Roughness Configuration Research Articles

Experimental study on heat transfer and friction characteristics for hybrid roughened duct used in solar heat collector

ABSTRACT This study presents a novel hybrid roughness configuration for solar heat collectors (SHCs), combining wired roughness and protrusion rib roughness to address the global demand for sustainable energy solutions. By leveraging wired roughness to induce localized turbulence near the absorber plate and protrusion ribs to disrupt the laminar sublayer without adding significant material mass, the proposed design enhances thermal performance while minimizing environmental and economic costs. The experimental investigation evaluates the effects of relative roughness width (W/w), gap width (d/e), and relative rib height (e/D) on the Nusselt number (Nu), friction factor (f), and thermohydraulic performance parameter (THPP). Results highlight the optimal configuration of W/w = 5, d/e = 4, and e/D = 0.045, achieving a maximum THPP of 3.20 at a Reynolds number of 19,000. This configuration significantly improves heat transfer efficiency while maintaining hydraulic resistance within acceptable limits, offering a sustainable and cost-effective pathway to enhanced solar thermal energy systems. These findings provide a robust framework for advancing SHC technology, aligning with global sustainability goals by promoting renewable energy utilization and reducing dependence on fossil fuels.

Enhancing triboelectrification via multiscale roughness dependent thermal dissipation

Polymer-based triboelectric nanogenerators (TENGs) have held promise due to their excellent interfacial conformity and ease of fabrication. However, the role of surface roughness in triboelectricity requires further study. In this study, we have manipulated the nano-/micro-scale roughness configuration in polydimethylsiloxane (PDMS) over a wide range of extents using various sandpaper-based templates. According to the power spectral density analysis, the spatial frequency of template-free PDMS exhibits several distinct bandwidth regions each with different fractal dimensions significantly higher than 2, despite having the lowest roughness value. In contrast, most template-based PDMS shows an entire spatial frequency region that scales nearly with a single power factor corresponding to a fractal dimension as low as 2, despite slight increases in roughness values. Consequently, the surface temperature gradient and output performance of TENG increased, following the trend of fractal dimension and roughness, but the surface potentials have remained almost invariant. However, excessive increases in the surface roughness cause the spatial frequency to be divided once again into several different bandwidth regions with different cutoffs and higher fractal dimensions. These results suggest that the performance of TENG can be controlled by tuning both surface roughness and self-affine properties over multiple scales. Specifically, adhesive interaction becomes dominant on surfaces with lower fractality, enhancing TENG performance due to the expanded contact area. This study sheds light on the relationship among triboelectricity, thermal dissipation, and topography.

Response surface-based optimization of thermal characteristics for frustum roughened solar air heater: An experimental and numerical study

Artificial roughness on the absorber plate of the solar air heater increases its thermal characteristics, thermohydraulic performance, and overall performance of the solar air heater. The roughness and flow parameters selected for artificial roughness play a vital role. Along with selection of geometry, range of roughness parameter at optimum values creates keen interest in research field. The present investigation is in line with the previous work of the authors in which an experimental and numerical simulation was carried out to explore the thermal characteristics of a frustum roughness solar air heater. Roughness configuration such as the relative height of the roughness (e/Dh), the relative diametral ratio(d1/d2), and the roughness to the diameter of the base diameter (e/d1) was calculated to optimize the thermal characteristic. To optimize thermal characteristics (Nusselt number and friction factor) a response surface optimization (RSM) was applied. The suggested design of the experiment (DOE) was performed experimentally and numerically. Data collected from experiment and simulation are used to develop mathematical model using analysis of variance (ANOVA) approach. Nusselt number achieves its maximum optimum value of 256.78 at e/Dh = 0.043, d1/d2 = 2.33, e/d1 = 0.98 and Re = 12500. Friction factor achieves its minimum optimum value of 0.009 at e/Dh = 0.014, d1/d2 = 2.06, e/d1 = 1.44 and Re = 12158.

Criteria for Evaluating the Tribological Effectiveness of 3D Roughness on Friction Surfaces

A new technique for finishing the surfaces of friction pairs has been proposed, which, in combination with the original test method, has shown a significant influence of the initial roughness configuration (surface texture) on friction and wear. Two types of finishing processing of the shaft friction surfaces were compared, and it was found that the friction and wear coefficients differ by more than 2–5 and 2–4 times, respectively. Based on a new methodology for analyzing standard roughness parameters, the tribological efficiency criteria (in the sense of reducing friction and wear) are proposed for the initial state of the friction surface of a radial plane sliding bearing shaft relative to the friction direction, which is consistent with its frictional characteristics. Comparison of the laboratory test results with the surface tribological efficiency criteria showed that these criteria are very promising for controlling existing technologies and optimizing new technologies for friction surface finishing in various friction systems.

