The Experts below are selected from a list of 321 Experts worldwide ranked by ideXlab platform

Bert Blocken - One of the best experts on this subject based on the ideXlab platform.

  • effect of the shaft on the aerodynamic performance of urban vertical axis wind turbines
    Energy Conversion and Management, 2017
    Co-Authors: Abdolrahim Rezaeiha, Ivo I Kalkman, H Montazeri, Bert Blocken
    Abstract:

    The central shaft is an inseparable part of a vertical axis wind turbine (VAWT). For small turbines such as those typically used in urban environments, the shaft could operate in the subcritical regime, resulting in large drag and considerable aerodynamic power loss. The current study aims to (i) quantify the turbine power loss due to the presence of the shaft for different shaft-to-turbine diameter ratios δ from 0 to 16%, (ii) investigate the impact of different operational and geometrical parameters on the quantified power loss and (iii) evaluate the impact of the addition of surface Roughness on turbine performance improvement. Unsteady Reynolds-averaged Navier-Stokes (URANS) calculations are performed on a high-resolution computational grid. The evaluation is based on validation with wind-tunnel measurements. The results show that the power loss increases asymptotically with increasing δ due to the higher width and length of the shaft wake as the blades pass through a larger region with lower velocity in the downwind area. A maximum power loss of 5.5% compared to the hypothetical case without shaft is observed for δ = 16%. The addition of surface Roughness is shown to be an effective approach to shift the flow over the shaft into the critical regime, reducing the shaft drag and wake width as a result of a delay in separation. For an optimal dimensionless Equivalent Sand-Grain Roughness height of 0.08, the turbine power coefficient at δ = 4% improves by 1.7%, which is Equivalent to a 69% recovery of the corresponding turbine power loss. The results are found to be virtually independent of the shaft-to-turbine rotational speed ratio.

Pascal Molton - One of the best experts on this subject based on the ideXlab platform.

  • Turbulent drag induced by low surface Roughness at transonic speeds: Experimental/numerical comparisons
    Physics of Fluids, 2020
    Co-Authors: David Hue, Pascal Molton
    Abstract:

    The paper gives the main results of an experimental test campaign aimed at quantifying the effects of very low Roughness levels on the flat plate turbulent drag in transonic conditions. This work, in a field not completely understood yet, the one of the transitionally rough regime, has revealed its importance by showing that some surfaces considered as hydrodynamically smooth in past studies might not have been. That issue can be strongly critical in the delicate exercise of experimental / numerical comparisons, and more specifically, when it comes to absolute drag predictions, if the fluid dynamics computations use only, as they almost always do, infinitely smooth surfaces. The experiments carried out in the S8Ch wind tunnel have involved high-level measurement techniques, such as micro-drag evaluation with a three-component balance and near-wall laser Doppler velocimetry, for Mach numbers from 0.55 to 0.8 and maximum Reynolds numbers based on the sample length and boundary layer thickness of about 2.6 million and 0.13 million respectively, the greatest friction Reynolds number being close to 5,000. A dozen surface samples were tested, with average Roughness values from less than 0.25 µm (mirror-polished aluminum) to more than 10 µm (commercial sandpapers), and in between standard-machined or painted samples (including pressure sensitive paintings). A pragmatic computational fluid dynamics study reproducing this test campaign was completed. It is based on the well-known Equivalent sand grain Roughness height approach and also on the Musker correlation, which was adapted. The results and validity of such Reynolds-averaged Navier-Stokes simulations in that particular regime are discussed. Anyway, both experimental and numerical outcomes of this 2 work are in agreement to indicate that drag can be significantly impacted even for Roughness Reynolds numbers potentially below the usual threshold values often considered in the engineering world (i.e. about 3.5 to 5). And the cross-analysis of surface drag production and Roughness characteristics has allowed the decisive role played by the rms (or average) Roughness height, the skewness and kurtosis coefficients and more especially the slope parameter to be confirmed.

