# Dr Philip Hancock

Emeritus Reader in Fluid Mechanics

### Research

### Research interests

- Aerodynamics, turbulence and boundary layer meteorology
- Wind-tunnel simulation of neutral, stable and convective atmospheric boundary layer flows
- Wind turbine wakes, wake-wake and wake turbine interactions
- Structure of complex turbulent flows, boundary layers, separated flows and wakes
- Aerodynamic and bluff body flows, wind power aerodynamics.

### My publications

### Publications

Hancock PE (1999) Measurements of mean and fluctuating wall shear stress beneath spanwise-invariant separation bubbles, EXPERIMENTS IN FLUIDS27(1)pp. 53-59 SPRINGER VERLAG

Hancock PE, Pascheke F (2013) Wind-Tunnel Simulation of the Wake of a Large Wind Turbine in a Stable Boundary Layer. Part 1: The Boundary-Layer Simulation,Boundary-Layer Meteorology151(1)pp. 3-21

Measurements have been made in both a neutral and a stable boundary layer as part of an investigation of the wakes of wind turbines in an offshore environment, in the EnFlo stratified flow wind tunnel. The working section is long enough for the flow to have become very nearly invariant with streamwise distance. In order to be systematic, the flow profile generators of Irwin-type spires and surface roughness were the same for both neutral and stable conditions. Achieving the required profiles by adjusting the flow generators, even for neutral flow, is a highly iterative art, and the present results indicate that it will be no less iterative for a stable flow (as well as there being more conditions to meet), so this was not attempted in the present investigation. The stable-case flow conformed in most respects to Monin-Obukhov similarity in the surface layer. A linear temperature profile was applied at the working section inlet, resulting in a near-linear profile in the developed flow above the boundary layer and 'strong' imposed stability, while the condition at the surface was 'weak'. Aerodynamic roughness length (mean velocity) was not affected by stability even though the roughness Reynolds number <1, while the thermal roughness length was much smaller, as is to be expected. The neutral case was Reynolds-number independent, and by inference, the stable case was also Reynolds-number independent. © 2013 Springer Science+Business Media Dordrecht.

Hancock PE, Pascheke F (2014) Wind-Tunnel Simulation of the Wake of a Large Wind Turbine in a Stable Boundary Layer: Part 2, the Wake Flow,Boundary-Layer Meteorology151(1)pp. 23-37

Measurements have been made in the wake of a model wind turbine in both a neutral and a stable atmospheric boundary layer, in the EnFlo stratified-flow wind tunnel, between 0.5 and 10 rotor diameters from the turbine, as part of an investigation of wakes in offshore winds. In the stable case the velocity deficit decreased more slowly than in the neutral case, partly because the boundary-layer turbulence levels are lower and the consequentially reduced level of mixing, an 'indirect' effect of stratification. A correlation for velocity deficit showed the effect of stratification to be the same over the whole of the measured extent, following a polynomial form from about five diameters. After about this distance (for the present stratification) the vertical growth of the wake became almost completely suppressed, though with an increased lateral growth; the wake in effect became 'squashed', with peaks of quantities occurring at a lower height, a 'direct' effect of stratification. Generally, the Reynolds stresses were lower in magnitude, though the effect of stratification was larger in the streamwise fluctuation than on the vertical fluctuations. The vertical heat flux did not change much from the undisturbed level in the first part of the wake, but became much larger in the later part, from about five diameters onwards, and exceeded the surface level at a point above hub height. © 2013 Springer Science+Business Media Dordrecht.

Schober M, Hancock PE, Siller H (1998) Pulsed-wire anemometry near walls, EXPERIMENTS IN FLUIDS25(2)pp. 151-159 SPRINGER VERLAG

Aubrun S, Espana G, Loyer S, Hayden P, hancock P (2010) Is the actuator disc concept sufficient to model the far-wake of a wind turbine?,

Ciampoli F, Hancock PE (2006) Effects of flow width in nominally two-dimensional turbulent separated flows, EXPERIMENTS IN FLUIDS40(2)pp. 196-202 SPRINGER

Hancock PE, Hardman JR (2000) Scaling of the near-wall layer beneath a three-dimensional turbulent separation bubble, ADVANCES IN TURBULENCE VIIIpp. 419-422 INT CENTER NUMERICAL METHODS ENGINEERING

