Equivalent Sand Grain Roughness Research Articles

Direct numerical simulations of turbulent channel flow over ratchet roughness

The influence of the orientation of ratchet-type rough surfaces on their fluid dynamic roughness effect is investigated using direct numerical simulations of turbulent channel flow at Re_{tau }=395. The ratchet length-to-height ratio is varied from ell /k=2 to 16 for a fixed ratchet height of k/delta =0.1 where delta is the mean channel half-height. The results show that both roughness function and mean flow and turbulence statistics strongly depend on the ratchet orientation. Existing empirical formulae, which estimate the roughness function Delta U^+ or the equivalent sand-grain roughness k_s based on surface-slope related parameters such as the effective slope or the Sigal-Danberg parameter, fail to accurately predict the differences between ratchet surfaces with high windward slopes and ratchet surfaces with high leeward slopes.

Effects of boundary layer thickness on the estimation of equivalent sandgrain roughness in zero-pressure-gradient boundary layers

Effects of boundary layer thickness on the estimation of equivalent sandgrain roughness in zero-pressure-gradient boundary layers

Questions on the effects of roughness and its analysis in non-equilibrium flows

The prediction of turbulent flows over rough surfaces is important for many applications in engineering and in the natural sciences. Since the resolution of the roughness requires significant computational resources, most modelling approaches rely on the related concepts of ‘equivalent sand-grain roughness’ and ‘wall similarity’. While the validity of these concepts is well established for zero-pressure-gradient boundary-layers and for channel flows, such is not the case for non-equilibrium conditions. This raises a number of important questions, some of which are discussed in this paper. We also suggest some possible paths to answering these questions.

Important Parameters for a Predictive Model of for Zero-Pressure-Gradient Flows

To predict drag on a rough surface under turbulent flow conditions, practitioners rely on roughness correlations that map topographical features of the surface to the equivalent sand-grain roughness . However, details of the data that underpin these empirical correlations are not always immediately evident for comparison and discussion. Therefore, here we compile a table of roughness correlations with unified notation, in chronological order, listing the parameter ranges and the roughness types used in their development, noting idiosyncrasies. Overall, the table shows that tested roughness types have increased in generality from regular roughness features to random surface elevations, and that the independent parameters of primary importance measure size (e.g., height), frontal area (e.g., slope), and coverage (e.g., skewness). In addition to the need for more data to populate the parameter space, outstanding questions facing practitioners are filtering and sampling of multiscale surfaces, and the treatment of heterogeneous surfaces, with answers appearing on the horizon.

Numerical study of effects of solid particle erosion on compressor and engine performance

Solid particle erosion (SPE) is a major source of damage to, and cause of failure of, turbomachinery, including centrifugal and axial compressors and gas turbines. There is considerable interest within the turbomachinery industry to develop the means to enhance durability and improve prediction of likely breakdowns of machinery operating in locations and conditions where ingestion of, for example, dust and sand grains is unavoidable during operation. The present study examines the implications of degradation of the compressor on gas turbine cycle performance, caused by operating in eroding environments, linking compressor aerodynamics with the thermodynamic cycle. The research engine employed in the study comprised a fan, bypass, and two stages of the low-pressure compressor (booster). A numerical simulation using Computational Fluid Dynamics (CFD) software was performed to predict the likely erosion patterns due to solid particles (SPs) impacting on the blades of the first two stages of an axial compressor. This study introduces a method that provides the freedom to apply roughness to a particular region of the blade following the SPE patterns identified as part of the CFD simulations so as to represent the effects of the erosion spots on the overall flow field. The localised roughness method employs a perturbation of the geometry of the blade in the regions corresponding to the location predicted by the SPE model. The intensity of the perturbation, which is governed by a “fraction factor” coded in a Matlab script, was calibrated by reference to a roughness case, applied over the entire blade surface, with equivalent sand-grain roughness (ks) of 60 μm. Turbomatch, the Cranfield in-house gas turbine performance simulation software, was employed to model the degradation of engine performance. The research confirmed that SPE is linked with significant decreases in engine performance parameters such as isentropic efficiency (ηis) and pressure ratio (PR). These quantities showed, for the SPE cases considered, a drop of 8.8% and 5.2%, respectively. These findings are useful to link local SPE events to global GTE cycle performance.

