Distributed Surface Roughness Research Articles

Hydrokinetic power conversion using Flow Induced Vibrations with nonlinear (adaptive piecewise-linear) springs

A design, adaptive to the flow velocity, is developed for a hydrokinetic energy converter based on an oscillator undergoing Flow Induced Vibrations (FIVs); primarily Vortex Induced Vibrations (VIV) and galloping. This Alternating-Lift Technology implements a nonlinear spring to passively optimize the oscillator. The FIV of a single, rigid, circular cylinder, with distributed surface roughness, suspended on end-springs with piecewise linear continuous restoring force, are studied for Reynolds number 24,000 ≤ Re ≤ 120,000. Linear viscous damping for energy harnessing and different piecewise linear spring functions are used as parameters. The harnessed power envelope is established based on the results of linear spring stiffness and two nonlinear piecewise stiffness functions. Selective roughness is applied to enhance FIV and increase the hydrokinetic energy captured by the converter at higher Reynolds numbers. The second generation of a virtual spring-damping system (Vck), developed in the Marine Renewable Energy Laboratory (MRELab), enables embedded, real-time, computer-controlled change of viscous damping and spring stiffness for fast implementation of oscillator particulars. Experimental results for amplitude response, frequency response, energy harvesting, and efficiency are presented and discussed. All experiments are conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main conclusions are: (1) Each nonlinear, piecewise-linear, stiffness function has its own merits in power harnessing; the differences lie in the FIV characteristics of the different regions of the flow. (2) The nonlinear spring converter can harness energy from flows as low as 0.275 m/s with no upper limit. (3) The new adaptive function exhibits higher harnessed power in the upper VIV branch, transition from VIV to galloping, and fully developed galloping regions. (4) The FIV response is predominantly periodic for all nonlinear spring functions used. (5) Optimal power harnessing is achieved by changing the nonlinear piecewise spring function and the linear viscous damping. (6) The optimally harnessed power envelope is established according to the experimental results of the linear and nonlinear stiffness springs; four performance zones are established based on FIV.

Transition due to Roughness on Blunt Capsule: Comparison with Transient Growth Correlation

The effect of uniformly distributed surface roughness on the onset of boundary-layer transition for a capsule geometry was experimentally investigated in a conventional hypersonic blowdown wind tunnel. The thin-walled scale model of the Orion Crew Exploration Vehicle used in this study was mounted at a 28 deg angle of attack in the Adjustable Contour Expansion wind tunnel. A test matrix of 24 runs was developed to experimentally estimate the location of transition and to relate the flow properties at this location and the roughness height to established transient growth scaling. Tests were conducted for four different Reynolds numbers at six quasi-uniform surface roughness height distributions. The tunnel was nominally set to Mach 6.0 and a total temperature of 430 K for each run. Infrared thermometry was used to measure the surface temperature distribution. Heat transfer rates generated through a series of surface temperature maps were processed into normalized Stanton number centerline plots. These plots directly indicated the boundary-layer transition location. Analysis of the results indicated that the experimental data matched the established transient growth scaling.

Swept-wing boundary-layer transition at various external perturbations: Scenarios, criteria, and problems of prediction

This experimental work is devoted to a parametric study of characteristics of the laminar-turbulent transition occurred in a model of a crossflow-dominated swept-wing boundary layer. The experiments are performed in 39 different regimes with low and elevated freestream turbulence levels both in the absence and in the presence of steady freestream vortices of various spanwise scales. The measurements are performed at several values of freestream velocity for two types of distributed surface roughness. Transition scenarios and criteria of turbulence onset are investigated and the problem of transition prediction is examined. It is shown that in all studied cases, the turbulence onset starts with the appearance of local high-frequency secondary instability of the base flow perturbed by primary instability modes. It is found that the 30% level of the threshold zero-to-peak amplitude of combined boundary-layer disturbances (i.e., the steady ones plus unsteady ones) can be used in all studied cases as a simplified turbulence-onset criterion, which was found in previous experiments at other flow and disturbance conditions. A very important, often decisive role of extremely weak steady free-steam disturbances produced by turbulence generating grids is investigated. A generalized combined variable N-factor method of transition prediction is suggested.