Sound absorption of acoustic resonant absorbers with rough oblique perforations

Micro-perforated panel sound absorbers are widely used in the field of noise reduction due to their good sound absorption performance. For micro-perforated panel, the acoustic impedance characteristics of the perforation hole play an extremely important role in the sound absorption performance. Here, the acoustic metamaterial in the form of a perforated panel with rough oblique holes is proposed for low-frequency sound absorption. By introducing surface roughness, the acoustic performance of the oblique perforation holes can be adjusted, and the low-frequency sound absorption performance of the perforated panel sound absorber is improved. The theoretical prediction of sound performance has been verified by finite element simulation, and a good consistency has been achieved. The results show that the introduction of surface roughness effectively reduces the resonant frequency and increases the value of sound absorption peak. The introduction of surface roughness and oblique configuration can provide additional acoustic resistance and reactance for impedance matching with air without increasing the thickness of the structure, resulting in more effective low-frequency sound absorption. Physically, the surface roughness causes the periodic concentration effect of the fluid vibration in the hole, which increases the viscosity loss and improves the sound absorption performance of the sound absorber. The theoretical and the numerical models, as well as the low-frequency perfect sound absorption performance (α>0.99) of the proposed metamaterial, are validated by experimental measurements. This work is of great significance to the application of perforated panel absorbers in low-frequency sound absorption.

Light‐fueled self‐oscillation of liquid crystal polymer network: Effect of photostabilizers

AbstractThe quest for harnessing light‐induced oscillatory motion, inspired by natural oscillations, has become a focus of scientific attention. Researchers are exploring the use of liquid crystal network (LCN) polymers and their integration with compatible materials as soft actuators. Of particular interest is the utilization of photostabilizers, which play a pivotal role in preserving the polymer against photodegradation. For this, three distinct derivatives of Tinuvin were chosen to explore their influence on the light‐fueled oscillatory motion of LCN polymers when exposed to polarized light to examine the impact of molecular orientation on the resultant motion. The research includes a comprehensive analysis of morphology, FT‐IR spectra, and elastic modulus assessments. The findings indicate that the Tinuvin derivatives along with light polarization have an impact on the resulting oscillatory characteristics. The findings include oscillation frequencies spanning from 8.95 to 14.96 Hz and oscillation amplitudes ranging from 0.47 to 1.73 mm. The dipole moment of Tinuvin types influences the oscillation frequency by altering the elastic modulus of the LCN, while its impact on surface roughness and structural configuration affects the resulting oscillation amplitude. Notably, the highest oscillation frequency is observed when all oriented molecules within the LCN respond to 45° polarized light. Investigating the impact of utilized photostabilizers on the polymer structural configuration was although possible through the examination of related FT‐IR specra.

Experimental analysis of the wake behind a small wind-turbine model in yaw

In this work we study the wake of a yawed wind-turbine model immersed in an atmospheric boundary layer (ABL). The ABL is replicated in the wind tunnel by means of a barrier-spires and distributed roughness configuration and is representative of a rural terrain. We quantify the properties of the wake in the horizontal plane at hub height and compare the predictions of available wake models to our data for different yaw angles. It is found that the model based on lifting-line theory performs best in predicting the velocity deficit without the need of tuning the parameters to the current setup. However, the wake deflection is slightly underestimated, most notably at the transition between near and far wake. Furthermore, a comparison with the turbine in a uniform incoming flow highlights the enhanced downward deflection of the wake which results from its interaction with the ABL.