Calibration of a Computational Fluid Dynamics Methodology for the Simulation of Roughness Effects on Friction and Heat Transfer in Additive Manufactured Components

Abstract It is well known from the literature that surface roughness significantly affects friction and heat transfer. This is even more evident for additive manufactured (AM) components, which are taking an increasingly important role in the gas turbine field. However, the exploitation of numerical approaches to improve their design is hindered by the lack of dedicated correlations and computational fluid dynamics (CFD) models developed for such high roughness conditions. Usually, the AM components are simulated considering the surfaces as smooth or applying an equivalent sand-grain roughness (ks) that results in a velocity shift in the boundary layer. However, determining a priori the most appropriate value of ks is challenging, as dozens of correlations are available, returning scattered and uncertain results. A previous work proved how the CFD prediction of friction and heat transfer returns significant deviations, even exploiting the ks values obtained from experimental tests on the very same test case. That work also allowed identification of a promising CFD methodology based on friction and thermal corrections proposed by Aupoix from ONERA. The aim of this work is to further the assessment and calibration activity of the model, by analyzing additional experimental data of friction factor and Nusselt number from new test cases considering different geometries and flow conditions. The new coupons consisted of straight circular channels and wavy channels. This work represents a further step in the generation of a more validated and general methodology for the high-fidelity CFD analysis of AM components.

Combined effect of passive vortex generators and leading-edge roughness on dynamic stall of the wind turbine airfoil

Dynamic stall causes the highly unsteady and nonlinear aerodynamic loads on wind turbines. Recently, dynamic stall with passive vortex generators (VGs) and leading-edge roughness (LER) has received considerable attention, but independently. Therefore, this paper presents a careful investigation into dynamic stall of the NREL S809 airfoil with both VGs and LER to reveal their combined effect. Fully-resolved URANS simulations are conducted to identify the unsteady flow characteristics, and equivalent sand-grain roughness is used to model LER. LER significantly increases the turbulence kinetic energy (TKE) and suction loss at the leading edge, thereby causing the earlier onsets of separated flow and dynamic stall. Increasing roughness height generally reduces linear lift-curve slope, increases aerodynamic hysteresis and makes the separated flow hard to be reattached. VGs increase near-wall TKE via streamwise vortices and effectively suppress the separated flow. Dynamic stall is therefore significantly delayed by VGs with higher maximum lift, and aerodynamic hysteresis is also greatly reduced with the flow reattachment accelerated. Interestingly, serious LER may cause strong vortical disturbances and change the dynamic stall behaviors from light stall into deep stall. Double-row VGs are also found better than single-row VGs in improving the aerodynamic performance of roughened airfoil. These findings imply that VGs effectively control dynamic stall and diminish the adverse LER effect. This study should advance the control of unsteady loads on wind turbines suffering from harsh environmental conditions.

Blade Roughness Effects on Compressor and Engine Performance—A CFD and Thermodynamic Study