Hancock PE, Pascheke F (2013) Wind-Tunnel Simulation of the Wake of a Large Wind Turbine in a Stable Boundary Layer: Part 2, the Wake Flow, Boundary-Layer Meteorologypp. 1-15

Measurements have been made in the wake of a model wind turbine in both a neutral and a stable atmospheric boundary layer, in the EnFlo stratified-flow wind tunnel, between 0.5 and 10 rotor diameters from the turbine, as part of an investigation of wakes in offshore winds. In the stable case the velocity deficit decreased more slowly than in the neutral case, partly because the boundary-layer turbulence levels are lower and the consequentially reduced level of mixing, an 'indirect' effect of stratification. A correlation for velocity deficit showed the effect of stratification to be the same over the whole of the measured extent, following a polynomial form from about five diameters. After about this distance (for the present stratification) the vertical growth of the wake became almost completely suppressed, though with an increased lateral growth; the wake in effect became 'squashed', with peaks of quantities occurring at a lower height, a 'direct' effect of stratification. Generally, the Reynolds stresses were lower in magnitude, though the effect of stratification was larger in the streamwise fluctuation than on the vertical fluctuations. The vertical heat flux did not change much from the undisturbed level in the first part of the wake, but became much larger in the later part, from about five diameters onwards, and exceeded the surface level at a point above hub height. © 2013 Springer Science+Business Media Dordrecht.

Hancock PE, Zhang S, Pascheke F, Hayden P (2014) Wind tunnel simulation of a wind turbine wake in neutral, stable and unstable wind flow, Journal of Physics: Conference Series555(1)

© Published under licence by IOP Publishing Ltd. Measurements of mean velocity, Reynolds stresses, temperature and heat flux have been made in the wake of a model wind turbine in the EnFlo meteorology wind tunnel, for three atmospheric boundary layer states: the base-line neutral case, stable and unstable. The full-to-model scale is approximately 300:1. Primary instrumentation is two-component LDA combine with cold-wire thermometry to measure heat flux. In terms of surface conditions, the stratified cases are weak, but there is a strong 'imposed' condition in the stable case. The measurements were made between 0.5D and 10D, where D is the turbine disk diameter. In the stable case the velocity deficit decreases more slowly; more quickly in the unstable case. Heights at which quantities are maximum or minimum are greater in the unstable case and smaller in the stable case. In the stable case the wake height is suppressed but the width is increased, while in the unstable case the height is increased and the width (at hub height) reaches a maximum and then decreases. The turbulence in the wake behaves in a complex way. Further work needs to be done, to cover stronger levels of surface condition, requiring more extensive measurements to properly capture the wake development.

Hardman JR, Hancock PE (1997) Moderately three-dimensional separated and reattaching turbulent flow,TURBULENCE, HEAT AND MASS TRANSFER 2pp. 541-548 DELFT UNIV PRESS DUP SCI

Johnston AE, Hancock PE (1997) Some aspects of centrifugal fan characteristics in blower windtunnels, AERONAUTICAL JOURNAL101(1010)pp. 481-485 ROYAL AERONAUTICAL SOC

Hancock PE (2004) Scaling of the near-wall layer beneath reattaching separated flow, IUTAM SYMPOSIUM ON REYNOLDS NUMBER SCALING IN TURBULENT FLOW74pp. 151-154 SPRINGER

Gough TD, Hancock PE (1996) Low Reynolds number turbulent near wakes, ADVANCES IN TURBULENCES VI36pp. 445-448 KLUWER ACADEMIC PUBL

Hardman JR, Hancock PE (2010) Moderately three-dimensional separated and reattaching turbulent flow,JOURNAL OF FLUID MECHANICS649pp. 341-370 CAMBRIDGE UNIV PRESS

Cao C, Hancock PE (2004) Boundary layer development after a region of three-dimensional separated flow, EUROPEAN JOURNAL OF MECHANICS B-FLUIDS23(3)pp. 519-533 GAUTHIER-VILLARS/EDITIONS ELSEVIER

Hancock PE, Zhang S (2015) A Wind-Tunnel Simulation of the Wake of a Large Wind Turbine in a Weakly Unstable Boundary Layer, Boundary-Layer Meteorology156(3)pp. 395-413

