Step Change In Surface Roughness Research Articles

Evaluation of wind farm performance over heterogeneously rough terrain using large eddy simulation

We evaluate the effect of an abrupt change in the surface aerodynamic roughness height on a wind farm sited on it using the large eddy simulation (LES). Compared to a wind farm sited on a uniformly rough surface, the alteration in aerodynamic surface roughness from a rough to smooth value leads to substantial changes in the first-order and second-order turbulent statistics. Specifically, the rough-to-smooth surface roughness transition leads to an acceleration of the flow downstream of it, which affects the wake recovery and wind farm power production. Different velocity deficits are formulated considering different definitions of “upstream” velocity. The usual deficit, i.e., the difference between the overall wind farm upstream velocities and downstream of a turbine, attains negative values near the ground, rendering it difficult to model within the usual Gaussian radial-shape framework. An alternative definition, i.e., the difference in velocity at the same location with and without turbines on a heterogeneous surface, consistently yields positive values and is amenable to Gaussian shape-based modelling. The power generation decreases as the step change in surface roughness progressively moves into the wind farm. Maximum power is produced when all turbines are placed downstream of the surface roughness jump and minimum power is generated for a homogeneously rough surface when the entire wind farm is placed on the rough surface.

Neutrally- and stably-stratified boundary layers adjustments to a step change in surface roughness

In this work, we study the development of the internal boundary layer (IBL) induced by a surface roughness discontinuity, where the downstream surface has a roughness length greater than that upstream. The work is carried out in the EnFlo meteorological wind tunnel, at the University of Surrey, in both thermally neutral and stable cases with varying degrees of stability. For the neutrally-stratified boundary layer, the IBL development in the log-law region shows good agreement with the diffusion model proposed by Panofsky and Dutton (Atmospheric turbulence, Wiley, New York, 1984) provided that a modified origin condition is introduced and its growth rate is dictated by a constant diffusion term. However, the model over-predicts the growth of the IBL in the outer layer, where the IBL depth grows slowly with fetch following a power function with exponent n being 0.61 (whereas the original model prescribes napprox 0.8). For the stably-stratified boundary layers, n is found to further reduce as the bulk Richardson number, textrm{Ri}_textrm{b}, increases. The analysis of the top region of the IBL shows that the slow growth rate is due to a combination of the decay of the diffusion term and a significantly negative mean wall-normal velocity, which transports fluid elements towards the wall. Considering these two effects, a modified diffusion model is proposed which well captures the growth of the IBL for both neutrally and stably-stratified boundary layers.Graphical abstract

Experimental observations on turbulent boundary layers subjected to a step change in surface roughness

Based on experimental data acquired with particle image velocimetry, we examine turbulent boundary layers that are subjected to an abrupt change in wall roughness in the streamwise direction. Three different sandpapers (P24, P36 and P60) together with a smooth wall are used to form a number of different surface transition cases, including both R $\rightarrow$ S (where upstream surface is rough and second surface is either smooth or smoother compared with the upstream surface) and S $\rightarrow$ R (where upstream surface is smoother compared with the downstream surface; both surfaces are rough). This enables us to investigate the effect of the surface transition strength ( $M = \ln [y_{02}/y_{01}]$ , where $y_{01}$ and $y_{02}$ are the roughness lengths of the upstream and downstream surfaces, respectively) on the growth of the internal boundary layer (IBL) and the corresponding flow structure. The results show that for each surface transition group (i.e. R $\rightarrow$ S and S $\rightarrow$ R), the thickness of the IBLs is proportional to the strength of the surface transition, and that the IBLs are thicker for the S $\rightarrow$ R cases compared with their R $\rightarrow$ S counterparts for similar $|M|$ , when normalised by the initial boundary layer thickness ( $\delta _0$ ). The results also show that the growth rates of the IBLs could be represented by a power law, consistent with the previous studies. However, despite a wide range of scatter in the literature for the power-law exponent, an average value of $0.75$ (varies between $0.71$ and $0.8$ with no clear trend) is obtained in the present study considering all the surface transition cases. The pre-factor for the power-law fit, on the other hand, is found to be related to the strength of the surface transition. In addition to the variations in the velocity defect and diagnostic plots with the growth of the IBLs, sweep and ejection events appear to differ significantly (depending on the type of the step change). Two-point spatial correlations, moreover, show that the structure is more elongated in the wall-normal and streamwise directions, as the flow accelerates over the downstream surface (i.e. R $\rightarrow$ S cases). For the reverse transition cases (i.e. S $\rightarrow$ R, where the flow decelerates over the downstream rougher surface), however, the correlation coefficients shrink in size in both directions.