On predicting receptivity to surface roughness in a compressible infinite swept wing boundary layer

The receptivity of crossflow disturbances on an infinite swept wing is investigated using solutions of the adjoint linearised Navier-Stokes equations. The adjoint based method for predicting the magnitude of stationary disturbances generated by randomly distributed surface roughness is described, with the analysis extended to include both surface curvature and compressible flow effects. Receptivity is predicted for a broad spectrum of spanwise wavenumbers, variable freestream Reynolds numbers, and subsonic Mach numbers. Curvature is found to play a significant role in the receptivity calculations, while compressible flow effects are only found to marginally affect the initial size of the crossflow instability. A Monte Carlo type analysis is undertaken to establish the mean amplitude and variance of crossflow disturbances generated by the randomly distributed surface roughness. Mean amplitudes are determined for a range of flow parameters that are maximised for roughness distributions containing a broad spectrum of roughness wavelengths, including those that are most effective in generating stationary crossflow disturbances. A control mechanism is then developed where the short scale roughness wavelengths are damped, leading to significant reductions in the receptivity amplitude.

Hydrokinetic energy conversion by two rough tandem-cylinders in flow induced motions: Effect of spacing and stiffness

Flow Induced Motions (FIMs) of rigid circular cylinders, and particularly VIV (Vortex Induced Vibrations) and galloping, are induced by alternating lift. The VIVACE (VIV for Aquatic Clean Energy) Converter uses single or multiple cylinders, in tandem, on elastic end-supports, in synergistic FIM, to convert MHK energy to electricity. Selectively distributed surface roughness is applied to enhance FIM and increase efficiency. In this paper, two cylinders are used in tandem with center-to-center spacing of 1.57, 2.0 and 2.57 diameters, harnessing damping ratio 0.00<ζ < 0.24, for Reynolds number 30,000 ≤ Re ≤ 120,000. The virtual spring-damping system Vck in the Marine Renewable Energy Laboratory (MRELab) enables embedded computer-controlled change of viscous-damping and spring-stiffness for fast and mathematically correct oscillator realization, without including the hydrodynamic force in the closed control loop. Experimental results for oscillatory response, energy harvesting, and efficiency are presented and the envelope of optimal power is derived. All the experiments were conducted in the Low Turbulence Free Surface Water (LTFSW) Channel of the MRELab of the University of Michigan. The main conclusions are: (1) For the tested cylinder spacing, two cylinders harness power is between 2.56 and 13.49 times the power of a single cylinder, the efficiency of two cylinders is between 2.0 and 6.68 of a single cylinder. (2) The MHK power harnessed by the upstream cylinder is increased by up to 100%, affected by the downstream cylinder. (3) The MHK power harnessed by the downstream cylinder and its FIM are affected to a lesser extent by the interaction. (4) VIVACE can harness energy from flows as slow as 0.4 m/s with no upper limit in flow velocity. (5) Close spacing and high spring stiffness yield highest harnessed power. (6) The optimal harnessed power shifts to softer springs as spacing increases.

Computational and Experimental Assessment of Turbulence Stimulation on Flow Induced Motion of a Circular Cylinder

Vortex-induced vibrations (VIVs) are highly nonlinear and it is hard to approach the problem analytically or computationally. Experimental investigation is therefore essential to address the problem and reveal some physical aspects of VIV. Although computational fluid dynamics (CFDs) offers powerful methods to generate solutions, it cannot replace experiments as yet. When used as a supplement to experiments, however, CFD can be an invaluable tool to explore some underlying issues associated with such complicated flows that could otherwise be impossible or very expensive to visualize or measure experimentally. In this paper, VIVs and galloping of a cylinder with selectively distributed surface roughness—termed passive turbulence control (PTC)—are investigated experimentally and computationally. The computational approach is first validated with benchmark experiments on smooth cylinders available in the literature. Then, experiments conducted in the Marine Renewable Energy Laboratory (MRELab) of the University of Michigan are replicated computationally to visualize the flow and understand the effects of thickness and width of roughness strips placed selectively on the cylinder. The major outcomes of this work are: (a) Thicker PTC initiates earlier galloping but wider PTC does not have a major impact on the response of the cylinder and (b) The amplitude response is restricted in VIV due to the dead fluid zone attached to the cylinder, which is not observed in galloping.

