Laminar Inflow Research Articles

Validation of Aeroelastic Actuator Line for Wind Turbine Modelling in Complex Flows

The actuator line method is a widely used technique to model wind turbines in computational fluid dynamics, as it significantly reduces the required computational expense in comparison to simulations using geometrically resolved blades. Actuator line coupled to an aeroelastic solver enables not only the study of detailed wake dynamics but also aeroelastic loads, flexible blade deformation and how this interacts with the flow. Validating aeroelastic actuator line predictions of blade loading, deflection and turbine wakes in complex inflow scenarios is particularly relevant for modern turbine designs and wind farm studies involving realistic inflows, wind shear or yaw misalignment. This work first implements a vortex-based smearing correction in an aeroelastic coupled actuator line, and performs a grid resolution and smearing parameter study which demonstrates significant improvement in the blade loading and in the numerical dependencies of predicted thrust and power output. A validation is then performed using a 2.3 MW turbine with R = 40 m radius, comparing against blade resolved fluid-structure interaction simulations and full-scale measurement data, in both laminar and turbulent inflows including both high shear and high yaw misalignment. For an axisymmetric laminar inflow case, the agreement between blade resolved and actuator line simulations is excellent, with prediction of integrated quantities within 0.2%. In more complex flow cases, good agreement is seen in overall trends but the actuator line predicts lower blade loading and flapwise deflection, leading to underpredictions of thrust by between 5.3% and 8.4%. The discrepancies seen can be attributed to differences in wake flow, induction, the reliance of the actuator line on the provided airfoil data and the force application into the computational domain. Comparing the wake between coupled actuator line and blade resolved simulations for turbulent flow cases also shows good agreement in wake deficit and redirection, even under high yaw conditions. Overall, this work validates the implementation of the vortex-based smearing correction and demonstrates the ability of the actuator line to closely match blade loading and deflection predictions of blade resolved simulations in complex flows, at a significantly lower computational cost.

Using The Helix Mixing Approach On Floating Offshore Wind Turbines

In recent years dynamic induction control has shown great potential in reducing wake-to-turbine interaction by increasing the mixing in the wake. With these wake mixing methods the thrust force will vary in time. If applied to a floating offshore wind turbine, it will cause the platform to move. In this paper the effect of the Helix mixing approach on a DTU10MW turbine on the TripleSpar platform and its wake is evaluated. When the Helix mixing approach is applied at Strouhal equal to 0.25, the yaw movement is excited close to the eigenfrequency of the platform resulting in significant yaw angles for small blade pitch angles. To understand the impact of the motion on the wake, the yaw motion is simulated using the free wake vortex method as implemented in Qblade. Under laminar inflow, results show that the windspeed at a distance of 5 rotor diameters downstream can be increased by up to 10% compared to a fixed-bottom turbine.

Experimental Investigation on the Effect of Lateral Turbine Spacing on Interactions of Wakes

In recent years, development in wind energy has put great attention on the manifold possibilities of increasing wind farm efficiencies. Motivated by multiple positive effects dualrotors promise at controlling and mitigating wakes, it is goal of this work to gain experimental insight on the characteristics of a double-rotor wake in dependency of the lateral spacing between the turbines. Additionally, the influence of the rotational direction of the model wind turbine showed to be of great relevance and will be studied under laminar inflow conditions between 4 – 7 ms−1. An array of 21 hot-wires at a sampling frequency of 6 kHz will be traversing until 8 D downstream of the double rotor plane to allow for one- and two-point statistics in the wake interaction region. It will be presented that both lateral distance and direction of rotation has great effect on the entrainment of undisturbed air into the dual wake. In terms of turbine control, decreasing the lateral distance increases the generated power by 4-9%. The knowledge on turbulence structures and their decay in the mixing area could be used to adapt, optimise and implement control strategies in wind farms for a better entrainment and thus an overall quicker wake recovery.