Turbulent pipe flow response to a step change in surface roughness

Abstract

OBSERVAÇÕES E MODELAGEM DA CAMADA LIMITE INTERNA NO ENTORNO DE EXTENSOS SISTEMAS AQUÁTICOS

The determination of the spatial extent of the internal boundary layer (IBL) across a step change in surface roughness and surface heat flux and temperature is an important environmental aspect to the operation of industries, fossil-fuel, nuclear and wind power plants. The IBL of the land-water transition zone, such as in coastal regions, lakes and reservoirs, is strongly influenced by surface properties, as the roughness and stability change considerably within this zone. An observational study was conducted to measure meteorological properties near a large tropical reservoir and investigate the IBL at the water-land transition. A 42 m height meteorological mast was instrumented with sonic wind anemometers, air temperature and relative humidity sensors at 3, 10, 20 and 40 m height. Comparisons of the log profile model with the Monin-Obukhov similarity theory (MOS) and the observed wind profiles, shown that the MOS estimated the wind profile more accurately, especially if winds were from water to land.

The distribution of wall shear stress downstream of a change in roughness

In the present work, six different experimental techniques are used to characterize the non-equilibrium flow downstream of a rough-to-smooth step change in surface roughness. Over the rough surface, wall shear stress results obtained through the form drag and the Reynolds stress methods are shown to be mutually consistent. Over the smooth surface, reference wall shear stress data is obtained through two optical methods: linear velocity profiles obtained through laser-Doppler anemometry and a sensor surface, the diverging fringe Doppler sensor. The work shows that the two most commonly used methods to determine the wall shear stress, the log-law gradient method and the Reynolds shear stress method, are completely inappropriate in the developing flow region. Preston tubes, on the other hand, are shown to perform well in the region of a non-equilibrium flow.

The Lagrangian stochastic model for estimating footprint and water vapor fluxes over inhomogeneous surfaces

This study investigated a two-dimensional Lagrangian stochastic dispersion model for estimating water vapor fluxes and footprint over homogeneous and inhomogeneous surfaces. Over the homogeneous surface, particle trajectories were computed from a 2-D Lagrangian model forced by Eulerian velocity statistics determined by Monin-Obukhov similarity theory (MOST). For an inhomogeneous surface, the velocity and atmospheric stability profiles were computed using a second-order Eulerian closure model, and these local profiles were then used to drive the Lagrangian model. The model simulations were compared with water vapor flux measurements carried out above an irrigated bare soil site and an irrigated potato site. The inhomogeneity involved a step change in surface roughness, humidity, and temperature. Good agreement between eddy-correlation-measured and Lagrangian-model-predicted water vapor fluxes was found for both sites. Hence, this analysis demonstrates the practical utility of second-order closure models in conjunction with Lagrangian analysis to estimate the scalar footprint in planar inhomogeneous flows.

A Flume Experiment on the Adjustment of the Mean and Turbulent Statistics to a Transition from Short to Tall Sparse Canopies

Water-flume experiments are conducted to study the structure of turbulent flow within and above a sparse model canopy consisting of two rigid canopies of different heights. This difference in height specifies a two-dimensional step change from a rough to a rougher surface, as opposed to a smooth-to-rough transition. Despite the fact that the flow is in transition from a rough to a rougher surface, the thickness of the internal boundary layer scales as x4/5, consistent with smooth-to-rough boundary layer adjustment studies, where x is the downstream distance from the step change. However, the analogy with smooth-to-rough transitions no longer holds when the flow inside the canopy and near the canopy top is considered. Results show that the step change in surface roughness significantly increases turbulence intensities and shear stress. In particular, there is an adjustment of the mean horizontal velocity and shear stress as the flow passes over the rougher canopy, so that their vertical profiles adjust to give maximum values at the top of this canopy. We also observe that the magnitude and shape of the inflection in the mean horizontal velocity profile is significantly affected by the transition. The horizontal and vertical turbulence spectra compare well with Kolmogorov’s theory, although a small deviation at high frequencies is observed in the horizontal spectrum within the canopy. Here, for relatively low leaf area index, shear is found to be a more effective mechanism for momentum transfer through the canopy structure than vortex shedding.

