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.

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.

Turbulence structure of a reattaching mixing layer

1981 ◽  
Vol 110 ◽  
pp. 171-194 ◽  
Author(s):  
C. Chandrsuda ◽  
P. Bradshaw
Keyword(s):  
Boundary Layer ◽  
Shear Stress ◽  
Reynolds Shear Stress ◽  
Mixing Layer ◽  
Three Dimensional ◽  
Flow Region ◽  
Turbulent Energy ◽  
Turbulence Structure ◽  
Hot Wire ◽  
Recirculating Flow

Hot-wire measurements of second- and third-order mean products of velocity fluctuations have been made in the flow behind a backward-facing step with a thin, laminar boundary layer at the top of the step. Measurements extend to a distance of about 12 step heights downstream of the step, and include parts of the recirculating-flow region: approximate limits of validity of hot-wire results are given. The Reynolds number based on step height is about 105, the mixing layer being fully turbulent (fully three-dimensional eddies) well before reattachment, and fairly close to self-preservation in contrast to the results of some previous workers. Rapid changes in turbulence quantities occur in the reattachment region: Reynolds shear stress and triple products decrease spectacularly, mainly because of the confinement of the large eddies by the solid surface. The terms in the turbulent energy and shear stress balances also change rapidly but are still far from the self-preserving boundary-layer state even at the end of the measurement region.

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.

scholarly journals The Evolution and Arrest of a Turbulent Stratified Oceanic Bottom Boundary Layer over a Slope: Downslope Regime

2019 ◽  
Vol 49 (2) ◽  
pp. 469-487 ◽  
Author(s):  
Xiaozhou Ruan ◽  
Andrew F. Thompson ◽  
John R. Taylor
Keyword(s):  
Boundary Layer ◽  
Mixed Layer ◽  
Three Dimensional ◽  
Wall Stress ◽  
Bottom Boundary Layer ◽  
Ekman Transport ◽  
Momentum Balance ◽  
Bottom Boundary ◽  
Obukhov Length ◽  
Turbulent Characteristics

AbstractThe dynamics of a stratified oceanic bottom boundary layer (BBL) over an insulating, sloping surface depend critically on the intersection of density surfaces with the bottom. For an imposed along-slope flow, the cross-slope Ekman transport advects density surfaces and generates a near-bottom geostrophic thermal wind shear that opposes the background flow. A limiting case occurs when a momentum balance is achieved between the Coriolis force and a restoring buoyancy force in response to the displacement of stratified fluid over the slope: this is known as Ekman arrest. However, the turbulent characteristics that accompany this adjustment have received less attention. We present two estimates to characterize the state of the BBL based on the mixed layer thickness: Ha and HL. The former characterizes the steady Ekman arrested state, and the latter characterizes a relaminarized state. The derivation of HL makes use of a newly defined slope Obukhov length Ls that characterizes the relative importance of shear production and cross-slope buoyancy advection. The value of Ha can be combined with the temporally evolving depth of the mixed layer H to form a nondimensional variable H/Ha that provides a similarity prediction of the BBL evolution across different turbulent regimes. The length scale Ls can also be used to obtain an expression for the wall stress when the BBL relaminarizes. We validate these relationships using output from a suite of three-dimensional large-eddy simulations. We conclude that the BBL reaches the relaminarized state before the steady Ekman arrested state. Calculating H/Ha and H/HL from measurements will provide information on the stage of oceanic BBL development being observed. These diagnostics may also help to improve numerical parameterizations of stratified BBL dynamics over sloping topography.

scholarly journals Influence of two-dimensional roughness element on boundary layer structure in the favourable pressure gradient region of the swept wing

2019 ◽  
Vol 234 (1) ◽  
pp. 20-27
Author(s):  
Stepan Tolkachev ◽  
Victor Kozlov ◽  
Valeriya Kaprilevskaya
Keyword(s):  
Boundary Layer ◽  
Pressure Gradient ◽  
Layer Structure ◽  
Roughness Element ◽  
Three Dimensional ◽  
Two Dimensional ◽  
Frequency Range ◽  
Swept Wing ◽  
Boundary Layer Structure ◽  
Roughness Elements

In this article, the results of research about stationary and secondary disturbances development behind the localized and two-dimensional roughness elements are presented. It is shown that the two-dimensional roughness element has a destabilizing effect on the disturbances induced by the three-dimensional roughness element lying upstream. In this case, the two-dimensional roughness element causes the appearance of stationary structures, and then secondary perturbations, whose frequency range lies lower than in the case of the stationary vortices excited by a three-dimensional roughness element.

scholarly journals Experimental observation of frequency lock-in of roughness-induced instabilities in a laminar boundary layer