Iteration-Based Recycling and Reshaping Method for Inflow Turbulence Generation and Its Evaluation

For the analysis of the aerodynamic characteristics of the buildings immersed in the atmospheric boundary layer (ABL), it is necessary to generate a turbulence velocity field with similar temporal and special characteristics to the ABL to obtain a reliable result. In this paper, an improved precursor simulation method called the recycling and reshaping method (RRM) is proposed to generate a turbulent boundary layer in an LES model. The laminar inflow is firstly disturbed by the virtual roughness blocks realized by adding drag force term in the momentum equation, then the inflow velocity profile is reshaped every several steps to adjust the streamwise velocity profile in the downstream target area to meet the requirements. The final turbulence field generated by RRM with virtual roughness blocks is in good agreement with the target velocity conditions. Then, the simulation of the wind-induced pressure on an isolated low-rise building surface is carried out, using the generated turbulence boundary layer as inflow. The comparison between numerical results and TPU aerodynamic database shows that the time-averaged wind-induced surface pressure obtained by LES can be considered in good accordance with the measurements over the whole building surface. However, the non-ignorable deviations for the fluctuating pressure result in the flow separation corners still exist.

Numerical Investigation of Droplet Properties of a Liquid Jet in Supersonic Crossflow

The atomization process of a liquid jet in supersonic crossflow with a Mach number of 1.94 was investigated numerically under the Eulerian-Lagrangian scheme. The droplet stripping process was calculated by the KH (Kelvin-Helmholtz) breakup model, and the secondary breakup due to the acceleration of shed droplets was calculated by the combination of the KH breakup model and the RT (Rayleigh-Taylor) breakup model. In our research, the existing KH-RT model was modified by optimizing the empirical constants incorporated in this model. Moreover, it was also found that the modified KH-RT breakup model is applied better to turbulent inflow of a liquid jet than laminar inflow concluded from the comparisons with experimental results. To validate the modified breakup model, three-dimensional spatial distribution and downstream distribution profiles of droplet properties of the liquid spray in the Ma = 1.94 airflow were successfully predicted in our simulations. Eventually, abundant numerical cases under different operational conditions were launched to investigate the correlations of SMD (Sauter Mean Diameter) with the nozzle diameter as well as the airflow Mach number, and at the same time, modified multivariate power functions were developed to describe the correlations.

An investigation on the forced convection heat transfer in the gap of two rotating disks with laminar inflow

AbstractThis study presents forced convection in the gap between two rotating disks with the laminar radial inward flow. The disk surfaces are held at a constant temperature different from the temperature of the fluid flowing. The disks' surfaces may also receive a heat flux. The temperature of the fluid flowing in the gap is predicted by solving the coupled equations of momentum, energy, and continuity in cylindrical coordinate numerically. The finite difference method is used to discretize the energy equation into nonlinear algebraic equations. The tridiagonal matrix algorithm is employed to solve the resulting algebraic equations. Predominantly, throughflow Reynolds number, rotational Reynolds number, gap ratio, speed ratio, and Peclet number are the parameters that affect the temperature distribution for the fixed disk temperature and for the heat flux boundary conditions. The Nusselt number compares reasonably well with the numerical results of other investigators. The heat flow into the fluid is higher for corotating disks than for contrarotating disks for both constant temperatures as well as heat flux boundary conditions. This is the first investigation that predicts temperature distribution due to forced convection in the gap of two rotating disks with laminar inflow.

High-Fidelity Simulations of Multi-Jet Impingement Cooling Flows

Abstract This article shows the first parametric study on turbulent multi-jet impingement cooling flows using large-eddy simulations (LES). We focus on assessing the influence of the inter-jet distance and the cross-flow conditions on the heat transfer at the impingement wall. The LES setup is thoroughly validated with both experimental and direct numerical simulation data, showing an excellent agreement. The inter-jet distance effect on the heat transfer is studied comparing three different distances, where the full Nusselt number profile decreases in amplitude when the jet distance is increased. To evaluate the cross-flow effects, we prescribe both laminar and turbulent inflow conditions at different cross-flow magnitudes ranging between 20% and 40% of the impinging jet speed. Large cross-flow intensities cause a jet deflection that reduces the maxima in the Nusselt number distribution, and it increases the heat transfer in the areas of the wall less affected by the jet impingement. Adding realistic turbulent fluctuations to the inflow enhances the cross-flow effects on the heat transfer at the impingement wall.