  • Turbulent drag induced by low surface Roughness at transonic speeds: Experimental/numerical comparisons
    Physics of Fluids, 2020
    Co-Authors: David Hue, Pascal Molton
    Abstract:

    This paper gives the main results of an experimental test campaign aimed at quantifying the effects of very low Roughness levels on the flat plate turbulent drag in transonic conditions. This work, in a field not completely understood yet, the one of the transitionally rough regime, has revealed its importance by showing that some surfaces considered as hydrodynamically smooth in past studies might not have been. This issue can be strongly critical in the delicate exercise of experimental/numerical comparisons, and more specifically, when it comes to absolute drag predictions, if the fluid dynamics computations use only, as they almost always do, infinitely smooth surfaces. The experiments on the S8Ch wind tunnel have involved high-level measurement techniques, such as micro-drag evaluation with a three-component balance and near-wall laser Doppler velocimetry, for Mach numbers from 0.55 to 0.8 and maximum Reynolds numbers based on the sample length and the boundary layer thickness of about 2.6 × 106 and 0.13 × 106, respectively, the highest friction Reynolds number being close to 5000. A dozen surface samples were tested, with average Roughness values from less than 0.25 μm (mirror-polished aluminum) to more than 10 μm (commercial sandpapers), and in between standard-machined or painted samples (including pressure sensitive paintings). A pragmatic computational fluid dynamics study reproducing this test campaign was completed. It is based on the well-known Equivalent sand grain Roughness height approach and also on the Musker correlation, which was adapted. The results and validity of such Reynolds-averaged Navier–Stokes simulations in that particular regime are discussed. Anyway, both experimental and numerical outcomes of this work are in agreement to indicate that drag can be significantly impacted even for Roughness Reynolds numbers potentially below the usual threshold values often considered in the engineering world (i.e., about 3.5–5). And the cross-analysis of surface drag production and Roughness characteristics has allowed the decisive role played by the rms (or average) Roughness height, the skewness and kurtosis coefficients, and especially the slope parameter to be confirmed.

H Montazeri - One of the best experts on this subject based on the ideXlab platform.

  • effect of the shaft on the aerodynamic performance of urban vertical axis wind turbines
    Energy Conversion and Management, 2017
    Co-Authors: Abdolrahim Rezaeiha, Ivo I Kalkman, H Montazeri, Bert Blocken
    Abstract:

    The central shaft is an inseparable part of a vertical axis wind turbine (VAWT). For small turbines such as those typically used in urban environments, the shaft could operate in the subcritical regime, resulting in large drag and considerable aerodynamic power loss. The current study aims to (i) quantify the turbine power loss due to the presence of the shaft for different shaft-to-turbine diameter ratios δ from 0 to 16%, (ii) investigate the impact of different operational and geometrical parameters on the quantified power loss and (iii) evaluate the impact of the addition of surface Roughness on turbine performance improvement. Unsteady Reynolds-averaged Navier-Stokes (URANS) calculations are performed on a high-resolution computational grid. The evaluation is based on validation with wind-tunnel measurements. The results show that the power loss increases asymptotically with increasing δ due to the higher width and length of the shaft wake as the blades pass through a larger region with lower velocity in the downwind area. A maximum power loss of 5.5% compared to the hypothetical case without shaft is observed for δ = 16%. The addition of surface Roughness is shown to be an effective approach to shift the flow over the shaft into the critical regime, reducing the shaft drag and wake width as a result of a delay in separation. For an optimal dimensionless Equivalent Sand-Grain Roughness height of 0.08, the turbine power coefficient at δ = 4% improves by 1.7%, which is Equivalent to a 69% recovery of the corresponding turbine power loss. The results are found to be virtually independent of the shaft-to-turbine rotational speed ratio.

David Hue - One of the best experts on this subject based on the ideXlab platform.