Combined experimental and numerical investigations on the roughness effects on the aerodynamic performances of LPT blades

The aerodynamic performance of a high-load low-pressure turbine blade cascade has been analyzed for three different distributed surface roughness levels (Ra) for steady and unsteady inflows. Results from CFD simulations and experiments are presented for two different Reynolds numbers (300000 and 70000 representative of take-off and cruise conditions, respectively) in order to evaluate the roughness effects for two typical operating conditions.

Roughness receptivity and shielding in a flat plate boundary layer

Surface roughness can affect boundary layer transition by acting as a receptivity mechanism for transient growth. While experiments have investigated transient growth of steady disturbances generated by discrete roughness elements, very few have studied distributed surface roughness. Some work predicts a ‘shielding’ effect, where smaller distributed roughness displaces the boundary layer away from the wall and lessens the impact of larger roughness peaks. This work describes an experiment specifically designed to study this effect. Three roughness configurations (a deterministic distributed roughness patch, a slanted rectangular prism, and the combination of the two) were manufactured using rapid prototyping and installed flush with the wall of a flat plate boundary layer. Naphthalene flow visualization and hotwire anemometry were used to characterize the boundary layer in the wakes of the different roughness configurations. Distributed roughness with roughness Reynolds numbers ($\mathit{Re}_{kk}$) between 113 and 182 initiated small-amplitude disturbances that underwent transient growth. The discrete roughness element created a pair of high- and low-speed steady streaks in the boundary layer at a sub-critical Reynolds number ($\mathit{Re}_{kk}=151$). At a higher Reynolds number ($\mathit{Re}_{kk}=220$), the discrete element created a turbulent wedge 15 boundary layer thicknesses downstream. When the distributed roughness was added around the discrete roughness, the discrete element’s wake amplitude was decreased. For the higher Reynolds number, this provided a small but measurable transition delay. The distributed roughness redirects energy from longer spanwise wavelength modes to shorter spanwise wavelength modes. The presence of the distributed roughness also decreased the growth rate of secondary instabilities in the roughness wake. This work demonstrates that shielding can delay roughness-induced transition and lays the ground work for future studies of roughness-induced transition.

RANS Simulation Versus Experiments of Flow Induced Motion of Circular Cylinder With Passive Turbulence Control at 35,000 &lt; RE &lt; 130,000

Two-dimensional (2D) Unsteady Reynolds-Averaged Navier–Stokes equations (URANS) equations with the Spalart–Allmaras turbulence model are used to simulate the flow and body kinematics of the transverse motion of spring-mounted circular cylinder. The flow is in the high-lift TrSL3 regime of a Reynolds number in the range 35,000 &lt; Re &lt; 130,000. Passive turbulence control (PTC) in the form of selectively distributed surface roughness is used to alter the cylinder flow induced motion (FIM). Simulation is performed using a solver based on the open source Computational Fluid Dynamics (CFD) tool OpenFOAM, which solves continuum mechanics problems with a finite-volume discretization method. Roughness parameters of PTC are chosen based on tests conducted in the Marine Renewable Energy Lab (MRELab) of the University of Michigan. The numerical tool is first tested on smooth cylinder in vortex-induced vibration (VIV) and results are compared with available experimental measurements and URANS simulations. For the cylinder with PTC cases, the sandpaper grit on the cylinder wall is modeled as a rough-wall boundary condition. Two sets of cases with different system parameters (spring, damping) are simulated and the results are compared with experimental data measured in the MRELab. The amplitude ratio curve shows clearly three different branches, including the VIV initial and upper branches, and a galloping branch. The numerical branches are similar to those observed experimentally. Frequency ratio, vortex patterns, transitional behavior, and lift are also predicted well for PTC cylinders at such high Reynolds numbers.