Degradation of compressors is a common concern for operators of gas turbine engines (GTEs). Surface roughness, due to erosion or fouling, is considered one of the major factors of the degradation phenomenon in compressors that can negatively affect the designed pressure rise, efficiency, and, therefore, the engine aero/thermodynamic performance. The understanding of the aerodynamic implications of varying the blade surface roughness plays a significant role in establishing the magnitude of performance degradation. The present work investigates the implications due to the degradation of the compressor caused by the operation in eroding environments on the gas turbine cycle performance linking, thereby, the compressor aerodynamics with a thermodynamic cycle. At the core of the present study is the numerical assessment of the effect of surface roughness on compressor performance employing the Computational Fluid Dynamics (CFD) tools. The research engine test case employed in the study comprised a fan, bypass, and two stages of the low pressure compressor (booster). Three operating conditions on the 100% speed-line, including the design point, were investigated. Five roughness cases, in addition to the smooth case, with equivalent sand-grain roughness (ks) of 15, 30, 45, 60, and 150 µm were simulated. Turbomatch the Cranfield in-house gas turbine performance simulation software, was employed to model the degraded engine performance. The study showed that the increase in the uniform roughness is associated with sizable drops in efficiency, booster pressure ratio (PR), non-dimensional mass flow (NDMF), and overall engine pressure ratio (EPR) together with rises in turbine entry temperature (TET) and specific fuel consumption (SFC). The performance degradation evaluation employed variables such as isentropic efficiency (ηis), low pressure compressor (LPC) PR, NDMF, TET, SFC, andEPR. The variation in these quantities showed, for the maximum blade surface degradation case, drops of 7.68%, 2.62% and 3.53%, rises of 1.14% and 0.69%, and a drop of 0.86%, respectively.

Direct numerical simulation of turbulent heat transfer on the Reynolds analogy over irregular rough surfaces

The effect of rough surface topography on heat and momentum transfer is studied by direct numerical simulations of turbulent heat transfer over uniformly heated three-dimensional irregular rough surfaces, where the effective slope and skewness values are systematically varied while maintaining a fixed root-mean-square roughness. The friction Reynolds number is fixed at 450, and the temperature is treated as a passive scalar with a Prandtl number of unity. Both the skin friction coefficient and Stanton number are enhanced by the wall roughness. However, the Reynolds analogy factor for the rough surface is lower than that for the smooth surface. The semi-analytical expression for the Reynolds analogy factor suggests that the Reynolds analogy factor is related to the skin friction coefficient and the difference between the temperature and velocity roughness functions, and the Reynolds analogy factor for the present rough surfaces is found to be predicted solely based on the equivalent sand-grain roughness. This suggests that the relationship between the Reynolds analogy factor and the equivalent sand-grain roughness is not affected by the effective slope and skewness values. Analysis of the heat and momentum transfer mechanisms based on the spatial- and time-averaged equations suggests that two factors decrease the Reynolds analogy factor. One is the increased effective Prandtl number within the rough surface in which the momentum diffusivity due to the combined effects of turbulence and dispersion is larger than the corresponding thermal diffusivity. The other is the significant increase in the pressure drag force term above the mean roughness height.

Surface roughness effects on film-cooling effectiveness in a fan-shaped cooling hole

Large eddy simulations (LES) were carried out to investigate the influence of surface roughness, which was applied to the inner walls of a cooling hole, on cooling performance and flow structures. The cooling hole was a 30-degree inclined, baseline 7-7-7 fan-shaped hole relative to a turbulent flat plate boundary layer of the mainstream. Numerical simulations were performed at two different blowing ratios (M=1.5 and 3.0) and at a constant density ratio (DR=1.5) for various configurations of surface roughness. In order to numerically consider the effects of surface roughness, the equivalent sand-grain roughness model was utilized. Different correlations between the equivalent sand-grain roughness height and arithmetic average roughness height were numerically tested to find an accurate correlation compared to the measurements. The time-averaged numerical results were validated by experiments looking at the velocity and thermal fields for the smooth and rough cooling holes. Results revealed that increasing the surface roughness applied to the cooling hole, increases the thickness of the boundary layers within the hole, especially at the higher blowing ratio. This leads to a higher jet core flow at the cooling hole exit and lower cooling effectiveness at the flat plate surface compared with a smooth cooling hole. The minimum area-averaged film-cooling performance showed a 58% reduction compared to a smooth hole in the case of the highest blowing ratio (M=3.0) and the largest surface roughness height. In addition, the time-space evaluation of the velocity fluctuations showed greater flow unsteadiness and increased wavy patterns within the cooling hole in the case of rough holes.