Performance based investigation of inclined spherical ball roughened solar air heater

Heat transfer augmentation for inclined spherical ball roughened solar air heaters has been investigated and reported in this work. The roughness geometry investigated has advantages of both protruded shape and inclined pattern. This investigation covers the determination of all performance parameters viz., thermal, thermohydraulic and exergetic efficiency and its variations as these parameters are essential to justify the practical utility of the roughened system. The impact of roughness on performance is determined and compared to smooth plate air heater at identical flow conditions. Experimental study on spherical ball roughness aligned inclined to flow direction has not been performed to the best of author’s knowledge. The key outcomes were inline with literature as the maximum thermal efficiency for roughened ducts obtained was 81 % compared to 38 % for smooth duct at flow rate of 0.0319 kg/s. With varying roughness configuration, maximum effective efficiency was found 0.67, 0.748, 0.725 and 0.739 respectively at flow rate 0.0372, 0.0391, 0.04 and 0.0418 kg/s. The determination of exergetic efficiency along with thermal and effective efficiency is something that makes present work different from other works since most of the work performed on roughened solar duct system only deals with thermal and effective efficiency determination. The variation in exergetic efficiency with Reynolds number and temperature rise parameter at different roughness geometrical orientations revealed that as Reynolds number changed, both smooth and rough absorber showed maximum exergetic efficiency at same Reynolds number but rough absorber showed maximum exergetic efficiency at temperature rise parameter of 0.0295, 0.0274, 0.031 and 0.045 with maximum exergetic efficiency being 0.0246, 0.037, 0.0262 and 0.0349 respectively.

Study of surface roughness with MHD and couple stress fluid on porous curved annular plates

The purpose of this article is to examine the effect of surface roughness on porous curved annular plates lubricated with couple stress fluid in the presence of magnetic field. The MHD-Stochastic Reynolds equation is derived using Christensen’s stochastic model and applied to predict the squeeze film characteristics of porous curved annular plates. The expressions for squeeze film pressure, load-carrying capacity, and squeeze film time are obtained analytically, the results are discussed for various values of operating parameters, and are plotted graphically. It is found that the squeeze film characteristics of porous curved annular plates are improved using a non-Newtonian fluid in the presence of an external magnetic field. The effect of roughness parameter is to increase (decrease) the squeeze film attributes for azimuthal (radial) roughness configuration as compared to the smooth case. Furthermore, the effect of permeability parameter is to decrease the pressure, load-carrying capacity, and squeeze-film time as compared to the non-porous case.

An experimental investigation on pressure loss and local heat transfer characteristics in additively manufactured ribbed cooling configurations

Pressure loss and transient heat transfer measurements are performed on real-scale turbine cooling configurations at engine-relevant flow conditions. To determine the influence of rib turbulators and surface roughness on heat transfer and pressure loss, five additively manufactured models and one milled model were investigated, which distinguish in their manufacturing-related surface roughness and/or rib turbulator configurations. The experimentally determined friction factors are compared with data from a computed tomography (CT) scan, which was carried out for one of the additively manufactured models. An inverse discrete volume approach is applied, where the temperature response of the accessible outer wall caused by a sudden change in the temperature of the fluid flowing through the channel is measured. This allows the determination of locally resolved heat transfer coefficients on the non-accessible inner surface and provides information about the internal heat transfer processes. From the variety of experiments performed on the six models, the influence of surface roughness, one-sided high-blockade ribs, combination of these two, and the Reynolds number on friction factor and heat transfer coefficient is determined. The effects of roughness and rib configuration are separated from each other, and the expected measurement uncertainties are estimated by means of a sensitivity analysis.

Effect of inclination angle on heat transport properties in two-dimensional Rayleigh–Bénard convection with smooth and rough boundaries

Using direct numerical simulations, two-dimensional tilted Rayleigh–Bénard convection (RBC) is studied in both smooth and roughness-facilitated convection cells of double aspect ratio ( $\varGamma =2$ ) for air as a working fluid. We investigate the effect of inclination angle ( $0^{\circ } \leq \phi \leq 90^{\circ }$ ) on heat flux ( $Nu$ ), Reynolds number ( $Re$ ) and flow structures. In a Rayleigh number range $10^{6}\leq Ra\leq 10^{9}$ , we address the $Ra$ dependence of $Nu(\phi )$ trend. In the smooth case, while greater tilt results in highest heat flux below $Ra=10^{8}$ , $Nu$ drops with $\phi$ monotonically above it (RBC transports heat most efficiently), which explains the different $Nu(\phi )$ trend observed in the previous studies due to $Ra$ dependence (Guo et al., J. Fluid Mech., vol. 762, 2015, pp. 273–287; Shishkina & Horn, J. Fluid Mech., vol. 790, 2016, R3; Khalilov et al., Phys. Rev. Fluids, vol. 3, 2018, 043503). For the smooth case, we identify the control parameters ( $\phi =75^{\circ }$ and $Ra=10^{7}$ ) that yield maximum heat flux (an increment of $18\,\%$ with respect to the level case). On the other hand, among the three roughness set-ups used in the present study, the tallest roughness configuration yields the maximum increment in heat flux ( $25\,\%$ ) in vertical convection ( $\phi =90^{\circ }$ ) at $Ra=10^{6}$ . With increase in $Ra$ , $Re$ changes with $\phi$ marginally in the smooth case, whereas it shows notable changes in its roughness counterpart. We find that the weakening of thermal stratification is related directly to the height of roughness peaks. While $Ra$ delays the onset of thermal stratification (in terms of inclination angle) in the smooth case, an increase in roughness height plays the same role in roughness-facilitated convection cells.

