oncoming stream. Insofar as the amplitude of all of the modulated signals increases exponentially in time and space, the excitation process gives rise to absolutely unstable disturbances in the streamwise direction. The computation confirms the theoretical prediction about the existence of upstream-advancing wave packets. They can be prevented from being persistently amplified only in a region ahead of the ribbon where nearly critical values of the Reynolds number are attained.The results achieved are shown to be broadly consistent with wind-tunnel measurements. Hence a conjecture is made that the onset of transition is probably associated, under some environmental conditions, with the mechanism of streamwise absolute instability in the supercritical range of the Reynolds numbers.

scholarly journals Receptivity of a laminar boundary layer to the interaction of a three-dimensional roughness element with time-harmonic free-stream disturbances

1992 ◽  
Vol 242 ◽  
pp. 701-720 ◽  
Author(s):  
M. Tadjfar ◽  
R. J. Bodonyi
Keyword(s):  
Boundary Layer ◽  
Laminar Boundary Layer ◽  
Stokes Flow ◽  
Free Stream ◽  
Flow Direction ◽  
Roughness Element ◽  
Three Dimensional ◽  
Unsteady Motion ◽  
Time Harmonic ◽  
Tollmien Schlichting

Receptivity of a laminar boundary layer to the interaction of time-harmonic free-stream disturbances with a three-dimensional roughness element is studied. The three-dimensional nonlinear triple–deck equations are solved numerically to provide the basic steady-state motion. At high Reynolds numbers, the governing equations for the unsteady motion are the unsteady linearized three-dimensional triple-deck equations. These equations can only be solved numerically. In the absence of any roughness element, the free-stream disturbances, to the first order, produce the classical Stokes flow, in the thin Stokes layer near the wall (on the order of our lower deck). However, with the introduction of a small three-dimensional roughness element, the interaction between the hump and the Stokes flow introduces a spectrum of all spatial disturbances inside the boundary layer. For supercritical values of the scaled Strouhal number, S0 > 2, these Tollmien–Schlichting waves are amplified in a wedge-shaped region, 15° to 18° to the basic-flow direction, extending downstream of the hump. The amplification rate approaches a value slightly higher than that of two-dimensional Tollmien–Schlichting waves, as calculated by the linearized analysis, far downstream of the roughness element.

Investigation of Formation and Development of Stationary and Secondary Disturbances in the Favourable Pressure Gradient Area on the Swept Wing

2013 ◽  
Vol 8 (2) ◽  
pp. 55-69
Author(s):  
Stepan Tolkachev ◽  
Vasily Gorev ◽  
Viktor Kozlov
Keyword(s):  
Boundary Layer ◽  
Roughness Element ◽  
Swept Wing ◽  
Liquid Crystal Thermography ◽  
Natural Case ◽  
Turbulent Transition ◽  
Stage Transition ◽  
The Moment ◽  
Combined Technique ◽  
Attachment Line

In this work the combined technique of liquid-crystal thermography and thermoanemometry measurements is used to trace the stationary disturbance development from the moment of formation to the nonlinear stage transition. It has been shown that the pair of stationary vortices are formed after the cylindrical roughness element. These vortices modify a boundary layer and destabilize it. There is the area of maximal receptivity to the roughness location, which in the experiment was distant from the attachment line. If the stationary disturbance has enough magnitude in its core the secondary disturbances excite and lead to the laminar-turbulent transition. Secondary disturbances are sensitive to the acoustics and achieve the magnitude in hundred times higher than for the natural case

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