  • Turbulent drag induced by low surface Roughness at transonic speeds: Experimental/numerical comparisons
    Physics of Fluids, 2020
    Co-Authors: David Hue, Pascal Molton
    Abstract:

    The paper gives the main results of an experimental test campaign aimed at quantifying the effects of very low Roughness levels on the flat plate turbulent drag in transonic conditions. This work, in a field not completely understood yet, the one of the transitionally rough regime, has revealed its importance by showing that some surfaces considered as hydrodynamically smooth in past studies might not have been. That issue can be strongly critical in the delicate exercise of experimental / numerical comparisons, and more specifically, when it comes to absolute drag predictions, if the fluid dynamics computations use only, as they almost always do, infinitely smooth surfaces. The experiments carried out in the S8Ch wind tunnel have involved high-level measurement techniques, such as micro-drag evaluation with a three-component balance and near-wall laser Doppler velocimetry, for Mach numbers from 0.55 to 0.8 and maximum Reynolds numbers based on the sample length and boundary layer thickness of about 2.6 million and 0.13 million respectively, the greatest friction Reynolds number being close to 5,000. A dozen surface samples were tested, with average Roughness values from less than 0.25 µm (mirror-polished aluminum) to more than 10 µm (commercial sandpapers), and in between standard-machined or painted samples (including pressure sensitive paintings). A pragmatic computational fluid dynamics study reproducing this test campaign was completed. It is based on the well-known Equivalent sand grain Roughness height approach and also on the Musker correlation, which was adapted. The results and validity of such Reynolds-averaged Navier-Stokes simulations in that particular regime are discussed. Anyway, both experimental and numerical outcomes of this 2 work are in agreement to indicate that drag can be significantly impacted even for Roughness Reynolds numbers potentially below the usual threshold values often considered in the engineering world (i.e. about 3.5 to 5). And the cross-analysis of surface drag production and Roughness characteristics has allowed the decisive role played by the rms (or average) Roughness height, the skewness and kurtosis coefficients and more especially the slope parameter to be confirmed.

  • Turbulent drag induced by low surface Roughness at transonic speeds: Experimental/numerical comparisons
    Physics of Fluids, 2020
    Co-Authors: David Hue, Pascal Molton
    Abstract:

    This paper gives the main results of an experimental test campaign aimed at quantifying the effects of very low Roughness levels on the flat plate turbulent drag in transonic conditions. This work, in a field not completely understood yet, the one of the transitionally rough regime, has revealed its importance by showing that some surfaces considered as hydrodynamically smooth in past studies might not have been. This issue can be strongly critical in the delicate exercise of experimental/numerical comparisons, and more specifically, when it comes to absolute drag predictions, if the fluid dynamics computations use only, as they almost always do, infinitely smooth surfaces. The experiments on the S8Ch wind tunnel have involved high-level measurement techniques, such as micro-drag evaluation with a three-component balance and near-wall laser Doppler velocimetry, for Mach numbers from 0.55 to 0.8 and maximum Reynolds numbers based on the sample length and the boundary layer thickness of about 2.6 × 106 and 0.13 × 106, respectively, the highest friction Reynolds number being close to 5000. A dozen surface samples were tested, with average Roughness values from less than 0.25 μm (mirror-polished aluminum) to more than 10 μm (commercial sandpapers), and in between standard-machined or painted samples (including pressure sensitive paintings). A pragmatic computational fluid dynamics study reproducing this test campaign was completed. It is based on the well-known Equivalent sand grain Roughness height approach and also on the Musker correlation, which was adapted. The results and validity of such Reynolds-averaged Navier–Stokes simulations in that particular regime are discussed. Anyway, both experimental and numerical outcomes of this work are in agreement to indicate that drag can be significantly impacted even for Roughness Reynolds numbers potentially below the usual threshold values often considered in the engineering world (i.e., about 3.5–5). And the cross-analysis of surface drag production and Roughness characteristics has allowed the decisive role played by the rms (or average) Roughness height, the skewness and kurtosis coefficients, and especially the slope parameter to be confirmed.

Neil D. Sandham - One of the best experts on this subject based on the ideXlab platform.