On the use of synthetic inflow turbulence for scale-resolving simulations of wetted and cavitating flows

The Delft Twist 11 Hydrofoil is a common test case for investigating the interaction between turbulence and cavitation modelling in computational fluid dynamics. Despite repeated investigations, results reported for the lift and drag coefficient are accompanied by significant uncertainties, both in experimental and numerical studies. When using scale-resolving approaches, it is known that turbulent fluctuations must be inserted into the domain in order to prevent the flow from remaining laminar around the body of interest, although this has been overlooked until now for the present test case. This work investigates the errors occurring when a laminar inflow is applied for mildly separated or attached flows, by employing the partially averaged Navier–Stokes equations with varying values for the ratio of modelled-to-total turbulence kinetic energy, and with varying grid densities. It is shown that depending on the grid resolution laminar leading edge separation can occur. When turbulent fluctuations are added to the inflow, the leading edge separation is suppressed completely, and the turbulent separation zone near the trailing edge reduces in size. The inflow turbulence has a large effect on the skin friction, which increases with increasing turbulence intensity to a limit determined by the grid resolution. In cavitating conditions the integral quantities are dominated by the shedding sheet cavity. The turbulence intensity has little effect on the pressure distribution, leading to a largely unaffected sheet cavitation, although the shedding behaviour is affected. It is shown that, especially in wetted flow conditions, with scale-resolving methods inflow turbulence is necessary to match the experimental flow field.

Aero‐hydro‐servo‐elastic coupling of a multi‐body finite‐element solver and a multi‐fidelity vortex method

AbstractThe manuscript presents a novel aero‐hydro‐servo‐elastic coupling framework, MIRAS‐HAWC2. In this coupling, the wind turbine blades and rotor‐wake aerodynamics are modeled using a modified lifting‐line theory which accounts for blade curvature, combined with a hybrid vortex method. The wind turbine structure and foundation are modeled using a finite‐element and multi‐body system approach. Last, hydrodynamics are modeled using Airy wave theory together with Morison's equation. An initial assessment of the performance of the aeroelastic coupling framework has been performed for steady rotor‐only cases, assuming laminar inflow without shear. This included a comparison against fully resolved computational fluid dynamics, for both stiff and flexible blades showing an excellent agreement. In a second stage, the aero‐hydro‐servo‐elastic coupling is used, comparing MIRAS‐HAWC2 as well as blade‐element momentum‐based simulations with selected results from the Offshore Code Comparison Collaboration projects (OC3 and OC4), which study the NREL 5 MW turbine mounted on different offshore support structures. A good agreement has been obtained for the simulations of a monopile with rigid foundation, a tripod, and jacket support structures.

Distinct Turbulent Regions in the Wake of a Wind Turbine and Their Inflow-Dependent Locations: The Creation of a Wake Map

Wind turbines are usually clustered in wind farms which causes the downstream turbines to operate in the turbulent wakes of upstream turbines. As turbulence is directly related to increased fatigue loads, knowledge of the turbulence in the wake and its evolution are important. Therefore, the main objective of this study is a comprehensive exploration of the turbulence evolution in the wind turbine’s wake to identify characteristic turbulence regions. For this, we present an experimental study of three model wind turbine wake scenarios that were scanned with hot-wire anemometry with a very high downstream resolution. The model wind turbine was exposed to three inflows: laminar inflow as a reference case, a central wind turbine wake, and half of the wake of an upstream turbine. A detailed turbulence analysis reveals four downstream turbulence regions by means of the mean velocity, variance, turbulence intensity, energy spectra, integral and Taylor length scales, and the Castaing parameter that indicates the intermittency, or gustiness, of turbulence. In addition, a wake core with features of homogeneous isotropic turbulence and a ring of high intermittency surrounding the wake can be identified. The results are important for turbulence modeling in wakes and optimization of wind farm wake control.

Vortex dynamics of a low aspect ratio cantilevered cylinder immersed in a boundary layer

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The effect of porosity on the drag of cylinders

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Aerodynamic characterization of barge and spar type floating offshore wind turbines at different sea states

