摘要 :
In this paper, a theoretical derivation from the perspective of the force on the slender bodies arranged along the hydrofoil span is performed to explain why the vortex circulation Γvortex should theoretically be equal to the max...
展开
In this paper, a theoretical derivation from the perspective of the force on the slender bodies arranged along the hydrofoil span is performed to explain why the vortex circulation Γvortex should theoretically be equal to the maximum bound circulation Γ0 around the hydrofoil. Then, a more accurate and operable prediction formulation based on hydrofoil load, Γ_(vortex) = kΓ_0, is proposed, where k = 0.9-1. Γ_0 is the averaged hydrofoil circulation along spanwise direction and its value can be obtained easily from hydrofoil load. To validate our new formulation, two typical vortex flows, i.e., tip leakage vortex (TLV) flow and tip vortex (TV) flow, are simulated with large eddy method. Our numerical results confirm that the variations of hydrofoil load along span would lead to streamwise vorticity, and then the rolling-up of latter leads to the vortex formation (Green, 1995). However, since the influential region of vortex is limited that not all streamwise vorticity could be entrained into the vortex and there are viscous and dissipation effects, the actual Γ_(vortex) cannot reach TO but it is quite close to Γ_0. The prediction formulation agrees well with the numerical results and available measured data.
收起
摘要 :
This paper presents an investigation of the effects of tip injection on the performance and near wake characteristics of a model wind turbine rotor. Experiments are conducted by placing a three-bladed horizontal axis wind turbine ...
展开
This paper presents an investigation of the effects of tip injection on the performance and near wake characteristics of a model wind turbine rotor. Experiments are conducted by placing a three-bladed horizontal axis wind turbine rotor at the exit of an open-jet wind tunnel. The rotor blades are non linearly twisted and tapered with NREL 5826 airfoil profile. The nacelle, hub and the blades are specifically designed to allow pressurized air to pass through and get injected from the blade tips while the rotor is rotating. Measurements of torque and thrust coefficient variations with the Tip Speed Ratio (TSR) as well as wake flow field surveys using Constant Temperature Anemometry are performed with and without tip injection. Results show that power and thrust coefficients both increase due to injection especially at TSR values higher than 3.5. Wake characteristics show a tip flow region that is radially pushed outwards with increased levels of turbulence occupying wider areas compared to the no injection case. Up to two diameters downstream within the wake, the boundary between the wake and the freestream, which is normally dominated by the presence of the tip vortices, gets more diffused and turbulence levels are significantly increased due to tip injection. (C) 2015 Elsevier Ltd. All rights reserved.
收起
摘要 :
An investigation of the dependence of the lift-induced drag coefficient C (Di) of a square-tipped, cambered wing model on Reynolds number for Re 展开
An investigation of the dependence of the lift-induced drag coefficient C (Di) of a square-tipped, cambered wing model on Reynolds number for Re 收起
摘要 :
The blade tip leakage flow with efficiency losses and cavitation phenomena is a concern for the low-head tidal power units. A simplified case of NACA0009 hydrofoil in a water tunnel is used to investigate the effects of tip cleara...
展开
The blade tip leakage flow with efficiency losses and cavitation phenomena is a concern for the low-head tidal power units. A simplified case of NACA0009 hydrofoil in a water tunnel is used to investigate the effects of tip clearance geometries including the foil tip shape and gap width on the flow features and foil performance. Steady non-cavitating simulations are implemented for a round tip foil and a sharp tip foil with two incidence angles (alpha = 10 degrees and 5 degrees) and different normalized gap width (tau). The minimum pressure is used to reflect the normalized vortex intensity (Gamma*) and cavitation characteristics. The Gamma*-tau curves at different streamwise positions show that the sharp tip foil generates relatively weaker tip leakage vortex with more flat curves, but its higher Gamma* of tip separation vortex in wider gaps increases the risk of clearance cavitation. The flow features on a cross section inside the gap suggest that the sharp tip reduces the leakage flow losses and increases the velocity gradient due to the boundary layer separation. The lift coefficient is a little higher for the sharp tip foil than the round tip foil, with small differences for alpha = 5 degrees but noticeable deviations for alpha = 10 degrees especially within 0.3收起
摘要 :
Three-dimensional unsteady calculations were performed to explore the unsteady nature of tip clearance flow and its possible linking with the spike stall inception. It was found that the interaction of the broken-down leakage vort...
展开
Three-dimensional unsteady calculations were performed to explore the unsteady nature of tip clearance flow and its possible linking with the spike stall inception. It was found that the interaction of the broken-down leakage vortex with the tip clearance flow formed another distinctive vortex, denoted as the tip separation vortex. It formed below the rotor blade tip section and propagated diagonally inward. This vortex propagated across the rotor passage from the pressure side to the suction side with its vortex core filled with low-energy fluid. The spike emergence during stall inception included the breakdown of the tip leakage vortex and the formation and movement of the tip separation vortex.
收起
摘要 :
Three-dimensional unsteady calculations were performed to explore the unsteady nature of tip clearance flow and its possible linking with the spike stall inception. It was found that the interaction of the broken-down leakage vort...
展开
Three-dimensional unsteady calculations were performed to explore the unsteady nature of tip clearance flow and its possible linking with the spike stall inception. It was found that the interaction of the broken-down leakage vortex with the tip clearance flow formed another distinctive vortex, denoted as the tip separation vortex. It formed below the rotor blade tip section and propagated diagonally inward. This vortex propagated across the rotor passage from the pressure side to the suction side with its vortex core filled with low-energy fluid. The spike emergence during stall inception included the breakdown of the tip leakage vortex and the formation and movement of the tip separation vortex.
