An Overset Mesh Methodology for CFD Modelling of Rotary Compressors

Full 2D and 3D unsteady CFD simulations are gradually replacing the widely used lumped parameters simulations for rotary compressors. Lumped parameter models predict the overall thermodynamic processes; however, they cannot reveal the spatial and temporal variations in the working chamber and account for complex fluid interactions and losses. The complex and continuously deforming working chamber geometry is challenging for CFD. The overset method has been implemented here for the simulation of a 2D Coupled Vane Compressor (CVC). The method was compared to a dynamic mesh formulation using a remeshing technique and has shown; (1) it is 3-4 times faster computationally and (2) capable of modelling tight clearances not possible in the dynamic mesh approach and are difficult, if not impossible to model using the lumped parameter models. The CFD results revealed complex fluid interactions inside the chamber, accounted for leakage and can be used to optimize the CVC designs further

OBJECT PERCEPTION IN UNDERWATER ENVIRONMENTS: A SURVEY ON SENSORS AND SENSING METHODOLOGIES

Underwater robots play a critical role in the marine industry. Object perception is the foundation for the automatic operations of submerged vehicles in dynamic aquatic environments. However, underwater perception encounters multiple environmental challenges, including rapid light attenuation, light refraction, or backscattering effect. These problems reduce the sensing devices’ signal-to-noise ratio (SNR), making underwater perception a complicated research topic. This paper describes the state-of-the-art sensing technologies and object perception techniques for underwater robots in different environmental conditions. Due to the current sensing modalities’ various constraints and characteristics, we divide the perception ranges into close-range, medium-range, and long-range. We survey and describe recent advances for each perception range and suggest some potential future research directions worthy of investigating in this field

Comparison between OpenFOAM CFD & BEM theory for variable speed – variable pitch HAWT

OpenFoam is used to compare computational fluid dynamics (CFD) with blade element momentum theory (BEM) for a variable speed - variable pitch HAWT (Horizontal Axis Wind Turbine). The wind turbine is first designed using the BEM to determine the blade chord, twist and operating conditions. The wind turbine blade has an outer diameter of 14 m, uses a NACA 63–415 profile for the entire blade and root to tip twist distribution of 15deg (Figure 3). The RPM varies from 20–75 for freestream velocities varying between 3–10.5 m/s (variable speed) and a constant RPM of 78.78 for velocities ranging between 11–25 m/s (variable pitch). OpenFOAM is used to investigate the wind turbine performance at several operating points including cut-in wind speed (3 m/s), rated wind speed (10.5 m/s) and in the variable pitch zone. Simulation results show that in the variable-speed operating range, both CFD and BEM compare reasonably well. This agreement can be attributed to the fact that the complex three-dimensional flow around the turbine blades can be split into two radial segments. For radii less than the mid-span, the flow is three-dimensional, whereas for radii greater than the mid-span, the flow is approximately two-dimensional. Since the majority of the power is produced from sections beyond the mid-span, the agreement between CFD and BEM is reasonable. For the variable-pitch operating range the CFD results and BEM deviate considerably. In this case the majority of the power is produced from the inner sections in which the flow is three-dimensional and can no longer be predicted by the BEM. The results show that differences in pitch angles up to 10deg can result to regulate the power for high wind speeds in the variable-pitch operation zone

Aerodynamic performance enhancement using active flow control on DU96-W-180 wind turbine airfoil

In this work, the objective was to investigate the influence of Active Flow Control on the improvement of a DU96-W-180 airfoil aerodynamic performance. A numerical simulation was done for incompressible unsteady low Reynolds Number flow at high angle of attack. The innovative approach was the use of an “Active Slat” where the periodic blowing effect was achieved by periodically opening and closing the slat passage. The major benefit of this concept is being flexible to a desired operating condition. A new OpenFOAM® solver was developed from the existing pisoFoam solver to simulate the active slat flow control technique. To get the best aerodynamic performance, the active slat should operate at the domain dominant frequencies. A Fast Fourier Transform (FFT) was performed to achieve the optimum slat excitation frequency. These frequencies will help in controlling the inherent instabilities in the boundary layer and thus improving the aerodynamic performance. Finally, active flow control simulations were applied using different excitation. Using the optimum FFT excitation frequency (f= 0.68 in the wake region) yields the best aerodynamic improvement of all tested frequencies. Improvements in lift coefficient were achieved up to 8%. Hence, the slatted airfoil is superior to the conventional clean configuration airfoil.Published versio