AbstractThe wake dynamics behind a compliant floating offshore wind turbine (FOWT) with two alternative supporting platforms of spar buoy and barge platform are studied numerically. The computational model is based on the large eddy simulation and the use of the actuator disk model of the FOWT rotor in which the circular actuator disk is discretized with an unstructured two‐dimensional triangular mesh in a structured three‐dimensional Cartesian grid of the fluid domain. The wake dynamics and platform motions are calculated for laminar and turbulent inflow conditions. The flow solver is verified through a series of experimental wake measurements done behind a porous disk model and is also cross‐validated against previously published results. The dynamics of the floating spar and barge structures are calculated from wave–structure interaction for three distinctive sea states. The time history of power extraction and wind force for the floating offshore wind turbine are recorded and compared against a fixed horizontal axis wind turbine. The motion of the barge platform is found to induce low‐frequency modulation of the wake, whereas the spar buoy primarily displays similar wake dynamics to the fixed turbine. It is discussed how a single turbine utilizing the spar concept can be more energy efficient under the wave‐induced motion. Moreover, for both laminar and turbulent inflow conditions, due to the large axial oscillation experienced by the barge concept, the turbine wake recovers more rapidly. This suggests that for a tighter spacing of the turbines in a farm, the barge platform concept can be leveraged to obtain higher energy‐capturing efficiency over a fixed area.

Reactor Design and Thermal Performance Analysis for Solar Thermal Energy Storage Application

Solar energy is a sustainable and low-cost renewable energy of enormous importance, especially at this time where non-renewable energy sources are unsustainable and costly. However, improving the thermal performance of a solar energy storage reactor poses some challenges. In this study, the location of fluid inlets and outlets in the given reactor design and its impact on the thermal performance were investigated. A P1 approximation radiation model coupled with shallow channel approximation of fluid flow was developed. By taking the frustum base as a reference, four fluid inlets along the edges of the frustum and two outlet locations at the base and side of the reactor were computed. Inlets located 4.81 cm from the base of the frustum and an outlet located at the side of the reactor were found to have a better thermal performance with a short conveyer energy flow system. It was also deduced that radiation applied at the edges of the frustum had better thermal performance than that applied at a quartz edge. Furthermore, increasing the laminar inflow rate from 0.36 (L/h) to 3.6 (L/h) increased the temperature distribution in the reactor. This study provides noteworthy insights of relevance to the power engineering industry and academia.

Actuator line simulations of wind turbine wakes using the lattice Boltzmann method

Abstract. The high computational demand of large-eddy simulations (LESs) remains the biggest obstacle for a wider applicability of the method in the field of wind energy. Recent progress of GPU-based (graphics processing unit) lattice Boltzmann frameworks provides significant performance gains alleviating such constraints. The presented work investigates the potential of LES of wind turbine wakes using the cumulant lattice Boltzmann method (CLBM). The wind turbine is represented by the actuator line model (ALM). The implementation is validated and discussed by means of a code-to-code comparison to an established finite-volume Navier–Stokes solver. To this end, the ALM is subjected to both laminar and turbulent inflow while a standard Smagorinsky sub-grid-scale model is employed in the two numerical approaches. The resulting wake characteristics are discussed in terms of the first- and second-order statistics as well the spectra of the turbulence kinetic energy. The near-wake characteristics in laminar inflow are shown to match closely with differences of less than 3 % in the wake deficit. Larger discrepancies are found in the far wake and relate to differences in the point of the laminar-turbulent transition of the wake. In line with other studies, these differences can be attributed to the different orders of accuracy of the two methods. Consistently better agreement is found in turbulent inflow due to the lower impact of the numerical scheme on the wake transition. In summary, the study outlines the feasibility of wind turbine simulations using the CLBM and further validates the presented set-up. Furthermore, it highlights the computational potential of GPU-based LBM implementations for wind energy applications. For the presented cases, near-real-time performance was achieved using a single, off-the-shelf GPU on a local workstation.

Analysis of a Multi-Porthole Injector Array in a Supersonic Crossflow

Computational fluid dynamic (CFD) simulations of hydrogen injection through a multi-porthole injector array into a Mach 10 enthalpy supersonic crossflow are presented and compared to experimental data. Reynolds-averaged Navier–Stokes simulations using a Menter shear-stress transport turbulence model are performed with the CFD code CFD++. The experimental results showed transient flow behavior in the boundary-layer separation upstream of the injector past the measurable test time. This complicated the validation process of the steady-state simulations to the experimental data. A hybrid simulation, which provides a laminar inflow boundary layer and turbulence production at the injector, matched the upstream separation length as measured from the experimental data. The simulation near-field shock structures and fuel penetration matched experimental schlieren photographs. The close jet-to-jet spacing leads to a blockage of injector-generated vortices. The simulations show that including three-dimensional spillage effects reduced the injection-induced separation length by 20%, and they resulted in a better match with the experimental data. The results also showed the injection-induced separation has a significant spanwise flow and alters the mixing process downstream of the multi-porthole injector array by enabling trailoff vortices to develop.