  • A DNS/URANS approach for simulating rough-wall turbulent flows
    International Journal of Heat and Fluid Flow, 2020
    Co-Authors: F. Alves Portela, Neil D. Sandham
    Abstract:

    Abstract A novel hybrid method combining direct numerical simulation (DNS) and the Reynolds-averaged Navier Stokes (RANS), denoted as a stress-blended method (SBM), has been developed. The SBM is targeted at simulating turbulent flows over arbitrary rough surfaces in which computational savings can be achieved by making the DNS domain as small as possible. Within the SBM framework, a RANS model is enforced above the Roughness layer to prevent the momentum build-up which arises in simulations where the computational domain is too small to represent the largest eddies. The SBM is validated for turbulent channel flow, both for smooth wall turbulence and using a parametric forcing approach to mimic Roughness effects, with a computational cost that scales linearly with Re τ . The method is then applied to selected subsets of a scanned grit-blasted surface. For the same subset, the Roughness function is found to be within 1 % of available DNS. Comparisons of small and large subsets showed differences of over a factor of two in Equivalent sand grain Roughness, indicating the importance of choosing representative surface samples. Simulations in the fully rough regime are carried out using one to two orders of magnitude fewer points than in a typical DNS. Since no assumptions on the Roughness properties or the flow structure (such as outer layer similarity) are made, we expect the SBM to be applicable to non-equilibrium turbulent boundary layer flows.

  • Dataset for 'DNS of turbulent channel flow over a surrogate for Nikuradse-type Roughness'
    2017
    Co-Authors: Manan Thakkar, Angela Busse, Neil D. Sandham
    Abstract:

    Data related to the publication: Thakkar, M., Busse, A. & Sandham, N.D. (2017) DNS of turbulent channel flow over a surrogate for Nikuradse-type Roughness.Table1.csv contains data from Table 1 in the paper. Additionally, it contains two more columns, showing the values of ks+ (Equivalent Sand-Grain Roughness height in wall-units) and Nikuradse's A parameter, for all cases considered.

  • Surface correlations of hydrodynamic drag for transitionally rough engineering surfaces
    Journal of Turbulence, 2016
    Co-Authors: Manan Thakkar, Angela Busse, Neil D. Sandham
    Abstract:

    ABSTRACTRough surfaces are usually characterised by a single Equivalent Sand-Grain Roughness height scale that typically needs to be determined from laboratory experiments. Recently, this method has been complemented by a direct numerical simulation approach, whereby representative surfaces can be scanned and the Roughness effects computed over a range of Reynolds number. This development raises the prospect over the coming years of having enough data for different types of rough surfaces to be able to relate surface characteristics to Roughness effects, such as the Roughness function that quantifies the downward displacement of the logarithmic law of the wall. In the present contribution, we use simulation data for 17 irregular surfaces at the same friction Reynolds number, for which they are in the transitionally rough regime. All surfaces are scaled to the same physical Roughness height. Mean streamwise velocity profiles show a wide range of Roughness function values, while the velocity defect profiles...

  • Parametric forcing approach to rough-wall turbulent channel flow
    Journal of Fluid Mechanics, 2012
    Co-Authors: Angela Busse, Neil D. Sandham
    Abstract:

    The effects of rough surfaces on turbulent channel flow are modelled by an extra force term in the Navier–Stokes equations. This force term contains two parameters, related to the density and the height of the Roughness elements, and a shape function, which regulates the influence of the force term with respect to the distance from the channel wall. This permits a more flexible specification of a rough surface than a single parameter such as the Equivalent sand grain Roughness. The effects of the Roughness force term on turbulent channel flow have been investigated for a large number of parameter combinations and several shape functions by direct numerical simulations. It is possible to cover the full spectrum of rough flows ranging from hydraulically smooth through transitionally rough to fully rough cases. By using different parameter combinations and shape functions, it is possible to match the effects of different types of rough surfaces. Mean flow and standard turbulence statistics have been used to compare the results to recent experimental and numerical studies and a good qualitative agreement has been found. Outer scaling is preserved for the streamwise velocity for both the mean profile as well as its mean square fluctuations in all but extremely rough cases. The structure of the turbulent flow shows a trend towards more isotropic turbulent states within the Roughness layer. In extremely rough cases, spanwise structures emerge near the wall and the turbulent state resembles a mixing layer. A direct comparison with the study of Ashrafian, Andersson & Manhart (Intl J. Heat Fluid Flow, vol. 25, 2004, pp. 373–383) shows a good quantitative agreement of the mean flow and Reynolds stresses everywhere except in the immediate vicinity of the rough wall. The proposed Roughness force term may be of benefit as a wall model for direct and large-eddy numerical simulations in cases where the exact details of the flow over a rough wall can be neglected