2.3 Interaction of the Propeller Jet with Free Surface
Propellerwake is a complicated flow having two major components: one directing towards the fluid surface and another towards the sea bed. In the case of ice management, typically the component interacting with fluid surface is more important. The component interacting with fluid surface causes turbulence at the interaction region, which was studied by numerous researchers including Anthony (1990), Anthony and Willmarth (1992), Madina and Bernal (1994), Walker, Chen and Willmarth (1995), and Bernal and Scherer (1997). One of the latest studies by Tian (2011) reported that when the jet interacts with the free surface, a surface current is produced which occupied a thin layer beneath the surface. When the surface wave and surface currents move in the same direction, the wave amplitude decreases and wave length increases. Madina and Bernal (1994) reported that surface waves are propagated as large scale vortical structures in the jet flow as they interact with water surface. They also reported that the waves propagate at an angle with respect to the direction of flow, which increases against the increase of Flow Froude number. The investigation made a comparison of jet interacting with the fluid surface against free jet, and it was found that the decay rate of the maximum mean velocity at the far field is reduced by a factor √2 comparing to free jet. The average growth rate of turbulent
Abstract: The propeller jet from a ship has a significant component directed upwards towards the free surface of the water, which can be used for ice management. This paper describes a comprehensive laboratory experiment where the influences of operational factors affecting a propellerwakevelocityfield were investigated. The experiment was done on a steady wakefield to investigate the characteristics of the axial velocity of the fluid in the wake and the corresponding variability downstream of the propeller. The axial velocities and the variability recorded were time-averaged. Propeller rotational speed was found to be the most significant factor, followed by propeller inclination. The experimental results also provide some idea about the change of the patterns of the mean axial velocity distribution against the factors considered for the test throughout the effective wakefield, as well as the relationships to predict the axial velocity for known factors.
simulations are carefully validated and recent flow visualizations methods such as Particle
Image Velocimetry (PIV) are great candidates for this purpose. PIV allows for practically non-
intrusive measurement of the velocityfield in contrast to point measurements using other
methods that provides plenty of data for validation. Recently, several studies have been done
5.6.3 Simulation of 2D Wind Tunnel Environment
It is widely known that the presence of wind tunnel walls affects the flowfield around a test article, and the effect is accentuated with high values of flow curvature and lift. The wind tunnel walls act as a no-flux plane and thus increase the chord-normal pressure gradient as the flow must return to the straight freestream conditions in less distance than if the walls were not present. To quantify the effects of the testing environment, a computational simulation was executed with the wind tunnel walls acting as the outer domain boundaries, as shown in Fig. 5.50. The quasi-two-dimensional simulation did not simulate the entire span of the airfoil due to computational cost and computing limitations. Instead, a slice of the airfoil was simulated with the wind tunnel walls. No-flux reflection-plane boundary conditions were specified for the top and bottom wind tunnel walls in addition to the reflection plane walls on the ends of the airfoil. The inlet plane was prescribed to be a characteristic inflow/outflow plane while the outlet plane was defined to be a full- extrapolation surface; this boundary condition was necessary to obtain computational convergence. A visualization of the computational domain and grid, with the wind tunnel walls defined by the outer edges of the domain, is presented in Fig. 5.50(a). The resulting flowfield, shown in Fig. 5.50(b), exhibits many of the same characteristics as the free-air computational simulations discussed earlier in this chapter. Results indicate a large, thick burst wake with a minimum velocity in the wake of approximately 0.2, which is significantly less than the experimentally-collected value of e U t = 0.71. These results indicate that the differences between the experimental and computational results are
The wake vortices generated by large aircraft pose danger to following aircraft, especially during landing and take-off , which limits the capacity of airports [3,4]. Therefore, the dynamics of wing tip vortices after roll-up and until their decay is an important subject which has been studied by many researchers [5,6].
Experiments were conducted on a micro air vehicle (MAV) model in a low speed wind tunnel to study the actual lift and drag experienced by the model under propeller induced flow by ostracizing the thrust force generated by the propeller by decoupling the motor-propeller from the model and mounting it on a separate arrangement with minimal flow interference. Tests were conducted on the model at actual flight conditions - at a freestream velocity of 9 m/s (Re = 135000 based on root chord) with the propeller running at 8000 rpm. The lift and drag coefficients obtained from the model with decoupled motor-propeller arrangement are compared to those obtained from the model with attached motor-propeller for the same test conditions and justification is made in favor of the former method. Effects of propeller induced flow with respect to increase in propeller rpm on the lift and drag characteristics of the model were also studied. Higher C L at higher angle of attack and increased C D were
Many Investigators have designed and used different shapes and sizes of probes based on the capacitance method. The experimental results obtained by this method, notably by Bakker and Heertzes (5) show the existence of three distinct zones* a sieve effect zone near the bed support, a zone of constant porosity reaching up to the initial bed height at incipient fluidization and a third zone at the top with increasing porosity, A serious criticism of this experi mental set up is due to the neglect of the interfering effect of the electrostatic field. The interference could have been eliminated by grounding the column wall. The extent of the influence caused by the lack of grounding during the experimental measurements using a capacitance probe may be catastrophic to the whole effort.