Enhancement of flow-induced motion of rigid circular cylinder on springs by localized surface roughness at 3×104≤Re≤1.2×105

Passive turbulence control (PTC) in the form of selectively distributed surface roughness is applied on a rigid circular cylinder on two end-springs. The cylinder is placed perpendicular to a uniform steady flow and the cylinder response is measured experimentally for 3×104≤Re≤1.2×105 and broad ranges of the main PTC parameters. PTC consists of two roughness strips placed parallel to the cylinder axis and symmetrically to the flow with thickness on the order of the boundary layer thickness. Different flow-induced motion (FIM) is observed depending primarily on the circumferential location of the two strips. FIM enhancement is studied in this paper in the soft galloping and the two hard galloping zones identified in the PTC-to-FIM Map. In galloping, amplitudes of oscillation reach 2.9 times the cylinder diameter limited only by the free-surface and bottom-boundary effects of the experimental facility. The galloping range follows the VIV range thus expanding dramatically the FIM range. Enhancement of FIM is needed to convert more hydrokinetic energy to mechanical and subsequently to electrical energy over broad velocity range. Use of laser broad field-of-view visualization reveals very different vortex structures between VIV and galloping confirming the fundamentally different driving mechanism of these two FIM kinds.

Stabilization of a swept-wing boundary layer by distributed roughness elements

AbstractThe stabilization of a swept-wing boundary layer by distributed surface roughness elements is studied by performing direct numerical simulations. The configuration resembles experiments studied by Saric and coworkers at Arizona State University, who employed this control method in order to delay transition. An array of cylindrical roughness elements are placed near the leading edge to excite subcritical cross-flow modes. Subcritical refers to the modes that are not critical with respect to transition. Their amplification to nonlinear amplitudes modifies the base flow such that the most unstable cross-flow mode and secondary instabilities are damped, resulting in downstream shift of the transition location. The experiments by Saric and coworkers were performed at low levels of free stream turbulence, and the boundary layer was therefore dominated by stationary cross-flow disturbances. Here, we consider a more complex disturbance field, which comprises both steady and unsteady instabilities of similar amplitudes. It is demonstrated that the control is robust with respect to complex disturbance fields as transition is shifted from 45 to 65 % chord.

Semi-empirical material removal rate distribution model for SiO2 chemical mechanical polishing (CMP) processes

► We propose a material removal rate (MRR) distribution model for SiO 2 CMP. ► Contact mechanics are used for modeling the average MRR of SiO 2 film. ► A spatial parameter for MRR distribution is obtained from semi-empirical study. ► The MRR distribution model consists of the average MRR and spatial parameter. ► The proposed model includes the newly proposed line density of particles. A novel semi-empirical model was developed for predicting the material removal rate (MRR) during chemical mechanical polishing (CMP) based on the following assumptions: plastic contact at the wafer–particle interface, elastic contact at the pad–particle interface, a particle size distribution, and a randomly distributed surface roughness of the polishing pad. The proposed model incorporates the effects of particle size, concentration, and distribution, as well as the slurry flow rate, pad surface topography, material properties, and chemical reactions during the silicon dioxide (SiO 2 ) CMP. To obtain the unknown parameters and ensure the validity of the model, a SiO 2 CMP experiment was conducted by using various-sized CMP slurries. The spatial distribution of the MRRs is expressed with respect to the normal contact stress distribution and the relative velocity distribution. The proposed MRR model can be used for the development of a CMP simulator, the optimization of CMP process parameters, and the design of next-generation CMP machines.