© 2015, Springer Science+Business Media Dordrecht.Measurements have been made in the wake of a model wind turbine in both a weakly unstable and a baseline neutral atmospheric boundary layer, in the EnFlo stratified-flow wind tunnel, between 0.5 and 10 rotor diameters from the turbine, as part of an investigation of wakes in offshore winds. In the unstable case the velocity deficit decreases more rapidly than in the neutral case, largely because the boundary-layer turbulence levels are higher with consequent increased mixing. The height and width increase more rapidly in the unstable case, though still in a linear manner. The vertical heat flux decreases rapidly through the turbine, recovering to the undisturbed level first in the lower part of the wake, and later in the upper part, through the growth of an internal layer. At 10 rotor diameters from the turbine, the wake has strong features associated with the surrounding atmospheric boundary layer. A distinction is drawn between direct effects of stratification, as necessarily arising from buoyant production, and indirect effects, which arise only because the mean shear and turbulence levels are altered. Some aspects of the wake follow a similarity-like behaviour. Sufficiently far downstream, the decay of the velocity deficit follows a power law in the unstable case as well as the neutral case, but does so after a shorter distance from the turbine. Tentatively, this distance is also shorter for a higher loading on the turbine, while the power law itself is unaffected by turbine loading.

Hancock PE, Zhang S (2015) A Wind-Tunnel Simulation of the Wake of a Large Wind Turbine in a Weakly Unstable Boundary Layer, Boundary-Layer Meteorology156(3)pp. 395-413 Springer Verlag

© 2015 Springer Science+Business Media Dordrecht Measurements have been made in the wake of a model wind turbine in both a weakly unstable and a baseline neutral atmospheric boundary layer, in the EnFlo stratified-flow wind tunnel, between 0.5 and 10 rotor diameters from the turbine, as part of an investigation of wakes in offshore winds. In the unstable case the velocity deficit decreases more rapidly than in the neutral case, largely because the boundary-layer turbulence levels are higher with consequent increased mixing. The height and width increase more rapidly in the unstable case, though still in a linear manner. The vertical heat flux decreases rapidly through the turbine, recovering to the undisturbed level first in the lower part of the wake, and later in the upper part, through the growth of an internal layer. At 10 rotor diameters from the turbine, the wake has strong features associated with the surrounding atmospheric boundary layer. A distinction is drawn between direct effects of stratification, as necessarily arising from buoyant production, and indirect effects, which arise only because the mean shear and turbulence levels are altered. Some aspects of the wake follow a similarity-like behaviour. Sufficiently far downstream, the decay of the velocity deficit follows a power law in the unstable case as well as the neutral case, but does so after a shorter distance from the turbine. Tentatively, this distance is also shorter for a higher loading on the turbine, while the power law itself is unaffected by turbine loading.

Hancock PE, Pascheke F, Zhang S (2014) Wind Tunnel Simulation of Wind Turbine Wakes in Neutral, Stable and Unstable Offshore Atmospheric Boundary Layers, WIND ENERGY - IMPACT OF TURBULENCE2pp. 109-114 SPRINGER-VERLAG BERLIN

Hancock PE, Pascheke F (2013) Wind-Tunnel Simulation of the Wake of a Large Wind Turbine in a Stable Boundary Layer. Part 1: The Boundary-Layer Simulation, Boundary-Layer Meteorologypp. 1-19

Measurements have been made in both a neutral and a stable boundary layer as part of an investigation of the wakes of wind turbines in an offshore environment, in the EnFlo stratified flow wind tunnel. The working section is long enough for the flow to have become very nearly invariant with streamwise distance. In order to be systematic, the flow profile generators of Irwin-type spires and surface roughness were the same for both neutral and stable conditions. Achieving the required profiles by adjusting the flow generators, even for neutral flow, is a highly iterative art, and the present results indicate that it will be no less iterative for a stable flow (as well as there being more conditions to meet), so this was not attempted in the present investigation. The stable-case flow conformed in most respects to Monin-Obukhov similarity in the surface layer. A linear temperature profile was applied at the working section inlet, resulting in a near-linear profile in the developed flow above the boundary layer and 'strong' imposed stability, while the condition at the surface was 'weak'. Aerodynamic roughness length (mean velocity) was not affected by stability even though the roughness Reynolds number {Mathematical expression}, while the thermal roughness length was much smaller, as is to be expected. The neutral case was Reynolds-number independent, and by inference, the stable case was also Reynolds-number independent. © 2013 Springer Science+Business Media Dordrecht.