Dimensionless Model of Frost Roughness on Cold Flat Plate Under Forced Convection

Laboratory experiments were conducted to generate empirical dimensionless correlations for estimating surface roughness of frost formed on a cold flat plate under forced convection. Environmental variables, including wall temperature , air velocity , relative humidity , and air temperature , were considered when generating the correlations. The test conditions were chosen to simulate the environmental conditions experienced by cold-soaked fuel frost formation on the upper surface of airplane wings. Experimental conditions included wall temperatures from to , freestream temperatures from 8 to , relative humidities from 60 to 91%, and air velocities from 0.5 to . The root-mean-square height, skewness, and equivalent sand-grain roughness height of frost were correlated as functions of the Reynolds number, absolute humidity, and dimensionless temperatures. The comparison between the correlation of frost equivalent sand-grain roughness height in this study and a previous model revealed the importance of test surface geometry on frost roughness formation.

The effect of cleaning and repainting on the ship drag penalty

Although the hull of a recently dry-docked large ship is expected to be relatively smooth, surface scanning and experimentation reveal that it can exhibit an “orange-peel” roughness pattern with an equivalent sand-grain roughness height = 0. 101 mm. Using the known value and integral boundary layer evolution, a recently cleaned and coated full-scale ship was predicted to experience a significant increase in the average coefficient of friction and total hydrodynamic resistance during operation. Here the report also discusses two recently reported empirical estimations that can estimate ks directly from measured surface topographical parameters, by-passing the need for experiments on replicated surfaces. The empirical estimations are found to have an accuracy of 4.5 − 5 percentage points in

CFD investigation on the maximum lift coefficient degradation of rough airfoils

<abstract> Ice accretion can reduce the performance of aircraft's wings, which results in higher fuel consumption and risk of accidents. Experiments proved that even in its very earlier stages (increased roughness), icing could cause a reduction of 25% in the maximum lift, and an increase of 90% in drag of an aspect ratio 6 wing. In this work, we propose a correlation to predict the degradation of the maximum lift coefficient caused by roughness effects on flows over two airfoils, a NACA 0012 and a model 5–6. In addition, a second correlation is proposed to find the minimum Reynolds number that are useful for higher Reynolds number applications when roughness is considered. The SA roughness extension is implemented into an open-source code called SU2. The verification and validation of the implementation is performed in two steps. First, the behavior of the flow over a smooth NACA 0012 is investigated to confirm whether the implementation has no influence on the original model when roughness is not activated. Then, roughness is activated, and estimations of lift coefficients and velocity profiles inside the boundary layer are evaluated and compared to numerical and experimental results. Finally, investigations on the maximum lift coefficient reduction caused by different equivalent sand grain roughness heights and Reynolds numbers are performed. Our results demonstrated that, for the equivalent sand grain roughness heights investigated, the variation of sufficiently small heights has no significant influence on the maximum lift coefficient degradation. Moreover, when roughness is continuously increased, a saturation point seems to be approached, in which the variation of the maximum lift coefficient degradation is reduced. We noticed that although the reduction of the maximum lift coefficient caused by different equivalent sand-grain roughness heights and Reynolds number present similar behavior, they fall into different curve formats. </abstract>

Numerical study of the equivalent roughness effect on the low-flow centrifugal compressor impeller characteristics