Analysis of Material Removal Rate and Surface Roughness on Advanced Ceramics Materials Micro-Hole Drilling

This study discussed the rotary ultrasonic machining (RUM) parameters and tool geometry parameters of optimized hole wall quality, and obtained better and uniformly distributed surface roughness of deep hole walls. The experimental configuration of this study was planned using Taguchi orthogonal array, where the hole diameter, spindle speed, feed rate and power were taken as the experimental factors. Drilling machining experiment was carried out using the parameter combination configured by the experimental design, and wall surface roughness of the drilled holes was measured. A relation model between the average surface roughness and standard deviation of the hole walls and processing parameters and tool geometry were established, so as to achieve optimized surface roughness of the overall deep hole walls. The optimization process respectively aimed at the minimum surface roughness and maximum metal removal rate. The experimental results proved that application of the optimized parameter combination in rotary ultrasonic machining of hole processing can obtain better and more uniform hole wall surface quality.The effect of control factors on the hole quality was discussed from the respects of the straightness deviation, minimum surface roughness and maximum metal removal rate of related factors on the hole when the Taguchi method was applied to rotary ultrasonic machining of hole drilling. The results showed that the level of straightness factors of produced holes can be fully presented. Keywords-Advanced Ceramics; Rotary Ultrasonic Machining; Surface Roughness; Material Removal Rate; Holes Deviation; Taguchi Method

Direct numerical simulations of roughness-induced transition in supersonic boundary layers

AbstractDirect numerical simulations are used to study the laminar to turbulent transition of a Mach 2.9 supersonic flat plate boundary layer flow due to distributed surface roughness. Roughness causes the near-wall fluid to slow down and generates a strong shear layer over the roughness elements. Examination of the mean wall pressure indicates that the roughness surface exerts an upward impulse on the fluid, generating counter-rotating pairs of streamwise vortices underneath the shear layer. These vortices transport near-wall low-momentum fluid away from the wall. Along the roughness region, the vortices grow stronger, longer and closer to each other, and result in periodic shedding. The vortices rise towards the shear layer as they advect downstream, and the resulting interaction causes the shear layer to break up, followed quickly by a transition to turbulence. The mean flow in the turbulent region shows a good agreement with available data for fully developed turbulent boundary layers. Simulations under varying conditions show that, where the shear is not as strong and the streamwise vortices are not as coherent, the flow remains laminar.

VIV and galloping of single circular cylinder with surface roughness at 3.0×10 4≤ Re≤1.2×10 5

Passive Turbulence Control (PTC) in the form of selectively distributed surface roughness is used to alter Flow Induced Motion (FIM) of a circular cylinder in a steady flow. The objective is to enhance FIM's synchronization range and amplitude, thus maximizing conversion of hydrokinetic energy into mechanical energy by oscillator in vortex-induced vibration and/or galloping. Through additional viscous damping, mechanical energy is converted to electrical harnessing clean and renewable energy from ocean/river currents. High Reynolds numbers ( Re) are required to reach the high-lift TrSL3 (Transition-Shear-Layer-3) flow regime. PTC trips flow separation and energizes the boundary layer, thus inducing higher vorticity and consequently lift. Roughness location, surface coverage, and size are studied using systematic model tests with broad-field laser visualization at 3.0×10 4< Re<1.2×10 5 in the low-turbulence free-surface water-channel of the Marine Renewable Energy Laboratory of the University of Michigan. Test results show that 16° roughness coverage is effective in the range (10°–80°) inducing reduced vortex-induced vibration (VIV), enhanced VIV, or galloping. Range of synchronization may increase or decrease, galloping amplitude of oscillation reaches three diameters; wake structures change dramatically reaching up to ten vortices per cycle. Conversion of hydrokinetic energy to mechanical is enhanced strongly with proper PTC.