Hancock PE (1998) Plane multiple screens in non-uniform flow, with particular application to wind tunnel settling chamber screens, EUROPEAN JOURNAL OF MECHANICS B-FLUIDS17(3)pp. 357-369 GAUTHIER-VILLARS/EDITIONS ELSEVIER

Hancock PE, Hayden P Stable boundary layer wind tunnel simulation 1, University of Surrey

Hancock PE (2004) Pulsed-wire measurements in the near-wall layer in a reattaching separated flow, EXPERIMENTS IN FLUIDS37(3)pp. 323-330 SPRINGER

Farr TD, Hancock PE (2014) Torque fluctuations caused by upstream mean flow and turbulence,SCIENCE OF MAKING TORQUE FROM WIND 2012555(1) IOP PUBLISHING LTD

© Published under licence by IOP Publishing Ltd.A series of studies are in progress investigating the effects of turbine-array-wake interactions for a range of atmospheric boundary layer states by means of the EnFlo meteorological wind tunnel. The small, three-blade model wind turbines drive 4-quadrant motor-generators. Only a single turbine in neutral flow is considered here. The motor-generator current can be measured with adequate sensitivity by means of a current sensor allowing the mean and fluctuating torque to be inferred. Spectra of torque fluctuations and streamwise velocity fluctuations ahead of the rotor, between 0.1 and 2 diameters, show that only the large-scale turbulent motions contribute significantly to the torque fluctuations. Time-lagged cross-correlation between upstream velocity and torque fluctuations are largest over the inner part of the blade. They also show the turbulence to be frozen in behaviour over the 2 diameters upstream of the turbine.

Pascheke F, Hancock PE (2008) Wake development and interactions within an array of large wind turbines, European Wind Energy Conference and Exhibition 20086pp. 3008-3016

This paper presents first test results from wind tunnel studies of mean and turbulent wake characteristics within an array of large wind turbines. Up to now, a single rotating speed controlled 1:300 scale model of a 5MW-rated machine with a rotor diameter of 126m and a hub height of 90m is tested in a realistic model off-shore atmospheric boundary layer. The blade design is based on blade-element theory for low Reynolds number blade aerodynamics to comply with modelling requirements. Preliminary tests in a low-turbulence flow at a tip speed ratio of TSR=6 yielded a thrust coefficient of CT=0.52 which is within 5% of the predicted value of the theoretical design case with a lift coefficient of C= 0.6 (but a larger blade chord to mimic a higher C). Velocity measurements in the modelled off-shore boundary layer at several downstream positions suggest a transition from near to far wake at a downstream distance of approximately 4 rotor diameters D. At a downstream distance of 10D turbulence intensities in the wake of the single model turbine are still approximately twice as large as in the undisturbed boundary layer. Along with the high turbulence levels a velocity deficit of about 25% is found. Time averaged flow fields and lateral profiles of the vertical velocity clearly illustrate the characteristic swirl generated by the blade rotation, which persists until about a downstream distance of 7D.

Hancock PE, McCluskey FM (1997) Spanwise-invariant three-dimensional separated flow, EXPERIMENTAL THERMAL AND FLUID SCIENCE14(1)pp. 25-34 ELSEVIER SCIENCE INC

Hancock PE, Johnson AE (1997) Close spacing of settling chamber screens, AERONAUTICAL JOURNAL101(1004)pp. 179-183 ROYAL AERONAUTICAL SOC

Hancock PE (2005) Velocity scales in the near-wall layer beneath reattaching turbulent separated and boundary layer flows, EUROPEAN JOURNAL OF MECHANICS B-FLUIDS24(4)pp. 425-438 GAUTHIER-VILLARS/EDITIONS ELSEVIER

Hancock PE, Hayden P, Zhang S (2013) A Wind-Tunnel Artificially-Thickened Simulated Weakly Unstable Atmospheric Boundary Layer, Boundary-Layer Meteorologypp. 1-26

A wind-tunnel simulation of an atmospheric boundary layer, artificially thickened as is often used in neutral flow wind-loading studies, has been investigated for weakly unstable stratification, including the effect of an overlying inversion. Rather than using a uniform inlet temperature profile, the inlet profile was adjusted iteratively by using measured downstream profiles. It was found that three cycles are sufficient for there to be no significant further change in profiles of temperature and other quantities. Development to nearly horizontally-homogeneous flow took a longer distance than in the neutral case because the simulated layer was deeper and therefore the length scales larger. Comparisons show first-order and second-order moments quantities are substantially larger than given by 'standard forms' in the mixed layer but are close in the surface layer. Modified functions, obtained by matching one to the other, are suggested that amount to an interpolation in the mixed layer between the strongly unstable and the weakly unstable cases. © 2013 Springer Science+Business Media Dordrecht.