05/02360 Turbulent flow in a rectangular duct with a smooth-to-rough step change in surface roughness

05/02360 Turbulent flow in a rectangular duct with a smooth-to-rough step change in surface roughness

Turbulent flow in a rectangular duct with a smooth-to-rough step change in surface roughness

Experimental study has been conducted on the response of turbulent flow in a rectangular duct to a smooth-to-rough step change in surface roughness on its short-side walls. The distributions of the primary flow velocity, secondary flow velocities, turbulence intensities, and turbulent shear stresses have been measured in seven cross-sections located from 2.0 d h to 37.5 d h downstream from the smooth-to-rough junction, and the relaxation process of this turbulent flow has been examined. It has been found that the relaxation process of this flow is considerably complex, but the flow characteristics except for the primary flow velocity in the core region as well as near the rough wall reach an almost equilibrium state, i.e., the fully developed rough-duct flow condition, within a distance of 37.5 d h from the change in the surface roughness.

Near-Wall Flow Development After A Step Change In Surface Roughness

An experimental study of the initial flow field downstream of a step change in surface roughness is presented. The roughness length of the downstream surface was approximately tenfold that of the upstream roughness and, unlike all previous studies, attention was concentrated on the roughness sublayer region beneath the inertial (log-law) region. The experiments were conducted at a boundary layer Reynolds number of about 6 × 104 (based on layer thickness and free-stream velocity) and around a longitudinal location where the (downstream) roughness length, zo2, was about 1% of the boundary-layer thickness at the roughness change point. The thickness of the roughness sublayer was found for the two roughness. It was observed that the vertical profiles of mean velocity and turbulence characteristics started to show similarity after about 160z02 downstream of the roughness change. The presence of a shear stress overshoot is shown to depend strongly on the precise location (with respect to the roughness elements) at which the measurements are made and the thickness of the equilibrium layer is shown to be very sensitive to the way it is defined. It is demonstrated that the growing equilibrium layer has first to encompass the roughness sublayer before any thickness of inertial sublayer can be developed. It follows that, in some practical cases, like flows across some urban environments, the latter (log-law) region may never exist at all.

Relaxation of Turbulent Flow in a Rectangular Duct with a Step Change in Surface Roughness.

The response of a turbulent flow in a rectangular cross-sectioned duct to a rough-to-smooth step change in the surface roughness of the short-side walls has been investigated experimentally. The distributions of the primary and secondary flow velocities, turbulence intensities, and turbulent shear stresses have been measured by hot-wire anemometers in seven cross-sections set up in the smooth-duct part. In the core region of the duct cross-section, large secondary currents are maintained over a relatively long distance after the rough-to-smooth junction, and they cause large velocity gradient in the primary flow and large turbulent shear stress there. The streamwise changes of the turbulence intensities are rather complex. In the core region they once overshoot the values in the rough duct just after the rough-to-smooth junction, then undershoot those in the smooth duct, and finally approach an equilibrium state, while near the short-side wall they are decreased quite rapidly and monotonously in the streamwise direction.

Relaxation of the Turbulent Boundary Layer After an Abrupt Change From Rough to Smooth Wall (Data Bank Contribution)

Measurements of velocity and turbulence intensity profiles and skin friction coefficient are presented for turbulent flat-plate boundary layer flow over a test surface with a rough-to-smooth step change in surface roughness. The first 0.9 m length of the test surface is roughened with 1.27 mm diameter hemispheres spaced 2 base diameters apart in a uniform staggered array, and the remaining 1.5 m length is smooth. The profiles are compared with previous data for all-rough cases under closely matched conditions in the same facility. The skin friction data are compared with previous data for both all-rough and all-smooth cases.

Heat Transfer in the Turbulent Boundary Layer With a Step Change in Surface Roughness

Measurements of Stanton numbers, velocity profiles, temperature profiles, and turbulence intensity profiles are reported for turbulent flat plate boundary layer flows with a step change in surface roughness. The first 0.9 m length of the test surface is roughened with 1.27-mm-dia hemispheres spaced 2 base diameters apart in a staggered array. The remaining 1.5 m length is smooth. The experiments show that the step change from a rough to a smooth surface has a dramatic effect on the convective heat transfer. In many cases, the Stanton number drops below the smooth-wall correlation immediately downstream of the change in roughness. The Stanton number measurements are compared with predictions using the discrete element method with excellent results.