THz beam shaping based on diffractive transformation for forming patterned simulation lightfields and wavefronts

In this paper, the traditional Gerchberg-Saxton (GS) algorithm is used to achieve a key phase recovery in the THz band via common Fourier transformation to realize desired beam manipulation. The main technological process includes a single mask UV-photolithography and a dual-step KOH wet etching, thus enabling a kind of THz diffractive optical element (THz-DOE) with the needed phase-step arrangement. The experimental results about THz beam shaping for generating patterns and wavefronts based on the THz-DOEs are given to demonstrate the feasibility of realizing complex lightfield generation and modulation for THz imaging applications. The developed THz-DOEs exhibit a significant THz amplitude modulation for constructing some particular patterns selected. The non-ideal surface roughness and phase-step configuration of the silicon-based DOEs for both the visible and infrared wavelength ranges is perfectly acceptable in the THz band due to its long wavelength nature. A better imaging exhibition based on the beam modulation of the developed THz-DOEs can be expected since the phase configuration accuracy can be further improved according to the technological and micro-structural parameter optimization.

Digital optofluidic compound eyes with natural structures and zooming capability for large-area fluorescence sensing

Compound eyes are ubiquitous natural biosensors that possess high temporal resolution and large fields of view (FOVs). While for solid materials based artificial imaging systems, flexible zooming ability while keeping the constant FOV is still challenging, as well as the low-cost fabrication. Herein, liquid compound eyes with natural structures are presented that synthesize optofluidics and bionics in a non-trivial manner, which enables the deformation-free zooming and flexible cell fluorescence sensing. Experimental results indicate that the innovatively manufactured bionic template possesses low roughness and uniform lens configuration with more than two thousands units, which endows the eyes with high-quality and low aberration imaging ability. Besides, digital controlled miscible liquids switching enables the focus of ommatidia simultaneously be adjusted from 150 μm to 5 mm with 100° view angle, and without bending the microlens curvature, to avoid FOV changing and image aberration. Due to large FOV and tunable ability, large-area cell fluorescence signal arrays and dynamic cell motion are imaged using this liquid compound eyes. This work presents novel strategy for compound lens manufacture at low-cost, and proposes deformation-free and continuous focus-tuning strategy, offering potentials for numerous applications, including biomedical sensing and adaptive imaging with large FOV.

Effect of nanostructured surface configuration on the interface properties and heat transfer of condensation process of argon inside nanochannels using molecular dynamics simulation

Present work studies the impact of surface structure on the heat transfer and liquid–vapor interface properties of condensation flow formed inside rough nanochannels compared to the smooth ones, using molecular dynamics simulation. Considering rectangular roughness configuration on the walls of nanochannel, simulations are performed under various saturated temperature conditions to obtain density, velocity, and temperature profiles. Furthermore, the effect of rough elements on the mechanism of liquid film formation and condensation rate as well as the thermal properties of two-phase flow has been investigated. It is observed that the amount of the oscillations in the vicinity of the walls at the end of the simulation time in rough nanochannels, is less than in smooth nanochannels. Results show that the effect of rough structures on the distribution of particles as well as velocity profile is greater at a lower temperature. Also, rough surfaces increase heat transfers at the beginning of the condensation process that leads to higher values of thermal conductivity in the rough nanochannel compared to the smooth nanochannel. Moreover, it is demonstrated that the thermal conductivity of argon condensation in rough nanochannel is approximately equivalent to the value of thermal conductivity in smooth nanochannel by adding one copper nanoparticle to the base fluid of argon.