The article presents the results of a numerical study of the effect of equivalent sandgrain roughness on the characteristics of a low-flow centrifugal compressor stage. Low-flow stages are widely used in high-pressure compressors as the last stages. These compressors are used for natural gas booster compressor stations, as well as in technological processes for the production of methanol, ammonia, high - pressure polyethylene, etc. The efficiency of the low-flow stages is lower than that of medium-and high-flow centrifugal compressors stages. Stages operate at high pressure and have low gas volume flow rates. For this reason they have narrow channels of the flow part, significant friction and leakage losses. The study was conducted in the Ansys CFX 19.2 software package. In the first part of the study, the low-flow stage numerical model is validated with the test results. Full-scale tests of the stage were carried out at the LPI-SPbPU earlie. In the second part of the study, impeller equivalent sand-grain roughness numerical study carried out. As a result, the characteristics of the stage were obtained at four different values of equivalent sand-grain roughness ks. The impeller losses were estimated. It is established that when the ks values increase, the polytropic head and efficiency values do not decrease equally over the entire characteristic. Characteristics are significantly reduced in areas of high expenditure and change slightly with minimal expenditure. The internal head coefficient varies slightly depending on ks.

Variation of Frost Roughness on a Flat Plate Under Forced Convection

Abstract Experiments were conducted on a cold flat aluminum plate to characterize the variation of frost roughness over both time and location on the surfaces. The testing conditions included air temperatures from 8 to 16 °C, wall temperatures from −20 to −10 °C, relative humidities from 60 to 80%, and air velocities from 0.5 to 2.5 m/s. Each test lasted 2 h. A 3D photogrammetric method was employed to measure the variation in frost root-mean-square height and skewness by location and time. These data were used to develop the equivalent sand-grain roughness for the frost at different locations and time. The experimental results showed that frost roughness varied by location and changed with time. For the environmental conditions in this study, relative humidity and air temperature were the most important factors determining changes in the peak frost roughness. For example, at an air temperature of 12 °C and a surface temperature of −15 °C, the frost roughness peaked at about 40 min for a relative humidity of 80% and 90 min for a relative humidity of 60%. Empirical correlations were provided to describe the relationships between the environmental conditions and the appearance of the peak frost roughness.

Effects of blade surface roughness on compressor performance and tonal noise emission in a marine diesel engine turbocharger

A numerical study was conducted to investigate the effects of blade surface roughness on compressor performance and tonal noise emission. The equivalent sand-grain roughness model was used to account for blade surface roughness, and a hybrid method that combines computational fluid dynamics and boundary element method was used to predict compressor performance and tonal noise. The numerical approach was validated against experimental data for a baseline compressor. Nine different cases with different blade surface roughness were studied in this paper, the global performance was analyzed under compressor design speed, and the tonal noise level was predicted under the design condition. The results indicate that compressor total-to-total pressure ratio and isentropic efficiency were gradually decreased with the increasing blade surface roughness. Besides, the blade total pressure loss coefficient and the efficiency loss coefficient were also increased. It was found that the reverse flow at the leading edge of compressor rotor blades reduced blade loading. The pressure fluctuation at the leading edge showed that the peak of pressure fluctuations increased as the blade surface roughness was increased. The sound pressure level at blade-passing frequency shows a significant change with variation in blade surface roughness, which results in an increased total noise level. Furthermore, it was shown that the blade surface roughness had nearly no influence on acoustic directivity, but the sound pressure level increased with the increase in roughness, especially at blade-passing frequency.

A novel indicator for ship hull and propeller performance: Examples from two shipping segments

Ship hulls and propellers suffer from both mechanical defects and marine growth, which increase surface roughness and thus lead to significant power/speed penalties. A recent standard, ISO 19030:2016, aimed at establishing a transparent and accurate method to compare a vessel's performance to itself over time. However, the standard performance value, percentage speed difference Vdiff, is both vessel-specific and sensitive to operation point, most importantly vessel speed. The current paper thus proposes a new performance indicator, based on the concept of equivalent sand-grain roughness height ks, which would enable comparisons between vessels, increased accuracy for comparison of a vessel to itself over time, and calculation of penalties under operating conditions differing from past data (e.g. slow steaming). Sand-grain height ks is determined iteratively, through modelling roughness penalties on propulsive power (Granville's similarity law scaling), comparing modelled and measured penalties, and stopping the iterative process at a given tolerance on power penalty. This new performance indicator is applied to onboard-collected data from a Panamax tanker and two Roll-on/Roll-off sister vessels, yielding qualitative agreement with hull condition reported from diving inspections. Comparative advantages and limitations of ks are discussed in relation to the ISO 19030 performance value.