2019 ◽  
Vol 870 ◽  
pp. 680-697
Author(s):  
Dominik K. Puckert ◽  
Ulrich Rist
Keyword(s):  
Boundary Layer ◽  
Aspect Ratio ◽  
Laminar Boundary Layer ◽  
Roughness Element ◽  
Water Channel ◽  
Three Dimensional ◽  
Boundary Layer Theory ◽  
Linear Stability Theory ◽  
Mode Decomposition ◽  
Lock In

The interaction of disturbance modes behind an isolated cylindrical roughness element in a laminar boundary layer is investigated by means of hot-film anemometry and particle image velocimetry in a low-turbulence laminar water channel. Both sinuous and varicose disturbance modes are found in the wake of a roughness with unit aspect ratio (diameter/height $=$ 1). Interestingly, the frequency of the varicose mode synchronizes with the first harmonic of the sinuous mode when the critical Reynolds number from three-dimensional global linear stability theory is exceeded. The coupled motion of sinuous and varicose modes is explained by frequency lock-in. This mechanism is of great importance in many aspects of nature, but has not yet received sufficient attention in the field of boundary-layer theory. A Fourier mode decomposition provides detailed analyses of sinuous and varicose modes. The observation is confirmed by a second experiment with the same aspect ratio at a different position in the laminar boundary layer. When the aspect ratio is increased, the flow is fully governed by the varicose mode. Thus, no frequency lock-in can be observed in this case. The significance of this work is to explain how sinuous and varicose modes can co-exist behind a roughness and to propose a mechanism which is well established in physics but not encountered often in boundary-layer theory.

Combined effects of flow curvature and rotation on uniformly sheared turbulence

2009 ◽  
Vol 628 ◽  
pp. 371-394 ◽  
Author(s):  
D. C. ROACH ◽  
A. G. L. HOLLOWAY
Keyword(s):  
Shear Stress ◽  
Wind Tunnel ◽  
Model Development ◽  
Three Dimensional ◽  
Simple Shear Flow ◽  
Test Case ◽  
Mean Velocity ◽  
Tunnel Section ◽  
Flow Curvature ◽  
The Mean

This paper describes an experiment in which a uniformly sheared turbulence was subjected to simultaneous streamwise flow curvature and rotation about the streamwise axis. The distortion of the turbulence is complex but well defined and may serve as a test case for turbulence model development. The uniformly sheared turbulence was developed in a straight wind tunnel and then passed into a curved tunnel section. At the start of the curved section the plane of the mean shear was normal to the plane of curvature so as to create a three-dimensional or ‘out of plane’ curvature configuration. On entering the curved tunnel, the flow developed a streamwise mean vorticity that rotated the mean shear about the tunnel centreline through approximately 70°, so that the shear was nearly in the plane of curvature and oriented so as to have a stabilizing effect on the turbulence. Hot wire measurements of the mean velocity, mean vorticity, mean rate of strain and Reynolds stress anisotropy development along the wind tunnel centreline are reported. The observed effect of the mean shear rotation on the turbulence was to diminish the shear stress in the plane normal to the plane of curvature while generating non-zero values of the shear stress in the plane of curvature. A rotating frame was identified for which the measured mean velocity field took the form of a simple shear flow. The turbulence anisotropy was transformed to this frame to estimate the effects of frame rotation on the structure of sheared turbulence.

A Design Study of a Supersonic Air Intake Device in a Wide Range of Main Flow Parameters

2020 ◽  
pp. 44-53
Author(s):  
E.S. Studennikov ◽  
R.S. Ayupov
Keyword(s):  
Boundary Layer ◽  
Rectangular Cross Section ◽  
Three Dimensional ◽  
Stability Margin ◽  
Aircraft Design ◽  
Software Systems ◽  
Flow Parameters ◽  
Wide Range ◽  
Air Intake ◽  
The Stability

This paper examines operation modes of a mixed compression air intake with a rectangular cross-section at Mach number 2.0. The perfect gas model was used for the calculation. Calculations were performed for three values of Mach numbers: 1.8, 2.0 and 3.0. k–ε turbulence model was chosen for describing flows with large adverse pressure gradients. Two-dimensional and three-dimensional configurations of the air intake device were examined. Versions of geometry with and without the boundary layer drain system were considered. The influence of the boundary layer drain system on the flow in the air intake and its characteristics was established. Throttle characteristic curves were formed for all the considered modes with regard to the averaged flow parameters. A comparison of the calculation and experimental data showed a good agreement of the results. The obtained results can serve as a basis for further optimization and improvement of the efficiency of the aircraft design layout, increase in the stability margin of air intakes, as well as development of software systems for regulating supersonic input devices.

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