Implementation of Menter's Transition Model on an Isolated Natural Laminar Flow Nacelle

This study evaluates the implementation of Menter's γ-Re θ Transition Model within the CFX12 solver for turbulent transition prediction on a natural laminar flow nacelle. Some challenges associated with this type of modeling have been identified. The computational fluid dynamics transitional flow simulation results are presented for a series of cruise cases with freestream Mach numbers ranging from 0.8 to 0.88, angles of attack from-2 to 0°, and mass flow ratios from 0.60 to 0.75. These were validated with a series of wind-tunnel tests on the nacelle by comparing the predicted and experimental surface pressure distributions and transition locations. A selection of the validation cases are presented in this paper. In all cases, computational fluid dynamics simulations agreed reasonably well with the experiments. The results indicate that Menter's γ-Re θ Transition Model is capable of predicting laminar boundary-layer transition to turbulence on a nacelle. Nonetheless, some limitations exist in both the Menter's γ-Re θ Transition Model and in the implementation of the computational fluid dynamics model. The implementation of a more comprehensive experimental correlation in Menter's γ-Re θ Transition Model, preferably the ones from nacelle experiments, including the effects of compressibility and streamline curvature, is necessary for an accurate transitional flow simulation on a nacelle. In addition, improvements to the computational fluid dynamics model are also suggested, including the consideration of varying distributed surface roughness and an appropriate empirical correction derived from nacelle experimental transition location data.

Hypersonic viscous flow over large roughness elements

Viscous flow over discrete or distributed surface roughness has great implications for hypersonic flight due to aerothermodynamic considerations related to laminar–turbulent transition. Current prediction capability is greatly hampered by the limited knowledge base for such flows. To help fill that gap, numerical computations are used to investigate the intricate flow physics involved. An unstructured mesh, compressible Navier–Stokes code based on the space–time conservation element, solution element (CESE) method is used to perform time-accurate Navier–Stokes calculations for two roughness shapes investigated in wind tunnel experiments at NASA Langley Research Center. It was found through 2D parametric study that at subcritical Reynolds numbers, spontaneous absolute instability accompanying by sustained vortex shedding downstream of the roughness is likely to take place at subsonic free-stream conditions. On the other hand, convective instability may be the dominant mechanism for supersonic boundary layers. Three-dimensional calculations for both a rectangular and a cylindrical roughness element at post-shock Mach numbers of 4.1 and 6.5 also confirm that no self-sustained vortex generation from the top face of the roughness is observed, despite the presence of flow unsteadiness for the smaller post-shock Mach number case.

Features of X-ray diffraction on sapphire single crystals with a nanostructured surface

Single-crystal sapphire plates with stepped nanostructures on the surface produced by thermal treatment and with a stochastically distributed surface roughness were studied by X-ray diffraction. It was found that the angular dependencies of intensities and half-widths of the curves of diffraction reflection measured under the Bragg geometry conditions appreciably changed versus the mutual orientation of the direction of propagation of the X-ray beam and the position of the steps, forming nanostructures on the surfaces of sapphire plates.

Three-dimensional instabilities of laminar flow in a rough channel and the concept of hydraulically smooth wall

Flow in a channel with distributed surface roughness is considered. Results of the linear stability analysis show that the presence of the roughness destabilizes the traveling-wave instability as well as introduces a new instability that manifests itself in the form of streamwise vortices. The critical conditions for the occurrence of both instabilities are given for different classes of roughness shape. It is shown that these conditions can be predicted with a reasonable accuracy in the case of an arbitrary (but Fourier transformable) roughness by considering only the leading Fourier mode (wavy-wall model). It is argued that the onset of instabilities provides a decision mechanism that determines whether a particular rough wall can be viewed as being hydraulically smooth in the case of transitional flows.

Flow management using natural instabilities

Accidental introduction of a small disturbance into an unstable flow leads to a large change in the form of the motion, e.g., laminar-turbulent transition. We wish to explore this phenomenon in the design of flow management strategies. We want to modulate the flow using a small input of external energy in such a way that the flow evolves through a natural instability process to a new, more desirable form. This presentation is focused on the use of distributed surface roughness for such purposes.