Visualization of hypersonic incident shock wave boundary layer interaction

The incident shock interactions with the hypersonic laminar and forced turbulent boundary layer are visualized by the planar laser scattering technique. The effect of interaction strength on flow structures is also investigated. The results show that the boundary layer shape has been greatly altered by the incident shock. In both the laminar and turbulent inflows, the boundary layer thickness has an abrupt decrease at the interaction region. In laminar inflow, the boundary layer transition rapidly takes place due to the incident shock. The greater the incident shock angle, the bigger the boundary layer thickness downstream the incident shock. In turbulent inflow, the thickness downstream the shock is less than that of the inflow. Fractal analysis is firstly carried on hypersonic shock boundary layer interaction. The effect of the shock angle and inflow condition on fractal dimension is studied. The results indicate that with the stronger shock, the fractal dimension value is bigger for both laminar and forced turbulent inflows.

Direct Numerical Simulations of Bypass Transition over Distributed Roughness

Bypass transition in a boundary layer subjected to freestream turbulence and distributed surface roughness is studied numerically. The distributed surface roughness is reproduced with an immersed boundary technique, and the freestream turbulence is artificially generated by a superposition of eigenmodes of the Orr–Sommerfeld and Squire equations. Both an undisturbed laminar inflow and a disturbed inflow with freestream turbulence are studied. In either case a parametric study on the effects of the roughness size and density is carried out. The simulations reveal that the presence of roughness induces streaks in the laminar flow. When the freestream is turbulent, both roughness height and density show an impact on the onset of transition. The superposition of surface roughness and freestream turbulence causes amplified streaks. As a result, the streak instability occurs earlier within the boundary layer. The results show good qualitative and quantitative agreement to both experimental and numerical studies available in the literature.

Transitional shock wave boundary layer interaction over a flexible panel

A three-dimensional transitional shock boundary layer interaction (SWBLI) over a finite-span flexible panel is investigated by performing direct numerical simulations (DNS). The laminar inflow is at Mach 2, on which an oblique shock of turn angle 5.62∘ is imposed. The shock impinges at the mid-chord length of the panel, giving rise to flow separation due to the induced adverse pressure gradient. The flow separation leads to streamline curvature in the laminar boundary layer, initiating a centrifugal instability similar to Görtler instability; where the nominally two-dimensional and steady SWBLI becomes three-dimensional and unsteady at a critical Reynolds number. The flow transition is characterized by the presence of counter-rotating streamwise-oriented Görtler-like vortices, engendering spanwise undulations for increasing Görtler number. The transitional SWBLI is analyzed over both the rigid and flexible panels by performing proper orthogonal decomposition (POD) in order to examine the onset of transition as well as the modal response of the flexible panel. The presence of flexible panel promotes flow transition, resulting in lower values of the critical Reynolds and Görtler numbers. Furthermore, at a post-critical Reynolds number, the interaction results in increased turbulence levels and exchange of energy at the low and high wavenumber modes.

Impact of turbulence on flying insects in tethered and free flight: High-resolution numerical experiments

Flapping insects are remarkably agile fliers, adapted to a highly turbulent environment. We present a series of high resolution numerical simulations of a bumblebee interacting with turbulent inflow. We consider both tethered and free flight, the latter with all six degrees of freedom coupled to the Navier--Stokes equations. To this end we vary the characteristics of the turbulent inflow, either changing the turbulence intensity or the spectral distribution of turbulent kinetic energy. Active control is excluded in order to quantify the passive response real animals exhibit during their reaction time delay, before the wing beat can be adapted. Modifying the turbulence intensity shows no significant impact on the cycle-averaged aerodynamical forces, moments and power, compared to laminar inflow conditions. The fluctuations of aerodynamic observables, however, significantly grow with increasing turbulence intensity. Changing the integral scale of turbulent perturbations, while keeping the turbulence intensity fixed, shows that the fluctuation level of forces and moments is significantly reduced if the integral scale is smaller than the wing length. Our study shows that the scale-dependent energy distribution in the surrounding turbulent flow is a relevant factor conditioning how flying insects control their body orientation.