HANCOCK PE, CASTRO IP (1993) END EFFECTS IN NOMINALLY 2-DIMENSIONAL SEPARATED FLOWS, APPLIED SCIENTIFIC RESEARCH51(1-2)pp. 173-178 KLUWER ACADEMIC PUBL

Nathan P, Hancock PE (2011) Two-Point Near-Wall Measurements of Velocity and Wall Shear Stress Beneath a Separating Turbulent Boundary Layer, PROGRESS IN WALL TURBULENCE: UNDERSTANDING AND MODELING14pp. 135-142 SPRINGER

Hancock PE (2007) Scaling of the near-wall layer beneath turbulent separated flow, EUROPEAN JOURNAL OF MECHANICS B-FLUIDS26(2)pp. 271-283 GAUTHIER-VILLARS/EDITIONS ELSEVIER

Hardman JR, Hancock PE (2000) The near-wall layer beneath a moderately converging three-dimensional turbulent separated and reattaching flow, EUROPEAN JOURNAL OF MECHANICS B-FLUIDS19(5)pp. 653-672 GAUTHIER-VILLARS/EDITIONS ELSEVIER

Nathan P, Hancock PE (2009) Near-wall velocity and wall shear stress correlations in a separating boundary layer, ADVANCES IN TURBULENCE XII - PROCEEDINGS OF THE 12TH EUROMECH EUROPEAN TURBULENCE CONFERENCE132pp. 939-939 SPRINGER-VERLAG BERLIN

Watson S, Hancock PE (2016) Wind Resource,In: Hogg S, Crabtree C (eds.), UK Wind Energy Technologies Routledge

This chapter considers the offshore wind resource and how it is likely to be translated into power production by large arrays of offshore wind turbines. Firstly, the characteristics of the offshore wind resource were studied using long-term reanalysis data from the ERA?40 dataset. For two offshore sites, a more in-depth prediction of the wind resource was made using a mesoscale model. Finally, the results of two studies of the characteristics of offshore wind farm wakes is presented, using a computational fluid dynamics (CFD) model and then inferred from scale model measurements in a meteorological wind tunnel.

Hancock PE (2000) Low Reynolds number two-dimensional separated and reattaching turbulent shear flow, JOURNAL OF FLUID MECHANICS410pp. 101-122 CAMBRIDGE UNIV PRESS

Hancock PE, Hayden P Wind turbine wake in stable boundary layer 1, University of Surrey

Hancock PE, Farr TD (2014) Wind-tunnel simulations of wind-turbine arrays in neutral and non-neutral winds, Journal of Physics: Conference Series524(1)

Wind tunnel simulations have been made of a neutral atmospheric boundary layer (ABL), a stable layer and an unstable layer, typical of offshore conditions, in order to better understand wake development and turbine-wake interactions. Measurements of the wake of a single turbine showed a slower reduction of the velocity deficit for the stable case, and a more rapid reduction for the unstable case, compared with the neutral. It is proposed that there are two effects of non-neutral conditions, indirect and direct. Indirect effects are seen in the earlier part of the wake, influenced by the turbulence level in the ABL but not by buoyancy forces directly; direct effects, caused by buoyancy forces, are seen further downstream. In the stable case, direct effects were seen from about 3 rotor diameters, while for the unstable case they were not seen until about 10 diameters. Two-point measurements in the wakes of four turbines aligned with the flow, compared with those of the ABL, exhibited very different flow characteristics, suggesting a lateral oscillation of the wakes of the later turbines. The effects of laterally adjacent turbines, in a 3-wide x 4-deep array, but with closer-than-typical lateral spacing (2.4 diameters) so as to give early interaction in the short array, were also investigated, and showed only limited interaction. © Published under licence by IOP Publishing Ltd.