A domain decomposition technique for small amplitude wave interactions with shock waves

In this paper, a domain decomposition technique in the finite volume framework is presented to propagate small amplitude acoustic and entropy waves in a linearized Euler region and simulate the interaction of these waves with an initially steady normal shock in a nonlinear region. An overset method is used to two-way couple the linear and nonlinear regions that overlap each other. Linearized solvers alone cannot capture this interaction due to the discontinuity encountered at shocks. On the other hand, nonlinear solvers based on second order shock-capturing schemes will result in excessive dissipation and dispersion for the small disturbances. The domain decomposition technique provides a good balance between minimizing dissipation and dispersion errors while enabling nonlinear shock-acoustic interactions. To preserve low dispersion and dissipation, a DRP scheme is used to simulate the incoming and outgoing waves in the linear region. To capture the shock wave interaction and motion, a hybrid central-upwind flux scheme is used in the nonlinear region that contains the shock. Grid sensitivity studies for an acoustic wave propagating in stationary flow were performed to compare the linear, nonlinear, and domain decomposition solvers. The nonlinear solver required ten times the mesh resolution to achieve similar accuracy as the linear solver, resulting in a forty-fold increase in computational time. For modest cell size ratios, the domain decomposition solver reduced the computational time by a factor of three compared to the nonlinear solver while achieving similar accuracy. Interaction of standing shocks with acoustic and entropy waves of amplitudes ϵ=±10−2 and ±10−5 was investigated using the domain decomposition technique. The numerical results for ϵ=±10−2 compared well with the linearized interaction analysis (LIA) with less than 3% discrepancy in terms of the amplification factors. The domain decomposition technique acts as a low pass filter that averages the post-shock oscillations generated by the slow-moving shocks in the nonlinear region, resulting in the correct amplification factors in the linear region. For the smaller amplitudes of ϵ=±10−5, the amplification factors deviated from LIA predictions by up to 70%. Numerical results suggest that the large discrepancy for the small amplitude cases is due to insufficient mesh resolution for capturing extremely slow-moving shocks.Ministry of Education (MOE)Submitted/Accepted versionThis research is supported by Ministry of Education, Singapore, under its Academic Research Fund Tier 1 (RG183/18)

Lattice Boltzmann simulation on the flow behaviour associated with Helmholtz cavity-backed acoustic liners

Noise from jet engines can be reduced by means of a Helmholtz cavity configuration. The resonance that occurs when a flow passes the neck of the Helmholtz resonator will dissipate acoustic energy. The mechanism for such dissipation is mainly due to the vortex shedding that occurs at the neck of the resonator where the vortex structures absorb acoustic energy and subsequently dissipate it through viscous effects. In this work, numerical simulations utilizing the lattice Boltzmann method are used to aid in visualizing the flow behaviour that is associated with Helmholtz cavity-backed acoustic liners. In both experiments and numerical simulations, the 1-neck cavity is found to result in an amplification of an applied acoustic source. For a 4-neck cavity, the configuration is able to achieve acoustic pressure reductions. Differences in the flow behaviour of the 1-neck and 4-neck cavities are detailed in this work. Results show that the stronger vortex shedding that occurs in the 4-neck cavity configuration could explain its increased effectiveness as a Helmholtz cavity-backed acoustic liner.National Research Foundation (NRF)Submitted/Accepted versionThis research is supported by the National Research Foundation, Singapore, under its NRF-NSFC joint grant (NRF2016NRF-NSFC001-102)

Unsteady aerodynamic modeling and control of pusher and tilt-rotor quadplane configurations

A nonlinear unsteady aerodynamics model is coupled with a three degree of freedom quadplane to control the forward and backward transition between hover and steady level flight. The unsteady lift and drag forces are modeled using a lumped vortex model for flat plates. Two variants for the quadplane are considered: (i) a pusher and (ii) a tilt-rotor configuration in the absence of control surfaces to assess the controllability for altitude, attitude and forward speed. Conventional PID control is applied to generate the control inputs. The simulation results conclude that the pusher quadplane configuration is effortless to control as all the selected states are controllable, whereas for the tilt-rotor configuration, even though the vehicle is stable, altitude control is significantly more challenging due to one less control input when compared to the pusher configuration