A comparison of characteristics exhibited by model with attached motor-propeller and model with decoupled motor-propeller arrangement is inevitable to justify the work carried out here. Figure 7 shows the comparison of C D at a freestream velocity of 9 m/s and with the propeller running at 8000 rpm for both the test cases. The C D of the model with attached motor-propeller shows lower values. In testing of model with attached motor-propeller, along the longitudinal direction of the model, the balance measures the resultant of two oppositely acting forces – the axial force experienced by the body and the thrust produced by the propeller. The resultant force measured by the balance may be positive or negative depending on the thrust produced. If the thrust dominates the axial force experienced, it would result in a negative axial force. Figure 8 compares the axial forces of the two test cases. The dominance of thrust can clearly be seen in terms of negative axial force, for the model with attached motor-propeller case. Since drag is calculated from this resultant axial force, it shows lower values. On the other hand, in testing the model with decoupled motor-propeller arrangement, the propeller induced flow is simulated over the model with the thrust produced not being measured. Hence the balance purely measures the axial force
The governing equations have been discretized using the finite-volume method on a fixed Cartesian-staggered grid with non- uniform grid spacing. The grids in the region of the cylinder boundaries are sufficiently fine in order to achieve the reasonable accuracy. The temporal discretization has been done in conformity with the fully implicit scheme. For the spatial discretization, the hybrid scheme has been employed. PISO (Pressure-implicit with splitting of operators) procedure was applied to calculate the flow field variables.
The wake behind the wind turbine rotor is due to kinetic energy extraction but also to blades and hub drag. There are two distinct regions in the wake: the near wake, immediately behind the rotor, where the individual presence of each rotor blade can be observed, and the far wake where the large-scale vortex structures become dominant. To understand the far wake development, it is necessary to have a good knowledge of the near wake structure immediately behind the rotor. The flow in the near wake is dominated by blade-tip vortices. These tip vortices play a significant role in wake meandering and diffusion. Emanated from blade tips, they remain close to the rotor and produce a very complex velocityfield with high gradients. These vortices are also a major source of unsteadiness, aerodynamic noise and aerodynamic interaction. Therefore, it is important to investigate the vortex trailed from blade tip in order to gather comprehensive information about vortex intensity, swirl velocity distribution and vortex diffusion depending on vortex age. After a certain distance from the rotor, tip vortex breakdown occurs and then the wake becomes highly turbulent with irregular vortex structures.
And the images captured from high speed camera were analyzed by 2D digital image correlation method (2D-DIC) software MatchID-2D to track the motion of drop-weight and beam. The DIC technique can obtain a continuous field of displacement, strain components on the surface of specimen by comparing of the gray level distribution between deformed images and reference images. A Region of Interesting (ROI) is chose firstly as Fig. 2 shown. In this region, a lot of small sparkles are
Abstract. This paper provides an investigation into the inuence of wake and skew on a ship propeller performance, based on the potential Boundary Element Method (BEM). Two types of inow wake from a ship (i.e. Seiun-Maru and MS689) have been investigated for two propeller types; a Conventional Propeller (CP) and a Highly Skewed Propeller (HSP). The computed results include pressure distribution, open water characteristics and thrust uctuation for one blade and for all blades of the propeller. Calculations of the unsteady pressure distributions, thrust and torque are in good agreement with experimental data. In addition, the eect of propeller skew angle on the performance of thrust and torque, is investigated. Keywords: Skewed propeller; Inow wake; Hydrodynamic performance.
Fig.6 and Fig.7 show the influence of cavity length and diameter on axis velocity attenuation of organ pipe nozzle, which can be observed that the greater the diameter was, the slower the attenuation of the axis velocity was, and there was an optimal cavity length (L = 21 mm),which could keep the axial speed characteristic good. Thus it could be seen that large diameter increase the feedback of interior vortex ring, which was helpful for self-resonating. The cavity length effected the pulse period, so it existed an optimal value.