A Computational Fluid Dynamics Investigation of using Large-Scale Geometric Roughness Elements in Open Channels

The hydraulic behavior of the flow can be changed by using large-scale geometric roughness elements in open channels. This change can help in controlling erosions and sedimentations along the mainstream of the channel. Roughness elements can be large stone or concrete blocks placed at the channel's bed to impose more resistance in the bed. The geometry of the roughness elements, numbers used, and configuration are parameters that can affect the flow's hydraulic characteristics. In this paper, velocity distribution along the flume was theoretically investigated using a series of tests of T-shape roughness elements, fixed height, arranged in three different configurations, differ in the number of lines of roughness element. These elements were used to find the best configuration of roughness elements that can be applied to change the flow's hydraulic characteristics. ANSYS Parametric Design Language, APDL, and Computational Fluid Dynamics, CFD, was used to simulate the flow in an open channel with roughness elements. CFD can be used to study the hydrodynamics of open channels under different conditions with inclusive details rather than relying on the costly field and time-consuming. Runs were implemented with different conditions, the discharge, and water depth in upstream and downstream of the flume. T-shape roughness elements with height equal to 3cm placed in three different configurations, two lines, four lines, and fully rough configurations were tested. The results show that the effect of roughness elements increasing with increasing the number of lines of roughness elements. Cases of four lines and fully rough configurations have almost the same hydraulic performance by having the same results of the velocity decrease percentage, which is decreased by approximately about 66% and 61% of the control case's velocity in the zone near the roughness elements consequently. But the difference is that four lines configuration is affected in a part of the test section. This behavior increases the velocity values by about 11% in the other side and by about 10% near the free surface in the case of four lines configuration and increased by about 32% above the roughness elements in a fully rough configuration.

A Computational Fluid Dynamics Investigation of using Large-Scale Geometric Roughness Elements in Open Channels

The hydraulic behavior of the flow can be changed by using large-scale geometric roughness elements in open channels. This change can help in controlling erosions and sedimentations along the mainstream of the channel. Roughness elements can be large stone or concrete blocks placed at the channel's bed to impose more resistance in the bed. The geometry of the roughness elements, numbers used, and configuration are parameters that can affect the flow's hydraulic characteristics. In this paper, velocity distribution along the flume was theoretically investigated using a series of tests of T-shape roughness elements, fixed height, arranged in three different configurations, differ in the number of lines of roughness element. These elements were used to find the best configuration of roughness elements that can be applied to change the flow's hydraulic characteristics. ANSYS Parametric Design Language, APDL, and Computational Fluid Dynamics, CFD, was used to simulate the flow in an open channel with roughness elements. CFD can be used to study the hydrodynamics of open channels under different conditions with inclusive details rather than relying on the costly field and time-consuming. Runs were implemented with different conditions, the discharge, and water depth in upstream and downstream of the flume. T-shape roughness elements with height equal to 3cm placed in three different configurations, two lines, four lines, and fully rough configurations were tested. The results show that the effect of roughness elements increasing with increasing the number of lines of roughness elements. Cases of four lines and fully rough configurations have almost the same hydraulic performance by having the same results of the velocity decrease percentage, which is decreased by approximately about 66% and 61% of the control case's velocity in the zone near the roughness elements consequently. But the difference is that four lines configuration is affected in a part of the test section. This behavior increases the velocity values by about 11% in the other side and by about 10% near the free surface in the case of four lines configuration and increased by about 32% above the roughness elements in a fully rough configuration.

Flow and heat transfer analysis of microchannels structured with rectangular surface roughness

Microchannels structured with hybrid rectangular surface roughness is numerically analyzed for thermal-hydraulic performances of laminar flow. The performances are compared between working fluids of air and water, and under different heating walls and roughness configuration conditions at constricted Reynolds number (Rec) of 50–250. The results show that both constricted friction factor and Nusselt number of water are slightly higher than those of air. The performance index (η) of air increases with increasing Rec but is decreasing for water, and the average deviation between them is 11 %. The convective heat transfer with one rough heated wall is almost 40 % lower than that of two heated walls, whereas the constricted friction factor has negligible difference. The channel structured with regular surface roughness (constant roughness height and with air flow) shows about 14 % higher values of η than those of the respective channel with hybrid surface roughness at Rec = 200. Structured surface roughness with suitable configuration can be an innovative way to improve the heat dissipation for electronic cooling.