Turbulence characteristics over k-type rib roughened porous walls

To understand the permeability effects on turbulent rib-roughened porous channel flows, particle image velocimetry measurements are performed at the bulk Reynolds number of 5000–20000. Solid impermeable and porous ribs are considered for the rib-roughness whose geometry is categorised in the k-type roughness whose pitch/rib-height is 10. Three isotropic porous media with nearly the same porosity: 0.8, and different permeabilities (0.004, 0.020, 0.033 mm2) are applied. It is observed that the recirculation between the ribs becomes weak and the recirculation vortex submerges into the porous wall as the wall permeability and Reynolds number increase for both solid and porous rib cases while the recirculation vanishes in high permeable cases. These phenomena result in characteristic difference in turbulence quantities. By fitting the mean velocity profiles to the log-law form, the permeability effects of both rib and bottom wall on the log-law parameters and the equivalent sand-grain roughness are discussed. It is concluded that the zero-plane displacement increases while the von Kármán constant and the equivalent sand-grain roughness decrease as the wall and rib permeability increases.

Skin Friction Measurements of Systematically-Varied Roughness: Probing the Role of Roughness Amplitude and Skewness

Skin-friction, roughness functions and predictive correlations are presented for random roughness that has a Gaussian power spectral density distribution of surface elevations. The root-mean-square (rms) roughness height and the skewness of the probability density function are parametrically varied to investigate the role of these parameters in generating the friction at the wall. Results are presented for all roughness regimes, from hydraulically-smooth to fully-rough. Negative skewness (pits) had a much smaller influence on drag than positive skewness (peaks). Predictive engineering correlations for the equivalent sandgrain roughness height indicate that the rms roughess height and skewness are important scaling parameters. However, the scaling does not appear to be universal as different correlations are needed for surface roughness with positive, negative and zero skewness. Most surfaces collapse to a single roughness function in the transitionally-rough regime similar to the one developed by Nikuradase (1933) for uniform sand-grain roughness. The exceptions are the wavy surface (low effective slope) and the surface with high positive skewness.

An Assessment of LEWICE Roughness and Convection Enhancement Models

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;During aircraft design and certification, in-flight ice accretions are simulated using ice prediction codes. LEWICE, the ice accretion prediction code developed by NASA, employs a time-stepping procedure coupled with a thermodynamic model to calculate the location, size and shape of an ice accretion. LEWICE has been extensively validated for a wide range of icing conditions. However, continuing improvements to LEWICE predictive capabilities require better understandings of 1) the fundamental physics of turbulent flow generated by ice accretion roughness during an icing event and 2) the mechanisms responsible for convective enhancement of real ice accretion roughness. Recent experiments in the Icing Research Tunnel (IRT) at NASA Glenn Research Center have enabled significant insights into the nature of ice accretion roughness spatial and temporal variations. Other recent investigations have employed scans from the IRT to generate scaled test surfaces to investigate convection enhancement and skin friction interactions of flow over the surfaces with real ice roughness. For this investigation, the measurements of ice roughness characteristics, skin friction, and convection heat transfer enhancement are compared directly to predictions calculated using the LEWICE models. Issues associated with the LEWICE model for predicting laminar-to-turbulent transition are also addressed. The comparisons show that while the LEWICE models significantly overestimate equivalent sand-grain roughness heights and the local skin friction, the resulting LEWICE predictions of convection heat transfer generally agree within 25% of the experimental values for surfaces constructed with low thermal conductivity materials. At each step of the LEWICE model, methods to improve the predictions are provided.&lt;/div&gt;&lt;/div&gt;