The research of car air vents is mostly focused on a complex view of airflow inside the car cabin. Yang  applied PIV method on a simplified model of passenger car cabin. Lee  performed experimental measurement using PIV method in a real cabin of a passenger car. He measured three vertical planes in total. The first plane led through the driver's seat, second divided the car in two halves and the last vertical plane led through the seat of co-driver. Significant differences in shape of airflow fields which led through the middle of both seats were explained by presence of driving wheel and brake. However, these components are often missing in idealized models of car cabins. Herwig  applied LDA method on
In the supersonic regime, the use of MVG is largely associated to the control of shock wave/boundary layer interactions (SWBLI), a phenomenon occurring in supersonic inlets among other devices. Several investigations have been conducted by means of experiments and numerical simulations. In the pioneering work of Holden and Babinsky , the effect of micro ramps and micro vanes installed upstream of a normal shock wave was investigated and a reduction of flow separation was observed. Also, the mechanism of the streamwise pair of vortices, initially visualized in subsonic flows [2,3], was also revealed in the supersonic regime. A detailed flow topology was inferred by Babinsky et al.  who conjectured the existence of additional secondary vortices using oil flow visualization. A further study of Nolan & Babinsky  quantified the velocityfield in the MVG wake by means of laser Doppler anemometry (LDA). The spatial velocity distribution in the micro ramp wake was also accessed with particle image velocimetry (PIV) within planes parallel to the flow floor by Blinde et al. . Inside the velocity cross-sections, wall-normal counter-rotating vortex pairs were visualized at the edge of the micro ramp wake and were interpreted as imprints of hairpin-like vortices. A conceptual hairpin vortex model was then proposed representing the vortical structure resulting from a micro ramp.
It might seem that this problem is already well described, however, there are many difficulties, when it comes to the model slightly differing from the simple geometry of regular and symmetric cylinder. The main difference is that we have a cylinder with variable diameter, thus the Reynolds number of the experiment varies from 40000 to 50000. To further complicate the problem, we are talking about the model submerged in the boundary layer, which is to be explained later, so the velocity of incoming flow changes as the altitude of the model changes. This means that the boundary layer thickness developed on the model will vary as well. Additionally, the model remained covered by small grains from the civil engineering experiments, hence our last piece of known theory fails. Such rough surface forces the transition in the boundary layer on the model to occur at higher Reynolds numbers. On the other hand, the high rate of turbulence of incoming flow has opposite effect.
Gaussian profile at any downstream location is U m and one of the key criteria of
interest is the rate at which it decays downstream. Figure 9(a) shows the trend in maximum velocity decay for both the current data and Petersson et al.’s [3, 6] data. Figure 9(a) shows that the decay gradient for the maximum velocity at 1500 rpm is lower than that at 3000 rpm. So, the jet is dissipating more of its initial mean kinetic energy at the high propeller speed. In addition, Petersson et al. [3, 6] obtained a much lower maximum axial velocity than the current data, which is possibly due to the blade profile variations and the emphasis on thrust versus mixing in the case of the smaller 20 mm propeller. It is also reasonable to find that the decay rate in co-flow is lower than that in counter flow and no ambient flow.
In spite of the fact that turbulence is one of the very old issues in the domain of fluid mechanics, it has been remained unresolved. This phenomenon generally exists in most of the issues related to energy transformation, fluid flow, transmission systems, etc. . The possible method for the description of turbulence, with the help of general laws of continuous mechanics, was established by Reynolds at the end of the previous century. In this method, the fieldvelocity of turbulence is decomposed into two main components: one of them is related to the average motion and the second one to the time- dependent fluctuations of fluid velocity. Therefore, the most logical method for describing turbulence is related to the theories which are formed based on the statistical hypotheses related to dynamic equations of fluid flow. The statistical theory requires some information about the investigation of the probability density distribution function and the corresponding correlation functions. The higher level co-relational functions (like skewness and flatness …) lead to the
In order to clarify the relationship between the aerodynamic noise and the flow regime around the rotating blade of a propeller fan operated at the maximum efficiency point and the off-design point, the characteristics of fans with differ- ent solidity impellers were analyzed experimentally. At the off-design point, the broadband noise of the high solidity fan was much larger than that of the low solidity fan because the relative velocity increased according to the solidity and the noise sources increased because of the number of blades. In the case of the low solidity fan, the broadband noise due to wake vortex shedding was generated at the off-design point in the low flow rate domain and the maximum effi- ciency point because the relative flow around the blade separated easily.
to time is drawn. Figure 4 shows the unsteady investigation of the cavitation at a rotational speed of 900 rpm, an axial velocity of 0.5 m/s and an operating pressure of 58000 Pascal. Thrust force oscillation is also existent in non-cavitating conditions, especially at low values of advance coecient, and is dependent on the dynamometer load cell, free stream turbulence and larger loading on propeller blades. But, the nature of the oscillation and its amplitude dierence is appreciable with respect to cavitating conditions, and the main source of oscillation in Figure 4 is the presence of unsteady cavitation.