Non-planarity, scale-dependent roughness and kinematic properties of the Pidima active normal fault scarp (Messinia, Greece) using high-resolution terrestrial LiDAR data

Terrestrial LiDAR (TLS), photogrammetric and field data were collected during the years 2014, 2015 and 2017, at a 20-m long limestone scarp locality along the N–S striking, 70° west-dipping, active Pidima fault (Messinia, SW Peloponnese, Greece). This locality presents an artificially exhumed portion of an active fault plane, thus providing a unique opportunity to study kinematics and limestone scarp morphology (including its curvature and roughness). The survey of 2015 offered a high-density point cloud of the fault scarp with 6-mm working resolution, creating a very close to real life representation 3D model. We found that scarp geometry is non-planar with increasing convexity up-dip and increasing northwesterly dip-directions from north towards south, along strike. Non-planarity is also recorded from the morphological data along strike with the appearance of several, slip-parallel troughs and ridges with a mean distance (half-wavelength) of 0.75 m. The fault-plane roughness (absolute values) is scale dependent. The roughness configuration attains a slip-parallel pattern for observation scales above 1-cm. At observations scales less than 1-cm a slip-normal pattern is weakly visible. The obtained TLS results agree with field data (manual compass measurements) and macroscopic observations. We infer an earthquake magnitude of 6.2 ± 0.2 for the last event along the Pidima fault based on the measured thickness of the smooth stripe that was imaged by t-LiDAR along the non-exhumed surface of the scarp.

Roof-level large- and small-scale coherent structures in a street canyon flow

The characteristics of large- and small-scale turbulent motions at roof-level in a street canyon flow were experimentally investigated along with their spatio-temporal organization and their mutual interaction in this region. Quadrant analysis was conducted to identify sweep and ejection events, whilst a spanwise spatial filter was used to identify the large-scale motions in the flow. The present analysis was conducted for six configurations; three upstream roughness arrays and two canyon width (W) to height (h) aspect ratios (AR = W/h = 1 and 3). The upstream roughness arrays consisted of three-dimensional cubes (plan area density, λp = 25%), 1h spaced two-dimensional bars (λp = 50%, corresponding to the skimming-flow regime) and 3h spaced two-dimensional bars (λp = 25%, corresponding to the wake-interference flow regime). It was found that the roughness configuration has a strong effect on the size of the quadrant events in the street canyon, with the wake-interference flow regimes producing significantly larger sweep and ejection events than the skimming-flow regimes. Upstream wake-interference flow regimes were found to have a significantly greater temporal correlation than in the skimming-flow regimes. Large-scale streamwise and spanwise velocity components were found to have large spatio-temporal correlation integral scales, in agreement with the large-scale structures existing in the overlying boundary layer. It was also confirmed that there is a coupling between large- and small-scales at roof-level of the street canyon. While sweep and ejection events were found to be due to the dynamical contribution of the small-scales, their occurrence is shown to be coupled with that of large-scale low- and high-momentum regions, respectively.

Turbulization of a wake behind the double roughness elements in a hypersonic boundary layer

The study of the effect of close placing of two individual elements of roughness with the cylindrical shape on the nature of laminar-turbulent transition were performed. Experiments were performed for a blunt conical model with the radius 9 mm at Mach number M = 5. These double elements of roughness with different heights and divergence angles between elements were allocated at the blunted nose of the tested model. Measurements with a hot-wire anemometer provided information about the averaged and unsteady parameters of the boundary layer in the wake behind the roughness elements. For all types of roughness, we confirmed existence of an effective height of roughness; for the heights above the effective one, we observe deformation of the boundary layer margin and the growth of non-equilibrium in the wake. Process of turbulization behind a double roughness element (similar to the case of single roughness) is accompanied by the generation of longitudinal vortices and by the deformation of the velocity profile. Depending on this deformation, pulsations in the wake either enhance or decline. In contrast to the single roughness configuration, the double element roughness decreases the mass flowrate for a narrow range of angles (and fullness of the boundary layer profile is more significant). Meanwhile, flow turbulization occurs right behind the single element of turbulization, for the case of double element turbulization, the main gain in the mass flowrate occurs with the wake (developing from the boundary between twin elements). Roughness has significant influence on unsteady characteristics of the boundary layer (when the height of roughness element is lower than the effective height). The wake downstream the double elements roughness exhibits the interaction between vortices, and this reduces the effective Reynolds number (compared to the case of single roughness).