收起
摘要 :
Propeller cavitation is a main source of fluctuating pressure and noise induced by propellers, and the tip vortex cavitation is the principal source. The present study measures the flow fields near the blade tip using the 2D-PIV t...
展开
Propeller cavitation is a main source of fluctuating pressure and noise induced by propellers, and the tip vortex cavitation is the principal source. The present study measures the flow fields near the blade tip using the 2D-PIV technique. The experimental setup and scheme are introduced. We monitor the process of generation and shedding of the propeller tip vortex in real time and analyse the dynamic structure of the tip vortex by testing the propeller wake field under different phases of the axial plane. The distribution characteristics of radial and axial velocity are also analysed. The influence range and the vorticity of the tip vortex and trailing vortex are obtained. All of the measured quantitative data are useful for future propeller design.
收起
摘要 :
The main purpose of this research is to develop a mesh refinement method for tip vortex cavitation (TVC) simulation. The mesh size of the blade surface is determined by studying the influence of mesh on the open water characterist...
展开
The main purpose of this research is to develop a mesh refinement method for tip vortex cavitation (TVC) simulation. The mesh size of the blade surface is determined by studying the influence of mesh on the open water characteristics. The mesh size of the tip vortex wake is determined by analyzing the influence of mesh on the structure of the tip vortex. In the process of increasing the mesh resolution, a hollow cylinder is used to refine the mesh of the tip vortex wake. Based on the structure of the tip vortex, the adaptive mesh refinement (AMR) technique is used to simulate the TVC behaviour. The Q-criterion and wall distance are used as field function variables to refine the mesh in the region where TVC is likely to occur. Numerical results show that the mesh refinement method proposed in this paper is very effective in simulating TVC.
收起
摘要 :
Tidal energy has attracted great attention due to its massive potential and eco-friendly nature. Performance of ducted turbines used in tidal energy conversion is significantly influenced by tip clearance and associated leakage fl...
展开
Tidal energy has attracted great attention due to its massive potential and eco-friendly nature. Performance of ducted turbines used in tidal energy conversion is significantly influenced by tip clearance and associated leakage flow. In this work, a method of variable-depth (VD) groove is developed to suppress the tip leakage vortex (TLV) and the tip leakage vortex cavitation (TLVC) around a NACA0009 hydrofoil, and different variable-depth laws are proposed to improve the suppressing effect. The flow pattern around the hydrofoil is investigated on basis of numerical method, which is validated by experiment results. Results show that this groove method not only effectively suppresses primary TLV by high-velocity flow impingement, but also reduces the primary TLVC by increasing local pressure in vortex core regions. In addition to the intrinsic primary TLV and secondary TLV, some local small-scale vortices are witnessed in vicinity of the groove. Among the VD grooves, the divergent groove shows an optimal effect on suppressing vortex and cavitation in general. Increasing the divergent groove depth at hydrofoil pressure side causes the shrinkage of the primary TLV but a larger scale of the additional vortices in vicinity of the groove. The suppressing effect of the divergent groove on the TLVC reaches a peak when the pressure side depth is increased to 50% of the tip clearance size. The lift coefficient of the hydrofoil is sensitive to the groove pressure side depth, by decreasing which can effectively improve the lift-drag perfor-mance of the hydrofoil.
收起
摘要 :
Boundary layer suction (BLS), or aspiration, is an effective methodology to eliminate flow separation and improve blade loading in axial compressors. The objective of this study is to reveal the flow physics and loss mechanism of ...
展开
Boundary layer suction (BLS), or aspiration, is an effective methodology to eliminate flow separation and improve blade loading in axial compressors. The objective of this study is to reveal the flow physics and loss mechanism of tip leakage vortex in aspirated compressor cascades with multiple loading levels under diffusion factor ranging from 0.42 to 0.71. The effect of blade pressure distribution on tip leakage flows was investigated numerically based on three aspirated blades, including the one designed with conventional method and the other two designed with curvature induced pressure recovery. The influences of tip clearance dimension and BLS schemes (BLS location, BLS height and BLS mass flow rate) on tip leakage vortex as well as its breakdown mechanism were investigated. Results show that a pseudo-shock was found in aspirated compressor cascades designed with conventional method and curvature induce pressure recovery concept. Compared with conventional compressor blades, the main flow phenomenon of tip leakage flow of aspirated compressor blades is the tip leakage vortex breakdown (TLVB) caused by strong adverse pressure gradient introduced by BLS. The TLVB phenomenon occurred both in conventionally loaded compressor cascade and highly loaded compressor cascade. For conventionally loaded blade, TLVB phenomenon occurred under low tip clearance condition but disappeared under high tip clearance condition due to increased strength of tip leakage vortex. Lower BLS mass flow resulted in weaker pseudo-shock, which reduced the swirl number of tip leakage vortex and improved the vortex breakdown phenomenon as a consequence. The vortex breakdown of highly loaded cascades was resulted from the joint action of high blade loading and pseudo-shock. Pseudo-shock designed near blade trailing edge resulted in higher leakage mass flow but promised superior tip leakage flow field. For all the aspirated cascades studied, the tip leakage loss decreased as tip clearance dimension increased, which was opposite to the results of conventional compressor cascade. (C) 2021 Elsevier Masson SAS. All rights reserved.
收起
原文获取