Analysis of transonic integral equations. II - Boundary element methods

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Internal boundary-layer height formulae ? A comparison with atmospheric data

The height of the internal boundary layer (IBL) downwind of a step change in surface roughness is computed using formulae of Elliott (1958), Jackson (1976) and Panofsky and Dutton (1984). The results are compared with neutral-stratification atmospheric data extracted from the set of wind-tunnel and atmospheric data summarized by Jackson (1976) as well as neutral-stratification data presented by Peterson et al. (1979) and new data measured at Cherrywood, Ontario. It is found that the Panofsky-Dutton formulation gives the least root-mean-square (RMS) absolute errors for atmospheric applications.

Numerical simulations of stratiform boundary-layer clouds on the meso-γ-scale. Part II: The influence of a step change in surface roughness and surface temperature

A higher order closure mesoscale model is used to study the influence of different surface properties on stratiform boundary-layer clouds. The model is hydrostatic, has a terrain-following coordinate system and a sub-grid scale condensation scheme. It also has a radiation parameterisation for shortwave and longwave radiation in order to calculate radiative cooling/heating. The simulations show the effects on a cloud field when cool or cold air is advected over warm water, the possible influence of local circulation systems on cloud fields in situations with weak synoptic forcing and the influence on a cloud field by growing internal boundary layers. Some of the results are compared with simpler physical models, and limitations in those are demonstrated.

The flow in a turbulent boundary layer upstream of a change in surface roughness

Modification of a turbulent flow upstream of a change in surface roughness has been studied by means of a stream function-vorticity model. A flow reduction is found upstream of a step change in surface roughness when a fluid flows from a smooth onto a rough surface. Above that layer and above the region of flow reduction downstream of a smooth-rough transition, a flow acceleration is observed. Similar flow modification can be seen at a rough-smooth transition with the exception that flow reduction and flow acceleration are reversed. Within a fetch of −500 < x/z 0< + 500 (z 0 is the maximum roughness length, the roughness transition is located at x/z 0 = 0), flow reduction (flow acceleration) upstream of a roughness transition is one order of magnitude smaller than the flow reduction (flow acceleration) downstream of a smooth-rough (rough-smooth) transition. The flow acceleration (flow reduction) above that layer is two orders of magnitude. The internal boundary layer (IBL) for horizontal mean velocity extends to roughly 300z 0 upstream of a roughness transition, whereas the IBL for turbulent shear stress as well as the distortion of flow equilibrium extend almost twice as far. For the friction velocity, an undershooting (overshooting) with respect to upstream equilibrium is predicted which precedes overshooting (undershooting) over new equilibrium just behind a roughness transition. The flow modification over a finite fetch of modified roughness is weaker than over a corresponding fetch downstream of a single step change in roughness and the flow stays closer to upstream equilibrium. Even in front of the first roughness change of a finite fetch of modified roughness, a distortion of flow equilibrium due to the second, downwind roughness change can be observed.

Dispersion in neutral boundary layer over a step change in surface roughness—I. Mean flow and turbulence structure

Dispersion in neutral boundary layer over a step change in surface roughness—I. Mean flow and turbulence structure

The Response of Turbulent Boundary Layers to Sudden Perturbations

The processes present in a turbulent boundary layer (BL) which experiences a sudden change in the surface roughness are discussed in terms of numerical models in comparison with empirical data. Consideration is limited to flow perturbations which preserve the BL approximation. Generalized continuity, momentum and turbulent energy transport equations are reviewed. The perturbed BL has two regions: one with large velocity gradients and an outer zone with smaller gradients. Expressions are defined for the velocity and temperature profiles and the roughness length after a step change in surface roughness, which causes the disturbance. A pertrurbation could also arise from a heat flux input of a pressure gradient. The effects of the shape of the perturbing step and the presence of several disturbing edges are explored. Finally, it is shown that closure forms and constants for self-preserving flows are worthy of further development because of their demonstrable abilities to describe simple perturbed flows.