Farzin Ghanadi

Analysis of the turbulent boundary layer in the vicinity of a self-excited cylindrical Helmholtz resonator

This study investigates the changes in the structure of a turbulent boundary layer downstream of a flow-excited Helmholtz resonator. To this end, a fully developed turbulent boundary layer over a resonator mounted flush with a flat plate was simulated by implementing a large eddy simulation (LES). To assist in understanding the effect of the resonator on the flow structure, a sensitivity study was undertaken by changing the main geometrical parameters of the resonator. The results demonstrated that when the boundary layer thickness equals the orifice length, the cross-stream component of velocity fluctuations penetrates the boundary layer, resulting in a reduction of the turbulence intensity by up to 12%. Therefore, it is concluded that a Helmholtz resonator has the potential to reduce the instabilities within the boundary layer. These investigations also assist in identifying the optimal parameters to delay turbulence events within the grazing flow using Helmholtz resonators.

Understanding of the flow behaviour on a Helmholtz resonator excited by grazing flow

In this study, a large eddy simulation of the three-dimensional shear flow over a flow-excited Helmholtz resonator has been implemented. The simulations have been performed over a wide range of flow speeds to analyse the effect of the inlet flow properties on the excitation condition. For validation proposes, the results obtained from the numerical simulations have been compared with published experimental data and show that numerical modelling provides an accurate representation of the pressure fluctuations inside the cavity. The main objective of this paper is to gain an understanding of the flow features over a flow-excited Helmholtz resonator. To this end, using the numerical model, the interaction of a turbulent boundary layer with a Helmholtz resonator has been considered, and the characteristics of the flow inside the resonator and over the orifice for various flow conditions are also analysed.

Attenuation of turbulence by the passive control of sweep events in a turbulent boundary layer using micro-cavities

Cavity arrays have been previously identified to disrupt the sweep events and consequently the bursting cycle in the boundary layer by capturing the structures responsible for the Reynolds stresses. In the present study, the sensitivity of a flushed-surface cavity array in reducing the turbulent energy production has been investigated. Two plates of varying thicknesses and four different backing cavity volumes were considered, at three different Reynolds numbers. The volume of the backing cavity was shown to be the most important characteristic in determining the attenuation of streamwise velocity fluctuations within the logarithmic region of the turbulent boundary layer. However, the results also demonstrated that the orifice length of the cavity array had negligible effect in modifying the reduction of the turbulent energy by the cavity array in this investigation. The results show that the maximum reduction in turbulence generation achieved for this study occurs when the backing volume is 3.1 × 106 tim...

Mechanism of sweep event attenuation using micro-cavities in a turbulent boundary layer

Cavity arrays have been identified as a potential passive device to disrupt and capture sweep events, which are responsible for the excess Reynolds stresses in the boundary layer. In the present study, the mechanism of the attenuation of captured sweep events has been analyzed, as well as the non-linear relationship between the volume of the backing cavity and the reduction in sweep intensity. The influence of cavity array on the turbulent boundary layer has been analyzed, with a total of six different backing cavity arrangements with varying volumes. Three of the backing cavities have been used to determine the non-linear relationship between the effectiveness of the cavity array in reducing sweep intensity and the volume of the backing cavity. The other three have been used to determine the mechanism by which the arrays manipulate the captured sweep events. The pre-multiplied energy spectra of multiple velocity histories were significantly reduced, by up to 12.5%, in the low and mid-range wavelength values (λx+<104), which is associated with the coherent structures. The results show that the maximum reduction in sweep intensity of approximately 7% may be obtained when Reθ = 3771. It has been demonstrated that the non-linear relationship between sweep event intensity reduction and cavity volume has reached an upper limit in this investigation. Results from this study have revealed that the cavity array weakens the sweep intensity of the captured sweep events by damping the energy of the events through the friction losses in the cavity array and also in the large volume of the backing cavity.Cavity arrays have been identified as a potential passive device to disrupt and capture sweep events, which are responsible for the excess Reynolds stresses in the boundary layer. In the present study, the mechanism of the attenuation of captured sweep events has been analyzed, as well as the non-linear relationship between the volume of the backing cavity and the reduction in sweep intensity. The influence of cavity array on the turbulent boundary layer has been analyzed, with a total of six different backing cavity arrangements with varying volumes. Three of the backing cavities have been used to determine the non-linear relationship between the effectiveness of the cavity array in reducing sweep intensity and the volume of the backing cavity. The other three have been used to determine the mechanism by which the arrays manipulate the captured sweep events. The pre-multiplied energy spectra of multiple velocity histories were significantly reduced, by up to 12.5%, in the low and mid-range wavelength val...

Numerical investigation of wind loads on an operating heliostat

The velocity fluctuations within the atmospheric boundary layer (ABL) and the wind direction are two important parameters which affect the resulting loads on the heliostats. In this study, the drag force on a square heliostat within the ABL at different turbulence intensities is simulated. To this end, numerical analysis of the wind loads have been conducted by implementing the three-dimensional Embedded Large Eddy Simulation (ELES). The results prove that in contrast with other models which are too dissipative for highly turbulent flow, the present model can accurately predict boundary effects and calculate the peak loads on heliostat at different elevation angles and turbulence intensities. Therefore, it is recommended that the model is used as a tool to provide new information about the relationship between wind loads and turbulence structures within ABL such as vortex length scale.The velocity fluctuations within the atmospheric boundary layer (ABL) and the wind direction are two important parameters which affect the resulting loads on the heliostats. In this study, the drag force on a square heliostat within the ABL at different turbulence intensities is simulated. To this end, numerical analysis of the wind loads have been conducted by implementing the three-dimensional Embedded Large Eddy Simulation (ELES). The results prove that in contrast with other models which are too dissipative for highly turbulent flow, the present model can accurately predict boundary effects and calculate the peak loads on heliostat at different elevation angles and turbulence intensities. Therefore, it is recommended that the model is used as a tool to provide new information about the relationship between wind loads and turbulence structures within ABL such as vortex length scale.

Investigation of the atmospheric boundary layer characteristics on gust factor for the calculation of wind load

Dynamic amplification and gust effects from turbulence can increase wind loads significantly over and above the static wind loads that have been used for heliostat design. This paper presents the results of analyzing the relationship between gust factor and turbulence intensity within the atmospheric boundary layer (ABL) based on the high fidelity measurements of wind velocity at the SLTEST facility in the Utah desert. Results showed that there are distinct characteristics of a low roughness ABL that deviate from semi-empirical relationships derived for open country and urban terrains with larger surface roughness heights. The analysis also indicated that gust factor is increased by 2.4% when lowering the gust period from 3s to 1s in the low roughness field experiment ABL, compared to a 3.6% increase in a suburban terrain at a 10m height. Although 3s gust periods are recommended in AS/NZS 1170.2 [1], comparison of gust factor data with a 1s gust period is recommended particularly in high roughness ABLs su...

Development of the ASTRI heliostat

The Australian Solar Thermal Research Initiative (ASTRI) aims to develop a high optical quality heliostat with target cost – manufactured, installed and operational – of 90 AUD/m2. Three different heliostat design concepts are described, each with features identified during a prior scoping study as having the potential to contribute to cost reduction compared to the current state-of-the-art. The three concepts which are being developed will be down-selected to a single concept for testing in late 2016. The heliostat concept development work is supported by technology development streams, developing novel sandwich panel mirror facet structures, analysing and testing wind loads on heliostats in both stow and operation positions, and developing new heliostat field layouts and software tools for optical analysis of heliostats design concepts.