Hancock PE, Zhang S, Hayden P (2013) A Wind-Tunnel Artificially-Thickened Simulated Weakly Unstable Atmospheric Boundary Layer,Boundary-Layer Meteorology149(3)pp. 355-380

A wind-tunnel simulation of an atmospheric boundary layer, artificially thickened as is often used in neutral flow wind-loading studies, has been investigated for weakly unstable stratification, including the effect of an overlying inversion. Rather than using a uniform inlet temperature profile, the inlet profile was adjusted iteratively by using measured downstream profiles. It was found that three cycles are sufficient for there to be no significant further change in profiles of temperature and other quantities. Development to nearly horizontally-homogeneous flow took a longer distance than in the neutral case because the simulated layer was deeper and therefore the length scales larger. Comparisons show first-order and second-order moments quantities are substantially larger than given by 'standard forms' in the mixed layer but are close in the surface layer. Modified functions, obtained by matching one to the other, are suggested that amount to an interpolation in the mixed layer between the strongly unstable and the weakly unstable cases. © 2013 Springer Science+Business Media Dordrecht.

Hancock PE (2013) Wind turbines in series; A parametric analysis, Wind Engineering37(1)pp. 37-58 Multi-Science Publishing

An analysis is made of wind turbines in a row by means of an extension to actuator disk theory and a representation of the turbulent diffusion in the wake by a velocity deficit scale and a single free parameter. Beyond this, no wake model is used. It is shown that when the thrust coefficient is 'high' a maximisation of overall power output leads to a large drop in power after the first turbine, followed by a fairly constant level and a rise at the end of the row; this behaviour is a natural consequence of optimisation, and on this basis a 'deep array effect' is to be expected. A variation of turbine size and the effect of impaired turbine performance are examined. The approach can also be used to calculate the turbine upstream velocity (with respect to a reference) from a distribution of measured power output and to make inferences about wake development. The approach could be useful in the assessment of wake models as well as turbine operation.

Aubrun S, Loyer S, Hancock PE, Hayden P (2013) Wind turbine wake properties: Comparison between a non-rotating simplified wind turbine model and a rotating model,Journal of Wind Engineering and Industrial Aerodynamics120pp. 1-8

Experimental results on the wake properties of a non-rotating simplified wind turbine model, based on the actuator disc concept, and a rotating model, a three-blade wind turbine, are presented. Tests were performed in two different test sections, one providing a nominally decaying isotropic turbulent inflow (turbulence intensity of 4% at rotor disc location) and one providing a neutral atmospheric boundary layer above a moderately rough terrain at a geometric scale of 1:300 (determined from the combination of several indicators), with 13% of turbulence intensity at hub height. The objective is to determine the limits of the simplified wind turbine model to reproduce a realistic wind turbine wake. Pressure and high-order velocity statistics are therefore compared in the wake of both rotor discs for two different inflow conditions in order to quantify the influence of the ambient turbulence. It has been shown that wakes of rotating model and porous disc developing in the modeled atmospheric boundary layer are indistinguishable after 3 rotor diameters downstream of the rotor discs, whereas few discrepancies are still visible at the same distance with the decaying isotropic turbulent inflow. copy; 2013 Elsevier Ltd.

Hardman JR, Hancock PE (1996) Measurements in a three-dimensional separation, ADVANCES IN TURBULENCES VI36pp. 453-456 KLUWER ACADEMIC PUBL

Hancock Philip, Hayden Paul (2018) Wind-Tunnel Simulation of Weakly and Moderately Stable Atmospheric Boundary Layers,Boundary-Layer Meteorology168(1)pp. 29-57 Springer Verlag

The simulation of horizontally homogeneous boundary layers that have characteristics of weakly and moderately stable atmospheric flow is investigated, where the well-established wind engineering practice of using ?flow generators? to provide a deep boundary layer is employed. Primary attention is given to the flow above the surface layer, in the absence of an overlying inversion, as assessed from first- and second-order moments of velocity and temperature. A uniform inlet temperature profile ahead of a deep layer, allowing initially neutral flow, results in the upper part of the boundary layer remaining neutral. A non-uniform inlet temperature profile is required but needs careful specification if odd characteristics are to be avoided, attributed to long-lasting effects inherent of stability, and to a reduced level of turbulent mixing. The first part of the wind-tunnel floor must not be cooled if turbulence quantities are to vary smoothly with height. Closely horizontally homogeneous flow is demonstrated, where profiles are comparable or closely comparable with atmospheric data in terms of local similarity and functions of normalized height. The ratio of boundary-layer height to surface Obukhov length, and the surface heat flux, are functions of the bulk Richardson number, independent of horizontal homogeneity. Surface heat flux rises to a maximum and then decreases.